Atmospheric entrained-flow gasification of biomass and lignite for decentralized applications

Fuel Processing Technology 152 (2016) 7282 Contents lists available at ScienceDirect Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc Research article Atmospheric entrained-ow gasication of biomass and lignite for decentralized applications Jens Schneider a, Christian Grube a, Andrộ Herrmann a, Stefan Rửnsch a,b, a b Syngas Technologies, Department Bioreneries, Deutsches Biomasseforschungszentrum gemeinnỹtzige GmbH, Torgauer Str 116, 04347 Leipzig, Germany Department of Industrial Engineering, Ernst-Abbe-Hochschule Jena, Carl-Zeiss-Promenade 2, 07745 Jena, Germany a r t i c l e i n f o Article history: Received February 2016 Received in revised form May 2016 Accepted 31 May 2016 Available online xxxx Keywords: Gasication Entrained-ow Biomass Syngas Decentralized a b s t r a c t The present study deals with the development of a small-scale entrained-ow gasication technology for the decentral use of biomass Gasication experiments with woody biomass in a wide range of particle diameters dS in the fractions 0.04 b dS b 0.11 mm, 0.20 b dS b 0.25 mm, 0.25 b dS b 0.50 mm, and 0.50 b dS b 1.0 mm were carried out in an atmospheric electrically-heated entrained-ow gasier at temperatures between 950 and 1100 C Power plant lignite in the fraction 0.05 b dS b 0.08 mm was gasied as well for comparison These low temperatures were chosen in order to verify that an entrained-ow gasication technology operating at mild conditions can be developed Low investment costs combined with the production of a tar-free syngas make this technology option attractive especially for decentralized applications (b5 MW fuel input power) The production of a high syngas quality during autothermal operation has still to be demonstrated A short review of studies prepared for entrained-ow gasication of biomass since 2006 points out the state of the art and most important ndings The concentrations of H2, CO, CO2 and N2 together with carbon conversion, cold gas efciency and syngas yield resulting from the present work are reported and compared to the respective literature values Carbon conversion, cold gas efciency, and specic syngas volume varied strongly with temperature and particle diameter showing values between 63 and 100 wt%, 14 and 61%, and 0.6 and 1.4 m3 kg1 (STP), respectively With the present set up, high cold gas efciencies were only obtained at temperatures of 1100 C and particle sizes of less than 0.2 mm Particle residence times in the gasier were measured at 25 C for three sawdust fractions and varied between 1.4 and 3.3 s These measurements indicate that the particle residence time is not equal to the gas residence time in general A model for the calculation of particle velocities and residence times at ambient and gasication conditions is presented The interrelationships between particle residence time, particle diameter, carbon conversion, and temperature are discussed â 2016 Elsevier B.V All rights reserved Introduction 1.1 Background For the transition of the energy system from fossil to renewable energies, economic, environmental and supply security concerns have to be considered [1,2] The use of bioenergy technologies provides a possibility to meet these concerns Bioenergy technologies allow a substantial reduction of net CO2 emissions [3,4], while providing energy reliably and weather-independent [5], which cannot be guaranteed by wind and solar power plants alone Corresponding author at: Syngas Technologies, Department Bioreneries, Deutsches Biomasseforschungszentrum gemeinnỹtzige GmbH, Torgauer Str 116, 04347 Leipzig, Germany E-mail address: Stefan.Roensch@dbfz.de (S Rửnsch) http://dx.doi.org/10.1016/j.fuproc.2016.05.047 0378-3820/â 2016 Elsevier B.V All rights reserved Especially the gasication of lignocellulosic biomass is a versatile biomass conversion pathway producing energy-rich gas that can be used for the generation of electrical power [68] and basic chemicals [914] However, for a competitive market implementation and use in small-scale bioenergy plants (b MW fuel input power), the investment costs of this technology have to be reduced Especially downstream processes like gas cleaning are complex and related to high specic investments in small-scale facilities For a reduction of the gas cleaning equipment [15,16] and the related investment costs, the production of a tar-free syngas is a crucial factor Entrained-ow gasiers are able to generate a tar-free syngas that is nearly free of hydrocarbons [17] They are commercially available e.g as large scale coal gasiers typically constructed for a few hundred megawatt fuel input power and operated at extreme pressures (4080 bar) and temperatures (14001600 C) [17,18] Commercial entrained-ow gasiers for biomass are under development J Schneider et al / Fuel Processing Technology 152 (2016) 7282 1.2 Objective of this work Comparably low investment costs for a gasier operating at mild conditions and being constructed of less expensive materials motivate this work Pressures above 40 bar and temperatures above 1400 C necessitate not only heavy and durable materials but also intensive safety precautions, which are expensive and seem to exclude the operation of small-scale bioenergy plants Hence, this work is aimed at investigating an entrained-ow biomass lab-scale gasier working at comparatively mild operation conditions (atmospheric pressure, temperatures 1200 C) and to prove its applicability for future small-scale facilities (b5 MW fuel input power) Experiments were carried out in order to identify a working point, in which the electrically heated lab-scale gasier reaches high carbon conversions and cold gas efciencies Nomenclature Symbol Description Unit C CG A G dS Fd Cd G p Re T u V_ Carbon conversion Cold gas efciency Cross section of tubing Density of gas Diameter of particles Drag force Drag force coefcient Dynamic viscosity of gas Equivalence ratio (excess air ratio) Minimal residence time Parameter for drag force coefcient Parameter for drag force coefcient Pressure Slip Reynolds number Temperature Velocity (particle, gas, slip) Volume ow rate wt% % m2 kg m-3 m N Pa s s Pa K m s-1 m3 h-1 Compact