Abstract
Improving the efficiency of coal-fired boiler is beneficial for greenhouse gas control, mainly for carbon dioxide (CO2). The low-temperature corrosion covering heating surfaces is a frequent threat for coal-fired thermal equipment. The corrosion is induced by a fouling layer, where the ash deposition and condensed acid in coal-fired flue gas react mutually. The corrosion experiments were designed to investigate the reactions of representative acid solution between basic oxides, non-basic oxides, and fly ash particles covering metal surfaces. Scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS), X-ray fluorescence (XRF) and X-ray diffraction (XRD) were used to analyze the reaction particles and metal samples collected from experiments. The corrosion rates of 316L steel, 20# steel, Corten steel and ND (09CrCuSb) steel by the sulfuric solution of different concentrations with and without particles were obtained. The results showed that corrosion rate could be reduced by reacted particles, followed as: basic oxides particles > fly ash particles > non-basic oxides particles. Meanwhile, the deposited ash particles with smaller sizes contribute to a deeper acid–ash reactions due to more alkaline oxides accumulated. Thus, the metal surfaces will be covered by denser attachments, playing a function of corrosion resistance. The effect of fouling layer on low-temperature corrosion was obtained, guiding a safe and efficient operation of heat equipment in coal-fired flue gas.
Highlights
To address the urgent global warming accelerated by excessive carbon dioxide (CO2) emissions, mainly from fossil fuels, it is imperative to reshape the energy supply system by increasing the share of renewable energy, such as wind and solar energy [1]
To better explain the average acid condensation characteristics, the acid solution concentrations in experiments under different reaction temperatures were theoretically obtained on the premise of 20 ppm sulfuric acid vapor content and 10% water vapor content [29]
The field tests proved that low-temperature corrosion and the viscous fouling layer occurred simultaneously, caused by the reaction of ash particles and condensed acid solution [16,18,21,22,32,33]
Summary
The flue gas with a lower temperature further leads to many operational problems, including bed agglomeration, deposit formation, corrosion and fly ash during the combustion of coal [2] Among these problems, corrosion is the most crucial risk along with the flue gas of boilers for thermal equipment, such as pre-heaters, emission control equipment and flue gas duct parts. The potential reaction between ash particles and the condensed acid solution (acid–ash reaction mechanism) related to the surface temperature of the tube has to be considered when the fouling layer and low-temperature corrosion of heat exchangers are studied. The characteristics of the fouling layer were mainly related to the size distribution of fly ash particles, characteristic parameters of heating surfaces and flow distribution of flue gas. In this study, only the difference in H2SO4 concentration caused by fixed temperatures was considered, while the mass of the H2SO4 solution was assumed to be unchanged throughout the experiments
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