The lowered NOx emission was believed to result from the combined contributions of DSL pyrolysis products including char, tar, and pyrolysis gas py-gas to the reduction of NOx via their
Introduction
Background
Fig 1.1 Distilled spirit lees (DSL) disposal
In term of beverage industries, China is a large spirits-producing country
Therefore, the output of solid residue such as distilled spirit lees (DSL) generated in the unique solid-state fermentation process (Fig 1.1) is enormous all over the country In fact, DSL amounts to 20 million tons per year (Deng and Luo, 2004) and can be considered as a good biomass resource due to its being rich in cellulose and hemicellulose Traditionally, the DSL have sufficiently high nutrition content for being feedstuff or protein substrate of animals such as pig With progress in the biotechnology, the digestible nutrition in the lees becomes so low that it cannot meet the requirement
Mechanism of Low-NOx Emission in Circulating Fluidized-Bed Decoupling Combustion
(Deng and Luo, 2004; Zhang et al., 1997) However, the high moisture in DSL, even up to 60 wt %, and the properties of easy putrescibility and strong acidity can cause serious environmental problems, such as smelly gas release and underground water pollution (Xu et al., 2009a) Therefore, highly reliable technologies for clean, rapid and large-scale utilization of DSL are in a great demand in the spirits industry of China, especially for large distiller factories
The nature of rich in cellulose in DSL makes it hardly treated via biological conversion technologies The thermal conversion of the lees into the energy usable in the distiller factories, such as steam or fuel gas, is thus considered to be viable Burning DSL to produce steam in fact not only limits the pollution as a result of DSL disposal but also offers a part of energy required by distilled spirits production (Xu et al., 2007)
However, as a result of its relatively high N content (about 3 – 5 wt % on a dry basis), direct combustion of DSL via the traditional way has to release high NOx emission, which is one of the main gas-phase pollutants released in fuel combustion, not only injures human health but also forms acid rain and photochemical smog (Winter et al., 1999; Calvert, 1997) The NOx content in the flue gas can even reach up to 550 ppm during burning this material in a laboratory fluidized bed (Zhu et al., 2015)
In order to reduce NOx emission in combusting high-N biomass waste and also improve the combustion efficiency of a conventional circulating fluidized bed, the so-called circulating fluidized-bed decoupling combustion (CFBDC) technology has been investigated and developed by our Advanced Energy Technology Laboratory (AET Lab), Institute of Process Engineering (IPE), Chinese Academy of Sciences (CAS) The technology has been well proven by an industrial system treating DSL The actual running data show that the NOx emission was lowered by about 70% comparing to the traditional CFB combustion (Han et al., 2015; Yao et al., 2011; Xu et al., 2010), making the NO content in its flue gas be 120 – 170 mg/Nm 3 for the DSL containing N of about 4.0 wt.% This kind of technologies can also be applied to many other lees and residues such as vinegar lees and Chinese herb residues generated in various light industrial processes (Yao et al., 2011)
The CFBDC technology is based on the reaction decoupling concept which separates the combustion process into drying/pyrolysis of fuel and combustion of pyrolysis-generated char and volatile Thus, the system is composed of a fluidized-bed pyrolysis reactor and a riser combustor, the pyrolysis-generated volatile consisting of non-condensable pyrolysis gas (py-gas) and condensable tar is sent to an intermediate position of the riser combustor to allow its co-burning with char, as conceptualized in Fig 1.2 The co-burning of fuel pyrolysis products in such an intermediate position can be considered as a reburning way that effectively reduces the NOx formed by burning char in the bottom zone Therefore, more fundamental studies refer to reburning chemistry are suggested to further understand the low-NOx emission mechanism in CFBDC system This would facilitate the technology scale-up and also contribute to update the technical designs for CFBDC
Fig 1.2 Principle conception of circulating fluidized bed decoupling combustion (CFBDC).
Objectives and Significance
The NOx reduction mechanisms in the developed coal/biomass combustion devices have been systematically studied by AET Lab to determine better control strategies associated with the technologies involving decoupling conception Thereby, the reduction effects of char and reducing gas on NO (as the main component of NOx in most practical flue gas) were analyzed in detail (Cai et al., 2013; Dong et al., 2010, 2009, 2007; He et al., 2006) Recently, tar species derived from the pyrolysis of DSL was found to reduce NOx significantly by the analysis in micro-fluidized bed reactor
Mechanism of Low-NOx Emission in Circulating Fluidized-Bed Decoupling Combustion of batch or semi-batch reactors adopted In addition, those studies are still far from fully understanding the interactions occurred within the practice system since the combined actions of NOx reduction in the reburning zone by different agents including char, tar and py-gas have not been previously considered Above all, the complexity of the NO reduction reactions required that further experimental work must be carried out in order to understand some key aspects of the process Therefore, it is highly worthwhile to deepen such studies for further understanding the mechanism of low-NOx emission in CFBDC treating not only DSL but also other fuels such as sawdust or coal To implement this plan, a continuous drop-tube reactor (DTR) was indeed adopted to facilitate the investigations on the characteristics of NO reduction using char, tar and py-gas from pyrolysis of different fuels as reagents, these aim to
(i) figure out the dominant NO reduction reactions for CFBDC, (ii) provide the optimal conditions for operating CFBDC system in term of achieving high NOx reduction, (iii) reveal the synergetic effect among char, tar and py-gas on NO reduction occurring during their reburning in CFBDC, (iv) understand the combined homogeneous–heterogeneous reaction mechanism for NO reduction, (v) understand the different NO reductions for tars, chars derived from different fuels and the contributions of their constituents to the achieved NO reduction.
Thesis Outline
both individual and combined actions, (ii) reduction of NO by reagents derived from different fuels in comparison with that by DSL-derived products
The contents are divided into seven chapters The first chapter served as an introduction where the brief background of CFBDC are given, followed by the research objectives and approach
Chapter 2 provides a literature review about the theoretical background and current state of research essential for the discussion of experiment results
Chapter 3 introduces the materials and the main experimental techniques used in this study Namely, the preparation of NO-reduction reagents from DSL, the design of experimental apparatus, the procedure and analysis of experiments were described in detail
Chapter 4 begins presenting the results of this study The NO reduction characteristics of DSL-derived char, tar and py-gas are shown and discussed, from which the influences of reagent feeding rate, reburning stoichiometric ratio (SR), reaction temperature, residence time, initial composition of flue gas on NO reduction by each reagent are put into evidence
Chapter 5 covers the synergetic effect of reagents on NO reduction The results of
NO reduction tests using the mixtures of binary reagents among char, tar and py-gas derived from DSL are given and discussed in function of SR, temperature and residence time Moreover, the effects of gas species in py-gas such as CO, H 2 , CH 4 on NO reduction by char/py-gas mixture are presented
Chapter 6 examines the NO reduction by tars, chars derived from different fuels such as such as SD and XF coal The results in this chapter are brought together with those in Chapter 4 in order to compare the capabilities of different reagents for reducing NO The difference in NO reduction by chars is discussed based on the analysis of catalytic matter in ashes, while the contribution of some components in tars to the overall NO reduction is provided in addition to explain the reactivity of tars for NO removal
Chapter 7 briefly summarizes the key conclusions of this study and provides a few recommendations for future research directions.
Literature Review
Nitric Oxides
2.1.1 Sources of NOx There are two sources of NOx emission generated by human activities such as mobile sources and stationary sources Automobiles and other mobile sources contribute about half of the NOx that is emitted, while electric power plant boilers produce about 40% of the NOx emissions from stationary sources (United States Environmental Protection Agency, 1999) The substantial emissions are also added by such sources as industrial boilers, incinerators, gas turbines, reciprocating spark ignition and diesel engines in stationary sources, iron and steel mills, cement manufacture, glass manufacture, petroleum refineries, and nitric acid manufacture In addition, biogenic or natural sources of nitrogen oxides including lightning, forest fires, grass fires, trees, bushes, grasses, and yeasts are also involved in the global NOx emission These various sources produce differing amounts of NOx but almost three-quarters of the total amount of NOx emission is contributed by human activities through combustion of fossil and alternative fuels (including field burning and forest fires) (Topsoe, 1997; Bosch and Janssen, 1988)
2.1.2 NOx Emission in China China is the largest NOx emission country in Asia contributing 41% – 57% of Asian NOx emissions (Fei et al., 2016; Wang et al., 2014; Wang and Hao, 2012; Zhao et al., 2008; Ohara et al., 2007; Mauzerall et al., 2005) With the rapid growth of energy consumption, NOx emissions were estimated to more than double from 11.0 Mt in 1995 to 26.1 Mt in 2010, with an annual growth rate of 5.9% Power plants, industry and transportation were major sources of NOx emissions, accounting for 28.4%, 34.0%, and 25.4% of the total NOx emissions in 2010, respectively (Zhao et al., 2013b) In 2014,
2.5 million tons of NOx were emitted from industrial boilers, which accounted for 12.3% of total Chinese NOx emissions Industrial boilers are the third largest emission source, after power plants and vehicles (Ministry of Environmental Protection of the People’s Republic of China, 2014) Based on current legislation and implementation status, defined as a business as usual (BAU) scenario according to Zhao et al (2013b), NOx emissions in China are estimated to increase by 36% in 2030 from 2010 level In detail, the trend in NOx emissions and that prediction for 2020 and 2030 of each province in China are listed in Table 2.1
Table 2.1 Provincial NOx emissions during 2005 – 2030 (Mt) in China (Zhao et al., 2013b)
Province 2005 2010 Business as Usual (BAU) Scenario
To mitigate the adverse effect of air pollution, the Chinese government released many policies assuring enforcement of the control programs to reduce NOx emissions during the 12 th Five-Year Plan (2011 – 2015) (The State Council of the People’s Republic of China, 2011b) This implied rapid installation of control measures, namely most coal-fired power plants are requested to be equipped with flue gas denitrification (The State Council of the People’s Republic of China, 2011a, 2010) In addition, emission standard for thermal power plants has been continually updated, and the Euro IV emission standard for heavy-duty diesel vehicles was implemented (Ministry of Environmental Protection of China, 2011) As a result, the governmental target of reducing 10% NOx emissions by 2015 (compared to 2010) has been achieved (Jin et al., 2016; Shi et al., 2014; Zhao et al., 2013a) However, the proportion of industry increases despite the control measures, reflecting the lower ambition of explored policies with respect to control of industrial NOx emissions For example, as shown in Table 2.1, Shandong, Henan, Jiangsu, Guangdong, and Hebei provinces were top five emitters of NOx emissions in 2010, each of which had over 1.5 Mt NOx emissions and contributed together over 35% of total emissions, and those continue to have the largest emissions in the next 5 – 10 years (Zhao et al., 2013b).