review concerning entrained-ow gasication of biomass Several studies concerning the gasication of various biomasses in entrained-ow reactors were conducted, especially since 2009 [19 42] The following presentation of research results in Section 2.1 focuses on the inuences of the process conditions on gas composition and deduced measures The conditions studied and major ndings are presented chronologically Entrained-ow gasication technologies that apply biomass and are commercialized or at an advanced development stage are presented in Section 2.2 2.1 Selected research results since 2006 Zhang et al conducted gasication experiments with steam and oxygen in nitrogen as well as pyrolysis experiments with N2 in an entrained drop-tube furnace at temperatures between 600 and 1400 C [20,26] Hinoki cypress sawdust with particle diameters below mm was applied at gas residence times between and s H2 yields increased with temperature and were high with high H2O feed ( 39 mol kg1 fuel (dry) at 1400 C) and low with high O2 feed (12 mol kg1 at 1400 C) as gasication agent Dehydrogenation of the fuel's constituents (cellulose, hemicellulose, and lignin) during pyrolysis and of the pyrolysis products (char, tar and hydrocarbons) during gasication led to the formation of H2 The decomposition of pyrolysis products was promoted at T N 1000 C CO yields increased from 600 to 800 C and at T N 1100 C in all atmospheres At 900 b T b 1100 C with H2O feed, CO yields decreased likely due to the water-gas shift reaction (CO + H2O CO2 + H2) At T N 1000 C with O2 and N2 as well as at T N 1100 C with H2O as gasication agent, increasing CO yields were explained by the Boudouard reaction 73 (C + CO2 2CO) and the steam gasication (C + H2O CO + H2) reaction CO2 yields were low with N2 feed at all temperatures ( mol kg1 fuel (dry)) and increased with temperature with H2O feed due to the water-gas shift reaction CO2 yields were high with O2 as gasication agent (17 mol kg1 fuel (dry)) at T b 1000 C and decreased at higher temperatures Furthermore, Zhang et al [20,26] showed that the concentrations of tars and char decreased with increasing temperature in all atmospheres and resulted in tar yields below 0.8 g kg1 fuel (dry) at a temperature of 1200 C [26] Coke (soot) yields increased at temperatures between 800 and 1100 C and decreased at higher temperatures in all atmospheres The activities in tar, char, and soot destruction followed the order with respect to gasication agent: O2 N H2O N N2 Between 600 and 900 C, carbon conversion increased with temperature with all atmospheres, decreased up to 1000 C with H2O and O2, decreased up to 1100 C with N2, and increased again at higher temperatures The intermediate decline of the carbon conversion was justied by the competition between hydrocarbon decomposition (forming coke) and carbon consuming reactions with H2O, O2 and CO2 [26] The effect of steam addition in the gasication of dealcoholized marc of grape (particle diameters below 0.5 mm) with air was studied by Hernỏndez et al [29] The effects of the steam-biomass ratio (between and 3.2 mol mol1) on gas yields as well as of the operation temperature (750 C T 1150 C) on gas yields in air (between and 2.6 mol air per mol biomass) and air-steam (0 to 100 wt% H2O) gasication are demonstrated Major ndings of Zhang et al [26] are supported Qin et al studied the air and air-steam gasication of beech sawdust (median diameter d50 = 280 m) and wheat straw (d50 = 170 m) at temperatures between 1000 and 1350 C in an atmospheric electrically-heated entrained-ow reactor [28] Molar steam-to-carbon ratios varied between and 1, while excess air ratios varied between 0.25 and 0.50 Gas residence times between and s were applied The effects of temperature, steam-to-carbon ratio, excess air ratio and biomass type on gas and soot yields are demonstrated The pyrolysis and steam gasication behaviors of beech sawdust in the fractions 0.313 b d S b 0.400 mm and 0.730 b dS b 0.900 mm were studied by Septien et al in an atmospheric, electrically-heated drop-tube reactor at temperatures of 1000, 1200, and 1400 C [33] Gas residence times varied between 2.2 and 4.4 s It was shown that soot yields decrease with temperature in a wet atmosphere (75 vol% N2, 25 vol% H2O) but increase with temperature in an inert atmosphere (amounting up to 22 wt% of the dry biomass) Tars were present at 1000 C but completely converted at T 1200 C Char yields were below wt% at all conditions studied and decreased with temperature in the wet atmosphere due to steam gasication Consequently, H and CO2 yields increased with temperature in the wet atmosphere, while C2H2, C2H4 and C6H6 yields decreased with temperature in both inert and wet atmospheres No signicant inuence of the particle size on product yields were found at the conditions studied (gas residence times of 2.2 and 4.4 s) It was speculated that particles with diameters of about mm could be gasied effectively in an entrained-ow reactor, which would decrease the pretreatment costs relative to lower diameters Yu et al investigated the gasication of rice straw with particle diameters below 0.3 mm and oxygen-enriched air [38] The inuences of O2 concentration (up to 60 vol%), equivalence ratio (0.15 0.35) and gasication temperature (800 T 1200 C) on gas composition, carbon conversion, lower heating value and tar yield are demonstrated Concentrations of H2 and CO2 increase with increasing O2 concentrations, while CO and CH4 concentrations decrease Considering the lower costs for the provision of oxygen-enriched air relative to pure air, = 0.25 and cO2 = 40 vol% are proposed as reference operation conditions in order to achieve nitrogen-diluted syngas with a lower heating value of 8.2 MJ m3 (STP) and tar yields of 2.6 mg g1 Yu et al suggest that membrane technologies could provide oxygen-enriched air with up to 40 vol% O2 in an economic way [38] 74 J Schneider et al / Fuel Processing Technology 152 (2016) 7282 Weiland et al studied an oxygen-blown autothermal pilot plant gasier at and bar with (thermal) fuel loads between 200 and 600 kW, excess air ratios between 0.25 and 0.51, and commercial stem wood pellets produced from sawdust of pine and spruce [41] The pellets were milled