||The neutrality of this article is disputed. (March 2015)|
Carbon-neutral fuels can refer to a variety of energy fuels or energy systems which have no net greenhouse gas emissions or carbon footprint. One class are synthetic fuel (including methane, gasoline, diesel fuel, jet fuel or ammonia) produced from sustainable or nuclear energy used to hydrogenate waste carbon dioxide recycled from power plant flue exhaust gas or derived from carbonic acid in seawater. Other types can be produced from renewable energy sources such as wind turbines, solar panels, and hydroelectric power plants. Such fuels are potentially carbon-neutral because they do not result in a net increase in atmospheric greenhouse gases. Until captured carbon is used for plastics feedstock, carbon neutral fuel synthesis is the primary means of carbon capture and utilization or recycling.
To the extent that carbon-neutral fuels displace fossil fuels, or if they are produced from waste carbon or seawater carbonic acid, and their combustion is subject to carbon capture at the flue or exhaust pipe, they result in negative carbon dioxide emission and net carbon dioxide removal from the atmosphere, and thus constitute a form of greenhouse gas remediation. Such power to gas carbon-neutral and carbon-negative fuels can be produced by the electrolysis of water to make hydrogen used in the Sabatier reaction to produce methane which may then be stored to be burned later in power plants as synthetic natural gas, transported by pipeline, truck, or tanker ship, or be used in gas to liquids processes such as the Fischer–Tropsch process to make traditional fuels for transportation or heating.
Carbon-neutral fuels are used in Germany and Iceland for distributed storage of renewable energy, minimizing problems of wind and solar intermittency, and enabling transmission of wind, water, and solar power through existing natural gas pipelines. Such renewable fuels could alleviate the costs and dependency issues of imported fossil fuels without requiring either electrification of the vehicle fleet or conversion to hydrogen or other fuels, enabling continued compatible and affordable vehicles. A 250 kilowatt synthetic methane plant has been built in Germany and it is being scaled up to 10 megawatts.
Carbon-neutral fuels are synthetic hydrocarbons. They can be produced in chemical reactions between carbon dioxide, which can be captured from power plants or the air, and hydrogen, which is created by the electrolysis of water using renewable energy. The fuel, often referred to as electrofuel, stores the energy that was used in the production of the hydrogen. Coal can also be used to produce the hydrogen, but that would not be a carbon-neutral source. Carbon dioxide can be captured and buried, making fossil fuels carbon-neutral, although not renewable. Carbon capture from exhaust gas can make carbon-neutral fuels carbon negative. Other hydrocarbons can be broken down to produce hydrogen and carbon dioxide which could then be stored while the hydrogen is used for energy or fuel, which would also be carbon-neutral.
The most energy-efficient fuel to produce is methanol, which is made from a chemical reaction of a carbon-dioxide molecule with three hydrogen molecules to produce methanol and water. The stored energy can be recovered by burning the methanol in a combustion engine, releasing carbon dioxide, water, and heat. Methane can be produced in a similar reaction. More energy can be used to combine methanol or methane into larger hydrocarbon fuel molecules.
Researchers have also suggested using methane to produce dimethyl ether. This fuel could be used as a substitute for diesel fuel due to its ability to self ignite under high pressure and temperature. It is already being used in some areas for heating and energy generation. It is nontoxic, but must be stored under pressure. Octane and ethanol can also be produced from carbon dioxide and hydrogen.
All synthetic hydrocarbons are generally produced at temperatures of 200–300 °C, and at pressures of 20 to 50 bar. Catalysts are usually used to improve the efficiency of the reaction and create the desired type of hydrocarbon fuel. Such reactions are exothermic and use about 3 mol of hydrogen per mole of carbon dioxide involved. They also produce large amounts of water as a byproduct.
Sources of carbon for recycling
The most economical source of carbon for recycling into fuel is flue-gas emissions from fossil-fuel combustion where it can be obtained for about USD $7.50 per ton. Automobile exhaust gas capture has also been seen as economical but would require extensive design changes or retrofitting. Since carbonic acid in seawater is in chemical equilibrium with atmospheric carbon dioxide, extraction of carbon from seawater has been studied. Researchers have estimated that carbon extraction from seawater would cost about $50 per ton. Carbon capture from ambient air is more costly, at between $600 and $1000 per ton and is considered impractical for fuel synthesis or carbon sequestration. Direct air capture is less developed than other methods. Proposals for this method involve using a caustic chemical to react with carbon dioxide in the air to produce carbonates. These can then be broken down and hydrated to release pure CO2 gas and regenerate the caustic chemical. This process requires more energy than other methods because carbon dioxide is at much lower concentrations in the atmosphere than in other sources.
