Experimental And Simulation Study On CaO-Based Carbon Capture Technology In Flue Gas

Recently,excessive use of fossil fuels has led to the annual increase of carbon dioxide emissions in China.Plenty of CO₂ emissions directly lead to a series of environmental problems such as global warming,so how to reduce carbon dioxide emissions reasonably and effectively is an urgent problem to be solved.Especially after President Xi Jinping pointed out at the UN General Assembly and the UN Climate Ambition Summit that China would achieve carbon peak by 2030 and strive for carbon neutrality by 2060.The exploration of carbon emission reduction technology has become one of the hottest research fields.As one of the most promising post-combustion carbon capture technologies,CaO-based carbon capture technology has the advantages of low cost and high efficiency,which has been widely studied all over the world for many years.However,the activity attenuation of CaO-based adsorbents at high temperature and the large-scale industrial application of CaO-based carbon capture system are the main problems restricting the development of this technology.In order to develop low-cost CaO-based adsorbents for industrial applications,the adsorption characteristics of different CaO-based adsorbents on thermogravimetric reactor,fixed bed reactor and fluidized bed reactor were studied in this paper.On the basis of experiments,a detailed simulation study of the CaO-based carbon capture system was carried out,and the technical and economic analysis of the system was carried out,which provided a reference for the large-scale application of the CaO-based carbon capture system.Firstly,the adsorption performance of CaO-based adsorbents doped with metal oxides was explored in a fixed-bed reactor.The effects of adsorbent,carbonation temperature,CO₂ concentration and carbonation time on the adsorption performance of the adsorbent were studied respectively.The experimental results show that at 700℃and 15%CO₂ concentration,CaO-based adsorbent doped with 5%Zr O₂ has excellent adsorption capacity,and the adsorption efficiency reaches about 38.4%after 20 cycles.The adsorption efficiency of CaO-based adsorbents doped with 5%Al₂O₃ and 5%Cu O is 34.5%and 34%after 20 cycles,respectively.The adsorption performance of the CaO-based adsorbent doped with 5%high alumina cement is basically the same as that of CaO-based adsorbent doped with 5%Al₂O₃.With the carbonation temperature increases gradually,the adsorption efficiency of the adsorbent also increases.For CaO-based adsorbent doped with 5%Zr O₂,the optimal carbonation temperature is 700℃.The adsorption efficiency of adsorbents under different CO₂ concentrations is also significantly changed.The adsorption efficiency of adsorbents is mainly determined by CO₂ concentration at 650℃,while temperature and CO₂ concentration jointly determine the carbon capture ability of adsorbent at 700℃.With the extension of carbonation time,the adsorption efficiency of the adsorbent increases gradually,and the adsorbent needs about 25 min to complete the carbonation reaction stage.On the basis of experiments,DFT was used to calculate the adsorption energy of different CaO-based adsorbents and CO₂ molecules for carbonation reaction.The results showed that the adsorption energy of CaO-based adsorbents doped with 5%Zr O₂,5%Al₂O₃,5%Cu O are 2.37 e V,1.98 e V,1.81 e V,respectively.The calculated results are consistent with the experimental results.Then,the carbonation reaction characteristics of CaO-based adsorbent doped with high alumina cement were studied in a thermogravimetric reactor.The reaction kinetic parameters of the adsorbent in the first carbonation-calcination cycle and the stable carbonation-calcination cycle were obtained.The experimental results show that the adsorption efficiency of the adsorbent doped with 5%high alumina cement increases gradually with the carbonation temperature increases in the first carbonation-calcination cycle.CO₂ concentration has little influence on the adsorption performance of the adsorbent,especially when the carbonation temperature is lower than 700℃.The activation energy in the rapid reaction and diffusion control stages are 37.9 k J/mol and119.7 k J/mol,respectively.When the transition from the first carbonate-calcination cycle to the stable carbonate-calcination cycle,the adsorption efficiency of the synthetic adsorbent gradually decreases.After 20 cycles,the adsorption efficiency is stable about23%.After 30 carbonation-calcination cycles,the activation energy of the adsorbent in the rapid reaction stage and diffusion control stage are 38.4 k J/mol and 122.6 k J/mol,respectively,which are almost the same as that in the first cycle.On the basis of experiment,the particle adsorption model was established to further success explained the sintering phenomenon of CaO-based adsorbent.The result shows that the sintering layer on the surface of the adsorbent particles leads to a prolonged diffusion time of CO₂ molecule to the inner CaO surface in the particles,and the reaction curve of the adsorbent begins to appear a delay section after multiple carbonation-calcination cycles.Further,the adsorption performance of CaO-based adsorbent in a small bubbling fluidized bed reactor was studied and the experimental verification of CaO-based adsorbent in a 100 ton/year CO₂adsorption CaO-based double fluidized bed reactor was completed.The experimental results show that the CaO-based adsorbent doped with 5%high alumina cement has the best adsorption capacity in small bubbling fluidized bed reactor.After 20 carbonation-calcination cycles,the adsorption efficiency reaches50.8%,which is significantly