co-gasification of coal and hardwood pellets: a case study - co-gasification of coal and... ·...

Columbia International Publishing American Journal of Biomass and Bioenergy (2013) Vol. 2 No. 1 pp. 25-40 doi:10.7726/ajbb.2013.1005

Research Article

______________________________________________________________________________________________________________________________ *Corresponding e-mail: [email protected] 1* Wood Science and Technology (Biofuels and Bioenergy), Division of Forestry and Natural Resources, Davis College of Agriculture, Natural Resources and Design, West Virginia University 2 Department of Chemical Engineering, College of Engineering and Mineral Resources, West Virginia University

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Co-Gasification of Coal and Hardwood Pellets: A Case Study

Jagpinder Singh Brar1, Kaushlendra Singh1*, John Zondlo2 and Jingxin Wang1 Received 24 August 2012; Published online 20 April 2013 © The author(s) 2013. Published with open access at www.uscip.org

Abstract This paper presents the effect of feedstock mixtures on the composition of syngas produced from a commercially available down-draft gasifier. Commercial hardwood pellets and bituminous coal (Pittsburgh seam) were used as fuel for the experiments. Five types of feedstock mixtures having composition(by weight) of 100% hardwood pellets, 20% coal-80% hardwood pellets, 50% coal-50% hardwood pellets, 80% coal-20% hardwood pellets and 100% coal were prepared. Air and an air plus water mist mixture were used as a gasifying agent. Syngas with 20.95% carbon monoxide and 16.05% hydrogen was produced from the gasification of wood pellets at 1200°C in the combustion zone and 450°C in the reduction zone of the reactor. The hydrogen and carbon monoxide content in the product gas decreased with an increase in the coal proportion in the mixtures. A significant increase in carbon efficiency was observed with an increase of coal proportion in the mixtures. Syngas carbon and syngas energy efficiency increased with injection of the water mist along with air in the system for the hardwood pellet gasification. Keywords: Renewable energy; Coal gasification; Biomass gasification; Syngas carbon efficiency and Syngas energy efficiency

1. Introduction Coal is gasified commercially to produce useful end products like synthetic natural gas, electricity, ethanol, ammonia, oxy-chemicals and hydrogen. Coal gasification not only increases the thermal efficiency of the overall conversion process but also increases the use of low-grade coal as compared to direct burning of coal (Minchener, 2005). According to NETL (2007), coal is used as the primary feedstock for about 55% of the gasification plants around the world. The 33% remaining feedstock comes from petroleum and 12% from others sources like pet coke and

Jagpinder Singh Brar, Kaushlendra Singh, John Zondlo, and Jingxin Wang / American Journal of Biomass and

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biomass waste (NETL, 2007). Coal gasification causes environmental problems like greenhouse gas, sulfur and nitrogen emissions. On the other hand, biomass gasification produces higher amounts of tar and consumes higher energy in feedstock processing than coal gasification (NETL, 2009). The mixture of biomass and coal has potential to reduce these environmental effects as it has low sulfur and ash contents. In addition, co-gasification of coal with high biomass ratio may produce a syngas, which can be used for liquid fuel synthesis (Kumabe et al., 2007). Moreover, co-gasification of coal and biomass also produces syngas with a higher amount of hydrogen and a higher heating value (Ruoppolo et al., 2010). Despite the benefits of co-gasification, some problems are also associated with this process. Due to the difference in the gasification behavior of coal and biomass, it is difficult to gasify them together in a same reactor. Biomass has high volatile matter and it requires temperature of about 400-500°C in the reduction zone. On the other hand, a temperature range of 800-900°C is required for gasification of coal in a downdraft gasfier. Moreover, different material handling properties of coal and biomass also cause problems for the simultaneous gasification. Therefore, more research is required to make the co-gasification process viable and economical (Brar et al., 2013). In this study, feedstock mixtures with the coal-biomass ratio varying from 0 to 1 were gasified in a fixed-bed down-draft gasfier designed for biomass gasification. Commercially available hardwood pellets and bituminous coal (Pittsburgh seam) were selected as the feedstock. Along with air, a mixture of air plus water mist was also used to quantify the effect of water vapors on the biomass gasification. The aim of this study was to investigate the effect of the biomass-to-coal ratio and gasifying agents (air, air-water mist) on the temperature profile, syngas composition, syngas energy efficiency and syngas carbon efficiency. The gasification of coal alone in this downdraft gasifier was not possible as the gas flow was restricted by the agglomeration of coal.

