A thermodynamic equilibrium comparison of the mobilities of trace elements when washed and unwashed coals are burnt under pf firing conditions

摘要

Thermodynamic equilibrium predictions of trace element partitioning between the gaseous and condensed products of combustion have been made for four coals as supplied and after washing to remove further mineral. The calculations modelled the products of combustion of the whole coal with excess oxygen (1 percent excess for each coal with additional calculations at 3 percent for Harworth coal and 5 percent for Gascoigne Wood); the trace elements examined were As, Cr, Cu, Mn, Ni, Pb and Zn. The calculations indicated that almost all thermodynamic equilibrium distributions included a greater percentage of the trace element in the gas phase at 1300-2100 K for feedstock with extra washing as compared with the standard feedstock. When the amounts volatilised were converted to moles per 100 g of coal burnt it was found that in some cases the relative amount of some elements volatilised from washed compared with unwashed coal was lower, in other cases the relative amount volatilised increased.Step cooling calculations were carried out using as initial concentrations the amounts of each element in the gas phase at 1300 K for washed and unwashed Gascoigne Wood coal. The results indicated deposition of sulphates in the lower part of the temperature range 1300-600 K, with a greater tendency to form alkali sulphate based melts for the washed coal.