Combustion characteristics of dried sewage sludge and control of trace-metal emission

While the volume of sludge is increasing, it is difficult to find new final disposal sites, especially in Japan. The incineration of sewage sludge is a suitable way to decrease the volume. However, one of the principal problems is the elevated content of trace metals in sewage sludge. These trace metals can escape post-combustion cleanup as a gaseous phase or enrichment in fine particles. In this paper, the combustion tests of dried sewage sludge are performed using a 2-m-tall, 40-mm-inner-diameter electrically heated drop tube furnace. Experimental procedures involve W the combustion characteristics of dried sewage sludge and (ii) baseline runs of sludge alone and several runs with the kaolin addition as a sorbent at 1073, 1148, and 1223 K. The sample is collected by an aerosol size fractionation system that is composed of a water-cooled rapid N-2 quenching probe and a low-pressure impactor. The results show that the ignition temperature of dried sewage sludge is < 900 K. Hollow cenospheres are formed during the sludge combustion. Without the sorbent addition, baseline measurements of chemical compositions in-the particle size distributions indicate that most of the lead and cadmium is contained in the fine particles (< 1 mu m), whereas the variation in chromium concentration with particle size shows a trend similar to the total particle mass distribution of the ash particles. The addition of 5% dispersed kaolin powder in the sewage sludge results in a shift of lead and cadmium from fine particles to coarse particles, especially at a temperature of 1223 K during the combustion of sewage sludge. This shift is caused by the chemical reaction between kaolin particles and trace-metal vapors.