(150c) 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

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

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.