Corrosion as a Result of Dew Points and Deliquescent Salts in the Boiler and the Flue Gas Treatment System

Boilers with difficult fuels such as waste, refuse derived fuels and biomass, may not only be hit by corrosion on the heat transfer surfaces of evaporators or superheaters, but also at the cold end, i.e. on ECO and heat transfer surfaces in the preheater area and even on the evaporator tubes or uncooled steel sheets located there. Corrosion may also occur during subsequent flue gas cleaning, on the raw gas and clean gas sides.

It is known from the combustion of fossil solid fuels that, in most cases, the problems of corrosion at the cold end are to be traced back to sulphuric acid, i.e., the dew point temperature of sulphuric acid constitutes a conditioning factor for operation processes. This dew point temperature is a function of the concentration of gaseous SO3 and the content of water vapour in the flue gas. In common terminology in the field of coal combustion, the term dew point corrosion is synonymously used with sulphuric acid dew point corrosion. This clear correlation has to do with the chemistry of coal as a fuel. If reference is made to the dew point of water, the water dew point is expressly referred to.

These common references to dew points and to sulphuric acid as the cause of any corrosion at the cold end (inasfar as the water dew point is not even reached) are not to be directly transferred to boilers with difficult fuels. Current findings suggest this. One of the causes here – compared to coal – lies in the significantly changed chemistry of these fuels. But also process-related aspects can play a role, particularly DENOx (SNCR, selective non-catalytic reduction).

This article demonstrates that, in the case of difficult fuels, not only sulphuric acid is to be considered as the cause of cold end corrosion, but also salts with hygroscopic and deliquescent properties. Deliquescent salts are hygroscopic to such extent that they deliquesce and form an electrolyte, thus causing corrosion through an aqueous electrolyte. This deliquescence corrosion can be distinguished from the dew point corrosion of sulphuric acid and water by the fact that the triggering process, the formation of halogen salts, does not constitute a dew point but, initially, the deposition of a salt resulting from the phase transition from a gaseous to a solid state or from the deposition of these salts as solid particles; it is only afterwards that, with sufficient humidity in the flue gas and appropriate temperatures, the deposited salts deliquesce and form a saturated salt solution, the corrosive electrolyte.

Copyright: © TK Verlag - Fachverlag für Kreislaufwirtschaft
Quelle: Waste Management, Volume 3 (Oktober 2012)
Seiten: 14
Preis inkl. MwSt.: € 0,00
Autor: Dr. rer. nat. Thomas Herzog
Dr. Wolfgang Spiegel
Dipl.-Mineraloge Wolfgang Müller

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