**1. Introduction**

In China, wet flue gas desulfurization technology is widely applied to coal-fired thermal power plants to remove SO2 in the flue gas because it has the advantages of high efficiency, low operating cost, and high reliability [1–4]. However, this technology produces large quantities of desulfurization wastewater, which contains many acidic ions, heavy metal ions, and suspended solids [5–7]. Releasing desulfurization wastewater into the environment is strictly prohibited [8,9]. Therefore, the methods for desulfurization wastewater disposal have gained extensive research interest in recent years [10].

Some technologies have been proposed to dispose of the desulfurization wastewater, such as chemical precipitation, membrane separation, evaporative crystallization, electrodialysis technology, etc. [11–14]. Of these desulfurization wastewater disposal technologies, desulfurization wastewater evaporation technology is an effective method to achieve zero-emission of desulfurization wastewater [14–17]. Significantly, flue gas after the air preheater is the best choice as a heating source to evaporate the desulfurization wastewater, as shown in Figure 1 [18]. The flue gas is injected into the spray drying tower through a special-designed channel, and the desulfurization wastewater is sprayed into a

**Citation:** Li, D.; Zhao, N.; Feng, Y.; Xie, Z. Numerical Investigation on the Evaporation Performance of Desulfurization Wastewater in a Spray Drying Tower without Deflectors. *Coatings* **2021**, *11*, 1022. https://doi.org/10.3390/ coatings11091022

Academic Editors: Eduardo Guzmán and Alexandru Enesca

Received: 20 July 2021 Accepted: 24 August 2021 Published: 26 August 2021

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spray drying tower through a high-speed rotating atomizer, and then evaporated under the preheating effect of the flue gas. The residual solid particles after evaporation are expected to be captured [19]. To improve the evaporation performance of desulfurization wastewater, some efforts have been performed. Liang et al. [20] investigated the evaporation and crystallization behaviors of the desulfurization wastewater droplet using thermogravimetric analysis. They found that the increase in heating rate can promote evaporation and crystallization rates simultaneously, while the final temperature has a limited effect on these rates. Deng et al. [21] numerically studied the effect of the position and number of nozzles, droplet size, and flue gas temperature on evaporation performance. Ma et al. [8] simulated the evaporation behavior of desulfurization wastewater and found that smaller droplet size, and higher flue gas flow rate and temperature could benefit the complete evaporation of the desulfurization wastewater. Zheng et al. [22] explored the chlorine migration of various chlorine salt solutions and typical desulfurization wastewater at high temperatures during the evaporation process of concentrated wastewater by a laboratory-scale tube furnace and a pilot-scale system. Although numerous efforts have been devoted to studies of the desulfurization wastewater evaporation, there are still few works that comprehensively investigate the desulfurization wastewater evaporation.

**Figure 1.** The desulfurization wastewater evaporation technology with flue gas.

This work is aimed to comprehensively investigate the effects of flue gas flow rate, flue gas temperature, wastewater flow rate, initial wastewater temperature, and droplet size on the desulfurization wastewater evaporation performance. These studied results can give more helpful information to guide the desulfurization wastewater evaporation technology with flue gas.

### **2. Physical Model and Numerical Method**
