OCR Text |
Show universities and companies in Japan. The high flame temperature and the high combustion intensity are the obvious advantages of the highly preheated air combustion technique. However, the over high flame temperature has limited the practical application of the technique because of the limited heat resistance of furnace materials and the increased N O x emission. In addition, for the requirement of heating processes in many industrial furnaces, a uniform temperature distribution is preferred rather than a high peak temperature. Recent investigations have shown that these limitations could be eliminated by lowering the oxygen concentration in the highly preheated air. The combustion of highly preheated air diluted by inert gases can provide not only the high combustion intensity, but also the uniform temperature inside the furnace and the low N O x emission. Furthermore, the air dilution by flue gases can also decrease the C O 2 emission. Using a small scale regenerative furnace, Hasegawa et al. [1, 2, 3] and Tanaka et al. [4] have experimentally studied the highly preheated and diluted air combustion. Combustion using high temperature and low oxygen concentration air has been shown to provide significant advantages for improving industrial furnace performance. The improvement on flame temperature profiles has been made by controlling the condition of air. The conditioned air can have air temperature in excess of 800 °C and less than 15 % oxygen by volume. The maximum flame temperature decreases with the decrease in oxygen concentration in the air and increases with the increase in air temperature. A suitable combination of the temperature and the oxygen concentration of air can therefore not only change the peak flame temperature but also control the distribution of flame temperature. The alleviation of peak flame temperatures while maintaining high and uniform thermal fields yield lower N O x emission levels, high thermal efficiency and reduced size of the equipment. In their experiments, air was diluted in nitrogen and carbon dioxide respectively. The dilution by flue gases has not been tested. Not only the air dilution but also the fuel dilution by inert gases can provide high thermal efficiency and low N O x emission [5]. The N O x formed in gas diffusion combustion is thermal N O x and prompt N O x , and therefore the N O x emission will greatly depend on the flame temperature and the mixing between fuel and air. The capability of lower N O x emission than that of conventional burners was proved experimentally by Katsuki and Ebisui [5] even if highly preheated air was used. L o w N Ox emission can be achieved by the dilution of air with flue gases before combustion as well as by the uniform distribution of fuel. These are the coupled effects of the high rate of exhaust gas |