It could be seen from Figure 9 that with the increase of the desulfurization time, the SO2 outlet concentration remained stable and then displayed a rapid augment, while the desulfurization efficiency kept stable first and then rapidly declined, indicating that desulfurizers could quickly fail after a period of operation. Under the circumstances of the same SO2 outlet concentration or the same desulfurization efficiency, the breakthrough of effect for 0# desulfurizer lasted the longest time, namely 460 min, followed by 2-1# desulfurizer, 1-1# desulfurizer, 2# desulfurizer, and 1# desulfurizer, and the breakthrough time of 1-1# desulfurizer and 2-1# desulfurizer was 180 min and 400 min, respectively, much longer than that of 1# desulfurizer and 2# desulfurizer. It was herein suggested that the apparent properties of the superfine desulfurizer were significantly improved, and the desulfurization efficiency increased rapidly. Additionally, it was displayed that 1# desulfurizer cannot maintain 100% desulfurization efficiency, while the efficiency of 1-1# desulfurizer was also lower than 100% in the initial stage of 0-40 min. As the desulfurization continued, the desulfurization efficiency reached 100% during 40 to 120 min, mirroring the second digestion of 1-1# desulfurizer in the initial stage of desulfurization under favorable reaction conditions provided by desulfurization circumstances. More importantly, further digestion strengthened the activity of the desulfurizer and its efficiency rapidly increased to 100%. Based on a performance comparison among the 5 desulfurizers, high-performance calcium-based desulfurizer was verified to be superior to sodium-based desulfurizers represented by NaHCO3 with similar particle size.
It has been generally recognized through experimental analysis that the desulfurization performance of traditional calcium-based desulfurizers is lower than that of sodium-based desulfurizers, and from previous literature (Ahmed et al., 2021; Min et al., 2010), as a result, to meet industrial SO2 emission standards, the calcium-sulfur ratio of calcium-based desulfurizers were 5:1~10:1, while that of sodium-based desulfurizers was only 1.3:1 to 1.6:1. Low desulfurization efficiency resulted in less frequent utilization of such desulfurizers, and calcium-based desulfurizer was not considered an option in dry desulfurization. However, the intensive processing of traditional calcium-based desulfurizers in this project contributed to its higher desulfurizer efficiency relative to sodium-based desulfurizers under the same desulfurization conditions, which broke through the conventional cognition and would significantly contribute to augmented efficiency of calcium-based desulfurizers in dry flue gas desulfurization.
Instead of the traditional digestion process of calcium-based desulfurizers, a steam jet mill with superheated steam as the pulverizing medium was employed in the present study for pulverization and quicklime digestion to prepare high-efficiency calcium-based desulfurizer which displayed overtly advantageous apparent properties including specific surface area and desulfurization performance in comparison to sodium-based desulfurizers. Additionally, effective advancements in apparent characteristics of particles and desulfurization performance could be achieved after pulverization of desulfurizers with the help of the jet mill with compressed air at normal temperature as the medium. Therefore, a steam jet mill or jet mill could contribute to the creation of novel processes to promote desulfurization performance, which is of significant importance in the promotion and application of dry flue gas desulfurization technology.
At an initial desulfurization temperature of 100 ℃ and up-regulation of 20 ℃ every 10 min, the correlation between desulfurization efficiency and temperatures of the different types of desulfurizers was investigated, as shown in Figure 10.
As depicted in Figure 10, the continuous increment in the temperature resulted in a gradual increase in SO2 outlet concentration but a decline in desulfurization efficiency, suggesting that temperature negatively modulated the efficiency of desulfurizers. The desulfurization efficiency of 1# desulfurizer could reach over 70% at 100 ℃ to 160 ℃, and dropped to only 56.47% at 180 ℃, reflecting that the critical temperature of 1# desulfurizer was 160 ℃. As for 1-1# desulfurizer, 100% efficiency could be maintained when the temperature was increased from 100 ℃ to 140 ℃, and the desulfurization efficiency dropped to 81.65% at 200 ℃ and rapidly decreased to 46.82% at 220 ℃, from which the critical temperature for 1-1# desulfurizer was suggested to be 200 ℃. Similarly, it was indicated from Figure 10 that the critical temperature for 2# desulfurizer and 2-1# desulfurizer was 180 ℃ and 200 ℃, respectively. Based on desulfurization characteristics with temperature augment among 1#, 1-1#, 2#, and 2-2# desulfurizers, we could conclude that the critical temperature at which desulfurization efficiency rapidly declined did not exceed 200 ℃. In other words, ordinary calcium-based desulfurizers and sodium-based desulfurizers cannot realize high-efficiency desulfurization under conditions higher than 200 ℃.
For 0# desulfurizer, it was testified in Figure 10 that 100% desulfurization could be maintained with the desulfurizer temperature increased from 100 ℃ to 240 ℃. And with the temperature rising to 260 ℃ and then 280 ℃, the desulfurization efficiency dropped slightly to 96.23% and obviously to 61.76% separately. It was indicated that the critical temperature inducing the rapid decrease of desulfurization efficiency of 0# desulfurizer was 260 ℃, and more importantly, the applicable high-efficiency desulfurization temperature of 0# desulfurizer was higher than that of the above four desulfurizers.
