Large-scale biogas
plants are a viable source of CH4 and CO2 to
be converted efficiently into high-value products.
Specifically, production of liquid hydrocarbons can enhance the availability
of green fuels while achieving significant CO2 reductions
on site. In this study, the production of liquid hydrocarbons is simulated
by dry reforming of biogas into lean-hydrogen syngas, further converted
in CO hydrogenation and oligomerization reactors. The process was
modeled by using CHEMCAD based on published experimental results with
the projected feed composition. A high molar feed ratio of CO2/CH4 (>1.7) was set for the reformer to minimize
steam requirement while avoiding carbon formation and reaching an
optimal H2 to CO molar ratio (0.7). Two options were techno-economically
evaluated based on a biogas plant with a capacity of 5000 N m3/h that produces between 13.8 and 15.7 million liters per
year of blending stock for transportation fuels. The economics of
the process depends mainly on the cost and availability of the biogas.
The minimum selling price of the liquid fuels is $1.47/L and $1.37/L
for options 1 (once-through conversion of syngas to liquid fuels)
and 2 (recycle of tail gas from oligomerization reactor), respectively,
and can be significantly reduced in case the biogas throughput is
increased to >20 000 N m3/h. Recycling of the
tail
gas (option 2) yielded higher productivity, resulting in higher carbon
yield (77.9% on the basis of methane) and energy efficiency (67.1%).
The economic viability of the process can be improved by implementing
CO2 tax or other incentives to reduce capital investment.
It provides a potential route for efficient conversion of biogas into
liquid hydrocarbons to meet the increased demand for renewable fuels
as blending stock in the transportation sector while improving the
sustainability of the plant.