contained in a gallon of ethanol (approximately 76,000 Btu). Corn-based ethanol has a net energy balance of 20,000 to 25,000 Btu per gallon, whereas cellulosic ethanol has a net energy balance of more than 60,000 Btu per gallon. In addition, cellulosic ethanol use can reduce greenhouse gas emissions. Cellulosic ethanol can produce an 8 to 10% reduction in greenhouse gas emissions when used in E10 and a 68 to 91% reduction when used in E85.
See also: Agricultural Residue, Agricultural Waste, Biomass, Alcohols.
Alcohol Fuels - Methanol
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 CnH2n+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 19th Century and in the early 20th 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.
Alcohol Fuels – Propanol and Butanol
Propanol and butanol are considerably less toxic and less volatile than methanol. In particular, butanol has a high flashpoint (35°C; 95°F), which is a benefit for fire safety, but may be a difficulty for starting engines in cold weather.
The fermentation processes to produce propanol and butanol from cellulose are fairly tricky to execute, and the Clostridium acetobutylicum currently used to perform these conversions produces an extremely unpleasant smell, and this must be taken into consideration when designing and locating a fermentation plant. This organism also dies when the butanol content of whatever it is fermenting rises to 7%. For comparison, yeast dies when the ethanol content of its feedstock reaches 14%. Specialized strains can tolerate even greater ethanol concentrations -so-called turbo yeast can withstand up to 16% ethanol. However, if ordinary Saccharomyces yeast can be modified to improve its ethanol resistance, scientists may yet one day produce a strain of the Weizmann organism with a butanol resistance higher than the natural boundary of 7%. This would be useful because butanol has a higher energy density than ethanol, and because waste fiber left over from sugar crops used to make ethanol could be made into butanol, raising the alcohol yield of fuel crops without there being a need for more crops to be planted.
See also: Alcohols.
Alcohols
Alcohol is the family name of a group of organic chemical compounds composed of carbon, hydrogen, and oxygen and has fuel properties (Table A-11). The molecules in the series vary in chain length and are composed of a hydrocarbon plus a hydroxyl group. Alcohols are oxygenated fuels insofar as the alcohol molecule has one or more oxygen, which decreases to the combustion heat. Practically, any of the organic molecules of the alcohol family can be used as a fuel. The alcohols can be used for motor fuels are methanol (CH3OH), ethanol (C2H5OH), propanol (C3H7OH), and butanol (C4H9OH). However, only methanol and ethanol fuels are technically and economically suitable for internal combustion engines.
Table A-11 Fuel properties of methanol and ethanol compared to the properties of iso-octane.
| Item | Iso-octane | Methanol | Ethanol |
|---|---|---|---|
| Formula | C8H18 | CH3OH | C2H5OH |
