Bioenergy systems for the future 16 biomass gasification producer gas cleanup

Bioenergy systems for the future 16 biomass gasification producer gas cleanup Bioenergy systems for the future 16 biomass gasification producer gas cleanup Bioenergy systems for the future 16 biomass gasification producer gas cleanup Bioenergy systems for the future 16 biomass gasification producer gas cleanup Bioenergy systems for the future 16 biomass gasification producer gas cleanup Bioenergy systems for the future 16 biomass gasification producer gas cleanup

Biomass gasification producer gas cleanup 16 S Adhikari*, N Abdoulmoumine†, H Nam*, O Oyedeji† *Auburn University, Auburn, AL, United States, †University of Tennessee, Knoxville, TN, United States 16.1 Introduction Biomass producer gas contains nonnegligible amount of impurities such as fine particulates, condensable organic compounds known as tar, sulfur-containing compounds, nitrogen-based compounds, hydrogen halides, and trace metals The presence of these impurities in biomass producer gas hinders its utilization and must therefore be removed to meet stringent environmental emission regulations and minimize deleterious effects on equipment and catalysts in downstream processes Producer gas cleaning is a vital step for large-scale commercial deployment of biomass gasification as it has the most impact on the cost of clean syngas and its potential use for several downstream processes 16.2 Producer gas impurities 16.2.1 Particulates Particulate matter (PM) impurities emanates from biochar, soot, and elutriated bed materials, if fluidized-bed systems are used, with particle diameter up to 100 μm The quantity of particulates in the producer gas prior to cleaning depends on the particle size of the starting feed, type of gasifiers (i.e., moving bed vs fluidized bed) and the process conditions (residence time and temperature) Particulate impurities are primarily composed of residual solid carbon and inorganic elements emanating from biomass ash Particulate impurities are classified according to aerodynamic diameter with PM10, for example, representing “particulate matter” with diameter smaller than 10 μm This classification is used to indicate particulate cleanliness requirements for specific applications For example, gas turbine applications require particulate concentrations less than 30 mg/m3 for PM5 and above Particulates in producer gas lead to air pollution, fouling, corrosion, and erosion, which adversely affect human health, efficiency, and safety in gasification plants 16.2.2 Tar Tars are remnants of volatile compounds of biomass devolatilization and are a complex group of organic compounds that condenses in transfer lines, conduits, and other equipment downstream of the gasifier The definition and classification of tar is not Bioenergy Systems for the Future http://dx.doi.org/10.1016/B978-0-08-101031-0.00016-8 © 2017 Elsevier Ltd All rights reserved 542 Bioenergy Systems for the Future uniform in literature because of the complex chemical nature of tar However, tars have been defined as “all gasification organic compounds with molecular weight greater than benzene” and classified into five classes of tars in several works: l l l l l Class I: This class represents tar compounds with seven or more rings Tar compounds belonging to this class are considered the heaviest and tend to condense even at high temperature and moderately low gas-phase concentrations These tar compounds exhibit low volatility and are seldom detected during gas chromatographic analysis Class II: Class II tar compounds encompass heterocyclic hydrocarbons with heteroatoms Examples of these compounds are phenol, cresol, and pyridine These compounds are highly water-soluble Class III: This class is composed of light aromatic compounds that not readily condense on surfaces and have poor solubility in water Examples of such compounds are toluene, styrene, and xylene Class IV: Light polyaromatic compounds with two or three rings belong to this class Unlike Class III compounds, these compounds readily condense at intermediate temperatures Common tar compounds in this class are naphthalene, phenanthrene, and anthracene Class V: This class covers heavy polyaromatic hydrocarbons with four to six ring compounds that condense even at high temperatures and low concentrations Examples of these compounds are fluoranthene and pyrene Tar is the most notorious impurity existing in producer gas from biomass gasification because of its high abundance and potential to polymerize As a result, tar has been the focus of most producer gas cleanup researches (Abdoulmoumine et al., 2015) 16.2.3 Nitrogenous impurities Nitrogen in biomass is converted to ammonia (NH3), hydrogen cyanide (HCN), and oxides of nitrogen (NO, NO2, N2O, and other NOx) (Zhou et al., 2000) Nitrogenous impurities in biomass producer gas