What are the most common alternative fuels?

#1 · September 28, 2023, 8:39 pm

Quote from chief_editor on September 28, 2023, 8:39 pm

With rising oil prices and growing concerns about climate change, there has been increasing interest in developing alternative liquid fuels that can supplement or replace traditional petroleum-based fuels like gasoline and diesel. Alternative liquid fuels, also known as non-conventional fuels, offer a number of potential benefits including reducing dependence on imported oil, lowering greenhouse gas emissions, and providing a domestically produced fuel source. This article will provide an overview of the major types of alternative liquid transportation fuels, including alcohols like ethanol and methanol, biodiesel, renewable diesel, vegetable oils, dimethyl ether (DME), and Fischer-Tropsch liquids. For each fuel, the production process, feedstocks, energy content, emissions profile, and current usage will be examined. The advantages and challenges associated with expanded usage of each alternative fuel will also be explored.

Ethanol

Ethanol, also known as ethyl alcohol, is a clear, colorless liquid alcohol fuel that can be produced through fermentation and distillation of crops high in sugar or starch such as corn, sugarcane, wheat, and sorghum. The fermentation process converts the sugars in the feedstock into ethanol using yeast or bacteria. Ethanol can be used as a blending component in gasoline in low level blends up to 10% (E10) in most vehicles or up to 85% (E85) in flexible fuel vehicles optimized for ethanol. As a renewable, domestically producible alcohol fuel, ethanol displaces the need for petroleum-based gasoline and can help reduce greenhouse gas emissions by 20-30% on a lifecycle basis compared to pure gasoline.

The main advantage of ethanol is that it is a renewable fuel made from agricultural crops that are abundant domestically in many countries around the world. Expanding ethanol production also creates value-added markets for commodities like corn and supports local agricultural economies. The challenges with ethanol include high production costs compared to gasoline, energy intensive distillation requirements, and potential food vs fuel conflicts when using crops like corn as the feedstock source. Critics also argue that the total lifecycle greenhouse gas reductions from corn-based ethanol are minimal once land use change impacts from expanding corn production are accounted for.

Methanol

Methanol, also known as methyl alcohol, is the simplest alcohol fuel. It is a light, volatile, colorless, flammable liquid produced through natural gas reforming or the gasification of sustainable biomass sources like forest harvest waste and agricultural residue. Methanol can be blended with gasoline or used on its own in flex-fuel vehicles optimized for high methanol blends such as M85 or M100. Methanol blends have been extensively used in China and racing applications. As a domestically producible alcohol fuel, methanol can displace petroleum imports, is biodegradable, and reduces tailpipe emissions.

The advantage of methanol is its feedstock flexibility. Methanol can be produced from any carbon-based feedstock, including natural gas, coal, and sustainable biomass which gives it significant production potential from domestic sources. Methanol combustion also produces much lower particulate emissions than gasoline while CO2 emissions can be reduced by 50-60% with biomass-based methanol. Challenges with methanol include its toxicity, corrosiveness, low energy density, and the fact that few existing vehicles can handle high methanol blends. Modifications would be required to deliver, store, and dispense a new methanol fuel infrastructure.

Biodiesel

Biodiesel is a renewable diesel fuel substitute produced through the chemical process of transesterification where organically derived oils like soybean, canola, palm, or waste cooking oil are combined with an alcohol (usually methanol) and a catalyst to produce methyl esters. The resulting fuel is then refined to produce a product comparable in combustion properties to petroleum-based diesel. Biodiesel can be used in any concentration with conventional diesel fuel in existing diesel engines with little or no modification required. The most common blends are B5 and B20 which contain 5% and 20% biodiesel mixed with conventional diesel respectively.

Biodiesel offers significant reductions in lifecycle greenhouse gas emissions of 50-80% compared to petroleum diesel depending on the feedstock used. Major advantages include its renewable, domestic production potential from sources like soybeans and its cleaner emissions profile. Disadvantages include high production costs, cold weather performance issues, high feedstock demand, and limits on the supplies of lower cost waste oils that are available. Most biodiesel today is used in low level petroleum diesel blends.

