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.

Viscous ash layers and low-temperature corrosion pose major challenges to the high-efficiency operation of heat exchange equipment in the flue gas of coal-biomass co-firing power stations. The field experiments on low-temperature fouling layers were carried out in a 135 MW coal-fired power station and a 35 MW biomass-fired power station. The micro characteristics of ash samples were tested to explore the forming mechanisms of viscous ash layers. The results show that the reactions of ash particles and ammonium salt crystallization (NH₄HSO₄, (NH₄)₂SO₄, NH₄Cl), acid condensation (H₂SO₄, HCl and HF), deliquescence of hygroscopic salts (CaCl₂, MgCl₂, NH₄Cl, etc.), and water condensation determine the viscous ash layer formation covering heating surfaces in coal-biomass co-firing power stations.