This invention relates to a method for the combustion of coal in a cyclone combustor, and more particularly, to a method which minimizes the emission of ash particles and other pollutants from such a combustor.
The apparatus known as a cyclone coal combustor consists of a cylindrical chamber into which pulverized coal is injected and centrifuged to the cylindrical wall of the chamber by a high velocity, swirling gas flow. Heretofore, most cyclone combustors have been designed for combustion to take place near or at the cylindrical wall, and at a temperature sufficiently high, on the order of about 3000.degree. F., to melt the coal ash. Conventionally, the melted coal ash, or slag, has been drained continuously from the combustor. Although there are two general types of cyclone combustors, the horizontal cyclone of interest in connection with the present application and the reverse flow vertical cyclone, most prior commercial experience is with the horizontal cyclone.
Commercial horizontal cyclone combustors heretofore used have removed from 70 to 85 percent of coal ash as slag. These commercial cyclone combustors, typically of 100 to 700 million BTU/per hour capacity, were used extensively in large utility and industrial boilers in the 1950's and early 1960's. Their inability however, to control the emission of NO.sub.x, led to the discontinuance of their use in the late 1960's, as environmental concerns grew. Renewed interest arose in the cyclone combustor in the mid 1970's due to the need for a low ash output combustor in conjunction with the U.S. Department of Energy's magnetohydrodynamics ("MHD") program. This work, as well as greatly expanded research and development efforts on coal combustion phenomena, led to an improved understanding of the cyclone combustor (C. S. Cook, et al., "Evaluation of Closed Cycle MHD Power", DOE Contract Report No. DE-AC01--78ET 10818, Nov., 1981) ("Ref. 1").
A one million BTU/hr. air cooled, horizontal cyclone coal combustor, which was tested as part of an MHD program activity, had a central oil gun located at the center of the closed end of the unit. The oil gun was used to preheat the combustor wall, which was ceramic lined, and to start coal combustion. Pulverized coal, in this combustor, was transported by a primary air stream in about a one-to-one air/coal mass ratio, and injected into the combustor in an annular region surrounding the oil gun. The ceramic liner was maintained at a temperature of about 2200.degree. to 2500.degree. F., a temperature high enough to keep the coal slag in a liquid free flowing state. The ceramic liner was cooled by an air stream which was also used as the secondary air stream. The cooling/secondary air stream was injected at the closed end of the combustor to produce the characteristic swirling gas motion around the axis of the combustor, and the injection velocity was high enough to obtain tangential gas velocities of several hundred feet per second at the cylindrical walls of the combustor. In the above-described apparatus, efforts were made to burn as much of the coal particles as possible in suspension near the cylindrical wall of the combustor, and the unit was operated with conventional pulverized coal, having a conventional particle size distribution. Nevertheless, the above-described prior art device achieved slag retention values in excess of 80 percent and, in some cases, when operated with a somewhat more coarse coal particle size distribution (70% through 200 mesh), slag retention above 90 percent was evident. Significant NO.sub.x emission reductions were obtained with this apparatus and method by operating it at sub-stoichiometric conditions in the range of 60-90% of the stoichiometric air/fuel ratio (Ref. 1). In addition, pulverized limestone was injected as a sorbent directly through the closed end of the combustor, and this resulted in 25 to 35 percent reductions in the SO.sub.2 emissions from the combustor. The reacted sorbent was embedded in the slag and removed with it through a slag tap located at the down stream end of the combustor.
Horizontal cyclone combustors which were in commercial use in the United States and Germany in the 1950's and '60's had the primary air (which carried the coal) and the secondary air (which produced the swirl) injected tangentially along most of the top axial length of the unit. (Babcock & Wilcox Co., STEAM, ch. 10 (1978 ed.) ("Ref. 2")). The U.S. units typically operated with coarse crushed coal (50 percent through 20 to 40 mesh), in order to conserve pulverization power and to provide a method for burning coals having low ash fusion temperatures. Due, however, to the use of such very large coal particles, it was assumed that most of the coal combustion process took place on the surface of the slag, so that excess air, scrubbing the slag layer, was necessary for complete combustion. This, of course, produced high NO.sub.x emissions, and ultimately led to discontinuance of the use of such combustors.
The German commercial horizontal cyclone coal combustors were similar in design to the above-described U.S. units. (H Seidl, "Development & Practice of Cyclone Firing in Germany", Proc. Jt. Conf. on Combustion ASME-I.Mech.Eng., MIT, Cambridge, Mass., June, 1955, p. 92) ("Ref. 3"). However, in the German case, the intended application was the combustion of very high ash (up to 40 percent) coal which had low volatile matter content (under 20 percent). Consequently, a finer, but still relatively coarse pulverized coal particle distribution (approximately 60 percent through 100 mesh, i.e., 150 micron diameter coal particles) was used. These units obtained up to 85 percent slag retention, as compared to only 70 percent in the U.S. units.
