Table of Contents
Introduction
Liquid biofuels are energy carriers produced from biomass that are designed to replace or blend with conventional petroleum fuels such as gasoline, diesel, or jet fuel. In the transport sector, they are particularly important because many vehicles, ships, and aircraft still rely on liquid fuels, and cannot easily be electrified in the short term. This chapter focuses on the main types of liquid biofuels for transport, how they are produced, how they are used in engines and fuel systems, and the key sustainability and technical issues that are specific to these fuels.
Main Types Of Liquid Biofuels For Transport
For road, air, and marine transport, several categories of liquid biofuels are most relevant. At the simplest level, it is useful to distinguish between alcohol-based fuels, biodiesel-type fuels, and advanced or “drop-in” biofuels that are designed to closely resemble fossil fuels in their properties.
Ethanol is the most widely used biofuel in the world and is primarily used as a gasoline substitute or blend component. It is an alcohol, chemically $C_2H_5OH$, and is typically produced by fermenting sugars or starches found in crops such as sugarcane and corn. Biodiesel refers to fatty acid methyl esters, often abbreviated as FAME, produced from vegetable oils, used cooking oils, or animal fats. These fuels are mainly used as a substitute for diesel in compression ignition engines.
Beyond these first-generation fuels, advanced liquid biofuels include hydrotreated vegetable oil, often called HVO or renewable diesel, various forms of biojet or sustainable aviation fuel for aircraft, and other synthetic fuels made from biomass. These advanced fuels are designed to be more compatible with existing fuel infrastructure and engines, and to use a broader range of feedstocks, including residues and wastes.
Production Pathways For Ethanol Fuels
Ethanol production for transport usually follows a biochemical conversion route. The feedstock determines the main processing steps, but the core idea is to convert carbohydrates into fermentable sugars, ferment these sugars into ethanol using microorganisms, and then purify the ethanol.
For sugar-rich crops such as sugarcane or sugar beet, sugars are extracted by crushing or diffusion, then fermented directly. For starch-based crops such as corn or wheat, the starch must first be broken down into simpler sugars using enzymes in a process called saccharification. These sugars are then fermented by yeast to produce ethanol and carbon dioxide. The resulting mixture is distilled to increase the ethanol concentration, and further dehydrated if fuel-grade ethanol is required.
Ethanol used in transport is rarely used pure. More commonly it is blended with gasoline at various proportions, such as E10, which contains 10 percent ethanol by volume, or E85, which can contain up to about 85 percent ethanol when used in specially designed flexible-fuel vehicles. Blending strategies depend on national regulations, vehicle compatibility, and fuel supply conditions.
Production Pathways For Biodiesel (FAME)
Biodiesel in the strict sense usually refers to fatty acid methyl esters produced by a chemical process called transesterification. The feedstock is a triglyceride, typically a vegetable oil such as rapeseed oil, soybean oil, palm oil, or a waste oil such as used cooking oil. In transesterification, these triglycerides react with an alcohol, most commonly methanol, in the presence of a catalyst, to form methyl esters and glycerol as a by-product.
The methyl esters are purified to remove catalyst residues, methanol, and glycerol, and the final product is biodiesel that can be blended with fossil diesel. A common blend in many countries is B5, which contains 5 percent biodiesel, or B7, B10, and higher for specific vehicle fleets. In some cases, engines and fuel systems are designed to operate on much higher blends, such as B20 or even B100, which is pure biodiesel.
The properties of biodiesel, such as viscosity, cold flow behavior, and oxidation stability, are strongly influenced by the type of feedstock oil. Cold climates, for example, require attention to the cloud point and pour point of the fuel in order to avoid problems with fuel gelling in low temperatures.
Advanced Liquid Biofuels And “Drop-in” Fuels
Advanced liquid biofuels aim to overcome some of the technical and sustainability limitations of first-generation biofuels by using different feedstocks and more sophisticated conversion technologies. One key family of advanced fuels is hydrotreated vegetable oil or HVO, also marketed as renewable diesel. Instead of transesterification, HVO production uses hydrotreatment, which is similar to processes in petroleum refineries. In this process, plant oils, animal fats, or waste oils are treated with hydrogen under pressure over a catalyst, which removes oxygen and saturates the molecules.
The product is a hydrocarbon fuel, chemically more similar to fossil diesel than FAME biodiesel. This “drop-in” characteristic means it can generally be used at any blend level in existing diesel engines and infrastructure without modification. HVO can also be tailored to meet specific standards, such as those for aviation fuels, creating hydroprocessed esters and fatty acids jet fuel, often called HEFA jet, which is one of the main types of sustainable aviation fuel.