According to the results of desulfurization characteristics with temperature increases, the applicable high-efficiency desulfurization temperature of the desulfurizer after jet mill pulverization was higher than that before pulverization, but the difference was not obvious. Additionally, high-efficiency calcium-based desulfurizer, as compared with ordinary calcium-based desulfurizers and sodium-based desulfurizers, could desulfurize at wider applicable temperatures, and achieve more desulfurization efficiency at the same temperature. Currently, in the industrial integrated flue gas desulfurization and denitration process, the denitration temperature generally should be maintained up to 200 ℃ for higher activity of the denitration catalysts, and the high-efficiency calcium-based desulfurizer could be applied without affecting desulfurization efficiency but with the flue gas temperature controlled at 200 ℃ to 260 ℃, which can not only meet the high desulfurization efficiency but also provide suitable reaction conditions for the catalysts in the denitration process.
Energy consumption analysis of desulfurizer preparation capacity
1# desulfurizer was prepared after full digestion of quicklime via a digester in a simple process. The production cost mainly included the cost of quicklime raw material and that of digestion. The quicklime was about 300 ￥∙T-1 and high-temperature water was utilized for digestion (Ding et al., 2019), in which the water-cement ratio was 0.84:1, cost roughly 50 to 100 ￥∙T-1, and taken together, the production cost of 1# desulfurizer was about 342 to 384 ￥∙T-1. 2# desulfurizer could be purchased directly at the price of roughly 2000 ￥∙T-1.
1-1# desulfurizer and 2-1# desulfurizer required pulverization utilizing a jet mill. The LNJ-240A air jet mill was applied for pulverization of the two desulfurizers to facilitate the calculation of industrial production. The main parameters of the LNJ-240A air jet mill were shown in Table 1. The rated power was 272 kW, and the actual energy consumption was 231.7 Kw∙h-1. The processing capacity of 1-1# desulfurizer was 320 Kg∙h-1, that is, the total power consumption for the production of 1 ton of 1-1# desulfurizer material was 724.06 Kwh∙T-1, and the total cost was 1024.06 ￥. Similarly, the output of 2-1# desulfurizer was 280 Kg∙h-1. In other words, the total power consumption of manufacturing 1 ton of 2-1# desulfurizer material was 827.5 Kwh∙T-1 and the total cost price was 2827.5 ￥∙T-1.
0# desulfurizer was prepared by pulverizing and digesting quicklime through a steam jet mill, whose parameters were shown in Table 1. The rated power was 7.5 kW, and the actual energy consumption was 6 Kw∙h-1. The output was 60.5 Kg∙h-1, that is, production of 1 ton of 0# desulfurization consumed 99.17 Kwh∙T-1 of power. Also, superheated steam was utilized as the pulverizing medium for the LNGS-80 steam jet mill. 72 Kg∙h-1 of system steam consumption needed to be taken into consideration and the production of 1 ton of material required 667 Kg of steam. The cost of superheated steam was 200 ￥∙T-1, so 133.4 ￥ was spent on the steam. Therefore, considering the cost of raw materials, power consumption, and steam, the cost of producing 1 ton of 0# desulfurizer was about 532.57 ￥.
According to current market circulation prices, the processing and preparation costs of different desulfurizers were analyzed. In the production of desulfurizer per unit mass, the cost of 1# desulfurizer was the lowest, followed by 0#, 1-1#, 2#, and 2-1# (the highest). Based on a comparative analysis of desulfurizers with higher efficiency, namely 0# and 2-1# desulfurizers, the production cost of the sodium-based desulfurizer after pulverization was about 5.36 times higher than that of the high-efficiency calcium-based desulfurizer. After 1 T of the flue, gas SO2 was removed through theoretical calculations, 1.156 tons of high-efficiency calcium-based desulfurizer were required at the price of 615.65 ￥, while 1.312 tons of pulverized sodium-based desulfurizer were needed at the price of 3,709.68 ￥, which was 6.02 times higher than that of high-efficiency calcium-based desulfurizer.
Ecological environment pollution has become the focus worldwide, and under such circumstances (Yi et al., 2008; Pan et al., 2019), in response to national policies on energy conservation and emission reduction, energy structure adjustment, carbon neutrality, and peaking carbon dioxide emissions, the industrial application of waste heat from industrial emissions which could be fully utilized to provide pulverization energy for steam jet mills, to a certain extent, is expected to further reduce the cost of desulfurization. As for the high-performance characteristics of high-performance calcium-based desulfurizers, the authors will further analyze and study them. Therefore, this technology, in which a steam jet mill is applied for quicklime digestion to prepare high-performance calcium-based desulfurizers, could realize relatively high desulfurization efficiency, and accordingly has great development potential in the field of flue gas desulfurization.