Experimental And Simulation Study On Zn Reduction CO₂ And Its Generation

As the most abundant carbon source on earth,the resource utilization of CO2 is attracting more and more attention.In particular,thermal chemical cycle conversion CO2 technology,using heat energy through a series of correlated thermochemical reactions,decomposing CO2 and(or)H2O into CO and(or)H2,O2,while CO and H2 can be used as raw material to further process methanol synthesis,hydrocarbon chemical products and transportation liquid fuel,and the resulting CO2 is returned to the thermochemical recycling system after centralized treatment by capture technology,it is a kind of CO2 resource utilization technology with high energy conversion efficiency.In recent years,it has gained the focus of attention and carried out a large number of basic research.At present,the research on the thermochemical decomposition of CO2 and(or)H20 is based on two step cycle via metal and its oxide.There is a general shortage of high reaction temperature,difficult product separation and overall thermal efficiency.On the basis of SI cycle decomposition of water hydrogen production,considering of the element electronegativity,the Zn reduction of CO2 reaction was introduced into the SI cycle,thus,a novel thermochemical ZnSI cycle was proposed,in which Zn would be prepared by ZnI2 thermal decomposition instead of ZnO thermal decomposition.The thermochemical ZnSI cycle for decomposition of CO2 and H20 reduces the maximum reaction temperature from 1773 to 1073 K.In this paper,some basic problems of CO2 reduction reaction with Zn and ZnI2 decomposition reaction in thermochemical ZnSI cycle are studied,as well as the reaction of ZnI2 and CO2.A fixed bed experimental system and an airflow bed experimental system were used to study the Zn reduction reaction of CO2.Results showed that the reaction mechanism of the two experimental systems varied with temperature,for which the temperature ranges were identical,that is,the chemical reaction control mechanism was taken in the 653-753 K and the>793 K interval,while the diffusion control mechanism is subordinate to the inside,i.e.the 753-793 K interval.High temperature and high CO2 concentration can achieve high Zn conversion rate.The study shows that the maximum Zn conversion rate of 973 K and 100%CO2 in the airflow bed experimental system can reach 96.05%.While,it can be up to 83.05%under the condition of 1073 K and 100%CO2 in the fixed bed experimental system.The effect of CO2 concentration on the conversion of Zn has a best value,and the higher the reaction temperature,the greater the optimum value.For example,the optimum CO2 concentration of 833 K is about 90%under the the airflow bed experimental system.In addition,the temperature and C02 concentration are increased under the two experimental systems,and the ZnO crystal becomes stronger and denser.The proportion of hollow particles,shells and holes would be increased at high temperature,but the micromorphology of the ZnO crystal is step shape under the fixed bed condition,obeying the dislocation control growth,but having a cubic cone under the airflow bed,obeying the nucleation control.Growth.At the nanoscale level,a gas-solid two phase reaction kinetic model of Zn reduction C02 was established,and the thickness of ZnO shell was quantitatively described.It is suggested that the apparent reaction of Zn reduction C02 can be divided into:a surface reaction stage,a diffusion stage,and a second surface reaction stage.On this basis,using the first principle on the molecular level,the surface reaction path of the unreacted Zn crystal and partially oxidized Zn crystal were calculated.Meanwhile,the bi-directional diffusion mechanism of Zn2+ and O2-in the ZnO crystal has been analyzed.The calculation results are corresponding to the model.Thermodynamic simulation of ZnI2 decomposition reaction and side reaction was carried out by FactSage software.The calculation shows that the ZnI2 decomposition reaction belongs to high endothermic reaction and can be carried out spontaneously in high temperature environment over 1000 K under normal pressure.The quantitative experiment shows that the effect of temperature on the decomposition rate of ZnI2 conforms to the Growth/sigmoidal-slogisticl model.In a certain temperature range,the decomposition rate of ZnI2 increased with the temperature rise,while the time required to reach the equilibrium state would be shortened.The effect of the initial sample amount on the decomposition rate of ZnI2 is effectively the effect of the pressure on the sample.The initial sample volume increases with the vapor partial pressure and the ZnI2 decomposition rate decreases.In addition,the volume rate of carrier gas is negatively correlated with the residence time,and when the carrier gas rate is constant,the residence time varies with temperature.Under the experimental conditions,the conversion rate of 0.1 g ZnI2 can reach 41.36%at 1073 K,while the volume rate of carrier gas was 100sccm.When C02 is present in the reaction atmosphere,the thermodynamic simulation shows that the decomposition rate of ZnI2 is significantly increased and the initial reaction temperature is reduced to 757.75 K.When ZnI2 and CO2 react as the molar ratio of 1:1,ZnI2 can be fully decomposed at 1565 K,whose decomposition ratio is increased by about 90.88%compared with the pure ZnI2 decomposition reaction.However,once the temperature was too high(>1452 K),the side reaction ZnO+CO(?)CO2 ?Zn became severe,resulting in the decrease of C02 decomposition rate.With the increase of ZnI2,both higher ZnI2 amd C02 decomposition ratio can be obtained at the same time.High CO2 decomposition ratio can be achieved when Zn exists,but it is not conducive to the improvement of ZnI2 decomposing.The results of the quantitative experiment system and the airflow bed experimental system show that when the atmosphere contains C02,the decomposition ratio of ZnI2 increases significantly.For example,when the concentration of CO2 in atmosphere is 50%,and temperature is 1173 K,the decomposition ratio of ZnI2 can be improved from 0.72%to 14.29%in the airflow bed,and enhanced from 31.02%to 42.15%in the fixed bed experimental system as the innital amount of ZnI2 is 1.0 g.Combined with the above research,the simplified design of thermochemical ZnSI cycle system was carried out by Aspen Plus.The traditional Bunsen reaction in the original system was replaced by electrochemical Bunsen reaction.The C02 decomposition reactor and the ZnI2 decomposition reactor were merged into ZnI2-CO2 reactor.Thus,modules for the Bunsen reaction purification,the H2SO4 phase purification,the HI phase purification,and some corresponding auxiliary equipment can be cancelled,leading to a greatly simplified system.Quality balance and energy balance analysis of the simplified system show that the CO yield 0.5mol/s can be achieved,when the rated hydrogen production rate was set to be 0.5mol/s.Compared with the original system,the heat load of the simplified one reduced by 80.0%.As well as the flow rate,the process steps are greatly simplified,but the thermal efficiency of the system is also reduced.Taking the total logistics volume of H2O and CO2 as an example,each decreased by 52.86%and 79%respectively.