Low-NOx Emission Strategy
2.2.1 NOx Formation During Fuel Combustion NOx emissions from combustion are commonly considered to be comprised of nitric oxide (NO) and nitrogen dioxide (NO2) For most combustion systems, significant evidence exists to show that NO is the predominant NOx species (over 95% of the total) (Meadows et al., 1996) However, for purposes of emissions control, NOx is defined as the sum of NO and NO 2 fully converted to NO 2 or vice versa This corresponds to the output of most NO measurement technique
The formation of NOx during fuel combustion is a complex interaction among chemical, physical, and thermal processes occurring simultaneously within the device
Generally, NOx is formed either from fixation of N 2 in the combustion air at high temperatures or from oxidation of nitrogen chemically bound in fuel (Glarborg et al., 2003) To help simplify the understanding of NOx formation and assist in identifying control strategies, NOx is typically considered to form through three following mechanisms
Thermal NOx is formed by the oxidation of atmospheric nitrogen by free oxygen atoms in the higher temperature regions of the combustion flame The formation of thermal NOx proceeds through the following reactions sequence known as Zeldovich mechanism (Miller and Bowman, 1989; Zeldovich, 1946) The first step (2.1) has high activation energy so that the formation of thermal NO is most important at temperatures above 1800 K
Prompt NOx is formed by chemical reactions between atmospheric nitrogen and fuel-derived hydrocarbon radicals and subsequent oxidation In this way, the CH radicals attack on the N2 triple bond, and then the reactive nitrogen compounds may subsequently be oxidized to NOx or recycled to N 2 dependent on reaction conditions (Williams et al., 1994; Miller and Bowman, 1989; Glarborg et al., 1986), as follows:
CH + N 2 ⟷ HCN + N , CH 2 + N 2 ⟷ HCN + NH , CH 2 + N 2 ⟷ H 2 CN + N , C + N 2 ⟷ CN + N
(2.4) (2.5) (2.6) (2.7) Those reactants continue to react with O to form NOx, as
HCN + O ⟷ NO + CH , NH + O → NO + H
The total mechanism of the prompt NOx formation is complex It was reported that the prompt NOx is usually formed in the fuel-rich zone close to the burner that is of high concentration of CH and low concentration of O2 However, the formation is often assumed negligible during biomass fuel combustion (Nussbaumer, 2003; Skreiberg et al., 1997)
Fuel NOx is formed from chemical reactions involving nitrogen atoms chemically bound within the fuel component species (fuel-N) The detailed fuel-N behavior during the thermal conversion of carbonaceous fuel is illustrated in Fig 2.1
Chapter 2 Literature Review like organic nitrogenated compounds (tar-N) and N2 The rest part of fuel-N is left in the char (char-N) In turn those NOx precursors, tar-N, char-N are emitted as NOx/N 2 O after being combusted and gasified (Mukadi et al., 2000; Tan and Li, 2000) Considering fuel-NOx is the main part which cover about 80% of NOx formed during fuel combustion, the control of NOx emission in thermal conversion of carbonaceous fuel is usually based on the fuel-N content
Fig 2.1 Fuel NOx released during the thermal conversion of carbonaceous fuel (Chen, 2012)
2.2.2 NOx Reduction Technologies Various techniques for reducing NOx emission, which are summarized in Fig 2.2, have been applied on fuel-fired systems (Baukal, 2004; Park et al., 2001; Tomita, 2001;
Hill and Smoot, 2000; Smoot et al., 1998; Teng and Huang, 1996; Mereb and Wendt, 1994) Those can be classified into two fundamentally different categories: combustion controls reducing NOx formation during the combustion process and post-combustion controls reducing NOx after it has been formed Combustion control techniques mainly include low-NOx burners (LNBs), reburning, overfire air (OFA), flue gas recirculation (FGR), operational modifications and so on, while selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR) are two well-known technologies in post-combustion controls (United States Department of Energy, 1999)
Fig 2.2 Most popular technologies for NOx emission control
Low-NOx Burners – LNBs are designed to control the mixing of fuel and air so as to achieve staged combustion This results in a lower maximum flame temperature and a reduced oxygen concentration during some phases of combustion, thus resulting in both lower thermal NOx and lower fuel NOx production
Overfire Air – OFA is the air that is injected into the burnout zone located above the normal combustion zone of furnace OFA is generally used in conjunction with operating the burners under fuel-rich conditions, which reduces NOx formation
The OFA is then added to achieve complete combustion In addition, OFA is frequently used together with LNBs in combustion system
Fuel Reburning (or fuel-staging) combustion system consists of three zones in series: (1) a combustion zone operating under slightly fuel-lean conditions; (2) a reburning zone, where added fuel results in a fuel-rich, reducing condition; and (3) a burnout zone, where the OFA is injected leading to complete of the fuel combustion With a typical reburning process, about 10% – 30% total heat input is added into reburning zone, where fuel-rich conditions allow reducing up to 70%
NOx formed in the normal combustion zone (Smoot et al., 1998) Coal, oil, gas
Selective Noncatalytic Absorptions Post-combustion controls Combustion controls
Flue Gas Recirculation – FGR, in which part of flue gas is recirculated to the combustion zone, can be used to modify the fuel-burning conditions, namely lowering temperature and reducing oxygen concentration to reduce NOx formation In practical system, FGR is also used as a carrier to inject fuel into the reburning zone to increase penetration and mixing
Operational Modifications involve several methods changing the operational parameters to create conditions in the furnace that can lower NOx formation, e.g., burners-out-of-service (BOOS), low excess air (LEA), and biased firing (BF) In BOOS, selected burners are suspended by temporarily stopping fuel injection, but air flow is maintained to create staged combustion in the furnace LEA involves operating at the lowest possible excess air level but maintaining good combustion, and BF creates staged combustion conditions in the furnace by injecting more fuel to some burners (typically the lower burners) while reducing fuel to other burners (typically the upper burners)
Selective Noncatalytic Reduction – SNCR is a simple process, referred to as
“thermal deNOx”, and involves the reduction of NOx to N2 in the presence of oxygen by reaction with a reducing agent (typically amine-based reagent such as ammonia or urea at 1073 – 1273 K The critical factors in applying SNCR are sufficient residence time in the appropriate temperature range and even distribution and mixing of the reducing agent across the full furnace cross section
Selective Catalytic Reduction – SCR is a typical technology which has been applied on stationary combustion units for effectively reducing NOx emission In SCR, a catalyst bed is installed downstream of the furnace A reducing agent such as ammonia is injected into the flue gas before it passes over the fixed-bed catalyst
Decoupling Combustion (DC) for Lowering NOx Emission
2.3.1 Principle of Decoupling and DC Technology Thermochemical conversion of solid carbonaceous fuels is generally shown with three types of process technologies: pyrolysis (including coking and carbonization), gasification, and combustion These processes actually involve a similar complex reaction network as shown in Fig 2.4, those are resulted from differentiating the oxygen amount fed to the conversion system and the typical operation temperatures, as follows - The combustion process requires an excessive oxygen supply to convert all
C and H elements in fuel into CO 2 and H 2 O
- The gasification refers to the conversion of a solid fuel into CO, H 2 and light
- The pyrolysis process indicates the conversion of fuel without oxygen requirement or with a certain amount but much less than its demand in gasification
Fig 2.4 Chemical behaviors occurring in thermochemical conversion of solid fuels (Zhang J et al., 2013)
In Fig 2.4, the reactions in the box occur in sequence and are intercorrelated or interactive With heat supplying, solid fuel is first dried and pyrolyzed to produce char, tar, steam, and non-condensable pyrolysis gas mainly consisting of H2, CO, CO2, and CH4 Then, other reactions start to occur, accelerating a series of interactions among various reactions such as cracking, hydrogenation, reforming and so on Among these interactions, some can facilitate the conversion to lower pollutant emission, increase process efficiency and product quality, and widen fuel adaptability, but some others are not In order to avoid or weaken the effects of undesired interactions for optimizing the conversion process, the related reactions should separately occur in the rearranged reaction zones This idea of reaction control has been termed “decoupling” and further developed by AET Lab (Zhang J et al., 2013; Zhang Y et al., 2013; Xu et al., 2010, 2009b, 2008; Zhang et al., 2010)
In conventional combustion (CC) process, solid carbonaceous fuels are generally burned in a single reaction vessel (combustor) and completely coupling all the involved reactions, resulting in difficult control of pollutant emission and combustion efficiency
Many technical ways have been developed to optimize the combustion process, such as air-staging (OFA), reburning, FGR, LNBs; as described in Subsection 2.2.2, aiming to lower NOx emission Those advanced combustion technologies are actually correlative with decoupling, but it was not commonly reported in literatures This conception was only introduced into the combustion area for the first time as a so-called decoupling combustion (DC) by researchers in IPE (Li et al., 1997) The technology has been developed to reduce the NOx emission during fuel combustion through separating the involved reactions of combustion process according to the principle illustrated in Fig 2.5
Fig 2.5 Principle of decoupling combustion
Up to now, many studies on the NOx emission and reduction during the solid fuel combustion have been carried out to demonstrate the advantage of DC in efficiently lowering NOx emission (Han et al., 2015; Xu et al., 2010; Dong et al., 2009, 2007; He et al., 2006) The decoupling conception is in fact implemented in combustion process via two typical ideas (decoupling modes) which are isolating and staging the fuel pyrolysis and char combustion reactions Table 2.2 thus summarizes two DC technologies representative for such two decoupling modes and their realized decoupling effects
Table 2.2 Typical combustion technologies and their features by involving decoupling (Zhang J et al., 2013)
Staging Grate-based DC (fixed bed) (Li et al., 1997)
Burning the released volatiles in combusting char bed to lower NO and CO emissions
Isolating CFB-based DC (fluidized bed) (Xu et al., 2010)
Reburning the released volatiles in transporting bed of char in burning to lower NO to CO emissions
2.3.2 Low-NOx Emission in Grate-Based DC
Chapter 2 Literature Review zone, a combustion zone and a post-combustion zone Firstly, fuels are pyrolyzed in the pyrolysis zone to produce char and volatiles including tar and py-gas Char is in turn moved into the combustion zone due to gravity so that it could be burned together with the released volatiles Then the unburned volatiles are forwarded to burnout in post-combustion zone In this way, the NOx reduction effects are believed to occur when the volatiles are passing through the char bed in combustion zone
Fig 2.6 Configuration of a domestic stove using DC
By comparing different combustion modes that may occur in real stove, He et al
(2006) reported that the obtained low NOx emission in DC stove burning coal is mainly attributed to the catalytic reduction of NO over hot char particles The reactions and mechanisms of NO reduction over char have been widely reported in literatures (Chambrion et al., 1998; Suzuki et al., 1994; Yamashita et al., 1993), also the NO generated in char combustion can be reduced by CH4, H2, and CO in py-gas and gaseous tar component as well (Giral and Alzueta, 2002; Rüdiger et al., 1997) Above all, the surface and structure of char particles were found to be the dominant factor in such DC stove because most of NO reduction reactions take place on char surfaces in combustion zone (Dong et al., 2010; Dong et al., 2007)
In view of application, the NOx emission from DC stove burning coal was reduced to less than 200 mg/m 3 (excess O2 at 7%), which is up to 40% lower than that of traditional stoves (Li et al., 1997) Moreover, complete combustion of the pyrolysis gas in DC also led to lower emissions of CO and soot, thus resulting in the higher combustion efficiency Dong et al (2009) also applied DC on grate furnace for burning biomass (rice husk, corn straw and sawdust) and biomass–coal blends Both their tests in a quartz dual-bed reactor or in a real stove demonstrated that the NO emission and the corresponding conversion of fuel-N to NO were obviously lower for DC than for CC Currently, different types of household stoves and industrial boilers using DC in capacities of 0.01 − 0.7 MW are widely equipped in rural areas of China for cooking, heating, and hot water supply (Zhang J et al., 2013)
2.3.3 Low-NOx Emission in CFBDC
Technical Description of CFBDC The DC can be also implemented in circulated fluidized-bed (CFB) combustor for lowering NOx emission and promoting combustion efficiency, which is in fact representative for the “isolating” decoupling approach The property of low NOx emission in the so-called circulating fluidized-bed decoupling combustion (CFBDC) was fully verified by the actual running data of a CFBDC system treating DSL in demonstration plant (35000 ton/a), namely the NOx emission was lowered by about 70% comparing to the traditional CFB combustion (Han et al., 2015), making the NOx content in its flue gas be 120 – 170 mg/Nm 3 for the DSL containing N of about 4.0 wt.%
Fig 2.7 A schematic diagram of CFBDC system and possible mechanisms for NO reduction
Fig 2.7 illustrated the schematic diagram of a CFBDC system, it involves two fluidized-bed chambers, such as a fuel pyrolyzer (or gasifier) and a riser char combustor, and the heat carrier particles (HCPs) between the two chambers are circulated to carry the heat of combustion into the pyrolyzer for the reactions of fuel pyrolysis Different from CFB combustion, in CFBDC the fuel is supplied into pyrolyzer and the generated volatiles are fed into the middle of the riser char combustor while the produced char mixed with HCPs is forwarded to the bottom of the riser combustor to generate heat
Therefore, the combustion in the middle of riser combustor could be considered as a reburning zone following commercialized reburning technology, in which not only the homogeneous interaction between py-gas and flue gas (i.e., gas reburning) (Cai et al., 2013) but also the reduction effect of the entrained char on NOx can contribute to lowering the NOx emission (Dong et al., 2010) In addition, the tar species derived from fuel pyrolysis can reduce NOx significantly (Song et al., 2014)
NOx-Reduction Chemistry of Reburning Fuel reburning is a well-known low-NOx combustion technique Many studies have been performed on reburning of different fuels including gas (e.g., natural gas and other hydrocarbon fuels), liquid (e.g., residual fuel oil) and solid (e.g., coal and biomass) (Meadows et al., 1996) Considering NO as the main component of NOx in most practical flue gas, the mechanisms of lowering NOx in reburning were almost studied through considering NO reduction reactions in terms of gas-phase homogeneous reactions between volatiles (gaseous fuel, tar and py-gas) and NO and gas-solid heterogeneous reactions between char and NO
As mentioned above, the pyrolysis products including char, tar and py-gas are considered as reburning fuels conducting to an intermediate position of the riser char combustor of CFBDC system Therefore, the occurred mechanism of NO reduction in the system is quite similar to that in the commercialized reburning technology The chemistry of reburning is very complex although many fundamental studies on it have been reported (Luan T et al., 2009; Casaca and Costa, 2005; Glarborg et al., 2000;
Smoot et al., 1998) The technologies with different types of reburning fuel or other features have their respective characteristics, for example in CFBDC system both heterogeneous and homogeneous mechanisms were involved to form a combined action of char, tar, and py-gas in reducing NO, as also illustrated in Fig 2.7 Following are the summaries of NO reduction reactions which may occur in CFBDC
Homogeneous Mechanism of NO Reduction:
In the homogeneous mechanism, light hydrocarbons from existing components and gaseous tar cracking decompose into C-containing radicals (e.g., CH i and HCCO) via partial oxidation (Shu et al., 2015b; Zhang R et al., 2014), as
The generated C-containing radicals via reaction (2.10) in turn react with NO existing in the reburning zone to generate nitrogenous radicals, for example (Shu et al., 2015b; Dagaut et al., 2000b):
CH i + NO → HCN + … , HCCO + NO → HCN + HCNO + …
Material and Methodology
Preparation of NO-Reduction Reagents
3.1.1 Feedstock Material As mentioned above, the experiments performed in this study simulate the reburning process that occurs within the CFBDC system treating distilled spirit lees (DSL) (Han et al., 2015) This kind of biomass waste, provided by Luzhou Laojiao Group Company of China, was thus chosen as the feedstock material to prepare NO-reduction reagents The characteristic of the adopted material was firstly examined by the proximate analysis following the national standard method of China for solid biofuels – GB/T 28731-2012 In addition, the ultimate analysis was analyzed by the CHNS element analyzer (VARIO MACRO CHNS analyzer with an oxygen kit;
Elementar Co., Langenselbold, Germany) Table 3.1 shows the proximate and ultimate analyses of the received DSL sample, the sample contained about 51.22 wt % water and had a relatively high content of N, say, 3.5 wt % on the dry and ash free basis (daf)
The size of DSL as received was in range of 2.0 – 5.0 mm
Table 3.1 Proximate and ultimate analyses of raw distilled spirit lees (DSL)
3.1.2 Pyrolysis Setup and Procedure NO-reduction reagents used in this study were prepared from DSL in a pyrolysis setup which was mainly composed of a reaction system and a volatile-collection system, as illustrated in Fig 3.1 In the reaction system, a horizontal fixed bed quartz reactor (reactor tube) of 42 mm in inner diameter and 1250 mm in length was mounted inside an annular electric furnace together with a K-type thermocouple to measure the temperature at an intermediate position of the furnace, and this temperature was treated as the pyrolysis temperature The volatile-collection system consisted of two condensing tubes, three acetone scrubbing bottles and a tar collection flask These bottles and flask were immersed in an ice-water bath to keep the expected cooling conditions for possibly highest condensable volatile (tar) yield while the condensing tubes were cooled down by cold-water stream from a circulating water bath.
