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Wood Energy ConversionTypes of conversion processes - Charcoal production - Briquetting - RWEDP activities - Further reading - Other issues in wood energy Before energy can be used for end-use activities, it often has to be converted from its primary form into a form that is more convenient for transport and use. Also wood is converted into other forms for energy purposes, such as charcoal, briquettes, gas, ethanol and electricity. Wood energy conversion technologies range from simple, traditional processes such as charcoal production in earth mounds to modern, highly efficient processes, such as dendro power and co-generation. Types of Conversion ProcessesThe main processes of woodfuel conversion are listed below:
Generally, biomass fuels are consumed by direct combustion in their primary form after drying and resizing, or in the form of charcoal. New conversion technologies such as gasification and anaerobic digestion have been developed to provide alternatives and to match biomass resources with modern end-use devices. However, as of yet these technologies are not widely used, so also in the near future wood energy conversion will consist mainly of resizing, drying and charcoal production. Nevertheless, in recent years there has been increasing use of wood and other biomass for energy conversion in modern applications, such as combined heat and power generation (CHP) and co-generation. Charcoal Production
The oldest and probably still the most widely used method for charcoal production is the earth kiln. Two varieties exist, the earth pit kiln and the earth mound kiln. An earth pit kiln is constructed by first digging a small pit in the ground. Then the wood is placed in the pit and lit from the bottom, after which the pit is first covered with green leaves or metal sheets and then with earth to prevent complete burning of the wood. The earth mound kiln is built by covering a mound or pile of wood on the ground with earth. The mound is preferred over the pit where the soil is rocky, hard or shallow, or the water table is close to the surface. Mounds can also be built over a long period, by stacking gathered wood in position and allowing it to dry before covering and burning.
Several other types of charcoal kilns have been developed, which generally have higher efficiencies but also require higher investments than the earth kiln. Two often-used types are fixed kilns made of mud, clay, bricks, and portable steel kilns. Fixed kilns usually have a beehive shape. Smaller beehives are usually made of mud and are not very durable. Larger beehives are made of bricks and have external chimneys. Beehive kilns have an opening for loading the wood and unloading the charcoal, which is closed after loading. Portable steel kilns can be made from oil drums, and can be used both in horizontal and vertical position. They generally have a short lifetime. When used in the horizontal position, an opening is made in the side, through which the wood is loaded. For the vertical kiln the top is cut out and used as a lid. BriquettingBriquetting or densification is used to improve characteristics of materials for transport and use as energy source. Raw materials include sawdust, loose crop residues, and charcoal fines. The material is compacted under pressure, and depending on the material, the pressure, and the speed of densification, additional binders may be needed to bind the material. The two main briquetting technologies are the piston press and the screw press. In the piston press the material is punched into a die by a ram with a high pressure. In the screw press, the material is compacted continuously by a screw. With the screw press, generally briquettes of higher quality can be produced. RWEDP activitiesIn cooperation with local organizations from Indonesia and Japan, RWEDP held a training of trainers on charcoal making to introduce new technologies. Participants came from selected organizations in countries where charcoal is an important fuel. On briquetting, RWEDP published an overview of the main technologies and practices. Further reading:
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