Techno-economic impact of lower emission standards for waste-to-energy acid gas emissions

Acid gas removal is one of the main drivers of operating costs in the flue gas cleaning lines of waste-to-energy (WtE) plants. In the light of updated technical and normative references, such as the revised Best Available Technology reference document for waste incineration in the EU, plants are required to comply with increasingly lower emission limit values (ELV). In the case of existing WtE plants, this requires selecting the appropriate option among three alternatives: intensification of current operations, installation of additional equipment (retrofitting) or substitution of equipment (revamping). The identification of the most cost-effective solution to meet the new ELVs is thus paramount. In the present study, a comparative techno-economic assessment is performed with reference to the relevant options available to WtE plants equipped with a dry acid gas treatment system, explicitly taking into account the influence of several technical and economic variables by a sensitivity analysis. The results show that retrofitting based on furnace sorbent injection is a competitive option especially in the presence of high acid gas loads in the flue gas. Despite the high investment cost, revamping based on conversion to wet scrubbing can also reduce the overall cost of treatment compared to intensification, but only if no constraints are present on flue gas temperature downstream of the acid gas treatment. If flue gas reheating is needed, e.g., for the compatibility with a downstream DeNOx treatment or to avoid plume visibility at stack, the associated costs make revamping not competitive with retrofitting or intensification. Sensitivity analysis confirms that these findings are robust even in presence of relevant variations in cost entries.

Scoping candidate minerals for stabilization of arsenic-bearing solid residuals

Arsenic Crystallization Technology (ACT) is a potentially eco-friendly, effective technology for stabilization of arsenic-bearing solid residuals (ABSRs). The strategy is to convert ABSRs generated by water treatment facilities into minerals with a high arsenic capacity and long-term stability in mature, municipal solid waste landfills. Candidate minerals considered in this study include scorodite, arsenate hydroxyapatites, ferrous arsenates (symplesite-type minerals), tooeleite, and arsenated-schwertmannite. These minerals were evaluated as to ease of synthesis, applicability to use of iron-based ABSRs as a starting material, and arsenic leachability. The Toxicity Characteristic Leaching Procedure (TCLP) was used for preliminary assessment of candidate mineral leaching. Minerals that passed the TCLP and whose synthesis route was promising were subjected to a more aggressive leaching test using a simulated landfill leachate (SLL) solution. Scorodite and arsenate hydroxyapatites were not considered further because their synthesis conditions were not found to be favorable for general application. Tooeleite and silica-amended tooeleite showed high TCLP arsenic leaching and were also not investigated further. The synthesis process and leaching of ferrous arsenate and arsenated-schwertmannite were promising and of these, arsenated-schwertmannite was most stable during SLL testing. The latter two candidate minerals warrant synthesis optimization and more extensive testing.