Alkali-ash reactions and deposit formation in pulverized-coal-fired boilers: the thermodynamic aspects involving silica,sodium, sulphur and chlorine

Thermodynamic calculations predict the behaviour of alkalis in the furnace gases of pulverized-coal-fired boilers. Results imply that a large proportion of the volatile sodium in the coal reacts with silica or silicate fly ash and forms a low viscosity surface layer of sodium silicate on the ash which, depending on its thickness, may enhance retention of the ash particles impinging on boiler tubes. The thickness of the sodium silicate layer depends mainly on the ratio of alkalis (non-chloride) to ash surface area in the furnace gases and the kinetics of the silicate-forming reaction in the temperature range 1300–1850K. The calculations show that sodium sulphate is stable only below 1300–1400 K in pulverized-coal-fired furnace atmosphere, and, therefore, sodium-ash reactions at higher temperatures will greatly reduce sulphur retention in the boiler as sulphate deposits.