Co-combustion of Bituminous Coal and Pickling Sludge in a Drop-Tube Furnace: Thermodynamic Study and Experimental Data on the Distribution of Cr, Ni, Mn, As, Cu, Sb, Pb, Cd, Zn, and Sn

The effect of bituminous coal and pickling sludge co-combustion on the distributions of Cr, Ni, Mn, As, Cu, Sb, Pb, Cd, Zn, and Sn in flue gas, fly ash, and bottom ash were studied in a drop-tube furnace. To simulate combustion conditions in suspension-firing boilers, experiments were carried out at temperatures ranging from 1100 to 1400 °C, with sludge amounts ranging from 0% to 10% by weight. The results show that the 10 selected heavy metals could be divided into three distinct classes according to bottom ash mass percentage, except for Sn, which showed an irregularity. Class I included Cr, Ni, Mn, and As and were identified as less volatile heavy metals because more than 95% was retained in the bottom ash; these heavy metals exhibit high-temperature stability. Class II contained Cu, Sb, Pb, and Cd and were identified as semivolatile heavy metals; nearly 20–40% was distributed among flue gas and fly ash. In addition, the bottom ash heavy metal percentage decreased markedly with increasing temperature. Zn belongs to class III and was identified as a volatile heavy metal; less than 20% was retained in bottom ash under all experimental conditions. Thermodynamic equilibrium calculations were used to forecast heavy metal compounds, and most calculation results were consistent with the actual outcomes. X-ray diffraction results indicated that Cr and Ni mainly reacted with MgO and Fe2O3 to form MgCr2O4 and NiFeO4 in solid phase during the co-combustion process. The heavy metal emissions in flue gas meet the national standard, while the bottom ash leaching results indicate that the bottom ash from bituminous coal and pickling sludge co-combustion cannot be disposed of as common waste through landfill disposal without further treatment.