Researchers have also suggested using biomass as a carbon source for fuel production. Adding hydrogen to the biomass would reduce its carbon to produce fuel. This method has the advantage of using plant matter to cheaply capture carbon dioxide. The plants also add some chemical energy to the fuel from biological molecules. This may be a more efficient use of biomass than conventional biofuel because it uses most of the carbon and chemical energy from the biomass instead of releasing as much energy and carbon. Its main disadvantage is, as with conventional ethanol production, it competes with food production.
|Part of a series on|
Renewable and nuclear energy costs
Nighttime wind power is considered the most economical form of electrical power with which to synthesize fuel, because the load curve for electricity peaks sharply during the warmest hours of the day, but wind tends to blow slightly more at night than during the day. Therefore, the price of nighttime wind power is often much less expensive than any alternative. Off-peak wind power prices in high wind penetration areas of the U.S. averaged 1.64 cents per kilowatt-hour in 2009, but only 0.71 cents/kWh during the least expensive six hours of the day. Typically, wholesale electricity costs 2 to 5 cents/kWh during the day. Commercial fuel synthesis companies suggest they can produce gasoline for less than petroleum fuels when oil costs more than $55 per barrel.
The U.S. Navy estimates that 100 megawatts of electricity can produce 41,000 gallons of jet fuel per day and shipboard production from nuclear power would cost about $6 per gallon. While that was about twice the petroleum fuel cost in 2010, it is expected to be much less than the market price in less than five years if recent trends continue. Moreover, since the delivery of fuel to a carrier battle group costs about $8 per gallon, shipboard production is already much less expensive.
Demonstration projects and commercial development
A 250 kilowatt methane synthesis plant was constructed by the Center for Solar Energy and Hydrogen Research (ZSW) at Baden-Württemberg and the Fraunhofer Society in Germany and began operating in 2010. It is being upgraded to 10 megawatts, scheduled for completion in autumn, 2012.
The George Olah carbon dioxide recycling plant operated by Carbon Recycling International in Grindavík, Iceland has been producing 2 million liters of methanol transportation fuel per year from flue exhaust of the Svartsengi Power Station since 2011. It has the capacity to produce 5 million liters per year.
Audi has constructed a carbon-neutral liquefied natural gas (LNG) plant in Werlte, Germany. The plant is intended to produce transportation fuel to offset LNG used in their A3 Sportback g-tron automobiles, and can keep 2,800 metric tons of CO2 out of the environment per year at its initial capacity.
Commercial developments are taking place in Columbia, South Carolina, Camarillo, California, and Darlington, England. A demonstration project in Berkeley, California proposes synthesizing both fuels and food oils from recovered flue gases.
Greenhouse gas remediation
Carbon-neutral fuels might lead to greenhouse gas remediation because carbon dioxide gas would be reused to produce fuel instead of being released into the atmosphere. Capturing the carbon dioxide in flue gas emissions from power plants would eliminate their greenhouse gas emissions, although burning the fuel in vehicles would release that carbon because there is no economical way to capture those emissions. This approach would reduce net carbon dioxide emission by about 50% if it were used on all fossil fuel power plants. Most coal and natural gas power plants have been predicted to be economically retrofittable with carbon dioxide scrubbers for carbon capture to recycle flue exhaust or for carbon sequestration. Such recycling is expected to not only cost less than the excess economic impacts of climate change if it were not done, but also to pay for itself as global fuel demand growth and peak oil shortages increase the price of petroleum and fungible natural gas.
Capturing CO2 directly from the air or extracting carbonic acid from seawater would also reduce the amount of carbon dioxide in the environment, and create a closed cycle of carbon to eliminate new carbon dioxide emissions. Use of these methods would eliminate the need for fossil fuels entirely, assuming that enough renewable energy could be generated to produce the fuel. Using synthetic hydrocarbons to produce synthetic materials such as plastics could result in permanent sequestration of carbon from the atmosphere.
