Experimental study on effects of premixed air distribution on preheating combustion characteristics and NOx emission of pulverized coal

Air distribution is the key factor affecting combustion characteristics and NOx emission of pulverized coal with the use of the self-preheating burning technology. Nevertheless, previous research is mainly about the effects of air distribution in the down-fired combustor, but not on the effects of premixed air distribution. To make up for such shortage, we focused on the effects of premixed air distribution on combustion characteristics and NOx emission. Relevant experimental variables included the ratio of premixed air flow to total secondary air flow (CP), premixed position, and the ratio of first-staged premixed air flow to second-staged premixed air flow (C12). During preheating, N2 was the predominant conversion form of released coal-N. During combustion, the influ-ence of the premixed air on temperature distribution differed among regions, and it may be regulated by the premixed position. At above 800 mm, the temperature was lower with the premixed air, and decreased and increased with the rising CP when premixed air entered from points A and B respectively. At below 1200 mm, the temperature with the premixed air was higher and lower when premixed air entered from points A and B respectively, but it always decreased as CP rose in these two conditions. As C12 increased, the temperature dropped at above 1200 mm, while climbed first and then declined at below 1200 mm. Introducing the premixed air improved the combustion efficiency (ri) and reduced the NOx emission, but the effect of CP on ri was regulated by the premixed position. Specifically, ri rose first and then decreased as CP increased when the premixed air entered from point A, while its regularity was just the opposite when the premixed air entered from point B. On the contrast, NOx emission always decreased in these two conditions. Additionally, increasing C12 properly can reduce NOx emission while improving ri.