Experimental investigation of nitrogen oxide emissions from emulsified fuel combustion incorporating a rapid internal mixing injector by using temperature–time scaling

Abstract

The aim of the present study was to clarify the effect of the operating parameters of the global equivalence, atomizing air, and water content ratios on nitrogen oxide (NOx) emissions from the combustion of emulsified fuel in furnaces incorporating a rapid internal mixing (RIM) injector. NOx concentration was measured at the exit of the furnace, and the temperature distributions were measured using thermocouples. The flame lengths under various conditions were evaluated by analyzing photographic images. On the basis of information on the temperature and flame length, the dominant factors determining the amount of NOx emission were investigated using the temperature–time scaling of NOx. As a result, the dominant factors were found to be the gas temperature in the flame holder and the flame length. The NOx emission index was observed to increase with increasing equivalence ratio; this is due to the increase in the flame length. The atomizing air ratio was found to have little influence on NOx emissions because the increase in the gas temperature in the holder produced by an increase in the atomizing air ratio balances the corresponding decrease in flame length. The water contained in the emulsified fuel significantly reduces NOx emissions because of the reduction in the gas temperature resulting from the latent heat of the water vapor. In addition, NOx scaling also revealed that when water is introduced, the dominant reaction mechanism of NOx production changes from the Zel’dovich mechanism to another mechanism, such as the prompt mechanism.