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Short term online corrosion measurements in biomass fired boilers. Part 2: Investigation of the corrosion behavior of three selected superheater steels for two biomass fuels

Retschitzegger, S., Gruber, T., Brunner, T., Obernberger, I.

Published 02.2016

Citation: Retschitzegger, S., Gruber, T., Brunner, T., Obernberger, I. Short term online corrosion measurements in biomass fired boilers. Part 2: Investigation of the corrosion behavior of three selected superheater steels for two biomass fuels. Fuel Processing Technology. Volume 142, February 2016, Pages 59-70.

Abstract

The high temperature corrosion behavior of the boiler steels 13CrMo4-5 (1.7335), P91 (1.4903) and 1.4541 has been investigated during short-term test runs (~ 500 h) at a biomass fired grate furnace combined with a drop tube. For the test runs performed with 13CrMo4-5 and P91 chemically untreated wood chips have been used as fuel, whereas waste wood has been used for test runs with P91 and 1.4541. Online corrosion probes and a mass loss probe have been used applying a methodology developed in a previous study to correct for a measurement error occurring during short-term measurements with online corrosion probes (mass loss correction). Furthermore, deposit probe measurements have been performed to evaluate the deposit build-up rate and the chemical composition of deposits. SEM/EDX analyses of the corrosion probes have been performed subsequently to the test runs to gain information regarding the chemical composition and structure of the deposits as well as the corrosion layers.

The furnace has been operated at constant load to ensure constant combustion conditions. The flue gas temperature at the probes has been varied between 740 and 900 °C and the probe surface temperature has been varied between 400 and 560 °C in order to determine their influence on the corrosion rate.

General trends determined by the variation of these temperatures were similar for all boiler steels: the corrosion rate increased with increasing flue gas temperature and also with increasing probe surface temperature. For chemically untreated wood chips combustion at low flue gas temperatures (740 °C) the corrosion rates were comparable for 13CrMo4-5 and P91 at all probe surface temperatures. However, at flue gas temperatures of 800 °C and higher P91 showed better corrosion resistance than 13CrMo4-5. For waste wood combustion 1.4541 generally showed a better corrosion resistance than P91.

The mass loss correction of the measurement error occurring in the initial phase resulted in different errors of 55% for 13CrMo4-5 and 32% for P91 for chemically untreated wood chips. For waste wood the mass loss correction resulted in errors of 55% for P91 and 77% for 1.4541. The results from the mass loss determination for the waste wood test runs scattered stronger compared to the wood chips test runs. Therefore, the fits were not that accurate and the error margin was higher. However, the results outline that the mass loss correction is relevant in order to achieve a meaningful comparison of different short-term test runs using online corrosion probes.

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