Predicting nitrogen oxides emissions of tangentially coal-fired boilers

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Predicting nitrogen oxides emissions of tangentially coal-fired boilers

 

Author: Lee, Chun-leung
Title: Predicting nitrogen oxides emissions of tangentially coal-fired boilers
Degree: Eng.D.
Year: 2010
Subject: Coal-fired furnaces
Boilers -- Fuel
Nitrogen oxides
Hong Kong Polytechnic University -- Dissertations
Department: Faculty of Engineering
Pages: xvi, 197 p. : ill. (some col.) ; 30 cm.
InnoPac Record: http://library.polyu.edu.hk/record=b2392265
URI: http://theses.lib.polyu.edu.hk/handle/200/6133
Abstract: Nitrogen oxides (NOx) is a collective designation under which the two most important gases are nitric oxide and nitrogen dioxide. NOx has been identified to be one of the atmospheric pollutants that leads to the production of photochemical smog and acid rain. Power industry using coal as fuel is one of the important emission sources of NOx. Increasing concern from the community, stringent legislation and advanced technologies have reduced the NOx emissions substantially in recent years. There are generally two methods to reduce NOx namely, the in-furnace method and the gas treatment method. However, no matter which method for NOx reduction is adopted, a simple, fast and accurate prediction of its emission level is desirable. On the other hand, the complex NOx formation and reduction mechanisms, and the inter-relationship with different operating conditions of a coal combustion system, make the prediction challenging. Many empirical and analytical models were proposed to estimate the emissions of NOx. However, very few of them are used to perform continuous emission prediction for a running boiler. The objective of this project is to develop a simple method for operating engineers of tangentially coal-fired boilers to predict the NOx emissions of their boilers operating at different conditions so that effective measures can be taken before it reaches an unacceptable level. Such operating conditions may include different boiler loads, using different coals, using different combinations of burners and with different air supply patterns. Lumped parametric modelling method was used for this project. The basic concept is to incorporate the structure of the combustion system, including the amount and the pattern of the air distribution, the firing pattern and the composition of the fuel, into the theoretical NOx formation and destruction mechanisms. Simplified global reactions for the coal combustion system and the NOx formation system were used to form the basis of the model. The theoretical results were then compared with the actual performance of the boiler to obtain the model constants. To simplify the process, calculations can be performed by spreadsheet software with the basic input parameters such as coal composition, coal flow rate, amount of excessive oxygen and burners to be used. To confirm the applicability of the model, it has been tried on both a conventional boiler and a boiler equipped with low NOx combustion system. The result shows that the model can effectively predict the NOx emissions of a conventional boiler with an error typically within ±5%; however, the error may reach over 10% for some cases when it is applied to a low NOx combustion system. Nevertheless, because of the great practical value of this method of NOx prediction, further investigation and refinement of the model is recommended.

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