Influence of operating parameters on the microwave pyrolysis of rice husk: biochar yield, energy yield, and property of biochar

Burning of agricultural waste like crop residues releases black carbon, a major source of air pollution in megacities like Delhi. Conversely, crop residues are a rich source of cellulose, hemicellulose, and lignin. The present study investigated the effect of heating time (HT) and MW power (P) on biochar yield from rice husk, an agricultural waste, using microwave pyrolysis. Biochar yield varied from 68% (HT 18 min and P 500 W) to 39% (at HT 15 min and P 900 W). A second-order polynomial model was obtained for predicting biochar yield, which signified that microwave power and interaction of heating time and microwave power positively affected the maximum yield of biochar. The high heating value and fuel ratio of biochar (25.46 MJ/kg and 2.82) showed improvement compared with that of rice husk (12.43 MJ/kg and 0.06). Also, the MW pyrolysis of rice husk resulted in increased fixed carbon content from 4.78% in rice husk to 52.2% in biochar. The property of rice husk and biochar indicated a decrease in O/C ratio from 1.5 to 0.66, improvement in surface area from 2.4 to 190 m2/g, and an increase in pH from 6.71 to 8.75. Also, energy density and energy yield in MW pyrolysis were found as 1.46 and 99.28% respectively. Hence, MW pyrolysis of rice husk resulted in a high quality of biochar that could be used as a potential source of energy, nutrient captive media, and soil amendment.