Other advanced liquid biofuels are produced through thermochemical routes. Biomass can be gasified to produce a mixture of hydrogen and carbon monoxide, called synthesis gas, or syngas. This syngas can then be converted into longer-chain hydrocarbons using Fischer Tropsch synthesis, creating synthetic diesel or synthetic jet fuels. Alternatively, pyrolysis of biomass can produce a bio oil that can be upgraded in a refinery. These advanced technologies are more complex and capital intensive than simple fermentation or transesterification, but they can use a wide range of feedstocks, including agricultural residues, forestry residues, and some types of municipal waste.
Liquid Biofuels In Road Transport
In road transport, liquid biofuels are most commonly used as blending components in conventional gasoline and diesel. Ethanol blends such as E5 or E10, containing 5 or 10 percent ethanol, are already common in many fuel markets. These low blends can usually be used by conventional spark ignition vehicles without modification. Higher blends, such as E85, require compatible engines and fuel systems that can handle different material interactions and combustion characteristics.
Ethanol affects some key fuel properties. It has a higher octane number than gasoline, which can improve engine knocking resistance, but it also has lower energy content per liter. This means that a liter of high-ethanol fuel contains less energy than a liter of pure gasoline, so vehicles running on high ethanol blends may show lower fuel economy measured in kilometers per liter, even if the overall energy conversion in the engine can be efficient.
For diesel engines, biodiesel in the form of FAME is blended with conventional diesel. Typical blends such as B5 or B7 are already compatible with most existing vehicles. Higher blends can alter fuel viscosity, cold start behavior, and can impact fuel filters and seals, particularly in older engines. As with ethanol, biodiesel has a different energy content compared to fossil diesel, and this can have a modest effect on volumetric fuel consumption.
HVO or renewable diesel provides an alternative option in road transport. Because it is chemically closer to fossil diesel, it can be used as a pure fuel or blended at any proportion without the technical constraints associated with FAME. This flexibility is one reason HVO is attracting attention, especially in heavy-duty road transport and bus fleets.
Liquid Biofuels For Aviation
Aviation is one of the most challenging sectors to decarbonize, because aircraft need very energy dense fuels and existing aircraft engines and fuel systems are tightly specified. Currently, most planes use Jet A or Jet A-1 kerosene derived from petroleum. Sustainable aviation fuels, or SAF, are liquid biofuels and other low carbon fuels that can be blended with fossil jet fuel to reduce lifecycle greenhouse gas emissions.
Several production pathways for bio-based SAF are certified. HEFA jet, produced from hydroprocessed esters and fatty acids, is one of the most commercially advanced. It uses similar feedstocks as HVO, such as vegetable oils, used cooking oils, and animal fats. Another pathway uses the alcohol to jet process, where ethanol or other alcohols are converted into jet-range hydrocarbons. Fischer Tropsch synthetic paraffinic kerosene, made from biomass-derived syngas, is another route.
For safety and performance reasons, biojet fuels are usually certified as blending components, not as pure fuels. Typical blends might be up to 50 percent SAF with conventional jet fuel, depending on the specification. The blended fuel must meet the same stringent standards for energy content, freezing point, and other properties as standard jet kerosene. This “drop-in” compatibility is essential, because aircraft and airport infrastructure are global and cannot be easily modified for different fuels at each location.
Liquid Biofuels For Shipping And Other Uses
International shipping has also started to explore the use of liquid biofuels to reduce emissions. Marine engines traditionally use heavy fuel oil or marine diesel, and both biodiesel and other advanced biofuels can be used as partial substitutes. For example, some ships have tested and implemented blends of FAME biodiesel with marine diesel. In these cases, fuel quality, stability, and microbial growth in fuel storage tanks are important considerations.
Advanced biofuels that more closely resemble marine fuels, such as bio-based diesel or bio-oil that has been upgraded to marine fuel specifications, are being explored. The shipping sector is also looking at other alternative fuels such as ammonia or methanol, which can be produced from biomass or renewable electricity. When methanol is produced from biomass, it becomes a biofuel suitable for ships designed to use methanol as a fuel.
Beyond large-scale shipping, liquid biofuels also appear in smaller applications such as agricultural machinery, construction equipment, and backup generators. In many cases, these engines can use similar biodiesel blends as road vehicles, but equipment manufacturers often specify allowable blend levels to avoid technical problems.
Fuel Properties, Engine Compatibility, And Infrastructure
A critical aspect of liquid biofuels for transport is how they fit into existing engines and fuel distribution systems. Fuel properties such as energy density, viscosity, lubricity, cetane number for diesel-type fuels, and octane number for gasoline-type fuels are central to compatibility and performance.
