Idaho National Lab Developing Highly Carbon-Efficient Biomass-to-Liquids Process Combining High Temperature Steam Electrolysis and Biomass Gasification
|Overview of the Bio-Syntrolysis process. Source: INL. Click to enlarge.|
Researchers at Idaho National Laboratory (INL) are developing a process—Bio-Syntrolysis—that combines high temperature steam electrolysis (HTSE) and biomass gasification to produce syngas for subsequent conversion into synthetic fuels and chemicals. The process results in the highly efficient conversion of biomass carbon to syngas (>90%).
Given the efficiencies of a typical Fischer-Tropsch process, Bio-Syntrolysis would thus convert about 90% of the carbon in biomass to liquid synthetic fuel, INL says. By comparison, INL notes, conventional biomass or coal gasification to liquid fuels converts only ~35% of the carbon to liquid fuel. Likewise, conventional biological routes for ethanol production convert only ~35% of biomass carbon to liquid fuel.
In Bio-Syntrolysis, process heat from the biomass gasifier produces the steam to improve the hydrogen production efficiency of the HTSE process, while the biomass itself is the source of the carbon. Hydrogen from HTSE allows a high utilization of the biomass carbon for syngas production, while the oxygen resulting from water splitting is used to control the gasification process. The new process is an evolution of INL’s earlier work on co-electrolysis (Syntrolysis).
Syntrolysis used high-temperature electrolysis with a solid-oxide electrolysis cell designed to take advantage of electricity from nuclear or renewable energy sources and industrial process heat to simultaneously convert water and carbon dioxide into syngas.
However, splitting pure CO2 is very energy intensive, noted Grant Hawkes, one of the INL team members.
We found that the amount of syngas produced per unit of electricity was a lot higher (~20%) for Bio-Syntrolysis compared to the Syntrolysis process. In the Bio-Syntrolysis process we make very little CO2 and mostly CO in the biomass gasifier. The amount of heat produced by making CO is just a perfect fit for how much heat we need to heat the water to the steam for the High Temperature Steam Electrolysis.—Grant Hawkes
|Carbon utilization rates (top group) and syngas production efficiencies (bottom group) for different feedstocks at different gasifier temperatures. Source: INL. Click to enlarge.|
In a modeling study, the INL team concluded that carbon utilization in the Bio-Syntrolysis process is only slightly affected by gasifier temperature, and ranges around 94-95% depending upon feedstock and gasifier temperature. Syngas production efficiency is closely tied to the power cycle efficiency. Assuming the thermal efficiency of the power cycle for electricity generation is 50%, (as expected from GEN IV nuclear reactors),the syngas production efficiency ranges around 70% to 73%.
The electrical power needs to be derived from a non-fossil source such as nuclear, hydro, wind or solar to keep the process carbon-neutral.
High-temperature electrolysis. INL researchers hit a milestone in September 2008 with a major scale-up of hydrogen production via high-temperature electrolysis, producing hydrogen at a rate of 5.6 cubic meters per hour—a major scale-up from earlier INL experiments on a smaller scale. (Earlier post.)
High-temperature electrolysis (HTE) adds in some of the energy needed to split water into its components (hydrogen and oxygen) as heat from a source such as high-temperature steam instead of electricity. Because the conversion efficiency of heat to electricity is low compared to using the heat directly, HTE reduces the overall energy required.
The electrolytic cell consists of a solid oxide electrolyte with conducting electrodes deposited on either side of the electrolyte. A high-temperature mixture of steam and hydrogen is supplied to the anode side of the electrolyte.
Regional Bio-Syntrolysis. INL is proposing locating Bio-Syntrolysis plants regionally, close to where the biomass is grown. A 25,000 barrel (1.05 million gallon US, 3.974 million liter) per day plant for full biomass to liquid fuels would entail a capital cost of around $2 billion and an annual operating cost of $1 billion per year.
The plant, according to INL analysis, would have a production cost of around $2.80 per gallon, and use 1,000 MW of electricity. Biomass would be gathered from an area 40-50 miles in diameter.
Widespread implementation of the process would require an enormous increase in non-fossil electrical power production.
INL began modeling and economic analysis of Bio-Syntrolysis in May 2008. That research is expected to continue through FY 2009. A patent for Bio-Syntrolysis is pending at the US Patent Office.
M. G. McKellar, G. L. Hawkes, J. E. O’Brien (2008) The Production of Syngas via High Temperature Electrolysis and Biomass Gasification (IMECE2008 - 68900)
Bio-Syntrolysis fact sheet