Coal combustion technologies

Sohd fuels are burned in a variety of systems, some of which are similar to those fired by Hquid fuels. In this article the most commonly burned soHd fuel, coal, is discussed. The main coal combustion technologies are fixed-bed, eg, stokers, for the largest particles pulverized-coal for the smallest particles and fluidized-bed for medium size particles (99,100) (see Coal). 

The use of more efficient technologies or process changes can reduce PIC emissions. Advanced coal combustion technologies such as coal gasification and fiuidized-bed combustion are examples of cleaner processes that may lower PICs by approximately 10%. Enclosed coal crushers and grinders emit lower PM. 

The studies on NO formation by the traditional MEIS have been performed at Melentiev Energy Systems Institute for a long time. In parallel with MEIS the use was made of kinetic models and full-scale experiments that assisted in turn to gain information for variant calculations on MEIS. The results of these calculations allowed the conditions for nitrogen oxides formation by different mechanisms to be determined and the ways for improvement of coal combustion technology to increase environmental safety of boiler units to be outlined. 

Coal combustion technology is a mature technology with potentially increasing the efficiency in progress of advanced materials. In the short term and medium term, it will play an important role in the world energy section. In the long term, the role of coal may be reduced but coal will be difficult to replace. 

Moreover, viable clean coal technologies also promote the continued use of coal, thereby offering some degree of energy security to those countries that are net oil importers but having plentiful supplies of coal. Clean coal combustion technologies can reduce emissions of sulfur oxides (SO), nitrogen oxides (NO), and other pollutants at various points of coal use from a mine to a power plant or factory. 

Fluidized-bed combustion It accomplishes coal combustion by mixing the coal with a sorbent such as limestone or other bed material. The fuel and bed material mixture is fluidized during the combustion process to allow complete combustion and removal of sulfur gases. Atmospheric FBC (AFBC) systems may be bubbling (BFBC) or circulating (CFBC). Pressurized FBC (PFBC) is an emerging coal combustion technology. 

Proceedings of the Conference on Coal Combustion Technology and Emission Control, California Institute of Technology, Pasadena, CA. 

Combustion. Coal combustion, not being in the strictest sense a process for the generation of gaseous synfuels, is nevertheless an important use of coal as a source of gaseous fuels. Coal combustion, an old art and probably the oldest known use of this fossil fuel, is an accumulation of complex chemical and physical phenomena. The complexity of coal itself and the variable process parameters all contribute to the overall process (8,10,47—50) (see also COLffiUSTION SCIENCE AND technology). 

The discussion of combustion fundamentals so far has focused on homogeneous systems. Heterogeneous combustion is the terminology often used to refer to the combustion of Hquids and soHds. From a technological viewpoint, combustion of Hquid hydrocarbons, mainly in sprays, and coal combustion are of greatest interest. 

Pulverized-Goal Firing. This is the most common technology used for coal combustion in utiUty appHcations because of the flexibiUty to use a range of coal types in a range of furnace sizes. Nevertheless, the selection of cmshing, combustion, and gas-cleanup equipment remains coal dependent (54,100,101). 

Some of the advantages of fluidized beds include flexibiUty in fuel use, easy removal of SO2, reduced NO production due to relatively low combustion temperatures, simplified operation due to reduced slagging, and finally lower costs in meeting environmental regulations compared to the conventional coal burning technologies. Consequently, fluidized-bed combustors are currently under intensive development and industrial size units (up to 150 MW) are commercially available (Fig. 10). 

Andrus, H.E. et al., Hybrid Combustion-Gasification Chemical Looping Coal Power Technology Development, ALSTOM Technical Report for Project DE-FC26-03NT41866,2006. 

Flagan, R.C. and Friedlander, S.K. "Particle Formation in Pulverized Coal Combustion-A Review," presented at Symposium on Aerosol Science and Technology, Eight-Second National Meeting of the American Institute of Chemical Engineers, Atlantic City, N.J. 29 August -1 September 1976. 

The description of the workings of the control technologies is beyond the scope of this article. However, it is worth noting that many of these technologies create substantial amounts of solid or liquid waste that needs to be disposed of properly. For example, in the USA the total amount of fly ash produced from coal combustion alone is about 57 Mt/y (Kalyoncu 2000). About one-third is utilized as secondary raw material (e.g., for aggregate and asphalt), but the rest is usually disposed of in landfills. The wet and dry scrubbers for S02 control produce a sludge or dry waste that finds little secondary use, and a large amount is disposed in landfills. 

Linak, W. P. Wendt, J. O. L. 1994. Trace-metal transformation mechanisms during coal combustion. Fuel Processing Technology, 39, 173-198. 

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