Bioenergy

Bioenergy is energy derived using organic material, especially plant matter, as fuel. The material burned or processed to produce bioenergy (the "feedstock") is called biomass. Biomass has been an energy source for as long as humans have used wood fires to warm themselves and cook food. Wood is still the most commonly used biomass fuel. In some developing countries, crop and logging residues, and a dried mixture of straw and animal dung are also common biomass fuels.

Unlike most other sources of energy, biomass is a potentially renewable resource. By definition, bioenergy sources also include coal, petroleum, and natural gas, because these fossil fuels are derived from ancient plant biomass laid down in early geologic ages. However, these are non-renewable sources of energy and by convention are omitted when discussing bioenergy. In fact, the big advantage of bioenergy is that its use reduces the use of such non-renewable energy sources.

There are three major ways in which the energy in plants is utilized: direct burning, conversion to gas, and conversion to alcohol.

Direct burning

Biomass materials can be burned directly as fuel, as when wood logs are put on a fire. A major environmental problem in many developing countries is that forests are disappearing as people use up nearby trees as fuel for cooking. Without a forest cover to hold soil, the land can erode and become unproductive, making it difficult to raise crops or regenerate another forest. Deforestation has contributed to severe environmental damage, including widespread famine, in large regions of Africa and elsewhere. The direct burning of biomass also releases many of the same pollutants into the air as does the burning of fossil fuels.

Less damaging than deforestation of natural forest is the use of biomass grown specifically for that purpose. As fossil-fuel resources become scarce, such bioenergy crops will undoubtedly be much more important sources of energy in the future. The organic material in municipal and industrial solid waste is also a biomass fuel, and can be combusted in "waste-to-energy" incinerators, which generate electricity for local use.

Conversion to gas

Biomass can also be converted into methane gas, also called biogas, which can be burned as a source of energy. When bacteria digest organic materials in the absence of oxygen (anaerobic digestion), the gas produced is about two-thirds methane, CH₄, which is the main component in natural gas. Methane is also produced in landfills as organic waste is digested anaerobically. The gas can be collected and piped out of landfills and used as a fuel. Biogas can also be obtained from the anaerobic digestion of sewage sludge.

In China, many farmers use small closed pits as anaerobic digesters. Agricultural waste and sewage is placed in the pit, and the biogas given off is used as a fuel for cooking. On a larger scale, some dairy farmers in the United States have begun to produce biogas from cow manure. The gas is used to run electrical generators. In addition, the heat given off by the generators can be fed back into the manure digesters to speed up the process, as well as into the barns for space heating.

Conversion to alcohol

About one-fourth of all energy use in the United States is for transportation. Biomass can be converted by microbial fermentation into liquid alcohol, which can be used to fuel vehicle engines. The fermentation is essentially the same natural process that has been used to make alcoholic beverages since civilization began: yeast feeds on sugars and starches in the plant biomass, producing ethyl alcohol, or ethanol, (C₂H₆O; also written as CH₃CH₂OH). A ratio of 1 part ethanol to 9 parts gasoline is used to produce a fuel known as gasohol, which can be burned in a standard automobile engine. Ethanol can also be used alone in engines that have been slightly modified. Brazil, which makes ethanol from sugarcane, has many cars that run on gasohol. Ethanol has a higher octane rating and produces less carbon monoxide than gasoline.

Most ethanol in the United States has been by a fermentation of corn and sorghum grain. This can be done in the U.S., where there is an excess of farmland, but in most countries the agricultural land is needed to grow food. Researchers are working on solving the problems of converting cornstalks, instead of grain, into ethanol. The industrial fermentation results in some emissions of pollutants to the atmosphere from the fossil fuel used in the distillation process. However, the burning of ethanol in vehicles can reduce the amount of carbon monoxide emitted.

The fermentation of organic material also gives off carbon monoxide and hydrogen gas, which can be heated in the presence of a catalyst to synthesize a poisonous but energy-efficient alcohol called methanol, or wood alcohol (CH₄O or CH₃OH). Methanol can also be substituted for gasoline in motor vehicles and other machinery. However, compared with the production of ethanol, that of methanol is not so efficient, and its wider use as a fuel is not yet commercially realistic. Also, emissions given off by burning methanol include toxic formaldehyde, although this could be controlled by pollution control equipment on vehicles.

There are four major sources of biomass: agricultural and forestry residues, municipal solid waste, industrial waste, and specifically grown bioenergy crops.