(1) Reactor (quartz tube); (2) Horizontal furnace; (3) Condensing tubes; (4) Ice-water bath;
(5) Tar collection flask; (6) Acetone scrubbing bottles; (7) Wet gas meter
Fig 3.1 A schematic diagram of pyrolysis experiment
The pyrolysis temperature was maintained at 500 o C by a temperature control unit, this temperature is quite close to those adopted for fuel pyrolysis in CFBDC that ensure the stable combustion in the riser (Yao et al., 2011) In addition, at high temperatures, cracking of tar should occur to decrease tar yield and make the collected tar be mainly heavy tars As in coking, the produced tar is mainly pitch or asphaltene The pyrolysis temperature is thus set at 500 °C in this study with the concerns of having higher tar yield (with least secondary reactions)
The temperature profiles obtained from maintaining the furnace temperature of 500 o C and 600 o C are shown in Fig 3.2 As shown, there was a central zone (approximately 400 mm in length) where the temperature could be considered fairly uniform Therefore, the material should be placed in this zone of reactor (reaction zone) to gain a high efficiency of pyrolysis process as well as a homogeneous feature for char particles obtained With the given sizes in range of 2.0 – 5.0 mm, about 100 g of the dried DSL, obtained by exposing raw material in air for 24 h to reach a moisture of approximately 10%, was fully loaded into the defined reaction zone for a batch process of DSL pyrolysis
Fig 3.2 Temperature profiles along the horizontal axis of furnace
After loading a certain amount of material, the air inside reactor tube was removed by introducing high purity N2 gas at a flowrate of 1 L/min for at least 10 min to form an inert atmosphere The pyrolysis process was started with heating the furnace to a preset temperature of 500 o C at which the reactor tube loaded with the given amount of DSL was quickly placed into the furnace and in turn quickly connected to the volatile-collection system The gaseous pyrolysis product coming out from the reactor was cooled immediately in the tar collection flask immersed in ice-water bath and the condensing tubes The formed liquid tar including water was collected in the flask while the other condensable light tar was further absorbed by acetone in the scrubbing bottles
The remaining non-condensable gas was burned before being released into the atmosphere The process was ended after about 30 min as no gas bubble appeared in the scrubbing bottle, this ensured the complete release of volatile matter from the residual char inside reactor tube The reactor tube was taken out of the furnace and cooled to room temperature in inert atmosphere by a N 2 flow similar to that in the preparing step
The residual char from the cooled reactor tube was ground and sieved to gain a particle size range of 0.05 – 0.1 mm suitable for NO-reduction experiments The liquids from tar collection flask, from scrubbing bottle and from washing the entire pipeline and condensing tubes (by acetone) were blended together and the blend was then treated in a rotary vacuum evaporator to primarily remove acetone The evaporated liquid was further dehydrated using MgSO4 and filtrated to remove any dust intake Finally, the filtrate was evaporated in the rotary vacuum evaporator to fully recover pure tar ready for NO-reduction evaluations Because the storage of non-condensable pyrolysis gas (py-gas) required a complicate system, the used py-gas for NO-reduction tests was a model gas made according to the analyzed composition of the sampling gas which was obtained in a batch pyrolysis
3.1.3 Characteristics of NO-Reduction Reagents Similar to the raw material, the characteristics of its pyrolysis products were firstly examined by proximate and ultimate analyses Table 3.2 shows the results of proximate and ultimate analyses for DSL pyrolysis-derived char, tar, and py-gas
Table 3.2 Proximate and ultimate analyses of DSL pyrolysis products
Sample Proximate (wt %, db) Ultimate (wt %, daf)
Such analyses of char or tar were carried out similarly to that of DSL, whereas the ultimate analysis of py-gas was calculated from the composition of the model py-gas used The non-condensable gas from a batch pyrolysis of DSL was collected to analyze its composition in a micro gas chromatography (micro GC) (Agilent GC 3000; Agilent Technologies Co., Santa Clara, USA) The composition analysis in Table 3.3 shows that the py-gas was composed mainly of H 2 , CO, CH 4 , C 2 H 6 , C 3 H 8 , and CO 2 Thus, these gas species (and N2 as balance gas) were used to prepare the model py-gas reagent for NO-reduction evaluation
Table 3.3 Composition analyses of DSL-derived py-gas (vol %)
H 2 CO CO 2 CH 4 C 2 H 6 C 3 H 8 Others a 5.993 27.00 12.97 11.97 0.682 0.0892 41.296 a by difference
Chapter 3 Material and Methodology silica capillary column, and helium was used as its carrier gas The temperature of the GC injector and detector was 280 °C The column temperature was first kept at 50 °C and subsequently heated to 280 °C via 6 °C/min and finally held at 280 °C for 10 min
The solvent delay time for GC–MS analysis was 1.7 min, and the scanning range of mass was from 20 to 900 m/z The concentration of each component was defined according to area percentage, the proportion of its corresponding peak area to the total area of all peaks Fig 3.3 shows in brief the major components detected in DSL-derived tar by GC–MS, the analysis revealed that phenol and phenolic compounds are the dominant chemicals in tar, in good agreement with other literature studies (Liu et al., 2012; Li and Suzuki, 2010; Vreugdenhil and Zwart, 2009) The detail of tar composition is presented in the Appendix A
Fig 3.3 Result of GC−MS analysis for DSL-derived tar
Furthermore, all products from a batch process of DSL pyrolysis were carefully collected and measured their amounts (weighted or volumetrically metered) to determine the pyrolysis products distribution, as shown in Table 3.4 This aims to ensure that the pyrolysis process carried out in this work for generating NO-reduction reagents is corresponding to that in CFBDC system (Han et al., 2015; Yao et al., 2011;
Table 3.4 Product distribution (wt %, db) from pyrolyzing DSL at 500 °C
Char Tar Py-gas Water a 50.25 13.18 10.95 25.62 a by difference.
Experimental Drop-Tube Reactor for NO-Reduction Evaluation
Fig 3.4 A schematic of experimental DTR system
Chapter 3 Material and Methodology and length of 850 mm, this guaranteed sufficient distance for stabilizing the simulated flue gas before entering the reaction zone The vertical chamber had an inner diameter of 100 mm and length of 1680 mm, including an effective reacting distance of 1320 mm which was considered as the reaction zone In order to minimize the heat loss, a few insulation layers made of ceramic fiber were wrapped around the main chamber
Furthermore, alloy steel was employed to cover outside the insulated main chamber body for good air tightness, and airtightness tests were performed before experiments
Fig 3.5 The main chamber in DTR system
3.2.2 Heating Control System The main chamber of DTR system was maintained at a high temperature by an electric heater of 9 kW for the horizontal (preheating zone) and that of 27 kW for the vertical chamber one (reaction zone) The heater of preheating zone consisted of three U-shaped silicon carbide (SiC) rods which enable the maximum temperature of 1300 oC, whereas that of the reaction zone included nine UL-shaped molybdenum disilicide (MoSi 2 ) rods could allow reaction temperature up to 1550 o C To maintain a constant temperature in the reaction zone, the heating control system, consisted of three temperature controllers (TCW–32B; Shanghai Guolong Co., Shanghai, China) corresponding to three sections of the vertical chamber, was employed to adjust the reaction temperature The temperature signals for heating control were collected from three Pt/Rh thermocouples (TI – TIII) measuring the temperatures inside such three sections Additionally, other five Pt/Rh thermocouples (T1 – T5) were distributed along the vertical chamber to monitor the temperatures at the local areas, ensuring a uniform temperature along the reaction zone The temperature inside the preheating zone was also adjusted by other TCW–32B controller and Pt/Rh thermocouple (TO) The positions of all heating rods, temperature controllers and thermocouples employed for the DTR system are indicated in Fig 3.5(a)
3.2.3 Reagent-Feeding System The fact is that the reagents for testing NO reduction in this study derived from fuel pyrolysis exist in different phases namely solid, liquid and gaseous phases corresponding to char, tar and py-gas reagents, respectively Therefore, a few types of feeder were designed suitable for different reagents, as shown in Fig 3.6, so that the feeder could continuously inject a specified amount of the related reagent into reactor during its test In addition, a cylinder of N2 and a mass flow controller were employed to supply a carrier gas flow for each reagent during its test
Fig 3.6 The reagent feeders: (a) Solid feeder, (b) Liquid feeder, (c) Gaseous feeder
Char, a solid reagent, was fed into reactor by a screw feeder placed at the top
Chapter 3 Material and Methodology char particles in appropriate size range of 0.05 – 0.1 mm supplied a stable amount of material loading on the trough An agitator was placed inside the hopper to avoid stagnant zones and promote a constant feeding rate The rotation of the screw feeder was controlled by a motor while the motor itself is controlled by an inverter, thus the feeding rate of char could reach desired values by adjusting the output frequency of inverter from a minimum value of 1.5 Hertz (Hz) and a minimum step of 0.1 Hz To guarantee the accuracy of char feeding rate, the screw feeder was carefully calibrated before each experiment
The liquid-reagent feeder consisted of a feeding probe, a thermostatic circulating water bath and a peristaltic pump, as shown in Fig 3.6(b) A constant amount of liquid reagent (tar) was continuously fed into reactor by a metering pump which is the peristaltic type (Lead-Fluid BT100S; Baoding Lead Fluid Technology Co., Hebei, China) The feeding rate of tar was adjustable by changing the frequency indicated on the peristaltic pump with a minimum step of 0.1 Hz and the pump was also calibrated before each experiment Fig 3.6(b) also presents two directions for placing the feeding probe into the entrance nozzles of reactor
The vertical direction was typically used in cases of individual-reagent tests, from which tar was fed from the top entrance of reactor similar to char reagent However, to enable char and tar feeding together in the experiments of Chapter 5, the horizontal direction must be used for tar feeding so that tar was fed from the side entrance of reactor The different locations of feeding probe actually did not affect feeding rate of tar and also the experimental results
The tar derived from fuel pyrolysis is well-known as a high-viscosity reagent that is indeed very concentrated at room temperature; the feeder was thus designed suitable for this feature of tar Namely, the tar container was placed in the thermostatic circulating water bath maintained at 60 o C while the stainless-steel pipes for transporting tar were entirely wrapped in a heating band which was also maintained at 60 o C by a temperature controller Moreover, the pump head which had a short part of silicon pipe for tar passing through was operated under a hot atmosphere caused by a 275 W incandescent lamp The purpose of all these tasks was to lower the viscosity of tar through heating On the other hand, tar is easy to be coked due to the high temperature at the entrance of reactor, leading to clog the feeding pipe with coke formed To prevent this problem, the feeding pipe was mounted inside a water-cooling probe (feeding probe) which was connected to the 60 o C thermostatic circulating water bath Therefore, the entire flow of tar reagent was maintained at around 60 o C from the container to the feeding probe, facilitating the feeding of such reagent and stabilizing its rates as well
As presented in Section 3.1 of this chapter, the py-gas reagent used for NO-reduction tests in this study was a model gas made according to the analysis of non-condensable gas obtained from DSL pyrolysis at 500 o C Therefore, the used py-gas was provided from a 9 MPa gas cylinder (Beijing AP BAIF Gas Industry Co., Beijing, China) which contained the mixture of individual gases with the compositions presented in Table 3.3 The flowrate of py-gas fed into reactor was controlled by a mass flow controller which was calibrated for maintaining N2 flow Thus, the read-out value must be converted to gain an accurate flowrate by multiple with a conversion factor (C) of 0.903 which was calculated by Eq (3.1)
∑ (ρ i × Cp i × V i %) (3.1) where N i is the structure factor of gas species “i”, ρ i is the density of gas species
“i” (g/L), Cpi is the specific heat of gas species “i” (cal/g o C), V i % is the concentration of gas species “i” in py-gas
Table 3.5 Data for calculating conversion factor of py-gas
Gas species Concentration in py-gas (vol %) Density (g/L) Specific heat
Chapter 3 Material and Methodology experiment to guarantee an accurate correlation between the specified feeding rate of py-gas and the read-out value from controller
