Life Cycle Assessment of Greenhouse Gas Emission of Bio-Oil Co-Firing with Coal for Electric Power Generation

Life Cycle Assessment of Greenhouse Gas Emission of Bio-oil Co-firing with Coal for Electric Power Generation

Qi Dang ^a, Mark Mba Wright ^b, Robert C. Brown ^a,b

^a Bioeconomy Institute, Iowa State University, Ames, IA 50011, USA

^b Department of Mechanical Engineering, Iowa State University, Ames, IA 50011, USA

Burning fossil fuels, primarily coal and natural gas, has accounted for over 70% of greenhouse gas (GHG) emissions from the electricity sector in the United States in 2012 and 2013. In order to reduce carbon pollution from power plants, biomass is expected to become an important energy source in the U.S. electricity mix under renewable portfolio standards. Bio-oil derived from biomass fast pyrolysis can be a promising enabler of renewable power production by overcoming biomass and coal co-firing limits. Therefore, a novel strategy of combing bio-oil with coal for electric power generation from existing power plants is proposed and a new vision of sequestering bio-char produced from this thermochemical process as soil amendment is adopted. This approach provides a sustainable and economical pathway to meeting lifecycle carbon reduction targets in the power sector.

This study aims at quantitative evaluation of GHG emissions of displacing coal via cofiring bio-oil with coal for electric power production. Bio-oil from fast pyrolysis of corn stover can be recovered into heavy end, middle end and light end fractions using a fractionation system developed at Iowa State University. Heavy end is assumed to be blended and cofired with bituminous coal to form a bio-oil cofire fuel (BCF) compatible with existing coal boiler systems to produce electric power. A heavy ends to coal mass ratio of 30%:70% has been demonstrated and forms the basis of this study. Furthermore, two scenarios including bio-char sequestration and bio-char combustion are investigated and compared based on different applications of bio-char. Simapro 7.3 and the IPCC 2007 method with a 100-year time horizon are selected to evaluate GHG emissions of different scenarios.

A fast pyrolysis facility with a processing capacity of 2000 dry metric tons per day (DMTPD) is designed using Aspen Plus. It can be seen from the process models that when coal consumption in the cofiring system decreases from 1594 to 455 DMTPD, the total output power decreases from 248 to 124 MW and the corresponding heavy end fraction varies from 30% to 60%. The sensitivity analysis for the process model assumptions indicates that heavy end fraction is the most influential factor to total power export followed by the isentropic efficiencies of power generating turbines among the specified range whereas corn stover moisture content and coal moisture content have lower impacts.

Since the environmental burdens can be allocated to all products by mass or energy distribution from fast pyrolysis as well as no allocation to light end, three different emission allocation cases are analyzed within each scenario. When the heavy end fraction changes from 30% to 60%, the GHG emissions per 1 kWh electricity produced from bio-char sequestration scenario vary from 0.69 to 0.25 kg CO₂-eq among various cases, which corresponds to a GHG emission reduction of 33.9% to 76% compared with that from traditional US coal power plants. For bio-char combustion scenario, the GHG emissions are reported to be between 0.70 to 0.35 kg CO₂-eq per kWh of electric power generated within different distribution cases. GHG reductions of 33.1% to 66.9% are observed. In order to meet EPA's new regulation for fossil fuel-fired power system (0.499 kg CO₂-eq/kWh), the heavy end fraction is estimated to be between 42.1% to 45.2% for bio-char sequestration scenario, and 45.9% to 49.5% for bio-char combustion scenario. The results suggest that bio-char sequestration is more beneficial from an environmental perspective. Sensitivity analysis of the life cycle emissions indicates that heavy end fraction and turbines efficiencies have higher impacts on GHG emissions, followed by electricity consumption in the corn stover pretreatment and pyrolysis process, corn stover moisture content, and corn stover transport distance. Uncertainty analysis is conducted as well to estimate the ranges of expected GHG emissions by incorporating probability distributions of selected parameters. The results show relatively large uncertainties exist in terms of wide ranges of heavy end fraction. These results suggest that heavy end fraction is a viable option for coal power plants to reduce their lifecycle emissions and that future study is needed to determine its performance in commercial-scale systems.