SO2 removal performance for the solution absorption system applied to coal-fired flue gas
Introduction
More than 75% of the power generated in China is contributed by coal-fired plants [1], and the wet flue gas desulfurization (WFGD) method, which removes SO2 effectively, is widely applied in the flue gas purification process [[2], [3], [4], [5]]. During the WFGD process, the moisture in the desulfurization slurry evaporates, making the flue gas saturated [6]. If the saturated flue gas is directly discharged, a large amount of water and latent heat will be wasted. Furthermore, the emission of saturated flue gas contributes to the formation of white plumes and secondary inorganic aerosols, causing environmental problems [[7], [8], [9], [10]]. Therefore, the recovery of water and latent heat from desulfurized flue gas is of significance.
Driven by the vapour pressure difference between the flue gas and the liquid desiccant, the solution absorption method avoids the limitation of the flue gas dew point and utilizes the latent heat better than condensation [[11], [12], [13]]. Riffat [14] also concluded that the solution absorption method system has a higher efficiency for latent heat recovery than condensing boiler when it is used for natural gas. Wang [15,16] also applied the solution absorption method to natural gas boilers and achieved optimal system performance by adjusting the real-time flow rate of the circulating solution. For industrial waste gas, Yang [17,18] proposed an open absorption heat pump system with high-temperature steam as the heat source for solution regeneration. Simulation results showed that the heat recovery capacity of the new system outperformed that of the condensing heat exchanger by 19.7–178.1%.
To decrease construction and operation costs, Chen [19] proposed a novel dehumidification system combined with the WFGD process. In this system, the desulfurization and dehumidification processes for flue gas were realized sequentially in a single spraying tower with a dual-loop. Case studies showed that the water recovered from the flue gas could supply nearly half of the water demand in the WFGD system.
To reduce energy consumption during solution regeneration, Ye [20] established a new open absorption heat pump system for the drying processes. This new system contained two absorbers and two generators and could work under both single-stage and double-stage modes. Simulation results showed that the coefficient of performance (COP) varied from 1.52 to 1.97 when the heat source temperature was varied from 160 to 175 °C. Based on flash evaporation, Zhang [21] designed a two-stage regeneration system and a partial regeneration system. The COPs of the optimized systems were raised by 4.2% and 17.3%, respectively, compared with the original one. The exergy analysis also indicated better exergy efficiencies for the optimized systems.
All of the above studies have proven the feasibility of applying solution absorption systems to coal-fired flue gas. However, very few studies have been conducted on the effects of the SO2 present in the flue gas on system operation. Although the ultra-low emission standard of SO2, <35 mg/m3, has been met in most power plants in China [22], the residual SO2 reacts and forms CaSO3 and CaSO4 precipitates, causing equipment corrosion and pipeline blockage [23]. In terms of this issue, Folkedahl [23] and Wei [24] have both explored pilot test systems, but the results were only suitable for certain dehumidification conditions. Furthermore, neither the reactions mechanism of SO2 and CaCl2 solution or the treatment of precipitation were discussed. For the solution absorption system, when applied to coal-fired flue gas, CaSO4 precipitates generated by the absorption of SO2 will affect the stable operation of the system. Therefore, the synergistic removal of SO2 and the generation and filtration of CaSO4 precipitate are crucial to the application of the solution absorption system. As a result, a falling-film reaction platform for solution absorption was established in this work, and the influence of various operating conditions, such as solution temperature and concentration, solution pH, oxygen content and catalyst concentration, on the SO2 removal performance was then investigated experimentally. In addition, composition analysis was performed on the precipitate, and the solubility of the precipitate was tested.
Section snippets
Chemical reaction mechanism
SO2 does not react directly with CaCl2. Under the action of O2, SO32− dissolved in the CaCl2 solution is oxidized to SO42− and then reacts with Ca2+ to form CaSO4 precipitate.
Thus, we have
The intermediate product CaSO3 is extremely unstable and can be decomposed into SO2 at high temperature, causing serious corrosion in the pipelines and heat exchangers in the solution absorption system. In addition, the
Experimental system
The falling-film reactor has fixed reaction areas, and the axial mixing of the materials can be ignored. Therefore it was used as the reactor in this experiment. The inner diameter of the falling-film tube was 20 mm, and the height was 200 mm. The dehumidification solution was introduced from the top of the falling-film reactor, and then it was distributed evenly along the inner wall of the tube through the distributor. CaCl2 solution was chosen as the dehumidification solution on account of
Effect of CaCl2 solution temperature and concentration
In this test, and according to the parameters of the solution dehumidification process, the temperature of the CaCl2 solution was varied from 35 to 55 °C, while the concentration ranged from 35 to 50% [21,23]. The O2 content in the flue gas was set at 5%, and no catalyst was added.
Fig. 2 shows the effect of CaCl2 solution temperature and concentration on the SO2 removal efficiency. When the CaCl2 solution concentration was constant, the removal efficiency of SO2 decreased with solution
Analysis of CaSO4 solubility in CaCl2 solution
The solubility of CaSO4 in the CaCl2 solution played a significant role in the design, optimization and operation of the solution absorption system. The filtration and removal of precipitates in the CaCl2 solution is crucial for circulation operation. The precipitate generated in the CaCl2 solution was analysed, and the results showed that calcium sulfate was precipitated as the dihydrate (CaSO4·2H2O). The solubility of dihydrate in single acid or salt solutions at ambient temperature was
Conclusion
The synergistic removal of SO2 and the subsequent problem of precipitation treatment is crucial for the application of a solution absorption system using CaCl2 solution as the liquid desiccant. In this paper, various factors affecting the SO2 removal efficiency of the CaCl2 solution were analysed, and the solubility of CaSO4·2H2O precipitated from the CaCl2 solution was measured. Some conclusions can be made as follows:
- (1)
CaCl2 solutions with lower temperatures, lower concentrations and the
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper
Acknowledgments
The research was sponsored by the Major Science and Technology Innovation Project of Shandong Province [2019JZZY010403], Xinjiang-Changji Vocational Education Alliance special Project.
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