Carbon-neutral fuel
Carbon-neutral fuel is fuel which produces no net-greenhouse gas emissions or carbon footprint. In practice, this usually means fuels that are made using carbon dioxide (CO2) as a feedstock. Proposed carbon-neutral fuels can broadly be grouped into synthetic fuels, which are made by chemically hydrogenating carbon dioxide, and biofuels, which are produced using natural CO2-consuming processes like photosynthesis.[1]
The carbon dioxide used to make synthetic fuels may be
If the
Production of synthetic hydrocarbons
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Synthetic hydrocarbons can be produced in chemical reactions between carbon dioxide, which can be captured from power plants or the air, and
Hydrogen fuel is typically prepared by the
Through the
There are a few more fuels that can be created using hydrogen. Formic acid for example can be made by reacting the hydrogen with CO2. Formic acid combined with CO2 can form isobutanol.[15]
Methanol can be 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.
Researchers have also suggested using methanol 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.[16] Larger hydrocarbons[17] and ethanol[18] 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.
Sources of carbon for recycling
The most economical source of carbon for recycling into fuel is
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.[5]
Renewable and nuclear energy costs
Nighttime
In 2010, a team of process chemists led by Heather Willauer of the U.S. Navy, estimates that 100 megawatts of electricity can produce 160 cubic metres (41,000 US gal) of jet fuel per day and shipboard production from nuclear power would cost about $1,600 per cubic metre ($6/US gal). 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.[needs update] Moreover, since the delivery of fuel to a carrier battle group costs about $2,100 per cubic metre ($8/US gal), shipboard production is already much less expensive.[25]
Willauer said seawater is the "best option" for a source of synthetic jet fuel.[26][27] By April 2014, Willauer's team had not yet made fuel to the standard required by military jets,[28][29] but they were able in September 2013 to use the fuel to fly a radio-controlled model airplane powered by a common two-stroke internal combustion engine.[30] Because the process requires a large input of electrical energy, a plausible first step of implementation would be for American nuclear-powered aircraft carriers (the Nimitz-class and the Gerald R. Ford-class) to manufacture their own jet fuel.[31] The U.S. Navy is expected to deploy the technology some time in the 2020s.[26]
In 2023, a study published by the NATO Energy Security Centre of Excellence, concluded that e-fuels offer one of the most promising decarbonization pathways for military mobility across the land, sea and air domains.[32]
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.[33][34]
The
Commercial developments are taking place in
Greenhouse gas remediation
Carbon-neutral fuels can 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.
Capturing CO2 directly from the air, known as direct air capture, 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.[5] 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.[4]
Technologies
Traditional fuels, methanol or ethanol
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.[16] It is possible to convert methanol into gasoline, jet fuel or other hydrocarbons, but that requires additional energy and more complex production facilities.[4] Methanol is slightly more corrosive than traditional fuels, requiring automobile modifications on the order of US$100 each to use it.[5][48]
In 2016, a method using carbon spikes, copper nanoparticles and nitrogen that converts carbon dioxide to ethanol was developed.[49]
Microalgae
This section's factual accuracy is disputed. (February 2020) |
Fuel made from microalgae could potentially have a low carbon footprint and is an active area of research, although no large-scale production system has been commercialized to date. Microalgae are aquatic unicellular organisms. Although they, unlike most plants, have extremely simple cell structures, they are still photoautotrophic, able to use solar energy to convert carbon dioxide into carbohydrates and fats via photosynthesis. These compounds can serve as raw materials for biofuels like bioethanol or biodiesel.[50] Therefore, even though combusting microalgae-based fuel for energy would still produce emissions like any other fuel, it could be close to carbon-neutral if they, as a whole, consumed as much carbon dioxide as is emitted during combustion.
