James G. Speight - Encyclopedia of Renewable Energy

See also: Geothermal Energy, Hydrogen, Hydroelectric Energy, Nuclear Energy, Ocean Energy, Solar Energy, Tidal Energy, Waste, Wind Energy.

Put simply, a biofuel is a fuel that is produced through contemporary processes from a biological source (biomass) rather than a fuel produced by the geological processes involved in the formation of fossil fuels. In addition, the word biofuel is usually reserved for liquid or gaseous fuels, used for transportation. Biofuels can be produced from plants (such as energy crops), or from agricultural, commercial, domestic, and/or industrial wastes (if the waste has a biological origin).

Renewable biofuel sources generally involve carbon fixation, such as those that occur in plants or in micro-algae through the process of photosynthesis. Furthermore, some observers argue that biofuel can be carbon-neutral because all biomass crops sequester carbon to a certain extent because all crops move carbon dioxide from aboveground circulation to below-ground storage in the roots and the surrounding soil. However, the simple proposal that biofuel is carbon-neutral requires that the total carbon sequestered by the root system of the energy crop must compensate for all the above-ground emissions which must include any emissions caused by direct or indirect land-use change. Many first generation biofuel projects are not carbon-neutral given these demands – in fact, some may even have higher total emissions of greenhouse gases than some fossil-based alternatives.

Generally, the classification of biofuels is related to the method of preparation which, in turn, is related to the starting material of the agent by which the biomass if converted to biofuels ( Table B-10). For convenience, biofuels are generally classified as being (i) first generation biofuels, (ii) second generation biofuels, and (iii) third generation biofuels. Examples of solid biofuels include wood, sawdust, grass cuttings, domestic refuse, charcoal, agricultural waste, non-food energy crops, and dried manure.

Table B-10General classification of biofuels.

When raw biomass is already in a suitable form (such as firewood), it can burn directly in a stove or furnace to provide heat or 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 1 to 3 cm, 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 feedstocks. The resulting densified fuel is easier to transport and feed into thermal generation systems such as boilers.

Raw biomass emits considerable amounts of pollutants such as particulate matter and polyaromatic hydrocarbon derivatives, (PAHs also known as polynuclear aromatic hydrocarbon derivative, PNAs, PAHs) during combustion. Nevertheless, biomass fuels appear to have significantly less impact on the environment than fuels based on fossil sources.

A derivative of solid biofuel is biochar, which is produced by biomass pyrolysis. Biochar made from agricultural waste can substitute for wood charcoal. As wood stock becomes scarce, this alternative fuel is gaining popularity.

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. Table 10 gives the biofuel yields for different feedstocks and countries.

See also: Alcohols, Algae Fuel, Bioalcohol, Bioethanol, Biodiesel.

Currently, there is a focus on biofuels made from crops, such as corn, sugar cane, and soybeans, for use as renewable energy sources. Though it may seem beneficial to use renewable plant materials for biofuel, the use of crop residues and other biomass for biofuels raises many concerns related to major environmental problems, including food shortages and serious destruction of vital soil resources. Tropical countries have the highest potential to produce biofuel crops: higher energy yields, better greenhouse gas (GHG) balance if properly produced, lower costs, and in some countries, large reserves of uncultivated cropland. Sugar cane and oil palm are the highest-yielding tropical biofuel crops and consequently provide the greatest carbon offsets. Industrialized countries with biofuels targets (such as the United States and the European Union countries) are unlikely to have the agricultural land base needed to meet their growing demand for current production of biofuels, which are largely produced from food and feed crops (e.g., maize, oil palm, rapeseed, soy).

Bioethanol feedstocks can be divided into three major groups: (i) sucrose-containing feedstocks, such as sugar cane, sugar beet, sweet sorghum, and fruits, (ii) starchy materials, such as corn, milo, wheat, rice, potatoes, cassava, sweet potatoes, and barley, and (iii) lignocellulosic biomass, such as wood, straw, and grasses). In the short term, the production of bioethanol as a vehicular fuel is almost entirely dependent on starch and sugars from existing food crops. The drawback in producing bioethanol from sugar or starch is that the feedstock tends to be expensive and demanded by other applications as well.

Sugarcane as a bio-fuel crop has much expanded in the last decade, yielding anhydrous bio-ethanol (gasoline additive) and hydrated bio-ethanol by fermentation and distillation of sugarcane juice and molasses. The yield of ethanol per hectare, is on the order of 7,000 L per hectare. Brazil is the largest single producer of sugarcane with approximately 31% of the global production and millions of hectares (1 ha = 2.47 ac) of sugarcane under cultivation. Brazilian bio-ethanol is less expensive than that produced in the United States from corn or in Europe from sugar beet, because of shorter processing times, lower labor costs, and lower transport costs and input costs.

Another type of feedstock, which can be used for bioethanol production, is starch- based materials. Starch is a high yield feedstock for bioethanol production, but its hydrolysis is required to produce bioethanol by fermentation. Starch is a biopolymer, defined as a homopolymer consisting of only one monomer, D-glucose. The starch-based bioethanol industry has been commercially viable for approximately 30 years; in that time, tremendous improvements have been made in enzyme efficiency, reducing process costs and time, and increasing bioethanol yields. This type of feedstock is the most utilized for bioethanol production in North America and Europe. Corn and wheat are mainly employed with these purposes. Corn-based bioethanol production in most of the countries assessed is limited, especially compared to the United States.