Biofuel
Biofuel is a fuel that is produced over a short time span from biomass, rather than by the very slow natural processes involved in the formation of fossil fuels such as oil. Biofuel can be produced from plants or from agricultural, domestic or industrial bio waste. Biofuels are mostly used for transportation, but can also be used for heating and electricity. Biofuels are regarded as a renewable energy source. The use of biofuel has been subject to criticism regarding the "food vs fuel" debate, varied assessments of their sustainability, and ongoing deforestation and biodiversity loss as a result of biofuel production.
In general, biofuels emit fewer greenhouse gas emissions when burned in an engine and are generally considered carbon-neutral fuels as the carbon emitted has been captured from the atmosphere by the crops used in production. However, life-cycle assessments of biofuels have shown large emissions associated with the potential land-use change required to produce additional biofuel feedstocks. The outcomes of lifecycle assessments for biofuels are highly situational and dependent on many factors including the type of feedstock, production routes, data variations, and methodological choices. Estimates about the climate impact from biofuels vary widely based on the methodology and exact situation examined. Therefore, the climate change mitigation potential of biofuel varies considerably: in some scenarios biofuel emission levels are comparable to fossil fuels, and in other scenarios the use of biofuel results in negative emissions.
Global demand for biofuels is predicted to increase by 56% over 2022–2027. By 2027 worldwide biofuel production is expected to supply 5.4% of the world's fuels for transport including 1% of aviation fuel. Demand for aviation biofuel is forecast to increase. However some policy has been criticised for favoring ground transportation over aviation.
The two most common types of biofuel are bioethanol and biodiesel. Brazil is the largest producer of bioethanol, while the EU is the largest producer of biodiesel. The energy content in the global production of bioethanol and biodiesel is 2.2 and 1.8 EJ per year, respectively.
Bioethanol is an alcohol made by fermentation, mostly from carbohydrates produced in sugar or starch crops such as maize, sugarcane, or sweet sorghum. Cellulosic biomass, derived from non-food sources, such as trees and grasses, is also being developed as a feedstock for ethanol production. Ethanol can be used as a fuel for vehicles in its pure form, but it is usually used as a gasoline additive to increase octane ratings and improve vehicle emissions.
Biodiesel is produced from oils or fats using transesterification. It can be used as a fuel for vehicles in its pure form, but it is usually used as a diesel additive to reduce levels of particulates, carbon monoxide, and hydrocarbons from diesel-powered vehicles.
Terminology
The term biofuel is used in different ways. One definition is "Biofuels are biobased products, in solid, liquid, or gaseous forms. They are produced from crops or natural products, such as wood, or agricultural residues, such as molasses and bagasse."Other publications reserve the term biofuel for liquid or gaseous fuels, used for transportation.
The IPCC Sixth Assessment Report defines biofuel as "A fuel, generally in liquid form, produced from biomass. Biofuels include bioethanol from sugarcane, sugar beet or maize, and biodiesel from canola or soybeans.". It goes on to define biomass in this context as "organic material excluding the material that is fossilised or embedded in geological formations". This means that coal or other fossil fuels is not a form of biomass in this context.
Conventional biofuels (first generation)
First-generation biofuels are made from food crops grown on arable land. The crop's sugar, starch, or oil content is converted into biodiesel or ethanol, using transesterification, or yeast fermentation.Advanced biofuels
To avoid a "food versus fuel" dilemma, second-generation biofuels and third-generation biofuels are made from feedstocks which do not directly compete with food or feed crop such as waste products and energy crops. A wide range of renewable residue feedstocks such as those derived from agriculture and forestry activities like rice straw, rice husk, wood chips, and sawdust can be used to produce advanced biofuels through biochemical and thermochemical processes.gThe feedstock used to make the fuels either grow on arable land but are byproducts of the main crop, or they are grown on marginal land. Second-generation feedstocks also include straw, bagasse, perennial grasses, jatropha, waste vegetable oil, municipal solid waste and so forth.
