Co-combustion of sewage sludge together with coal or wood has been investigated in two circulating fluidized bed (CFB) plants, a laboratory scale plant and a pilot scale 12MWth CFB boiler, in both of which the gas residence times are comparable to those in commercial plant. The investigation focuses on emissions of harmful gases from co-combustion compared to mono-combustion in CFB and the influence of air supply. The result shows that co-combustion can be carried out in CFB plant designed for the base fuel without exceeding EU or German emission limits for sludge energy fractions of less than 25%, except for the chlorine emission that may have to be reduced by flue gas treatment. Although sewage sludge contains large quantities of nitrogen and sulfur, the beneficial properties of CFB lead to considerable reduction of nitrogen oxides, and only a few percent of the nitrogen was effectively converted to NO or N2O. Sulfur can be captured by conventional limestone addition, but for wood as a base fuel this method is not as efficient as for coal.