3.2.4 Flue-Gas Supplying System The flue gas used in this study was the simulated gas supplied from different gas cylinders such as N 2 , O 2 , NO In order to simplify the reactions of NO with different reagents, only O2 gas was considered as a major constituent of flue gas beside NO and N2 in all experiments The presence of other constituents (i.e., CO2, CO, moisture) which may affect the NO reduction would be evaluated as influent parameters in some specific tests The cylinders of N 2 and O 2 contained high purity gases (99.99 vol %) while that of NO included 10 vol % NO and balanced N 2 Each gas flowrate was controlled by a mass flow controller calibrated suitable for a gas type used Before each experiment, the real flowrate of each gas was carefully measured by a wet gas meter to match the setup value
Fig 3.7 A schematic of flue-gas mixer
Because the concentration of NO in the adopted flue gas was at ppm level, the diluted limit might cause an un-uniform flue-gas flow during experiment Therefore, a gas mixer was equipped for mixing the NO and N 2 flows, ensuring a well-mixed gas flow before introducing to reactor Fig 3.7 shows the schematic of the flue-gas mixer, it consisted of a mixing chamber and a pipe coil, both were placed inside a furnace to promote the gas diffusion at a high temperature up to 800 o C
3.2.5 Sampling and Analyzing System In order to make the reactor flexible for sampling and analyzing, five nozzles marked as N.1 – N.5 were distributed vertically along the axial direction of the reaction zone, with a 260 mm gap between adjacent nozzles, to take flue gas for measuring its composition Reacting flue gas was sampled by five water-cooling sampling probes mounted into such nozzles as shown in Fig 3.8(a) Due to the diluted limit of reagent injected, there might be a radial gradient of flue gas composition along the radius of the reactor; the head of the sampling probe made of a corundum tube was thus drilled with nine holes of 3 mm diameter equally distributed across the cross section of sampling zone to measure the average concentrations of flue gas species (see Fig 3.8(b))
(b) Design of the head of probe
Fig 3.8 The water-cooling sampling probe
The outlet concentrations of NO, CO, CO2 and O2 in the flue gas from all five sampling probes were continually monitored using an online gas analyzer (SDL M3080;
Beijing SDL Technology Co., Beijing, China), all data were recorded every second and saved in a computer The flue gas analyzer consisted of a gas-pretreating module and
Protection The gas-pretreating module was equipped a cooling device as an efficient stage for reducing water vapor in flue gas at 2 o C and a built-in NOx converter that converting NO 2 to NO with efficiency more than 95% to measure the total NOx emission The infrared-analyzing module was composed of four infrared sensors for measuring concentration of NO, SO2, CO2, and CO, and an electrochemical sensor for O2 concentration measurement The specification of each sensor is presented in Table 3.6
Table 3.6 The specification of gas sensors in the flue-gas analyzer
Gas component Range Error Sensor type
Experimental Procedure
3.3.1 Procedure and Analysis The simulated flue gas consisted of a specified content of NO, together with varied concentrations of O2 and balanced N2, and the total flow rate of such flue gas was 45 L/min (STP) After heating in the preheating zone of the DTR, the simulated gas entered the reaction zone The O 2 concentration in the simulated flue gas was adjusted for each test to achieve a similar reburning stoichiometric ratio (SR) for all reagents tested (char, tar, and py-gas) The SR referred to the ratio of the adopted O2 flow rate in each experiment to the O2 flow rate required for stoichiometric combustion of the NO reduction reagent fed into the reactor, as calculated by
V O2, stoi , (3.2) where VO 2 , in is the flowrate of O2 introduced to reactor and VO 2 , stoi is the flowrate of O 2 required for stoichiometric combustion of the injected reagent Once the gas flow and temperatures in the reactor reached their steady states, a reagent of char, tar, or py-gas was fed continuously into the reactor by 4 L/min (STP) of N2 carrier gas to start the reduction of NO in the simulated flue gas The outlet gas from the DTR was sampled and quenched in the water-cooling probe, and the sampled gas further passed through a dust filter before entering the flue gas analyzer Because the formed N2O and NO2 were very few (below 10 ppm), the NOx reduction efficiency was evaluated only in terms of NO reduction Table 3.7 summarized the setup parameters for typical experiments, the variation of these parameters in specific characterizations would be presented in the related chapter
Parameter SR T ( o C) τ(s) [NO] 0 (ppmv) Flue gas
It is well-known that the ash in char can catalyze either NO reduction reactions or NO formation reactions The analysis in this study, however, was based on the overall reduction ability of char, which includes the catalytic effect of ash In the experiment, the ash was thus not removed from char while it is actually not consumed during reaction (as a catalyst) Similarly, some gas species in py-gas such as N2 and CO2 do not have any contribution to NO reduction Consequently, both ash and such inert gases were not considered in normalizing the feeding rate of char and py-gas as a NO reaction reactant (not as a catalyst) In this way, the fed amount of each reagent was carefully determined, so that all three reagents had a similar normalized mass rate of reductant (g/min), specified excluding the amount of ash or inert gas for a comparison of NO reduction, as expressed by
100% - %inert × 100% , (3.3) where FMr is the calculated feeding rate for reagent testing (g/min), SMr is the normalized (specified) mass rate of reagent (g/min) and %inert is the content of ash in char or inert gas (CO2, N2) in py-gas (wt %) Generally speaking, the comparisons in this study are all based on such a mass flow rate of reagent This is also the most practical way for control in real processes
The NO reduction efficiency (η) was estimated as η = 1 NO out
NO in × 100% , (3.4) where [NO] is the NO concentration measured at the sampling port, [NO] is the NO
Chapter 3 Material and Methodology where [NO]0 is the NO concentration measured at the inlet of preheating zone, and Q0 and Q 1 are the flow rates of flue gas and carrier gas (for carrying reagent), respectively
Because the reaction occurred under very diluted atmosphere, the gas volume variation through the reactor was neglected in estimating the NO reduction efficiency according to the measured NO concentrations
Although the setup feeding rate for reagent was based on the specified mass rate (SMr), the actual read-out (FMr) may not accurately reach the desired value, and the errors are different for different kind of reagents In order to effectively compare the overall NO reduction efficiencies realized by different reagents at the same mass rate, all NO reduction efficiency data (realized for actual feeding rate of reagents) were normalized to the values corresponding to such a mass rate of 0.15 g/min which was determined by an investigation in Chapter 4 In this way, the efficiency is assumed to be proportional to the amount of reagent fed into the reactor As a result, the NO reduction efficiency in Eq (3.4), η (%), was modified to the normalized one, η e (%), in the following equation: η e 1 [NO] out
SMr × 100% , (3.6) here SMr was the value including the error of reagent feeder, which was calculated from the read-out value of FMr by Eq (3.3) The normalized NO reduction efficiency (η e ) was thus mostly adopted in this study for comparing the efficiency of different reagents in reducing NO, except for characterizing the effect of reagent feeding rate
3.3.2 Validation of Experimental Setup Conditions
Blank Tests Prior to the NO-reduction tests, the steady state of main parameters inside the reactor such as flue gas temperature, concentration of constituent in flue gas should be determined A series of blank tests was conducted following the same experimental procedure but excluding the reagent injection The conditions for blank tests were based on the setup parameters in Table 3.7, from which the simulated flue gas flowrate was 45 L/min (STP), the carrier gas flowrate was 4 L/min (STP) and the NO concentration at the inlet nozzle was about 800 ppmv
Fig 3.9 The temperature profile along the vertical chamber
The temperature inside the vertical chamber was controlled by adjusting three sections of the electrically heated furnace as described above, and Fig 3.9 shows the temperature profile along the reactor which was almost uniform in each blank test of 800, 900, or 1000 o C Therefore, this temperature profile could be considered as the temperature of flue gas flow passing through the reactor The flue gas temperature was maintained within a margin of ±10 o C around the setpoint
In addition, the temperature drop caused by feeding reagent was less than 5 o C in most experiments, thus it could be negligible in the NO-reduction tests
Simulated flue gas from the preheating zone mixed with the carrier gas intensely at cross section of two chambers Although these two flows impacted each other for a short distance and flow deflection disappeared quickly downstream, the residual deflection in the main flow from impingement of horizontal flue gas might affect the mixing at cross section (Luan J et al., 2009), resulting a gas composition gradient along the axial direction of the reaction zone
Therefore, the initial NO concentration profile along the reaction zone must be
Chapter 3 Material and Methodology relatively uniform except for the value at port N.1 which was slightly higher than the others about 15 ppmv This indicated that only a short distance close to the cross section was affected by the deflection At the inlet NO concentration of 806 ppmv, the difference between the average value from ports N.2 – N.5 and the value calculated by Eq (3.5) was less than 1%, the Eq (3.5) was thus verified to gain an initial NO concentration, [NO]in, which is the baseline value for determining NO reduction efficiency in Eq (3.4)
Fig 3.10 Initial NO concentration at different sampling ports
Reagent Feeding Rate To ensure continuously feeding a desired amount of reagent into reactor during experiments, the feeding curve for each reagent which correlated the actual feeding rate with an adjustable parameter of the feeder was determined Each point in the obtained curve was verified at least three times and the uncertainty for the actual feeding rate was less than 2% This means that the reagent was introduced into reactor at a constant rate during an experiment, and the accurate results was thus achieved in the NO-reduction tests of this study
The feeding curve for char reagent was determined by continually weighting the char from outlet of the screw feeder at different electrical frequency adjusted by inverter Fig 3.11(a) presents a good linear relationship of the feeding curve which is the char feeding rate (g/min) varying with the electrical frequency (Hz)
This enabled a stability of char reagent feeding during NO-reduction tests
Furthermore, Fig 3.11(b) shows an example for a stable feeding of char that the mass accumulation varied linearly with time during a period of 60 min as a typical duration of each experiment
(a) Feeding rate vs electrical frequency (b) Accumulated mass vs time
Fig 3.11 Calibration curve of char feeder
Similar to the process for char, the feeding curve for tar reagent was determined by continually weighting the tar drained off from feeding probe at different frequency indicated on pump A good linear relationship of tar feeding curve was also performed in Fig 3.12
(a) Feeding rate vs pump frequency (b) Feeding rate vs time
Fig 3.12 Calibration curve of tar feeder
Fig 3.13 Calibration curve of py-gas feeder
Fig 3.13 shows the correlation between the py-gas volume flowrate measured by a bubble flowmeter at room temperature and the read-out value from controller The read-out value in this case was the volume flowrate of N2 gas at standard condition (calibrated for N2 gas) The measured flowrate was then converted to mass flowrate by an average density of py-gas On the other hand, calculating mass flowrate of py-gas from the read-out value by the following equation also performed a similar result, m ρ × V 0 × C , (3.7) where m is the mass flowrate of py-gas (g/min), ρ is the average density of py-gas (g/L), V 0 is the read-out value from controller (L/min), C is the conversion factor calculated by Eq (3.1) Therefore, the calculation method provided by the controller manufacturer was verified to determine the setup values corresponding to the specified mass flow rates of py-gas reagent.
NO Reduction by Biomass Pyrolysis Products
Introduction
According to the preliminary finding about the high reactivity of DSL-derived tar, as compared to DSL-derived char, in reducing NO in a micro-fluidized bed reactor (Song et al., 2014), such kind of tar was suggested as an attractive reagent for lowering NOx emission in the CFBDC process Additionally, the ability of py-gas among three DSL pyrolysis products (char, tar, py-gas) in reducing NO should be evaluated
Therefore, in this chapter, the characteristics of NO reduction using char, tar and py-gas from DSL pyrolysis as reagents were investigated in the experimental DTR to further understand the low-NOx mechanism of CFBDC and figure out the dominant NO reduction reactions for CFBDC.
