OCR Text |
Show has already reached almost at the possible high level owing to the remarkable progress of modern technologies. However, it is a common scenery that chimneys are rejecting hot flue gases. The utilization of a fraction of the waste heat contributes to fuel saving of the process, hence the reduction of carbon dioxide emission. Therefore, the approved goal of carbon dioxide emission at C O P 3 , held in Kyoto last year, requires advanced level of energy saving far ahead of the present state of the technology. Preheating of combustion air by recycled heat from exhaust gases is called "heat-recirculating combustion" [1,2] and has been used as an effective method to recover waste heat for saving energy. Heat exchangers transferring heat from exhaust gas to combustion air have made considerable progress due to the advancement in material and structure during last two decades. Among them, a heat-recirculating furnace equipped with two sets of staging combustion burner and ceramic regenerator of honeycomb-type, which was operated with high alternating flow cycle, achieved considerable high performance of regeneration. With this system high temperature combustion air above 1273 K can easily be obtained for the operating furnace temperature of 1473 K, which yields more than 30 percent fuel saving. It has been generally held that the emission of nitric oxides increases with the temperature rise of combustion air when we use preheated air, and that the increase of thermal efficiency and the reduction of nitric oxides emission are incompatible requirements in practical operation of furnaces. However, unexpected low nitric oxides emission was observed even with high temperature combustion air between 1073 and 1373 K [3], in the process of developing a heat-recirculating furnace equipped with two sets of regenerative burner operated with high alternating flow cycle (Fig.1). If higher flame temperatures were produced in the furnace due to the raised air temperature, it is difficult to interpret the results by our knowledge on nitric oxides formation mechanisms. This consequently lead to an extensive collaboration among industries and academia and a national project supported by N E D O in Japan[4-20]. Studies in the same concept have been carried out in North America and Europe during the same period [21-25] and the concept of new combustion technology is clearly explained by the term of |