Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 140

Biologically produced alcohols, such as ethanol, propanol and butanol, are produced by the action of microorganisms and enzymes through the fermentation of starches, sugars, or cellulose. Alcohol fuels are produced by fermentation of sugars derived from corn, molasses, sugar beets, sugar cane, wheat, as well as potato and fruit waste.

However, as an example, there is no difference in properties between ethyl alcohol produced in a petrochemical facility and ethanol from biomass; it is merely the use of the prefix “bio” to define the origin of the alcohol. However, in some cases, ethanol (C₂H₅OH) that is derived from crude oil should not be considered safe for consumption as this alcohol contains approximately 5% v/v methanol (CH₃OH) and may cause blindness or death. This mixture may also not be purified by simple distillation, as it forms an azeotropic mixture.

Bioalcohols are a useful type of liquid fuel because they combust rapidly and are often cheap to produce. However, their acceptance is hampered by the fact that their production often requires as much or even more fossil fuel than they replaced since they are typically not primary sources of energy; however, they are a convenient way to store the energy for transportation.

Biomethanol

Methanol (the simplest alcohol, CH₃OH) is the lowest molecular weight and simplest alcohol, produced from natural gas (methane). It is also called methyl alcohol or wood alcohol, the latter because it was formerly produced from the distillation of wood. Currently, the majority of the methanol produced is manufactured from natural gas, a nonrenewable fossil fuel, and modern methanol is also produced in a catalytic industrial process directly from carbon monoxide, carbon dioxide, and hydrogen. However, methanol can also be produced from biomass (as biomethanol) using similar chemical processes.

Bioethanol

Ethanol, also known as grain alcohol or ethyl alcohol, is most commonly used in alcoholic beverages. The ethanol production methods used are enzyme digestion (to release sugars from stored starches), fermentation of the sugars, distillation, and drying. Ethanol is produced mostly from carbohydrates produced in starch and sugar crops such as corn or sugar cane. The distillation process requires significant energy input for heat (often unsustainable natural gas, but cellulosic biomass such as bagasse, the waste left after sugar cane is pressed to extract its juice, can also be used more sustainably). Ethanol can be used as a fuel for vehicles in its pure form, but it is usually used as a gasoline additive to increase octane and improve vehicle emissions.

Two types of grain ethanol production are currently available in the United States, dry milling and wet milling, with the latter predominating among large-scale producers.

Corn dry milling is the most common type of ethanol production in the United States. In the dry grind process, the entire corn kernel is first ground into flour and the starch in the flour is converted to ethanol via fermentation. The other products are carbon dioxide (used in the carbonated beverage industry) and an animal feed called distillers’ dried grain with soluble matter.

Corn wet milling is the process of separating the corn kernel into starch, protein, germ, and fiber in an aqueous medium prior to fermentation. The primary products of wet milling include starch and starch-derived products (e.g., high fructose corn syrup and ethanol), corn oil, and corn gluten.

Ethanol can also be produced from other forms of biomass that do not fall under the nomenclature of food crops. One set of production processes is applicable to what is known as cellulosic biomass, including fast-growing fuel crops, agricultural waste such as bagasse and corn stover, and forest and wood processing waste, while a second set of techniques can make use of practically any form of inexpensive biomass including the cellulosic sort as well as the black liquor produced by paper mills, animal wastes, and some types of landfill wastes.

Ethanol can be readily produced by fermentation of simple sugars that are converted from starch crops. Feedstocks for such fermentation ethanol include corn, barley, potato, rice, and wheat. This type of ethanol may be called grain ethanol, whereas ethanol produced from cellulose biomass such as trees and grasses is called bioethanol or biomass ethanol.” Both grain ethanol and bioethanol are produced via biochemical processes, while chemical ethanol is synthesized by chemical synthesis routes that do not involve fermentation.

Cellulosic biofuels, such as cellulosic ethanol, began to be produced in commercial-scale plants in 2013. These fuels are made from cellulose-containing organic material. Cellulose forms the primary structural component of green plants and is by far the most abundant organic (carbon-containing) compound on Earth. The primary cell wall of green plants is made primarily of cellulose; the secondary wall contains cellulose with variable amounts of lignin. Lignin and cellulose, considered together, are termed lignocellulose, which (as wood) is the most common biopolymer on Earth. Cellulosic biomass, derived from non-food sources, such as trees and grasses, is also being developed as a feedstock for ethanol production. Even dry ethanol has roughly one-third lower energy content per unit of volume compared to gasoline, so larger (and therefore heavier) fuel tanks are required to travel the same distance, or more fuel stops are required.

Ethanol may also be used as a fuel, most often in combination with gasoline. For the most part, it is used in a 9:1 ratio of gasoline to ethanol to reduce the negative environmental effects of gasoline. There is increasing interest in the use of a blend of 85% fuel ethanol blended with 15% gasoline. This fuel blend, called E85, has a higher fuel octane than premium gasoline, allowing in properly optimized engine increases in both power and fuel economy over gasoline.

Biopropanol

Propanol (propyl alcohol, C₃H₇OH) is the next member of the alcohol series followed by butanol (butyl alcohol, C₄H₉OH). Both alcohols may be used as a fuel with the normal combustion engine. The advantages of propanol and butanol are the high octane rating (over 100) and high energy content, only approximately 10% lower than gasoline, and subsequently more energy-dense than methanol and ethanol. The major disadvantage of propanol and butanol as fuels for the internal combustion engine is the relative high flashpoint of these alcohols.

Biobutanol

Butanol can be produced from biomass (as biobutanol) as well as fossil fuels, but biobutanol and crude oil-derived butanol (sometimes referred to as petro-butanol) have the same chemical properties. Butanol may be used as a fuel in an internal combustion engine. Because its longer hydrocarbon chain causes it to be fairly non-polar, it is more similar to gasoline than it is to ethanol. Butanol has been demonstrated to work in vehicles designed for use with gasoline without modification, and is thus often claimed to provide a direct replacement for gasoline (in a similar way to biodiesel in diesel engines). Biobutanol has the advantage in combustion engines in that its energy density is closer to gasoline than the simpler alcohols (while still retaining over 25% higher octane rating); however, biobutanol is currently more difficult to produce than ethanol or methanol.

See also: Biodiesel, Biofuels, Biogas, Vegetable Oil.