This invention is related to a combustor for a heat engine, such as a Stirling cycle heat engine, and particularly to an improved combustor for a heat engine capable of using high temperature combustion inlet air and having low NOx emission characteristics.
Combustors in heat engines are used to burn a fuel, such as natural gas, gasoline or diesel fuel, to produce heat. Heat from the combustion gas produced by burning the fuel is transferred to a working fluid circulating within the heat engine by a heater assembly on the heat engine. The working fluid undergoes a thermodynamic cycle within the heat engine which converts thermal energy in the working fluid into mechanical output energy. This mechanical output energy can be used for a variety of purposes, such as to drive an electrical generator to produce electricity or to drive other mechanical components, such as a vehicle drive train, an irrigation pump, etc.
The heat engine used in conjunction with the inventive combustor can comprise a Stirling cycle heat engine similar to those previously developed by the assignee of the present invention, Stirling Thermal Motors, Inc., including those described in U.S. Pat. Nos. 4,481,771; 4,532,855; 4,615,261; 4,579,046; 4,669,736; 4,836,094; 4,885,980; 4,707,990; 4,439,169; 4,994,004; 4,977,742; 4,074,114, 4,966,841, and 5,611,021, which are hereby incorporated by reference. Basic features of many of the Stirling cycle heat engines described in the above referenced patents may be implemented in connection with a heat engine incorporating the present invention.
Combustion of fuel typically produces three types of hazardous material emissions: volatile organic compounds ("VOCs"), carbon monoxide ("CO"), and oxides of nitrogen ("NOx compounds"), such as nitric oxide (NO), nitrous oxide (NO.sub.2), N.sub.2 O.sub.2, etc. Due to their relatively unstable chemical nature, VOCs and CO are typically comparatively easy to reduce or substantially eliminate, such as through the use of catalyst materials in the exhaust system. NOx compounds, on the other hand, are more chemically stable and more difficult to eliminate after they have been formed during the combustion process.
NOx compounds are formed during a combustion process when the combustion inlet air and fuel are less than thoroughly mixed as the fuel is burned. The quantity of NOx compounds formed also tends to increase as the temperature at which combustion takes place is raised. The most common method for reducing NOx emissions from a combustion process is to optimize the mixing and combustion process and lower the combustion temperature. The lowest emission rates of NOx compounds are currently obtained from combustion systems in which the fuel and combustion inlet air are thoroughly pre-mixed prior to combustion and where the combustion inlet air is at approximately room temperature.
Developing a steady state combustor using pre-mixed fuel and combustion inlet air to reduce the quantity of NOx compounds formed during the combustion process is relatively straightforward when the combustion inlet air is at approximately room temperature. Heat engines, however, typically improve their thermal efficiency (and thereby reduce fuel consumption) by transferring heat from the exhaust combustion gas to the incoming combustion inlet air. This reduces the amount of heat lost in the exhaust gas and substantially increases the overall operating efficiency of the system. By using high efficiency combustion inlet air pre-heaters (a type of heat exchanger), the incoming combustion inlet air can be heated to very high temperatures, approaching 800.degree. C., prior to being mixed with the fuel. Conventional low NOx combustors are not designed or built to operate under such extreme operating conditions. It is also impossible to develop a pre-mixed combustor system if the temperature of the combustion inlet air substantially exceeds the autoignition temperature of the fuel/air mixture. When the temperature of the combustion inlet air substantially exceeds the autoignition temperature of the fuel, the use of such a pre-mixed system would result in the premature ignition of the fuel/air mixture and could lead to the eventual destruction of the combustor assembly.
The inventive combustor allows the use of high temperature combustion inlet air while at the same time substantially limiting the formation of NOx compounds during the combustion process. The combustor incorporates a large number of nozzles that each mix a portion of the fuel and combustion inlet air together in an internal mixing chamber before the swirling fuel/air mixture is discharged into a collective combustion chamber. Low pressure regions are created as fuel/air mixture is discharged from the nozzles, which helps to circulate the combustion gas back into the wakes produced by the nozzle discharge. This stable aerodynamic swirling pattern and circulation of the combustion gas within the combustion chamber provides a continuous combustion process so that an igniter (i.e. a spark plug) is only required to start the combustion process. The stability of the combustion process allows for a wide range of operating conditions without additional mechanical contrivances.
The inventive combustor is provided with an igniter that initiates combustion of the fuel/air mixture when the heat engine is being started. As the components of the heat engine warm, the temperature of the combustion inlet air is raised until the combustion inlet air temperature has increased sufficiently to allow the temperature of the fuel/air mixture to exceed its autoignition temperature. The nozzles in the inventive combustor have been designed to provide rapid and efficient mixing of the combustion inlet air and fuel and combustion of the fuel/air mixture even when the temperature of the combustion inlet air substantially exceeds the autoignition temperature of the fuel/air mixture. This results in very low production of NOx compounds, even at very high combustion inlet air temperatures. In tests performed on the inventive combustor in which the temperature of the combustion inlet air approached 800.degree. C., the production of NOx compounds was so low that the levels could not be measured by the laboratory test equipment (i.e. the quantity of NOx compounds in the exhaust combustion gas was less than 1 part per million). By manufacturing the components of the innovative combustor from high-temperature alloys, such as Inconel 713C, the combustor is able to operate properly even under severe operating conditions, such as when the combustion inlet air temperature approaches 800.degree. C.
The inventive combustor also features a short flame length, which helps to reduce the size of the required combustion chamber. Having a relatively small combustion chamber is particularly important for mobile heat engine applications, such as motor vehicle applications.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.