Traditional fuels or methanol
Some authorities have recommended producing methanol instead of traditional transportation fuels. It is a liquid at normal temperatures and can be toxic if ingested. Methanol has a higher octane rating than gasoline but a lower energy density, and can be mixed with other fuels or used on its own. It may also be used in the production of more complex hydrocarbons and polymers. Direct methanol fuel cells have been developed by Caltech's Jet Propulsion Laboratory to convert methanol and oxygen into electricity. It is possible to convert methanol into gasoline, jet fuel or other hydrocarbons, but that requires additional energy and more complex production facilities. Methanol is slightly more corrosive than traditional fuels, requiring automobile modifications on the order of USD $100 each to use it.
Investigation of carbon-neutral fuels has been ongoing for decades. A 1965 report suggested synthesizing methanol from carbon dioxide in air using nuclear power for a mobile fuel depot. Shipboard production of synthetic fuel using nuclear power was studied in 1977 and 1995. A 1984 report studied the recovery of carbon dioxide from fossil fuel plants. A 1995 report compared converting vehicle fleets for the use of carbon-neutral methanol with the further synthesis of gasoline.
- Leighty and Holbrook (2012) "Running the World on Renewables: Alternatives for Transmission and Low-cost Firming Storage of Stranded Renewable as Hydrogen and Ammonia Fuels via Underground Pipelines" Proceedings of the ASME 2012 International Mechanical Engineering Congress & Exposition November 9–15, 2012, Houston, Texas
- Zeman, Frank S.; Keith, David W. (2008). "Carbon neutral hydrocarbons" (PDF). Philosophical Transactions of the Royal Society A 366: 3901–18. doi:10.1098/rsta.2008.0143. Retrieved September 7, 2012. (Review.)
- Wang, Wei; Wang, Shengping; Ma, Xinbin; Gong, Jinlong (2011). "Recent advances in catalytic hydrogenation of carbon dioxide". Chemical Society Reviews 40 (7): 3703–27. doi:10.1039/C1CS15008A. Retrieved July 6, 2013. (Review.)
- MacDowell, Niall et al. (2010). "An overview of CO2 capture technologies". Energy and Environmental Science 3 (11): 1645–69. doi:10.1039/C004106H. Retrieved September 7, 2012. (Review.)
- Eisaman, Matthew D. et al. (2012). "CO2 extraction from seawater using bipolar membrane electrodialysis" (PDF). Energy and Environmental Science 5 (6): 7346–52. doi:10.1039/C2EE03393C. Retrieved July 6, 2013.
- Graves, Christopher; Ebbesen, Sune D.; Mogensen, Mogens; Lackner, Klaus S. (2011). "Sustainable hydrocarbon fuels by recycling CO2 and H2O with renewable or nuclear energy". Renewable and Sustainable Energy Reviews 15 (1): 1–23. doi:10.1016/j.rser.2010.07.014. (Review.)
- Socolow, Robert et al. (June 1, 2011). Direct Air Capture of CO2 with Chemicals: A Technology Assessment for the APS Panel on Public Affairs (PDF) (peer reviewed literature review). American Physical Society. Retrieved September 7, 2012.
- Conference on Carbon Dioxide as Feedstock for Chemistry and Polymers (Essen, Gerrmany, October 10–11, 2012; post-conference program)
- Goeppert, Alain; Czaun, Miklos; Prakash, G.K. Surya; Olah, George A. (2012). "Air as the renewable carbon source of the future: an overview of CO2 capture from the atmosphere". Energy and Environmental Science 5 (7): 7833–53. doi:10.1039/C2EE21586A. Retrieved September 7, 2012. (Review.)
- House, K.Z.; Baclig, A.C.; Ranjan, M.; van Nierop, E.A.; Wilcox, J.; Herzog, H.J. (2011). "Economic and energetic analysis of capturing CO2 from ambient air" (PDF). Proceedings of the National Academy of Sciences 108 (51): 20428–33. doi:10.1073/pnas.1012253108. Retrieved September 7, 2012. (Review.)
- Lackner, Klaus S. et al. (2012). "The urgency of the development of CO2 capture from ambient air". Proceedings of the National Academy of Sciences of the United States of America 109 (33): 13156–62. doi:10.1073/pnas.1108765109. PMID 22843674. Retrieved September 7, 2012.
