Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 63

Alcohols are organic compounds that contain at least one hydroxyl functional group (-OH) bound to a saturate carbon atom. The term alcohol originally referred to the primary alcohol ethanol (ethyl alcohol, CH₃CH₂OH or C₂H₅OH), which is used as a drug or as a relaxant-intoxicant in the form of alcoholic beverages. An important class of alcohols, of which methanol (methyl alcohol, CH₃OH) and ethanol are the simplest members, includes all compounds for which the general formula is C_nH_2n+1OH. Simple mono-alcohols (one hydroxyl group per molecule) include the primary alcohol derivatives (RCH₂OH), the secondary alcohol derivatives (R₂CHOH), and the tertiary alcohol derivatives (R₃COH) alcohols.

Methanol is a colorless, odorless, and nearly tasteless alcohol and is also produced from crops and is also used as a fuel. Methanol, like ethanol, burns more completely but releases as much or more carbon dioxide than its gasoline counterpart. The balance is often seen as the various bi-processes that draw carbon dioxide from the atmosphere so there is no net modern release, as there is for fossil fuels.

Methanol and other chemicals were routinely extracted from wood in the 19^th Century and in the early 20^th Century. However, the original route for methanol recovery from biomass was quite different to current routes. Methanol was originally recovered from wood as a by-product of charcoal manufacture, and was often called wood alcohol. Pyrolysis (heating wood in the absence of air) to above 270°C in a retort causes thermal cracking or breakdown of the wood and allows much of the wood to be recovered as charcoal. The watery condensate leaving the retort contained methanol, among other compounds. In 1923, commercial production of methanol from synthesis gas by a catalytic process was commenced. Now, almost all of the methanol used worldwide comes from the processing of natural gas.

In general, methanol production from natural gas feed consists of three steps: (i) generation of synthesis gas – in the case of natural gas feed, synthesis gas production consists of converting methane (CH₄) into carbon monoxide (CO) and hydrogen (H₂) via steam reforming, (ii) synthesis gas upgrading - primarily removal of carbon dioxide, plus any contaminants such as sulfur, and (iii) methanol synthesis and purification - reacting the carbon monoxide, hydrogen, and steam over a catalyst in the presence of a small amount of carbon dioxide and at elevated temperature and pressure. The methanol synthesis is an equilibrium reaction, and excess reactants must be recycled to optimize yields.

Modern methods proposed for the production of methanol from biomass involve the conversion of the biomass to a suitable synthesis gas, after which processing steps are very similar to those developed for methanol from natural gas. However, the gasification techniques proposed are still at a relatively early stage of development using biomass feed and the methods are based on similar techniques used widely already with natural gas as feed.

Before biomass can be gasified, it must be pre-treated to meet the processing constraints of the gasifier. This typically involves size reduction, and drying to keep moisture contents below specific levels. Thereafter, biomass gasification involves heating biomass in the presence of low levels of oxygen (i.e., less than required for complete combustion to carbon dioxide and water). Above certain temperatures, the biomass will break down into a gas stream and a solid residue. The composition of the gas stream is influenced by the operating conditions for the gasifier, with some gasification processes more suited than others to producing a gas for methanol production. In particular, simple gasification with air creates a synthesis gas stream that is diluted with large quantities of nitrogen. This nitrogen is detrimental to subsequent processing to methanol, and so techniques using indirect gasification or an oxygen feed are preferred. For large-scale gasification, pressurized systems are considered to be more economic than atmospheric systems.

Once the economic optimum synthesis gas is available, the methanol synthesis takes place. This typically uses a copper-zinc catalyst at temperatures of 200 to 280°C and pressures of 50 to 100 atmospheres. The crude methanol from the synthesis loop contains water produced during synthesis as well as other minor by-products. Purification is achieved in multistage distillation, with the complexity of distillation dictated by the final methanol purity required.

See also: Alcohols.