Spreader Stoker Firing of Wood and Coal in Multiple Fuel Furnaces

Use of steam generating units designed to burn several types of fuel are becoming common because they are flexible enough to adapt to the changing fuel supply and price situation. There has been a trend to the spreader stoker furnace for multiple fuel firing. The spreader stoker scatters or spreads the fuel over the burning fuel bed. The heavy particles fall to the grate and burn, while the small particles quickly ignite and burn in suspension. Grates for the spreader stoker can be stationary, dumping, or traveling. Units designed with furnace volume below 25,000 BTU’s per cubic foot per hour give improved efficiency and reduce flyash. Optimum performance can be obtained only with the use of jets to create furnace turbulence. Grate size, furnace design, and flyash collection can be arranged to maintain low flyash emission. The moisture content of wood affects the overall efficiency of the steam generating unit and also affects the burning characteristics of the wood. Curves are given for moisture and hydrogen loss in wood refuse firing, dry gas loss, carbon loss, efficiency, products of combustion, and theoretical flame temperature. Wood produces a greater quantity of combustion products and moisture than coal, oil, or gas, and the resulting higher quantities of flue gases require ample provision for air supply and draft. In burning multiple fuels, requirements for either induced draft or forced draft fans must be calculated for wood fuels.

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