Bottom ash characteristics and pollutant emission during the co-combustion of pulverized coal with high mass-percentage sewage sludge

A municipal and a coal-coking industrial sludge (MS and CS) were separately co-combusted with bituminous coal at a sludge mass ratio (SMR) of 10%–50%. Quartz, larnite, gehlenite, mayenite, and calcium sulfate were formed during co-combustion. Slight or medium slagging was predicted to occur when slagging tendency was assessed based on the mineral components and fusion temperature of the ash. The SO2 emission peak at the devolatilization stage increased with increasing SMR of MS but only slightly changed with increasing SMR of CS. SO2 emission at the char combustion stage was suppressed due to the sulfur retention effects of sludge ash. NO, HCN, and N2O were the main emission forms of N-containing gases. With increasing SMRs of MS and CS, HCN emissions became increasingly prominent, whereas NO showed only a slight increase and even decreased. In addition, the volatilization percentages (VP) of Cd, Zn, Cr, and Ni decreased. By contrast, the VP of Pb increased when coal was combusted with CS because Pb was considerably influenced by high Cl. The concentrations of Cd, Cr, Ni, Pb, and Zn in the bottom ashes were determined and satisfied China's threshold limit value for landscaping land application or soil amendment. •Co-combustion of sludge and coal at high sludge mass ratio (SMR) was carried out.•Ash fusion temperatures (AFTs) were high although they decreased with increasing SMR.•Slight slagging was predicted due to high AFTs and low combustion temperature.•SO2 emission peaks were decreased by ash components at char combustion stage.•Minerals of sludge immobilized the heavy metals and decreased their volatility.