Regeneration Behavior Of Iron Oxide High Temperature Coal Gas Desulfurization Sorbent In SO₂-O₂ Atmophere

With the development of society and economy, energy resource has become a mainstay of the national economics. The coal, which is and will be the major of China’s energy consumption, occupies the top position in once energy consumption in our country and the coal consumption of China accounts for about 68%. However, the mining and direct combustion of coal have lead to serious ecological and environmental pollution problem. The direct combustion of coal is responsible for more than 70-80% pollutants such as SO2, NOx, Hg, particles and CO2. With the continuous development and utilization of coal resources, the low rank coal will be the main type of future use of coal resources in China. Therefore, the conversion and utilization of clean coal especially the low rank coal is of great significance to China’s national economy, social development and environmental protection. Due to the high thermal efficiency and low pollution, techniques such as IGCC (integrated gasification combined recycle), FC (fuel cell) and CPG (coal polygeneration) have become the most advanced technologies in research and develop of clean coal conversion and utilization technology nowadays. The removal of sulfide especially at medium or high temperature is vitally important for the above-mentioned technologies. To meet the requirement of industrial application, hot coal gas desulfurization sorbent should have excellent performance in desulfurization, regeneration and sulfur recovery, and must be sTable after hundreds of times of desulfurization/regeneration cycles use. Regeneration of hot coal gas desulfurization sorbent is a critical step in its desulfurization/regeneration cycles use.On the basis of our research group’s previous study, this paper chooses iron oxide hot coal gas desulfurization sorbent as the research object. The influences of the regeneration temperature and the density and GHSV of regeneration gas on the regeneration behavior of sorbent were investigated respectively in O2 atmosphere, SO2 atmosphere and SO2-O2 cooperative atmosphere in differential and integral fixed-bed. The composition, element distribution and surface microstructure of sorbent before and after regeneration were studied through XRD, XPS, SEM and BET characterization methods, and the regeneration mechanism of iron oxide hot coal gas desulfurization sorbent in different regeneration gas were also further discussed.The experimental results showed that under O2 atmosphere the main regeneration product of iron oxide hot coal gas desulfurization sorbent were Fe2O3, SO2 and small amounts of sulfur. In the temperature range of 500-600℃, increasing the regeneration temperature can shorten the regeneration reaction time effectively; the regeneration temperature of below 500℃ was easy to the formation of stable sulfate; when the regeneration temperature was higher than 600℃, the sorbent was easy to sinter and deactivation in O2 atmosphere. Both on differential and integral bed, the concentration of sulfur in regeneration exhausts were the highest under 500℃. With the increase of O2 concentration, the selectivity of sulfur increased, while the yields of sulfur were low. With further increasing of O2 concentration, sulfate formed and may lead to sinter of sorbent. On the other hand, the regeneration GHSV had a strong impact on the regeneration process. Under the same regeneration GHSV, the sorbent on integral bed was easier to sinter than that on differential bed. The increase of GHSV can decrease the heat of reaction bed effectively, avoiding sorbent to sinter due to the high temperature, and can take away SO2 and sulfur, which is beneficial for decreasing the formation of sulfate and increasing the selectivity of sulfur. The optimal regeneration conditions for iron oxide hot coal gas desulfurization sorbent in O2 atmosphere are as following:on differential bed, 500℃,1.2%(vol) O2, GHSV of 60000 h-1; on integral bed,550℃,1.5%(vol) O2, GHSV of 6000 h-1.Under the regeneration atmosphere of SO2, the main regeneration product of iron oxide hot coal gas desulfurization sorbent was Fe2O3 and sulfur. The increase of regeneration temperature, GHSV and inlet SO2 concentration all can improve the sulfur concentration of exhausts. The order of the influences of the three conditions was as follows:regeneration temperature> regeneration GHSV> inlet SO2 concentration. Both on differential and integral bed, the concentration of sulfur in exhausts was the highest when the regeneration was 600℃. So, it can be concluded that the regeneration temperature of 600℃is helpful to the sulfur recovery. Regeneration GHSV had a great effect on regeneration process. The raise of GHSV can alleviate the weighting of sorbent, but also can make its regeneration rate decline. Moreover, the reaction of SO2 and FeS is an endothermic reaction, and the increase of GHSV will decrease the heat released by reaction bed,, leading to the decrease of localized temperature and the decline of reaction rate of SO2 and FeS, which is helpful to the formation of sulfate and unbeneficial to the regeneration of sorbent. SO2 may react with FeS through two-step path under the regeneration atmosphere of SO2, i.e. FeS →①SO2FE3O4→②SO2+SFe2O3. The first reaction is fast and the second is slow. As the extension of reaction time, the second reaction can be complete. The optimal regeneration conditions for iron oxide hot coal gas desulfurization sorbent in SO2 atmosphere are as following:on differential bed,600℃, 4.08%(vol) SO2, GHSV of 60000 h-1; on integral bed,600℃,4.08%(vol) SO2, GHSV of 4000 h-1.When under SO2-O2 cooperative atmosphere, the main regeneration product of iron oxide hot coal gas desulfurization sorbent was Fe2O3, SO2 and sulfur. The forms of sulfate in regeneration product were different under different temperature. The sulfate was FeSO4 when at low regeneration temperature, while at high temperature the sulfate was Fe2(SO4)3. The increase of regeneration temperature, GHSV and SO2/O2 ratio all can improve the sulfur concentration of exhausts. The order of the influences of the three conditions was as follows:regeneration temperature> SO2/O2 ratio> regeneration GHSV. Both on differential and integral bed, the concentration of sulfur in exhausts was the highest when the regeneration was 600℃. When the temperature was 550℃, the results of regeneration remained basically the same with 600℃. When the regeneration temperature further increased to 700℃, sinter of sorbent did not occur, which had appeared under O2 atmosphere, suggested that the fast strongly exothermic reaction between O2 and FeS is mutually energy-complementary with the slow endothermic reaction between SO2 and FeS. Under the same regeneration conditions, the SO2-O2 cooperative atmosphere can significantly shorten the regeneration reaction time and increase the regeneration rate of sorbent, which is complementary in dynamics. The optimal regeneration conditions for iron oxide hot coal gas desulfurization sorbent in SO2-O2 cooperative atmosphere are as following:on differential bed,600℃,5.3%(vol) SO2,3%(vol) O2, GHSV of 60000 h-1,a maximum selectivity of 53% can be achieved under this conditions; on integral bed,600℃,5.3%(vol) SO2,2%(vol) O2, GHSV of 6000 h-1, a maximum selectivity of 50% can be achieved under this conditions.