to particle sizes with characteristic size distribution numbers d50 between 125 and 180 m and d90 between 230 and 410 m Weiland et al found carbon conversions of 100% within 0.35 b b 0.50 (equivalent to process temperatures of 1300 b Tp b 1500 C) and of 95 and 80% at = 0.30 (Tp 1150 C) and 0.25 (Tp 1090 C), respectively The variation in fuel particle size did not have a statistically signicant effect on the gasication results This could result from the low variation in particle diameters The highest CO yield as well as low CH4 and benzene yields below vol% and 100 ppm (on dry, N2-free basis), respectively, were received at 0.425 (Tp 1400 C) The gasier was optimized for the production of syngas, which is applicable in downstream synthesis reactors, e.g for fuel production Detailed analyses of tars formed during pyrolysis and gasication of biomasses with steam and oxygen at temperatures between 600 and 1400 C were conducted by Zhang et al [26] and Hernỏndez et al [34] More than 20 individual tar species were detected and quantied The total amount of tars included in the product gas decreases with increasing temperature and increasing oxygen-to-fuel ratio in the feed Higher temperatures enhance reaction rates of tar cracking and reforming reactions O2 was found to be more effective in tar cracking than H2O or CO2 due to its higher reactivity It was found that the BTX group of tars (being represented by benzene, toluene and o-, m- and p-xylene) is the most abundant one and needs temperatures above 1200 C to be completely destructed [26] Phenol, naphthalene, biphenylene and acenaphthylene could also represent a signicant share of the tars produced The amount of individual species depends strongly on temperature, gasication agent, oxygen-to-fuel ratio and fuel applied as demonstrated by Zhang et al and Hernỏndez et al In order to produce a syngas with low tar amounts and high heating values, entrained-ow gasiers should be operated at T 1200 C [26] and steam contents below 50 wt% in the gasication agent [34] demonstrate the economic processing of biomass in highly integrated bioreneries KIT applies a gasier that is based on rst experiments of the Gaskombinat Schwarze Pumpe (GSP) technology [44] and built by Air Liquide in the bioliq project [45] Heated, homogenized bioslurries made by fast pyrolysis of lignocellulosic biomass are fed to the gasier with screw or plunger pumps and atomized with oxygen Extreme pressures up to 80 bar are targeted for the large-scale gasier Linde acquired the Carbo-V technology from Choren Industrietechnik in 2012 and redesigns it since then [46] It combines horizontal stirred-tank pyrolysis as pretreatment, oxygen-blown entrained-ow gasication of pyrolysis gas and chemical quenching of the syngas with the char from pretreatment Choren's original design was sophisticated and had several drawbacks Linde expects their improved design to show better robustness and availability [47] ThyssenKrupp Uhde offers the PRENFLO technology, which was designed for the oxygen-blown entrained-ow gasication of coal, but can be fed with pretreated biomass as well [48] Its applicability in a full biomass-to-liquids (BTL) process will be demonstrated in the BioTfueL project in France [49] All entrained-ow gasication technologies described in Table besides the one of ETC apply thermally pretreated biomass These pretreatments change the properties of the original biomass and shift them closer to the properties of lignite or generate slurries with high heating values This facilitates the application of coal gasication technologies At least the PRENFLO technology originates from coal gasiers Most designs aim at large-scale applications with thermal input powers of more than 100 MW and are typically operated at pressures above 25 bar This is justied by the economy-of-scale especially if Syngasto-Fuel applications are the target [45] Nevertheless, small-scale applications of entrained-ow biomass gasiers investigated in this work are of interest especially in order to decentralize the energy system Experimental 3.1 Test facility 2.2 Technologies commercialized or at advanced development stage Entrained-ow gasication technologies that apply biomass and are commercialized or at an advanced development stage are summarized in Table Chemrec is involved in the eld of pulp and paper mills and offers atmospheric as well as pressurized entrained-ow oxygen-blown gasiers for the conversion of black liquor into syngas [43] A pressurized demonstration plant started up in 2005 in Piteồ The Energy Technology Centre (ETC) in Piteồ constructed an autothermal oxygen-blown entrained-ow gasier for woody biomass [40,41] It is a pilot gasier with a maximum thermal throughput of MW and participated in the SUPRABIO project nanced by the European Commission within the 7th Framework program It is the target to The experimental setup included gas supply, fuel feeding, entrainedow reactor, coarse gas cleaning and gas analysis (Fig 1) Compressed air, oxygen and carbon dioxide at typical ow rates V_ STP between and m3 h at standard temperature and pressure (STP) could be used as gasication agents at varying nitrogen dilutions A screw-conveyor fed fuel at a rate between and kg h1 into a venturi nozzle A nitrogen ow (2 m3 h (STP)) transports the fuel from the venturi nozzle to the top of the reactor (inner diameter: 160 mm) in a tube with an inner diameter of 20 mm An electrical oven heats the reactor tube in three independent sections to a temperature of up to 1200 C The reactor tube was made of stainless steel Eight type N thermocouples measured the temperature at the vertical axis of the reactor tube with a length of 2.1 m The ow rates of Table Entrained-ow gasication technologies for biomass that are commercially available or at advanced development stage [40,41,43,45,47,48,50,51] Company/Institution Technology Fuel Gasication conditions Pretreatment Chemrec, Sweden Black Liquor Gasication (BLG) not specied Concentrated black liquor 140 bar, ~1000 C Milled woody biomass 10 bar, 10501550 C (demo) GSP, modied Bioslurry 4080 bar, N1200 C Carbo-V Hot pyrolysis vapors and char powder from pretreatment Pulverized torreed material from wood, straw, and energy crops 45 bar, N1400 