- Pearson, R.J.; Eisaman, M.D. et al. (2012). "Energy Storage via Carbon-Neutral Fuels Made From CO2, Water, and Renewable Energy" (PDF). Proceedings of the IEEE 100 (2): 440–60. doi:10.1109/JPROC.2011.2168369. Retrieved September 7, 2012. (Review.)
- Pennline, Henry W. et al. (2010). "Separation of CO2 from flue gas using electrochemical cells". Fuel 89 (6): 1307–14. doi:10.1016/j.fuel.2009.11.036.
- Graves, Christopher; Ebbesen, Sune D.; Mogensen, Mogens (2011). "Co-electrolysis of CO2 and H2O in solid oxide cells: Performance and durability". Solid State Ionics 192 (1): 398–403. doi:10.1016/j.ssi.2010.06.014.
- Fraunhofer-Gesellschaft (May 5, 2010). "Storing green electricity as natural gas". fraunhofer.de. Retrieved September 9, 2012.
- Pearson, Richard; Eisaman (2011). "Energy Storage Via Carbon-Neutral Fuels Made From Carbon dioxide, Water, and Renewable Energy" (PDF). Proceedings of the IEEE. Retrieved 18 October 2012.
- Kleiner, kurt (17 January 2009). "Carbon Neutral Fuel; a new approach". The Globe and Mail: F4. Retrieved 23 October 2012.
- Olah, George; Alain Geoppert; G. K. Surya Prakash (2009). "Chemical recycling of Carbon Dioxide to Methanol and Dimethyl Ether: From Greenhouse Gas to Renewable, Environmentally Carbon Neutral Fuels and Synthetic Hydrocarbons". Journal of Organic Chemistry 74 (2): 487–98. doi:10.1021/jo801260f. PMID 19063591. Retrieved 23 October 2012.
- Musadi, M.R.; Martin, P.; Garforth, A.; Mann, R. (2011). "Carbon neutral gasoline re-synthesised from on-board sequestrated CO2" (PDF). Chemical Engineering Transactions 24: 1525–30. doi:10.3303/CET1124255. Retrieved September 7, 2012.
- DiMascio, Felice; Willauer, Heather D.; Hardy, Dennis R.; Lewis, M. Kathleen; Williams, Frederick W. (July 23, 2010). Extraction of Carbon Dioxide from Seawater by an Electrochemical Acidification Cell. Part 1 – Initial Feasibility Studies (PDF) (memorandum report). Washington, DC: Chemistry Division, Navy Technology Center for Safety and Survivability, U.S. Naval Research Laboratory. Retrieved September 7, 2012.
- Willauer, Heather D.; DiMascio, Felice; Hardy, Dennis R.; Lewis, M. Kathleen; Williams, Frederick W. (April 11, 2011). Extraction of Carbon Dioxide from Seawater by an Electrochemical Acidification Cell. Part 2 – Laboratory Scaling Studies (PDF) (memorandum report). Washington, DC: Chemistry Division, Navy Technology Center for Safety and Survivability, U.S. Naval Research Laboratory. Retrieved September 7, 2012.
- Electricity Price NewFuelist.com (compare to off-peak wind power price graph.) Retrieved September 7, 2012.
- Holte, Laura L.; Doty, Glenn N.; McCree, David L.; Doty, Judy M.; Doty, F. David (2010). Sustainable Transportation Fuels From Off-peak Wind Energy, CO2 and Water (PDF). 4th International Conference on Energy Sustainability, May 17–22, 2010. Phoenix, Arizona: American Society of Mechanical Engineers. Retrieved September 7, 2012.
- Willauer, Heather D.; Hardy, Dennis R.; Williams, Frederick W. (September 29, 2010). Feasibility and Current Estimated Capital Costs of Producing Jet Fuel at Sea (PDF) (memorandum report). Washington, DC: Chemistry Division, Navy Technology Center for Safety and Survivability, U.S. Naval Research Laboratory. Retrieved September 7, 2012.
- Center for Solar Energy and Hydrogen Research Baden-Württemberg (2011). "Verbundprojekt 'Power-to-Gas'" (in German). zsw-bw.de. Retrieved September 9, 2012.
- Center for Solar Energy and Hydrogen Research (July 24, 2012). "Bundesumweltminister Altmaier und Ministerpräsident Kretschmann zeigen sich beeindruckt von Power-to-Gas-Anlage des ZSW" (in German). zsw-bw.de. Retrieved September 9, 2012.
