| OCR Text |
Show 2 efficiency and low pollution has long been hoped for as an innovative technology. The heat regeneration from flue gas has been a popular technology to raise the thermal efficiency of a heating process as well as to generate high temperature flames [1]. In practice, the technology was first applied to melting and reheating furnaces, where high temperature flames were essentially required, and preheated air of 873 K was utilized by a recuperative burner [2]. Subsequent improvement in materials has permitted higher preheat, and glass melting regenerative furnaces have been working with preheated air exceeding 1273 K [3]. Regarding the emissions of the system, it has been generally held that the emission of nitric oxides increases with the temperature rise of combustion air when we use preheated air. Therefore, the practical trade-off between the thermal efficiency and the emission control has been always a critical issue for the designers and engineers in industries. Numerous efforts to overcome the conflict interest between fuel saving and reduction of nitric oxides emission have been made during the last decade [4-9]. Practically, direct injection of fuel into a furnace, and high momentum ejection of staging air, and mixing control were found effective to some extent in reducing nitric oxide in regenerative combustion. Several years ago, it was reported that extremely low nitric oxides was observed in a test furnace using combined technologies of the highly preheated air between 1073 and 1473 K generated by a high-frequency alternating flow regenerator, the fuel-staged combustion and the extremely high m o m e n t u m of inlet air[10]. However, it was difficult to interpret the observed facts based on the established knowledge on flame structures and chemical kinetics of nitric oxides formation mechanisms. Carefully discussing the conditions on which the above mentioned unexperienced combustion regime was realized, we decided to study combustion in highly preheated air in a relatively simple system. In the present study we have observed the change of nitric oxides emission and the combustion regime of nonpremixed flame of natural gas in highly preheated air paying a special attention on the mixing process in the combustion chamber. Experimental apparatus and procedure |
