|Boiler Efficiency and NOx Optimisation Through Advanced Monitoring and Control of Local Combustion Conditions|
|Written by A. Copado, F. Rodriguez|
|Tuesday, 08 June 2010 12:37|
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This paper presents the methodology, development and results of a power station optimisation programme performed by INERCO, AICIA and ENDESA. The overall aims of this programme have been the improvement of boiler efficiency and the reduction of the environmental impact associated to NOX emissions, by means of the following specifically developed tools:
The combustion process in large power plant boilers is almost a “black box” for the operators. It is significant that in an activity in which the largest production cost is the cost of fuel, the lack of information about the process involved is greatest precisely in the area of how this fuel is utilised. In this respect, the limited degree of boiler monitoring is largely related to combustion conditions inside the furnace, and to the adequate characterisation of fuel and air supplies (Scott (1995); Rodríguez (1998)).
This limitation concerning monitoring of the boilers, which has a significant effect on their optimisation, is caused by the fact that the majority of these power plants were designed at a time when the plant’s production (and not its emissions or efficiency) was the critical operating parameter. The inadequate monitoring described means that, in general, the operation of the boilers is based on the use of certain combinations of global or indirect variables, derived either from the recommendations of the boiler supplier or from the accumulated experience of the operators of each particular facility. These combinations frequently have more to do with operational stability and historical inertia, i.e. following customary practices, than with true optimum operating conditions.
A clear example of this situation is the adjustment of combustion air. The minimisation of excess air is, in fact, one of the most direct and effective primary measures (combustion regulation adjustments) for optimising performance and NOx emissions in any type of boiler. However, boiler operators are extremely loath to use this type of adjustment, due to the possible creation of sub- stoichiometric areas in the furnace which may cause high levels of unburnt fuel or even a plant shutdown.
Therefore, relatively high base levels of excess air are habitually used, in spite of its negative effect on heat rate and on the generation of NOx, with priority being given to considerations of operational safety. However, this “critical” parameter is usually calculated as the average of measurements taken at only 4 or 6 points in the boiler outlet, whose representativeness, with regard to the combustion conditions inside the furnace near each burner, is very limited. A similar case could be made for the monitoring of air and fuel feed to the furnace, and the control of these as a means of optimising the combustion process.
In this context, and taking into account the increasing competitiveness among power plants, INERCO, a Spanish engineering and consultancy firm, has been researching a new approach to optimisation of heat rate and NOx emissions in coal- fired power plants. This research has been carried out with the collaboration of ENDESA, the leading electric company in Spain, and AICIA, a research centre linked to the University of Seville. It has also been partially financed by the ECSC.
The work has focused on the evaluation and development of the most appropriate computer tools for the optimisation process and on the analysis and experimentation of new systems for characterising and monitoring the operation of boilers.
A new philosophy is proposed that relies on furnace specific monitoring systems and the computer architecture shown in Figure 1. According to this structure the global system comprises the following components:
Figure 1 – Optimisation system structure
This article describes the main aspects of the research carried out to develop these systems and their characteristics, with particular emphasis on the following aspects:
Finally, the article describes the results obtained by the application of this development at a full-scale Spanish power plant and author’s future plans in the field of optimisation.