2. Materials and Methods 2.1 Gasifier Description A downdraft gasifier system, designed for biomass gasification (Model: 10 kW Power pellet, All Power Labs, Berkeley, CA) was used for the gasification experiments. This unit is an integrated Gasification Combined Cycle System (IGCC) in which the gas produced is combusted in a natural gas engine (Model: Kubota DG 972-10 kW, Kubota Engine America, Gainesville, GA). This engine drives an electric generator (Model: Mecc Alte, McHenry, IL) to produce electricity. Generally, this system consumes biomass at the rate of 12 kg/hr and produces 3-10 KW-hr of electricity (All Power Labs, Berkeley, CA) depending on feedstock moisture. Temperatures in the top and bottom of the reduction zone, combustion zone, pyrolysis zone and gas exit zone along with the gas flow rate, and pressure drop inside reactor are continuously monitored by a Gasification Control Unit (GCU). Fig. 1 shows a schematic diagram of the gasification system. Feedstock is fed from the feed input port (#9 in Fig. 1) into a double-jacketed feed input container which also serves as a drying area for the feed (#8 in Fig. 1). Most of the moisture from feedstock is removed in this area. Feed is transported by an automatic auger (#6 in Fig. 1) into the pyrolysis zone where it is heated up to 200-400°C and most of the volatile matter is lost in this zone.


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Fig 1. Schematic diagram of the gasification system: 1-auger; 2- outer cylinder of reactor; 3-grate; 4- air/steam port in inner cylinder; 5- charcoal inlet port; 6- auger; 7- cyclone separator; 8- outer jacket for gasses; 9- feed input 10- compressed air input; 11- Charcoal; 12- Orifice flow meter. After the pyrolysis zone, the feed is moved to the combustion zone. Air or other gasifying agents are fed into the combustion zone to produce carbon-dioxide and water vapor. After that, the carbon-dioxide and water vapor are reduced in the absence of oxygen to produce carbon-monoxide and hydrogen, also called syngas, in the reduction zone. The syngas then moves from the bottom of the reactor to the cyclone separator (#7 in Fig. 1) and char moves into the char collecting unit. In the cyclone separator solid impurities like particulate matter are removed. Other impurities like un-burnt carbon particles and tars are further removed in the syngas filter. This filter is made up of three main layers. The bottom layer (1”-2” thick) of the filter is made up 1”-2” wood chips and the top layer (5” thick) is made up of very fine materials like cotton t-shirts, sponge etc. In between, the middle layer is filled with sawdust of particle size less than 1/8 inch. The gasifier reactor and syngas filter operate at a negative pressure of approximately 1” to 3” of water column. The negative pressure is created by the initial fuel ignition stage when the system is running in the “flare mode”, using compressed air and a venturi mounted just above the syngas filter. In the “power mode”, the natural gas engine generates the required suction. Purified gas from the syngas filter may be sent to either the flare or the natural gas engine by switching a directional valve.


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2.2 Gasification Experiments Wood charcoal was filled inside the reduction zone of the reactor in a layer of up to 6” thick to provide the initial heating and reducing agent (coke) during the startup period. The feedstock was filled above the wood charcoal as shown as in the Fig. 1. Negative pressure (approximately 3” of water column) was then created inside the reactor using compressed air (port #10 in Fig. 1). Initial ignition of the feed was assisted by 10 ml of diesel fuel and a propane torch. Once the temperature of the top of the reduction zone reached 80°C, the air inlet port was opened (#4 in Fig. 1). Heating by the propane torch was stopped when the reduction zone reached 280°C. The gas produced from the gasifier was sent to the burner for producing the flare. The flare started after the temperature of the reduction zone reached 400°C. The gasifier was kept running for approximately one hour after the start of flare. The natural gas engine was not operated with the gas produced from any coal-containing feedstock since the sulfur present in the coal could have deteriorating effects on the engine. Gas samples were taken by attaching air-tight syringes (Model: 1100 Gastight syringes, Hamilton Company, Reno, NV) to the gas sampling port. Four gas samples were taken at 20-minute intervals after the flare was ignited. These samples were tested in a gas chromatograph (Model: Clarus GC 580, Perk Elmer Waltham, MA) equipped with thermal conductivity detector to determine the gas composition of the syngas. The GC columns were specially customized for light gas analysis. A gas standard (Catalog: 501697 Gas standard, Supelco, PA, USA) was used for calibration of the gas chromatograph for detecting carbon dioxide, carbon monoxide, oxygen, methane and nitrogen. Another gas standard (Product: N6107201 Gas standard, Perkin Elmer, Shelton CT, USA) was used to calibrate the gas chromatograph for hydrogen. 2.3 Flow Rate Measurements Measurements on the flow rate of the gas produced in the gasfier were required to calculate the carbon efficiency and energy efficiency of the system. It was measured by installing an orifice flow meter supplied by All Power Labs (APL Berkeley, CA) in the gas outlet line. Two pressure sensors were installed across the orifice plate to measure pressure difference. The fluctuation of temperature and other gasification conditions could also affect the gas flow rate inside the gasifier. Equation 1 was used for calculating the flow rate of gas produced by the gasifier.

