A Study On The Effect Of Coal Combustion On CaO Regeneration And Its Cyclic Reactivity In High Steam Atmosphere

An increase in the emission of greenhouse gas CO₂ intensifies global warming.The emission of CO₂ is mainly from coal-fired power plants.Post-combustion CO₂ capture is an applicable method to be applied in current power plants reformation.CaO-based sorbent,which is from limestone decomposition,can be used to cyclic CO₂ capture.However,a large amount of heat is needed to drive limestone decomposition.It's an effective approach to calcine limestone integrating with coal combustion in O₂/CO₂/Steam atmosphere.Steam is introduced to calcination atmosphere to enhance the reactivity of CaO sorbent.To apply this novel method into practical application,there are still some issues involved:?1?the influence of steam on CaCO₃ decomposition and CaO carbonation reactions;?2?the distributions of coal ash,sulfur and other pollutant gases in CaO product and CO₂ exhaust gas,and how do they influence the CaO-based sorbent reactivity;?3?the influence of high concentration of steam on limestone decomposition and coal combustion reactions.In this study,the effects of steam on CaCO₃ decomposition,CaO carbonation and hydration reactions were investigated in a fixed bed reactor.Ca?OH?₂ with high strength was obtained to capture CO₂.The influence of coal combustion on the cyclic reactivity of CaO sorbent was analyzed in a TG analyzer.The distributions of gas and solid product,the reactivity of CaO product and steam gasification behavior during limestone decomposition and coal combustion in O₂/CO₂/Steam atmosphere were both experimentally studied in a continuously operating fluidized bed reactor.Introducing steam into both calcination and carbonation atmospheres can promote the decomposition rate of CaCO₃ and the reactivity of CaO.On the one hand,the CO₂ partial pressure decreases when steam is introduced into calcination atmosphere,resulting in the increase of CaCO₃ decomposition rate.On the other hand,the heat-transfer during CaCO₃ decomposition is enhanced in CO₂/Steam atmosphere,which also promotes CaCO₃ decomposition.CaCO₃ decomposition reaction occurs at a relative low temperature since steam accelerates the CaCO₃ decomposition.Low decomposition temperature results in low sorbent sintering,thus improving the sorbent reactivity.When steam is introduced into carbonation atmosphere,H₂O dissociation occurrs on CaO particle surface and generates OH-.Later,OH-reacts with CO₂ to form CO₃^2-,thus promoting the absorption of CO₂.When high pressure steam is introduced into hydration atmosphere,both the reactivity and strength of the Ca?OH?₂ sorbent are improved.The porosity and internal stress of Ca?OH?₂ particle decrease in high pressure steam atmosphere.As a result,the particle strength increases.When coal combustion heat is supplied for limestone decomposition,it is better to conduct limestone decomposition and coal combustion in steam atmosphere.In this study,limestone decomposition and coal combustion were performed in a continuously operating fluidized bed reactor.The result shows that increasing O₂ supply reduces the emissions of SO₂?CO and H₂ in the CO₂ exhaust gas.Increasing steam supply reduces the emissions of CO and SO₂ and enhances the emission of H₂.Optimizing O₂ and steam supply can obtain high concentration of CO₂ exhaust gas in the experiment.Compared to conventional coal combustion,the combustion reaction becomes much different in O₂/CO₂/Steam atmosphere.In this study,we innovatively proposed ¹⁸O isotope tracer method to clarify the combustion/gasification reaction mechanism in O₂/CO₂/Steam atmosphere.The result shows that local low-oxygen environment is generated within the coal particle internal pores in both low-oxygen and oxygen-rich atmospheres.As a result,steam(H₂¹⁸O)gasification reaction occurs and generates C¹⁶O,C¹⁸O and H₂.Then,C¹⁶O,C¹⁸O and H₂ diffuse to char surface boundary layer and react with O₂ to form C¹⁶O¹⁶O,C¹⁶O¹⁸O and H₂¹⁶O.In summary,H₂O molecule is primarily involved in the gasification reaction and then reproduced during boundary layer reactions.Besides,the reactivity of CaO decreases when coal combustion heat is supplied for limestone decomposition.Coal ash remains in the CaO sorbent after limestone decomposition and coal combustion,but it doesn't react with the CaO sorbent in the fixed bed reactor.Besides,the sulfur content is very low in the sorbent?only a little CaSO₄?.Particularly in fluidized bed reactor,coal sulfur and ash are mainly transferred into small particles?cyclone?.In practical application,only large particle is used to capture CO₂ in the flue gas from coal power plant.The experiment result shows that coal sulfur and ash are very low in large CaO sorbent particle?overflow holder?.Therefore,coal sulfur and ash have little influence on the reactivity of CaO sorbent.However,the sorbent reactivity is decreased under the condition of coal combustion.The main explanation is that high temperature environment occurs in coal combustion atmosphere,which results in sorbent sintering.Although coal combustion results in the decrease of sorbent reactivity,but it can be enhanced through steam activation.The process flow of this approach is built through Aspen Plus simulation software.The result shows that although high concentration of steam increases the system energy consumption,the energy consumption based on per active CaO sorbent decreases after considering the reactivity enhancement in steam atmosphere.In conclusion,limestone decomposition integrating with coal combustion in O₂/CO₂/Steam atmosphere has good application prospect in the field of Ca-looping process for CO₂ capture.