| Hydrogen energy is the clean energy with the most development potential in the 21st century.Based on the Steam Methane Reforming(SMR),Reactive Sorption Enhanced Reforming(ReSER)couples the calcium cycle with the SMR reaction,which reduces the reaction temperature and increases the raw materials,gas conversion rate,simplified process flow,etc.As the core of the calcium cycle and a key part of the ReSER reaction,the nano-CaO CO2 adsorbent has become a research hotspot because its directly affects the realization of the ReSER hydrogen production process.At present,the decomposition of calcium carbonate in calcium-based adsorbents still has the problems of high temperature and slow decomposition rate,which are affected by heating methods and equilibrium limitations.Therefore,research to solve the problems of efficient decomposition of calcium carbonate and utilization of released CO2 has theoretical and important research significance for the industrial application of ReSER hydrogen production technology.In this paper,a new idea was proposed to use a chemical reaction to enhance the decomposition of calcium carbonate through the preparation of a composite adsorbent containing nickel and CaO components,the CH4 gas is introduced into the decomposition process of calcium carbonate in the composite adsorbent,so that the CO2 generated by the decomposition of calcium carbonate will undergo dry reforming reaction with methane to generate syngas under high temperature,which not only makes the equilibrium of the decomposition reaction of calcium carbonate move to the direction of generating CO2,but also achieves the purpose of CO2 conversion and utilization to produce syngas.In this paper,the feasibility of methane-drying reforming on calcium carbonate decomposition was studied theoretically and experimentally.Preparation method of composite adsorbent NiO-nano CaCO3/Al2O3,the influence of reaction conditions such as coupling reaction temperature and airspeed on regeneration and adsorption properties of NiO-nano CaCO3/Al2O3 adsorbents were also studied.Firstly,a thermodynamic model for the decomposition of methane dry reforming coupled calcium carbonate was established,and the decomposition conversion rate of calcium carbonate before and after the coupling reaction and the relationship between the conversion rate of methane dry reforming and the temperature were calculated.The results show that:when the reaction temperature is 706.3℃ and the methane conversion rate is 78.1%,the calcium carbonate can be completely decomposed,indicating that the decomposition of calcium carbonate can be realized at a temperature lower than the thermal decomposition temperature of 827.3℃.Reaction strengthening can reduce the decomposition temperature of nano-calcium carbonate by about 120℃.Secondly,NiO-nano CaCO3/Al2O3 composite adsorbents with different NiO contents were prepared by urea hydrolysis method combined with wet mixing method,the microstructure of NiO-nano CaCO3/Al2O3 and the size of the active component NiO grain size,the catalytic component reduction performance and the adsorption component CO2 adsorption performance were studied by means of BET,XRD,H2-TPR,TGA,etc.The results show that the NiO content has little effect on the microstructure of the composite adsorbent.When NiO content is 10 wt.%,the particle size of NiO is 17.2 nm,which theoretically has a good catalytic performance of methane dry reforming reaction.The maximum adsorption capacity of the composite adsorbent decreased with the increase of NiO content in the composite adsorbent,among which the adsorption capacity of the composite adsorbent with 5 wt.%NiO content was the highest(9.7 molCO2/kg agent),which met the CO2 adsorption capacity requirements for hydrogen production by reaction strengthening ReSER.Thirdly,the feasibility of the coupling reaction to strengthen the decomposition of calcium carbonate was verified in a laboratory-scale fixed bed reactor,the adsorption performance and reaction rate of the composite adsorbent under the conditions of reaction intensification and thermal decomposition was compared,and the relationship between the content of NiO,temperature,space velocity and dry reforming of methane and the decomposition rate of calcium carbonate in the coupling reaction were studied.The results showed that the decomposition rate of calcium carbonate in the compound was more than 90%when the reaction temperature was above 740℃.The temperature is lower than the theoretical thermal decomposition temperature of 824 ℃ with the same 90%decomposition rate.At 800℃,the decomposition time of calcium carbonate in the composite adsorbent was reduced from 128 min under thermal decomposition condition to 22 min due to the strengthening of methane-dry reforming reaction,indicating the decomposition rate of calcium carbonate increased by more than 5 times,and the product of methane dry reforming coupling calcium carbonate decomposition reaction H2:CO:CO2=10:8.3:1,which could be used for Fischer-Tropsch synthesis after treatment.In the coupled reaction system,the higher the content of NiO in the composite adsorbent,the better the catalytic effect on the dry reforming reaction.The composite adsorbent with a NiO content of 15 wt.%at 800 ℃ corresponds to a methane conversion rate of up to 79.7%and a calcium carbonate decomposition conversion rate of 92.7%.The higher the reaction temperature or space velocity,the faster the calcium carbonate decomposition rate.At 750℃,the adsorption capacity of the composite adsorbent with 10 wt.%NiO content obtained by the coupling reaction enhanced regeneration is 7.0 molCO2/kg,which has better adsorption performance than the 6.2 molCO2/kg agent by thermal decomposition.At 700 and 750℃,the reaction rate constants of calcium carbonate under reaction strengthening conditions are higher than thermal decomposition.Finally,a spray granulation method was used to prepare a NiO-nano CaCO3/Al2O3 composite adsorbent with smooth surface,spherical shape,particle size of 40-50 μm,and 10 wt.%NiO content.The effects of temperature and space velocity on the decomposition rate of calcium carbonate in the coupling reaction were studied,and the decompositon effects of calcium carbonate in fixed and fluidized bed were compared.The results show that the higher the temperature,the faster the decomposition rate of calcium carbonate and the higher the decomposition rate.When methane space velocity is 400 h-1 at 800℃,the decomposition rate of calcium carbonate is the fastest.The decomposition conversion rate of calcium carbonate can reach 97%within 15 minutes,which is 10 minutes shorter than the decomposition time on the fixed bed under the same conditions.Through calculation,at 700 ℃ and 750℃,when the methane space velocity is 400 h-1,the decomposition rate constants of calcium carbonate on the fluidized bed are higher than those on the fixed bed under the same conditions.It shows that the regeneration effect of fluidized bed is better than that of fixed bed.The results of this study have important guiding significance for the improvement of calcium carbonate decomposition efficiency and the comprehensive utilization of CO2 in the ReSER reforming hydrogen production technology. |
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