Techno-economic analysis (TEA) of auto-thermal pyrolysis of lignocellulosic biomass to transportation fuels
Wenqin Li.b, Joseph P. Polin. b, Wright M. M.ab, Brown, R. C.ab, aBioeconomy Institute
bDepartment of Mechanical Engineering, Iowa State University
The objective of this project is to evaluate the economic performance of auto-thermal fast pyrolysis of lignocellulosic biomass to transportation fuels on a 2000 metric tons per day (MT/d) commercial-scale. Auto-thermal fast pyrolysis employs oxygen at 0.06 equivalence ratio to convert biomass into bio-oil without the need of indirect heating sources. Process models have been built in Aspen Plus including biomass pretreatment, fast pyrolysis, bio-oil fractionation, stabilization, upgrading, H2 reforming and steam generation. Biomass is first chopped and ground into fine particles of around 3 mm and dried to moisture content of less than 10 wt.%. Pretreated biomass is then sent to fluidized bed reactor with 25 kg/hr air flow to produce bio-oil, non-condensable gases (NCG) and biochar. A novel six-stage fractionation system developed by Iowa State University has been implemented to separate bio-oil into six stage fractions with distinctive characteristics. Heavy and middle bio-oil stage fractions will be hydro-deoxygenated into stable oil via a two-stage stabilization process. Stable bio-oil is further sent to bio-refinery and upgraded into gasoline and diesel- range fuels. Light bio-oil stage fractions react with make- up natural gas and water to produce H2 for the hydrotreating process. NCG and part of biochar are combusted to heat the whole system. The rest biochar is sold as a byproduct at $20/MT.
Iowa State University has demonstrated higher bio-oil yields and better oil qualities are obtainable with auto-thermal pyrolysis. The experimental results indicate that auto-thermal pyrolysis could balance the endothermal heat required for conventional fast pyrolysis with released heat from oxidization reactions of NCG and lignin content. To evaluate the economic performance of the auto-thermal pyrolysis conversion process, a discounted cash flow rate of return (DCFROR) analysis has been conducted for a plant lifetime of 30 years and 10% internal rate of return. The preliminary economic results show that the capital cost could be reduced by 26% compared to conventional pyrolysis, and the minimum bio-oil selling price decreases from $0.91/gal to $0.76/gal. The minimum biofuel selling prices are further evaluated and compared. Sensitivity analysis has been applied to evaluate the impacts of various parameters on the minimum product selling price, which could help improve the biomass-to-biofuel conversion process from a technical and economic perspective.