generally emanate from the decomposition of protein and/or heterocyclic aromatic structures in the biomass feedstock (Hansson et al., 2003, 2004) It has been hypothesized that biomass protein is first decomposed to form 2,5-diketopiperazines that are later decomposed to form HCN and HNCO, while NH3 is mainly form in the solid phase (Hansson et al., 2004) The nitrogen-containing compounds in producer gas may deactivate catalysts and cause air pollution Among all nitrogenous impurities, NH3 is the most abundant with widely varying concentrations typically between 350 and 18,000 ppmv depending on the nitrogen content in the feedstock and process conditions The primary incentive for NH3 removal is the reduction of NOx emissions in downstream applications such as burners, gas engines, and turbines 16.2.4 Sulfur impurities Sulfur in the biomass is converted primarily to hydrogen sulfide (H2S), carbonyl sulfide (COS), carbon disulfide (CS2), and other minor sulfur-containing compounds with H2S being the most dominant in gasification The quantity of sulfur-based impurities in biomass producer gas is in lower quantity compared with coal producer gas Most downstream applications require the removal of sulfur-based impurities to avoid Biomass gasification producer gas cleanup 543 equipment corrosion and catalyst deactivation and to comply with emission regulations Since concentrations as low as few parts per million of sulfur in gas stream can severely deactivate catalysts, producer gas requires extensive cleaning of H2S and other sulfur-based impurities if it is intended for use in catalytic chemical processes downstream 16.2.5 Hydrogen halide impurities Halogens, like chlorine in biomass feedstocks, are released as hydrogen halides such as hydrogen chloride (HCl) and their respective salts by binding to metals in biomass such as alkali metals Among all hydrogen halides, HCl is the most abundant (Ohtsuka et al., 2009) Hydrogen halide removal is very important to minimize corrosion and fouling/slagging of equipment such as filters, turbine blades, and heat exchanger surfaces and to meet often stringent requirements for downstream application Fuel cell applications particularly have very stringent requirements for hydrogen halide concentrations due to the susceptibility of electrolytes and electrodes attack by halogen ions 16.2.6 Trace metal impurities Metals such as Ca, Mg, P, Si, Na, K, Fe, Al, Cu, Mn, Fe, Zn, Mo, As, Cd, Hg, and Pb are contained in biomass intrinsically and sometimes added through technogenic activities Intrinsic biomass metals are taken during the plant growth from soil, water, and air, and technogenic biomass metals are added to biomass during pregasification processes such as harvesting and transportation During gasification, intrinsic biomass metal is partitioned into char and gas product The gas-phase metals are the major sources of concern as they must be captured downstream prior to exhausting the gas into the atmosphere Consequently, they must be removed as they are a source of concern for human health and environmental pollution In addition, some of these elements can contribute to catalyst deactivation and corrosion and fouling of equipment Technogenic biomass metal may be detached from biomass especially in fluidized-bed systems causing slagging and severe defluidization issues Trace metal impurities during gasification may also originate from catalysts and bed material in the case of fluidized-bed systems 16.2.7 Mercury and other toxic impurities Several heavy metals such as Hg, As, Se, Cu, Pd, Cd, and Zn are found in producer gas in trace quantities In the context of the environment and human health, these metals are highly toxic Biomass in its raw form contains less than 40 ppb of Hg (Thy and Jenkins, 2010) However, Hg receives special attention being a severe environmental pollutant that is capable of accumulating in the ecosystem and causing serious human health problems Vaporized Hg may remain in the atmosphere for months and move across intercontinental distances Hg exists in producer gas mainly as elemental Hg and rarely as HgCl2 and HgS depending on gasification operating conditions In downstream applications, Hg forms amalgams with metals, especially aluminum, leading to 544 Bioenergy Systems for the Future the failure of metal components of equipment Dioxins and dioxin-like compounds (DCLs) are another toxic impurity in producer gas DCLs are formed at low gasification temperatures by hydrocarbons and chlorine, in the presence of oxygen and metals Similar to Hg, DCLs have the capacity to bioaccumulate in the tissue of animals and have been found to cause cancer and damage to the human hormonal and immune system 16.3 Operating conditions and their implications on producer gas impurities 16.3.1 Particulates Particulate matter is primarily affected by the temperature that