Economic and environmental benefits by improved process control strategies in HCl removal from waste-to-energy flue gas

The control of HCl emission in waste-to-energy (WtE) facilities is a challenging flue gas treatment problem: the release of HCl from waste combustion is highly variable in time and the HCl emission standards are typically far lower in WtE than in any other industry. Traditional process control approaches in dry HCl removal processes are generally based on feeding a large excess of solid reactants to the system, to ensure robustness and a wide safety margin in the compliance to environmental regulations. This results in the production of a high amount of unreacted sorbents, strongly increasing the generation of solid wastes that need to be disposed. In the present study, an approach was developed to allow the implementation of improved control strategies for dry HCl abatement systems in operating full-scale facilities. Its objective is the reduction of the reactant feed and the waste production, while still providing an adequate safety margin for emission compliance. The approach was based on the reproduction of the behaviour of the real system in a virtual console that allows the extensive testing of alternative control strategies, limiting the need of demanding test-runs at the real plant. A test case on an Italian WtE facility demonstrated the capability of a control logic tuned in the virtual console to achieve a 13% reduction in the consumption of reactants and generation of process residues, with unchanged HCl removal efficiency. The results evidence the wide opportunities for optimisation of dry acid gas removal systems, in particular when multistage systems are implemented.