Identifying and Measuring Corrosion of Boiler Materials in Various Combustion Environments

The urgent need to reduce greenhouse gas emissions is driving the development of cleaner energy technologies. This thesis examines various boiler material corrosion-related issues associated with biomass combustion, which is generally regarded as an enabling technology for carbon-neutral energy generation. To meet ambitious climate targets, advanced combustion techniques like chemical looping combustion (CLC) are necessary. CLC enables direct CO2 capture during combustion, making it a carbon-negative technology. CLC involves two reactors: an air reactor and a fuel reactor. In the fuel reactor, a metal oxide carrier transfers oxygen to the fuel, producing a flue gas stream primarily composed of CO2 and H2O. Although CLC using biomass fuel shows promise, it faces significant challenges similar to those in burning waste-derived fuels. One major issue is the presence of corrosive elements in biomass, such as potassium and chlorine, which can lead to severe corrosion of heat-transfer materials, particularly at the elevated temperatures required for efficient operation. This corrosion can compromise the long-term viability and efficiency of CLC systems.

The main focus of the thesis was to investigate the practical challenges and benefits of different methods for analyzing and measuring high-temperature corrosion under various conditions. A secondary aim was to assess how temperature, boiler atmosphere, and the presence of potassium and chloride affect the corrosion of different alloys in Chemical Looping Combustion.

Different techniques and analytical methods were used to measure and characterize the oxide scales formed due to synthetic K2SO4 and Na2SO4 mixtures containing KCl additions to provide two K-to-Cl ratios in the deposit. These analysis methods included Scanning Electron Microscopy (SEM-EDX), X-ray fluorescence (XRF), Inductively coupled plasma optical emission spectroscopy (ICP-OES), gravimetry, and 3D surface profilometry. The same sulfate mixture was employed to compare the influence of potassium halides (KCl, KBr, and KF) in the deposit on the corrosion of three alloys under conditions typical for waste combustion. Additionally, the thesis compared different methods for analyzing the composition of fly ashes produced from various coal and biomass co-combustion boilers.

The laboratory-scale studies that mimicked typical conditions in the CLC fuel reactor indicated that austenitic steels exhibit better corrosion resistance than ferritic steels in a reducing atmosphere containing HCl or a combination of HCl and oxygen at temperatures above 450°C.

Trace amounts of chlorine and bromine present in the K2SO4-Na2SO4 deposit led to increased corrosion, which was seen as thicker oxide scales and sintering of the deposit due to melt formation. This phenomenon should be taken into account when selecting materials for boilers, likely limiting the use of 10CrMo9-10 and AISI347 steels when the fuels contain halides.

The methods employed to measure and compare the corrosion of the boiler materials exposed to synthetic salt containing the corrosive species potassium and chlorine provided consistent results. Each method required specific sample pretreatments before analysis. Furthermore, expressing material behavior in terms of material loss or thinning required certain assumptions and data conversions to obtain the desired characteristics. A straightforward mass change of the alloy after removing the deposit and corrosion layer was found to be suitable for comparing material loss and thinning. However, SEM-EDX cross-sectional analyses of the metal samples with fixed deposits allowed for detailed investigations of the phenomena occurring within the samples in various environments. A new procedure and software developed for studying the exposed materials facilitated quick, efficient processing of the SEM images, ultimately yielding oxide layer data. One of the main advantages of 3D optical measurements of metal surfaces is their ability to provide a detailed description of localized corrosion effects. This research offered valuable insights into material selection, ash characterization, and corrosion assessment in biomass combustion systems, contributing to improved boiler performance.