Study Of Zhundong Coal Gasification Characteristics With In-situ CO₂ Capture Based On Calcium Looping Process

The status that coal is the primary energy in China will not change in a short period of time.With increasing global attention to CO2 emissions,clean and efficient coal utilization is of great significance.The coal gasification with in-situ CO2 capture based on calcium looping process is capturing CO2 meanwhile shifting the thermodynamic equilibrium of the water-gas shift reaction to enhance H2 production.Since the carbonation reaction is endothermic,the temperature in the gasifier is in the range of 650?800?,that might have a negative impact on reaction rate.As representative of high alkali coal,Zhundong coal is rich in reserves.The high alkali metal content in coal has catalytic effects during the gasification,and meanwhile the temperature range of this gasification process might reduce the the agglomeration and erosion caused by the volatilization of the alkali metal in coal.Therefore,based on the Zhundong coal gasification process with in-situ CO2 capture by CaO,the gasification kinetics and gas production characteristics were investigated in this work.The CaO absorbent modification experiments were also carried out to deal with the problem of deactivation during the cyclic carbonation/calcination.The specific work is as follows:1.The influence of alkali metal occurrence modes in Zhundong coal on thegasification reaction kineticsThe Zhundong coal was treated by different solvents to extract the alkali metals of different occurrence modes,and the alkali metal content of the corresponding occurrence mode was determined.The reaction kinetics of the raw coal,water washed-coal and hydrochloric acid washed-coal during the steam gasification were investigated using pressurized TGA.The effects of different occurrence modes of alkali metals on gasification kinetic characteristics were clarified.The experimental data were fitted using the shrinking core model,the homogeneous model and the modified volumetric model,respectively.The results showed that the modified volumetric model is the most appropriate model to fit the conversion curves,which have trend of increasing first and then decreasing during the gasification.The steam gasification kinetic parameters of Zhundong coal in the medium temperature range(700?750?)were obtained.2.The influence of added CaO on the kinetic characteristics of Zhundong coalgasificationThe reaction kinetics of Zhundong coal steam gasification with CaO addition was investigated using pressurized TGA.The influences of the gasification temperature,the steam partial pressure and the calcium to carbon mole ratio on coal gasification kinetics were examined.The experimental results showed that there was synergistic catalytic effect between the alkali metal in coal and the introduced CaO.The synergistic effect was related to the gasification temperature,and it became obvious when the gasification temperature was higher than 735?.The gasification kinetic parameters of Zhundong coal gasified with CaO in medium temperature range were obtained using modified volumetric model.The results showed that the addition of CaO could make more active sites of the coal char to play the synergistic catalytic effect with alkali metal.Combined with the expetimental and calculated results,the synergistic catalytic mechanism was further analyzed from the perspective of the influence of Ca on the the formation of C(O)as active intermediate.3.Study on gas production characteristics of Zhundong coal gasification within-situ CO2 capture based on calcium looping processThe experiments of Zhundong coal gasification with in-situ CO2 capture based on calcium looping process in bubbling bed were conducted to investigate the gas production characteristics.The influences of temperature and pressure on gas production characteristics were investigated.The results showed that the reaction rate and the effective gas production would be enhanced because of the synergistic effect.Among the bubbling bed gasification experiments,the reaction rate and the effective gas production of water washed-coal were the highest.The results also showed that reducing the gasification temperature is beneficial to the carbonation and increasing H2 concentration.However,reducing temperature had a negative impact on gasification rate.The increase of gasification temperature can significantly enhance the gas production and the coal char conversion.Considering both product gas composition and yield,the appropriate gasification temperature range is 700 to 750?.The increase of gasification pressure is beneficial to carbonation,and can enhance the H2 production to increase H2 concentration and reduce CO2 and CO.For the circulating experiment of coal gasification,the CaO reactivity gradually decreased with the increase of cycle numbers,and thus the H2 concentration gradually decreased and the CO2 concentration increased.After seven cycles of gasification,CaO sintering was serious,and there was no obvious effect on enhancing H2 production.4.Study on the modification of CaO-based absorbent and the cyclic propertiesIn order to improve the cyclic stability of CaO,peanuts husk ash was used as additive to modify the calcium-based absorbent.The relevant preparation parameters were obtained.When the hydration time was 12 hours and the mole ratio of SiO2 to CaO was 0.07,the modified calcium-based absorbent had the best stability,and the carbonation conversion rate was 2.4 times as that of pure CaO af-ter 22 cycles under the same carbonation/calcination reaction conditions.The predicted carbonation conversion of the modified sorbent using a semi-empirical equation can be maintained around 0.35 after 100 cycles.In addition,the influences of steam,calcination temperature and steam activation on the modified absorbent were also studied.The results showed that the addition of steam during the carbonation reaction can improve the pore structure of the absorbent in multiple carbonation/calcination cycles,thereby keep more surface area for the fast kinetic reaction and decrease the diffusion resistance of the thick product layer to enhance the reaction at product layer controlled stage,relieving the influence of sintering on the carbonation.The modified calcium-based absorbent after multiple carbonation/calcination cycles was hydrated by steam at a temperature of 350?.The steam activation could effectively recover the pore structure and improve the reactivity of the sorbent,and the hydration time of 10 minutes is enough.