Experimental Conditions
Mechanism of Low-NOx Emission in Circulating Fluidized-Bed Decoupling Combustion oxygen and reagent were varied simultaneously to maintain the desired stoichiometric ratio (SR) in range of 0.44 – 1.32, represented the reburning conditions ranging from fuel-rich to fuel-lean
Table 4.1 Variation range of experimental parameters
Concerning the reagent feeding rate determined for comparing NO reduction, the inert contents were not included in the specified mass flow rate as mentioned in Chapter 3; based on Eq (3.3), Table 4.2 thus correlates the specified mass flow rate (SMr) and the actual feeding rate (FMr) of reagents char, tar, and py-gas in the feeding-rate investigation In the table, the FMr values of py-gas were converted to L/min suitable to gas flowrate adjustment The experiments were conducted over eight feeding levels covering specified mass flow rates of 0.034 – 0.24 (g/min)
Table 4.2 Correspondence of specified mass flow rate (SMr) and feeding mass flow rate (FMr) of reagents
Char Tar Py-gas (STP)
Results and Discussion
4.3.1 NO Reduction Varying with Reagent Feeding Rate A series of experiments with different amounts of fed reagent have been performed in order to investigate the influence of reductant concentration in reaction zone on NO reduction Fig 4.1 shows the NO reduction efficiencies (η) varying with mass feeding rate of reagents defined in Table 4.2 The tests are for all reagents of char, tar and py-gas but at a reaction temperature of 900 °C, the typical condition for the riser combustor in a CFBDC system The results indicate that at the same SR, the achieved NO reduction increased with increasing the feeding rate of all reagents The fact is that in fuel reburning the actual NO reductants are C*, CH i , H and many others generated from the C and H elements in the fed reagent; thus the molar ratios of total C and H elements in reagent to fed NO (CH/NO ratios) corresponding to the specified mass flow rate of char, tar and py-gas in tested conditions are also given in Fig 4.1 for reference The values of CH/NO ratio were calculated based on the ultimate analysis shown in Table 3.2
Fig 4.1 Variation of NO reduction efficiency with specified mass flow rate at different SR values for individual reagents: (a) char, (b) tar, (c) py-gas
Mechanism of Low-NOx Emission in Circulating Fluidized-Bed Decoupling Combustion more available reductants consisting of C and H Exposure of NO to reducing radicals or char-active surface area was thus increased for both the gas-phase homogeneous reactions (2.13) – (2.19) or heterogeneous reactions (2.24) – (2.32), respectively In addition, quick consumption of oxygen by the high amounts of reagent fed into reaction zone must also enhance the formation of low-oxygen atmosphere, which is more effective for reducing NO (Su et al., 2012; Luan J et al., 2009)
On the other hand, the panels (a) – (c) of Fig 4.1 show that the realized NO reduction efficiency η by individual reagent increases rapidly with raising the reagent mass flow rate only to a determined value, which is called the dilution limit (Bilbao et al., 1997) From that limit, the improvement on NO reduction becomes slower against the increase in the reagent mass flow rate due to the less efficient mixing between reagent and flue gas (Luan J et al., 2009) The dilution limit is different for different reagents In Fig 4.1(a) and Fig 4.1(b) it is respectively 0.153 g/min and 0.078 g/min for char and tar reagents, and the corresponding CH/NO ratios are 7.70 and 6.26 The dilution limit for tar is obviously shifted to a lower reagent mass rate than that of char does This is in agreement with the fact that the reagent tar is easier to decompose into many active species or radicals, as compared to the active sites on char surface (Song et al., 2014; Zhang R et al., 2011; Liu et al., 2009) Therefore, there was effective NO reduction for tar, even though its concentration is highly diluted in the reaction zone
Their corresponding CH/NO ratios are nearly equivalent, suggesting the suitable CH/NO ratio of 6.0 – 8.0 and it can be used to determine the dilution limits of individual reagents including char and tar
However, in a CFBDC system the generated tar exists in the gas phase and it is transferred from the pyrolysis bed to the reburning zone of the combustor with the flow of py-gas at high temperature Thus, the dilution of tar should be better than that in our experiments For py-gas reagent in Fig 4.1(c), the obtained limit value seems to be 0.15 g/min, but the phenomenon is not as distinct as that for the char, tar reagents It is well known that the gas-phase reagent is easier to diffuse into the simulated flue gas than the solid- or liquid-phase reagents does Py-gas should be well diluted even in experiments As a result, the dilution limit is negligible for py-gas reagent, and this would be much close to the actual operation in a CFBDC system
In order to minimize the effect of dilution in comparing NO reduction capability among different reagents, the conditions for all following tests of this chapter were fixed at 0.15 g/min of the specified mass flow rate of reagents Consequently, the amount of char injected into reaction zone was below its limit value of 0.153 g/min, while the influence from dilution limit is ignorable for py-gas and tar Regarding to the sufficiency of such reductant amount in NO reduction reaction, Table 4.3 lists the corresponding molar ratios of total C and H elements in reagent to fed NO (CH/NO ratio), as presented in Fig 4.1, and that of total C to NO (C/NO ratio) which was also calculated on basis of the data in Table 3.2 While the CH/NO ratios are about 10.0, the C/NO ratios are 3.8 – 5.0 to ensure the sufficient reduction of NO (Song et al., 2014)
The tar reagent had the highest CH/NO and C/NO ratios, but char and py-gas had the lowest CH/NO and C/NO ratios, respectively
Table 4.3 The feeding rates of char, tar, py-gas for NO reduction efficiency comparison
Sample Char Tar Py-gas
Inert compound Ash (37.92 wt.%) - N 2 , CO 2 (54.26 vol.%)
FMr 0.24 g/min 0.15 g/min 0.34 l/min (STP)
4.3.2 NO Reduction Varying with SR Literature studies (Shu et al., 2015a; Lu et al., 2011; Duan et al., 2007) have reported that the reburning stoichiometric ratio (SR), as defined in Chapter 3, plays an important role on the realized NO reduction Fig 4.2 shows the variation of realized NO reduction by char, tar and py-gas reagents with SR at a reaction temperature of 900 oC in the experimental DTR This temperature is typical for the char combustor of a CFBDC system (Han et al., 2015) As we know, the SR values below and above 1.0 can be classified as fuel-rich conditions and fuel-lean conditions, respectively In Fig
4.2, the plotted NO reduction efficiencies (η e ) enabled by tar and py-gas reagents exhibited a variation trend of first increasing and then decreasing with raising SR On the other hand, the realized NO reduction by char gradually decreased with the rise of SR The maximal η e was achieved at SR of 0.66 by tar, indicating that NO-reduction species and radicals were easily formed from decomposition of tar under fuel-rich conditions Similarly, the optimal SR was found to be 0.89 for NO reduction by py-gas,
Mechanism of Low-NOx Emission in Circulating Fluidized-Bed Decoupling Combustion should be operated under the fuel-rich conditions (SR < 1.0 or insufficient O2 for full combustion) in order to achieve the highest NO reduction through reburning of DSL pyrolysis products
Fig 4.2 Variation of NO reduction efficiency with SR at the typical reaction temperature
Panels (a) – (c) of Fig 4.3 show the NO reduction efficiencies varying with SR at reaction temperatures of 800, 900 and 1000 o C for reagents char, tar, py-gas, respectively The achieved NO reduction generally increased with increasing temperature for all reagents but different temperatures caused more or less different NO reduction variations with SR The effect of SR on realized NO reduction was obviously slight at 800 o C but became apparent at rather high temperatures Raising temperature increased not only the activity for NO reduction of each reductant but accelerated also the oxidation of reagent (Lu et al., 2011; Bilbao et al., 1994) The competition between NO and oxygen for their reactions with reagent caused such results and a deep analysis is presented below for a clear understanding
From Fig 4.3(a) one can see that with increasing SR the NO reduction efficiency realized by char decreased, and the decrease became more significant at higher temperatures It is well known that the heterogeneous NO reduction by char requires the fuel-rich condition (Shu et al., 2015a; Zhong et al., 2002) so that NO is reduced to N2 by reacting with free-active sites C* and surface complexes C(O) on the char surface through reactions (2.24) – (2.30) With the increase of SR, the raised oxygen concentration must cause a fraction of C to burn out and to form CO 2 rather than to create free-active sites C* and surface complexes C(O) Also, the C–O reaction is several orders faster than C–NO reaction (Dong et al., 2010; Lu et al., 2009), the acceleration of char burning with raising temperature is greater This thus hinders the C–NO reaction, so that the more significant decrease in NO reduction at high temperatures is due to the more increased char burning
Fig 4.3 Variation of NO reduction with SR at different temperatures for reagents:
(a) char, (b) tar, and (c) py-gas
For tar and py-gas reagents, known as the released volatiles of DSL pyrolysis, they reduce NO via the homogeneous gas-phase reactions described in Chapter 2, as (2.10) – (2.19) Fig 4.3(c) shows that elevating SR caused the achieved NO reduction efficiency by py-gas to gradually increase first and decrease later, so that there was a
Mechanism of Low-NOx Emission in Circulating Fluidized-Bed Decoupling Combustion require oxygen radicals to initiate such chain reactions that generate lots of reduction radicals such as CH i , HCCO and so on (Myerson, 1974) These radicals in turn react with NO present in flue gas via reactions (2.11), (2.12) and (2.14) – (2.19) to implement NO reduction With low SR values, the generation of reduction radicals was enhanced by increasing oxygen amount to have thus the gradually elevated NO reduction When SR was high enough, oxidation of reagent has to excessively occur convert py-gas into CO or CO 2 Few reduction radicals then were generated to decrease thus the achieved NO reduction (Alzueta et al., 1997) Dagaut et al have reported similar findings by using acetylene (Dagaut et al., 1999), propane(Dagaut et al., 2001), and biomass pyrolysis gas (Dagaut and Lecomte, 2003) as reactants to reduce NO In these literature studies, the maximum of achieved NO reduction was also observed at SR only slightly below 1.0 (near stoichiometric condition)
Fig 4.3(b) shows the similar variation features with SR, as in Fig 4.3(c), for the NO reduction efficiency realized by tar reagent Nonetheless, the realized NO reduction by tar was higher than that by py-gas in the whole range of tested SR At 800, 900 and 1000 o C, the maximal η e were 24%, 58% and 87% at SR values of 0.9, 0.66 and 0.55, respectively These SR values are smaller than those required for obtaining the maximal η e for reagent py-gas, indicating less oxygen needed for achieving maximal η e by tar
When raising reaction temperature, the NO reduction by tar tended to reach its maximum at lower SR This trend is observed also for py-gas reagent in Fig 4.3(c), but the phenomenon is rather clearer for tar reagent in Fig 4.3(b) The general result is that the higher temperature, the more fuel-rich condition is needed for maximizing homogeneous NO reduction
Conclusions
Suppressing the conversion of N containing in tar and char into NO was suggested to be the major cause for increasing NO reduction efficiency with raising NO as well as CO content in the reaction atmosphere (or simulated flue gas) Meanwhile, the effects of CO2 and moisture on NO reduction were found to be little for all tested reagents For py-gas, the dilution effect caused the realized NO reduction to decrease with increasing the initial NO concentration as well as CO concentration in simulated flue gas
Concerning the CFBDC process, the mechanism of NO reduction by reburning char, tar, and py-gas in term of the synergetic action existing in the system is still unclear and will be investigated in Chapter 5.
Synergetic Effect Among Pyrolysis Products in
Introduction
Following the previous findings about the characteristics of NO reduction using char, tar and py-gas from DSL pyrolysis as reagents in Chapter 4, this chapter further investigates the interactions between two reagents among char, tar and py-gas in reducing NO to revealing their synergetic NO reduction occurring in the CFBDC process The effects of reagent proportion, reburning stoichiometric ratio (SR), reaction temperature and residence time on the synergetic NO reduction by some binary reagents such as char/py-gas, tar/py-gas, tar/char mixtures were systematically carried out in an experimental drop-tube reactor In addition, the influences of some reducing gases (i.e., CO, H2, and CH4) on the NO reduction by char/py-gas mixture were evaluated to understand the combined homogeneous–heterogeneous reaction mechanism for NO reduction.
