Experimental Studies On Structure And Reactivity Of Char In Co-Gasification Of Shenmu Coal And Corn Straw

As the supporting energy in China,coal could emit a lot of carbon dioxide during its utilization.Excessive emission of carbon dioxide can cause many ecological problems and environmental pollution problems,such as desertification,greenhouse effect,extreme weather and glacier melting.Therefore,China has put forward the goal of"double carbon".In this context,biomass with the advantages of low pollution and carbon dioxide neutral is favored.Its co-gasification with coal is an important mean to achieve efficient and clean energy utilization.At present,the nature of the interaction between coal and biomass during co-gasification is not fully understood,for example,issues such as the correlation between the physical structure of the gasification char and the reactivity of the char remain controversial;the action mechanism of active alkali and alkaline earth metals(Ac-AAEMs)in the feedstock on the co-gasification reaction,in particular,the influence mechanism of Ac-AAEMs on the characteristics of the co-gasification char and of Ac AAEMs on the volatile substances-char interaction are still unclear.To this end,in this paper,the in-situ co-pyrolysis-gasification coupling method is used to explore the effects of different reaction conditions and Ac-AAEMs in different samples on the physico-chemical structure and reactivity of co-gasification chars,and also investigate the interaction mechanism of volatile gas and volatile AAEMs with char during co-pyrolysis and co-gasification processes and the interaction mechanism of volatile AAEMs with char by subsequent char gasification process.To eliminate the errors brought by the gasification method with cooled pyrolysis char,the in-situ co-pyrolysis-gasification coupling method is adopted to investigate the effects of gasification time,pyrolysis time,and corn straw blending ratio on the physicochemical structure and reactivity of co-gasification chars from coal(RSM)and corn straw(RCS).Meanwhile,the correlation between char physicochemical structure and reactivity is discussed,and the optimal conditions for co-gasification char with high reactivity are finally obtained.The results show that the physicochemical structure of char is correlated with the reactivity of char during co-gasification.Appropriate prolongation of pyrolysis time(<10 minutes)is beneficial to promote the development of pore structure and specific surface area of co-gasification chars,and reduce the graphitization degree of char structure.Finally,the char reaction is promoted.The optimum conditions for obtaining co-gasification char with high reactivity are pyrolysis for 10 minutes,gasification for 20 minutes and corn straw blending ratio greater than 70%.To investigate the effects of Ac-AAEMs in coal,Ac-AAEMs in corn straw and Ac-AAEMs in the mixture of coal and corn straw on the characteristics of co-gasification chars,co-gasification chars of four mixtures(acid washed samples and raw samples are mixed in pairs)with different gasification time are prepared by a fixed-bed/fluidized-bed combined reactor system(FFCRS),and the char content and gasification carbon conversion rate of each gasification char are measured.The physicochemical structure of chars are characterized by nitrogen adsorption analyzer,scanning electron microscope,and Raman spectroscopy,respectively.In addition,the oxidation reactivity and combustion reactivity of chars are measured by thermogravimetric analysis and micro fluidized bed reaction analyzer,respectively.The outcomes indicate that three forms of Ac-AAEMs can promote the development of specific surface area of char and reduce the graphitization of char structure,ultimately leading to the enhancement of char reaction.Besides,the content of co-gasification chars is affected by influence-1(Ac-AAEMs promote the increase of char content during pyrolysis)and influence-2(Ac-AAEMs catalyze the char reaction during gasification,resulting in the decrease of char content)during the whole co-gasification process.In the first 5 minutes of gasification,influence-1 occupies the core position.After gasification for 10 minutes,influence-2 plays a major role.To decouple volatile gas-char interaction,the samples for removing Ac-AAEMs are prepared by dilute sulfuric acid deionization method,and the experiments on volatile substances-char interaction as well as volatile gas-char interaction are performed by FFCRS.The role of volatile AAEMs in volatile substances-char interaction is revealed by comparing the results of volatile substances-char interaction and volatile gas-char interaction on char characteristics.The results show that the volatile gas-char interaction inhibits the reactivity of coal char.The presence of volatile AAEMs weakens the inhibition of volatile gas on the development of char pore structure and significantly promotes the reaction of coal char.The existence of volatile AAEMs could inhibit the depletion of oxygen-containing groups caused by volatile gas-char interactions.Among them,volatile AAEMs from RCS inhibit the consumption of oxygen-containing groups by about 11%.Volatile AAEMs from RSM suppress the depletion of 4%oxygen-containing groups.To explore the interaction mechanism between volatile substances and char in the co-gasification process and the interaction mechanism between volatile AAEMs and char in the subsequent gasification process,the effects of volatile substances on the physicochemical structure and reactivity of the char samples are analyzed by nitrogen adsorption analyzer,Raman system and thermogravimetry.Meanwhile,the carbon conversion rate curves are determined and the synergistic index A_R(X_i)under different carbon conversion rates is calculated by a combined mass spectrometer and FFCRS.The results show that the volatile AAEMs released from corn straw char during gasification have a significant effect on the co-gasification reaction,and their presence facilitates the development of pore structure of coal char and the improvement of reactivity.Among them,the content of microporous structure increases by 22%,the content of S-mesoporous structure increases by 144%,and the reaction index K increases by 178%.