Renewable Diesel

Renewable diesel is similar in chemical composition to petroleum diesel but is made through modern catalytic refining processes like hydrotreating, gasification, or pyrolysis using feedstocks like crop oils, animal fats, waste greases, and municipal solid waste. The result is a hydrocarbon fuel that is chemically equivalent to conventional petroleum-based diesel. Renewable diesel can be used as a direct replacement for diesel without blending and used in engines and infrastructure designed for conventional diesel fuel.

The main advantage of renewable diesel is it requires no engine modifications and its drop-in compatibility makes it easy to integrate into the existing diesel fuel supply system. The emissions profile of renewable diesel features significantly lower particulate matter compared to regular diesel. Disadvantages are the high cost of production and questions about the scalability of non-food feedstocks availability. Renewable diesel also provides no reduction in tailpipe CO2 emissions compared to conventional diesel.

Vegetable Oils

Vegetable oils are oils extracted from oil crops like soybeans, rapeseed, and palm. The oils have long carbon chains similar to diesel fuel but higher viscosity. Vegetable oils can generally be used as direct replacements for diesel fuel if they are preheated. This requires engine modifications like conversion to multi-fuel engines and fuel preheaters to account for vegetable oils higher viscosity compared to diesel. Vegetable oil fuels like soybean oil or rapeseed oil are biodegradable and provide the advantage of renewable, domestically producible feedstocks. Lifecycle greenhouse gas reductions can reach 60% compared to diesel. Disadvantages include poor cold weather performance and dilution of lubricating oil. Most vegetable oil fuels today are used for niche applications rather than as broad fuel alternatives.

Dimethyl Ether (DME)

Dimethyl ether is a gas at ambient temperatures but can be liquefied under mild pressures. DME is synthetically produced by combining (dehydrating) methanol. As a clean burning fuel, DME can supplement or replace diesel fuel in compression ignition engines with some system modifications. Blends with diesel are also feasible. DME produces virtually no particulate emissions, very low NOx emissions, and reduces CO2 by 80-90% compared to diesel using biomass-based production pathways.

The advantage of DME is it can utilize existing liquid fuel infrastructure. Disadvantages include its lower volumetric energy density compared to diesel and that it is a gas at ambient conditions, requiring pressurized storage. While not widely used today, DME has properties that make it attractive as a potential substitute for diesel in heavy-duty transportation applications if production costs can be reduced in the future through advanced synthesis processes.

Fischer-Tropsch Liquids

Fischer-Tropsch synthesis converts syngas (hydrogen and carbon monoxide) into liquid hydrocarbons that can be refined into diesel and jet fuel substitutes. The syngas feedstock can be derived from coal, natural gas, or biomass sources. Fischer-Tropsch diesel can be used as a direct replacement for petroleum-based diesel while the jet fuel is a kerosene substitute. Fischer-Tropsch fuels are sulfur and aromatics free, providing ultra clean burning characteristics. CO2 reductions depend on the carbon feedstock but can reach 80-95% compared to conventional diesel using biomass resources.

The advantage is these "designer" fuels have very high quality tailored for optimal performance in engines and turbines. The primary challenge is the high production cost and energy requirements of the Fischer-Tropsch process. Currently, Fischer-Tropsch fuels are used mostly in niche applications such as feeding an alternative jet fuel supply. If production costs can be brought down through improved catalysts and processes, the potential to produce these premium grade synfuels from abundant resources like natural gas and biomass could make them a major fuel option.

Summary/Conclusions

In summary, there are a variety of promising alternatives to supplement or replace conventional liquid transportation fuels like gasoline and diesel that can be domestically produced, lower greenhouse gas emissions, and reduce reliance on petroleum. These include alcohols like ethanol and methanol, biodiesel and renewable diesel from fats and oils, Fischer-Tropsch liquids derived from various feedstocks, and potentially DME and vegetable oils with further development. Each comes with its own advantages and challenges. Going forward, investing in research to improve production economics, testing engine and infrastructure compatibility, and comprehensive lifecycle assessments will be key to realizing the potential of these alternative liquid transportation fuel options. An "all of the above" strategy will likely be needed to displace a meaningful amount of petroleum-based fuels, provide security against oil price volatility, and meet emissions reduction goals.