Experimental Conditions
initial concentration of NO in the flue gas was 800 ppmv Table 5.1 lists the experimental conditions adopted in this chapter
Table 5.1 Experimental conditions adopted for comparing NO reduction by reagents derived from different fuel
Stoichiometry ratio – SR 0.6 – 1.0 Residence time – , (s) 0.6 – 2.9 Flue gas flowrate (L/min – STP) 45 Inlet NO concentration (ppmv) 800
The synergetic NO reduction was investigated between a pair of reagents among char, tar and py-gas Thus, two of such three reagents were simultaneously fed into the DTR reactor for each test Concerning the dilution limit of reagent as increasing its feeding rate which was discussed in the Subsection 4.3.1, the issue becomes more important in this investigation because a bad mixing of multi-reagents with flue gas may obviously inhibit the NO reduction capability of the reagents so that the identification of the synergetic effect become difficult In order to minimize the effect of dilution, the conditions for all experiments were fixed at 0.15 g/min of the total mass flow rate of reagents similar to a typical level of reagent feeding rate in previous tests
The combined reagents were also called the binary reagents or reagent mixtures, and the contents of individual reagents were determined according to experimental conditions as summarized in Table 5.2 There, the different mass proportion of a binary reagent shows the variation of its mass ratio between the flowrates of two individual reagents, but the work maintained the total mass rate of the binary reagent to be about 0.15 g/min as specified The feeding rates of reagents listed in Table 5.2 are all defined on basis of reagent mass flow rate (g/min) excluding amount of ash or inert gas, their actual feeding mass rates were detailed as the FMr values in Table 4.3 of the previous chapter
Chapter 5 Synergetic Effect Among Pyrolysis Products in Reducing NO
Table 5.2 The specified mass flow rates of reagents in testing NO reduction by binary reagents
Reagent mass flow rate (g/min)
Results and Discussion
5.3.1 Synergetic Effect of Binary Reagent The synergetic NO reduction was investigated for several binary reagents specified in Table 5.2 In order to effectively identify the synergetic effect on NO reduction by such binary reagents, the synergetic difference was defined, Δη e (%), to be Δη e η e ∑ η i , (5.1) where η e (%) is the measured NO reduction efficiency of a binary reagent at the specified mass flow rate of 0.15 g/min, and η i (%) is the absolute NO reduction efficiency of the related single reagent “i” (char, tar, or py-gas) fed at the similar mass rate of reagent "i" as in the binary case, the values of η i were obtained from the preceding results in Subsection 4.3.1 where the NO reductions by char, tar and py-gas varying with reagent feeding rates were individually tested This synergetic difference measures the improved or diminished NO reduction efficiency through the combining reagents
Fig 5.1 – Fig 5.3 show respectively the results from testing the binary reagents of char/py-gas, tar/py-gas and tar/char with different reagent proportions (normalized to mass percentage) The plotted data are the actual efficiencies (η e ) and synergetic differences (Δη e ) in NO reduction estimated according to Eq (5.1) under different SRs
For an effective comparison, the NO reductions for individual reagents of char, tar and py-gas obtained in Chapter 4 were also presented in Fig 5.1 – Fig 5.3
Fig 5.1 Synergetic NO reduction by binary reagent of char/py-gas at varied SRs but a typical reaction temperature of 900 o C: (a) NO reduction, (b) Synergetic difference: “Sets 1 – 3” in Table 5.2
For the char/py-gas binary reagent, the realized η e sharply decreased with the rise in SR, and the decrease was more apparent than that obtained for individual reagents of char and py-gas at a similar specified mass rate (Fig 5.1(a)) Moreover, increasing the percentage of char in the binary mixture, the realized NO reduction diminished but was still higher than that achieved by 100% char, especially under the fuel-rich conditions
Chapter 5 Synergetic Effect Among Pyrolysis Products in Reducing NO char at SR of 0.6 and 1.0 were respectively 30.4% and 7.4%, but those for a mixture of 75% char under such SRs were respectively 5.6% and –1.8% Thus, the synergetic promotion effect shown by Δη e was obviously higher for the fuel-rich condition at lower SR and also for the binary mixture with the lower char content (or higher py-gas content) Indeed, the fuel-rich reburning condition (lower SRs) forms a reducing atmosphere to enhance NO reduction by all reagents including char, tar and py-gas (Shen et al., 2015; Lu et al., 2009) On the other hand, the gas components CO, H 2 , and CH 4 in py-gas would positively affect the NO reduction reactions by char, as will be analyzed in the Subsection 5.3.4 This refers actually to the identified synergetic effect between py-gas and char on NO reduction
Fig 5.2 Synergetic NO reduction by binary reagent of tar/py-gas at varied SRs but a typical reaction temperature of 900 o C: (a) NO reduction, (b) Synergetic difference: “Sets 4 – 6” in Table 5.2
Fig 5.2(a) shows that the realized η e by the binary reagent tar/py-gas increased with raising the percentage of tar in the mixture at all the tested SRs, but it obviously did not exceed the level achieved by 100% tar (tar reagent) It was previously proved in Chapter 4 that tar enables the best NO reduction among char, tar and py-gas The result suggests that the positive synergetic effect on NO reduction was not observed for the tar/py-gas mixture, and mixing py-gas into gaseous tar led to negative effect on NO reduction by tar and thus had the negative Δη e in Fig 5.2(b) Consistent results were obtained also by Duan et al (2007) who investigated NO reduction by biogas with and without tar mixed They reported that the mixing model tar compound such as toluene played a positive role on reducing NO by reburning biogas
Fig 5.3 Synergetic NO reduction by binary reagent of tar/char at varied SRs but a typical reaction temperature of 900 o C: (a) NO reduction, (b) Synergetic difference: “Sets 7 – 9” in Table 5.2
Fig 5.3(a) shows the η e realized by the binary reagent tar/char at varied SRs and mixing percentages of tar Again, η e increased with raising tar content at all SRs, and at the same mass rate of 0.15 g/min only the η e obtained at 77% tar in the tar/char mixture exceeded the level achieved by 100% tar Nonetheless, the corresponding synergetic difference shows that the capability of this binary reagent for reducing NO was not as good as expected because in Fig 5.3(b) the Δη e for 77% tar mixture at all tested SRs were negative (~ –5%) Similar to Fig 5.2(b), only the mixture with 26% tar caused a positive Δη e Thus, for the mixture of tar with another reagent (char or py-gas), the lower tar content, the more synergetic effect could be
Overall, we can summarize that the interaction between char and non-condensable volatile gas (py-gas) improved their NO reduction capability, and the synergetic effect was more significant for the mixture with the higher py-gas proportion Between tar and other reagents (either char or py-gas) it caused a positive synergetic effect only when the tar content is relatively low, for example below 26% As discussed in Chapter
Chapter 5 Synergetic Effect Among Pyrolysis Products in Reducing NO atmosphere for oxidative cracking of tar, and coke/soot may be conversely formed through polymerization and graphitization (Zhang R et al., 2014; Ruiz et al., 2007) It is well known that soot particles are nonporous and have limited porosity (Ruiz et al., 2007; Akhter et al., 1985), indicating a weak ability to occur the heterogeneous NO reduction reaction, even though there were some studies reporting positive effect (Mendiara et al., 2009, 2007) In comparison, a strong reducing atmosphere was reported to effectively enhance the heterogeneous NO reduction by char, possibly due to the porous structure and catalytic ash components in char (Allen and Hayhurst, 2015;
Shu et al., 2015a; Lu et al., 2011; Zhong and Tang, 2007) Consequently, the competition in capturing oxygen by other reagents would surely accelerate the NO reduction reactions by char to demonstrate the positive synergetic effect
Fig 5.4 SEM images and EDS spectra of reburning residues collected from experiments (Sets 2 and 5 in Table 5.2) at typical temperature of 900 o C and an SR of 0.7
Fig 5.4 shows the surface morphology and elemental distribution given by the scanning electron microscope – energy dispersive spectroscopy (SEM–EDS) for the residue samples collected in our reburning experiments The SEM images and EDS spectra of unreacted char and tar-derived coke were respectively from the tests using char/py-gas and tar/py-gas mixtures (Sets 2 and 5) at 900 o C and a reburning SR of 0.7
It is obvious in the SEM images that char has the typical porous morphology (Septien et al., 2012) even though it was partially burned during reburning, whereas the surface of coke particle exhibits a flat, smooth, and less textured appearance (Huang et al., 2016;
Hays et al., 1983) The EDS spectra show that the composition of the char surface consists of K and Ca metals which are known to be catalytic for NO reduction reactions (Zhong and Tang, 2007; Chen and Ma, 1996) These identifications further explain the distinctive effect of each reagent on the above-observed synergetic NO reduction results
5.3.2 Synergetic NO Reduction Varying with Reaction Temperature
Fig 5.5 NO reduction efficiencies by binary reagents with a mass ratio of 1:1 at different temperatures but an SR of 0.7 (Sets 2, 5, 8 in Table 5.2) and their comparison with efficiencies for individual reagents
Fig 5.5 shows the achieved η e at different temperatures of 800, 900, and 1000 o C under a typical fuel-rich condition (SR = 0.7) for three binary reagents of char/py-gas, tar/py-gas and tar/char at equal mass proportion of approximately 1:1 For comparison the NO reduction efficiencies realized by individual reagents at the same feeding mass
Chapter 5 Synergetic Effect Among Pyrolysis Products in Reducing NO
39.2%, respectively The order is in correspondence of their CH/NO ratios of 8.56, 11.21, and 9.88 Thus, the higher CH/NO ratio is beneficial to NO reduction so that here the highest dependence on temperature is for the tar/py-gas mixture
Nonetheless, for all binary reagents their increases in η e from 900 to 1000 o C were less significant than from 800 – 900 o C Comparing to the realized NO reduction by individual reagents of char, tar and py-gas in Chapter 4, the increases in η e with raising temperature did not diminish in 900 – 1000 o C It means that the synergetic effect between two reagents for NO reduction was not enhanced by increasing reaction temperature As discussed in the Subsection 5.3.1, only the interaction between char and py-gas improved the NO reduction by their mixture at 900 o C At rather higher temperatures of up to 1000 o C, the accelerated char oxidation as well as ash melting may inhibit NO reduction by char (Lu et al., 2011; Aarna and Suuberg, 1997), while in the oxygen-lean atmosphere tar would be decomposed into coke/soot other than reducing radicals at high temperatures (Zhang R et al., 2014; Ruiz et al., 2007) It is because at high temperatures the oxidation reactions for all reagents are accelerated more Thus, for a CFBDC system treating DSL (Han et al., 2015), the reburning zone temperature at the intermediate position of its riser should be around 900 o C in order to develop the synergetic NO reduction effects of DSL pyrolysis products
Conclusion
Residence time of reagents in reaction zone exhibited certain effect on NO reduction because gaseous tar and py-gas raised efficient homogeneous reactions even at short residence time The main gas species in py-gas such as CO, H2, CH4 manifested different effects on NO reduction by char/py-gas mixture (gas components adjusts py-gas composition), and the identified effect of gas component in py-gas was closely related to the corresponding CH/NO ratio Both H and CH played important roles in
NO Reduction by Reagents Derived from Different Fuels
Materials and Experimental Conditions
Fig 6.1 The adopted raw materials: (a) Sawdust (SD), (b) Xianfeng (XF) lignite
Sawdust (SD) and Xianfeng (XF) coal, as shown in Fig 6.1, were selected as the raw materials to prepare reagents for further evaluating NO reduction, those were collected from Heilongjiang and Yunnan provinces of China, respectively The raw materials were sieved to gain the particle size of 8 – 10 mm for SD and 2.0 – 2.5 mm for XF coal, then they were naturally dried in air for 24 h before implementing the pyrolysis process
1 Furnace; 2 Reactor; 3 Pressure gauge; 4 Condenser; 5 Collection bottle; 6 Acetone washing bottle; 7 Filter; 8 Buffer flask; 9 Vacuum pump; 10 Wet gas meter; 11 NaHCO 3 washing bottle;
12 Silica gel bottle; 13 Valve; 14 Gas sampling; 15 Gas exhaust
Fig 6.2 A schematic diagram of indirectly heated fixed bed with internals (Zhang C et al., 2014)
The procedure for preparing char and tar reagents from SD and XF coal generally followed that described in Chapter 3 The SD was pyrolyzed in a horizontal fixed-bed reactor similar to the DSL in preceding chapters; whereas the XF coal pyrolysis followed a typical process which was described in detail elsewhere (Siramard et al., 2016; Lin et al., 2015; Zhang C et al., 2014), from which coal was pyrolyzed in an indirectly heated fixed bed with internals (see Fig 6.2) The inner diameter of this stainless-steel reactor was 100 mm, and its effective volume for loading coal was 1500 mL Usually 1200 g of XF coal was loaded into the reactor The pyrolysis batch was started with preheating furnace to a preset temperature of 1000 o C at which the reactor loaded with the given amount of coal and connected to the gas cleaning line was quickly placed into the furnace Each batch was ended when the coal temperature at the reactor central line or near the central gas collection pipe wall reached 500 o C and the reactor was lifted out of the furnace to be cooled down naturally
Table 6.1 Proximate and ultimate analysis of raw materials and their derived chars, tars
Sample Proximate (wt %, db) Ultimate (wt %, daf)
Chapter 6 NO Reduction by Reagents Derived from Different Fuels
The collected tar reagents were dehydrated over anhydrous MgSO4 prior to evaluate NO reduction, and the char particles were selected in range of 0.1 – 0.5 mm suitable for experiments Table 6.1 shows the proximate and ultimate analyses of char, tar reagents and their raw materials The analyses of SD and its char were carried out similar to those of DSL, whereas XF lignite and the derived char were analyzed according to the National Standard method of China for coal – GB/T 212-2008
Additionally, the chemical composition of tar reagents derived from SD and XF coal was obtained using a GC–MS analysis In Appendix A, the original GC–MS spectra of such tar samples and also the DSL tar were presented The content of each component refers to the percentage of its peak area, say, the proportion of each peak area to the total area of all peaks in a GC–MS spectrum; and the identified compounds in tar were further divided into some functional groups which were also detailed in the appendix Their content (area) percentages were listed in Table 6.2 (the total area for each group over the total area of all peaks) Obviously, phenols, acids and ester are the major components of biomass tars, whereas aromatics and aliphatic hydrocarbons are the major components of coal tar Consequently, in this chapter phenol, acetic acid, methyl acetate, heptane, and benzene were adopted as the model compounds of the tested tars to further evaluate the NO reductions realized by individual chemical compounds containing in the tars, justifying the NO reduction by different tar reagents
Table 6.2 Composition analyses of used tar reagents (based on GC–MS results)
Composition (% area) Representative reagents SD tar DSL tar XF coal tar
6.2.2 Experimental Conditions Similar to previous chapters, the DTR experiments were carried out following the procedure described in Chapter 3 In this chapter, the experiments were performed on the same experimental conditions as in Chapter 4; namely the reburning SR and the reaction temperature were further considered as key variables in the NO-reduction evaluation, while the specified mass flow rate of reagents was kept at 0.15 g/min to effectively compare the NO reduction efficiency of different reagents derived from different fuel The typical conditions adopted in such experiments are listed again in Table 6.3
Table 6.3 Experimental conditions adopted for comparing NO reduction by different reagents
Stoichiometry ratio – SR 0.40 – 1.30 Inlet NO concentration (ppmv) 800 Flue gas flowrate (L/min – STP) 45 Residence time – , (s) 2.90 Specified mass flow rate (g/min) 0.15
To determine the basis feeding rate for different reagents, the ash content in char was ignored Table 6.4 thus correlates the specified mass flow rate and the actual feeding rate (including ash) of SD char and XF coal char; whereas the actual feeding rate of tars and model tar compounds was equal to the specified mass flow rate, say, 0.15 g/min
Table 6.4 The feeding rates of char, tar, and model compounds for NO reduction efficiency comparison
Sample SD char XF coal char Tars & Model compounds Inert compound (wt %) 13.79 (ash) 7.08 (ash) -
Calculated feeding rate (g/min) 0.209 0.161 0.150 Specified mass rate (g/min) 0.150 0.150 0.150
Char and tar from SD and XF coal were injected to DTR by the suitable feeder same as those for DSL, that was the screw feeder for char and the peristaltic pump for tar (see Fig 3.6) For the model tar compounds, except phenol is solid at room temperature, the others are liquid, and their viscosity is relatively low Therefore, in a specific test one of such model compounds including benzene, heptane, methyl acetate, acetic acid was simply fed into reactor by a peristaltic pump In case of phenol feeding,
Chapter 6 NO Reduction by Reagents Derived from Different Fuels maintained at 60 o C like the tar feeding The feeders were calibrated to obtain feeding curve for each reagent so that its set up value was determined The calibration curves for tested reagents such as SD char, XF coal char, SD tar, XF coal tar and five model tar compounds were shown in the Appendix B.
