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Show to study the release of volatile material and nitrogen during combustion. The objective of this research was to use data from both fundamental and burner studies to determine the effect of coal quality on NOx emissions from pulverised coal flames. The fundamental work involved coal characterisation studies in which coals were subjected to conditions similar to pulverized coal flames. The burner studies involved low-NOx burner concepts which were designed to simulate full scale burners and operate at the semi-industrial scale (1-4 MW) . Although previous work has been carried out on the effect of coal quality on NOx formation, no work has combined characterisation of the coal in conditions similar to pulverized coal flames with measurements on burners sufficiently similar to those used in industry. Therefore, it is believed that this work yielded significant insight into the relative roles of coal quality and burner design on NOx emissions. 2 BACKGROUND There are two sources of nitrogen in coal flames, the air and the coal. The coal typically contains less than 1% of the total nitrogen input, but results in up to 80% of the NOx formed [6, 7]. Simply stated, the NOx formation mechanisms for converting coal bound N2 are much more efficient than the mechanisms for converting N2 from the air. Thus, techniques to reduce the amount of NOx by coal combustion are centred on fuel-NOx routes. The formation of NOx from N2 in the air proceeds via two paths: thermal and prompt. The majority of the NOx by these paths comes from the thermal route. The term thermal NOx arises from the high activation energy of this path and it can be calculated that the rate is about 10 ppm/s at 1400 0 C [8]. It has been shown that typical levels of NOx in pulverized coal flames are about 100-200 ppm [7]. However at higher temperatures, this path can produce very high levels of NOx. The main way to control the amount of thermal NOx is to keep the peak flame temperature below 1500-1550 oC [8]. The formation of NOx from fuel-bound N2 also proceeds via two main paths: volatile nitrogen and char nitrogen. It is generally believed that in unstaged pulverized coal flames, the volatile nitrogen path contributes 60 to 80% of the fuel NOx [6,7]. This follows from the facts that under pulverized-coal-flame conditions: the volatile matter contains most of the coal N2 [6, 10-13], and the volatile path for NOx production is more efficient [6, 9]. The efficiency of coal-bound N2 conversion to NOx has been estimated to be 20-25% for the char and up to 60% for the volatile matter [6, 9]. 2 |