The advantages of microalgae are their higher CO2-fixation efficiency compared to most plants[51] and their ability to thrive in a wide variety of aquatic habitats.[52] Their main disadvantage is their high cost. It has been argued that their unique and highly variable chemical compositions may make it attractive for specific applications.[50]
Microalgae also can be used as livestock feed due to their proteins. Even more, some species of microalgae produce valuable compounds such as pigments and pharmaceuticals.[53]
Production
Two main ways of cultivating microalgae are raceway pond systems and photo-bioreactors. Raceway pond systems are constructed by a closed loop oval channel that has a paddle wheel to circulate water and prevent sedimentation. The channel is open to the air and its depth is in the range of 0.25–0.4 m (0.82–1.31 ft).[50] The pond needs to be kept shallow since self-shading and optical absorption can cause the limitation of light penetration through the solution of algae broth. PBRs's culture medium is constructed by closed transparent array of tubes. It has a central reservoir which circulated the microalgae broth. PBRs is an easier system to be controlled compare to the raceway pond system, yet it costs a larger overall production expenses.[citation needed]
The carbon emissions from microalgae biomass produced in raceway ponds could be compared to the emissions from conventional biodiesel by having inputs of energy and nutrients as carbon-intensive. The corresponding emissions from microalgae biomass produced in PBRs could also be compared and might even exceed the emissions from conventional fossil diesel. The inefficiency is due to the amount of electricity used to pump the algae broth around the system. Using co-product to generate electricity is one strategy that might improve the overall carbon balance. Another thing that needs to be acknowledged is that environmental impacts can also come from water management, carbon dioxide handling, and nutrient supply, several aspects that could constrain system design and implementation options. But, in general, Raceway Pond systems demonstrate a more attractive energy balance than PBR systems.[citation needed]
Economy
Production cost of microalgae-biofuel through implementation of raceway pond systems is dominated by the operational cost which includes labour, raw materials, and utilities. In raceway pond system, during the cultivation process, electricity takes up the largest energy fraction of total operational energy requirements. It is used to circulate the microalgae cultures. It takes up an energy fraction ranging from 22% to 79%.[50] In contrast, capital cost dominates the cost of production of microalgae-biofuel in PBRs. This system has a high installation cost though the operational cost is relatively lower than raceway pond systems.[citation needed]
Microalgae-biofuel production costs a larger amount of money compared to fossil fuel production. The cost estimation of producing microalgae-biofuel is around $3.1 per litre ($11.57/US gal),[54] which is considerably more expensive than conventional gasoline. However, when compared with electrification of the vehicle fleet – a key advantage of such biofuel is the avoidance of the costly distribution of large amounts of electrical energy (as is required to convert existing vehicle fleets to battery electric technology), therein allowing for the re-use of the existing liquid-fuel transportation infrastructure. Biofuel such as ethanol is also greatly more energy dense than current battery technologies (approximately 6x as much[55]) further promoting its economic viability.
Environmental impact
The construction of large-scale microalgae cultivation facilities would inevitably result in negative environmental impacts related to land use change, such as the destruction of existing natural habitats. Microalgae can also under certain conditions emit greenhouse gases, like methane or nitrous oxide, or foul-smelling gases, like hydrogen sulfide, although this has not been widely studied to date. If poorly managed, toxins naturally produced by microalgae may leak into the surrounding soil or ground water.[56]
Production
Water undergoes electrolysis at high temperatures to form hydrogen gas and oxygen gas. The energy to perform this is extracted from renewable sources such as wind power. Then, the hydrogen is reacted with compressed carbon dioxide captured by direct air capture. The reaction produces blue crude which consists of hydrocarbon. The blue crude is then refined to produce high efficiency E-diesel.[57][58] This method is, however, still debatable because with the current production capability it can only produce 3,000 liters in a few months, 0.0002% of the daily production of fuel in the US.[59] Furthermore, the thermodynamic and economic feasibility of this technology have been questioned. An article suggests that this technology does not create an alternative to fossil fuel but rather converting renewable energy into liquid fuel. The article also states that the energy return on energy invested using fossil diesel is 18 times higher than that for e-diesel.[60]
History
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.
See also
- Artificial photosynthesis
- Butanol fuel
- Carbon-neutral hydrogen production
- Carbon cycle re-balancing
- Carbon sink
- Climate change mitigation scenarios
- Climate engineering (geoengineering)
- Compressed CO2 as a fuel
- Fossil-fuel phase-out
- Hydrogen vehicle
- Fourth generation biofuels
- Low-carbon economy
- Sustainable energy
- Synthetic Liquid Fuels Program
References
Books and reports
- Sustainable Synthetic Carbon Based Fuels for Transport. London: Royal Society. 2019. OCLC 1181251736.
Notes
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Further reading
- 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. S2CID 207079044. — has 10 citing articlesas 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. . — claims US$100/ton CO2 extraction from air, not counting capital expenses.
- Holligan, Anna (2019-10-01). "Jet fuel from thin air: Aviation's hope or hype?". BBC News. Retrieved 2019-10-24.
External links
- Doty Windfuels (Columbia, South Carolina)
- Cost Model for US Navy Zero Carbon Nuclear Synfuel Process Archived 2013-05-13 at the Wayback Machine spreadsheet by John Morgan (January 2013; source)
- Interview with Kathy Lewis of the US Naval Research Laboratory