Rajan Wilcox
While methane is used as an automotive fuel, several factors hinder its widespread adoption. Firstly, the multi-use nature of natural gas (NG) and varying import sources result in substantial differences in gas composition, which is a critical consideration when using methane as a vehicle fuel. Additionally, the final quality and composition of biomethane depend on operational parameters and upgrading technology, requiring tight control of trace components like siloxanes, hydrogen, water, and hydrogen sulfide to mitigate associated risks. Furthermore, methane-fuelled cars have lower energy consumption than gasoline-fuelled cars, and while methane has a higher hydrogen-carbon ratio, leading to lower or comparable tailpipe CO2 emissions, it has a high global warming potential due to methane slippage. These factors, along with the specific engine requirements and the need for exhaust aftertreatment devices to reduce methane emissions, present challenges to utilizing methane as a primary automotive fuel.
Why methane is not used as a fuel for cars
| Characteristics | Values |
|---|---|
| Energy consumption | Lower than gasoline-fuelled cars |
| Carbon intensity | Better than diesel fuel due to a higher hydrogen-carbon ratio |
| Tailpipe CO2 emissions | Lower than comparable gasoline engines |
| Methane emissions | High global warming potential with emissions from NG-fuelled vehicles hard to reduce for all engine concepts |
| Trace components | Siloxanes, hydrogen, water, and hydrogen sulfide need to be controlled when using biomethane as a vehicle fuel to prevent corrosion, driveability problems, and engine damage |
| Storage and transportation | Compressed methane (CNG, CBG) and liquefied methane (LNG, LBG) are used for storage and transportation; LNG requires on-board cryogenic tanks |
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What You'll Learn
Methane has a high global warming potential
Methane (CH4) has a much shorter atmospheric lifetime than CO2 (around 10-12 years compared with centuries for CO2), but it absorbs much more energy while it exists in the atmosphere. This means that, although methane emitted today will only last about a decade on average, its GWP is much higher than that of CO2. The net effect of this shorter lifetime and higher energy absorption is reflected in the GWP of methane.
The Intergovernmental Panel on Climate Change (IPCC) has indicated a GWP for methane of between 84-87 when considering its impact over a 20-year timeframe (GWP20) and between 27-36 when considering its impact over a 100-year timeframe (GWP100). This means that, for example, a leak of one tonne of methane is equivalent to emitting 81.2-87 tonnes of CO2 measured over 20 years.
Burning methane to CO2 would reduce the global warming impact, but by a smaller factor than 25:1 because the mass of methane burned is less than the mass of CO2 released (a ratio of 1:2.74). This highlights the complexity of considering the global warming potential of different gases and the importance of accurate measurements and calculations.
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Methane engines are less fuel-efficient than gasoline engines
While methane has been considered as an alternative fuel source for automobiles, there are several reasons why it is not yet widely used for this purpose. One important consideration is the fuel efficiency of methane engines compared to traditional gasoline engines.
Methane engines may have lower energy consumption than gasoline engines, but they also come with a unique set of challenges that can impact their overall efficiency. One key issue is the risk of methane slipping as methane emissions, which have a high global warming potential. While methane emissions from natural gas-fuelled vehicles can be reduced using exhaust after-treatment devices, this is not always effective for all engine concepts. For example, older studies have reported varying levels of methane emissions from CNG buses, with some as high as 2750 mg/km.
Additionally, the composition of biomethane, which is often used as a vehicle fuel, can vary depending on its source. This means that several trace components, such as siloxanes, hydrogen, water, and hydrogen sulfide, need to be carefully controlled to mitigate risks such as abrasion, embrittlement of metallic materials, corrosion, and driveability problems. The presence of these impurities in the fuel can impact the overall efficiency of methane engines, as they may require additional treatment processes to ensure safe and proper functioning.
The operational parameters and upgrading technology used also play a role in the final quality and composition of biomethane. This adds another layer of complexity to the use of methane as a fuel source, as the variability in gas composition can impact engine performance and efficiency.
In summary, while methane has the potential to be a viable alternative fuel for automobiles, the challenges associated with methane emissions, fuel composition, and engine compatibility currently limit its widespread adoption. Further advancements in engine technology and fuel processing may help overcome these obstacles and improve the fuel efficiency of methane engines in the future.
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Methane requires different storage conditions than gasoline
The composition of biomethane, in particular, depends on the operational parameters of its final use and the upgrading technology employed. Several trace components, such as siloxanes, hydrogen, water, and hydrogen sulfide, must be carefully monitored when using biomethane as vehicle fuel. These components can cause issues like abrasion, knocking, embrittlement of metallic materials, corrosion, driveability problems, and engine valve damage.
