Based on studies which indicated a large potential for significantly increased coal-firing in the commercial sector, the development of advanced coal combustion systems is currently being pursued.
Fluid heating systems known in the art include conventional combustion systems in communication with boiler assemblies. Oil or gas is primarily used in these conventional combustion systems to provide heat to water passing through boiler systems. The heated water or steam is then forwarded for its desired application, such as space heating, turbine operation, or otherwise.
Conventional oil and gas systems suffer from a major drawback--availability and price stability, both of which are subject to the turbulent conditions in the Middle East. Solid domestic fuels, on the other hand, are generally plentiful at the present time and are not faced with the same concerns.
A major concern, however, with utilizing solid fuels such as coal, particularly the cheaper low grade sulfur-containing coals, for supplying heat to conventional fluid heating devices such as a fire tube boiler, is the amount of particulates and other contaminants produced by combustion that are carried over in the combustion gas stream. A particulate-contaminant-laden gas stream operating such systems can adversely impact the atmosphere as such particulate is released thereto. Although conventional devices such as cyclones may be used to remove larger particulate matter from combustion gas streams, these devices generally fail to remove smaller particulates such as fly ash from the streams. Similar problems also exist in other gas streams in which the suspended particulate matter originates from other than combustion.
Fuel-bound nitrogen also causes nitrogen oxide (NO.sub.x) emissions to form in the gas stream. Methods and processes to either reduce the production of nitrogen oxides or to destroy or remove such pollutants from the flue gas stream are necessary to meet the requirements of the Clean Air Act. Economically viable means for removing these pollutants from the exhaust stream before discharging such exhaust into the atmosphere have not heretofore been available.
Various attempts have been made to overcome the above and other problems and to provide an economically feasible and efficient process for treating fluids using a solid fuel. One such attempt has focused on ultracleaning the coal prior to combustion to reduce coal-based contaminants. The coal must be extensively cleaned in an attempt to remove ash and sulfur from the fuel prior to firing. Generally, a cold water slurry is made from micronized, deeply cleaned coal and then used as fuel. This approach is very expensive and imposes delays in time before the coal may be used. It does, however, produce an essentially oil-like slurry fuel made from coal.
In summary, effective reduction of suspended particulates and other contaminants in a gas stream created by combustion remains a paramount problem due to the lack of a cost effective, efficient system for particulate and contaminant removal. Available particulate collection/removal systems are limited by combustor operating conditions. Any new systems should possess a number of attributes, such as high combustion efficiency, high sulfur capture capability, high solid fuel particulate removal, low nitrogen oxide emissions, and high removal of alkali vapors created by the combustion of the fuel. New systems providing these attributes should be relatively inexpensive and should not require substantial preparation and pre-cleaning of the fuel used for combustion.
Acoustic agglomeration is a process in which high intensity sound is used to agglomerate submicron- and micron-sized particles in aerosols. This concept is a pretreatment process to increase the average size of entrained particulates to permit high collection/removal efficiencies using cyclone or other conventional separators. Sound waves cause relative motion between the solid particles, and hence, increase their collision frequency. Once the particles collide, they are likely to stick together. As an overall result of sound treatment, the particle size distribution in the aerosol shifts significantly from small to larger sizes relatively quickly. Larger particles may be more effectively filtered from the carrying gas stream by conventional particulate removal devices such as cyclones. The combination of an acoustic agglomeration chamber with one or more cyclones in series provides a promising high-efficiency system to clean particulate-laden gases such as hot flue gases from pressurized combustors.
Acoustic agglomeration of small particles in hot combustion gases and other sources of fine dust-bearing effluent streams has been studied intermittently for many years. Although effective in producing larger-sized particles (5 to 20 microns) for more efficient removal by conventional devices, the prior art methods of acoustic agglomeration are not generally viewed as potential clean-up devices due to their large power requirements. For example, fine fly ash particulates (less than 5 microns in size) have been agglomerated using high-intensity acoustic fields at high frequencies in the 1,000-4,000 Hz range. These higher frequencies were necessary for the disentrainment of the fine particulate so as to effect collisions therebetween, and hence, agglomeration of the fine particles.
In these prior art acoustic agglomeration devices, the acoustic fields have been produced by sirens, air horns, or electromagnetic speakers. The resulting acoustic generation for sonic agglomeration requires power estimated to be in the range of 0.5 to 2 hp/1,000 cfm. This, of course, is a significant parasitic power loss even for efficient horns and sirens which normally have efficiencies ranging from 8 to 10%.
Furthermore, the sirens and air horns require auxiliary compressors to pressurize air. Electromagnetic devices require special designs and precautions to provide the desired equipment reliability, availability and life. In addition, powerful amplifiers are required to drive such speakers to deliver 160 decibels (dB) or more of sound pressure.
In addition to the foregoing, desired system performance goals include dual fuel capability (i.e., coal as primary fuel and a premium fuel as secondary fuel), combustion efficiency exceeding 99 percent, thermal efficiency greater than 80 percent, turndown of at least 3:1, dust-free and semi-automatic dry ash removal, fully automatic start-up with system purge and ignition verification, emissions performance exceeding new source performance standards and approaching those produced by fuel oil-fired commercial-scale units, and reliability, safety, operability, maintainability, and service life comparable to the oil-fired units currently employed for heating fluids.
The apparatus and process according to the present invention overcome most, if not all, of the above-noted problems of the prior art and generally possess the desired attributes set forth above by using a pulse combustor to produce a heat source for enhancing the generation of fluid heat, such as the creation of steam. The present invention may be designed to operate in both a slagging and a non-slagging mode.