Energy derived from resources that are regenerative or for all practical purposes can not be depleted. Types of renewable energy resources include moving water (hydro, tidal and wave power), thermal gradients in ocean water, biomass, geothermal energy, solar energy, and wind energy.
Biomass is any sort of vegetation - trees, grasses, plant parts such as leaves, stems and twigs, and ocean plants. From it, we can extract a wealth of stored energy. During photosynthesis, plants combine carbon dioxide from the air and water from the ground to form carbohydrates, which form the building blocks of biomass. The solar energy that drives photosynthesis is stored in the chemical bonds of the structural components of biomass. While the actual ratio of components varies among species, biomass averages 75% carbohydrates or sugars and 25% lignin.
If we burn biomass efficiently (which extracts the energy stored in the chemical bonds), then oxygen from the atmosphere combines with the carbon in plants to produce carbon dioxide and water. Biomass can produce electricity, heat, liquid fuels, gaseous fuels, and a variety of useful chemicals, including those currently manufactured from fossil fuels. Industry and agriculture need superior energy crops and cost-effective conversion technologies to expand the use of renewable biomass. Biomass is available from various industries - including agriculture, forest products, transportation, and construction - that dispose of large quantities of wood and plant products. Whether cultivated or growing wild, biomass represents a huge renewable energy source.
The most common crops grown in the UK for bioenergy are:
SRC consists of densely planted, high-yielding varieties of either willow or poplar, harvested on a 2 to 5 year cycle, although commonly every 3 years. SRC is a woody, perennial crop, the rootstock or stools remaining in the ground after harvest with new shoots emerging the following spring. A plantation could be viable for up to 30 years before re-planting becomes necessary, although this depends on the productivity of the stools. In the UK , yields achievable from willow SRC at first harvest are expected to be in the range 7 to 12 oven dry tonnes per hectare per year depending on site and efficiency of establishment. New varieties are expected to greatly increase yields.
Miscanthus species are woody, perennial, rhizomatous grasses, originating from Asia . Once established the crop can be harvested annually for at least 15 years. By the third year harvestable yields are between 10-13 tonnes per hectare. Peak harvestable yields of 20 tonnes per hectare have been recorded. Miscanthus has a net calorific value, on a dry basis, of 17 MJ/kg, with a 2.7% ash content. The energy value of 20 t of dry miscanthus would be equivalent to that of 12 t of coal.
Around the world other crops are favoured depending on the climate and surrounding condition, both economic and environmental. Brazil has an extensive program for the production of bioethanol from sugar, whilst the U.S. relies heavily on corn for its ethanol production.
Worldwide, biomass is the fourth largest energy resource after coal, oil, and natural gas. It is used for heating (such as wood stoves in homes and for process heat and steam in industries such as pulp and paper), cooking (especially in many parts of the developing world), transportation (fuels such as ethanol and biodiesel) and for electric power generation. It is estimated that there are about 278 Quadrillion Btu of installed biomass capacity worldwide. Most of this capacity is in the pulp and paper industry using combined heat and power systems.
In practice, we tend to use these three different terms for three different end uses - transportation, electric power or heat, and products such as chemicals and materials. "Biofuel" is short for "biomass fuel." We use the term "biofuels" for liquid fuels for transportation, such as ethanol and biodiesel that can be purely from biomass such as B100 or, in part, such as E10 (the number after the letter represents the percentage of biodiesel or ethanol in the fuel). We tend to use "BioPower" for "biomass power" systems that generate electricity or industrial process heat and steam, such as from combined heat and power (CHP) systems. The term "bioproduct" is short for biomass products, and can be used to describe a chemical, material, or other product derived from renewable biomass resources.
Ethanol is the most widely used biofuel today. Ethanol is an alcohol, and most is made using a process similar to brewing beer where starch crops are converted into sugars, the sugars are fermented into ethanol, and then the ethanol is distilled into its final form. Bioethanol is ethanol (C2H5OH) produced by the biological fermentation of carbohydrates derived from plant material. In terms of fuel use, ethanol is mainly of interest as a petrol additive or substitute. Both synthetic ethanol and bioethanol are suitable for fuel use, but synthetic ethanol is classed as a fossil fuel.
The use of bioethanol as a transport fuel dates back to the early part of the century. In the 1920's Henry Ford's Model T was designed to run on alcohol, petrol or any mix of the two. Brazil initiated a national fuel ethanol programme (Proalcohol) in 1975 to produce ethanol for blending with petrol, in order to reduce dependency on imported oil, and a similar programme was established in the USA in 1979. The Brazilian programme was later extended to produce ethanol for use as a complete substitute for petrol, and by 1988 ethanol fuelled a third of the cars in Brazil .
Studies in the US suggest that when bioethanol is produced the reabsorption of carbon dioxide by the growing feedstock can balance its emission from the use of the fuel, and there would be no net increase in carbon dioxide emitted to the atmosphere (Holman et al., 1991). However the studies also point out that, if the crop is grown specifically for bioethanol generation, the inputs of modern agriculture and the emissions produced during the ethanol production would mean that emissions are increased when bioethanol is used.
