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In recent years the efforts to reduce nitrogen oxide (NO_x) emissions from power stations have resulted in operational modifications including the fitting of low NO_x burners. These modifications are expensive and generally have an adverse effect upon plant performance, resulting in an increase in unburnt carbon. To reduce these adverse effects, on-line optimisers have been developed as an enhancement to the power station s digital control system (DCS).

The success of the boiler optimisation models has suggested that on-line optimisation can be used in other parts of the power station, eg thermal efficiency, electrostatic precipitator (ESP). Although each local optimiser is able to perform its task well individually there will be occasions when the individual packages will provide conflicting advice.

The purpose of this unit optimisation project is to develop an integrated approach to unit optimisation and develop an overall optimi ser thatis able to resolve any conflicts between the individual optimisers. A substantial demonstration project has been conductedat Southern Company s Plant Hammond over recent years.

Financial support for the project has been provided by the UK DTI, US DOE and EPRI together with some participant companies. Considerable effort has been put into the project by staff in E.ON UK, URS Corporation, Energy Technologies Enterprises Corporation, Tennessee Technological University, SCS E ngineering and Syngenco engineering

Unit optimisers provided by E.ON UK and Synengco have been considered during this project together with the following individual optimisers:

• On-line thermal efficiency package
• GNOCIS boiler optimisation
• GNOCIS steam side optmisation
• ESP Optimisation
• Intelligent Sootblowing System (ISBS)

There has been a substantial amount of software development during the Unit Optimisation project at P lant Hammond. Access to the results of this large project has been very valuable in showing the level of detail and complexity of the optimiser models. The project has also focussed thinking in the UK ahead of actually running multiple optimisers on a single unit.

There has been substantial development of individual optimisers during the project and in several instances the development of the individual optimisers has been difficult. This has meant that the evaluation of the unit optimi ser hasnot progressed as much as originally hoped.

SCS have installed the Synengco Sentient software as the unit optimiser. Use of this software together with Excel has resulted in the individual models being quite remote from the DCS making closed loop installation difficult. It is not clear that this approach will be used in the future since it would seem preferable to keep the individual optimisers close to the DCS.

Potential conflict between optimiserscan be reduced by eith er prioritising the objectives of different optimisers (eg environmental objectives achieved ahead of efficiency ones) or adding rules to optimisers (eg including a steam temperature model within the boiler optimiser, GNOCIS).

The limited feedback on the performance of the E.ON UK unit optimisation algorithm was favourable. Convergence was usually obtained in about 5 iterations and the algorithm proved stable and reliable.