PERFORMANCE EVALUATION OF DOWNDRAFT GASIFIER FOR SYNGAS PRODUCTION USING RICE HUSK

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Department of Engineering

ABSTRACT
Biomass gasification is a thermochemical process that converts biomass to a combination of gases known as syngas comprising mainly of CO, H2and CH4 as a result of partial combustion with a gasifying agent. It is considered to be a promising clean energy option for reduction of greenhouse gas emissions and a way of utilizing agricultural wastes like rice husk. The syngas can be used not only to produce heat and power but in synthesis of liquid fuels and chemicals. This research comprises of rice husk characterization, mathematical model formulation to predict rice husk gasification theoretically and gasification of rice husk using both air and oxygen-enriched air as gasifying agents experimentally. Theoretical rice husk air gasification was done by inputting the composition of the characterizedrice husk into set of mathematical equations derived based on thermodynamics, mass and energy balances using equilibrium approach and resulting equationswere computed using MATLAB as a toolto predictsyngas composition and calorific value between temperature of 500 and 1100 °C. Experimental ricehusk gasification was conducted using a downdraft gasification system installed at National Research Institute for Chemical Technology, Zaria, Nigeria, comprising of a gasifier as reactor, cyclone, filter and air blower. The gasification was done with two different gasifying agents; air and oxygen-enriched air. For the air gasification, effect of 6.4, 3.0 and 0.7 L/min flow rates were studied while for the oxygen-enriched air gasification, 30 to 100% oxygen enrichment in air were examined. Temperature, syngas composition and calorific value were monitored during the experiment using online portable infrared syngas analyser (Gasboard 3100P series), digital thermometer (UT 350) and K-type (chromel-alumel) thermocouple. The results of the model indicated an optimum temperature at 800 °C with syngas composition of 18.72 CO%, 16.68% H2, 13.05% CO2, and 0.39% CH4, and 4.47 MJ/m3 calorific value. The best experimentalsyngas composition was at 6.4 L/min air flow rate with composition of 10.83 CO%, 9.51% CO2, 2.12% H2 and 1.18 CH4%, desired syngas composition of 14.13 % and equivalence ratio of 0.128, with an average temperature of 567°C and 2.53 MJ/Nm3 calorific value.Root mean square error value of 7.58 was calculated when the model developed was validated with the best results obtained from rice husk air gasification. For oxygen enriched- air rice husk gasification,the best point was considered at 50% oxygen enrichment in air having the highest CO to CO2ratio of 1.63 with equivalence ratio of 0.494, desired syngas of 24.34%, syngas composition of 19.8% CO, 12.16% CO2, 2.26% H2, 2.28% CH4, and calorific value of 3.67 MJ/m3.Performance analysis shows that for air gasification the highest Carbon Conversion efficiency (CCE) and Cold Gas Efficiency (CGE)was achieved at the highest air flow rate (6.4L/min) as 21.27 and 12.55% respectively.While for oxygen- enriched air gasification, 50 % oxygen enrichment in air gave the best values of both CCE and CGE as 46.72 and 26.24%, respectively.