Ethanol is used to increase octane and improve the emissions quality of petrol. Petrol/ethanol blends may be used in an unmodified engine, although some adjustment may be necessary to optimise performance. There is no clear maximum at which ethanol may be blended for use in unmodified engines. Within the EU 5% blends have been used and in the US blends containing up to 10% are covered by the warranty of all US cars.
On February 3 rd , 2004 the European Commission approved a UK reduction of 20p per litre in excise duty in favour of bioethanol used for road transport. The excise duty reduction shall take effect from January 2005 and last until 31 December 2010 .
In the UK we already apply a reduced rate of excise duty on biodiesel used for road transport. For bioethanol 20p per litre reduction in duty will put it below that on ultra low sulphur petrol and sulphur free fuel. The duty reduction will be granted automatically to any producer of bioethanol in the UK , or importer of bioethanol provided that the fuel concerned meets the specified definition of bioethanol.
Renewable diesel fuels are fuels that are used in diesel engines in place of or blended with petroleum diesel, but are made from renewable resources such as vegetable oils, animal fats, or other types of biomass such as grasses and trees. Biodiesel is an example of a renewable diesel fuel that is available today.
Biodiesel is a fuel that performs like a mineral diesel, or derv, but is derived from a renewable energy source. In the UK interest is mainly focussed on producing biodiesel from oilseed rape. The technical term for the resulting fuel is rape-methyl ester (RME). Any vegetable oil, including used frying oil, generally has potential for the production of biodiesel.
Crude rape seed oil undergoes the process of esterification, which removes the glycerin allowing the oil to perform like mineral diesel. Biodiesel can be mixed with normal diesel, so vehicles require no modifications to use it. No significant differences in engine performance were recorded when comparisons were made between biodiesel and conventional diesel.
Glycerol is a valuable by-product of the reaction, and it is used in over 1,500 applications and products, including drugs, polymers, paints and textiles. The sale of glycerol can offset some of the production costs.
Biodiesel is significantly safer than petroleum-derived diesel; it has a lower flashpoint, and so does not ignite easily, does not produce explosive vapours and even has a low degree of toxicity to humans and animals if ingested. It is biodegradable, so if spilled will not cause lasting damage to the environment. Emission benefits include less particulate matter (the soot associated with diesel vehicles), reduced levels of carbon monoxide and total hydrocarbons, and an improved odour. In contrast to conventional diesel, it is also essentially free of aromatic compounds and sulphur, both of which are toxic and subject to legislation.
e-diesel may be the next new renewable diesel fuel. e-diesel is a blend of ethanol and diesel fuel with other chemicals to improve the performance of the blend. The ethanol portion of e-diesel is renewable because it is made from used vegetable oil.
e-diesel can be blended with conventional diesel in any concentration. The blend level depends on economics and availability. Typically, e-diesel is supplied at 100%, 20% or 5% concentrations blended with Ultra Low Sulphur Diesel. e-diesel is completely bio-degradable and far less toxic than petroleum diesel.
Biomass power is the use of biomass feedstocks instead of conventional fossil fuels (natural gas or coal) to generate electricity or industrial process heat and steam. Biomass is one of the oldest fuels known to humanity. Although basic, the primitive campfire illustrates the nature of using biomass for power. When the biomass is burned, it produces heat. In a power plant, this heat is used to turn water into steam. The steam is then used to turn turbines, which are connected to electric generators. Gasifiers heat the biomass to convert it into a gas that can be used in highly efficient power systems, such as combustion turbines or fuel cells.
Electricity generated from a renewable biomass supply is a Carbon neutral option for power generation and can provide overall Carbon emission reductions when used to replace electricity generated from coal burning. It has been proposed as a feasible option to supply many areas in developing countries that do not yet have an electricity supply.
Renewable bioproducts are products created from plant- or crop-based resources such as agricultural crops and crop residues, forestry, pastures, and rangelands. Many of the products that could be made from renewable bioproducts are now made from petroleum.
In 2003, production of ethanol from biomass reached 2.81 billion gallons and was made primarily from corn (Renewable Fuels Association, Ethanol Industry Outlook 2004). Nearly all gasoline oxygenated to reduce carbon monoxide during winter months contains ethanol, although this is a relatively small market. A modest but growing portion of reformulated gasoline for reducing ground-level ozone (smog) also contains ethanol. This market is growing because of groundwater pollution concerns with and bans on methyl tertiary butyl ether (MTBE), the alternative oxygenate. About half of current ethanol use, however is as an octane booster in regular gasoline. For this market, ethanol competes with petroleum-derived additives such as aromatics and alkylates, as well as MTBE.
Ethanol is not yet being commercially produced from cellulose and hemicellulose, the fibrous sugar polymers that make up the bulk of plant material. However, several companies are moving toward commercial production.