With rising oil prices and growing concerns about climate change, there has been increasing interest in developing alternative liquid fuels that can supplement or replace traditional petroleum-based fuels like gasoline and diesel. Alternative liquid fuels, also known as non-conventional fuels, offer a number of potential benefits including reducing dependence on imported oil, lowering greenhouse gas emissions, and providing a domestically produced fuel source. This article will provide an overview of the major types of alternative liquid transportation fuels, including alcohols like ethanol and methanol, biodiesel, renewable diesel, vegetable oils, dimethyl ether (DME), and Fischer-Tropsch liquids. For each fuel, the production process, feedstocks, energy content, emissions profile, and current usage will be examined. The advantages and challenges associated with expanded usage of each alternative fuel will also be explored.

Ethanol

Ethanol, also known as ethyl alcohol, is a clear, colorless liquid alcohol fuel that can be produced through fermentation and distillation of crops high in sugar or starch such as corn, sugarcane, wheat, and sorghum. The fermentation process converts the sugars in the feedstock into ethanol using yeast or bacteria. Ethanol can be used as a blending component in gasoline in low level blends up to 10% (E10) in most vehicles or up to 85% (E85) in flexible fuel vehicles optimized for ethanol. As a renewable, domestically producible alcohol fuel, ethanol displaces the need for petroleum-based gasoline and can help reduce greenhouse gas emissions by 20-30% on a lifecycle basis compared to pure gasoline.

The main advantage of ethanol is that it is a renewable fuel made from agricultural crops that are abundant domestically in many countries around the world. Expanding ethanol production also creates value-added markets for commodities like corn and supports local agricultural economies. The challenges with ethanol include high production costs compared to gasoline, energy intensive distillation requirements, and potential food vs fuel conflicts when using crops like corn as the feedstock source. Critics also argue that the total lifecycle greenhouse gas reductions from corn-based ethanol are minimal once land use change impacts from expanding corn production are accounted for.

Methanol

Methanol, also known as methyl alcohol, is the simplest alcohol fuel. It is a light, volatile, colorless, flammable liquid produced through natural gas reforming or the gasification of sustainable biomass sources like forest harvest waste and agricultural residue. Methanol can be blended with gasoline or used on its own in flex-fuel vehicles optimized for high methanol blends such as M85 or M100. Methanol blends have been extensively used in China and racing applications. As a domestically producible alcohol fuel, methanol can displace petroleum imports, is biodegradable, and reduces tailpipe emissions.

The advantage of methanol is its feedstock flexibility. Methanol can be produced from any carbon-based feedstock, including natural gas, coal, and sustainable biomass which gives it significant production potential from domestic sources. Methanol combustion also produces much lower particulate emissions than gasoline while CO2 emissions can be reduced by 50-60% with biomass-based methanol. Challenges with methanol include its toxicity, corrosiveness, low energy density, and the fact that few existing vehicles can handle high methanol blends. Modifications would be required to deliver, store, and dispense a new methanol fuel infrastructure.

Biodiesel

Biodiesel is a renewable diesel fuel substitute produced through the chemical process of transesterification where organically derived oils like soybean, canola, palm, or waste cooking oil are combined with an alcohol (usually methanol) and a catalyst to produce methyl esters. The resulting fuel is then refined to produce a product comparable in combustion properties to petroleum-based diesel. Biodiesel can be used in any concentration with conventional diesel fuel in existing diesel engines with little or no modification required. The most common blends are B5 and B20 which contain 5% and 20% biodiesel mixed with conventional diesel respectively.

Biodiesel offers significant reductions in lifecycle greenhouse gas emissions of 50-80% compared to petroleum diesel depending on the feedstock used. Major advantages include its renewable, domestic production potential from sources like soybeans and its cleaner emissions profile. Disadvantages include high production costs, cold weather performance issues, high feedstock demand, and limits on the supplies of lower cost waste oils that are available. Most biodiesel today is used in low level petroleum diesel blends.

