Applications Of Metal Compounds In The System Of Thermochemical Energy Storage And Hydrogen Production Cycle Reaction

With the rapid development of science and technology,and the huge increase in fossil energy consumption,the world is facing a serious energy crisis.Renewable energy represented by solar energy,wind energy,tidal energy,etc.,is widely used in many fields due to its clean and renewable characteristics,but its energy supply stability is insufficient as a resulte of the daytime or weather changes.Energy storage technology has received widespread attention for improving the performances and reliabilities of renewable energy systems.Among many energy storage technologies,thermal energy storage(TES)uses materials for energy storage,absorbing and releasing heat when needed.Among the TES technologies,thermochemical energy storage(TCES)technologies are carried out through reversible reactions to charge,and discharge.The charge process absorbs thermal energy,and converts thermal energy into chemical energy through an endothermic reaction(positive reaction)of metal compounds and stores the energy in the products.The unique advantage of TCES is that energy can be stored for a long period of time.The energy supply process releases heat through an exothermic reaction(reverse reaction).Commonly used TCES media include metal oxides,metal hydrides,carbonates,ammonia and inorganic hydroxides.In addition,H2 can also be used as an energy storage medium and it is a clean fuel.The thermochemical water splitting(TCWS)not only can store energy,but also produce H2.Therefore,TCWS is of great significance in dealing with energy crisis.Compared with the direct thermal decomposition of water,TCWS has advantages:the reaction temperature of TCWS is lower,and H2 and O2 are generated in different steps making sepration easier.In general,a large number of thermochemical cycles for water splitting can be grouped into two broad categories:high-temperature two-step processes and low-temperature multistep processes.Compared with a two-step cycle,a multi-step cycle has a lower reaction temperature and less fossil fuel consumption,which is of great significance in terms of energy saving and emission reduction.This thesis applies metal compounds to thermochemical cycle reaction systems.The work is divided into two parts:the first part is to utilize metal compounds for TCES to improve reaction efficiency and reduce energy waste;the second part is to utilize metal compounds for TCWS to produce H2 and take measures to lower the reaction temperature,making the reaction easier to proceed,effectively reducing production costs and unnecessary heat loss.This thesis screens possible reversible reactions,and takes the following TCES reaction systems as the research objects:Mg(OH)2/MgCO3,MgO/MgCO3,CaCO3/CaO,CaMg(CO3)2/CaO&MgO,Mn2O3/Mn3O4 and Co3O4/CoO.The reactor was customized,and the reactants were placed in the corresponding reactors or synchronous thermal analyzer for TCES applications.When reactions completed,the cycle performance evaluation and thermodynamic and kinetic analysis were performed.For the Mg(OH)2/MgCO3 and MgO/MgCO3 reaction pairs,the CO2 adsorption by Mg(OH)2 and MgO is an exothermic reaction.This reaction can be used for energy supply.The process is carried out and evaluated in a fixed bed reactor.The experimental energy discharge efficiency of the Mg(OH)2/MgCO3 system and the MgO/MgCO3 system are 62%-75%and 86%-99%,respectively.The experimental results verifies the reaction kinetics of Mg(OH)2/MgCO3 and MgO/MgCO3 systems by the Jander 3D diffusion model:when the contact time is 20-40 s,the theoretical energy discharge efficiency of the Mg(OH)2/MgCO3 system is 60%-70%,and the theoretical energy discharge efficiency of the MgO/MgCO3 system is greater than 90%.Therefore,Mg(OH)2 and MgO have great application potential in TCES.The CaCO3/CaO and CaMg(CO3)2/CaO&MgO systems can store energy and produce lime at a relatively low temperature with a certain concentration of steam,reducing unnecessary heat loss.This thesis studies the thermal decomposition reaction of CaCO3 and CaMg(CO3)2 with steam.The reaction can be applied for energy storage.The reaction data fit the first-order reaction kinetic model well.The introduction of steam reduces the activation energy(Ea)of the thermal decomposition reaction of dolomite from 143.5 kJ/mol to 97.3 kJ/mol,which makes the reaction easier to proceed and promotes the reaction.Adding steam also greatly reduces the decomposition temperature of dolomite from 900℃ to 800℃ and reduces the decomposition temperature of marble from 900℃ to 825℃.The reaction can not only store energy,but also be applied for industrial production of quicklime.The specific surface area of commercial quicklime and high-quality quicklime are generally about 3 m2/g and5 m2/g respectively,while that of the self-produce quicklime can reach 14.67 m2/g,exhibiting high quality.The reaction temperature of this system is low,which can not only greatly reduce the production cost of quicklime,but also reduce the consumption of fossil.The research can bring considerable economic benifits to the industrial production of quicklime,and contribute to energy saving and emission reduction.The Mn2O3/Mn3O4 and Co3O4/CoO redox reaction pairs were applied to high temperature TCES reaction systems.In this thesis,Mn2O3 micron particles and Co3O4 nanoparticles were synthesized,which showed good cycle performances in multiple cycles of reactions.The kinetics results show that the reduction reaction rate of Mn2O3 is about 4 times than the reoxidation reaction rate of Mn2O3.SEM and BET characterization results show that the reduction products(Mn3O4 and CoO)exhibit slight sintering phenomena,not affecting their cycle performances.So selfmade Mn2O3 and Co3O4 are suitable for TCES.Besides,different amounts of Al2O3 were doped into Mn2O3 to synthesize MnAlOx(9.5%Al and 18.2%Al),but their cycle performances were not significantly improved.Co3O4 was also doped with different amounts of Fe2O3,and FexCo3-xO4(5%Fe,10%Fe,and 15%Fe)nanoparticles were synthesized and applied to TCES.Among these FexCo3-xO4 nanoparticles,FexCo3-xO4(5%Fe)has the best cycle effect.In addition to laboratory-scale experiments,a 10 kW large scale TCES experiment was designed.Since the reaction enthalpy change of Co3O4(ΔHR=844 kJ/kg)is higher than the reaction enthalpy change of Mn2O3(ΔHR=185 kJ/kg),when the energy stored is the same,the amount of Co3O4 is smaller,so Co3O4 is more suitable for TCES amplification experiments.After screening the TCWS reaction system,it is finally determined that the research object is the Mn3O4 and Na2CO3 multi-step TCWS reaction system.The reaction temperature of the system was lowered by reducing the particle size of the reactants.For the first cycle of commercial Mn3O4 and Na2CO3 reacting with H2O at 825℃,reducing the particle size of the reactants made the H2 yield increase from 72.3%to 78.3%.In the TCWS reaction of Mn3O4 and Na2CO3 at 775℃,almost no H2 was generated,but the H2 yield after pulverization reached 56.4%.The pulverized MnO and Na2CO3 were directly utilized for TCWS,and the H2 yield reached 96.4%.So the cycle performance of Mn3O4 and Na2CO3 needs to be further improved.The reactants first generated intermediate product MnO at the high temperature,but at this step,the reactants converged into large particles,which reduced the chance of contacting with H2O and reduced the H2 yield.In this thesis,the utilization of reactants with smaller particle diameters successfully reduced the reaction temperature,reduced unnecessary heat loss,made the reaction easier to occur,and greatly reduced the cost of hydrogen production.This thesis deeply explored the application of metal compounds in TCES and TCWS systems,which greatly reduced the reaction temperatures of the systems,reduced the amount of fossil fuels,greatly reduced the reaction cost,and reduced greenhouse gas emissions.It has extremely important significance in energy conservation and emission reduction and has broad application prospects.