Boiler fuels, stoker-fired

A difference between these firing methods may also be manifested in the initial fuel cost. For efficient operation of a spreader-stoker-fired boiler, the coal must consist of a proper mixture of coarse and fine particles. Normally, double-screened coal is purchased because less expensive run-of-mine coal does not provide the optimum balance oFcoarse and fine material. 

Figure 23.5 A stoker-fired watertube boiler of 36,300 kg/h steam capacity at 28 bar and 385°C (from British Coal Publication, Boiler House Design for Solid Fuel, 1980, and kind permission from College of Fuel Technology)...

Densified fuel pellets may be used as boiler fuel in place of coal in stoker-fired furnaces. Direct substitution of fuel pellets was demonstrated in two informally reported instances with no furnace adjustments. The particulate emission from each of these installations were monitored with the emission rate results shown in Figure 2. As a comparison, the North Carolina Administrative Code allowable particulate emission rate schedule (a function of heating rate) is also shown. The environmentally acceptable particulate emission contributes to the economic justification of the conversion of small-to-medium size steam generators to the use of pelleted biomass fuels. Two such installations are now using pelleted fuels on a routine basis, and a third has concluded the economic benefit by entering into a contract agreement to purchase pelleted fuel. 

Figure 2. Stoker fired boiler emissions grate burning pelleted fuels...

AFBC is a commercial technology that is widespread across the world. Coal-fueled AFBC competes with conventional stoker-fired boilers, with practical capacity of 20,000 to 300,000 MJ/h fuel energy input, all the way up to the lower end of base-load pulverized fuel-fired boilers, having practical capacities of 250,000 to 9,000,000 MJ/h. Among the many claimed practical advantages of AFBC are the following ... 

Stoker Firing, Stokers are the oldest continuous coal combustion devices, having been in operation for more than 150 years. They are still used to fire small-capacity boilers. Table 17.16 gives the approximate range of capacity for each kind of stoker. There are several types of stokers, which are classified by the method of coal feeding and the relative direction of the fuel and air. The three main... 

Formation of Airborne Emissions. Airborne emissions are formed from combustion of waste fuels as a function of certain physical and chemical reactions and mechanisms. In grate-fired systems, particulate emissions result from particles being swept through the furnace and boiler in the gaseous combustion products, and from incomplete oxidation of the soHd particles, with consequent char carryover. If pile burning is used, eg, the mass bum units employed for unprocessed MSW, typically only 20—25% of the unbumed soHds and inerts exit the combustion system as flyash. If spreader-stoker technologies are employed, between 75 and 90% of the unbumed soHds and inerts may exit the combustion system in the form of flyash. 

Utilities and industrial power plant fuel-fired boilers coal - pulverized coal - cyclone cole - stoker oil wood - bark bagasse fluid coke... 

The chamber is externally insulated and clad. Combustion equipment for solid fuel may be spreader or traveling-grate stokers or by pulverized fuel or fluid bed. Oil and gas burners may be fitted either as main or auxiliary firing equipment. The boilers will incorporate superheaters, economizers and, where necessary, air preheaters, grit arresters, and gas-cleaning equipment to meet clean air legislation. 

Varied boiler firing configurations are found in hog fuel boiler applications, including dutch oven, fuel cell, spreader stoker with traveling or vibrating grates, and cyclone stoker types. As stated previously, the spreader stoker is the most widely used of these configurations. Spreader/stoker boilers in the pulp and paper industry often have an air swept spout added to the front of the boiler to feed bark down on top of the coal.14 Wood is puffed at one-... 

---