Chlorine Induced Corrosion in Biomass and Waste Fired Boilers: Laboratory and Field Investigations

Doktorsavhandling, 2010

The fireside corrosion in biomass and waste fired boilers is widely believed to arise from the presence of chlorine-containing compounds in the fuels. Although many researchers have studied the influence of such compounds on the high temperature corrosion of various steels, the mechanisms underlying chlorine-induced corrosion are still poorly understood. In this thesis, the chlorine induced corrosion of austenitic and ferritic steels is investigated through careful laboratory exposures, detailed microscopy experiments and field exposures in actual boilers. In the laboratory, the effect of adding 500 ppm HCl(g) to a dry atmosphere containing 5% O2 in N2, and to a wet atmosphere containing 40% H2O and 5% O2 in N2, on the oxidation of the austenitic alloy 310S at 500°C was investigated, and also the effect of 0.1 mg/cm2 KCl(s) on the oxidation of the ferritic steel Fe-2.25Cr-1Mo at 400 and 500°C in an atmosphere with 40% H2O and 5% O2 in N2. Polished steel coupons were isothermally exposed for up to 168 hours. After exposure, the samples were analyzed using SEM/EDX, AES, FIB and BIB cross sections, TEM, XRD, and IC. The Fe-2.25Cr-1Mo steel was also oxidized in the presence of KCl, in the specimen chamber of an ESEM, and imaged in situ to follow the evolution of the corrosion morphology in real time. Austenitic and ferritic steels were also exposed in both biomass and waste fired boilers, using air cooled corrosion probes. The field exposures were made to relate the corrosion mechanisms in the laboratory to the actual applications and to evaluate the effect of adding sulphur to the fuel as a measure to mitigate the corrosion. The laboratory investigation showed that the presence of chlorine-containing compounds caused a rapid increase in mass gain, the formation of transition metal chlorides and, after longer exposure times, poorly adherent and buckled oxide scales. The HCl(g) caused formation of transition metal chloride and accelerated corrosion mainly along steel grain boundaries and at slag inclusions in the steel surface of 310S. The KCl caused a significant acceleration of the oxidation of Fe-2.25Cr-1Mo at 400°C and after short exposure times at 500°C. A new mechanism for the chlorination of steel, based on the simultaneous oxidation of O2 and dissociation of adsorbed HCl or KCl at the scale surface and on diffusion of chloride ions along oxide grain boundaries towards the steel surface, is proposed. Transition metal chlorides and high concentrations of alkali chlorides were detected, and also chromate, on austenitic stainless steels, in the field exposures when no sulphur was added to the fuel. With sulphur added, the amount of alkali chloride was suppressed in the waste fired boiler and eliminated in the biomass fired boiler. The average corrosion rage was reduced by around 50% for all materials in the waste fired boiler. In the biomass fired boiler, the corrosion rate of the ferritic material was reduced by almost 50%, while the corrosion rates of the austenitic materials were reduced by more than 90% when sulphur was added.