Pulverized coal (pulverized fuel-pf) has been fired in rotary cement kilns (see Kilns) and boiler furnaces (see Boilers). The latter are basically boxes lined with tubes in which water is evaporated and contain a water/steam mixture. The coal is pulverized to a fine powder, usually so that 70% is less than about 75 μm in size, before being carried by part of the combustion air stream to the burners. These coal burners are usually mounted on one vertical wall, two opposing vertical walls or grouped one above the other at the four corners of the furnace (see Figure 1 and Figure 2). Corner burners fire tangentially into the furnace giving a single ball of flame in a central vortex. Other designs, for example, using down-firing for low volatile coals and burners for low-grade high-moisture coals, have been discussed by Dryden (1975) and Lawn (1987).
In all the furnaces described so far hot dry ash falls to the "dry bottom" of the furnace chamber where it is removed. The exception to this is in cyclone fired boilers, which are usually of two types. In the vertical cyclone furnace, a variant of the tangentially fired design, larger coal particles are centrifuged out of the gas flow to burn on the refractory-lined walls of the lower part of the chamber, the slag running out of the "wet bottom" chamber. The other type of cyclone furnace uses individual cylindrical refractory-lined chambers in which the coal burns to give hot gases, which are exhausted into the main furnace (see previous references).
Large pf furnaces have a fuel input of about 200 tons per hour of coal (to provide a 500 MW electrical load) and chambers 35 m high with a cross-sectional area of about 300 m2 are typical. The furnace designer has to ensure that there is the right amount of heat transferred from the flames to the wall tubes to evaporate the desired amount of water and to still have the correct gas temperature at the furnace exit. This must be done without any excessive local heat fluxes damaging the tubes, while at the same time complete combustion of the coal particles has to be achieved. There is the additional consideration that pollutants, e.g., oxides of nitrogen, must be kept to a minimum.
One of the problems specific to coal fired furnaces is the build up of ash or slag on the- furnace walls leading to changes in temperature and emissivity. Raask (1985) has dealt at length with ash and slag deposits on furnace walls and the heat transfer properties of boiler deposits. He also describes measures to combat Fouling in boilers (e.g., coal cleaning and blending, installation of soot-blowers and water-jets). Data on combustion efficiency (typically over 98%), furnace exit gas temperatures of between about 1300 K and 1600 K and measured heat fluxes up to about 320 KWm−2 (increasing by about 10% after soot-blowing) have been reported in large pf fired plant by Godridge and Read (1976).
Furnace designers make use of physical and/or mathematical models (see Furnaces). The latter use either heat balances or Computational Fluid Dynamics, CFD. In the first method the furnace is divided into regions or zones, see Hottel and Sarofim (1967) and Field et al. (1967), and a specific application to a pf fired furnace, also making use of a physical model to provide mass transfer information has been described by Cooper and Gibb (1984). The CFD method is based on the finite difference solution of momentum, enthalpy and species conservation equations. Application of the method to coal fired cyclone combustors has been reported by Boyson et al. (1986).
Figure 1. Water-tube boiler furnaces and gas flow patterns, (a) front-wall-fired furnace, (b) opposed-wall-fired furnace, (c) corner-fired furnace (horizontal section) x burners.
Figure 2. Wall-tubes and burner openings in a PF-fired water-tube boiler-furnace under construction. (Reproduced by permission of PowerGen.)
Boyson, F., Weber, R., Swithenbank, J., and Lawn, C. J. (1986) Modelling coal fired cyclone combustors, Combustion and Flame, 63, 73—86. DOI: 10.1016/0010-2180(86)90112-4
Cooper, S. and Gibb, J. (1984) High temperature heat transfer: furnace performance assessment, First U.K. Heat Transfer Conf., Leeds.
Dryden, L. G. C. Ed. (1975) The Efficient Use of Energy, I.P.C. Press, London.
Field, M. A., Gill, D. W., Morgan, B. B., and Hawksley, P. G. W. (1967) Combustion of Pulverised Coal, B.C.U.R.A., Inst. Energy, London.
Godridge, A. M. and Read, A. W. (1976) Combustion and heat transfer in large boiler furnaces, Prog. Energy and Comb. Sci., 2, 83-95. DOI: 10.1016/0360-1285(76)90018-6
Hottel, H. C. and Sarofim, A. F. (1967) Radiative Transfer, McGraw-Hill, NY.
Lawn, C. J. Ed. (1987) Principles of Combustion Engineering for Boilers, Ch. 1, 3 and 5, Acad. Press, London.
Raask, E. (1985) Mineral Impurities in Coal Combustion, Hemisphere Pub. Corp. London.