causes morphological, physical, and chemical changes As temperature is increased, particulate diameter decreases following a shrinking core phenomena However, particle-size reduction is more strongly impacted by elutriation, especially when pellets and chips in fluidized-bed and entrained-flow gasifiers by fragmentation Furthermore, temperature significantly alters the chemical properties of particulates and increases residual solid carbon and inorganic content ER and steam not influence the morphology or physical properties and mildly affect the chemical properties 16.3.2 Tar The quantity and nature of tar in producer gas can be affected by temperature, residence time, equivalence ratio (ER), and steam-to-biomass ratio by promoting thermal cracking, oxidation, and steam-reforming reactions Temperature plays a vital role on tar content and on the nature of tar compounds in producer gas by promoting thermal cracking and favoring the formation of multiring aromatic tar compounds In general, tar concentration usually decreases as temperature increases (Narvaez et al., 1996), but tar refractoriness increases with temperature The role of residence time is similar to that of temperature because the severity of reaction is directly proportional to both temperature and residence time ER has a significant effect on tar reduction by promoting oxidation of volatiles formed during devolatilization In general, an increase ER decreases tar concentration Tar concentration was decreased from 10 to near 2.5 g/Nm3 by increasing the equivalence ratio from 0.26 to 0.45, everything else being equal (Narvaez et al., 1996) Catalyst has been found to affect tar yield and composition with tar yield following Co/Al2O3 > Fe/Al2O3 > Ni/Al2O3 when corn stover was gasified in a microwave-assisted system at 900°C (Xie et al., 2014) 16.3.3 Nitrogenous impurities Since nitrogen impurities evolve due to the presence of nitrogen in biomass, it is expected that biomass feedstock type will play a major role in their concentrations In addition, biomass gasification operating parameters impact the yield of individual nitrogenous impurities When gasification temperature is increased, the decomposition Biomass gasification producer gas cleanup 545 of NH3 to N2 is increased For example, using sawdust, the concentration of NH3 decreased by half over temperatures ranging from 700°C to 900°C at ER ¼ 0.25 (Zhou et al., 2000) Similar trends are observed for HCN and NO (Zhou et al., 2000) in sawdust at ER ¼ 0.25 Besides temperature, ER can also affect the proportion of NH3, HCN, NO, and other nitrogen-containing species in producer gas It was observed that NH3, HCN, and NO decrease as ER is increased slightly from 0.18 to 0.37 at 800°C (Zhou et al., 2000) However, it is evident that temperature, rather than ER, has the most impact in reducing nitrogen impurities as illustrated in Fig 16.1 Besides temperature and ER, the type of gasifying medium also plays a role in the concentration of NH3, HCN, and other nitrogenous compounds The presence of steam in gasification enhances the formation of NH3 A comparison of two fuels with similar nitrogen contents gasified in a circulated fluidized-bed reactor with air and steam at similar temperature showed that NH3 concentration is doubled when steam is used (Wilk and Hofbauer, 2013; Van der Drift et al., 2001) Fuel nitrogen conversion to HCN is also affected by gasifying agents As the use of steam increases the concentration of H2 in the syngas, it is likely that the increasing reducing environment favors the formation of NH3 16.3.4 Sulfur impurities Fuel N conversion to NH3 (%) There are diverging reports of the effect of temperature on H2S content in syngas In a study of a mixture of 70% refuse-derived fuel and coal (0.62 wt% dry ash-free basis sulfur), it was reported that H2S concentration in syngas increased from 808 to 1081 ppmv from 720°C to 850°C but subsequently decreased to 823 ppmv as temperature was further increased to 900°C (Dias and Gulyurtlu, 2008) However, Carpenter et al (2010) reported an increase in H2S for switchgrass, Vermont wood, and wheatgrass as temperature was increased from 650°C to 875°C Gasifying agent (air, steam, air/ steam, or O2/steam) significantly affects the composition of primary gases as discussed earlier These primary gases, in turn, can be involved in various reactions with H2S and 100 ER 0.18 80 0.25 0.32 60 40 20 700 800 900 Gasification temperature (°C) 1000 Fig 16.1 Effect of temperature and ER on fuel-bound nitrogen conversion to NH3 Reproduced from tabulated data Zhou, J., Masutani, S.M., Ishimura, D.M., Turn, S.Q., Kinoshita, C.M., 2000 Release of fuel-bound nitrogen during biomass gasification Ind Eng Chem Res 39(3), 626–634 546 Bioenergy Systems for the Future other sulfur impurities, thus affecting their final concentrations Several reactions are involved in