The evaporation of desulfurization wastewater using flue gas has been considered as a promising technique. However, the release of HCl during the evaporation hinders the application of this technique. Herein, we investigated the technique in a laboratory-scale flue duct through experimental and numerical methods. The influences of operating parameters on characteristics of evaporation and HCl release were experimentally explored. The results show that when inlet gas temperature increases from 200°C to 350°C, gaseous HCl concentration significantly increases from 5.02 ppm to 70.96 ppm and HCl release ratio accordingly increases from 1.57% to 17.32%. However, when the wastewater only contains CaCl₂, the HCl release is effectively inhibited. Subsequently, the established CFD model was validated with the experiments. It was found that relative deviations between predicted and experimental outlet gas temperatures mostly locate within the range of -10%∼0%. The negative deviations are attributed to the neglect of the crust formation. Numerical studies also reveal that droplet evaporation temperature is close to the wet-bulb temperature of the inlet gas ranging from 321.47 K to 337.85 K. Moreover, thermodynamic analysis shows that when the wastewater contains equimolar MgCl₂ and CaCl₂, MgCl₂·6H₂O first crystalizes at about d/d₀ = 0.3. However, when the wastewater only contains CaCl₂, CaCl₂·2H₂O begins to crystallize at about d/d₀ = 0.25. Before crystallization, the equilibrium partial pressure of HCl is close to 0 Pa, indicating the HCl is mainly released at the particle drying stage. Further analysis suggests HCl should be mainly released from the decomposition of MgCl₂·6H₂O into MgOHCl. Finally, several potential solutions to inhibit the HCl release were proposed.

The increasing penetration of renewable energy into the grid has given rise to an emerging challenge of maintaining the supply–demand balance. Conventional supply-side regulation is now insufficient to maintain this balance, necessitating flexible resources from the demand side to address this challenge. Buildings, as important energy end-use consumers, possess abundant flexible resources and can play a significant role in responding to grid dispatch via demand response. Therefore, grid-interactive buildings (GIBs) have garnered widespread attention. This technology coordinates the scheduling of distributed renewable energies, energy storage, and adjustable loads via advanced control methodologies, leading to the reshaping of building load profiles to enhance grid flexibility. However, the realization of energy flexibility in GIBs has not yet been comprehensively identified in the literature. To narrow the knowledge gap, this review compared GIBs with other technologies of building energy management to highlight the distinct features of GIBs. Additionally, the flexible energy strategies of GIBs were explored, combined with flexible resources within buildings, and the feasible pathways for these strategies were also addressed. Based on the scheduling scenarios in GIBs, the performance characteristics of various control methodologies were compared and analyzed. Finally, an evaluation framework for GIBs was established. This review will facilitate the shift of buildings from traditional energy consumers to flexible resources that actively respond to the grid and provide critical support for the grid stability and reliability.