Hydrogen Chloride Removal from Flue Gas by Low-Temperature Reaction with Calcium Hydroxide

Municipal solid waste incineration (MSWI) is a method of waste valorization whose overall sustainability depends on the effective removal of the gaseous contaminants generated. Hydrogen chloride (HCl) is a typical pollutant formed in waste combustion. Dry processes based on its reaction with basic powders such as calcium hydroxide are among the state-of-the-art best available technologies for MSWI flue gas treatment. An experimental investigation of the heterogeneous reaction process between hydrogen chloride and calcium hydroxide in the temperature range between 120 and 180 °C was carried out. A laboratory-scale fixed bed reactor connected to a Fourier transfrom infrared (FTIR) spectrometer was used for the online continuous monitoring of HCl conversion. Solid reaction products were characterized using thermogravimetric analysis and X-ray diffractometry. The experimental data collected were used to validate a fundamental kinetic model for the description of the gas–solid reaction between Ca­(OH)₂ and HCl. A sensitivity analysis was carried out to assess the importance of the different temperature-dependent parameters in the model. The results allow an improved understanding of the heterogeneous reaction process that is applied in acid gas dry removal processes

CO₂ Uptake Potential of Ca-Based Air Pollution Control Residues over Repeated Carbonation–Calcination Cycles

The operation of dry processes for acid gas removal from flue gas in waste-to-energy plants based on the use of calcium hydroxide as a solid sorbent generates a solid waste stream containing fly ash, unreacted calcium hydroxide, and the products of its reaction with acid pollutants in the flue gas (HCl and SO₂). To date, the fate of the solid waste stream is to be put into a landfill in the absence of commercially viable recycling approaches. The present study investigates the potential of these residues as CO₂ sorbents in the calcium looping process. Samples collected in different waste-to-energy plants were tested over multiple carbonation–calcination cycles, comparing their performance to that of limestone. Although inferior, the CO₂ sorption capacity of the residues resulted in values comparable to that of limestone and that steadily increased for a significant number of cycles. This peculiar behavior was attributed to the presence of a chlorinated phase, which enhances the CO₂ uptake in the diffusion-controlled stage of carbonation by reducing the product layer resistance to CO₂ diffusion. No significant release of acid gases was observed at the characteristic temperatures of calcium looping carbonation