C, quenching with char to b1000 C 2542 bar, N1200 C Integrated into pulp mill Milling, optional torrefaction Fast pyrolysis at bar, 500 C Pyrolysis at b500 C Energy Technology Centre, Sweden Karlsruhe Institute of Technology, Germany Linde, Germany ThyssenKrupp Uhde, Germany PRENFLO Torrefaction J Schneider et al / Fuel Processing Technology 152 (2016) 7282 gasication agent and transport gas (N2) were set by mass ow controllers Both uid streams entered the hot reactor tube without preheating In order to reach approximately isothermal conditions during gasication, rst and last section of the oven were set to 30 K higher temperatures than the middle section Typically, maximum deviations between the target temperature of the middle oven section and temperatures measured inside of the reactor tube amounted to less than 30 K within the region 0.3 b x b 1.7 m at steady-state gasication conditions (Fig 2) The temperature distributions measured for beech wood at the four target temperatures are displayed in Fig Higher deviations from the target temperature are present close to the end of the reactor tube (x N 1.7 m) since this part was outside of the oven and connected via anges to the water quench (injection of ca 2.6 l min1 at 15 C) The tubing above and below the oven is insulated only with ceramic ber tape without external heating Raw gas, ash, and non-converted fuel particles passed a water quench downstream to the reactor tube, where four hollow cone nozzles (1.1 mm drilling with 0.66 l min1 throughput at bar water pressure, spray cone of 60) were placed and injected water with a temperature of about 15 C The gas trajectory included a 180 curve in the water quench Thereby, a fast cooling to temperatures between 40 and 60 C occurred and solids as well as liquid droplets were separated from the gas ow Most of the tars produced by the gasier were condensed here and pumped out of the quench together with water Neither weighing nor compositional analysis of these tars were carried out Further particles were separated by a cyclone following the water quench Finally, a fraction of the gas passed a ber lter prior to the gas analysis by a Fourier transform infrared (FTIR) spectroscope and a gas chromatograph with thermal conductivity detector (GC-TCD) 3.2 Gas analysis FTIR spectroscope and GC-TCD were the main devices for analyzing the gas composition The FTIR spectroscope was of the type Gasmet CX4000 from the company ANSYCO Analytische Systeme und Componenten GmbH It was connected to the test facility behind the cyclone (compare Fig 1) via a heated tube (T = 150 C) The GC-TCD was a GCM Microbox II from the company Elster GmbH It was also connected to the heated tube Additionally, detectors for H2 (TCD CONTHOS from the company LFE GmbH & Co KG) and O2 (paramagnetic oxygen 75 Fig Measured temperature distributions at the vertical axis of the reactor tube at steady-state gasication conditions with beech wood (BW) and four target temperatures analyzer PMA100 from the company M&C TechGroup Germany GmbH) were combined with the FTIR spectroscope The gas phase species CO, CO2, N2, O2, H2, H2O, CH4, C2H6, C2H4, benzene, toluene and xylene were detected and quantied by the quoted techniques The concentrations of the most important species (CO, CO2, H2, CH4) were measured by FTIR spectroscope and GC-TCD independently 3.3 Fuels Two types of woody biomass were used in the experiments: a ne beech wood (BW) fraction with particle diameters of 0.04 dS 0.11 mm and waste wood (WW) from a local joinery, which was a mixture of beech and spruce The waste wood was fractionated with a sieving unit into samples with particle diameters of 0.2 dS 0.25 mm, 0.25 dS 0.5 mm, and 0.5 dS 1.0 mm Additionally, power plant lignite (LI) with particle diameters of 0.05 dS 0.08 mm served as a reference Results of proximate and ultimate analyses are shown in Table Moisture content, ash content, xed carbon, and lower heating value mentioned in proximate analysis were determined by procedures regulated in DIN EN 147741, DIN 51719, VDI 2465 (part 2) and DIN EN 14918, respectively The applied methods for the determination of carbon, hydrogen, and nitrogen as Fig Experimental setup of the atmospheric entrained-ow test facility 76 J Schneider et al / Fuel Processing Technology 152 (2016) 7282 well as of sulfur (and chlorine) stated in the ultimate analysis are regulated in DIN EN 15104 and DIN EN 15289, respectively Chlorine contents were below 200 ppm in all cases While the values from proximate analysis for BW and WW were comparable, lignite contained about 12% (absolute) more carbon and 24% (absolute) less oxygen than the wood samples Ash, volatiles and xed carbon contents for beach wood (BW) and waste wood (WW) were comparable at values of about 1%, 87%, and 12%, respectively The WW had a higher moisture content of 7.6% relative to 5.1% for BW and a slightly smaller lower heating value of 18.1 MJ kg1 Lignite had much higher ash and xed carbon contents of 15 and 43%, respectively, a higher moisture content of nearly 12%, and a higher lower heating value of 24.6 MJ kg1 than the two wood fractions Comparable results for beech wood are presented in [33] 3.4 Test series Gasication experiments were carried out with air as gasication agent at oven target temperatures between 950 and 1100 C Flow and feeding rates, calculated O2 concentration as well as the equivalence ratio (excess air ratio, i.e mass of oxygen applied to mass of oxygen needed for total combustion) for the different fuel fractions are summarized in Table 3.5 Data evaluation The measured H2O concentrations were affected by the injection of water in the quench at the reactor tube outlet and did not represent the water content of the product gas They varied between 3.5 and vol% in all cases without showing a clear trend No dependences for the gas species CH4, C2 (C2H4, C2H6) as well as for BTX tars (benzene, toluene and xylene) are discussed since their concentrations were low or affected by the water quench CH4, C2 and BTX tar concentrations were measured by the FTIR spectroscope and amounted to values below 1.0, 0.2 and 0.2 vol%, respectively, in all cases A carbon mass balance was formulated based on the measured species