Effect of nitriding treatment on char reducing nitric oxide at high temperature

Abstract

To reduce nitric oxide (NO) emissions from coal-fired industrial boilers, char waste could be collected from the tail flue and fed into the furnace to reduce NO, which would also decrease the incomplete combustion loss. Nitriding treatment could be used to enhance the NO reducibility of char waste. In this paper, char with nitriding treatment (nitriding char) was prepared from Shenhua char and urea. The effect of nitriding treatment on char reducing NO at high temperatures (700 °C–900 °C) was studied using a fixed-bed experimental system and various urea masses and pretreatment times. Increasing the urea mass and decreasing the pretreatment time resulted in more urea residue being present in the char, which increased the nitrogen complex which existed in the form of C(N) and caused the surface enrichment of nitrogen element, but blocked the pore structure. In an inert atmosphere, the NO reduction rates of all the nitriding chars were higher than that of the raw char, but there was an optimal limit parameter for the urea mass and pretreatment time. The presence of 2 vol% O2 was not conducive to the NO reduction by nitriding char because of the higher char-nitrogen (char-N) content resulting from the nitriding treatment, which generated more NO and offset the benefit of the nitriding treatment. This paper provides a useful NO control strategy in coal-fired industrial boilers, and the relevant process route has achieved a 20%–35% decrease in NO emissions in engineering applications.