In bed catalytic tar reduction in the autothermal fluidized bed gasification of rice husk: Extraction of silica, energy and cost analysis

•Rice husk catalytic air gasification in bubbling fluidized bed was carried out.•Raw dolomite and untreated olivine were added in sand bed in different proportions.•On line method for the tar + particulate matter measurement developed and used.•Catalysts reduced tar + suspended particulate matter content by 41–50%.•60% silica was extracted from rice husk ash.

Fluidized bed gasifiers are becoming quite popular because of their higher conversion efficiencies, ability to handle wide variety of fuels and recent development in the field of catalytic gasification. Air gasification of rice husk was carried out in a pilot scale bubbling fluidized bed gasifier (ID: 197 mm and H: 1.6 m) at a constant air flow rate of 43 m3/h. Burning charcoal was initially used for the heating of bed material instead of electric heater in order to make it more suitable for the rural areas. For the bed of sand experiments were carried at four different equivalence ratios (ER) 0.28, 0.3, 0.33 and 0.35 to determine the suitable ERs for the catalytic gasification. Two naturally occurring catalyst: raw dolomite and untreated olivine were added separately in different proportions (10–60 wt.%) in the sand bed in order to determine their effectiveness at its higher and lower content in the sand bed and to decide the optimum combination of inert bed material and catalyst. Compact on-line method for the measurement of tar + suspended particulate matter (SPM) was developed and used. ER of 0.3 and 0.33 were found the most suitable ERs for the catalytic gasification. Dolomite provided 45 and 50% while olivine provided comparatively less 41 and 44% tar + SPM reduction at the ER of 0.3 and 0.33, respectively. However, olivine generated less fine particles and consumed 66% less energy for bed heating than dolomite. Improvement in the carbon conversion and thermal efficiency by 5–7% were obtained with the addition of catalysts. 60% silica was extracted from the rice husk ash which was taken from the discharge of the cyclone separators.