ABSTRACT

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:

• Advanced automatic monitoring system of local combustion conditions (OPTICOM system). This system enables the collection and analysis of gas samples from the boiler furnace that provides on-line measurements of local furnace parameters as temperature and gas concentrations (O2, CO, NOx, SO2).
• Signal quality assurance system. This software package detects anomalous values of measured variables, substituting them for the best allowable estimations.
• On-line calculation and supervision system of unit energy efficiency. This system continuously displays the global boiler efficiency and the performance of significant parts of the installation, providing a reliable evaluation of the influence of any parameter adjustment in the overall facility performance.
• An advisory and modelling software package for optimisation of boiler efficiency and NOX emissions (SIRE system).

INTRODUCTION

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:

• Monitoring systems and instrumentation specifically installed for the optimisation process. The use of these specific measurement systems is one of the novelties of the new optimisation approach proposed. Specially of note is the application of the OPTICOM system.
• A Control System responsible for information exchanges. This module receives and stores data from: operation, coal analysis, emission measurements and mill classifier and combustion air damper positions, and includes an operating signal quality assurance system.
• An on-line calculation and supervision system (CESAR system), which provides a reliable assessment of the unit’s performance. This module includes the program libraries necessary for calculating the boiler local and global parameters.
• SIRE system. This module advises plant operators on decision-making for optimised boiler settings. It allows joint on-line optimisation of the performance/NOx equation for any boiler operating condition and makes use of cutting-edge modelling and mathematical algorithms.

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:

• New monitoring tools. Activities related to the characterisation and monitoring of the plant. Detailed description of the OPTICOM system.
• Signal quality assurance system.
• On-line calculation and supervision system (CESAR system).
• Optimisation system of boiler efficiency and NOx emissions (SIRE system). Different philosophies behind the modelling and optimisation module. Mathematical and computer tools implemented.

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.