Metabolism of metals from co-processing of energy-from-waste air pollution control residue in cement kilns

Abstract

Co-processing of air pollution control residues (APCR) from energy-from-waste (EfW) facilities in the cement kiln is a potential alternative to landfill and an opportunity to reduce the virgin raw material demand of cement production. However, the high concentrations of trace metals and soluble salts in these residues are of concern. In the present work, material and substance flow analyses were conducted to estimate the potential effects of co-processing raw and washed APCR from UK EfW facilities as alternative raw materials for cement manufacture on the metal concentrations in cement powder. The results suggest that co-processing APCR can significantly increase the concentration of several toxic trace metals in cement, but may decrease the concentration of others (e.g., Tl) whose background concentrations in virgin raw materials typically used in the UK are higher than those in APCR. The high chlorine concentration in APCR leads to a disproportional increase in the fraction of raw gas bypassed from recirculation to the preheater, from 1 to 81%, when the co-processed amount is increased from 0 to 35% of the raw materials. This effect can be mitigated by pre-washing of APCR. However, loss of soluble salts during water washing increases the concentrations of insoluble trace metals in the residue (Co, Cr, Mn, Ni and V), and, thus, cement. The collection of more UK-specific data would increase the robustness of the model and life cycle assessment of the different disposal options (landfilling, co-processing with and without washing) could help develop sustainable policies regarding the fate of APCR.