Ethanol, for example, is miscible with water and has different solvent properties compared to gasoline. This can affect seals, gaskets, and fuel lines, especially in older vehicles not designed for ethanol blends. It also can lead to phase separation if water contamination occurs in storage tanks. For biodiesel, properties like higher viscosity, higher cloud point in some cases, and greater tendency to oxidize over time can cause problems if not managed, such as clogged filters or fuel degradation.
Conventional fuel infrastructure, including pipelines, storage tanks, and dispensing equipment, was mainly designed around petroleum fuels. Low-level blends of liquid biofuels can often use this infrastructure without major changes. Higher blends, or pure biofuels, may require separate storage, dedicated tanks, or specific handling procedures to avoid contamination and maintain fuel quality. These logistical issues influence how quickly and at what scale liquid biofuels can penetrate existing fuel markets.
HVO and other drop-in fuels have an advantage because they are chemically similar to fossil fuels and can generally move through the same infrastructure and be used in engines without significant redesign. However, their production costs and feedstock availability are important factors that determine their market share.
Energy Content And Emissions Considerations
Although detailed lifecycle assessment belongs elsewhere, there are some fuel specific aspects worth noting. One important characteristic is the lower heating value of a fuel, commonly expressed in megajoules per liter or per kilogram. Ethanol and biodiesel both have lower energy content per liter than the fossil fuels they replace. For example, the lower heating value of ethanol is roughly two thirds that of gasoline, and biodiesel has slightly less energy per liter than fossil diesel. This affects the distance a vehicle can travel per unit volume of fuel.
For liquid fuels, the useful energy released during combustion is often expressed as the lower heating value (LHV). Comparing LHVs per liter or per kilogram is essential when evaluating fuel economy and planning fuel storage for transport applications.
In terms of engine-out emissions, biodiesel typically burns with lower particulate and carbon monoxide emissions compared to conventional diesel, because of its oxygen content and combustion characteristics. However, some biodiesel blends may lead to higher nitrogen oxide emissions in certain engines if not managed carefully. Ethanol blends in gasoline can reduce carbon monoxide and some hydrocarbon emissions, but again the exact effect depends on the engine and blend level.
When considering greenhouse gas emissions, pure tailpipe measurements do not capture the whole picture, because plants absorbed carbon dioxide while growing. The concept of lifecycle greenhouse gas emissions, from feedstock cultivation through processing and use, is critical for assessing how much biofuels actually reduce emissions compared to fossil fuels. Most policy frameworks for liquid biofuels require some accounting of these lifecycle emissions and set minimum greenhouse gas savings thresholds for biofuels to qualify for support.
Policy Context And Market Uptake
The use of liquid biofuels in transport is strongly influenced by policy. Governments often set blending mandates that require a certain percentage of biofuel in gasoline or diesel. They may also define sustainability criteria that feedstocks and production processes must meet in order for the fuel to count toward targets or to receive subsidies.
Fuel quality standards, such as those for gasoline, diesel, and jet fuel, specify the exact properties that blended fuels must meet. This affects which biofuels can be used and at what blend levels. Certification schemes and traceability systems are often put in place to verify that biofuels meet environmental and social criteria.
Market uptake varies significantly by region. Some countries with abundant sugarcane resources have high shares of ethanol in road transport, while others focus more on biodiesel from oilseeds or waste oils. In aviation and shipping, the use of liquid biofuels is still comparatively small but growing, encouraged by voluntary commitments, corporate climate strategies, and emerging regulations that aim to reduce emissions from international transport.
Future Prospects For Liquid Biofuels In Transport
Looking ahead, liquid biofuels are likely to play different roles in different transport segments. For light duty vehicles, the growing adoption of electric vehicles will reduce the long term demand for gasoline and diesel, and therefore for some forms of biofuel. In contrast, for long haul aviation, heavy duty road transport, and international shipping, liquid fuels remain difficult to replace, so advanced biofuels and other low carbon fuels are expected to be particularly important.
Research and development is focusing on improving conversion efficiencies, expanding the range of sustainable feedstocks, and reducing production costs. Efforts are also under way to improve fuel quality, develop new pathways such as algae based fuels or cellulosic ethanol that make better use of residues and non food biomass, and to integrate biofuel production with other renewable energy systems.
The extent to which liquid biofuels can contribute to a sustainable transport system will depend on how they are produced, what feedstocks they use, how they interact with land, water, and ecosystems, and how they complement other solutions such as electrification, hydrogen, and improved transport efficiency. Within this broader context, understanding their specific technical characteristics and uses in different transport modes is essential for making informed decisions about their role in future energy systems.