Results and Discussion
Fig 6.3 Variation of NO reduction efficiency with SR for char reagents at different temperatures: (a) 800 o C, (b) 900 o C, (c) 1000 o C
In this part, the reburning SR was further considered as a key parameter in evaluation of the NO reduction by chars Fig 6.3(a) – (c) show the NO reduction efficiencies (η e ) varying with SR at reaction temperatures of 800, 900, and 1000 C for char reagents The results of DSL char were also replotted to make an effective comparison Generally, the realized NO reduction by all tested chars decreased with the rise of SR and the decreases became greater as reaction temperature increased The variation tendency is quite similar to the preceding results for DSL char, so that it is consistent with the heterogeneous NO reduction mechanism discussed in Chapter 4
However, the NO reduction capability is apparently different for different char reagents
Noting that the comparison is based on the same mass feeding rate of 0.15 g/min (without ash content) and the same amount of NO fed into reactor, the results indicated that DSL char and SD char could reduce more NO than XF coal char, that is to say biomass char is more efficient than coal char in reducing NO This should be due to the fact that the volatile content was apparently higher for biomass than for coal (see Table 6.1) and the char made from biomass thus has larger surface area and more active sites, which indeed enhanced the heterogeneous reaction for reducing NO (Shu et al., 2015a)
Similar result was also reported by Dong et al (2007) who analyzed kinetic reactions between some kinds of char and NO and claimed that the NO reduction capability of the tested chars should relate to their content of catalytic active sites and their specific surface area Table 6.5 lists the specific surface area for three kinds of char which was examined by Brunauer-Emmett-Teller (BET) analyzer (Quantachrome NOVA; Quantachrome Instrument Co., Boynton Beach, USA), from which DSL char has the largest BET surface area, the followings were SD char and XF coal char, respectively Thus, the larger specific surface area of char, the more efficient reduction of NO could be achieved
Table 6.5 BET specific surface areas of different char reagents
Sample DSL char SD char XF coal char
In addition, the catalytic effect of inherent metal in char (ash) on the NO–char reaction cannot be ignored These metals may be associated with the organic matrix or bound in char particles, thus increasing the number of catalytic active sites to make the NO reduction by chars more efficiently (Sứrensen et al., 2001) This means that the char contains more catalytic species such as alkali, alkali earth metals, and some first-series
Chapter 6 NO Reduction by Reagents Derived from Different Fuels oxide compounds, was analyzed by X-ray fluorescence (XRF) (Axios MAX;
PANalytical Co., Nottingham, UK) as presented in Table 6.6
Table 6.6 Ash composition of different char reagents (wt %, oxide basis)
XRF analysis SiO 2 CaO Na 2 O K 2 O MgO Al 2 O 3 Fe 2 O 3 TiO 2 Others a DSL char 64.126 20.271 0.104 4.186 1.078 1.762 1.372 0.153 6.947 SD char 25.427 26.585 0.972 7.917 3.495 9.894 6.298 8.260 11.152 XF coal char 25.547 10.810 0.193 0.585 0.444 35.023 7.357 4.304 15.737 a by difference
From the table, one can see that the contents of some major metal oxides such as CaO, MgO, K2O, as the well-known NO reduction catalysts (GarciaGarcia et al., 1997;
Ohtsuka et al., 1997; Chen and Ma, 1996; Hansen et al., 1992), are obviously higher for biomass char than for coal char Those inherent metal catalyzed the char–NO reaction rate to some degree and resulted in a decrease in the apparent activation energy (Illangomez et al., 1995b, c), thus shifting NO reduction by biomass char to a higher level in comparing with that by coal char
Noting that the comparison in Fig 6.3 was based on mass feeding rate of char without ash, the results indicated that the DSL char enabled somehow higher efficiency, or at least it did not significantly lower the efficiency than SD char This would be due to the fact that the higher ash content of DSL char (given in Table 3.2) lead to higher amount of catalytic metal oxides for unit mass reactant fed into the reactor It caused an important role of catalytic effect in the efficient reduction of NO by DSL char
Therefore, the DSL char de-ashed was then taken to investigate its capability in reducing NO, as presented in the next subsection
Contribution of Ash Content to NO Reduction by Char In order to clarify the contribution of catalytic ash during NO reduction, the de-ashed char gained from DSL-char treatment was tested for the capability of reducing NO in the DTR similar to the preceding experiments In view of composition shown in Table 6.6, the ash content in DSL char is relatively high but is dominated by CaO and SiO2 so that it can easily be removed by acidic treatment and in turn alkali treatment
However, de-ashed process was never complete, so the evaluation of ash contribution on NO reduction by char was only based on the amount of ash removed (Aarna and Suuberg, 1999)
De-ashed procedure was based on the previous study of Yang et al (2012), as follows The char was first drenched with HCl solution (2 mol/L) according to a solid/liquid ratio was of 1:5 (g/mL) as the acidic treatment The mixture was refluxed in a glass flask with magnetic stirring for 2 h After natural cooling, the sample was separated from the solution by vacuum filtration and washed with distilled water After that, the sample was further drenched in NaOH solution (2 mol/L) as the alkali treatment The procedure was the same as the acidic treatment Finally, de-ashed sample was washed with distilled water until the pH value of the washing solution was about 7
Then, the sample was dried in oven at 105 °C for 24 h to remove the moisture for the use in subsequent tests This de-ashed process can remove about 73% of the inherent char ash, which can be found in Table 6.7 with the proximate analysis In the table, XRF analysis of the remaining ash content in the de-ashed DSL char was also presented
Table 6.7 Analyses of original and de-ashed DSL chars
DSL char Proximate (wt %, db) XRF (wt %, db and total char base)
Volatile Ash FC SiO 2 CaO K 2 O MgO Al 2 O 3 Fe 2 O 3 TiO 2
Fig 6.4 presents the variation of η e by de-ashed DSL char with SR at 900 o C and demonstrates the appreciable catalytic effects of ash on NO reduction by DSL char It can be seen from the figure that the char de-ashed obviously exhibited lower η e comparing to the original char The realized efficiency was even less than that by SD char despite their similar ash contents (~ 14%) The main reason may be the difference of heterogeneous reactions between NO and char, which can be catalyzed by different mineral matter in the ash Thus, in the following analysis it is meaningful to further explain the distinctive NO reduction capability of different char based on their great variations in contents of metal oxides
Considering the differences of proximate analysis for different chars; a normalized parameter, was proposed following the study of Zhang J et al (2011), as m c = m MO m FC (6.1)
Chapter 6 NO Reduction by Reagents Derived from Different Fuels is the mass fraction of fixed carbon of the corresponding char The calculated results for all chars are listed in Table 6.8
Table 6.8 Ratios of metal oxide contents to fixed carbon in chars (mc, wt./wt.)
Sample CaO K 2 O MgO Fe 2 O 3 Al 2 O 3 TiO 2 Sum
DSL char 0.1682 0.0347 0.0089 0.0114 0.0146 0.0013 0.2233 SD char 0.0563 0.0168 0.0074 0.0133 0.0209 0.0175 0.0937 De-ashed DSL char 0.0219 0.0047 0.0051 0.0043 0.0066 0.0010 0.0359 XF coal char 0.0101 0.0005 0.0004 0.0069 0.0329 0.0040 0.0180
From the data in Table 6.8, it could be seen that the level of a metal oxide in different char reagents are obviously consistent with the capabilities of such chars in reducing NO as observed from Fig 6.4 Generally, at a specified mass rate of 0.15 g/min of reagents, the higher level of m c of a char the higher η e was realized by that char due to the dominant effect of catalytic matters such as CaO, K2O, MgO, Fe2O3 However, this observation was inappropriate in case of Al2O3 and TiO2 These oxides might not affect the NO–char reaction although they covered large percentages in SD char and XF coal char (Table 6.6) The catalytic effect of Al or Ti compounds on NO reduction by char actually has not been reported in literatures Above all, the total content of catalytic metal oxides in char ashes (excluding Al2O3 and TiO2), as presented by the sum of m c in the last column of Table 6.8, can be used to estimate the catalytic activity of ash during char reburning
Fig 6.4 The variation of NO reduction efficiency by char and de-ashed char with SR
The contribution of ash catalyst on NO reduction by DSL char was further investigated at different temperatures Fig 6.5 shows the variation with reaction temperature of the realized η e by de-ashed chars When the temperature increased from 750 to 1050 °C, η e by de-ashed char totally increased 57.9% It is noteworthy that the increase of η e by de-ashed char was about 1.3 times higher than that by original DSL char which was obtained in Chapter 4 and also replotted in Fig 6.5 Thus, a high temperature was more beneficial to NO reduction for de-ashed char although the realized η e by itself was still lower in comparison with that for original char The results suggested that the catalytic contribution of ash should be more important at lower reaction temperature
Fig 6.5 NO reduction varying with reaction temperature for char and de-ashed char reagents
The catalytic contributions of ash to NO reduction by DSL char at different temperatures are listed in Table 6.9, in which the contribution values were calculated from the data presented in Fig 6.5 via the following equation: β ash = η char η de η char × 100 η ash η char × 100 , (6.2) where β ash (%) is the contribution of catalytic effect to NO reduction, η char (%) is the η e of original char, η de (%) is the η e of de-ashed char The differences between η char and η de , considered as the catalytic efficiencies, η ash (%), are also listed in Table 6.9
Table 6.9 Contribution of catalytic effect to the NO reduction by DSL char
Chapter 6 NO Reduction by Reagents Derived from Different Fuels
Conclusions
The adopted char and tar reagents were prepared by pyrolysis of sawdust (SD) and Xianfeng (XF) coal, respectively The results were compared to that obtained by using char and tar from DSL Also, the NO reductions by five typical model tar compounds, which are phenol, benzene, acetic acid, methyl acetate and heptane, were investigated to further understand the mechanism involving in NO reduction by tars
All reagents char, tar derived from biomass and coal have considerable capabilities to reduce NO and their realized reduction efficiencies increased with increasing temperature in the range of 800 – 1000 o C While the NO reduction by char diminished as increasing SR, maximal efficiency was obtained for both kinds of tar at SR values of 0.6 – 0.8, which is similar to the optimal SR values proved for CFBDC In addition, tar could enable better NO reduction than char did, and it is possibly related to the higher molar ratio of H/C element for tar than for char The great contribution of ash as catalysis on NO reduction by char was also demonstrated so that catalytic matters such as CaO, K 2 O, MgO were responsible to the highest NO reduction efficiency for DSL char in comparison with SD char and XF coal char
For tar reagents at the same mass feeding rate (0.15 g/min), the SD tar enabled the highest NO reduction comparing to the XF coal tar and DSL tar Among five tested model tar compounds, phenol showed the highest, while benzene took the next NO reduction capability in comparison with that realized by other components Thus, the major contributor for reducing NO in biomass tar is phenols while that in coal tar is aromatic compounds This caused the significant higher NO reduction efficiency for biomass tar than that for coal tar because of the highest content of phenols in biomass tar and that of aromatics in coal tar.
