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The boiler has already been described above. The only difference between the hot water and the steam boiler is the addition of a steam drum mounted above the boiler in the steam applications.
The water medium supplying hot water or low pressure steam follows conventional system applications as recommended by ASHRAE and industry practices, and requires no further explanation. The one exception is that this boiler has a large reservoir to accommodate the prolonged heat transfer from solid fuels after the feed system is stopped. Consequently, the medium is in prolonged storage within the water jacket. For this reason we do not use a glycol mixture inside the boiler, since glycol is subject to fossil fuel ""cracking" thereby glazing and insulating the heat transfer surfaces and reducing boiler efficiency.
Various forms of fuel can be used to fire this boiler. The most common fuels used are shavings and sawdust, wood chips, wood pellets, off-cuts, and cordwood. Granular wastes are fed through an underhearth stoker, while offcuts can be fed through an inclined overhead conveyor system. Cordwood can be hand-fed into the hearth.
Additionally, a fossil fuel (propane, natural gas, or fuel-oil) burner can be installed to fire the boiler in the absence of wood fuels.
The average composition of wood fuel by weight is:
Carbon 49.56%
Hydrogen 6.11%
Oxygen 43.83%
Ash 0.05%
Some additional trace components can be found but in quantities too small for concern.
This composition represents dry wood. A percentage of water is always absorbed in wood, and during combustion, a part of the heat produced will be used to evaporate this water. Another part of the heat produced will be used to warm up the incoming fuel to combustion temperature. Additional heat will be lost to the exhaust gases and as well as some conducted through the various components to the surrounding space. The remainder will be transferred to the water medium.
There is, therefore, a maximum moisture content in wood waste that will still allow ignition for combustion. Beyond this level, heat taken to evaporate the moisture will exhaust available heat required for combustion. Boiler designs can accommodate additional moisture content in fuels, but at increased capital costs, and decreased thermal efficiencies. (See Figures 3 & 4.)
Much emphasis is being placed on environmental concerns, and rightly so. Standards established by regulatory agencies are constantly being challenged and changed.
The developing approach is to define the difference between ""noxious" and "annoying" pollution. For example, when coal or fuel oil is being burned, the sulphur content emitted is '"noxious"' for health reasons. But since wood contains almost only carbon, hydrogen, and oxygen, its combustion products would be classified as "'annoying" rather than ""noxious". In addition, the more complete the combustion of the carbons in wood, the less annoying the results to the environment. It should be noted that environmental guardian agencies are now considering the combustion of wood fuels to be pollution neutral, and of no effect to environmental concerns and ozone depletion causes.
By burning at very high hearth temperatures, virtually all carbons are combusted, and only fly-ash remains as a pollutant. Existing allowable emission standards range from undefined levels (subject to community acceptance) to 200 mg./cu. M., to sliding scales approaching 500 mg./cu. M. for all dust effluent and 250 mg./cu. M. for dust particles below 10 microns in size. (Fly-ash particles range in size from 8 to 20 microns.) The emerging allowable emission level inter-nationally accepted appears to be 150 mg./cu. m.
Since fly-ash is heavy, multi-clones or efficient cyclones effectively control fly-ash emission below 150 mg./cu. m. provided equipment is well maintained, properly operated, and fuel supply quality is controlled. (See Figure 5.)
Combustion air is brought into the hearth by means of an induced draft fan. Combustion air enters the hearth at two levels:
Primary air from under the hearth floor through openings in the refractory,
Secondary air above the fuel peak through the front loading doors and through the secondary air supply ports.
The primary air provides partial combustion for gasification of the fuel that then burns cleanly at the secondary level. The combustion principles of *"'time", "turbulence". and "temperature", are incorporated into the hearth design to:
Ensure slow gasification of incoming fuels Produce high secondary combustion temperatures Reduce stratification Allow complete combustion of carbons.
These design features include a large vertical hearth, strategic locations of air inlet ports, and an 80% excess combustion air mixture.
The APSCO wood-waste burning boiler is designed to ASME Code Section IV, "'Heating Boilers"' , and CSA B-51 '*"Code for the Construction and Inspection of Boilers and Pressure Vessels"'. Operation of these boilers falls under the ASME Code, Section VI, "'Recommended Rules for Care and Operation of Heating Boilers"'.
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