The storage and transportation of methane in a compressed or liquefied state present unique challenges and requirements compared to gasoline. The extreme temperatures and pressures needed for liquefied methane, for example, demand specialised equipment like cryogenic tanks. Additionally, the variability in methane's gas composition, stemming from its diverse import sources and multi-use nature as an energy carrier, adds complexity to its utilisation as automotive fuel.
Furthermore, the carbon intensity of methane is another factor to consider when comparing it with gasoline. Methane's higher hydrogen-carbon ratio leads to lower or comparable tailpipe CO2 emissions than diesel and gasoline engines, depending on engine efficiency. However, methane's high global warming potential, with a GWP of 28 over 100 years, underscores the necessity for careful management of methane emissions from vehicles. The conversion of methane from landfills into energy is an example of how methane can be harnessed as a fuel source while addressing its environmental impact.
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Methane engines have higher hydrocarbon emissions
While methane has been described as a "'clean and promising fuel for internal combustion engines'", there are some challenges associated with its use. One of these challenges is the issue of higher hydrocarbon emissions from methane engines, particularly in the case of methane direct-injection diesel engines.
Several studies have found that as the methane energy share in an engine increases, so do hydrocarbon emissions. For example, Song et al. (2016) analysed methane direct-injection spark ignition engines and found that higher methane supplementation resulted in increased hydrocarbon emissions. Similarly, Zhang et al. (2006) reported that in methane direct-injection diesel engines, higher methane supplementation led to an increase in hydrocarbon emissions. These findings indicate a direct relationship between the amount of methane used and the level of hydrocarbon emissions produced.
The reason for this correlation can be attributed to incomplete combustion. As Tripathi et al. (2020) explain, when more methane is used, a larger proportion of it remains unburned and is emitted as hydrocarbon emissions. This incomplete combustion also results in lower combustion pressure and temperature, which in turn affects NOx emissions. Thus, there is a trade-off between NOx and hydrocarbon emissions when using methane fuel.
However, it is important to note that the problem of higher hydrocarbon emissions in methane engines can be mitigated through various methods. For instance, Liu et al. (2019) suggest that the variation of diesel injection timing can effectively reduce hydrocarbon emissions in methane direct-injection diesel engines. Additionally, the use of efficient exhaust after-treatment devices can help lower emissions in both methane and diesel-fuelled vehicles, bringing them to similar levels.
In conclusion, while methane engines may have higher hydrocarbon emissions, this issue is not without potential solutions. Through further research and the development of more advanced engine technologies, it may be possible to optimise the use of methane as a fuel and minimise its environmental impact.
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Methane requires engine modifications
Using methane as an automotive fuel requires careful consideration of the gas composition, which can vary depending on the import sources and multi-use nature of natural gas. This variation in gas composition is a crucial factor when employing methane as a fuel for vehicles.
When using biomethane, which can be produced locally, several trace components must be meticulously controlled. These include siloxanes, which can cause abrasion and an increased likelihood of knocking; hydrogen, which can lead to embrittlement of metallic materials; water, which can result in corrosion and driveability issues; and hydrogen sulfide (H2S), which can be corrosive in the presence of water and impact after-treatment devices, potentially causing problems with engine valves.
The quality and composition of biomethane is dependent on the operational parameters of its final use and the upgrading technology employed. This means that the final product can vary significantly depending on the specific parameters and technology used during the production process.
To address these challenges, Michael Swain, an associate professor in the Department of Mechanical and Aerospace Engineering, is working on converting engines to run on methane gas. Swain's team purchases used automobile engines and modifies them to create Redesigned during Remanufacture (RDR) internal combustion engines specifically adapted to utilise methane. These modified engines can provide more energy output than larger, more expensive diesel generators, offering a cost-effective solution.
It is worth noting that methane emissions from natural gas-fuelled vehicles can be minimised through the use of exhaust aftertreatment devices, although this may not be applicable to all engine designs.
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Frequently asked questions
Methane is a greenhouse gas with a global warming potential 25 times greater than carbon dioxide. As a fuel, it may slip as methane emissions, which have a high global warming potential.
Several trace components need to be controlled when using methane as a vehicle fuel, including siloxanes, hydrogen, water, and hydrogen sulfide. These components can cause engine problems and corrosion.
Methane-fuelled cars have lower energy consumption than gasoline-fuelled cars. Methane also has a better carbon intensity than diesel fuel due to its higher hydrogen-to-carbon ratio, resulting in lower or comparable tailpipe CO2 emissions.
The Environmental Protection Agency (EPA) has implemented the Landfill Methane Outreach Program (LMOP) to convert methane from landfills into energy at hundreds of sites nationwide. This involves using modified diesel engines to generate energy from methane.
