1 Department of Chemical and Biochemical Engineering, Technical University of Denmark
Review, Experiments and Engineering Modeling
The objective of this study is to improve the understanding of nitrogen oxide formation and carbon burnout during the combustion of pulverized coal, and to contribute to addressing the potential of chemical engineering models for the prediction of furnace temperatures, NO emissions and the amount of carbon in ash. To this purpose, the effect of coal quality on NO and burnout has been investigated experimentally, a radiation heat balance has been developed based on simple chemical engineering methodology, and a mixing study has been conducted in order to describe the near burner macro mixing in terms of a reactor configuration. Results of the individual studies are described below.The influence of burner operating conditions, burner geometry, and fuel parameters on the formation of nitrogen oxide during combustion of pulverized coal has been reviewed. Main attention is paid to combustion facilities with self-sustaining flames, while extensions are made to full scale boilers and furnace modeling. Since coal combustion and flame aerodynamics are reviewed elsewhere, these phenomena are only treated briefly. The influence of coal type and process conditions on NO formation and carbon burnout has been investigated experimentally in a 400 MWe corner fired boiler with over fire air, a 350 MWe opposed fired boiler, and in a 160 kWt pilot scale test rig. Three different coals were fired in each of the furnaces as part of the activities in group 3 of the European Union JOULE 2 Extension project "Atmospheric Pressure Combustion of Pulverized Coal and Coal Based Blends for Power Generation". On the pilot scale test rig both single stage and air staged tests were performed. The pilot scale test rig was able to reproduce quantitatively the NO emissions from the corner fired boiler, while only qualitative results could be obtained for the opposed fired boiler. The better agreement with the corner fired boiler is presumed to be related to the existence of a distinct primary zone with a relatively low stoichiometry, which diminishes the influence of the near burner air and fuel mixing rate on NO formation. Trends between coal type and NO concentrations were identical in all three furnaces. No clear trends have been observed between coal type and carbon in ash content. This is mainly due to the fact that the burnout in large furnaces is high, and differences between coals become small.A simple, one-dimensional combustion and radiation heat transfer model has been developed for the furnace of full scale boilers. The model has been applied to the two boilers mentioned above, and is validated against measured temperatures and carbon in ash concentrations. The model is able to predict the temperature profiles in the boilers reasonably well. However, the influence of coal quality on the temperature profile and the carbon in ash content is less satisfying. The discrepancy between measured and predicted carbon in ash contents may be partly due to the simple mixing pattern and volatile combustion model, but it is also expected that char reactivity is not constant, but varies as a function of the conditions in the furnace. The inability to predict the influence of coal quality on temperature is presumed to be directly connected to the indistinct relation between coal type and wall reflectivity/absorptivity. This is supported by temperature measurements, which showed significantly different furnace temperatures for the three coals at constant thermal load. A better knowledge on reactivity and wall absorptivity is required before the influence of coal quality on furnace temperature and burnout can be predicted.A mixing study has been performed in order to initiate an investigation of the potential of chemical engineering models to predict NO from pulverized fuel burners. The success of chemical engineering modeling is strongly connected to the simplification of the flow pattern into a reactor configuration. The radiation model already showed the importance of mixing, and a model for the prediction of NO will be even more sensitive to the mixing pattern of the fuel with the combustion air and with (recirculated) flue gases. Therefore, an experimental study was initiated in order to describe the near burner macro mixing pattern of the primary air with the secondary air. To this purpose residence time distributions in a confined, cold swirling flow were measured with a fast response probe and helium as a tracer. The test rig represented a scaled down version of a burner. The effect of variation of flow velocities and swirl number on the flow pattern in the near burner zone of the laboratory furnace-model were studied. Experimentally obtained residence time distributions have been used to derive a chemical reaction engineering model for the mixing process. The model is based on a combination of plug flow reactors and continuous stirred tank reactors, which represent the main flow characteristics in regard of mixing in the near burner zone. Simulated residence time distribution curves compared well with those of the experiment at the two swirl numbers studied. The reactor model developed here can be the basis for further development of a chemical reaction engineering combustion model.