OCR Text |
Show airflow to that level. The bed temperature is elevated, due to a decrease in the bed airflow and an increase in the radiation from the freeboard. Besides these modifications in the structure of the furnace, also different combustion air distributions were tested. In Case 5, the lowest air level is closed and an equivalent airflow is added to the 2nd and the 3rd air levels. The opposite change is tested in Case 6 where the uppermost air level is closed and the airflow to the 1st level is added. Usually, the nozzles of the rear wall are closed, but in this case they are in use. The locations of the combustion and temperature peaks depend on the staging of the combustion air. In Case 5, combustion is partly shifted to the upper parts of the furnace and the temperatures are higher between the 2nd and the 3rd air levels than in other cases. Radiation to the bed decreases distinctly. In Case 6, combustion is very intensive at the 1st air level, and the temperature there rises locally. Anyway, the total radiation to bed decreases some, because the air fed through the nozzles of the rear wall just cools the rear part of the furnace to where fuel is not thrown. Case 7 is the same as Case 2, except for the reduced bed air flow. By comparing these cases, it can be noticed that the bed temperature goes down when the airflow through the bed increases. The extra airflow makes both the bed and the freeboard right above the surface of the bed cooler. However, with more bed air, the temperature in the freeboard rises up more rapidly. With lesser bed air, the combustion is more intensive and the temperature is higher at the 2nd air level, and the final flue gas temperature is lower, due to the smaller flue gas flow. The aim of furnace modifications is to gain a uniform combustion in the lower part of freeboard without any local high temperature peaks that could cause problems with sintering. The most evident remedy is a more uniform fuel feed that would bring combustion toward the rear part of the furnace. Naturally, there has to be enough air for combustion in the lower part of the furnace, but it is unnecessary to inject extra air to such places where fuel is not spread. A reduction in the bed area reduces also the need for fluidisation air and moreover the fuel is spread relatively more homogeneously to bed, when the rear part of the bed is removed. 4. CONCLUSIONS The applicability of the computational fluid dynamics based design and analysis tool to bubbling fluidised bed combustion has been demonstrated. The benefits of the CFD-based modelling lie in the identifying of the essential mechanisms as well as trends and cause - consequence relationships, which to some extent can substitute a lack of experience and experimental data. In troubleshooting cases, modelling is one of the most descriptive and useful tools in finding out the reasons as to why things are not as they should be and to give advice on what should be done to reach the desired state. Because of the fundamental physics and reduced elementary chemistry applied, a good generality and qualitative accuracy have been achieved. However, a high quantitative accuracy in the prediction of complex chemical phenomena like soot, N O * and slagging formation, which are not even theoretically completely understood, cannot be expected. Because of this, the CFD-based combustion modelling is not, in general, capable of predicting exact values. Preferably, modelling should be applied to ensuring that the process to be designed includes adequate mechanisms for the desired operating conditions to be reached, at least with in situ fine-tuning. In most cases, the final design should be complemented by experimental knowledge. In addition, knowledge of the accuracy, suitability and limitations of the particular models is essential for the quality and correct interpretation of the results. A n experienced eye is also required to sort out the essential and major trends, mechanisms and phenomena from the huge amount of three-dimensional data generated by the m o d e m C F D codes. Hence, a proper analysis and refinement of the results play an important role in successful modelling. 7 |