Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 176

Liquid biofuels, as their name suggests, are fuels derived from biomass and processed to produce a combustible liquid fuel. There are two main categories: (i) alcohol fuels, such as methanol and ethanol, and (ii) vegetable oils, which are derived from plant seeds, such as sunflower, sesame, linseed, and rapeseed.

Methanol is produced by a process of chemical conversion from any biomass with a moisture content of less than 60%. Potential feedstocks include forest and agricultural residues, wood, and various energy crops. As with ethanol, it can either be blended with gasoline to improve the octane rating of the fuel or used in its neat form. Both methanol and ethanol are often preferred fuels for racing cars.

Ethanol is the most widely used liquid biofuel and is produced by fermentation of sugars and starches or cellulosic biomass. Most commercial production of ethanol is from sugar cane or sugar beet, as starches and cellulosic biomass usually require expensive pre-treatment. Ethanol is used as a renewable energy fuel source as well as being used for manufacture of cosmetics, pharmaceuticals, and also for the production of alcoholic beverages.

Vegetable oils are used to produce biodiesel. The process of oil extraction from the biomass is carried out the same way as for extraction of edible oil from plants. There are many crops grown in rural areas of the developing world which are suitable for oil production – sunflower, coconut, cotton seed, palm, rapeseed, soy bean, peanut, hemp, and more. Sunflower oil, for example, has an energy content approximately 85% that of diesel fuel.

There are two well-established technologies for oil extraction: (i) the screw press and (ii) solvent extraction. The simple screw press, which is a device for physically extracting the oil from the plant - this technology is well suited to small-scale production of oil as fuel or as foodstuff in rural areas. The press can be motor-driven or hand-operated. On the other hand, solvent extraction is a chemical process which requires large, sophisticated equipment. This method is more efficient - that is, it extracts a greater percentage of the oil from the plant - but is less suited to rural applications.

Biodiesel production is not complex – the vegetable oil is converted to a useable fuel by adding ethanol or methanol alcohol along with a catalyst to improve the reaction. Small amounts of potassium hydroxide or sodium hydroxide (commonly called lye or caustic soda, which is used in soap making) are used as the catalyst material. Glycerine separates out as the reaction takes place and sinks to the bottom of the container. This removes the component that gums up the engine so that a standard diesel engine can be used. The glycerine can be used as a degreasing soap or refined to make other products.

See also: Bioalcohols, Biodiesel, Biofuels, Cellulosic Biomass, Methanol, Ethanol, Vegetable Oil.