/2ACQ of (1) Where Q is the volumetric flow; Cf is friction coefficient dependent on meter design, Ao is the orifice area, ∆P is the measured pressure difference across the orifice and ρ is fluid density. Values of Cf*Ao = 0.421233334 in2 and the density of syngas = 0.95 Kg/m3 were used in the calculations (APL, 2010). It is important to mention here that the flow rate measurements are usually effected by the temperature fluctuations at the orifice. However, in this particular experimental set-up, the orifice was located (#12 in Fig. 1) at the place where only cold syngas passes through it. There were no experimental temperature variations at that location. 2.4 Feed-Oxidizer Equivalence Ratio The feed-oxidizer equivalence ratio (ERFO) is defined as the ratio of the feed-to-oxidizer ratio to the stoichiometric feed-to-oxidizer ratio as given by Eq.2.


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Stox

feed

actualox

feed

FO

n

n

n

n

ER

(2) Here nfeed is the molar carbon feed rate and nox is the molar oxygen flow rate for the experimental (actual) and stoichiometric (St) conditions. To calculate actual molar oxygen flow rate, Eq.1 was used with air density value of 1.2929 kg/m3 along with the ∆P measured during first 20 minutes of the experiment. During this time, there was no gasification taking place as the reactor started to warm-up, hence only air supplied into the system came out of the system through the orifice. Airflow was maintained such that the system remained at the vacuum 3-inch of water column for each experiment. Assuming that air contains 21% oxygen and a mol of air occupies 22.4 L at standard temperature and pressure, the molar oxygen flow rate was calculated. To calculate molar carbon feed rate, the total feed rate was multiplied by the carbon content of the feed. The resulting value was then divided by the atomic weight of carbon to calculate molar carbon feed rate. After that, the actual feed-to-oxidizer ratio was calculated for a given sample. To calculate stoichiometric feed-to-oxidizer ratio, an empirical formula was developed for each sample tested using elemental composition data measured for wood pellets and coal. Elemental composition data for mixtures of wood pellet and coal were estimated from the measured data (Table 1). After developing the empirical formula, estimation was made for molar oxygen needed from an external source to oxidize carbon to produce carbon dioxide, and hydrogen to produce water. During these calculations, oxidation of sulfur and nitrogen were neglected. The molar oxygen requirement was used to calculate the stoichiometric feed-to-oxidizer ratio for a given sample. Table 1 Ultimate analysis (%) and heating value (MJ/Kg) of bituminous coal and hardwood pellets

Sample C (%) H (%) N (%) S (%) HHV (MJ/Kg) Wood Pellets 48.8 ± 0.11 6.09 ± 0.09 0.18 ± 0.04 0.36 ± 0.01 17.95 ± 0.29

Coal 73.96 ± 0.76 4.81 ± 0.06 1.63 ± 0.04 1.09 ± 0.09

31.97 ± 0.94

2.5 Syngas Carbon Conversion Efficiency and Syngas Energy Efficiency The syngas carbon efficiency is the ratio of the total carbon present in the syngas to that of the feedstock (Kumar et al., 2009). The total carbon present in the syngas was calculated by measuring the amounts of carbon dioxide, carbon monoxide and methane in the syngas. By using the mole concept and with the known gas flow rate, the carbon content in the syngas was calculated by means of Equation 3. The amount of carbon in the feedstock was calculated from ultimate analysis data (Model: Clarus 2400 CHNS analyzer, Perkin Elmer, Waltham, MA).

C/100 * /100)X-(1 * /100)X-(1 * F

1000*F *12/22.4*)CH + CO + (COC

ashmfeed

gas42eff

(3)


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Here CO2, CO and CH4 are the percentage composition of these components of the Syngas (%v/v), Fgas is the flow rate of syngas (l/min), Ffeed is the flow rate of the feedstock (kg/min), Xm (%wb) and Xash are the moisture and ash weight percentages of feedstock respectively, and C (%daf) is the carbon percentage in the feedstock. The syngas energy efficiency is the ratio of the energy contained in the syngas to the total energy supplied through the feedstock (Kumar et al., 2009). The syngas energy efficiency was calculated by using Equation 4.

