Abstract
Computational simulation has been carried out to investigate the NO formation/depletion in pulverized coal combustion process. Newlands Bituminous coal is injected along with career air through a central hole of an axi-symmetric burner. A certain amount of co-flow methane is injected coaxially as reburn fuel. The effect of overall equivalence ratio on NO formation and NO reburn are mainly focused in this study. Species concentration for various species are also investigated, because both NO formation and depletion are related closely to various species concentration. From the study it is observed that, at overall equivalence ratio φ=0.8 and 1.0, although the rate of Thermal-NO, Prompt-NO and Fuel-NO formation is high but due to narrow reaction zone and higher air velocity, a weak NO concentration field is observed. On the other hand, a higher NO concentration has been observed with higher equivalence ratio (ratio φ=3.0, 6.0 and 9.0). It also has been observed, the maximum NO reduction efficiency at φ=0.8, 1.0 and 3.0 is in between 1% to 7%, whereas for φ=6.0 and 9.0, the maximum NO reduction efficiency is 27% and 34% respectively. Therefore, co-flow methane injection NO reduction method is more suitable for highly rich mixture conditions. Moreover, the percentage of coal burnout is also relatively higher for higher equivalence ratio condition.
Highlights
Coal is the most abundant fuel available on earth
Various research works are performed on low nitrogen oxide (NOx) burner for coal combustion to investigate the effect of equivalence ratio on NO formation and other combustion characteristics of flame [8, 9]
Model Validation In order to establish the acceptability of current computational model, few results are compared with an experimental work performed in Japanese Central Research Institute of Electric Power Industry (CRIEPI) [21]
Summary
Coal is the most abundant fuel available on earth. it is economical to use in power generation and various metal extraction process. The effect of overall equivalence ratio on NO formation in low NOx burner is an important parameter. Various research works are performed on low NOx burner for coal combustion to investigate the effect of equivalence ratio on NO formation and other combustion characteristics of flame [8, 9]. They reported that, NOx emissions increased with increase in equivalence ratio under fuel-lean conditions, declined dramatically after when equivalence ratio become 0.8 Fuel biasing is another simple method used to reduce NO. In this work, a computational simulation is performed to investigate the effect of overall equivalence ratio on total NO formation and reduction. For NO reduction, NO-reburn reaction mechanism is used, where co-flow methane is used as reburn fuel Both Newlands bituminous coal-air and co-flow methane are injected from the same plane. The mass flow rate of coal and methane are calculated in such a way that, 50% of the total heat is contributed by coal and remaining 50% heat is contributed by methane reaction
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