Effect of Temperature on NO Release during the Combustion of Coals with Different Ranks

It is highly important to study the effect of the actual combustion environment temperature in a boiler on NOx formation and its types during the coal combustion. In this paper, a horizontal high temperature tube furnace system was established. Four typical Chinese coals with different ranks including a lignite coal, a high volatile bituminous coal, a high ash bituminous coal, and a low-volatile anthracite coal were employed in a series of isothermal combustion experiments in order to find out the combustion environment temperature in a boiler ranging from 1000 to 1600 degrees C affecting total NO transient release, as well as thermal NO transient release characteristics under the condition of fast heating. The results show, with the combustion environment temperature increasing, NO generates gradually in advance, the peak of NO appears earlier, and the peak value of NO increases. The effect of the combustion environment temperature on NO release amounts of per unit mass coal samples exhibited the change trend of "W"-type curve. At low temperature (1000 degrees C) and high temperature (1600 degrees C), the four test coals have higher emissions of NO. NO release amounts from four types of coal samples completely support the relative nitrogen contents in them. The NO release amount for the coal sample with high nitrogen content is relatively more. About 30-60% of coal nitrogen of four types of coals can be converted to NO during their combustion. The conversion rate of bituminous coal is generally the largest but that of anthracite is the smallest. At lower temperature (below 1200 degrees C), low volatile content or high ash content in coal will lead to the longer release process of NO. However, at high temperature (above 1300 degrees C), the effect of volatile content and ash content in coal on the NO release time will greatly be reduced. The release of fuel nitrogen is not in accordance with the fuel burnout degree. The impact of the combustion environment temperature on the release extent of nitrogen is very significant. As the combustion environment temperature increases, the NO formation proportions at all stages of combustion will be closer to the burnout proportion. High ash content in coal will weaken the improvement of the uniform release of final char nitrogen that results from the increase of combustion environment temperature. During the coal samples combustion, the fuel NO formation will produce a certain inhibiting effect on the thermal NO formation. In the actual furnace of the boiler with the region of high temperature of 1600 degrees C, the main part of all NO generation from anthracite combustion is still fuel NO. In a word, these results provide a theoretical foundation for developing the technique of reducing NO emission in coal combustion engineering.