Research On Stimulative Effect Of Additives On Sulfur-retention At High Temperature During Coal Combustion

Coal is a primary energy resource of the widest distribution, richest reserves and largest consumption in China. It has become an important issue for sustainable development to solve the pollution problems caused by coal combustion, especially to reduce sulfur dioxide emission. In general, there are three categories of coal desulfuration technologies including fuel-desulfuration before combustion, sulfur-retention during combustion and fume-desulfuration after combustion. Due to its less investment, low operation-cost and simple process, the technology of sulfur-retention using sorbent in the furnace is suitable to the condition of China. However, there are shortcomings in the technology using calcium-based sorbent singly, such as low utilization percent of calcium, inconsistent rate between sulfur-retention reaction and sulfur emission, decomposition of the production at high temperature, which all lead to low sulfur-retention efficiency. Recently scholars have discovered that the addition of small quantity additives can increase the utilization rate of calcium sorbent and promote the sulfur-retention rate at high temperature greatly. So it has become a key problem to select cheap and effective additives for the research of sulfur-retention during coal combustion at high temperature.In this paper, the process of coal combustion was simulated in an electric tube furnace, the sulfur content in the fume was determined by neutralization method and then the sulfur-retention rate was calculated according to it. Based on the optimization of sulfur-retention conditions, the effect on the sulfur-retention during coal combustion were studied of unitary, binary and ternary additives respectively at the temperature of 1100℃systematically.Among the various factor influencing sulfur-retention efficiency, temperature were fixed at 1100℃and the other four factors Ca/S molar ratio, sulfur content, retaining time, size of coal particle were considered in the experiment to explore the optimum experimental condition. Based on the condition, Ca/S molar ratio, characteristic of capture sorbents, kinds of additive and quantity of additive were selected as four factors to do the orthogonal experiment and the result indicated that when the Ca/S mol ratio is 2, CaCO3-Ca(OH)2 compound sorbent mixed by 2:3 quantitatively is main sorbent and the total addictive quantity is 1.0%of coal, the maximum value of sulfur-retention efficiency is obtained.Under the determinate optimum condition of sulfur-retention additive, the additive effect of 8 different single-composition was investigated by comparing the sulfur-retention efficiency. Based on the results of single-composition additive experiment,16 kinds double-composition additive of which one main component is respectively K2CO3, SiO2, Al2O3were researched. The results shows that when K2CO3 mixed respectively with NaCl,Fe2O3by molar ration 0.2, SiO2 mixed with Fe2O3by molar ration 1.0, mixed with K2CO3by molar ration 10.0, Al2O3 mixed with NaCl by molar ration 5.0, mixed with ZnO by molar ration 1.0, the sulfur-retention efficiency is relative high over 50%.Based on the experiment and related results above, this paper do research on three composition additive considering the different effect of different additive in order to reserve the problem that the desynchronization between sulfide and oxide and low utility-efficiency of CaO. Through the experiment of three different compound additive K2CO3-Fe2O3-SiO2, SiO2-K2CO3-NaCl, ZnO-Al2O3-NaCl, we found that three-composition addictive can promote the sulfur-retention efficiency to the value over 65%. In order to further validate the promote effect of three-composition additive to the sulfur-retention reaction, we do the experiment to the three different coals added with three composite additives in the muffle at different higher temperature. The experimental result shows that the promote effect of K2CO3-Fe2O3-SiO2, SiO2-K2CO3-NaCl, ZnO-Al2O3-NaCl three-composition additive to coal particles at high temperature is far-flung applicable. The cinder of coal with additive were characterized by XRD partly and the mechanism of promotion to the sulfur-retention reaction of additive are discussed.