Pulverized coal combustion technology

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). 

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. 

Ash from pulverized coal combustion is a strategic material that has many critical applications from a source of aggregate to the most important source of pozzolan for addition to Portland cement concrete. Environmental control measures on the emissions of coal combustion have resulted in a loss of quality for these materials. In response we have seen the advent of beneficiation processes applying both proven and new technologies to produce high-quality consistent products from these materials. Currently we estimate that about one-fifth of all ash products marketed are processed through some form of beneficiation method. We expect that the demand for quality and consistency will continue and the relative amount of process ash products will increase in the future. 

Pulverized coal combustion systems are most commonly used in power plants. In pulverized coal combustion, temperatures typically reach around 1480 °C at atmospheric pressure. In the past couple of decades, fluidized bed combustion (FBC) technologies have been commercialized. These combustors often use limestone bed materials to capture sulfur gases. They operate at about 880 °C and usually at atmospheric pressure (Smoot and Smith, 1985), 38. 

The suspension-fired boilers are similar to the pulverized coal firing technology and involve combusting the fuel in the form of small particles as they are fed into the boiler. A great deal of pretreatment is required of the fuel, which is a potential disadvantage. However, the higher boiler efficiency is an advantage. 

Each of the coal properties interacts in a significant way with generation technology to affect performance. For example, higher-sulfur content reduces efficiency of pulverized coal combustion (PCC) due to the added ena gy consumption and operating costs to remove sulfur oxides (SOx) from the flue gas. 

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). 

Fluidized-bed combustion (FBC) is a different technology from conventional pulverized-coal-fired boilers. A mixture of powdered rather than pulverized coal and a sorbent (usually limestone) are injected directly into the furnace and burned while suspended on a bed of high-velocity air. The turbulent mixing of coal and sorbent causes the solids to behave as a pseudo-fluid, enhancing heat generation and transfer at much lower temperatures (800-900 °C) compared to conventional pulverized-coal-fired furnaces (1500 °C) (US Department of Energy 1992). 

Levendis, Y. A., Atal, A., Carlson, J. B., Dunayevskiy, Y. Vouros, P. 1996. Comparative study on the combustion and emissions of waste tire crumb and pulverized coal. Environmental Science and Technology, 30, 2742-2754. 

In a suspended bed or entrained flow reactor technology, the coal is crushed, dried, and then pulverized to fine powder in a crusher and mill. As Table 9.1 shows, the coal particles used in entrained flow reactors are very small. The pulverized coal is transported with air to the furnace (primary air), and secondary air is heated and fed into the combustor to ensure complete combustion. The residence time of the coal in the furnace is typically around 1-2 s, which usually suffices for complete combustion. However, not all coal burns completely, and fly ash will be generated (see Table 9.1). 

Hydrocarbon Technologies, Inc. integrated gasification combined-cycle Kellogg-Rust-Westinghouse process molten carbonate fuel cell methanol-to-gasoline process once-through Fischer-Tropsch process phosphoric acid fuel cell pulverized coal polymer electrolyte fuel cell pressurized fluidized bed combustion 1015 Btu... 

A demonstration unit sponsored by the Environemtal Protection Agency (EPA) to produce RDF at St. Louis proved the basic feasibility of mechanical separation processes, transport and storage techniques, and combustion of fluff RDF to replace 5 to 27 percent of the pulverized coal used in suspension-fired utility boilers. However, the refinement of equipment components and the technical and economic optimization of the basic technology still require a great deal of work. 

T. F. Wall, A. Lowe, L. J. Wibberley, T. Mai-Viet, and R. P. Gupta, Fly Ash Characteristics and Radiative Heat Transfer in Pulverized-Coal-Fired Furnaces, Combustion Science and Technology, 26, pp. 107-121,1981. 

A technology employed in some coal-fired power stations. Instead of feeding pulverized coal directly to the boilers to raise steam, it is first converted to a gaseous mixture of carbon monoxide, hydrogen and nitrogen that is then combusted in a gas turbine. 

FIGURE 14.26 Pulverized fuel combustion for low-rank and medium-rank coal. (From Field, M.A. et al.. Combustion of Pulverized Coal, British Coal Utilization Research Association, Leatherhead, Surrey, 1967 Berkowitz, N., An Introduction to Coal Technology, Academic Press, Inc., New York, 1979.)... 

---