OCR Text |
Show Since highly preheated air combustion seems going successful in heating and melting furnaces, it is worth trying to apply this attractive technology to other fields, such as boilers, gas turbines, and IC engines. When we think about highly preheated air combustion, the exhaust gas higher than the preheatnig is essentially required. This gives the ground to negative opinions such that the application to boilers is hopeless because the present thermal efficiency of boilers is sufficiently high and the temperature level of the flue gas from boilers is far below than the autoignition temperature. It is true, but w e need not set regenerators at the end of exhaust line. As shown in Fig.8, highly preheated air combustion is possible if a regenerator is added at appropreate stage of the heating process and a fraction of burned gas is utilized for regeneration. Even though the obtainable thermal efficiency does not change, it is a remarkable advantage as far as low NOx emission is easily realized by the technology. This is totally a system optimization problem which should be taken into account for future design of combustion related devices and processes. Concluding remarks Although the attractive characteristics of highly preheated air combustion have not been studied intensively due to its short history, the recent advancement and potentials of the technology have been reviewed. Waste heat rejected with furnace flue gas can be minimized by adopting a high frequency alternating flow regenerator, and combustion air preheated higher than the autoignition temperature for the fuel is easily obtained. Practically, preheated combustion air within 60 K lower than the furnace exit temperature was obtained by a ceramic honeycomb type regenerator, and the temperature variation during a cycle operation was kept less than 50 K. The point of highly preheated air combustion to compromise between pollution control and energy saving is combustion in low oxygen concentration, |