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Superheater corrosion in a fluidised boiler burning biomass with a steam temp of 550 deg C

Superheater corrosion in a fluidised boiler burning biomass with a steam temp of 550 deg C is reduced by (a) use of a suitable additive (ChlorOut) and (b) use of a suitable material (austenitic stainless steel)

One of the major drawbacks to the combustion of 100% wood and waste wood fuels in power station boilers is the increase in the fouling and corrosion of superheaters, which increases operation and maintenance costs. Wood fuels have a high content of potassium and chlorine, but they contain very little sulphur compared to fossil fuels.

To combat corrosion, boiler tests have been performed with ammonium sulphate, which is sprayed into the flue gases after combustion but before the superheaters and effectively converts gaseous KCl into potassium sulphate, K2SO4. This is much less corrosive than KCl. Long-term probe measurements showed that the corrosion rates were reduced by 50%. Deposit growth rates were also reduced by about 50%. This concept, together with an instrument for measuring alkali chloride levels in the flue gas has been patented and is known as “ChlorOut”. Probe tests with different steels showed that superheater corrosion can also be greatly reduced by the choice of the right material. The stainless steel Esshete 1250, which contains only 15% Cr, performed very well in all the boiler tests.

A complete set of superheaters for a 100MW combined heat and power boiler costs about 1.5 MEuro. The durability of superheaters is thus an important factor in determining the long-term production costs. Unplanned outages due to leaking superheaters are also very expensive. As well as causing corrosion problems, the build-up of deposits reduces the heat uptake to the superheaters, which leads to lower efficiency. The use of ammonium sulphate additive, which approximately halves the corrosion and fouling rates in wood-fired boilers, is thus expected to greatly reduce the cost of producing electricity from biomass. This means that it becomes more technically viable and the amount of electricity produced from biofuels will therefore increase.

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Vattenfall Utveckling AB
814 26 Aelvkarleby
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