Types
Liquid
Ethanol
Biologically produced alcohols, most commonly ethanol, and less commonly propanol and butanol, are produced by the action of microorganisms and enzymes through the fermentation of sugars or starches or cellulose.The IEA estimates that ethanol production used 20% of sugar supplies and 13% of corn supplies in 2021.Ethanol fuel is the most common biofuel worldwide, particularly in Brazil. Alcohol fuels are produced by fermentation of sugars derived from wheat, corn, sugar beets, sugar cane, molasses and any sugar or starch from which alcoholic beverages such as whiskey, can be made. Production methods used are enzyme digestion, fermentation of the sugars, distillation and drying. The distillation process requires significant energy input to generate heat. Heat is sometimes generated with unsustainable natural gas fossil fuel, but cellulosic biomass such as bagasse is the most common fuel in Brazil, while pellets, wood chips and also waste heat are more common in Europe. Corn-to-ethanol and other food stocks has led to the development of cellulosic ethanol. Ethanol fuel can be combined with gasoline to create a more environmentally friendly fuel though there are more viable substitutions to gasoline such as Butanol.
Other biofuels
Methanol is currently produced from natural gas, a non-renewable fossil fuel. In the future it is hoped to be produced from biomass as biomethanol. This is technically feasible, but the production is currently being postponed for concerns that the economic viability is still pending. The methanol economy is an alternative to the hydrogen economy to be contrasted with today's hydrogen production from natural gas.Butanol is formed by ABE fermentation and experimental modifications of the process show potentially high net energy gains with biobutanol as the only liquid product. Biobutanol is often claimed to provide a direct replacement for gasoline, because it will produce more energy than ethanol and allegedly can be burned "straight" in existing gasoline engines, is less corrosive and less water-soluble than ethanol, and could be distributed via existing infrastructures. Escherichia coli strains have also been successfully engineered to produce butanol by modifying their amino acid metabolism. One drawback to butanol production in E. coli remains the high cost of nutrient rich media, however, recent work has demonstrated E. coli can produce butanol with minimal nutritional supplementation. Biobutanol is sometimes called biogasoline, which is incorrect as it is chemically different, being an alcohol and not a hydrocarbon like gasoline.
Biodiesel
Biodiesel is the most common biofuel in Europe. It is produced from oils or fats using transesterification and is a liquid similar in composition to fossil/mineral diesel. Chemically, it consists mostly of fatty acid methyl esters. Feedstocks for biodiesel include animal fats, vegetable oils, soy, rapeseed, jatropha, mahua, mustard, flax, sunflower, palm oil, hemp, field pennycress, Pongamia pinnata and algae. Pure biodiesel currently reduces emissions with up to 60% compared to diesel Second generation B100., researchers at Australia's CSIRO have been studying safflower oil as an engine lubricant, and researchers at Montana State University's Advanced Fuels Center in the US have been studying the oil's performance in a large diesel engine, with results described as a "breakthrough".Biodiesel can be used in any diesel engine and modified equipment when mixed with mineral diesel. It can also be used in its pure form in diesel engines, but some maintenance and performance problems may occur during wintertime utilization, since the fuel becomes somewhat more viscous at lower temperatures, depending on the feedstock used.
Electronically controlled 'common rail' and 'Unit Injector' type systems from the late 1990s onwards can only use biodiesel blended with conventional diesel fuel. These engines have finely metered and atomized multiple-stage injection systems that are very sensitive to the viscosity of the fuel. Many current-generation diesel engines are designed to run on B100 without altering the engine itself, although this depends on the fuel rail design. Since biodiesel is an effective solvent and cleans residues deposited by mineral diesel, engine filters may need to be replaced more often, as the biofuel dissolves old deposits in the fuel tank and pipes. It also effectively cleans the engine combustion chamber of carbon deposits, helping to maintain efficiency.
Biodiesel is an oxygenated fuel, meaning it contains a reduced amount of carbon and higher hydrogen and oxygen content than fossil diesel. This improves the combustion of biodiesel and reduces the particulate emissions from unburnt carbon. However, using pure biodiesel may increase NOx-emissions Biodiesel is also safe to handle and transport because it is non-toxic and biodegradable, and has a high flash point of about 300 °F compared to petroleum diesel fuel, which has a flash point of 125 °F.
In many European countries, a 5% biodiesel blend is widely used and is available at thousands of gas stations. In France, biodiesel is incorporated at a rate of 8% in the fuel used by all French diesel vehicles. Avril Group produces under the brand Diester, a fifth of 11 million tons of biodiesel consumed annually by the European Union. It is the leading European producer of biodiesel.