E*/100)X-(1 *F

F * )H*10.71+CO*12.62+CH*(35.81E

mfeed

gas24eff

(4) Here H2, CO and CH4 are the percentage composition of these components of the Syngas (%v/v), Fgas is the flow rate of syngas (l/min), Ffeed is the flow rate of the feedstock (kg/min), Xm (%wb) is the moisture content of feedstock and E is the gross heating value (KJ/kg). The energy contained in the syngas was estimated by using the gas composition data and the standard heating values of carbon monoxide, methane and hydrogen. The energy of the feedstock was calculated by using gross heating values measured by a bomb calorimeter (Model: Parr 6300 Calorimeter, Parr Instrument Company, Moline, IL., USA). Feed rate was determined by calculating the total amount of feed used from the starting of flare till the shutting down of the system. Moisture content was determined by the oven drying method. Ash content was measured by weighting the residue after heating the feedstock at 700°C in the presence of oxygen for 15 minutes. 2.6 Experiment Design Bituminous coal (Pittsburgh seam) and hardwood pellets were used for the experiments. Table 1 shows the ultimate analysis and heating values of coal and hardwood pellets. In the first set of experiments, five feedstocks (coal, hardwood pellets, 50% coal-50% hardwood pellets, 80% coal-20% hardwood pellets and 20% coal-80% hardwood pellets) were tested in this gasification system with air as a gasification agent using completely randomized design. In the second set of experiments, a mixture of air-water mist was used as gasifying agent during the gasification of wood pellets only to observe if it improves the hydrogen content in the syngas via the water-gas shift reaction. Water mist was generated by using a misting nozzle and the mist was injected at the volume discharge of 0.047 L/min and pressure of 60 psi into the gasification system for the air-mist test. For these tests, two treatments (air and air-water mist) were analyzed statistically for the wood pellets only. Two repetitions with each feedstock and gasification agent were performed. Four gas samples were taken per gasification run. The data were tested for significance difference by statistical software R (Version 2.12.1, Auckland, NZ) at 95% CI.


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3. Results 3.1 Effect of Coal and Biomass mixtures 3.1.1 Gas Composition Table 2 shows empirical formulae, actual carbon-oxygen ratio, stoichiometric carbon-oxygen ratio, feed-oxidizer equivalence ratios (ERFO), and the oxidizer-feed equivalence ratios (EROF) for the various fuels tested. Both ERFO and EROF are presented because it makes for easy comparison with published data. For example, Wang et al. (2011) reported EROF, whereas Hernández et al. (2010) reported ERFO. The higher the ERFO values, the lower the external oxygen requirement because the feed itself has high oxygen present internally. This is vice-versa for EROF. The ERFO was 8.05 for 100% wood pellets, which reduced to 4.33 for mixture of 80% wood pellets and 20% coal and to 3.73 for the mixture of 20% wood pellets and 80% coal. The ERFO for 50% wood pellets and 50% coal was 3.89. Gas composition data presented in Table 3 for this particular mixture may be compared with that reported by Hernández et al. (2010) at the ERFO value of 3.61. Table 2 Empirical formulae, actual carbon-oxygen ratio, stoichiometric carbon-oxygen ratio, feed-oxidizer equivalence ratios (ERFO), oxidizer-feed equivalence ratios (EROF) for various fuels tested

Sample Empirical Formula*

Actual Carbon-Oxygen (mol/mol)

Stoichiometric Carbon-Oxygen (mol/mol)

Feed-Oxidizer ERFO

Oxidizer-Feed EROF

WP C H1.62 O0.74 3.89 0.48 8.05 0.12 80WP20C C H1.30 O0.55 2.06 0.48 4.33 0.23 50WP50C C H1.07 O0.39 1.81 0.47 3.89 0.26 20WP80C C H0.88 O0.26 1.71 0.46 3.73 0.27

* Oxidation reactions for nitrogen and sulfur were neglected Table 3 shows the gas composition obtained by mixing coal to the biomass in the fixed- bed downdraft gasifier. Hydrogen composition of 16.05 (%v/v) was obtained in the biomass gasification (100% wood pellets) and it decreased significantly when 50% of coal was mixed with the biomass. The addition of 20% coal did not affect the hydrogen content in the syngas significantly. A similar observation was reported by Pinto et al. (2003) using a fluidized bed gasfier with air plus steam mixture as the gasification agent and the mixture of 60% coal, 20% wood chips and 20% polyethylene as the feedstock for the gasification. In their experiments, the temperature of the fluidized bed system was raised from 750°C to 890°C by providing external electrical heat. It was found that high temperature favored hydrogen production in the co-gasification process due to the endothermic nature of most of the gasification reactions. Similar effects of temperature were found by Kumar et al. (2009).


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Table 3 Syngas composition obtained by gasification of WP (Hard wood pellets), 80WP20C (80% hardwood pellets and 20% coal), 50WP50C (50% hardwood pellets and 50% coal), 20WP80C (20% hardwood pellets and 80% coal) and coal samples with air as the gasification agent in a fixed bed down-draft gasifier.

