Coal-Fired Heating Apparatus and Method

Because of the erosive and corrosive character of the constituents that they contain, the combustion products of certain "dirty" fuels, notably coal, are often unsuitable for use directly as the working fluid for gas turbines and the like. It is of course common practice to utilize heat exchangers to convectively transfer thermal energy from a hot, dirty gas to a heat-transfer fluid, thereby reducing the deleterious effects of the gas by subjecting only stationary parts to it. Further protection can be afforded by sweeping the exposed surfaces of the heat exchanger with a relatively clean (or only moderately dirty) gas; it is believed that proposals have been made to aerodynamically clean high temperature heat exchangers for use in magnetohydrodynamic recuporators. (Hoover et al; NASA Final Report No. NAS-3-19407, 1976)

As far as is known, no method or apparatus has heretofore been provided by which the energy produced by the burning of coal, char, and other dirty fuels, can be recovered in a highly efficient and yet practical manner while, at the same time, effectively shielding the energy-recovery structures from erosive and corrosive components of the combustion gases.

The objects of the present invention are therefore to provide heating apparatus, and a method utilizing the same, by which the radiant energy of a fuel can be recovered efficiently while protecting the components of the heat-exchange unit utilized against deleterious effects of combustion product constituents.

Certain of the foregoing and related objects of the invention are attained by the provision of heating apparatus comprised of means defining a heating chamber, a burner for producing a flame within the chamber, heat exchanger means, and gas introducing and flow-directing means. The heat exchanger means includes a plurality of tubes extending within the heating chamber, which tubes have surfaces disposed to receive radiant energy from a proximate flame produced by the burner, and the gas introduction means so directs the flow as to pass over the radiation-receiving surfaces. The fluid passing through the tubes can therefore be heated, directly or indirectly, by radiant energy from the flame absorbed by the fluid or by the heat-exchange tubes, with the radiation-receiving surfaces of the tubes being protected from deleterious substances that may be contained in the burner flame by a flow of gas issuing from the introducing means.

Other objects of the invention are attained by the provision of a method for heating a fluid by passing it through a heat-exchange tube in a heating chamber, as described. A flame is produced proximate the radiation-receiving surface of the heat-exchange tube, by effecting combustion of a first, usually relatively unclean, fuel, to directly or indirectly heat the fluid passing through the tube. Substantial contact of the irradiated surface of the tube by the deleterious combustion products is prevented by sweeping the tube with a flow of clean gas (i.e., a gas that is at least relatively free from erosive and corrosive substances).

FIG. 1 is a schematic representation of a furnace which incorporates heating apparatus embodying the present invention;

FIG. 2 is a schematic representation of the heating apparatus of FIG. 1, drawn to an enlarged scale;

FIG. 3 is a view of the apparatus of FIG. 2, taken along line 3--3 thereof;

FIG. 4 is a schematic representation of a portion of a heating unit of the kind shown in the previous Figures, illustrating the cross-sectional configuration of one form of heat-exchange tubes suitable for use therein; and

FIG. 5 is a similar representation, showing an alternative heat-exchange tube arrangement.

Turning now in detail to the appended drawings, FIG. 1 shows a furnace comprised of a housing 10 having gas flow and slag outlets at 12 and 14, respectively, and incorporating four heat-exchange units embodying the invention, each being generally designated by the numeral 16 and having gas inlet lines as at 17. Combustion products flowing through the outlet 12 may be used, for example, in an associated preheater system (not illustrated) from which the lines 17 extend; air heated in the units 16 exits through the lines 19.

Details of the heat-exchange units 16 are shown in FIGS. 2 and 3. Each unit consists of an insulating refractory sidewall 18, a coal burner 20, arrays of heat-exchange tubes 22, and auxiliary burners 24. The burners 20, 24 are located at a common end of the sidewall 18; inlet ducts 32 admit combustion air thereat, and pulverized coal, burned to produce a flame 36, is introduced through burner inlet 34. The inlet and the outlet ends, 26 and 28 respectively, of the tubes 22 are so disposed that the fluid passing through them exits through the leg closest to the flame 36.

The tubes 22', illustrated in FIG. 4, are presently regarded to comprise a preferred embodiment of the invention. They have a generally arcuate, aerodynamic cross-sectional configuration, uniform throughout at least the major portion of their lengths, and they cooperate with one another to promote sweeping of the gas from the burners 24 across their flame-irradiated faces. Each tube 22' consists of a head portion 38 and neck portion 40; the head portions 38 provide radiation-receiving faces 44, which are directed inwardly and present an optically large projected area to the coal flame 36. Needless to say, the energy absorbed or transmitted by or through the tubes 22' serves to heat the fluid flowing through the ducts 42.

As suggested by the flow lines, the clean combustion gas produced at the burners 24 is directed toward the tubes 22'. The gas passes between the confronting surfaces 46, 48 on the neck portions 40 of the adjacent tubes, being channelled thereby to sweep, upon exit, across the faces 44. This aerodynamic design serves to maintain the faces 44 free from ash deposits, and also to shield them from the deleterious effects of the particles and corrosive substances contained in the coal flame 36.

