Staged desulphurization by direct sorbent injection in pulverized-coal boilers

Emissions of the atmospheric pollutants SO2, H2S, CO2, NO, etc. from coal-fired PCC boilers and desulphurization of flue-gas by direct sorbent injection into the furnace are considered. The mathematical model used is based on equilibrium calculations for the coal/water/limestone/air system (50 gas-phase and six solid-phase chemical species) and the concept of freezing of the gas composition at low temperatures. Experiments were performed on bench-scale, coal-fired boilers. To improve the emission performance, a staged-desulphurization method is proposed. This method involves successive sulphur retention in the form of CaS in the high-temperature reducing zone of the furnace and as CaSO4 in the low-temperature oxidizing zone. It is shown that staging of sorbent supply increases the desulphurization efficiency to 80–85%. Experimental data agree with numerical predictions.