Sample N2 CO H2 CO2 CH4 O2 H2/CO

WP 47.73 ± 3.82 20.95 ± 2.42 16.05 ± 2.95 a 11.27 ± 2.78b 1.85 ± 0.45 a 2.14 ± 1.51 0.77

80WP20C 54.93 ± 3.74 16.74 ± 2.45 12.78 ± 1.77 a,b 13.6 ± 1.00a,b 1.21 ± 0.5 a 0.97 ± 0.41 0.76

50WP50C 59.59 ± 3.05 13.22 ± 1.02 10.36 ± 1.52 b 14.89 ± 1.65a,b 0.92 ± 0.99 a 1.03 ± 0.05 0.78

20WP80C 64.17 ± 1.84 6.57 ± 0.63 9.99 ± 1.43 b 16.7 ± 1.48a 1.43 ± 0.64 a 1.16 ± 0.2 1.52

Coal 74.78 ± 2.34 3.00 ± 1.68 3.09 ± 2.04 16.5 ± 1.06a 0.44 ± 0.22 a 2.19 ± 0.5 1.03

a, b No significant difference at 95% CI Additionally, carbon monoxide composition of 20.95 (%v/v) was obtained by gasification of the hardwood pellets. It decreased significantly upon addition of coal into the feedstock. A mixture of 80% hardwood pellets and 20% coal produced syngas with 16.74 (%v/v) of carbon monoxide. Similarly, Pan et al. (2000) observed a reduction in carbon monoxide composition with increase of coal in the feedstock when mixtures of pine chips; Black coal and Sabero coal were used for gasification in a fluidized bed gasifier. In their study, the temperature of the system was in the range of 840-910°C and an air plus steam mixture was used as the gasifying agent. In the present study, the carbon monoxide composition continued decreasing as the coal percentage in the feedstock increased, which is consistent with the findings of Hernández et al. (2010). Hernández et al. documented the influence of ERFO and the biomass (dealcoholized grape marc) proportion in coal-biomass mixtures during entrained flow gasification performed at 1150°C. Their report suggests that for a given ERFO, both CO and H2 contents were reduced when the amount of coal was increased from 0 to 100%. According to Hernández et al., the percent volumetric compositions (%v/v) for CO and H2 were (12.27, 8.47) for 0% coal, (11.32, 7.52) for 20% coal, (7.65, 5.31) for 50% coal, and (1.81, 0.91) for 90% coal at air/fuel ratios within the range of 3.93 to 4.85. In the present study, there was 13.22(%v/v) CO and 10.36(%v/v) H2 present at the ERFO of 3.89 for the 50% wood pellet and 50% coal mixture. Hernández et al. (2010) state that 5.23(%v/v) CO and 3.05(%v/v) H2 were present at the ERFO of 3.61 for 50% coal and 50% grape marc biomass. In another study, Wang et al. (2011) reported coal-wood entrained flow gasification using modeling studies. In their study, the gasifier was operated at 1350°C temperature and 30 bar pressure, which are ideal conditions for coal gasification. According to Wang’s report, increasing biomass proportion in coal from 5% to 20% decreases the molar concentration of CO from 66.8% to 64.5% and the molar concentration of H2 from 30% to 29.5%. There are several reasons for the difference between the gas composition results reported here and reported in literature, for example, gasifier temperature profile and operating conditions. Changes in the gasifier temperature profile alter the reaction conditions. The cause of the decrease in hydrogen content in the present study could be the decrease of the temperature of the gasfier for the gasification of mixture of 50% coal and 50% hardwood pellets (Fig. 2). Note that the gasifier


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used in this study was not heated externally. Therefore, addition of coal into the feedstock changed the temperature profiles inside the gasifier in the combustion as well as in the reduction zone (Fig. 2). Usually, the maximum temperature in the combustion zone for gasification of 100% hardwood pellets was around 1200°C and for the pyrolysis and reduction zones, it was a little over 400°C. The combustion zone temperature dropped to about 800°C and reduction zone temperature rose to little more than 800°C when 50% or more coal was added to the hardwood pellets. This change in the temperature profile may be explained by the influence of the feed-to-oxidizer ratio and the geometric length of the gasifier that contains the pyrolysis, combustion and reduction zones. It is a fact that these three zones (pyrolysis, combustion, and reduction zones) exist virtually inside downdraft gasifier, which is not heated externally. The length of these zones also varies depending upon the thermal degradation behavior and duration (residence time) of the feedstock. Changes in the feedstock thermal degradation behavior not only change the length of these zones but also shift these zones upward or downward within the gasifier. Therefore, the gasifiers height and approximate length of the three zones are decided according to the thermal degradation behavior of the feedstock and its approximate residence time in each zone. Feedstocks (wood, switchgrass, etc) which degrade easily and quickly require smaller geometric length gasifiers than coal type feedstocks, which take a longer time to degrade. Using a long-residence time feedstock like coal in a gasifier designed specifically for biomass may not create these virtual reaction zones leading to significant differences in temperatures. In addition, in the present study coal agglomerated forming an impervious barrier just below the combustion zone, which caused the gas flow to stop. As result, there was no distinction between the combustion and reduction zones. The temperature of the reduction zone became similar to the combustion zone and caused an overall reduction of the temperature. Most of the published studies on co-gasification used external heaters to maintain gasifier temperature (Kumabe et al., 2007), therefore, similar observations from the literature could not be cited.