Another class of cyclone combustors is the vertical, reverse flow type, whose design principles are very similar to those of conventional cyclone dust separators. A detailed experimental study of such a device was performed by Hoy (H. R. Hoy, et al., "Some Investigations with a Small Cyclone Combustor" in Jrn. Inst. Fuel, Oct., 1958, p. 429) ("Ref. 4"). In the unit studied by Hoy, which had a relatively large 20 million BTU/per hr. coal energy throughout, 80 to 85% of the slag was retained in the combustor. It was observed, however, that the reverse flow had a tendency to re-entrain slag from the walls in some cases.
One observation of importance in relation to the present invention should be made here in connection with the above-described cyclone combustors and other cyclone combustors being developed for MHD applications. In this regard, it is of significance that in these units coal was injected very close to the hot, liquid slag-covered walls, and as a result, the coal particles impinged very rapidly on the slag-covered walls. Thus, in such units, even the smaller particles that remained in suspension in the gas stream for but brief periods were in a gas temperature environment in the 3000.degree. F. range. Under these conditions, convective heating of the particles results in rapid pyrolysis and devolatilization of the coal particles. In the cyclone combustors developed for MHD applications, discussed below, in which different coal injection techniques were used, the use of very high temperature air preheat (in the 2000.degree. to 3000.degree. F. range) produces a similar effect. By contrast, in the air-cooled cyclone combustor of the present invention, coal injection occurs in a relatively cold gas environment, and therefore, considerably longer time periods are required and employed for coal pyrolysis.
The MHD program gave rise to a need for a high slag retention combustor. Several investigators designed and tested different versions of cyclone combustors intended for the MHD application. The Pittsburgh Energy Technology Center designed and tested a vertical unit similar in concept to Hoy's device. (W. S. Lewellen, et al., "Modeling Two Phase Flow in a Swirl Combustor", Aeronautical Research Rpt., (00-4062-5 (1977)) Princenton, N.J.) ("Ref. 5"). TRW designed a horizontal unit, similar to the early United States and German units described above. (J. A. Hardgrove, "MHD Cost Fired Combustor Dev.", 9th Energy Tech. Conf. Proc., Wash., D.C. Feb., 1982) ("Ref. 6"). The TRW unit, however, differed in that it used axial coal injection, as distinguished from the tangential injection used in the commercial units.
One significant operational difference between the processes performed by the cyclone combustors intended for the MHD application and the U.S. and German commercial units is the extremely rapid devolatilization that occurs in the MHD units due to the very high temperature air preheat which is used. This condition, coupled with the general use of relatively fine coal particle size distributions in such units appears to have resulted in the MHD units not only in rapid devolatilization, but also very rapid char gasification of much of the fuel while the coal particles were in suspension in the gas stream. These conditions account for very high carbon conversion, but low slag retention (in some cases as low as 30 percent) (Ref. 6).
Another prior art technique for the control of certain emissions in the combustion of coal is the injection into combustion chambers and furnaces of limestone or similar calcium oxide compounds as sorbents or binders for sulphur compounds. In the case of the million BTU/hr. combustor described above, 25 to 35 percent reductions in SO.sub.2 emissions were observed with the injection of pulverized limestone, and the reaction products of the limestone were removed with the coal slag. No explanation for this capture process was given (Ref. 1). Generally, the physical states of the sulphur capture process using calcium oxide compounds is by now well understood. The first step in the process is calcination, wherein CaCO.sub.3 is converted to CaO by removal of CO.sub.2 from the CaCO.sub.3 particle. A porous structure is left after calcination. With excess oxygen, sulphur capture leads to the formation of CaSO.sub.4. Under equilibrium conditions, this compound moves toward dissociation above about 2000.degree. F. Sulfation takes place heterogeneously by SO.sub.x contact with CaO, and it is affected by SO.sub.x diffusion through the CaO pore structure. Eventually, however, a layer of CaSO.sub.4 encapsulates the particle and stops the reaction.
For cyclone combustor applications, the average gas temperature is 3000.degree. F., which is much too high for equilibrium sulphur capture. However, it is now hypothesized in accordance with this invention that sulphur capture takes place during the time in which the CaCO.sub.3 particle is suspended in the combustion gases, the time for sulphur capture is in the 100 milliseconds time range.
Still another mechanism by which pollutants are removed from cyclone combustors utilizes the slag layer. The slag removes the mineral matter from the coal in liquid form, and serves as well as a base upon which one can burn up the remaining particles of char in the coal which floats on the slag and to remove the calcium-sulphur compounds resulting from the limestone injection. One problem, however, with prior art techniques has been the reevolution of SO.sub.2 from the slag layer. This occurs because at the temperatures of the slag layer, approximately 2200.degree. to 2500.degree. F., calcium sulphate in the slag will melt and react with species in the slag such as iron compounds or gases above the slag layer such as 0.sub.2 or CO.sub.2. For partial pressures of oxygen of less than 0.1 atmospheres, a condition likely to exist in a cyclone combustor, the rate of SO.sub.2, evolution from slag is in the time range of 15 to 20 minutes. It is, therefore, essential to remove the slag in a time less than this to avoid reevolution of sulphur. Reevolution is retarded somewhat, it has been found, by maintaining local reducing conditions above the slag. However, as noted, compounds such as those of iron can catalyze the reaction which converts chemically bound sulphur in the slag to gaseous form.