improved as comparing to pure CaO-based adsorbent.With carbonation temperature increases from 650℃to 700℃,the adsorption efficiency of the adsorbent also shows an increasing trend.Scanning electron microscopy（SEM）test results further found that the adsorbent doped with 5%high alumina cement has good pore structure.At the same time,the adsorption efficiency of CaO-based adsorbent doped with 5%high aluminum cement in thermogravimetric reactor,fixed bed reactor and small bubbling fluidized bed reactor is gradually improved,which also provides a reference for the industrial scale application of CaO-based carbon capture system.The experimental results of 100 ton/year CO₂adsorption CaO-based double fluidized bed reactor show that adsorbent-CA（doped with compound containing aluminum）,adsorbent-LCS（doped with carbide slag）and adsorbent doped with 5%Al₂O₃ have good CO₂adsorption capacity.The adsorption performance of adsorbent-CA（doped with compound containing aluminum）and adsorbent-LCS（doped with carbide slag）is basically the same.The carbon capture efficiency of the system increases with the increase of adsorbent load.When the carbonator temperature is about 700℃,the gas flow rate is 20 Nm³/h,the CO₂concentration is 15%,the adsorbent CA and the CaO-based adsorbent doped with 5%Al₂O₃ are 8 Kg,respectively,the carbon capture efficiency of the system is more than90%,respectively.SEM test results show that the adsorbent has good sintering resistance ability.The CaO-based carbon capture system for the flue gas of a 600 MWe coal-fired power plant was independently established,the flue gas of the coal-fired unit enters the carbonator and reacts with the calcium base adsorbent to complete carbon dioxide capture.After the carbonation reaction,the CaO-based adsorbent was transported to the calciner for high temperature regeneration.The energy consumption of calcination was provided by oxygen-rich combustion of coal.Through the system simulation,the flue gas volume,carbon capture efficiency,carbon capture energy consumption,carbon dioxide concentration and other parameters were obtained.Meanwhile,the conservation of materials and energy of the high-temperature CaO-based carbon capture system was verified.The effects of temperature,Ca/C ratio and fresh Ca CO₃supplementation on the system performance were also completed.The simulation results by Aspen plus show that when the carbonator temperature is 645℃,the carbon capture efficiency of the CaO-based carbon capture system is about 90.7%.The carbon capture energy consumption is 0.94 GJ/t,the waste heat of flue gas at the outlet of carbonator and calciner is 308.1 MW and 178.7 MW respectively.The concentration of carbon dioxide in flue gas at the outlet of calciner is about 85.9%.With the increase of Ca/C ratio,the energy consumption of the system increases.Supplementing a small amount of fresh Ca CO₃ is beneficial to improve the adsorption efficiency of the adsorbent.Finally,Aspen Plus software was used to establish the new 600 MWe coal-fired power plant integrated with CaO-based carbon capture system and retrofitted 600 MWe coal-fired power plant integrated with CaO-based carbon capture system.The technical and economic analysis of the coal-fired power plant were also performed.The technical scheme of the retrofitted coal-fired power plant was as follows:coal was transported to the coal-fired boiler unit and then heated the feed water to generate electricity.The flue gas entered the CaO-based carbon capture unit to complete the carbon dioxide capture process.The flue gas of the coal-fired boiler unit heated the feed water to generate high-temperature steam for power generation.The high-temperature flue gas of the CaO-based carbon capture unit heated the independent feed water into the new turbine set for power generation.The technical scheme of the new coal-fired power plant was as follows:after the coal entered the coal-fired boiler unit for combustion,the flue gas entered the CaO-based carbon capture unit to complete carbon dioxide capture.The high-temperature flue gas of the coal-fired boiler unit and the CaO-based carbon capture unit together heated the feed water to produce high-parameter steam for power generation.The thermodynamic results show that the system efficiency of the new 600MWe coal-fired power plant and the retrofitted 600 MWe coal-fired power plant are32.9%and 32.6%respectively,which have obvious advantages as comparing to the600 MWe oxygen-rich combustion power plant（29%）.However,the system efficiency of the two power plants is 8.3%and 8.6%lower than that of the benchmark 600 MWe coal-fired power plant,respectively.The economic results show that the payback period of the new 600 MWe coal-fired power plant and the retrofitted 600 MWe coal-fired power plant is 18.72 years and 16.73 years respectively.And the IRR is 3.66%and4.41%,respectively.The results show that the new 600 MWe coal-fired power plant and retrofitted 600 MWe coal-fired power plant have a good industrial application prospect.Aiming at the new 600 MWe coal-fired power plant,three heating surface layout cases are further proposed and compared.Based on the principle of energy cascade utilization,the coupling design of different high temperature flue gas heats feed water in new power plant under three cases is carried out.The results show that the heat exchange area of the new coal-fired power plant under case 1 is about 37333 m²,which is lower than that of the new coal-fired power plant under case 2（41188 m²）and case3（42979 m²）.The device cost could decrease effectively under case 1.On the whole,the new coal-fired power plant under three cases has broad market application value.