Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 257

The processes available for the production of briquettes are many and, often, not too varied. The production of coal briquettes is closely related to the carbonization process by which fuel-grade coke is produced. In fact, briquettes are manufactured from coals that are normally considered to be of insufficient quality to produce coke. A non-caking coal having a low content of volatile matter might be briquetted with a binder at 750 to 800°C (1,380 to 1,470°F).

A recent emphasis on the use of coal is the production of added-value products using a variety of processes by which the total coal can be used with little wastage of the products. Such is the character of the mild gasification process and a variety of other clean coal processes which have brought related to renewed interest in briquetting and pelletization of coal.

The mild gasification process is a modification of the conventional coal gasification and produces gaseous, liquid, and solid products by heating coal in an oxygen-free reactor. The process is less of a gasification process and more of a pyrolysis process insofar as volatile matter is removed by thermal means to leave a carbonaceous char/residue. The char can be upgraded further to remove both ash and pyritic sulfur, mixed back with coal-derived liquids, and burned in both coal- and oil-fired boilers.

The process is of interest since a slurry of the liquid fuel and the upgraded char has the potential of being a very versatile fuel that can be burned in both coal- and oil-fired boilers. If the char is upgraded to a high degree, even feedstock coal with a high sulfur content can be used without alternating heat rates or capacity factors.

The production of briquettes (and/or pellets) from coal is a thermal decomposition process, akin to the carbonization process.

Carbonization is essentially a thermal treatment process for the production of a carbonaceous residue (with the simultaneous removal of distillate) from a variety of organic substances:

The process may also be referred to as destructive distillation and has been applied to a range of organic materials, but more particularly to naturally-occurring materials such as wood, sugar, and vegetable matter. The carbonaceous residue from the thermal decomposition of coal is usually referred to as coke (which is physically dissimilar from charcoal) and has the more familiar honeycomb-type structure.

The thermal decomposition of the carbonaceous feedstock is a complex sequence of events which can be described in terms of several important physicochemical changes, such as the tendency of the feedstock to soften and flow when heated or the relationship to carbon-type in the feedstock. In fact, some feedstocks become quite fluid at temperatures of the order of 400 to 500°C (750 to 930°F) and there is a considerable variation in the degree of maximum plasticity, the temperature of maximum plasticity, as well as the plasticity temperature range for some feedstocks. Significant changes also occur in the structure of the char during the various stages of devolatilization.

A briquetting plant typically consists of the mixer-dryer with drive, the fan, and the gas ducts. From the mixing chamber, flue gas at 300 to 500°C (570 to 930°F) passes through the coal-binder mixture, suction being provided by the fan. After cooling, the material enters roll presses from the mixer-dryer.

Water, among other factors, influences the wetting and adhesion between coal and bituminous binder and on the strength of the resultant briquette. Indeed, interfacial interactions between the coal and the binder are extremely important in determining the strength of the briquette. The moisture content of the feedstock, before the addition of the bituminous binder, should be as low as possible, preferably not exceeding 4% w/w.

See also: Briquette, Briquette Binder, Briquette Manufacture, Briquette Properties.