CO, CO2, CH4 and BTX tars, while all other gaseous hydrocarbons (C2) were omitted Since a certain but not quantied amount of tars was condensed in the water quench (compare Section 4.4) and unconverted char was pumped out of the water quench without weighing, the carbon mass balance could never be closed The missing value to 100% was attributed to char and tars washed out in the water quench The molar ow rate of the product gas was determined by the ratio of the N2 concentrations present in educt and product gas Based on this, the mass of carbon-containing species in the product gas was deduced The carbon conversion was derived by calculating the ratios of the carbon mass present in the product gas and in the raw fuel Furthermore, the volume ow rates of H2, CO and CO2 were calculated, summed up and divided by the fuel's ow rate yielding the specic syngas volume (volumes of H2, CO and CO2 per mass of wet fuel) The cold gas efciency was determined by dividing the chemical energy (product of massbased ow rate and heating value) in the produced syngas by the energy input (product of feeding rate and heating value) from the fuel (wet basis) Lower heating values were applied in all cases Results and discussion 4.1 Particle residence time and slip velocity Prior to gasication experiments, measurements of the particle residence time were carried out with a modied setup The steel reactor was replaced by a quartz tube of comparable size to allow a visual observation of injected biomass particles The motion of a particle fraction through the tube was monitored by dosing g of biomass into the venturi nozzle The times when particles entered the top and reached the bottom of the quartz tube were measured These measurements were reproduced ten times for each fraction Standard deviations of up to 11% of the mean value for each fraction were realized Since big particles have a higher velocity than small particles [23,33], the residence times of the bigger particles in each fraction were determined Particle residence times measured for sawdust fractions with particle diameters of dS b 0.25 mm, 0.25 b dS b 0.5 mm and 0.5 b dS b 1.0 mm at a total ow rate of appr m3 h (STP) are displayed in Fig 3(A) The residence times of mm and 0.25 mm particles amounted to 1.4 0.1 s and 3.3 0.2 s, respectively, while the gas residence time was about 50.7 s These measurements indicate a strong dependence of the particle residence time on the diameter of the uidized particles and support theoretical predictions of Dupont et al [23], Umeki et al [52] and Kirtania and Bhattacharya [53] This will be discussed in the following with the help of a general model In the model, the gas velocity uG is determined by Eq .(1) from the volumetric ow rate at standard temperature and pressure V_ STP , actual temperature T and pressure p in the tube, and the cross section A of the tube Particles were injected into the reactor tube by uidization in a N2 ow with V_ N ẳ 2:0 m3 h through a tube with an inner diameter of 2 cm in the lab-scale gasier Assuming that the particles move with the velocity of the uidization agent at the end of the feeding tube, their starting velocity at the top of the reactor tube amounts to uS,0 = 1.93 m s1 (Eq .(1)) Since the reactor tube is eight times wider than the feeding tube, the gas decelerates quickly, e.g to uG = 0.07 m s1 when 2.3 m3 h1 of air (total gas ow rate of 4.3 m3 h1 assumed in Fig 3A) are applied as gasication agent at 25 C T 101325 Pa uG ẳ V_ STP pA 298:15 K 1ị The progress of the particle velocity from the top to the bottom of the reactor tube is governed by the attractive gravitational force and a decelerating drag force Fd as a simple assumption in this model The Table Proximate and ultimate analyses of beech wood (BW), waste wood (WW) and lignite (LI) Parameter Beech wood (BW) Waste wood (WW) Lignite (LI) Proximate analysis (wt%) Moisture Ash (dry) Volatiles (by difference) (dry) Fixed carbon (dry) Lower heating value (MJ kg1) 5.09 0.86 86.94 12.20 18.64 7.56 1.17 86.53 12.30 18.05 11.90 15.10 42.00 42.90 24.57 Ultimate analysis (wt% dry, ash-free basis) Carbon Hydrogen Oxygen (by difference) Nitrogen Sulfur 50.90 6.25 41.58 0.32 0.09 49.50 6.03 43.06 0.19 0.05 62.20 4.97 19.11 0.58 3.01 J Schneider et al / Fuel Processing Technology 152 (2016) 7282 77 Table Flow rates of the gasication agent (air), feeding rates of the fuel, calculated O2 concentration at the reactor inlet, equivalence ratio, and gas residence time at 950 T 1100 C with beech wood (BW), waste wood (WW), and lignite (LI) Fuel fraction 0.04 dS 0.11 mm (BW) 0.2 dS 0.25 mm (WW) 0.25 dS 0.5 mm (WW) 0.5 dS 1.0 mm (WW) 0.05 dS 0.08 mm (LI) Flow rate gasication agent in m3 h1 (STP) Feeding rate fuel in kg h1 O2 concentration at inlet in vol% Equivalence ratio Gas residence time in s 2.3 1.44 11.2 0.38 78 2.0 1.20 10.5 0.40 89 2.3 1.20 11.2 0.42 78 2.3 3.16 1.15 1.28 11.2 12.8 0.43 0.31 78 67 drag force is calculated by Eq (2) while the drag coefcient Cd is described by Eq (3) [54] Spherical particles are assumed in Eq (2) for simplicity More advanced models, which can describe the motion of non-spherical particles explicitly, are presented by Chhabra et al [55] and Loth [56] and need the aspect ratio or the shape factor as additional input data Eq (3) includes the tting parameters and Eq (3) represents the Schiller-Naumann expression [56,57] when the values = 0.15 and = 0.687 are used The residence times calculated with these values were signicantly lower than the measured values at 25 C That is why the values of the tting parameters were evaluated from our data and determined to = 0.20 and = 1.0 Using these values in the model presented (Eqs 24), residence times determined by experiment and by modelling agree well with each other The stated values of and are specic for the grinded biomass used and should not be generalized Furthermore, Eq (3) includes the slip Reynolds number Re of fuel particles, which is displayed in Eq (4) Therein, G Fig A: Measured and simulated residence times of fuel particles as well as the particle slip velocities at 25 and 1100 C in dependence of the particle diameter without fuel conversion at a total ow rate of m3 h1 B: Development of the particle velocity