Low-Temperature Corrosion in Biomass-Fired Combustion Plants - Online Measurement of Corrosion Rates, Acid Dew Points and Deliquescence Corrosion

Low temperature corrosion affecting the surface of heat exchanger tubes is a main issue when it comes to efficient heat recovery technologies in flue gas flows from biomass combustion plants. Acid dew point corrosion as well as deliquescence corrosion caused by hygroscopic salts are supposed to be the main activators. In this paper the results from test runs performed in two grate combustion units in Austria firing chemically untreated wood fuels applying a newly developed low temperature corrosion probe shall be presented. Furthermore, a new methodology regarding the determination of SO3 has been applied enabling the quantification of low SO3 contents in the flue gas in biomass combustion plants. The test runs with the corrosion probe indicated, that corrosion due to acid dew points plays a minor role at these applications. Measurements of dust emissions and ash deposits on the sampling probes showed considerable amounts of KCl and K2CO3 present which are both hygroscopic salts. The deliquescence characteristics of these salts are most likely responsible for the elevated temperatures (considerably above the water due point) at which corrosion signals started to occur.