Conclusions and Recommendations
Conclusions
An example is the distilled spirit lees (DSL) In order to achieve this goal, the char, tar and non-condensable pyrolysis gas (py-gas) from pyrolyzing DSL were firstly adopted as reagents to test their NO reduction capabilities in Chapter 4 Then the synergetic effects among such reagents on NO reduction were further investigated in Chapter 5
At last, Chapter 6 presents the NO reduction characteristics by pyrolysis products derived from sawdust (SD) and Xianfeng (XF) lignite, in comparison with those of DSL-derived products, to get the better understanding of NOx-reduction mechanism in the CFBDC treating different fuels All experiments in this study were carried out in a lab-scale drop-tube reactor (DTR) to simulate the reburning conditions of pyrolysis products involved in a CFBDC process The major results are summarized below
(1) For reagents derived from DSL pyrolysis, varying the stoichiometric ratio (SR) of reburning caused a maximal NO reduction efficiencies (η e ) for both tar and py-gas at SR values of 0.6 – 0.9, whereas the realized η e by char gradually decreased with raising SR The effect of varying SR on NO reduction appeared higher at the higher reaction temperatures, which was especially evident at temperatures above 900 o C
(2) All such reagents have considerable capability to reduce NO, and their reaction activity and reduction efficiency increased with increasing temperature
Among them, the realized NO reduction by tar reagent showed the most sensitive variation with reaction temperature On the other hand, increasing reaction temperature from 800 to 1000 o C, the realized NO reduction by binary reagents generally increased but the synergetic effects of a pair of two pyrolysis products were actually not enhanced
(3) In short reaction time, the homogeneous NO reduction by tar and py-gas should be most important, while heterogeneous NO reduction by char would become dominant in a long reaction time Therefore, the residence time of reagents in reaction zone exhibited certain effect on NO reduction when they were fed together
(4) At a specified mass feeding rate (0.15 g/min), tar enabled the highest NO reduction capability comparing with char and py-gas in testing individual reagents under specified temperatures and flue gas conditions However, for binary reagent tests under such conditions, the higher py-gas proportion enabled the better synergetic effect on NO reduction by char/py-gas reagent, whereas the mixtures consisting of tar caused a positive effect on NO reduction only when the tar fraction was below 26%
(5) Suppressing the conversion of N containing in tar and char into NO was suggested to be the major cause for increasing NO reduction efficiency with raising NO as well as CO content in the reaction atmosphere (or simulated flue gas) For py-gas, the dilution effect caused the realized NO reduction to decrease with increasing the initial NO concentration as well as CO concentration in simulated flue gas Meanwhile, the effects of other gas species in flue gas such as CO2 and H2O on NO reduction were found to be little for all tested reagents
(6) The main gas species in py-gas such as CO, H2, CH4 manifested different effects on the synergetic NO reduction of char/py-gas mixture, and the identified effect of such components was closely related to the corresponding CH/NO ratio Both H 2 and CH4 played important roles in the synergetic NO reduction, whereas the effect of CO was more insignificant
(7) All reagents derived from pyrolyzing other fuels such as sawdust (SD) and Xianfeng (XF) lignite also exhibited the similar tendencies in reducing NO as the DSL- derived reagents did The realized η e increased with increasing temperature in the range of 800 – 1000 o C While the NO reduction by char diminished as with increasing SR, the maximal efficiency was obtained for all kinds of tar at SR values of 0.6 – 0.8, the optimal SR values suggested for operating CFBDC systems Besides, all tar reagents enabled better NO reduction than chars did, and it is possibly related to the higher molar ratio of H/C element for tar than for char
(8) An obvious contribution by ash as a kind of catalyst to NO reduction by char was also demonstrated so that the catalytic matters such as CaO, K2O, MgO in char were responsible to the highest NO reduction efficiency for the DSL char in comparison
Chapter 7 Conclusions and Recommendations reductions by model tar compounds clarified that phenol plays an important role in enabling the good NO reduction for the SD tar Also, the work found that the compounds in tar containing at least an aromatic ring are the major contributor for reducing NO in either biomass tar or coal tar.
Innovation
Especially little information is available about the NO reduction by tar in reburning
The following points are for the first time clarified in this work
(1) The comparison of NO reduction by all char, tar and py-gas reagents derived from biomass pyrolysis under conditions simulating that in the CFBDC system
(2) The evaluation of synergetic effects among char, tar and py-gas on NO reduction during reburning of pyrolysis products
(3) The investigation of NO reduction by tars derived from various fuels and by the typical model tar compounds, which clarified the major contributor for reducing NO by tars
Therefore, the results of this work not only enable further understanding of low-NOx emission in the CFBDC system treating various fuels but are also helpful for the application of tar reburning at commercial scales.
Recommendations for Future Work
(1) Although tar reagent manifested the higher NO reduction efficiency than char or py-gas did in testing individual reagents, it did not cause a significant synergetic effect on NO reduction in the experiments of Chapter 5 Therefore, further studies can focus on the mechanism of tar decomposition during reburning of tar-containing reagents In this way, micro-fluidized bed reaction analyzer integrated with mass spectrometer (MFBRA–MS) is suggested to be used to get the kinetic data for the homogeneous–heterogeneous combined reaction of NO reduction
(2) The results in Chapter 6 suggested that there are possibly synergetic effects of all chemicals containing in tar on NO reduction The combined actions of model tar compounds in reactions with NO are consequently needed to be investigated for revealing the mechanism involved in NO reduction by tar and its components
(3) The char, tar and py-gas reagents used in this study was just prepared at the pyrolysis temperature of 500 o C for adapting to that adopted in the pyrolysis bed of practical CFBDC systems It would be worthwhile to further investigate the NO-reduction capabilities of pyrolysis products from different temperatures
Nomenclatures ar = as-received basis BET = Brunauer-Emmett-Teller CFB = Circulating Fluidized Bed CFBDC = Circulating Fluidized-Bed Decoupling Combustion CH/NO = Molar ratio of C, H elements in reagent to fed NO C/NO = Molar ratio of C element in reagent to fed NO daf = dry and ash-free basis db = dry basis DC = Decoupling Combustion DSL = Distilled Spirit Lees DTR = Drop-Tube Reactor EDS = Energy Dispersive Spectroscopy Eq (x.x) = Equation (x.x)
FC = Fixed Carbon FMr = Actual feeding rate of reagent GC−MS = Gas Chromatography – Mass Spectrometry H/C = Molar ratios of H element to C element in reagent HCPs = Heat Carrier Particles m c = Mass ratio of metal oxides to fixed carbon in chars [NO]in = NO concentration at inlet of reaction zone
[NO]out = NO concentration measured at sampling port [NO]0 = NO concentration measured at inlet of preheating zone Py-gas = Pyrolysis gas
SD = Sawdust SEM = Scanning Electron Microscope SMr = Normalized (Specified) mass rate of reagent SR = Stoichiometric Ratio
STP = Standard Temperature and Pressure T = Temperature
XF = Xianfeng XRF = X-ray fluorescence
i = Content of the defined chemical compound “i” in tar reagent
i = Contribution of component “i” to overall NO reduction by tar reagent β ash = Contribution of catalytic effect to NO reduction by char reagent Δη e = Synergetic difference
i = Absolute NO reduction efficiency of a chemical compound “i” η = NO reduction efficiency η e = NO reduction efficiency normalized to SMr value of 0.15 g/min η ash = Catalytic efficiencies on NO reduction τ = Residence time
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Appendix A Chemical Compositions of Tested Tar Reagents
* The names of identified species marked as numbers are presented in Table A.1
Fig A.1 GC–MS spectra of all tested tar reagents
Mechanism of Low-NOx Emission in Circulating Fluidized-Bed Decoupling Combustion
Identified Compounds of Tar
Table A.1 Identified compounds of tar composition corresponding to GC–MS spectra in Fig A.1
Identified compounds in tar corresponding the functional groups
Sawdust (SD) tar Distilled spirit lees (DSL) tar Xianfeng (XF) coal tar
Acid 1 Acetic acid 26 n-Hexadecanoic acid
Ester 4 Propanoic acid, 2-oxo-, methyl ester 24 1,2-Benzenedicarboxylic acid, bis (2- methylpropyl) ester 27 Hexadecanoic acid, ethyl ester 30 Ethyl Oleate
Aliphatic 11 2,4-Hexadiene, 2,3-dimethyl 2 Butane, 2-bromo 15 Cyclopropane,1,1-dichloro-2,2,3,3- tetramethyl
23 Tetradecane27 Heptadecane29 1-Heptadecene30 Heptadecane31 Heptadecane32 Heneicosane 33 1-Nonadecene34 1-Octadecene35 Heneicosane36 1-Nonadecene 37 Heneicosane38 Heneicosane
Identified compounds in tar corresponding the functional groups
Sawdust (SD) tar Distilled spirit lees (DSL) tar Xianfeng (XF) coal tar
Aromatic 15 Benzene, 1-ethyl-4-methoxy 1 Pyridine 1 Toluene
25 1,1'-Biphenyl-3,4,4'-trimethoxy-6'-formyl 6 1,3,5-Cycloheptatriene, 1-methoxy 2 Toluene
5 Benzene, 1,3-dimethyl 6 Benzene, 1,3-dimethyl 7 Benzene, 1-ethyl-3-methyl 8 Benzene, 1,2,3-trimethyl 9 Mesitylene
13 Indene 14 Acetophenone 17 Benzene, 1-ethenyl-4-ethyl 18 Benzene, 1-methyl-2-(2-propenyl) 19 Azulene
22 Naphthalene, 2-methyl 24 Naphthalene, 2,3-dimethyl 25 Naphthalene, 1,3-dimethyl
Furans 2 Furan-2-carbonyl chloride 3 2-Furanmethanol 26 Dibenzofuran
6 2(5H)-Furanone, 4 Ethanone, 1-(2-furanyl) 28 Dibenzofuran, 4-methyl
Hydroxyl 10 4-Octyne-3,6-diol 23 1-Dodecanol, 3,7,11-trimethyl
Ketone 3 2-Pentanone 7 2-Cyclopenten-1-one, 2,3-dimethyl
Mechanism of Low-NOx Emission in Circulating Fluidized-Bed Decoupling Combustion
Identified compounds in tar corresponding the functional groups
Sawdust (SD) tar Distilled spirit lees (DSL) tar Xianfeng (XF) coal tar
5 2-Propanone 25 Hexahydropyrrolo[1,2-a]pyrazine-1,4- dione 7 Cyclohexanol
14 Phenol, 4-ethyl-2-methoxy 10 Phenol, 2-ethyl
17 Phenol, 2-methoxy-4-(1-propenyl) 12 Phenol, 2,5-dimethyl- 18 Phenol, 2-methoxy-4-(1-propenyl) 13 Phenol, 2-methoxy-4-methyl 19 Ethanone, 1-(4-hydroxy-3- methoxyphenyl)
Appendix B Calibration Curves of Feeders for Different Reagents
Fig B.1 The correlation curves between feeding rate (g/min) of char, tar reagents and frequency of feeders (Hz)
Mechanism of Low-NOx Emission in Circulating Fluidized-Bed Decoupling Combustion
Fig B.2 The correlation curves between reductant feeding rate (g/min) and pump frequency (Hz)
This thesis would not be completed without the help, support and guidance of many people Therefore, I would like to express my gratitude to all those who have made these studies possible
My deepest gratitude goes first and foremost to my principal supervisor, Professor Guangwen Xu, for giving me the opportunity to become a doctoral student at the University of Chinese of Academy of Sciences (UCAS), as well as to perform my Ph.D research at Institute of Process Engineering (IPE), Chinese Academy of Sciences (CAS)
It is my great honor to work under him I also want to express my warmest thanks to Professor Shiqiu Gao for his instructive advice and useful suggestions on my thesis I am deeply grateful of his help in the completion of this thesis
I gratefully thank the UCAS and Advanced Energy Technology (AET) research group for the provision of Ph.D scholarship and maintenance grant for the duration of my study Appreciation is also extended to Professor Dinh-Tuan Phan in my country, who recommended me to the AET group and for the receipt of this full scholarship
My sincere thanks to all past and present members of AET group, where I learned a lot and made great friends during the last 4 years Especially, I wish to express my sincere thanks to Dr Jian Yu, Dr Guangyi Zhang, Dr Xi Zeng, Dr Fang Wang, Mr
Changbin Yao and all other teachers for their invaluable assistance all through this work
I also deeply appreciate to Mr Yutthasin Bunman, who cooperated with me in the experiments of Chapter 6 My gratitude is extended to Mr Yusheng Zhang, Mr Chao Yu, Mr Shuai Cheng, Mr Huan Liu and Dr Zhennan Han for their friendship and good collaboration during the research work
Most important of all, special thanks are expressed to my beloved family for their continuous support and encouragement Last, but not least, I express my special thanks to my wife (also my colleague) Tuyet-Suong Tran for her endless love and the great assistance in my work as well
Beijing, June 2018 Hai-Sam Do