at 25 and 1100 C for 0.25 and 1.0 mm wood particles at a total ow rate of 4.3 m3 h1 and G are representing density and dynamic viscosity of the gas surrounding the particles Fd ẳ dS G C d juS uG j uS uG ị 2ị   C d ẳ 24 ỵ Re Re1 3ị Re ẳ G juS uG j dS G 4ị Particle residence times and slip velocities uslip = uS uG at the reactor end at 25 and 1100 C without fuel conversion were simulated with the model described above They are included also in Fig 3(A) In order to discuss Fig 3(A), the spatial development of the velocity of particles inside of the reactor tube is shown in the right part Particles enter the reactor tube at its top with a velocity of uS 1.9 m s1, where the gas phase decelerates suddenly As a consequence, particles are affected by a drag force that decelerates them at the top of the reactor tube 0.25 mm particles decelerate within the rst 0.2 m of the reactor tube (Fig 3B) They not reach the velocity of the gas because the drag force equals the gravitational force at a higher particle velocity A noticeable particle slip velocity is a result thereof (Fig 3A) 1.0 mm particles decelerate within 0.4 m of the reactor tube at 25 C since the drag force per mass acting on them is lower than for 0.25 mm particles (Fig 3B) Furthermore, a higher slip velocity than for 0.25 mm particles is needed for 1.0 mm particles in order to compensate the accelerating gravitational force This is justied as the gravitational force grows with d3S , while the drag force grows with d2S only (Eq .(2)) At 1100 C, gas velocities are increased by a factor of 4.4 and gas densities are decreased accordingly 1.0 mm particles accelerate to about 2.5 m s1 until the end of the reactor tube in this case since the drag force is strongly reduced Eqs .(1) to (4) demonstrate the relationships between particle velocity, particle diameter, temperature, and pressure mathematically Fig shows these relationships in a more comprehensive way Particle residence times increase as the particle diameter decreases (Fig 3A) This is a consequence of increasing drag forces per mass with decreasing particle diameters At diameters below 0.1 mm, the particle slip velocity gets close to zero so that particles have the same velocity as the surrounding gas phase For dS N 0.35 mm, uslip is typically higher at 1100 C than at 25 C, while it depends only to a minor extend on temperature at dS b 0.35 mm (Fig 3A) The particle slip velocity was modelled also by Dupont et al [23] for a comparable entrained-ow reactor at 150300 K lower temperatures Dupont et al quoted slip velocities for 0.2 and 1.1 mm particles as high as 0.20 and N2 m s1 at the end of their reactor tube, respectively These values are lower than the ones displayed in Fig 3(A) at 1100 C amounting to 0.26 and 2.4 m s1, respectively The principal dependence of the particle slip velocity on the particle diameter is reproduced well, while there is some uncertainty about the exact height of the slip velocity 78 J Schneider et al / Fuel Processing Technology 152 (2016) 7282 Umeki et al [52] and Kirtania and Bhattacharya [53] also applied Eq .24 but used = 0.15 and = 0.687 (Schiller-Naumann expression [57]) in order to calculate the drag force coefcient Residence times of and 0.7 s for 0.1 mm and mm particles, respectively, were calculated for a reactor length of 1.9 m at 1000 C by Umeki et al [52] These residence times are higher than the ones presented in Fig 3(A) since the gas velocity was lower in the setup of Umeki et al [52] The values of the residence time in [53] is in the same range as in [52] for particle sizes between 0.1 and mm but the shape of this function differs The latter results from the incorporation of accurate pyrolysis kinetics into their model (consisting of particle motion, heat transfer and pyrolysis) and the changes in particle motion caused by density variations due to pyrolysis This approach is beyond the scope of the present article but of major importance when modelling the gasication process Deviations between the results of different models may not only be attributed to the partly different process conditions but also to deviating particle shapes, which inuence the drag force and the tting parameters and consequently Hence, it is recommended to measure the particle velocity or residence time at low temperatures and calculate the parameters and with the model described above Particle velocities at gasication temperatures can be determined subsequently 4.2 Gas concentrations Gasication experiments were carried out at the conditions listed in Table in order to study the effects of temperature and particle diameter on product gas composition and carbon conversion The gas concentrations measured for H2, CO, CO2 and N2 after the cyclone are presented in Fig The concentrations varied between and 10% for H2, and 16% for CO, and 11% for CO2 as well as 67 and 83% for N2 depending on target temperature and particle diameter Decreasing N2 concentrations are a direct consequence of pyrolysis, char gasication, and gas-phase reactions [26,41], which release gases and dilute the educt gas consisting of 87.289.5% N2 with other species A discussion of the variations in gas concentrations is complicated since pyrolysis was not complete in all cases (especially for particles with dS N 0.1 mm and temperatures below 1050 C) Nevertheless, basic trends found by Zhang et al [22] for partial oxidation conditions could be reproduced by this work E.g CO and CO2 concentrations were nearly independent on target temperature at T 1000 C for biomass, while CO2 concentrations decreased and CO concentrations increased at higher temperatures These dependencies were more pronounced with lignite (LI) than with biomass over the whole temperature range studied H2 concentrations generally increased with increasing target temperature and decreasing particle diameter due to improved dehydrogenation of the fuel (cleavage of C\\H or C\\O bonds) and decomposition of primary products (hydrocarbons, char, and tar) [20,26] They were similar for biomass and lignite particles with comparable particle sizes With oxygen or air as gasication agent, it is characteristic that H2 and CO concentrations increase with reactor temperature and decrease with