Furnaces – Combined – With boiler
Patent
1988-01-12
1990-02-27
Yuen, Henry C.
Furnaces
Combined
With boiler
110211, 110213, 110214, F23B 700, F23J 504
Patent
active
049036165
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention is for use with a solid-fuel-fired boiler with high combustion and system efficiency. The high level of emissions and low efficiency associated with the use of solid fuels has been an obstacle to the transition from oil to solid fuels. There is a clear need for a suitable solid-fuel-fired boiler that fulfills strict environmental standards and achieves high efficiency performance as well.
A solid fuel, e.g. wood in various forms such as logs, chips, pellets or peat, differs fundamentally from oil in its combustion properties. For example, wood burns in two widely differing phases: the Gas Combustion Phase and the Charcoal Phase. Both emissions and heat are formed and emitted in these two phases. In the former phase about 80% of the fuel mass is converted to gases in a relatively short time. Thus the gas volume and the rate of emission of the volatile matter depend on an important factor, the moisture content of the fuel. High moisture levels result in a long gas combustion phase. For a conventional boiler it has been shown that the gas combustion phase is critical from the environmental and heat transfer viewpoint. There are many physical and chemical factors at work during the gas combustion phase that affect the pattern of emissions. They will not be dealt with here. The most important factor in this context is the air supply, which will be discussed shortly.
In general, the charcoal phase involves about 20% of the total fuel mass, although the combustion time can actually be longer than that for the gas phase. The charcoal phase is not so much of concern regarding emissions, mainly because of the even and uncomplicated combustion. Even so, the supporting grate should be designed and shaped correctly to maintain a high combustion efficiency.
SUMMARY OF THE INVENTION
The present invention provides combustion apparatus which assures high efficiency combustion and low emissions during both the gas and charcoal phases of combustion. The apparatus may comprise a thermally insulated wall structure defining an upwardly open primary combustion chamber, means disposed in the primary combustion chamber for supporting a solid fuel for combustion, and a secondary combustion air supply device mounted above the primary combustion chamber. In accordance with the invention, the secondary air supply device comprises a frusto-conical shell open at both axial ends and oriented with the larger axial end opening to the primary combustion chamber. The shell has spaced inner and outer walls defining a frusto-conical air space therebetween, with the inner and outer walls being circumferentially sealed to one another near the axial ends of the shell, and the inner wall being perforated. Secondary air supply duct means is connected to the shell in communication with the frusto-conical air space for supplying secondary air to be ejected through the perforations of the inner wall into the exhaust gas stream generated by the fuel burning in the primary combustion chamber.
In a preferred form of the invention, the fuel supporting means comprises a grate having a horizontal central section and two upwardly inclined side sections, so that charcoal formed during the combustion process will accumulate on the horizontal section. The side sections are provided with primary air guide vanes. As the pressure drop across the central portion increases with increasing charcoal accumulation, the guide vanes redirect the primary air flow through the side grates toward the accumulated charcoal, thus ensuring continued intense combustion.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in detail hereinafter with reference to the accompanying drawings in which:
FIG. 1 is a diagrammatic part-sectional view of a combustion unit according to the invention;
FIG. 1A is an enlarged sectional view of a wall portion of the combustion unit;
FIG. 2 is a side view of a secondary air supply device for the unit of FIG. 1;
FIG. 2A is a top plan view of the device of FIG. 2A;
FIG. 3 is a plot showi
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