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The objectives of this project were;

• To determine the effect of co-firing biomass with coal on boiler tube fireside corrosion rates in existing subcritical coal fired power plants.
• To determine the likely fireside corrosion rates that could be expected on boiler tubing in future advanced (ultra) supercritical power plant.

Typical and potential boiler tube alloys have been exposed to simulated furnace wall and superheater/reheater environments in the 1MWTh Combustion Test Facility (CTF) at Power Technology.

A total of four nominally 50 hour duration exposures have been completed. Specimens were exposed to a range of metal temperatures, heat fluxes and gaseous environments, representative of pulverised coal combustion under low NO_x conditions with biomass additions. Biomass was co-fired withDaw Mill coal on 20 and 10 thermal or heat input basis (approx 35 and 17 by mass). Specimens were exposed to the combustion environment on air-cooled, precision metrology, corrosion probes.

When co-firing with wood there was no discernable worsening of either furnace wall or superheater / reheater corrosion when compared with firing coal alone. As would be expected, the austenitic stainless steel superheater/reheater specimens exhibited corrosion rates substantially reduced compared to low alloy ferritic T22 specimens. Co-firing with Cereal Co-Product (CCP) yielded high furnace corrosion rates under reducing conditions, comparable with that expected when firing coal alone. Under oxidising conditions the furnace corrosion rates were modestly increased from the expected low rates normally encountered. The T22 superheater/reheater specimens also exhibited metal wastage rates slightly increased compared with those measured previously. However, a marked increase in the corro sion was noted in the case of the austenitic superheater/reheater samples, where localised pitting attack resulted in peak wastage rates similar to those measured for the T22 samples.

The data indicate that plant operating at relatively low final steam temperatures ( 540 C), employing only low chromium containing ferritic alloys, could safely operate whilst co-firing CCP or similar fuel, and expect only slight worsening of existingcorrosion rates. However, plant operating at higher steam temperatures (= 560 C), which contains austenitic alloys, are potentially vulnerable to greatly enhanced superheater and reheater fireside corrosion attack. Such enhanced attack would lead to a marked reduction in expected tube operating lives when compared to plant firing only coal.

With only two biomass fuels examined it is impossible to fully identify the reasons for the changes in corrosion rates measured. However, it is likely that high alkali metal contents, and specifi cally potassium, in some biomass materials, are particularly aggressive. Potassium in biomass tends to be bound organically and hence is very reactive. This compares with potassium in coals, which tends to be un-reactive, being well bound to mineral matter.

Consideration should be given to extending the scope of this work to include other potential biomass fuels, in order to confirm the effects of the fuel composition on wastage rates and identify which fuels can be burnt safely without adversely affecting the operating life of superheater and reheater stages. Corrosion probe exposures in actual operating plant would be required to confirm whether the high wastage rates observed in the short term CTF trials are reproduced over the longer term.