Enhancement Methods and Mechanisms of Calcium Oxide Reaction Reactivity and Stability during Desulphurization and Heat Storage

Author:Han Rui

Supervisor:qin yu zuo gao ji hui


Degree Year:2019





Combustion power generation will still be the main battlefield of the national economy in the short term.At the same time,a series of high-efficiency pollutant control technologies have been extensively studied to prevent environmental pollution caused by fossil fuel combustion.Solar thermal power generation technology uses clean solar as an energy source to fundamentally avoid environmental pollution and greenhouse effect.In recent years,it has received extensive attention.However,the development of this technology requires advances in high-temperature heat storage technology.Fortunately,CaO own many advantages such as wide source of raw materials,low cost and environmental friendliness.Its sulfation reaction can be used for high temperature SO2 removal,and its carbonation reaction can service in high temperature thermochemical heat storage.Moreover,these two reactions share common scientific connotations.That is,the diffusion,adsorption and reaction processes of SO2/CO2 in porous CaO.However,low CaO reactivity and stability are the common problems faced by calcium-based sulfur fixation and heat storage technology,which need to forestall.In this regard,this paper focuses on the enhancement of CaO reactivity,the enhancement of CaO reactivity and stability,and the enhancement of CaO volume storage density.These will lay the foundation for the development of advanced calcium-based sulfur fixation and heat storage technology.Aiming at the low reactivity of calcium oxide sulfation/carbonation in high temperature sulfur-fixing/thermal chemical storage technology,this paper explores in detail the influence rule of Na2CO3 doping on the reaction performance of calcium oxide sulfation/carbonation.Traditional studies have suggested that high doses of Na2CO3 doping have a positive effect on calcium oxide reactivity,but this study found that when a very small amount(0.1 mol%)of Na2CO3 was added,the conversion of sulfation and carbonation would also significantly improved.And this can also effectively avoid the problem of material agglomeration caused by the addition of Na2CO3.By correlating the pore structure of calcium oxide,lattice distortion and conversion rate of sulfation/carbonation reaction,the influence mechanism of Na2CO3 on calcium oxide sulfation/carbonation reaction was obtained:Na2CO3 co-melt with calcium oxide under high temperature conditions and a eutectic film forms on the surface of calcium oxide,which introduce two opposite effects on calcium oxide sulfation/carbonation reaction.On one hand,the eutectic film accelerates the sintering of calcium oxide and changes the surface area and pore structure of the calcium oxide;on the other hand,the eutectic film acts as a medium,affects the interfacial reaction of calcium oxide.Aiming at the low stability of calcium carbonate cyclic carbonation reaction in high temperature thermal chemical storage technology,this paper developed a simple and scalable application of limited-space gas phase chemical deposition method to achieve uniform deposition of inert stabilizer on the surface of calcium oxide.Compared with the traditional stabilizer doping method,the method can effectively reduce the amount of stabilizer with decreasing the particle size of the stabilizer,and achieve uniform mixing of the stabilizer and calcium oxide at the nanometer scale.The results show that only 10 mol%of Al2O3 stabilizer deposited on the sorbents can significantly improve the structural stability of calcium oxide.Stabilizer deposited on the sorbents mitigates the sintering by inhibiting surface diffusion and grain boundary migration,thereby hindering the growth of the grains and the disappearance of the pores.While the increase of calcination temperature and concentration of CO2 in the atmosphere will aggravate the sintering of calcium oxide and weaken cycle stability of the composite material gradually.Namely when the calcination reaction temperature and the concentration of CO2 in the atmosphere increase,the stabilizer particles aggregate,resulting in a weakening of the inhibition of CaO grain sintering.In view of the problem that carbon material carrier is easily etched by CO2 under high temperature heat storage conditions,this paper selects high graphitization carbon material with H3BO3 modifying as carrier for the high temperature oxidation resistance.Based on the surfactant-induced self-assembly process,the anti-oxidation carbon skeleton/calcium carbonate composite was synthesized in one step,and the influence of the carbon skeleton on the cyclic carbonation reaction of calcium oxide was investigated.The results show that the H3BO3 impregnation increases the oxidation initiation temperature of the graphite nanosheets to above 1000°C.What’s more,graphite nanosheets can effectively inhibit the sintering of CaO particles with accelerating the diffusion of CO2 gas and heat transfer during the reaction.So that significantly improvement of the activity and stability of calcium carbonate carbonation was obtained.When the amount of graphite nanosheet added was 3 wt%,the CO2 absorption of the composite in the 20th cycle was 0.57 g CO2/gsorbent,which was 290%of the unsupported sample.The type of carbon material has a significant effect on the cyclic carbonation performance of the composite.The choice of graphite nanosheets or carbon nanotubes as a carrier can achieve better stability of the cyclic carbonation reaction.Aiming at the current problem of low volumetric energy storage density in calcium-based heat storage materials,the composite composites were densified to increase the volumetric heat storage density of the composites.And the densification of the graphite nanosheet-loaded composite material is achieved by a compression process.The compression treatment can effectively eliminate the ineffective macropores in the composite material.At the same time,thanks to the dispersion of CaO particles by graphite nanosheets,the composite still has high cycle stability.After 20 cycles,the composite containing 12 wt%graphite nanosheets had a volumetric CO2 absorption of 0.42 g CO2/cm3sorbent and a corresponding volumetric storage density of 1.69 GJ/m3.Moreover,the densified Al2O3 stabilized composite was obtained by selecting dense calcium formate as the calcium source and depositing Al2O3 stabilizer on the surface.The composite does not contain oversized pores,while the surface-deposited Al2O3 stabilizer ensures the cycle stability of the composite.After 20 cycles,the composite containing 10 mol%of Al2O3 stabilizer had a volumetric CO2 absorption of 0.51 g/cm3,and the corresponding volumetric energy storage density was 2.07 GJ/m3.