Fig 2. Temperature profiles obtained during the gasification of (a) hardwood pellets and (b)

mixture of 50WP50C (50% hardwood pellets and 50% Coal) in a fixed bed down-draft gasifier. The second factor is ERFO or EROF. Hernández et al. (2010) demonstrated, in a performance evaluation study, that increasing ERFO for a given mixture and keeping the gasifier temperature constant improves the CO and H2 content of the product syngas. In another study, Wang et al.


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(2011) used thermodynamic models to determine the effect of EROF on gasifier temperature stability and product gas composition. The study used an oxygen-fed entrained-flow gasifier and coal-wood mixtures as the feedstock. Wang et al. (2011) reported that EROF was 0.687 for gasification of coal alone, which was reduced to 0.64 with addition of 20% wood into coal to keep the gasifier temperature stable. This is because biomass acts as an oxygen carrier and thus requires less external oxygen. While results of the present study may not be compared directly with Wang et al. (2011), there is one common documented result and that is EROF decreased with increased biomass proportion in the coal-biomass mixtures. Unlike the present work, the modeling study of Wang et al. did not consider coal agglomeration. The agglomerated layer formed above the combustion zone during the gasification of mixtures with more than 50% coal. The maximum temperature of the gasification system reached up to 1000°C for the gasification of the feedstock having 80% of coal and 20% of hardwood pellets. Despite this temperature, the carbon monoxide content reduced in the syngas. The formation of agglomerates caused blockage of the gas flow and restricted carbon combustion. We assert that coal agglomeration was the reason for the significant decrease of carbon monoxide in the syngas. As a result, a higher amount of carbon dioxide but less carbon monoxide was produced when coal was added, which indicates that the reduction reactions responsible producing CO and H2, were not occurring effectively. 3.1.2 Syngas Carbon Efficiency and Syngas Energy Efficiency Syngas carbon and syngas energy efficiency values with different fuel mixtures are presented in Table 4. Syngas carbon efficiency of 50.73% was observed in the gasification of wood pellets. An increase in syngas carbon conversion efficiency was observed when up to 50% coal was mixed with the hardwood pellets. The carbon conversion of the feedstock carbon to gas with 80% coal decreased from 70.02% to 58.48% for feedstock with 50% coal. There are three explanations behind this increase in carbon conversion. First, at high coal proportions in the mixtures, most of the carbon ended up in form of carbon dioxide, a non-energy component of the syngas. Second, the agglomeration of coal could be another reason for the changes in the carbon conversion. However, the increase in the carbon efficiency was due to increase in the useless and non-energy carbon dioxide content of the syngas. Third, the increase in carbon dioxide was the result of an increased EROF ratio. The EROF ratios were 0.12 for wood pellets, 0.23 for 80% wood pellet and 20% coal mixture, 0.26 for 50% wood pellets and 50% coal mixture, and 0.27 for 20% wood pellets and 80% coal mixture. Increased EROF may lead to increased CO2 content in the product gas but this may not lead to an increase in the overall energy output from the syngas. Energy efficiency was not disturbed by the inclusion of coal in the feedstock. It remained the same for wood pellets, 80WP20C and 50WP50C but it reduced significantly for 20WP80C. Pan et al., (2000) found a decrease in the heating value of syngas produced from coal-rich feedstock. The blockage caused by agglomeration of coal could also be the reason for this reduction. The energy efficiency was 38.94% for 100% wood pellets, 37.82% for the mixture of 80% wood pellets and 20% coal, 35.02% for the mixture of 50% wood pellets and 50% coal, and 26.63% for the mixture of 20% wood pellets and 80% coal. The reported thermodynamic efficiency for wood gasification is close to 70% and it is close to 75% for coal gasification (Ptasinski, 2008). However, cold gas efficiency in entrained flow gasification is reported to be in the range of 7% for coal to 30% for the mixture of 80% biomass and 20% coal (Hernádez, 2010). Other reported experimental cold gas


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efficiencies for downdraft gasification are 56.87% for furniture waste at EROF of 0.205 (Seth and Babu, 2009) and 33.72% for furniture wood plus charcoal at EROF of 0.388 (Zainal et al., 2002). Table 4 Syngas carbon efficiency and syngas energy efficiency values obtained by gasification of WP (hardwood pellets), 80WP20C (80% hardwood pellets and 20% coal), 50WP50C (50% hardwood pellets and 50% coal), 20WP80C (20% hardwood pellets and 80% coal) and coal samples with air as a gasification agent in a fixed bed down-draft gasifier.

