Encyclopedia of Renewable Energy. James G. Speight

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      Stoichiometry suggests that the carbon monoxide/hydrogen ratio is optimum at 2, but the simultaneous presence of water-gas shift leads to an optimum ratio closer to 1.

      As in other synthesis gas conversion processes, the synthesis of higher molecular weight alcohols generates significant heat and an important aspect is choice of the proper reactor to maintain even temperature control which then maintains catalyst activity and selectivity. In fact, the synthesis of higher molecular weight alcohols is carried out in reactors similar to those used in methanol and Fischer-Tropsch synthesis. These include shell and tube reactors with shell-side cooling, trickle-bed, and slurry bed reactors.

      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/Cr₂O₃, (ii) modified low pressure methanol catalysts, such as alkali-doped Cu/ZnO and Cu/ZnO/Al₂O₃, (iii) modified Fischer-Tropsch catalysts, such as alkali-doped CuO/CoO/Al2O3, and (iv) alkali-doped sulfides, such as mainly molybdenum sulfide (MoS₂).

      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.