- "George Olah CO2 to Renewable Methanol Plant, Reykjanes, Iceland" (Chemicals-Technology.com)
- "First Commercial Plant" (Carbon Recycling International)
- Okulski, Travis (June 26, 2012). "Audi's Carbon Neutral E-Gas Is Real And They're Actually Making It". Jalopnik (Gawker Media). Retrieved 29 July 2013.
- Rousseau, Steve (June 25, 2013). "Audi's New E-Gas Plant Will Make Carbon-Neutral Fuel". Popular Mechanics. Retrieved 29 July 2013.
- Doty Windfuels
- CoolPlanet Energy Systems
- Air Fuel Synthesis, Ltd.
- Kiverdi, Inc. (September 5, 2012). "Kiverdi Receives Energy Commission Funding for Its Pioneering Carbon Conversion Platform" (press release). Retrieved September 12, 2012.
- DiPietro, Phil; Nichols, Chris; Marquis, Michael (January 2011). Coal-Fired Power Plants in the United States: Examination of the Costs of Retrofitting with CO2 Capture Technology, Revision 3 (PDF) (report NETL-402/102309). National Energy Technology Laboratory, U.S. Department of Energy. DOE contract DE-AC26-04NT41817. Retrieved September 7, 2012.
- Steinberg, Meyer (August 1995). The Carnol Process for CO2 Mitigation from Power Plants and the Transportation Sector (PDF) (informal report BNL–62110). Upton, New York: Department of Advanced Technology, Brookhaven National Laboratory. (Prepared for the U.S. Department of Energy under Contract No. DE-AC02-76CH00016). Retrieved September 7, 2012.
- Beller, M.; Steinberg, M. (November 1965). Liquid fuel synthesis using nuclear power in a mobile energy depot system (research report BNL 955 / T–396). Upton, New York: Brookhaven National Laboratory, under contract with the U.S. Atomic Energy Commission. (General, Miscellaneous, and Progress Reports — TID–4500, 46th Ed.). Retrieved September 7, 2012.
- Bushore, U.S. Navy Lieutenant Robin Paul (May 1977). Synthetic Fuel Generation Capabilities of Nuclear Power Plants with Applications to Naval Ship Technology (PDF) (M.Sc. thesis). Cambridge, Mass.: Department of Ocean Engineering, Massachusetts Institute of Technology. Retrieved September 7, 2012.
- Terry, U.S. Navy Lieutenant Kevin B. (June 1995). Synthetic Fuels for Naval Applications Produced Using Shipboard Nuclear Power (PDF) (M.Sc. thesis). Cambridge, Mass.: Department of Nuclear Engineering, Massachusetts Institute of Technology. Retrieved September 7, 2012.
- Steinberg, M. et al. (1984). A Systems Study for the Removal, Recovery and Disposal of Carbon Dioxide from Fossil Power Plants in the U.S. (technical report DOE/CH/0016-2). Washington, D.C.: U.S. Department of Energy, Office of Energy Research, Carbon Dioxide Research Division. Retrieved September 8, 2012.
- McDonald, Thomas M.; Lee, Woo Ram; Mason, Jarad A.; Wiers, Brian M.; Hong, Chang Seop; Long, Jeffrey R. (2012). "Capture of Carbon Dioxide from Air and Flue Gas in the Alkylamine-Appended Metal–Organic Framework mmen-Mg2(dobpdc)". Journal of the American Chemical Society 134 (16): 7056–65. doi:10.1021/ja300034j. — has 10 citing articles as of September 2012, many of which discuss efficiency and cost of air and flue recovery.
- Kulkarni, Ambarish R.; Sholl, David S. (2012). "Analysis of Equilibrium-Based TSA Processes for Direct Capture of CO2 from Air". Industrial and Engineering Chemistry Research 51 (25): 8631–45. doi:10.1021/ie300691c. — claims USD $100/ton CO2 extraction from air, not counting capital expenses.
- Doty Windfuels (Columbia, South Carolina)
- Air Fuel Synthesis, Ltd. (Stockton-on-Tees, UK)
- CoolPlanet Energy Systems (Camarillo, California)
- Cost Model for US Navy Zero Carbon Nuclear Synfuel Process spreadsheet by John Morgan (January 2013; source)
- Interview with Kathy Lewis of the US Naval Research Laboratory