How to reduce pollutant emissions from small fluidised-bed combustors

As a result of 27 series of tests, it was concluded that the maximum reduction of NOx emissions occurred when the sulphur retention was also at its highest, so emphasising the important role that CaSO4 plays as a catalyst in pollution-reducing reactions. Although the minimal emissions of both SO2 and NOx (at 85 and 45 ppm, respectively) presently recorded occurred at a bed temperature of 800°C, under two-stage combustion and a low oxygen-in-the-flue concentration (=2%), the combustion efficiency under these conditions was relatively low (at 72·9% without limestone and 84·5% with limestone added to the fluidised bed). Optimal conditions for achieving maximum combustion efficiency and minimum pollutant emission occurred at the highest bed-temperature (=1000°C) employed, under two-stage combustion conditions for a moderately high (~4%) oxygen-in-the-flue concentration. Under these conditions, the CaS, formed from the lime in the substoichiometric bed, was completely oxidised to CaSO4 by the moderately high O2 concentrations in the freeboard, so optimising the reductions of both the NOx and SO2 emissions. The addition of limestone was found to increase the combustion efficiency by just under 3%, to a maximum of 91·3%, under these conditions. Further, the presence of limestone (which gave an added Ca:S mole ratio of 2), resulted in reductions in the NOx emissions of 83% (i.e. from 283 to 47 ppm) and in the SO2 emission of 74% (i.e. from 455 to 117 ppm). Both the NOx and SO2 emissions were greatly reduced by this addition of limestone, under most operating conditions. The magnitude of the reduction varied according to the bed's temperature, e.g. at a bed temperature of 800°C, under two-stage conditions, the NOx emissions were reduced by 71% and teh SO2 emission by 76%, provided sufficient limestone was added to the bed to give a 2:1 Ca:S ratio. Similarly, the use of recycled gas, to achieve bed attemperation during these tests, led not only to a reduction in the NOx emissions of 33%, compared with only 15% achieved in previous experiments, but also to a 26% reduction in the SO2 emission. The latter was a direct result of the increased residence time for SO2 gas in the contact with the limestone/ash particles within the combustor. When burning S.A. Duff with: (i) the exhaust-gas recycled back to the bed; (ii) limestone added to the bed (to give a Ca:S mole ratio of 2); and (iii) the fluidised bed operated at a relatively high bed-temperature (~1000°C) under two-stage combustion conditions with a 4% concentration of O2-in-the-flue; then a 90% overall reduction in NOx emissions (compared with those occurring under oxidising conditions with no limestone added) and a sulphur retention of 74% were achieved. Larger sulphur retentions ensued by reducing the bed temperature to 800°C and using lower oxygen-in-the-flue concentrations (~2%), but this occurred to the detriment of the combustion efficiency. Nevertheless, the lower bed-temperature of ~800°C was needed to avoid the formation of clinker when burning a low-ash fusion coal, such as Maryport smalls. By contrast, the use of a high bed-temperature (~1000°C) with low values of the oxygen-in-the-flue concentration, resulted in no sulphur retention; all the CaS being partially oxidised to SO2 under these operating conditions, with or without limestone present.