Effect of the staged secondary air on NOx emission of pulverized semi-coke flameless combustion with coal preheating technology

Abstract Coal self-preheating combustion technology has been proved its potential on NOx reduction. This study adopted the technology to mainly investigate the effect of the staged secondary air on NOx emission with two different types of coaxial jet nozzles. Besides, the difference between flameless and flame combustion of the high temperature preheated fuel was also discussed. After being preheated, more than half of nitrogen was released to coal gas, and the released N was mainly converted to N2. It was confirmed that flameless combustion of the preheated fuel (coal gas + coal char) could be easily realized in a coaxial jet triple channel straight nozzle, and (1) the down-fired combustor was transparent without visible flame fronts; and (2) compared with flame combustion, NOx emission was far lower, and the lowest was 50.8 mg/m3 (@6% O2). By comparing the temperature eigenvalues, it was impossible to effectively distinguish between flameless and flame combustion of the preheated fuel. However, it could still be found that T′ (normalized temperature fluctuation) and δT (difference between the maximum temperature and the inlet temperature of the preheated fuel) of flame combustion were always higher than that of flameless combustion, and the differences were enlarged as increasing R12 (ratio of the inner secondary air flow rate to the external secondary air flow rate). With the increase of R12, the exit NOx emission increased first and then decreased whether it was flameless or flame, and the best R12 for NOx emission was about 1.