Techno-economic and environmental assessment of decentralized pyrolysis for crop residue management: Rice and wheat cultivation system in India

Abstract

The current study evaluates a decentralized biorefinery's economic and environmental performance, which uses two-step pyrolysis for converting rice and wheat straw to bio-oil and biochar. The biorefinery was located in Punjab (India), where open burning of residue is prevalent. The decentralized biorefinery was evaluated regarding the energy required for pyrolysis, product yield and applications, and global warming potential (GWP). Pyrolysis of unwashed, water-washed, and acid-washed straws was performed. The net GWP for pyrolysis of unwashed rice straw was −121 kg CO2eq/ton when biochar was used as a carbon sink. But pyrolysis of water-washed (159 kg CO2eq/ton) and acid-washed rice straw (311 kg CO2eq/ton) was a net contributor to GWP, which was undesirable. Similar trends were observed for wheat straw pyrolysis.The GWP of washed rice and wheat straw pyrolysis was higher than unwashed straw because 37–50% biochar generated was used for drying the washed straw, leaving less biochar for further application. Amongst the biochar applications considered, its use as a carbon sink offered more GWP reduction than its use as a substitute for coal in power and heat generation. The net GWP for direct transfer of residues to a centralized refinery for pyrolysis was −75 kg CO2eq/ton for rice straw and −384 kg CO2eq/ton for wheat straw. Therefore, it was environmentally beneficial to treat biomass locally rather than in a centralized unit. Finally, the minimum selling price for biochar was calculated to be 172–623 USD/ton, which was within the range of commercial biochar price. Therefore, the proposed biorefinery was expected to be environmentally and economically viable with an appropriate selection of pretreatment options and end-uses of the products.