OCR Text |
Show scale (5/ 14]. Residence time in the fuel rich zone increases as burner scale and flame size increase. Thus there may be a minimum scale, which is rank dependant, which allows sufficient time for all the nitrogen species reaction to occur. Another variable is temperature. The flame temperature profile varied greatly with the coal combusted. However, the data is difficult to interpret because the most important temperature is that to which the reacting volatile species are exposed. This is not easily identified. Additionally, the effect of temperature on NOx formation is not well understood. Because of the aforementioned problems, it is that the operating conditions of any burner adjusted to the coal used. This will ensure the of a sufficiently fuel-rich zone. Secondly residence time must be available for the NOx mechanisms. 5 CONCL~SIONS believed must be creation sufficient reduction In this paper, a comparison of data gained in a well controlled simulation of pulverized coal combustion was compared to data gained from experiments in 2.5 MW low-NOx burners. Two different burner concepts were considered. Because of the unique combination of fundamental coal information and semi-industrial scale burners, significant insight into the relative roles of the coal and the burner were gained. When the burners were operated under unstaged conditions, NOx emissions could be clearly correlated with the coal properties. As volatile matter and nitrogen contents increased, NOx emissions increased. This information was related to the relative ease of NOx formation from volatile nitrogen species in fuel lean conditions. When the burners were operated under staged conditions, the correlation of NOx emission with coal properties became more difficult. The differences between the NOx emissions for the coals studied decr~ased. However a trend was observed indicating that coals with high volatile matter contents and high nitrogen contents resulted in high NOx emissions. This behaviour was in opposition to that predicted by theory. The reasons for the discrepancies between theory and the burner performance is differences in the success of staging for the various coals. This behaviour was related to the different gas compositions and residence times that each of the coals were exposed to. The final conclusion is that the design and operation of 10 |