Sample Carbon Conversion Efficiency Syngas Energy Efficiency

Wood Pellets 50.73 ± 1.80 38.94 ± 0.11 a

80WP20C 62.48 ± 3.97a 37.82 ± 3.55 a

50WP50C 70.02 ± 3.95a 35.02 ± 1.20 a,b

20WP80C 58.48 ± 0.21 a 26.63 ± 2.77b a, b No significant difference at 95%CI 3.2 Problems with Coal Gasification in the Fixed Bed Down-Draft Gasifier Coal, with particle size in the range of 1”-2”, was fed into the fixed-bed downdraft gasifier and syngas with only 3% carbon monoxide and 3.09% hydrogen was produced even though the temperatures of the combustion zone and reduction zone were around 1100°C. A flare was not detected when only coal was used as the feedstock due to low quantity of combustible gases in the syngas. The main reason was the un-burnt coal particles and blockage of the reactor caused by the agglomeration of the coal (Fig. 3), which physically caused combustion in the reduction zone and hence the temperature of the physical reduction zone increased. The carbon dioxide content of syngas increased because of the combustion of the coal. The agglomeration of coal is usually caused by the devolatilization of the coal as it goes thru its softening point. As the volatiles emerge, the “plastic “coal fuses and forms a coke. This usually happens around 400°C, way below the ash fusion temperature (Collot et al., 2006). In our case, the agglomeration was mainly caused by temperature non-homogeneity. At certain points where temperature was close to 400°C, coal agglomerated due to softening as discussed above. Additionally, temperature near the air inlet valve was in the range of 1100°C, which caused the coal and inorganic matter in the coal to melt. Once the agglomerated layer of coal is formed, it reduced the flow of fuel hence the oxygen/fuel ratio increased. This was the cause of combustion in the reduction zone and hence high carbon dioxide in the syngas. The problem of un-burnt carbon particles may be reduced by using smaller particle size of coal and by increasing the cross-sectional area of the combustion zone. More area for the combustion zone should provide more exposure of heat to the coal and thermal breakdown of coal should become easy. Melting of inorganic ash may be avoided by lowering the bed temperature and making it less than the melting points of the ash constituents in the coal (Bartels et al., 2008). The bed temperature can be reduced by reducing the air intake into the system. Pretreatment of coal with some salts helps to reduce amount of agglomeration of a coal in a gasifier (Vuthaluru et al., 1996). Finally a non-agglomerating coal, such as sub-bituminous or lignite, can be selected.


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(a) (b)

Fig 3. (a) A layer formed by the coal agglomeration during the gasification of coal rich feedstock and (b) pieces of coal agglomeration obtained from the reactor.

3.3 Effect of Water Mist on Biomass Gasification 3.3.1 Gas Composition A mist of water at pressure of 60 psi and volume discharge of 0.047 L/min was injected along with the air inside the reactor through the air inlet port to investigate the effect of steam on biomass gasification. A mist/biomass ratio of 0.50 (w/w) was used for the run. An increase in hydrogen composition with injection of steam in the gasification system has been detected by various authors (Wang et al., 2007, Pinto et al., 2003, and Franco et al., 2003) who had external heating for their gasifier to control temperature. This increase in the hydrogen composition has been attributed to the extent of water-gas shift reaction in which carbon monoxide reacts with water vapor to produce hydrogen and carbon dioxide. Additional hydrogen is produced from the char/steam reaction. In this study, there was no significant variation in syngas composition obtained by mist injection (Table 5). Unexpectedly, a decrease (not significant at 95% CI) was observed in the hydrogen content of the syngas when air-mist was injected. It was also found that the maximum temperature of the gasification system reduced from 1200°C to 800°C with addition of mist into the system. Fig. 4 shows the temperature of the gasification system during the experiment for air and air-mist gasification. Throughout the run, the temperature with the air-mist remained lower than that of the air alone. With air plus water mist as the gasification agent, the temperature of gasifer was in the range of 800-850°C and with only air as the gasification agent, the gasifier temperature of 1000-1100°C was obtained. Probably, so much liquid water was added to the system that as it boiled it took heat from the reacting mass and dropping the temperature. High temperature favors higher hydrogen content as it provides more energy for the endothermic reactions, which produce more hydrogen (Kumar et al., 2009 and Pinto et al., 2003). The effect of temperature on the hydrogen content was also studied by many others and they have found that an increase of temperature leads to an increase of hydrogen in the product syngas (Lv et al., 2004; Lv et al., 2007 and Turn et al., 1998). The impact of temperature was found to be more prominent than


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that of the addition of steam for increasing the hydrogen content of the syngas. Similarly, the carbon monoxide composition did not change significantly due to the decrease in temperature. Table 5 Syngas composition obtained by gasification of wood pellets in a down-draft gasifier with air and air-mist as the gasification agents.

