Natural
Gas and Cellulosic Biomass: A Clean Fuel Combination?
Determining the Natural Gas Blending Wall in Biofuel Production
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Abstract
Natural
gas has the potential to increase the biofuel production
output by combining gas- and biomass-to-liquids (GBTL) processes followed
by naphtha and diesel fuel synthesis via Fischer–Tropsch (FT).
This study reflects on the use of commercial-ready configurations
of GBTL technologies and the environmental impact of enhancing biofuels
with natural gas. The autothermal and steam-methane reforming processes
for natural gas conversion and the gasification of biomass for FT
fuel synthesis are modeled to estimate system well-to-wheel emissions
and compare them to limits established by U.S. renewable fuel mandates.
We show that natural gas can enhance FT biofuel production by reducing
the need for water–gas shift (WGS) of biomass-derived syngas
to achieve appropriate H<sub>2</sub>/CO ratios. Specifically, fuel
yields are increased from less than 60 gallons per ton to over 100
gallons per ton with increasing natural gas input. However, GBTL facilities
would need to limit natural gas use to less than 19.1% on a LHV energy
basis (7.83 wt %) to avoid exceeding the emissions limits established
by the Renewable Fuels Standard (RFS2) for clean, advanced biofuels.
This effectively constitutes a <i>blending</i> limit that
constrains the use of natural gas for enhancing the biomass-to-liquids
(BTL) process