Coating integrated optical fibres for monitoring of boiler heat transfer surfaces

High temperature corrosion is often a problem in biomass combustion and waste incineration boilers. Critical components are heat transfer surfaces, such as water walls and superheaters. Instead of using more corrosion resistant high alloy steels for critical components more economical low alloy steels can be protected by coatings. These coatings can be applied by thermal spray techniques. This work investigates the possibility of integrating thin copper jacketed optical sensing fibres in this kind of coatings during the spray process. This kind of sensing fibres would provide new methods for managing boiler life and controlling the combustion process. The coating embedded fibres can be used for example to locate hot spots and places where corrosion and erosion are destroying the coating. In this work the cheapest possible method (direct spraying) for embedding the fibre was chosen even if it was known that this is close to the limits of what the fibre can sustain in order to retain its optical and mechanical integrity. The results of the direct spraying method are presented and some more advanced methods will be discussed. Methods for monitoring the state of the sprayed coating can extend from simple on-off methods to advanced distributed sensing methods. On-off methods mean monitoring the light transmission of the embedded fibre; when the corrosion or erosion reaches the fibre and destroys it the light transmission will cease. Using optical time domain reflectometry (OTDR) also onset of fibre destruction can be observed as well as the locations of these critical points. More advanced techniques are the use of in-fibre Bragg gratings for temperature and strain monitoring and the use of distributed temperature sensing (DTS) for hot spot detection. Both OTDR and DTS techniques will be presented in this work. The work presented was mainly done in the CEC funded Craft project PROCOMO (Protective coatings with combined monitoring system to control process conditions in boilers). The DTS demo shown in this work was carried out within the STYX project, which was a part of the Finnish nuclear safety research.