Renewable Diesel

Renewable diesel is similar in chemical composition to petroleum diesel but is made through modern catalytic refining processes like hydrotreating, gasification, or pyrolysis using feedstocks like crop oils, animal fats, waste greases, and municipal solid waste. The result is a hydrocarbon fuel that is chemically equivalent to conventional petroleum-based diesel. Renewable diesel can be used as a direct replacement for diesel without blending and used in engines and infrastructure designed for conventional diesel fuel.

The main advantage of renewable diesel is it requires no engine modifications and its drop-in compatibility makes it easy to integrate into the existing diesel fuel supply system. The emissions profile of renewable diesel features significantly lower particulate matter compared to regular diesel. Disadvantages are the high cost of production and questions about the scalability of non-food feedstocks availability. Renewable diesel also provides no reduction in tailpipe CO2 emissions compared to conventional diesel.

Vegetable Oils

Vegetable oils are oils extracted from oil crops like soybeans, rapeseed, and palm. The oils have long carbon chains similar to diesel fuel but higher viscosity. Vegetable oils can generally be used as direct replacements for diesel fuel if they are preheated. This requires engine modifications like conversion to multi-fuel engines and fuel preheaters to account for vegetable oils higher viscosity compared to diesel. Vegetable oil fuels like soybean oil or rapeseed oil are biodegradable and provide the advantage of renewable, domestically producible feedstocks. Lifecycle greenhouse gas reductions can reach 60% compared to diesel. Disadvantages include poor cold weather performance and dilution of lubricating oil. Most vegetable oil fuels today are used for niche applications rather than as broad fuel alternatives.

Dimethyl Ether (DME)

Dimethyl ether is a gas at ambient temperatures but can be liquefied under mild pressures. DME is synthetically produced by combining (dehydrating) methanol. As a clean burning fuel, DME can supplement or replace diesel fuel in compression ignition engines with some system modifications. Blends with diesel are also feasible. DME produces virtually no particulate emissions, very low NOx emissions, and reduces CO2 by 80-90% compared to diesel using biomass-based production pathways.

The advantage of DME is it can utilize existing liquid fuel infrastructure. Disadvantages include its lower volumetric energy density compared to diesel and that it is a gas at ambient conditions, requiring pressurized storage. While not widely used today, DME has properties that make it attractive as a potential substitute for diesel in heavy-duty transportation applications if production costs can be reduced in the future through advanced synthesis processes.

Fischer-Tropsch Liquids

Fischer-Tropsch synthesis converts syngas (hydrogen and carbon monoxide) into liquid hydrocarbons that can be refined into diesel and jet fuel substitutes. The syngas feedstock can be derived from coal, natural gas, or biomass sources. Fischer-Tropsch diesel can be used as a direct replacement for petroleum-based diesel while the jet fuel is a kerosene substitute. Fischer-Tropsch fuels are sulfur and aromatics free, providing ultra clean burning characteristics. CO2 reductions depend on the carbon feedstock but can reach 80-95% compared to conventional diesel using biomass resources.

The advantage is these "designer" fuels have very high quality tailored for optimal performance in engines and turbines. The primary challenge is the high production cost and energy requirements of the Fischer-Tropsch process. Currently, Fischer-Tropsch fuels are used mostly in niche applications such as feeding an alternative jet fuel supply. If production costs can be brought down through improved catalysts and processes, the potential to produce these premium grade synfuels from abundant resources like natural gas and biomass could make them a major fuel option.

Summary/Conclusions

In summary, there are a variety of promising alternatives to supplement or replace conventional liquid transportation fuels like gasoline and diesel that can be domestically produced, lower greenhouse gas emissions, and reduce reliance on petroleum. These include alcohols like ethanol and methanol, biodiesel and renewable diesel from fats and oils, Fischer-Tropsch liquids derived from various feedstocks, and potentially DME and vegetable oils with further development. Each comes with its own advantages and challenges. Going forward, investing in research to improve production economics, testing engine and infrastructure compatibility, and comprehensive lifecycle assessments will be key to realizing the potential of these alternative liquid transportation fuel options. An "all of the above" strategy will likely be needed to displace a meaningful amount of petroleum-based fuels, provide security against oil price volatility, and meet emissions reduction goals.