James G. Speight - Encyclopedia of Renewable Energy

Biofuels are fuels derived from plant materials – entering the market, driven by factors such as oil price spikes and the need for increased energy security. Examples of solid biofuels include wood, sawdust, grass cuttings, domestic refuse, charcoal, agricultural waste, non-food energy crops, and dried manure. Biofuels are also known as non-conventional fuels or alternative fuels. Alternative fuels can be classified as any fuel that is not derived from conventional sources like natural gas, crude oil, and coal.

Other than biofuels, some more examples of alternative fuels are solar, wind and tidal power, hydrogen, air engine power, and non-conventional oil. Since mankind discovered fire, wood has been the first biofuel used for heating and cooking which has also been used to produce electricity, and liquid biofuel have been used in the automotive industry since its inception. Unlike fossil fuels, which are necessarily derived from long deceased and metamorphosed biological organisms, biofuels (otherwise known as agrofuels) are obtained from only recently deceased or from living biological organisms, or in other words, derived from biomass or bio-waste.

When raw biomass is already in a suitable form (such as firewood), it can be combusted directly in a stove or furnace to provide heat or (industrially) to raise steam. When raw biomass is in an inconvenient form (such as sawdust, wood chips, grass, urban waste wood, and agricultural residues), the typical process is to densify the biomass. This process includes grinding the raw biomass to an appropriate particulate size, which depending on the densification type can be from 0.5 to 1.5 in., which is then concentrated into a fuel product. The current types of processes are pellet, cube, or puck. The pellet process is most common in Europe and is typically a pure wood product. The other types of densification are larger in size compared to a pellet and are compatible with a broad range of input feed-stocks. The resulting densified fuel is easier transport and feed into thermal generation systems such as boilers.

Bioprocessing routes have a number of compelling advantages over conventional petrochemicals production ( Table B-9). However, it is only in the last decade that rapid progress in biotechnology has facilitated the commercialization of a number of plant-based chemical processes.

Table B-9Example of sources and use of biomass for energy products.

It is widely recognized that further significant production of plant-based chemicals will only be economically viable in highly integrated and efficient production complexes producing a diverse range of chemical products. Also contrasting with fossil fuels, biofuels do not contribute to the stock of total carbon dioxide in the atmosphere, since the sources of many biofuels are plants which normally remove carbon dioxide from the atmosphere, and then give off the same amount when burned and are, therefore, considered to be carbon dioxide neutral, although some observers would doubt this balance as being the result of mathematical manipulation without taking into consideration other important and influential factors.

In any given location, the type of biofuel used will depend on, among other minor factors, the available natural resources and local energy needed for processing requirements. Some of the main reasons for the shifting of interest from fossil fuels to biofuels are the rising prices of oil and increasing emissions of greenhouse gases, low barriers to entry of biofuels, and increasing government support. There is also a requirement for the utilization of biofuels which is the reduction or total elimination of noxious emissions in an economical manner.

Biofuels can be classified into (i) first generation biofuels, (ii) second generation biofuels, and (iii) third generation biofuels, otherwise known as advanced biofuels, and the composition varies according to the feedstock and conversion techniques used. First-generation biofuels are derived from simple materials, such as mono and disaccharides (simple sugars), amylose and amylopectin (starch), esters of glycerol and fatty acids, free fatty acids and mono and diglycerides (fats and vegetable oils for biodiesel), methanol, ethanol, propanol and butanol (bioalcohols), methyl-tertiary-butyl-ether (or MTBE) and ethyl-tertiary-butyl-ether (or ethyl-t-butyl-ether, ETBE) – also known as bioether derivatives – methane, carbon-dioxide, nitrogen, hydrogen, hydrogen sulfide, oxygen, carbon monoxide and hydrogen (synthesis gas, often referred to as syngas) and cellulose, hemicellulose derivatives and lignin (wood for solid biofuels). Second generation biofuels are typically produced by the biomass to liquid (BTL) technology and are fuels such as ethanol from lignocellulose, biohydrogen, and wood diesel. The third generation type of biofuel is a biofuel from algae otherwise known as oilgae. Botryococcus braunii and Chlorella vulgaris, as well as macroalgae (seaweed), are typically used to produce such biofuels.

In fact, the free fatty acid from non-edible oils can be mixed with crude oil-derived diesel fuel (up to 30% w/w of the non-edible oil with diesel) may be usable without having too much of an adverse effect on the properties and efficiency of the diesel fuel. However, it must be remembered that olefins in fuel (such as gasoline or diesel fuel) can often result in the form of gums that have an adverse effect on fuel flow and efficiency. This tendency can be mitigated if the fatty acids are modified to produce more suitable fuels (i.e., fuels that do not for gum products) that can act as a diesel substitute. One method involves cracking at high temperatures on certain metal oxide catalysts (decarboxylation), which yields hydrocarbon products that can be hydrogenated to the saturated hydrocarbon derivative (n-heptadecane, C 17H 36). Using linoleic acid as the example, the pyrolysis product is heptadecadiene which hydrogenates to heptadecane:

More generally, many food sources and plant crops are the main feedstock for biofuels, and as such, one must take into consideration the deleterious effects of biofuel production on the natural resources of any particular region. These feedstocks may enter into animal as well as human food chains, and as biofuel production has increased, there has been criticism for diverting food away from human consumption, which can lead to food shortages and increased food prices. While the demand for biofuels is directly correlated to the price of oil, other issues also arise such as deforestation, soil erosion, water usage, carbon emissions, and biofuel prices which complicate the food-vsfuel debate with regard to energy balance and efficiency. Henceforth, while discussing the benefits of biofuels, the pitfalls must be kept apparent, before sustainable biofuel production can be realized.

In conclusion, biofuels are classed according to source and type. They are derived from forest, agricultural, or fishery products or municipal wastes, as well as from agro-industry, food industry, and food service by-products and wastes. They may be solid, such as fuelwood, charcoal and wood-pellets; liquid, such as ethanol, biodiesel and pyrolysis oils; or gaseous, such as biogas.