James G. Speight - Encyclopedia of Renewable Energy

Co-firing refers to mixing biomass with fossil fuels in conventional power plants. Coal-fired power plants can use co-firing systems to significantly reduce emissions, especially sulfur dioxide emissions. Gasification systems use high temperatures and an oxygen-starved environment to convert biomass into synthesis gas, a mixture of hydrogen and carbon monoxide. The synthesis gas (syngas) can be converted into other fuels or products, burned in a conventional boiler, or used instead of natural gas in a gas turbine. Gas turbines are very much like jet engines, only they turn electric generators instead of propelling a jet. Highly efficient to begin with, they can be made to operate in a “combined cycle,” in which their exhaust gases are used to boil water for steam, a second round of power generation, and even higher efficiency.

Using a similar thermochemical process but different conditions (totally excluding rather than limiting oxygen, in a simplified sense) will pyrolyze biomass to a liquid rather than gasify it. As with syngas, pyrolysis oil can be burned to generate electricity or used as a chemical source for making plastics, adhesives, or other bioproducts.

The natural decay of biomass produces methane, which can be captured and used for power production. In landfills, wells can be drilled to release the methane from decaying organic matter. Then, pipes from each well carry the methane to a central point, where it is filtered and cleaned before burning. This produces electricity and reduces the release of methane (a very potent greenhouse gas) into the atmosphere.

Methane can also be produced from biomass through a process called anaerobic digestion. Natural consortia of bacteria are used to decompose organic matter in the absence of oxygen in closed reactors. Gas suitable for power production is produced, and possibly troublesome wastes (such as those at sewage treatment plants or feedlots) are turned to usable compost.

Gasification, anaerobic digestion, and other biomass power technologies can be used in small, modular systems with internal combustion or other generators. These could be helpful for providing electrical power to villages remote from the electrical grid—particularly if they can use the waste heat for crop drying or other local industries. Small, modular systems can also fit well with distributed energy generation systems.

See also: Bioenergy, Biofuels, Biomass to Energy.

A bioprocess is any process that uses complete living cells or organisms or their components (e.g., bacteria, enzymes) to effect desired a physical change and/or a chemical change in the feedstock. Transport of energy and mass is fundamental to many biological and environmental processes.

Modern bioprocess technology used this principle and is actually an extension of older methods for developing useful products by taking advantage of natural biological activities. Although more sophisticated, modern bioprocess technology is based on the same principle: combining living matter (whole organisms or enzymes) with nutrients under the conditions necessary to make the desired end product. Bioprocesses have become widely used in several fields of commercial biotechnology, such as production of enzymes (used, for example, in food processing and waste management) and antibiotics.

Since bioprocesses use living material, they offer several advantages over conventional chemical methods of production. Bioprocesses usually require lower temperature, pressure, and pH (the measure of acidity) and can use renewable resources (biomass) as raw materials. In addition, greater quantities can be produced with less energy consumption.

In most bioprocesses, enzymes are used to catalyze the biochemical reactions of whole microorganisms or their cellular components. The biological catalyst causes the reactions to occur but is not changed. After a series of such reactions (which take place in large vessels (fermenters or fermentation tanks), the initial raw materials are chemically changed to form the desired end product. Nevertheless, there are challenges to the use of bioprocesses in the production of synthetic fuels.

First, the conditions under which the reactions occur must be rigidly maintained. Temperature, pressure, pH, oxygen content, and flow rate are some of the process parameters that must be kept at specific levels. With the development of automated and computerized equipment, it is becoming much easier to accurately monitor reaction conditions and thus increase production efficiency.

Second, the reactions can result in the formation of many unwanted by-products. The presence of contaminating waste material often poses a two-fold problem related to (i) the means to recover (or separate) the end product in a way that leaves as little residue as possible in the catalytic system, and (ii) the means by which the desired product can be isolated in pure form.

See also: Bioconversion Platform.

One of the major areas of research concerning landfill is the use of bioreactors. A bioreactor is formed under specific land filling conditions. Bioreactor land filling is a process in which water and air are circulated into a specially-designed landfill, in order to cause accelerated biological decomposition of the waste material. The intention for this type of landfill operation is to maximize the generation of biogas, which is captured using a network of perforated pipes and burnt to generate electricity. Another desired outcome is the rapid stabilization of organic waste material (in order to minimize the length of time required to manage the landfill site or to make use of the decomposed material as compost).

By adding and recalculating liquids in bioreactors, decomposition is accelerated under anaerobic conditions, increasing the production of landfill gas by 2 to 10 times, approximately half of which is methane with at least 23 times the warming potential as carbon dioxide. The result is to shift substantial volumes of methane production, which otherwise would not occur for decades hence, to the present.

In the landfill, which acts as a bioreactor, anaerobic digestion occurs insofar as the naturally occurring processes of anaerobic degradation are harnessed and contained. Anaerobic digestion has a long history dating back to the 10 thcentury BC and involves four stages which are (i) hydrolysis, (ii) acidogenesis, (iii) acetogenesis, and (iv) methanogenesis. These stages result from the biological treatment of organic waste by two key bacterial groups – acetogens, and methanogens.

The hydrolysis stage is the chemical reaction where complex organic molecules are broken down into simple sugars, amino acids, and fatty acids with the addition of hydroxyl groups. The acidogenesis stage is the process where a further breakdown by acidogens into simpler molecules, volatile fatty acids occurs, producing ammonia, carbon dioxide, and hydrogen sulfide as by-products. The acetogenesis stage is the biochemical process where the simple molecules from acidogenesis are further digested by acetogens to produce carbon dioxide, hydrogen, and mainly acetic acid. The methanogenesis fourth stage is the biochemical process where methane, carbon dioxide, and water are produced by methanogens.

A simplified overall chemical reaction for the degradation of sugars produced by the hydrolysis of cellulose is:

Thus, the desirable product of the landfill bioreactor is methane. Other products may (depending upon the composition of the waste material in the landfill) include (i) a solid fibrous material, which is spread without further treatment, or after post composting (maturation), to provide organic matter for improvement of soil quality and fertility (improves soil structure and reduces summer irrigation demand) and (ii) a liquid fraction which contains nutrients and can be spread as a fertilizer and sprayed on crops. If the solid and liquid fractions are not separated, the slurry can be spread on the soil.