Life cycle assessment of rice husk torrefaction and prospects for decentralized facilities at rice mills

Abstract

Utilization of rice husk, an abundant global residual biomass, represent a large opportunity for increasing the share of bioenergy in the power sector. The present work assesses the life cycle emissions of rice husk torrefaction in different reaction conditions and the techno-economic prospects for decentralized torrefaction facilities at rice mills. First, a cradle-to-gate life cycle assessment (LCA) is performed for processing rice husk into torrefied pellets for energy generation. Four system configurations of different temperature and torrefaction medium are considered for torrefied rice husk (TRH) pellet production. Medium torrefaction in inert medium shows the highest GHG emissions of about 0.043 kg CO2 eq/MJ of TRH pellet whereas severe torrefaction in oxidative medium shows the lowest GHG emissions of 0.021 kg CO2 eq/MJ of TRH pellet. The environmental impacts of torrefaction in partially oxidative medium and at higher temperatures are lower, only if a significant portion of the torgas and condensate is efficiently utilized. Second, a techno-enviro-economic evaluation is performed for multiple scenarios of utilizing raw or torrefied rice husk (partially oxidative conditions) for power generation. A case study of the rice mills in the Sacramento valley of California is considered, with an annual supply of about 0.2 million tons of rice husk, to operate a 26.5 MW biopower plant. A centralized facility at the power plant resulted in highest GHG emissions, and a rented mobile facility resulted in highest annual costs. The most desired scenario of having localized facilities at the rice mills proved most cost-effective, with excess TRH breakeven price of 61 USD/ton for medium torrefaction and 471 USD/ton for severe torrefaction. Instead of excess TRH, if the entire TRH is available for sale, the same scenario estimated the average breakeven price of 14.9 USD/ton and 21.1 USD/ton respectively. Sensitivity analysis for a single mill explains the role of factors such as availability of rice husk (milling capacity), transportation distance, and solid mass yield on the breakeven price, costs, and the GHG emissions.