Researchers increase carbon efficiency of BtL process from 38 to >90% by adding hydrogen from renewable sources; 2.4x more fuel produced
Researchers in Norway report that the carbon efficiency of a conventional Biomass-to-Liquid (BtL) process can be increased from 38 to more than 90% by adding hydrogen from renewable energy sources. The increased carbon efficiency is possible because the water gas shift reaction is avoided and instead a reversed water gas shift is introduced to convert CO2 to CO.
This means that the amount of fuel can be increased by a factor of 2.4 with the same amount of biomass. In addition, the emitted amount of CO2 per produced unit can be reduced by a factor of 16.
They also determined that there is a near linear relationship between added hydrogen and production of surplus FT-products—meaning that maximum hydrogen addition is favorable.
Based on their evaluation of process technology and economics, the researchers concluded that converting excess renewable electric power to advanced biofuels in a power and biomass to Fischer-Tropsch liquid fuels (PBtL) plant is a sensible way of storing energy as a fuel with a relatively high energy density. Their open-access paper is published in the journal Fuel.
Energy flow of the staged PBtL concept. The steam produced from the FT reactors (FT cooling) is energy that partly can be utilized.
In their process, hydrogen is produced through high temperature steam electrolysis in a solid oxide electrolysis cell (SOEC), with high temperature steam generated from the hot syngas from the Fischer-Tropsch biomass-to-liquids process. The oxygen produced from the SOEC is sufficient as oxidant in the gasifier, thereby eliminating the need for a cryogenic air separation unit.
The use of high-temperature electrolysis allows some of the required energy to be supplied by available heat.
The required electrical power for the extra production is estimated to be 11.6 kWh per liter syncrude (C5+). By operating the SOEC iso-thermally close to 850 °C the electric energy may be reduced to 9.5 kWh per liter—close to the energy density of jet fuel.
With an electrical power price of 0.05 $/kWh and with SOEC investment cost of the 1000 $/kW(el), the levelized cost of producing advanced biofuel with the PBtL concept is $1.70/liter ($6.40/gallons US)—approximately 30% lower than for conventional BtL.
M. Hillestad, M. Ostadi, G.d. Alamo Serrano, E. Rytter, B. Austbø, J.G. Pharoah, O.S. Burheim (2018) “Improving carbon efficiency and profitability of the biomass to liquid process with hydrogen from renewable power,” Fuel, Volume 234, Pages 1431-1451 doi: 10.1016/j.fuel.2018.08.004