Content

1. Introduction
    
2. Optimised furnace cleaning 600 MWe unit3
   2.1   Cleaning equipment
   2.2   Local detection of slagging ón furnace walls
   2.3   Intelligent on-load cleaning concept
   
3. Results
4. Conclusion

Abstract

Slagging and fouling are the most common reasons for reduced boiler availability and efficiency. The number of unscheduled shutdowns as well as the degree of efficiency loss is strongly dependent on the fuel quality and increase with the use of low rank fuels and fuel mixtures. The most efficient measure to increase boiler efficiency and availability is the selective on-load cleaning of different boiler sections with appropriate cleaning equipment controlled by intelligent diagnostic systems.

In this paper an efficient on-load cleaning system is presented, combining water cannon technology with advanced heat flux measurements to a closed loop furnace optimization system for detecting and cleaning boiler regions with unacceptable high ash deposition. The main focus is put on the optimized selective cleaning of the furnace region to achieve a low furnace exit gas temperature since the latter is a direct measure for the evaporator efficiency and an indicator for fouling on the heat exchanger surfaces in the convective pass.

The on-load boiler optimization system was applied to a 600 MWe utility boiler fired with lignite and bituminous coal. Already during the first weeks of operation with the optimization system a significant improvement of the boiler efficiency was observed. The average heat transfer from the furnace to the steam cycle increased by 30% and the furnace exit gas temperature decreased by about 80 degrees.

2.3 Intelligent on-load cleaning concept

The overall concept of an intelligent on-load cleaning system is sketched in Figure 8. The heat flux sensors are installed at preselected, optimum locations on the evaporator walls. The measuring signal of the sensors is transmitted through a multiplexer to the data acquisition system where it is complemented with available plant data such as boiler load, coal mill operation, etc. The optimisation system is monitoring the situation online and is calculating and analysing the current slagging situation of the furnace. In parallel, the system is doing the optimisation calculation following conditions such as:

• Optimum point in time for starting the cleaning action
• Selection of required cleaning zone
• Selection of optimum parameters for the individual cleaning zones (e.g. jet velocity, water quantity, cleaning pattern)

As a result, the optimisation system determines the required cleaning actions and controls the PLC based water cannons.

4. Conclusions

Ash deposition in utility boilers increases with the use of low rank fuels and fuel mixtures. Above a boiler specific critical deposition rate the overall availability and efficiency of the unit may decrease significantly. A first measure to control the overall slagging and fouling situation in a boiler is the optimized and efficient cleaning of the heat exchanger surfaces. Hereby the furnace exit gas temperature can be controlled and kept below the fuel specific ash melting temperature. Furthermore, the heat transfer rate of the evaporator will be increased and so will be the efficiency of the steam generator.

The presented work deals with the installation of an intelligent on-load boiler cleaning system to a 600 MWe steam generator fired with lignite and bituminous coal. In this closed loop optimization system water cannons are combined with advanced heat flux sensors to efficiently clean the furnace depending on the current operational condition of the boiler.

Operational experience over several months indicated that the presented furnace optimization system enables to minimize furnace slagging and superheater fouling significantly, to reduce waste gas losses and to increase the overall boiler availability and efficiency.