particle diameter, while CO2 concentrations decrease slightly with reactor temperature and increase slightly with particle diameter This behavior is observed in entrained-ow reactors [21,22,25,26,28] as well as in other reactor types [58,59] when biomass or coal are applied as fuel Higher temperatures and lower particle diameters support the complete pyrolysis and char gasication of solid fuels and the approach towards the chemical equilibrium of all reactions involved especially of the water-gas shift reaction The calculated H2/CO ratio is displayed in Fig Typically, the H2/CO ratio increased with target temperature and varied between 0.55 and 0.81 for all biomass fractions Such values are typical for gasiers operated with relatively dry biomass and oxygen or air as gasication agent [25,26,41] No trend relating the H2/CO ratio with the particle diameter is observed Interestingly, the H2/CO ratio decreased with target temperature for lignite (Fig 5) from 0.82 at 950 C to 0.63 at 1100 C This might be attributed to the higher C/H ratio present in lignite Fig Dependence of the concentrations of H2 (A), CO (B), CO2 (C) and N2 (D) on temperature and particle diameter dS for beech wood (BW), waste wood (WW) and lignite (LI) J Schneider et al / Fuel Processing Technology 152 (2016) 7282 79 (12.5, mass basis) relative to biomass (8.2) (Table 2) and an improved carbon conversion with increasing temperature 4.3 Carbon conversion, cold gas efciency, and gas yield At 950 C, all waste wood (WW) fractions and lignite (LI) showed carbon conversions between 63 and 66 wt% displayed in Fig 6(A), which indicate that pyrolysis was not completed yet At the same time, beech wood (BW) showed a carbon conversion amounting to nearly 80 wt%, which suggests complete devolatilization of the fuel into permanent gases, tars, water, and char [20,26,33] due to its low particle diameters Carbon conversions increased for all fuels applied with target temperature At 1100 C, the largest WW fraction and the very ne BW fraction showed carbon conversions of 72 and 93 wt% BW with particles sizes below 0.12 mm was fully pyrolyzed and the resulting char and tars were partly converted into gas Higher amounts of char and tars were generated with the larger biomass fractions At the same time, lignite with particle sizes below 0.09 mm was fully converted into permanent gases and water with minimal tar concentrations At 950 C, cold gas efciencies varied between 15% and 36% for the largest WW and the BW fraction in Fig 6(B), respectively Increasing the target temperature, cold gas efciencies increased steadily for all fuels applied At 1100 C, nest and largest biomass fraction showed cold gas efciencies of 54 and 27%, respectively, while lignite reached a value of 61% Cold gas efciencies steadily increased with decreasing biomass particle diameters at all target temperatures Hernỏndez et al found cold gas efciencies between 35 and 38% with dealcoholized marc of grape with particle diameters below 0.5 mm studying air gasication in the same temperature range [29] Weiland et al reported cold gas efciencies (assuming fuel applications) between 62 and 70% in their oxygen-blown pilot gasier at a comparable equivalence ratio (lambda value of ca 0.4 equivalent to a temperature of ca 1400 C) and increased pressures (up to bar) [41] These comparisons demonstrate that the gasier works relatively well at 1100 C with very ne wood particles (dS 0.11 mm) With particle sizes of 0.2 mm and above as well as at temperatures below 1100 C, cold gas efciencies are relatively low and hamper a commercial application of the technology The results of Weiland et al [41] indicate that a few hundred kelvin higher temperatures (equivalent to lambda values of around 0.4 in autothermal gasication) are necessary to reach commercially viable cold gas efciencies The specic syngas volume (gas yield) is another measure to evaluate the gasication process The obtained values between 0.6 and 1.4 m3 kg1 (STP) in Fig 6(C) are in the range reported by other authors [58,6063] Principally, the specic syngas volume shows the same dependences on temperature and particle diameter as carbon conversion and cold gas efciency (Fig 6A and B) The highest specic syngas Fig Dependence of carbon conversion (A), cold gas efciency (B) and specic syngas volume (C) on temperature and particle diameter dS for beech wood (BW), waste wood (WW) and lignite (LI) volumes are observed for lignite and the very ne beech wood fraction at 1100 C which correlates with the highest carbon conversions and cold gas efciencies 4.4 Discussion of the results Fig H2/CO ratio resulting from the measured gas concentrations for beech wood (BW), waste wood (WW) and lignite (LI) As shown in Table 4, the particle residence time depends only slightly on temperature for very ne particles (dS 0.11 mm), while it is practically independent of temperature for larger particles Hence, the higher carbon conversions obtained at higher temperatures are clearly attributed to enhanced pyrolysis, char gasication and tar decomposition reactions Although both surface-to-volume-ratio (assuming spherical particles with a mean diameter) and minimal residence time (Table 4) increase signicantly as particle diameters decrease, the increase in carbon conversion is less pronounced A signicant increase 80 J Schneider et al / Fuel Processing Technology 152 (2016) 7282 is observed only for the nest biomass fraction, which is attributed to doublings of residence time and surface-to-carbon-ratio At the same time, the increase in cold gas efciency can be clearly attributed to lower particle diameters and higher gasication temperatures These results support ndings made in entrained-ow [25], uidized-bed [64], and xed-bed gasiers [65], after which temperature and particle diameter are the most important parameters inuencing their performance As the gasication temperature in entrained-ow reactors increases to temperatures above 1200 C, the inuence of particle diameters on carbon conversion disappears progressively for submillimeter particles [33,41] This is true for gasiers that guarantee a sufciently high particle residence time since pyrolysis and char gasication are close to completion (no solid