Investigation On The Desulfurization Mechanism Of Ca-based Sorbents Under O₂/CO₂ Atmosphere

Today, economic development and population growth put more and more stress on environment, and cause all kinds of environmental problems. These affect directly or indirectly ecological balance and our health and living. The most serious three environmental problems that people must be confronted are greenhouse effect, acid rain and ozonosphere destruction now. And all of these are concerned with coal fired. The main reason for above environmental problems is CO₂,SO₂ and NO_x that emitted in course of coal burning. It is pressing to control and reduce CO₂,SO₂ and NO_x emissions in course of coal burning.Recently the O₂/CO₂ coal combustion technology is gradually paid extensive attention by people. This process uses pure oxygen instead of air and recycles most of the flue gas. The CO₂ concentration in flue gas may be enriched up to 95% and easy CO₂ recovery, therefore, becomes possible. Furthermore, it can greatly decrease NO_x emissions because in the combustion atmosphere there is no N₂ and most of the flue gas is recycled compared with coal combustion in air. Besides, above 90% desulfurization efficiency is also possible by limestone injection into furnace. Therefore, the O₂/CO₂ coal combustion technology is an innovative combustion technology that can control CO₂,SO₂ and NO_x emissions simultaneously. In present work the experimental and theoretical investigation on the characteristics of SO₂ and NO_x emissions and desulfurization of limestone in O₂/CO₂ combustion atmosphere was studied. It aimed at providing valuable experimental dates and theoretical foundation for the O₂/CO₂ coal combustion technology being applied.In O₂/CO₂ combustion atmosphere, CO₂ concentration in the flue gas is much higher compared with conventional combustion atmosphere. Under so high CO₂ concentration, the calcination, sintering and sulfation behaviors of limestone are not clear. Therefore, it is necessary to study the sulfation mechanism of Ca-based sorbents under O₂/CO₂ atmosphere. The calcination and sintering of limestone in O₂/CO₂ atmosphere were investigated systematically for the first time. The measurements showed that the specific surface area and porosity of calcined CaO in N₂/CO₂ atmosphere are bigger than that of in O₂/CO₂ atmosphere, but the latter has bigger average pore radius than the former. The fact is also testified by the SEM tests. This result is associated with the fact that the limestone completely decomposes into CaO in O₂/CO₂ atmosphere need much longer time than in N₂/CO₂ atmosphere, so the calcined CaO undergoes the longer sintering time in O₂/CO₂ atmosphere. Undergoing the same sintering time, the sintering has less influence on the pore structure of CaO calcined in O₂/CO₂ atmosphere than that of in N₂/CO₂ atmosphere, because CaO calcined in O₂/CO₂ atmosphere has lower surface energy. The crystal structures of CaO were measured with XRD. It is shown that the specific surface area of calcined CaO has a good relativity with the crystallite size. The decreasing in specific surface area with crystallite size rising shows that sintering influences the pore structure of calcined CaO by means of influencing the crystallite size.The direct sulfation reaction between limestone and SO₂ in O₂/CO₂ atmosphere was investigated. It is shown that the direct sulfation of limestone has a higher conversion degree than the sulfation of precalcined limestone. The Na2CO3 additives can improve the direct sulfation rate and Ca conversion degrees of limestone. SEM energy spectrum analysis proves that there is higher defect concentration in CaSO₄ product layer produced by limestone with Na2CO3 additives direct sulfation than that without additives. The increase in diffusivity in CaSO₄ product layer with defect concentration rising results in the increase in sulfation rate and Ca conversion degree. The kinetic studies of limestone direct sulfationreaction were performed. It is found that the reaction is chemically controlled in the initial stage and gradually controlled by diffusion through CaSO₄ product layer with CaSO₄ product layer forming. The higher value of the activation energy and the lower value of the diffusivity in CaSO₄ product layer suggest that mass transport through the product layer occurs by means of a solid-state ion diffusion mechanism instead of by molecular diffusion or Knudsen diffusion.The sulfation characteristics of limestone in O₂/CO₂ atmosphere at high temperature were studied by means of combined TGA with XRD phase quantitative analysis for the first time. The results suggest that there is a much higher sulfation rate before CaCO3 completely decomposed than after CaCO3 completely decomposed. This result is associated with the fact that CaO has higher specific surface area and porosity because of new CaO produced before CaCO3 completely decomposed. However, the specific surface area and porosity of CaO decreases quickly because there is not new CaO produced and CaO is sintered at high temperature. Therefore, the reactivity of CaO and the rate of sulfation reaction decrease quickly. The sulfation rate and Ca conversion degree of limestone in O₂/CO₂ atmosphere increase with CO₂ partial pressure when the temperature is above the equilibrium temperature.The calcination and sulfation characteristics of limestone in O₂/CO₂ atmosphere were investigated by drop-tube test facility and compared with in air. It is found that the limestone has an optimal desulfurization temperature in O₂/CO₂ atmosphere as in air. However, the optimal desulfurization temperature and corresponding desulfurization efficiency in O₂/CO₂ atmosphere is much higher than in air. It indicates that O₂/CO₂ atmosphere is in favor of limestone desulfurization at high temperature. Combined the results of TGA -XRD phase quantitative analysis, the desulfurization mechanism of limestone in O₂/CO₂ atmosphere was suggested. High CO₂ concentration slowed the limestone calcination in O₂/CO₂ atmosphere, so the system keeps on producing high activity CaO for a long time. At the same time, the diffusional resistance through CaSO₄ product layer is reduced because of the CO₂ being formed at the CaO/CaCO3 interface. The above two factors make CaO keep on a high sulfation rate for a long time. The characteristics of NO_x and SO₂ released in O₂/CO₂ atmosphere were performed and compared with in air. The results show that the released NO_x is much lower in O₂/CO₂ atmosphere than in air. The atmosphere and temperature have no effect on SO₂ emissions when the limestone is not added. After the limestone is added, the released SO₂ is much higher in air than in O₂/CO₂ atmosphere, because the limestone can achieve higher desulfurization efficiency.The calcination and sulfation model was established based on grain-micrograin model to simulate the calcination and sulfation reaction of limestone in O₂/CO₂ atmosphere. Considering that the high CO₂ partial pressure in O₂/CO₂ atmosphere results in slower calcination rate, the limestone calcination, calcined CaO sintering and CaO sulfation were combined to establish the integrated model of limestone desulfurization. At the same time, the true control mechanism of CaO sulfation-solid-state ion diffusion through the product layer is considered. The calcination degree, the specific surface area and porosity of calcined CaO and the desulfurization efficiency of limestone have been calculated based on this model. The calculated results are accord with the experimental values.