Fluidised-bed incineration of acid tar wastes

Problems experienced with the land fill disposal of highly corrosive acid tar wastes mean that alternative, less environmentally damaging methods of disposal must be employed. The incineration of acid tars, although possessing several advantages over landfill disposal, can lead to unacceptably high SO2-emission concentrations in the flue gases unless precautions are taken. However, such incinerations have been achieved successfully in the present tests with a two-stage fluidised-bed combustor: the attained combustion efficiencies were high (at around 99%). The use of a fluidised-bed combustor for burning acid tar requires that limestone be added to the bed material in order to provide some degree of sulphur capture. The sensitivity of the SO2 emission to the limestone concentration (as indicated by the Ca: S mole ratio) fell as the Ca: S ratio was increased. At a Ca: S mole ratio of 4:1, 91% sulphur retention was achieved. During this test, 340 kg/h acid tar was pumped into the bed and burnt there successfully with 98 kg/h coal: the bed's temperature and height, and the excess oxygen-in-flue concentration were kept constant. Ultimately, average SO2 emission-levels (in the flue gas) of around 417 ppm were thereby achieved; i.e., well below the 600 ppm SO2-emission limit stipulated by the Commission of the European Communities. Nevertheless, the fluctuations in the SO2 emissions meant that a slightly higher Ca: S ratio of 5:1 was recommended in order to ensure that the emission levels were consistently below the required limit, i.e., such that 97% of all the mean 48-hourly values would not exceed 110% of the stipulated emission limit.