residues containing carbon) In this case, gasication temperature, gasication agent (composition and amount), gas residence time, and possibly available catalytic materials determine the nal gas composition (permanent gases, water, and tars) Particles with diameters of ca 0.2 mm and above can be subject to extra- und intra-particle heat and mass transfer limitations at gasication conditions, which decrease pyrolysis and char gasication rates [64,66] Taking this into account together with the decreasing residence time of particles with increasing diameter (Table 4), complete carbon conversion during entrained-ow gasication can be obtained only with particles of a certain size Septien et al speculates that this limit lies at a particle diameter of about mm [33] At this limit, a carbon conversion of 72 wt% was obtained at 1100 C in the test facility Studies with torreed biomass show that this pretreatment can enhance the carbon conversion efciency of biomass [40,67,68] due to the decomposition of hemicellulose and restructuring of the pore system [69,70] Hence, the particle size applicable in entrained-ow gasiers can be enhanced by torrefaction Additionally, grinding costs are strongly reduced [71,72] The tar concentrations measured by FTIR in this work (including benzene, toluene and xylenes (BTX) only) after water quench and cyclone varied between 400 and 2000 ppmv (equivalent to 1.68 g m3 (STP)) while commercial co-current and counter-current xed-bed as well as uidized-bed gasiers show values in the ranges of 0.52, 50 150 and 710 g m3 (STP) [73,74], respectively A signicant fraction of the tars from gasication condensed by the water that was injected into the quench The amount of these tars was not quantied Considering the works of Zhang et al [26] and Hernỏndez et al [34], BTX are the dominating tar components at the conditions studied with lower amounts of naphthalene, phenol, and biphenylene Regarding statements made in [26,34], BTX concentrations should be by a factor of ve to 20 higher in the product gas of the test facility in front of the water quench than were measured after the cyclone These considerations indicate that most of the tars generated during mild entrainedow gasication can be separated from the product gas by injection of water This represents a simple condensation technique at the expense of gas temperature and cold gas efciency aim of this work developing an entrained-ow biomass gasication technology that operates at mild conditions and makes decentralized applications more likely important ndings were made: Maximum carbon conversion at T 1100 C was 93 wt% with very ne biomass particles (dS b 0.12 mm) and amounted to 72 wt% for particles with dS b mm, Entrained-ow fuel feeding with N2 and application of air as gasication agent resulted in N2 concentrations of 67 vol% (dry), Particle residence times resulting from the applied fuel feeding were slightly higher than from free fall techniques, Tar concentrations behind the cyclone were below g m3 (STP) due to water quenching, but a factor of ve to 20 higher in front of the water quench indicating insufcient reactor length or gasication temperature, and H2/CO ratio of 0.75 was obtained at the highest carbon conversion This work reveals high nitrogen dilutions of the product gas and short residence times as main obstacles of the gasier concept studied This situation could be improved if the gasication agent is used as uidization agent at the same time Alternatively, instead of a pneumatic feeding, screw or belt conveyors can be used since they are inexpensive and have low energy consumption [75] Increasing the length of the reactor tube is another option to increase carbon conversion and decrease tar concentrations by improving the residence times of particles and gas High residence times are especially important for particles with diameters of 0.2 mm and above since they can be subject to signicant intra- and extra-particle heat and mass transfer limitations [64], which counteract fast gasication Measures for thermal insulation have to be improved when longer reactor lengths are applied By considering the improvement options discussed above, the development of an entrained-ow biomass gasier for decentralized applications seems to be feasible Abbreviation Description BLG BTL ETC FTIR GC-TCD GSP KIT Black liquor gasication Biomass-to-liquids Energy Technology Centre Fourier transform infrared Gas chromatograph with thermal conductivity detector Gaskombinat Schwarze Pumpe Karlsruhe Institute of Technology Acknowledgement Summary and conclusions A technology for the atmospheric decentralized entrained-ow gasication of biomass was investigated at temperatures between 950 and 1100 C and equivalent ratios between 0.380.43 With reference to the This work was nanced partly by the European Regional Development Fund (ERDF) (100116093) provided by the Sọchsische AufbauBank (SAB) We thank ERDF and SAB for the nancial support of the project Table Comparison of surface-to-volume-ratio (S2VR), carbon conversion (C), cold gas efciency (CG) and minimal particle residence time (calculated with the model presented in Section 4.1) of all biomass fractions applied at 1000 and 1100 C Biomass fraction S2VR in m1 C,1000 in wt% CG,1000 in % min,1000 in s C,1100 in wt% CG,1100 in % min,1100 in s 0.04 dS 0.11 mm (BW) 0.2 dS 0.25 mm (WW) 0.25 dS 0.5 mm (WW) 0.5 dS 1.0 mm (WW) 80 27 16 88 70 69 64 44 29 23 16 4.9 2.54 1.26 0.78 93 85 83 72 54 45 36 27 4.6 2.52 1.24 0.76 J Schneider et al / Fuel Processing Technology 152 (2016) 7282 References [1] S Jacobsson, V Lauber, The politics and policy of energy system transformationexplaining the German diffusion of renewable energy technology, Energy Policy 34 (2006) 256276 [2] A Kitous, P Criqui, E Bellevrat, B Chateau, Transformation patterns of the worldwide energy system scenarios for the century with the POLES model, Energy J 31 (2010) 4982 [3] A.J Ragauskas, C.K Williams, B.H Davison, G Britovsek, J Cairney, C.A Eckert, W.J Frederick Jr., J.P Hallet, D.J Leak, C.L Liotta, J.R Mielenz, R Murphy, R Templer, T Tschaplinski, The path forward for biofuels and biomaterials, Science 311 (2006) 484489 [4] S 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