A second form of heat-exchange units embodying the invention is illustrated in FIG. 5, and is generally designated by the numeral 16'. In it, the heat-exchange tube, generally designated by the numeral 22", is oriented with its gas flow axis parallel to that of the flame 36, rather than perpendicular to it, as in the preceding Figures. Also, the wall 18' of the unit 16' tapers inwardly toward the flame axis, thus providing a restriction (which would of course be conical on a cylindrical wall), cooperating with the auxiliary burner 24' in promoting the flow of sweep gas 46 so as to pass about the tube 22", including of course the surface 44' thereof.

It is well known that the potential problems that are associated with the use of coal combustion products, as working fluids for gas turbines and the like, are primarily attributable to diminished aerodynamic blade properties, and to erosion and corrosion, caused by ash, sulfur, and organically bound alkali and alkaline earth metal constituents. The same erosion and corrosion effects would be produced on heat-exchange elements exposed to such combustion products, in addition to which ash deposits would compromise the efficiency of thermal energy transfer through them. It will be appreciated that the cleansing action of the clean gas sweep, employed in accordance with the instant method and apparatus, ameliorates those adverse effects upon the heat exchanger tubes.

A particularly unique aspect of the invention resides of course in the proximate positioning of the heat-exchange tubes and the coal flame, so as to most efficiently recover the large amount of radiant energy that the flame contains. The aerodynamic cleaning effect herein described makes that technically feasible and effective, as a practical matter. Consequently, while the heat-exchange tubes may have any suitable form and arrangement, they will most desirably be so configured and disposed as to cooperate with one another in producing effective gas flows across their irradiated surfaces. It is believed that the desired aerodynamic effect is achieved by inducing increased velocities, and generally laminar, or low-turbulence, flow in the sweep streams. It is regarded to be of importance that turbulence in the coal flame, as well as in the sweep stream, be maintained at levels that are sufficiently low to avoid excessive intermixing of the two flows, as would compromise the sweep-gas shielding effect; on the other hand, some mixing may be desirable, so as to maximize temperatures within the unit.

It will be appreciated that an optimal arrangement of components, in any heating apparatus embodying the invention, will therefore depend upon aerodynamic and thermodynamic factors, as well as upon mechanical factors dictated, for example, by the simple need to provide adequate support for the tubes. Thus, the configuration of the heat-exchange unit may vary widely within the scope of the invention. It should be noted, for example, that the tubes may have their primary flow axes oriented either substantially perpendicular to the axis of the flame or parallel to it, both as illustrated.

The tubes and associated auxiliary burners, or flow-directing means, will desirably be positioned at spaced locations about the periphery of the heating apparatus. Although only two locations are shown in FIGS. 2 and 3 of the drawings, surrounding the flame with tubes, and providing suitable sweep-gas discharge locations associated therewith, will often be found to maximize the efficiency of energy recovery. For the same reason it will usually be desirable to so dispose the tubes that the heat-exchange fluid exits therethrough from the hottest part of the chamber. It will be appreciated that a plurality of heating units will desirably be employed in a given furnace, arranged in any suitable manner.

As will be evident to those skilled in the art, effective radiation absorption characteristics (i.e., high emissivity values at the temperatures prevailing within the heating chamber) can be afforded by fabricating the heat-exchange tubes from a suitable ceramic material, such as silicon carbide, silicon nitride, and the like. Such tubes will of course be made to efficiently absorb energy from the flame in at least a portion of the infrared spectral region, for indirect heating of the heat exchanger fluid. When, on the other hand, the fluid is to absorb the radiant energy directly, it will comprises a substance other than air, e.g., carbon dioxide, water, soot dispersions, and other substances of high infrared absorptive coefficient. In the latter case, the heat-exchange tube will normally be made from a material that is substantially transparent to radiation in at least the portion of the spectral region at which the fluid is efficiently absorptive.

In most instances, the sweep-gas will constitute the combustion product of a clean fuel such as natural gas, methane, and products of coal pyrolysis, carbonization, or gasification. Although it is conceivable that a hot, non-combustible gas may provide the sweep gas, the use of fuels burned in situ will generally provide optimal energy production and economics.

A principal attribute of the present method and apparatus is that they enable radiative and convective heat transfer from a moderately clean sweep stream, coupled with radiative heat transfer from the flame of a much larger, unclean fuel stream. These characteristics permit the attainment of significantly increased heat-exchange rates, as compared to those that would be realized by combustion of the gases downstream of the flame, while also shielding the heat-exchange surfaces against high concentrations of alkalis, sulfur, and ash; this in turn allows fabrication of the heat-exchange components from a wider selection of materials, and enables operation at higher temperatures, than would otherwise be possible. Nevertheless, it should be appreciated that benefit may be derived in some instances from carrying out the method of the invention using a relatively clean primary flame fuel, the combustion product of which is free from deleterious substances.

Thus, it can be seen that the present method and apparatus satisfies the expressed objects of the invention. Efficient recovery of radiant energy from a relatively unclean fuel is enabled, while the exposed heat exchanger components are protected against the deleterious effects of its combustion products. The thus heated fluid may be utilized in any suitable application, including of course that of serving as the working fluid for a gas turbine.