Furnace stoker-fired

An advantage of a stoker-fired furnace is its easy adaptability to firing almost any unsized solid fuels. Bark, bagasse, or refuse can normally be fired on a stoker to supplement the coal with a minimum amount of additional equipment. Thus, such supplementaiy waste fuels may be able to contribute a higher percentage of the total heat input in a stoker-fired furnace than in a PC furnace without expensive equipment modifications. 

Table 27-21 gives typical values or ranges of these criteria for gas, oil, and coal. The furnace release rates are important, for they establish maximum local absorption rates within safe hmits. They also have a bearing on completeness of combustion and therefore on efficiency and particulate emissions. Limiting heat release on grates (in stoker firing) will minimize carbon loss, control smoke, ana avoid excessive fly a. ... 

Ford N., Cooke M.J., and Pettit M.D.,"The use of a laboratory fixed-grate furnace to simulate industrial stoker-fired plant", J. of the Institute of Energy 65, 137-143 (1992)... 

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. 

Typical locations for furnace cameras in waste-fo-en-ergy stoker-fired boilers ... 

The furnace volume is less for stoker firing because burning lower in the furnace gives more effective use of total furnace surface. 

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. 

The three principal fuels for domestic central heating systems are coal, fuel oil, and gas. The growth of fuel oil has been mostly at the expense of the hand-fired coal furnace. One of the main attractions of fuel oil was automatic heat, and the automatic coal stoker was for a time a lively competitor. Lately its popularity has very much declined. Within recent years the extension of natural gas lines has resulted in increased popularity for gas heating, and in 1950 the sales of gas burners actually surpassed the sales of oil burners, as shown by the following percentage figures on sales of automatic heating equipment gas burners, 56.3% oil burners, 42.6% and automatic stokers, 1.1%. 

These principles underlie the operation of smokeless furnaces and mechanical stokers. Stokers vary greatly in maintenance cost and in the associated cost of maintaining brickwork. In large plants, they save labor. A modern form of the old Dutch oven extension furnace, will give little smoke even when hand fired, if the furnace is not driven hard. The flames are kept hot, because they do not strike the boiler surface until combustion is complete. 

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