James G. Speight - Encyclopedia of Renewable Energy

Catalysts for the synthesis of higher molecular weight alcohols generally fall mainly into four groups: (i) modified high pressure methanol synthesis catalysts, such as alkali-doped ZnO/Cr 2O 3, (ii) modified low pressure methanol catalysts, such as alkali-doped Cu/ZnO and Cu/ZnO/Al 2O 3, (iii) modified Fischer-Tropsch catalysts, such as alkali-doped CuO/CoO/Al2O3, and (iv) alkali-doped sulfides, such as mainly molybdenum sulfide (MoS 2).

The catalytic synthesis process makes several different alcohols depending, in part, on residence time in the reactor and the nature of the catalyst. The alcohols can be separated by distillation and dried to remove water.

A further aspect of the waste-to-alcohols concept is the use of a plasma field in which temperatures are reputed (but not yet proved) to reach 30,000°C (54,000°F). The feedstock can be materials such as waste coal, used tires, wood wastes, raw sewage, municipal solid wastes, biomass, discarded roofing shingles, coal waste (culm), and discarded corn stalks. The plasma field breaks down the feedstock into their core elements in a clean and efficient manner.

Most efforts for alcohol production have centered on bacteria, enzyme, and acid/solvent hydrolysis fermentation– based ethanol production. However, the action of bacteria and enzymes is often very dependent on the feedstock. In addition, these processes generate a small quantity of ethanol over a period of days and the dilute aqueous ethanol product must be distilled to recover the ethanol. In the gasification-synthesis process, various carbonaceous feedstocks can be used, with appropriate modifications in the synthesis gas production step.

A new system for converting trash into ethanol and methanol could help reduce the amount of waste piling up in landfills while displacing a large fraction of the fossil fuels used to power vehicles in the United States. The technology developed does not incinerate refuse, so it does not produce the pollutants that have historically plagued efforts to convert waste into energy. Instead, the technology vaporizes organic materials to produce synthesis gas that can be used to synthesize a wide variety of fuels and chemicals. In addition to processing municipal waste, the technology can be used to create ethanol out of agricultural biomass waste, providing a potentially less expensive way to make ethanol than current corn-based plants.

The new system makes synthesis gas in two stages. In the first stage, waste is heated in a 1,200°C (2,190°F) chamber into which a controlled amount of oxygen is added to partially oxidize carbon and free hydrogen. Not all of the organic material is converted, and some forms char which is then gasified when researchers pass it through arcs of plasma. The remaining inorganic materials, including toxic substances, are oxidized and incorporated into a pool of molten glass which hardens into a material that can be used for building roads or discarded as a safe material in landfills. The second stage is a catalyst-based process for converting synthesis gas into equal ethanol and methanol.

Ethanol from cellulosic biomass materials (such as agricultural residues, trees, and grasses) is made by first using pretreatment and hydrolysis processes to extract sugars, followed by fermentation of the sugars. Although producing ethanol from cellulosic biomass is currently more costly than producing ethanol from starch crops, several countries (including the United States) have launched biofuel initiatives with the objective of the economic production of ethanol from bio-sources. In the process, the carbohydrate from the biomass is converted to a sugar derivative; this is converted to ethanol in a fermentation process that is similar to the process for brewing beer; typical feedstocks for the process are sugar beets and from molasses.

Ethanol can also be produced by synthesis from the chemical compound ethylene, which is derived from crude oil or natural gas, or by the fermentation of carbohydrates.

Methanol (methyl alcohol, wood alcohol, CH 3OH) is mainly manufactured from natural gas, but biomass can also be gasified to methanol. Methanol can be made with any renewable resource containing carbon such as seaweed, waste wood, and garbage. Methanol is stored and handled like gasoline because it is produced as a liquid. Methanol is currently made from natural gas, but it can also be made from a wide range of renewable biomass sources, such as wood or waste paper.

Methanol can be used as one possible replacement for conventional motor fuels and, like ethyl alcohol, has a high octane rating; hence an Otto engine is preferable. If an ignition booster is used, methanol can be used in a diesel engine.

Methanol also offers important emissions benefits compared with gasoline. It can reduce hydrocarbon emissions by 30 to 40% with M85 and up to 80% with M100 fuels. Methanol costs less than gasoline, but has lower energy content. Taking this into account, costs for methanol in a conventional vehicle are slightly higher than those for gasoline.

However, methanol is not miscible with hydrocarbon derivatives and separation ensues readily in the presence of small quantities of water, particularly with reduction in temperature. Anhydrous ethanol, on the other hand, is completely miscible in all proportions with gasoline, although separation may be effected by water addition or by cooling. If water is already present, the water tolerance is higher for ethanol than for methanol, and can be improved by the addition of higher alcohols, such as butanol. Also, benzene or Acetone can be used.

See also: P-Series Fuels.

See also: Alcohols.

Algaculture is a form of aquaculture involving the farming of species of algae. The majority of algae that are intentionally cultivated fall into the category of microalgae (also referred to as phytoplankton, microphytes, or planktonic algae).

Macroalgae (seaweed) also have many commercial and industrial uses, but due to their size and the specific requirements of the environment in which they need to grow, they do not lend themselves as readily to cultivation. However, as the algae grow and multiply, the culture becomes so dense that it blocks light from reaching deeper into the water. Direct sunlight is too strong for most algae, which need only approximately 10% of the amount of light they receive from direct sunlight.

Algae can be cultured in open-ponds which are vulnerable to contamination by other microorganisms, such as other algal species or bacteria. Thus cultivators usually choose closed systems for monocultures. Open systems also do not offer control over temperature and lighting. The growing season is largely dependent on location and, aside from tropical areas, is limited to the warmer months.

Open pond systems are cheaper to construct, at the minimum requiring only a trench or pond. Large ponds have the largest production capacities relative to other systems of comparable cost. Open pond cultivation can exploit unusual conditions that suit only specific algae and can also work if there is a system of culling the desired algae and inoculating new ponds with a high starting concentration of the desired algae.

Enclosing a pond with a transparent or translucent barrier effectively turns it into a greenhouse. This allows more species to be grown; it allows the species that are being grown to stay dominant; and it extends the growing season – and if heated, the pond can produce year round.

Algae can also be cultured in a photobioreactor (PBR) which is a bioreactor that incorporates a light source. However, because photobioreactor systems are closed, the cultivator must provide all nutrients, including carbon dioxide. A photobioreactor can operate in batch mode, which involves restocking the reactor after each harvest, but it is also possible to grow and harvest continuously. Continuous operation requires precise control of all elements to prevent immediate collapse. The grower provides sterilized water, nutrients, air, and carbon dioxide at the correct rates. This allows the reactor to operate for long periods.