Design consideration for packed-bed downdraft biomass gasifier: Optimization of syngas composition and controlling factors

Biomass has a great potential as a clean, renewable feedstock for production of power generation either in the form of thermal or electrical energy. The gasifier at present study is downdraft throatless type with 30 cm uniform diameter throughout the length and 10 cm diameter opening at the base for the gas exit. It has five air entrain pipes extending from top of the reactor with pre heating of air before it is injected to the oxidation zone. The air temperature reaches maximum up to 322°C and enters to the oxidation zone at 15 cm above the reactor gas exit grate. The gas leaving the reactor, cyclone, heat exchanger and the water bubbler has the temperature 375°C, 310°C, 65°C and 53°C respectively in series combination. The gas composition from the water bubbler has CO-25%, CO₂-9.8%, H₂-15.2%, CH₄-1.5%, N₂-48.5%, on dry basis on air being the gasification medium for small wood particle size and CO-8%, CO₂-27%, H₂-16%, CH₄-0.5%, N₂-48.5%, on dry basis. The HHV of the syngas is 4.5-5.5 MJ/Nm³. The temperature profile for the two tests with different wood size is presented. The maximum temperature reached after 10 minutes of the ignition for larger wood particle size (5 cm × 5 cm × 10 cm) is 473°C at the oxidation zone where as, for the same time, the temperature of the smaller wood particle size (1.5 cm × 1.5 cm × 2 cm) is 574°C. The temperature profile also demonstrates the effect of particle size on the temperature plot. The air supply for the both of the test was the same.