Sample N2 CO H2 CO2 CH4 O2

Air 47.73 ± 3.82 a 20.95 ± 2.42 a 16.05 ± 2.95 a 11.27 ± 2.78 a 1.85 ± 0.45 a 2.14 ± 1.51 a

Air-mist 47.21 ± 2.32 a 22.16 ± 1.2 a 13.84 ± 2.54 a 13.04 ± 1.3 a 2.08 ± 0.38 a 1.68 ± 0.31 a

a No significant difference at 95%CI

Fig 4. Temperature profiles of the combustion zone obtained by gasification of wood pellets with

air and air-mist as the gasification agents. 3.3.2 Syngas Energy and Syngas Carbon efficiency The syngas energy and syngas carbon efficiency increased significantly with injection of mist. Table 6 shows the syngas carbon efficiency and syngas energy efficiency for gasification of wood pellets with air and air plus water mist as the gasification agent. It was observed that both the carbon conversion and syngas energy efficiency increased when water mist was added to the air. Wang et al. (2007) used a two-stage gasification system, which consisted of a fixed-bed gasfier and a non-catalytic reformer. Wood chips of cedar were used as the biomass feedstock and an air-steam mixture was used as the gasification agent. In their study, the maximum heating value of syngas and the maximum cold gas efficiency was observed when the steam/ biomass ratio was 0.5 (w/w). Kumar et al. (2009) used a fluidized-bed gasifier with air plus steam as the gasification agent. They tested distillers grains and solid byproducts of ethanol production. Kumar et al. (2009) also found an increase in syngas energy and syngas carbon efficiencies with an increase of the steam/biomass ratio. In the present study, the increase in the carbon efficiency was due to the compound effect of the increase in the carbon dioxide and the carbon monoxide content of the syngas, which were individually not significantly different. The char-steam reaction causes an increase of carbon monoxide content. Higher carbon monoxide and carbon dioxide in the syngas caused higher carbon efficiency of the system.


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A significant increase in the syngas energy efficiency was observed when water mist was mixed with air as the gasification agent. An increase in energy content and high heating value of syngas was also observed by Kumar et al. (2009) and Wang et al. (2007). An increase in methane composition, which produces about 35.81 KJ/Kg, was the reason for the increase in the heating value. Table 6 Syngas carbon and energy efficiency values obtained by gasification of wood pellets with air and air-mist as gasification agent in a fixed bed down-draft gasifier.

Sample Syngas Carbon Efficiency Syngas Energy Efficiency

Air 50.73 ± 1.80 38.94 ± 0.11

Air-mist 60.4 ± 0.82 42.38 ± 0.18

4. Summary In this study, mixtures of bituminous coal (Pittsburgh seam) and hardwood pellets were gasified in a fixed-bed downdraft gasifier with air as the gasifying agent to study the effect of the biomass/coal ratio on the syngas composition, syngas carbon efficiency and syngas energy efficiency. It was found that the hydrogen and carbon monoxide content of the syngas reduced significantly with an increase of coal into the feedstock. Syngas energy efficiency remained almost the same for feedstock containing up to 50% coal and then reduced significantly when the coal content was increased to 80% in the feedstock. Syngas carbon efficiency increased significantly with the addition of coal. To study the effect of using water mist on biomass gasification, a mixture of air plus water mist was used as the gasification agent. The hydrogen content was expected to increase with inclusion of water but it was counterbalanced by the slow char/steam reaction due to the lower temperature attained in the gasifier. Hence, the hydrogen content remained similar for both the air and air-water mist gasification. In addition, the carbon monoxide composition did not change with the addition of water mist into the system but there was a significant increase in the syngas energy efficiency and syngas carbon efficiency. Co-gasification of coal and biomass was not successful in this downdraft fixed-bed biomass gasification system designed by All Power Labs (APL). By addition of coal, combustion started taking place in the region, which was supposed to be the reduction zone, and hence increased the carbon dioxide content in the syngas produced. Moreover, the agglomerated coal plugged the system. It can be concluded that this downdraft gasifier is not suitable for either coal gasification or co-gasification of coal and biomass.

Acknowledgement This research was funded by the U.S. Department of Energy awarded through the Consortium for Fossil Fuel Science with G. P. Huffman as the director and the United States Department of Agriculture through McStennis Grant.


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