To guarantee a safe compliance with the ambitious targets of acid gas (HCl, SO2) removal set by recent European regulations, existing waste-to-energy (WtE) plants equipped with single-stage acid gas treatment systems might take into consideration the installation of an additional treatment stage as retrofit. Multi-stage treatment systems can achieve higher acid gas removal efficiency and flexibility to cope with highly variable inlet pollutant concentrations, resulting from variability in waste composition.
From a sustainability viewpoint, the choice of the most suited retrofitting option has to be driven not only by considerations on performance at the WtE plant but also by considerations on the indirect environmental burdens related to the material and energy consumption and waste generation of the selected options.
The present study analyses the full environmental profile of two competitive dry retrofitting solutions (the low-temperature injection of hydrated lime and the high-temperature injection of calcined dolomite) by considering their life cycle impacts associated with the supply of reactants and the disposal of process residues. Modelling of the material balance of the alternatives as a function of pollutant load in the inlet flue gas allowed estimating the effect of flue gas composition and mode of operation of the two-stage system on the overall environmental impact. Both retrofitting options were found to reduce the life cycle impacts of the base case, but the use of hydrated lime showed the greatest potential. The generation of process residues constitutes a relevant fraction of the overall impacts. An alternative scenario for the management of process residues was evaluated to quantify the reduction of impact achievable with a commercially available valorisation route.