Particle coal-fired furnace

Coal is burned in many of the world s power plants to generate electricity. Significant air pollution is produced if the stack gases from coal-fired furnaces are not treated before being released to the atmosphere. Untreated gases contain soot (fine unburned carbon particles), nitrogen oxides (NO.x), ash, and sulfur dioxide (SO2). 

The devolatilized coal particles are transported to a direct-fired multihearth furnace where they are activated by holding the temperature of the furnace at about 1000°C. Product quaUty is maintained by controlling coal feed rate and bed temperature. As before, dust particles in the furnace off-gas are combusted in an afterburner before discharge of the gas to the atmosphere. Finally, the granular product is screened to provide the desired particle size. A typical yield of activated carbon is about 30—35% by weight based on the raw coal. 

The sulfation reaction, (D), in Section 9.1.1, is the second step of a process to remove SO, from the flue gas of a coal-fired utility furnace. The first step is the calcination of limestone (CaC03) particles, injected either dry or in an aqueous slurry, to produce CaO ... 

Size distribution studies (Shen et al., 1976) indicate that particulates emitted from coal-fired boiler stacks generally have a higher mean diameter than those from oil-fired boilers, but there is a large overlap, especially as particles from different coal combustion techniques also appear to show morphological and size differences due to differing furnace temperatures (McCrone Delly, 1973 Falster Jacobsen, 1982). Particles cannot therefore be classified to fuel-type on size criteria alone. 

The greatest single problem in operation of coal-fired units is the accumulation of coal ash on boiler heat transfer surfaces. Coal ash causes three main problems in large furnaces (1) buildup of ash on furnace wall tubes, (2) accumulation of small, sticky, molten particles of ash on superheater and reheater tube banks, and (3) corrosion. 

Dry injection of limestone and hydrated lime particles in desulfurization processes is currently under active development (Stouffer et al, 1986 Weinstein, 1989 Yoon et al, 1986). Furnace dry sorbent injection for SO2 control consists of injecting a dry pulverized calcium based material directly into furnace cavity of a coal fired boiler. The product CaS04 and unreacted sorbent particles are removed from the furnace together with fly ash. Fan and Satija (1984), Milne et al (1990b) Krammer and Staudinger (1991) reported kinetic data and model expressions which are applicable to dry injection systems. 

CycUme Furnaces In cyclone firing (Fig. 24-l4d) the coal is not pulverized but is crushed to 4-mesh (4.76-mm) size and admitted tangentially with primary air to a horizontal cylindrical chamber, called a cydone furnace, which is connected peripherally to a boiler furnace. Secondary air also is admitted, so that most all of the coal bums within the chamber. The combustion gas then flows into the boiler furnace. In the cyclone furnace, finer coal particles burn in suspension and the... 

Aerodynamics can play a role in the ash deposition process in all furnaces regardless of the type of firing recent interest in micro-fine grinding of coal is testimony to this fact. It has been postulated that smaller ash particles will follow gas streamlines and be less likely to strike heat transfer surfaces. This is a logical hypothesis for those ash particles that cause deposition due to an impact mechanism. In addition to particle size, particle density and shape also affect aerodynamic behavior. Due to the difference in drag forces, molten, spherical particles will be less likely to follow gas streamlines than angular or irregular particles of the same mass. 

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