Sewage sludge and wood sawdust co-firing: Gaseous emissions and particulate matter size distribution

Abstract

Sewage sludge (SS) is an inevitable by-product of wastewater treatment and its disposal is a continuing challenge for sustainable development and environmental management. Co-firing dried SS and wood sawdust (WS) seems to be a feasible SS disposal method. In the presented work, composite pellets made at 100% WS/0% SS, 95/5, 90/10, 85/15 and 80/20 ratios were combusted in a 25-kW grate-fired boiler and the effects of SS on the emissions and particulate matter (PM) size distributions were studied. An online stack analyzer was used to measure gaseous emission values and the PM concentrations were determined by three different principles. HT-DLPI+ was deployed to measure the particle mass distribution of particles between 13 nm and 50 μm and a SMPS-3080 with a CPC-3775 by TSI was used to measure particle number concentrations between 19 and 1000 nm. The results reveal that with increasing SS content in the pellets, the NOx and SO2 emissions increased, while the CO emissions significantly decreased. PM emissions also showed a strong correlation with the amount of SS in the pellets. SS addition increased the mass median diameter, nonetheless the contribution of particles over 0.6 μm in aerodynamic diameter was found to be insignificant in all cases.