Study Of Typical Coal-burning Gas/Solid Impurities Impacting On CO₂ Sorption/desorption Of Calcium Looping And Reaction Mechanism

Calcium looping is mainly composed of carbonation and calcination,which can be coupled to CO₂ capture process of coal-burning flue gas.Therefore,it is regarded as a CO₂ emission reduction technology with great industrial application potential.In the practical industrial process,coal-burning flue gas will carry impurities such as coal ash,steam and SO₂ into the carbonator.While in the regenerator,in order to achieve the high concentration separation of CO₂,the heat of the calcining process is provided by the oxygen-rich combustion of coal.The oxygen-rich combustion of coal also inevitably produces coal ash,steam?6-12 vol.%?,SO₂?500-3000 ppm?,unconsumed O₂?3-5vol.%?and variety of trace acid gases.Therefore,the influence of typical coal-burning gas/solid impurities on the CO₂ sorption/desorption process of CaL cannot be ignored.At present,a large number of literatures have studied the characteristics of CO₂sorption/desorption of Ca-based sorbents,but most of the work is based on the ideal carbonation/calcining atmosphere with CO₂/N₂ as the main,so the characteristics of CO₂ sorption/desorption cannot be correctly explained.In view of these problems,this paper firstly investigated the influence of coal ash on the CO₂ sorption characteristics of Ca-based sorbents under different combustion conditions through experiments.Then,the surface morphology evolution of sorbents and the rule of element migration were revealed to clarify the physical/chemical mechanism of coal ash impacting on the sorption process.Secondly,through experiment and quantum chemical simulation,the effect of steam on the regeneration process of sorbents was investigated.And the reaction mechanism of steam catalyzing CaCO₃ decomposition was also explained.In addition,a series of experiments were conducted in this paper to investigate the characteristics of CO₂ sorption under the combined action of H₂O and SO₂.Besides,the mechanism of CO₂/SO₂ competitive sorption under the action of steam was proposed.Finally,in view of the serious negative impact of SO₂ on the CaL,the technical route of CO₂-capturing first and then adsorb SO₂ was proposed.And the sulfation mechanism of deactivated CO₂-capturing sorbents was clarified.Based on the above research,this paper systematically investigates the influence of typical gas/solid impurities on the CO₂ sorption/desorption process of CaL and the reaction mechanism.The conclusions will provide important guidance for the optimization of CaL conditions and the parameter selection of sorbent particles in practical application.The main innovative contents of this paper are as follows:The effects of coal ash on the characteristics of CO₂ sorption of Ca-based sorbents were systematically studied.Then the physical/chemical action mechanisms were revealed.Furthermore,the influence of coal ash deposition and the following grain aggregation in the early stage of CaL was clarified.Besides,the continuous effect of the solid-solid reaction between coal ash and sorbent during the carbonation/calcination cycle was also revealed.By investigating the influence of different coal ash content and particle size on the characteristics of CO₂ sorption of various Ca-based sorbents,it was found that coal ash had obvious negative influence on the characteristics of CO₂sorption under the air combustion condition.Besides,inhibition of CO₂ sorption was enhanced with the increase content of coal ash.While the particle size of coal ash had little influence on the CO₂ sorption process.On the other hand,the influences of coal ash categories,contents and ash preparation/calcination temperature on CO₂ sorption were also investigated under the oxygen-enriched combustion condition.Furthermore,the morphological evolution of surface and the migration rules of elements were further revealed.By testing the surface and pore parameters of the sorbents after different cycles,it was found that the deposition and following grain aggregation of coal ash would seriously affect the sorption performance and pore structure of the sorbents in the early stage of calcium looping.As CaL proceeding,the aluminum and silicon elements in the coal ash would react with the Ca-based sorbents in a solid-solid reaction and generate complex inorganic substances composed of calcium,aluminum and silicon.And the complex inorganic substances would further affect the diffusion control stage in the CO₂ sorption reaction,resulting in the continuous influence of coal ash on the CO₂sorption performance.The effect of steam on the regeneration process of sorbents was investigated through experiments and quantitative calculation.The catalytic mechanism of steam accelerating the decomposition of CaCO₃ was firstly revealed at the atomic level.The effect of steam with different concentrations on the regeneration process of sorbents was investigated through experiments.It was found that steam not only reduced the decomposition temperature of CaCO₃,but also accelerated the decomposition process of CaCO₃.According to the Arrhenius formula,it can be qualitatively demonstrated that steam reduces the activation energy of CaCO₃ decomposition reaction,that is to say,steam catalyzes the decomposition reaction of CaCO₃.Based on density functional theory,sorption sites of H₂O molecules on CaCO₃ surface are clarified through the quantum chemical calculation.And the sorption and dissociation process of H₂O on the surface of CaCO₃ are revealed.Furthermore,the reaction path of CaCO₃ releasing CO₂and the reaction energy barrier are acquired.Therefore,it suggested that the existence of steam could decrease the reaction energy barrier of CaCO₃ decomposition,thus the catalytic mechanism of steam on CaCO₃ decomposition reaction were clarified.Through a series of comparative experiments,the effects of steam,SO₂ and their coexistence on the CO₂ sorption characteristics of Ca-based sorbents were investigated.And the competitive mechanism of CO₂/SO₂ sorption under the action of steam was further proposed.By changing steam and SO₂ concentration and steam/SO₂ ratio,the characteristics of CO₂ sorption under different atmosphere are studied.It was found that steam not only strengthened the CO₂ sorption process,but also enhanced the sulfation in the carbonation process.When steam and SO₂ co-existed,the promoting effect of steam on the sulfation induced by SO₂ was obviously stronger than the strengthening effect of steam on the CO₂ sorption process.Hence,a more obvious negative influence on the CO₂ sorption process was caused.Based on the above results,the competitive sorption mechanism of CO₂/SO₂ under the action of steam was further proposed.Based on the fact that the sulfation caused by SO₂ can seriously hinder the process of CO₂ sorption,a technical route that sorbents capture CO₂ first and then used for the desulfurization process was proposed.The reaction characteristics of deactivated CO₂sorbents were systematically investigated.Then the SO₂-retention mechanism of limestone after cyclic sorption of CO₂ is revealed.By changing the sorbents type,particle size,CaL cycles and calcination conditions,the SO₂ sorption characteristics of deactivated sorbents were firstly investigated.Furthermore,it proved that the changes of the limestones'sorption parameters and pore structure were suitable for the technology process of SO₂ sorption after CO₂ capturing.It provides important guidance for the optimization of working conditions and the parameter selection of sorbent particle in this technology.