|A flow-sheet for 100,000 gallons per day of CO2-free synfuel. Click to enlarge.|
A team from General Atomics is proposing the use of hydrogen provided from non-fossil sources (solar, wind or nuclear) and CO2 captured from coal-fired power plants or from the air to produce enough Fischer-Tropsch synthetics to meet the fuel needs of the transportation sector.
With such an approach, proposed in a poster session at the NHA hydrogen conference, the total net US release of CO2 could be halved, even factoring in the release of CO2 from the ongoing combustion of hydrocarbon—although not fossil—fuels, according to the researchers’ analysis.
The production rate of CO2 from coal power plants in the US is 1,875 million metric tons/year. If this CO2 were captured using proven absorption processes and used with hydrogen produced by solar, wind or nuclear energy to make synfuel, it would provide all the hydrocarbon fuel needed for our transportation economy.
Since that transportation economy produces 1,850 million metric tons of CO2 per year, this synfuel process would cut our CO2 production in half. We could shift from a petroleum-based transportation economy to a synfuel transportation economy.
This would reduce our petroleum use by 75%, and reduce our CO2 production by 50% with no increase in coal use. It would require significant quantities of hydrogen (255 million metric tons/year, or 25 times our current production) that would be produced from water using solar, wind or nuclear energy.
This hydrogen synfuel concept would allow us to significantly reduce our use of petroleum, and cut our CO2 emissions in half, while still using our existing hydrocarbon-based transportation infrastructure. It could provide a bridge to a pure hydrogen economy.
The Fischer-Tropsch process takes a synthesis gas (syngas) rich in hydrogen and carbon monoxide and converts it catalytically to liquid fuels and chemicals. The synthesis gas is produced by the gasification of carbon-bearing feedstocks (coal, biomass) or by the reforming of natural gas.
The gasification and reforming processes are energy, emissions and cost-intensive. The basic gasification reaction (for coal, for example) is:
2C + ½O2 + H2O → 2CO + H2
The Water-gas Shift reaction is then used to produce additional hydrogen:
CO + H2O → H2 + CO2
The reaction for producing Fischer-Tropsch products (generically [CH2]n) from synthesis gas (CO and H2) is:
CO + 2H2 → [CH2]n + H2O
The simultaneous Fischer-Tropsch and Water-Gas Shift reactions in the reactor leads directly to the complete reaction:
2C + H2O + ½O2 → [CH2]n + CO2
In summary, the process uses two carbons and half an O2 for every CH2 produced. Substituting such synfuels for petroleum-based fuels in transportations would triple US coal use and double current CO2 emissions.
Adding hydrogen from an external source into the process, however, cuts the carbon need in half compared to synfuel from standard coal gasification, and eliminated the production of CO2 from the process reactions.
|Adding Water-split H2 into the F-T Process|
|Gasification||C + ¼O2 + ½H2O → CO + ½H2|
|Water-splitting||3/2H2O + Energy →3/2H2 + ¾O2|
|F-T Reaction||CO + 2H2 → CH2 + H2O|
|Net Reaction||C + H2O + Energy → CH2 + ½O2|
The US currently produces 11 million tons of hydrogen annually primarily through steam reformation of CH4 (methane). The process is fossil-dependent and produces 100 million tons/yr of CO2.
Using low-temperature electrolysis, high-temperature electrolysis or thermochemical conversion of water (assuming the electricity is provided by wind, solar or nuclear and the heat is provided by solar or nuclear), eliminates the generation of CO2 from hydrogen production.
If CO2 is used as the source for carbon in the FT process, the gasification step is replaced by a reverse water-gas shift reaction and the outcome becomes even more attractive.
|Synfuel by CO2 Capture + H2 from Water-splitting|
|Reverse water shift||CO2 + H2 → CO + H2O|
|F-T reaction||CO + 2H2 → CH2 + H2O|
|Water-splitting||3H2O + Energy → 3H2 + 3/2O2|
|Net Reaction||CO2 + H2O + energy → CH2 + 3/2O2|
No coal or methane is needed, and one CO2 is consumed for each CH2 produced. When the CH2 is burned, the process is net carbon-neutral.
If used to replace oil, the use of these synfuels in transportation would cut US carbon dioxide emissions in half.
Carbon dioxide is readily available, the General Atomics team points out, from flue gas from fossil power plants. A 1,000MW coal-fired power plant produces 5.5 million tons of CO2/yr. (14,500 tons/day).
Coal-fired plants, which account for 53% (0.38TWh) of US electricity generation, generate 2 billion tons of CO2/yr—meeting the total annual CO2 requirement to make synfuel for US transportation needs.
Carbon dioxide could also be captured from the atmosphere. Membrane separation of CO2 from air followed by its absorption by either amine or inorganic solvents is an emerging technology, but has been demonstrated on a laboratory scale.
Large airflow would be required due to low concentration of CO2 in air, and the resulting CO2 would be costly: about $0.10/kg (about $1.0/gallon cost added to the synfuel).
All the component pieces have been demonstrated, the General Atomics team points out. What is required is an integrated demonstration.
To actually implement the proposal would require enormous amounts of both feedstocks.
The US consumes 260 million gallons of transportation fuel per day. (13 million barrels of crude)
2.5 million tonnes of CO2 (2.5 billion kg) and 0.35 million tonnes of H2 (350 million kg) per day would be required to make synfuel to replace crude.
General Atomics naturally points to the potential for using nuclear reactors for high volume high-temperature electrolysis or thermochemical conversion of water to hydrogen.
But assuming sufficient hydrogen from renewable or zero-carbon processes, and factoring in a modest carbon credit, the production of synthetics in this manner could make economic sense as well as environmental sense, according to the team.