The ash deposition mechanism in boilers burning Zhundong coal with high contents of sodium and calcium: A study from ash evaporating to condensing

The high contents of sodium and calcium in Zhundong coal induce severe slagging and ash deposition in boilers. In this study, the ash deposition mechanism was investigated based on the results obtained from a full-scale boiler (350 MW) burning Zhundong coal, and a fixed bed reactor used for ash evaporatingcondensing. In the full-scale boiler, the condensing and depositing of sodium and calcium sulfates play an important role on ash depositing on convection heating surfaces. Sulfates start to significantly condense and deposit at the flue gas temperature of about 850 circle C on the medium and high temperature reheater surfaces. Ash evaporating tests proved that, with the increasing in temperature from 400 circle C to 1200 circle C, the ash evaporating process is divided into three stages: 1) 400-800 circle C, 80% of sodium, and 100% of chlorine are released; 2) 800-1000 circle C, all the left sodium evaporates and sulfur starts to be released with the formation of partial aluminosilicates; 3) 1000-1200 circle C, all the left sulfur is released through the decomposition of calcium sulfates and then calcium starts to evaporate, while silicon oxides disappear due to the formation of new complex silicates. Ash condensing tests further proved that, the sodium in Zhundong coal was released mainly in the forms of atom, oxide, and chloride, in which sodium chloride account for about 50%. When the evaporating temperature increased higher than 1000 circle C, partial alkali and alkaline earth metals were released as gaseous sulfates, and afterward condense and deposit on the heating surfaces. At last, a temperature-dependent ash deposition mechanism in Zhundong coal combustion was proposed. (C) 2015 Elsevier Ltd. All rights reserved.