Agricultural and forest residues

In some respects, the conversion of crop-quality biomass to energy is a dubious strategy. It is less efficient to produce biofuel than food on Earth's limited arable land. However, many people in developing countries use crop residues and woody debris left after logging as a fuel source. Although they may seem a viable energy resource, crop and forest residues are often parts of plants containing high concentrations of nutrients. If the residues are harvested as biofuel, the nutrients are lost to the field or forest, and synthetic chemical fertilizers may have to be applied later. However, the burning of secondary wastes from processing, such as rice hulls, sawdust, or paper sludge, is an efficient use of material that might otherwise have been wasted.

Municipal solid waste

Municipal solid waste (MSW), much of which is organic, is being incinerated in some cities both as a means of keeping the waste out of landfills, and also as a way to acquire relatively inexpensive fuel to generate electrical power. On average, the burning of one ton of typical MSW produces as much heat as one barrel of oil. More than 70 waste-to-energy plants are now installed in the United States, and about that many others are being planned.

However, unexpected problems can arise in cities that do such a good job of recycling that not enough high-energy organic waste is available for the waste-to-energy incinerator. Plastics, for example, are a high-energy waste that can be efficiently recycled, while food waste can be composted. Another problem is that the content of MSW is usually not known. It often contains materials that after incineration can send toxic pollutants into the air, or that make the final residue toxic, so it must be treated as hazardous waste.

Industrial waste

Some industrial wastes are a good source of bioenergy, especially if its use involves a cogeneration facility. Cogeneration is the use of heat or material left over from manufacturing, often in combination with a conventional fuel such as oil, to produce electricity. The electricity can be used to run the factory, with any power left over put into the regional transmission grid. The U.S. pulp and paper industry satisfies about 8% of its energy needs by the cogeneration of wood waste. In South Florida's Palm Beach County, an extremely large cogeneration facility produces enough electricity to power 46,000 homes. It burns bagasse, a byproduct of milling sugar, plus wood waste obtained from building and demolition firms.

Bioenergy crops

Wood and other high-biomass crops can be grown specifically for use as bioenergy sources. The growing of annual crops such as corn may not be efficient enough in the long run for this purpose; too much labor and energy are required to grow this annual plant. However, perennial grasses such as switchgrass can be more efficiently cultivated for bioenergy. The grass biomass can be compressed into dense pellets, and efficiently transported and burned in this form. Woody crops, such as high-yield hybrid poplar trees, can be grown in plantations with the biomass harvested on a 3-to-10-year cycle, and then regenerated from stump sprouts. New trees might have to be replanted every 15-20 years.

The best biomass crops grow fast, use nutrients and water efficiently, grow densely, are hardy, are perennial, and regenerate easily after harvesting. Ideally, they would also have nitrogen-fixing capability, which would limit the need for fertilizer application. In the future, bioengineering may increase the nitrogen-fixing capability of certain plants and increase their rate of productivity. Bioengineering has already improved the bioenergy qualities of such trees as black cottonwood and hybrid poplar. Other trees with good potential as bioenergy sources include eucalyptus, sweetgum, and black locust.

Although the burning or conversion of biomass does not fully relieve pollution of the atmosphere, it does have several major benefits. In many regions, biomass is more reliable than solar or wind energy. This is because the energy in plants is captured and stored, while in solar and wind energy this must be done by manufactured technology. Another advantage of bioenergy is that it can be produced using organic waste material that might otherwise be discarded; this saves the environmental and economic costs of their disposal. Used in mass quantities, bioenergy could boost the economy of any nation that must now import fossil fuels. If crops grown for their biomass increase the biomass of growing plants on the planet, this would reduce the amount of carbon dioxide in the atmosphere. Perhaps the most significant advantage of bioenergy is that it is a potentially renewable natural resource that would help supply energy needs indefinitely.

However, there are some disadvantages to using bioenergy. Biomass has a smaller energy content for its bulk than fossil fuels. Therefore the costs of labor, transportation, and storage are higher. Water and nutrients, which are in short supply in many areas, must be used to grow biomass crops.

Perhaps the major difficulty with bioenergy, however, is the same problem that has arisen with recycling. People will not demand bioenergy until there is a considerable cost saving in doing so, but there will not be much savings until there is a much larger demand for bioenergy, or the non-renewable sources become significantly more expensive.