Alkali-ash reactions and deposit formation in pulverized-coal-fired boilers: experimental aspects of sodium silicate formation and the formation of deposits

The experiments investigate the formation of sodium silicates in reactions between silica particles and a synthetic furnace gas containing sodium, chlorine, sulphur and water vapour. A thin layer of silicate forms on the silica, having a thickness ranging from 0.02 μm to 0.31 μm, and an average composition from 9 to 36 wt% Na2O. Sulphur in the furnace gases reduces the silicate formation at temperatures below 1300–1400 K, and chlorine reduces its formation at higher temperatures. The thin, low viscosity layer of sodium silicate allows particles to collect on a cool metal surface (870 K) free from condensed material. Calculations suggest that in large pulverized-coal-fired boilers the formation of silicates by alkali-ash reactions can be predicted by thermodynamic considerations.