H2S and COS formation and conversion during gasification: H2 S + CO2 ! COS + H2 O H2 S + CO ! COS + H2 H2 S + 3=2O2 $ SO2 + H2 O COS + H2 S $ CS2 + H2 O In a study of corn straw gasification in a downdraft gasifier, it was observed that as ER increased from 0.20 to 0.40, H2S first increased from 473 to 512 ppmv from 0.20 to 0.30 ER and subsequently decreased to 459 ppmv from 0.30 to 0.40 ER (Gai et al., 2014) Dias and Gulyurtlu (2008) also observed that as ER is increased from to 0.40 in a mixture of 70% refuse-derived fuel and coal (0.62 wt% dry-ash-free basis) and H2S concentration increased from 672 to 1204 ppmv However, as S/B ratio increased, H2S concentration decreased according to various studies (Gai et al., 2014; Meng et al., 2010) 16.3.5 Hydrogen halide impurities Temperature impacts hydrogen halides concentration in syngas by enhancing the formation of alkali halides using fuel-bound metals (Kuramochi et al., 2005) At ER of 0.20, it was noticed that HCl concentration decreased from 95 to 65 ppmv as temperature was increased from 720°C to 900°C for a mixture of 70% refuse-derived fuel and coal with a weighted average of 0.07 wt% dry-ash-free basis chloride (Dias and Gulyurtlu, 2008) The impact of gasifying agent on hydrogen halides is not very clear due to the lack of information particularly in air/steam and steam gasification As ER is increased from to 0.40, HCl concentration increased from 78 to 85 ppmv at 850°C (Dias and Gulyurtlu, 2008) 16.3.6 Trace metal impurities Temperature plays a crucial role in the level of trace metals detected during gasification Alkali and other trace metals are commonly bound to halogen and other inorganic elements in biomass with the formation of alkali halides commonly increasing as temperature is increased due to favorable thermodynamics (Porbatzki et al., 2011; Dolan et al., 2012) Porbatzki et al (2011) investigated the release of metals in wood and miscanthus during gasification at 800°C, 900°C, and 1000°C in a fluidized bed and observed that the release of potassium decreased as temperature increased for wood but increased as temperature increased for miscanthus In a thermodynamic modeling of fluidized-bed gasifier, it was reported that Na, K, Fe, and Mn are not appreciably release into the gas phase even as gasification high temperature is increased to 1000°C (Konttinen et al., 2013) Similar conclusions were drawn by Froment et al (2013) where Ba, Mg, K, P, and Mn are not released at temperatures lower than 1000°C On the other hand, Pb, As, Zn, Hg, Sd, Sn, and Cd are completely released in the gas phase at 750°C (Konttinen et al., 2013) Biomass gasification producer gas cleanup 547 16.3.7 Mercury and other toxic impurities The form in which Hg exists in producer gas is important and determined by the gasification operating conditions It is more difficult to remove elemental Hg than it is to remove oxidized Hg in the form of HgS or HgCl2 When reducing conditions is prevalent during gasification, the oxidation of Hg to form Hg2+ compounds is inhibited This is typically the case for gasification process Gasification conditions not favor formation of DCLs; therefore, the amount of DCLs in producer gas may be in few parts by quadrillion The low oxygen levels in gasification condition inhibit the formation of DCLs Hence, the formation of DCLs is more prevalent in combustion conditions (Cheng and Hu, 2010) In addition, DCLs are decomposed at gasification temperatures higher than 850°C (Kalisz et al., 2008) There is however the potential for the formation of DCLs during cold gas cleanup process where biomass producer gas is cooled down The presence of particulate matter at the producer gas cool phase may facilitate the formation of DLCs (Lemmens et al., 2007) 16.4 Producer gas cleanup Prior to its utilization, biomass-derived syngas must be purified to adhere to the specific downstream applications (Table 16.1) Consequently, depending on the type of feedstock and gasification process, syngas purification and conditioning must be adapted to target desired impurities In addition to aforementioned impurities, carbon dioxide and other light hydrocarbons might require removal for optimal operations in catalytic reactors 16.4.1 Particulate cleanup Particulate removal is essentially achieved by producer gas filtration since the mass of the particulate matter is larger than that of the producer gas Particulate cleanup technologies are more mature relative to other cleanup technologies and operate by inertial separation, barrier filtration, and electrostatic interaction Many cleanup technology options Upper limits of impurities in gasification syngas for selected applications (Woolcock and Brown, 2013) Table 16.1 Applications Gas turbine FT synthesis Methanol synthesis Tars (mg/Nm3) Sulfur impurities (ppmv) Nitrogen impurities (ppmv) Alkali (ppmv) Halides (ppmv) n/a