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An Introduction APSCO Wood-waste Heating Boilers
APSCO Engineering Ltd. has been a front-runner in the market development and manufacture of wood-waste burning heating boilers since 1977. The interest was initiated by the sudden increase in fossil fuel prices resulting from the infamous Arab oil embargo.
The APSCO wood-waste burning boiler is a vertical, three-pass, fire-tube heating boiler, utilizing a large vertical hearth to allow the full development of the woodburning process. Using this boiler with waste fuel satisfies large thermal energy requirements at low operating costs. In some cases, capital costs have been amortized in less than one operating season.
Figure I shows a cut-away schematic of this boiler. Hearth temperatures as high as 2600 degrees F. ensure complete, virtually smoke-free combustion. In fact it is almost impossible to determine if the boiler is in operation by looking at the stack. The hearth is fully water jacketed, and partially refractory lined.
Heat is transferred to the surrounding water medium throughout each of the three passes that the hot flue gases make. Initially in the hearth, radiant heat (accounting for approximately 70% of the heat transferred) is transferred through the firewall to the water as the gasses pass into the upper transfer plenum. The hot gasses then travel down a number of firetubes to the lower transfer plenum. During this pass, gas temperatures have been reduced to approximately 800 degrees F. Finally the flue gases travel back up through another group of firetubes and out the induced-fan breaching. Stack temperatures are normally 50 degrees F. to 75 degrees F. higher than the water medium temperature. Thermal boiler efficiencies of 80 to 85% are commonplace with dry fuels.
The APSCO wood-waste boiler consists of five separate systems: (See Figure 2.)
1. The boiler with a water medium producing either hot water (212 degrees F.) or low pressure steam (15 psig).
2. The water or steam loop to the plant or building thermal requirements.
3. The fuel-feed system, automatically feeding either granular materials or offcuts to the hearth.
4. The flue-gas treatment system, including the induced draft fan, the fly-ash separators and ash handling equipment, the breaching, and the chimney or stack
5. The regulatory and operating control system.
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