Research on formation mechanism of typical low-temperature fouling layers in biomass-fired boilers

Ash deposition in flue gas covering low-temperature surfaces (<150 degrees C) of biomass-fired power plants have two typical characteristics that are distinct from that of coal-fired power plant. First, NH4Cl is present in the deposited ash when the surface temperature is adequately low. Second, hygroscopic salts, such as CaCl2, MgCl2, NaCl etc. are present in the deposited ash, which may cause the ash to adhere to the surface when the salts absorb water. In this study, a test system with transformable cooling media was designed according to the measured temperature needs for on-site ash deposition experiments in a 35 MW biomass-fired power plant. Scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) were used to analyze the ash samples collected during the experiment. The results show that four typical ash deposition layers were obtained by varying the tested pipe temperature during the cooling process, i.e., dry-loose ash layer (>120 degrees C), multi-loose ash layer (120-90 degrees C), adhering ash layer (90-60 degrees C), and viscous ash layer (<60 degrees C). In order to study the formation mechanism of typical low-temperature fouling layers, the following analyses were performed: the boundary temperature of NH4Cl crystallization was calculated, the condensations of H2SO4 and HCl were analyzed, critical deliquescence temperatures (CDTs) of deliquescent chlorides were calculated, and water uptake of deliquescent chlorides under different temperatures and moisture were proposed.