| Development of the conversion technology of solid fuels based on gasification method is the important direction for utilizing coal with high efficiency and environmentfriendly condition.It is also a significant source of energy systems and chemical raw materials.Low-rank coal,mainly lignite and sub-bituminous coal,accounts for 45.7% of China’s coal reserves,which is regarded as the essential part for supplying sustainably energy in the future.Low-rank coals are particularly suitable for gasification-based technologies due to their special features,in sharp contrast to the high-rank coal.The temperature needed for low-rank coal,corresponding to the minimum exergy rate of thermal energy required for the complete steam gasification,is much lower than that of high-rank coal.Therein,char structure and inherent alkali and alkaline earth metallic species are the main reasons to the high reactivity.In particular,ion-exchangeable Na and Ca are generally considered as excellent in-situ catalysts for low-rank coal.In this paper,the Zhundong coal is used as the experimental sample.The roles of ion-exchangeable Na and Ca in the whole process of gasification are studied,which lays the theoretical foundation for the improvement of char reactivity and the development of novel catalytic gasification technology.The catalytic characteristics of ion-exchangeable Na and Ca during pyrolysis,char gasification and volatile-char interaction were studied,respectively.Firstly,the pyrolysis experiments were conducted by the fluidized-bed/fixed-bed combination reactor.The behaviors of ion-exchangeable Na and Ca including migration,transformation and occurrence were analyzed.Effects of pyrolysis on the catalytic behaviors and char reactivity were analyzed through the changes of char reactivity.The results showed the evolutions of char structure were the main reasons for influencing char reactivity.The active structure index ID/I(S+A1+A2+D)measured by Raman spectra was well corresponding to the char reactivity.The changes of ion-exchangeable Na and Ca were the main reasons for influencing char reactivity in the catalytic pyrolysis.The reactivity of Na-Char and Ca-Char derived from the pyrolysis at 700°C was highest among the different pyrolysis temperatures.The ion-exchangeable Na was volatilized much more easily than the Ca species during pyrolysis,which related to the amount of catalytic sites in char.However,the magnitude of char reactivity was not directly dependent on the content of Na and Ca.With the increase of pyrolysis temperature,the changes of Na occurrence were well corresponding to the catalytic activity.Meanwhile,the dispersion of Ca in char was the main reason for controlling the reactivity.In conclusion,the pyrolysis had a significant effect on the catalyst,which played a decisive role in char reactivity.Based on the above results,controlling the pyrolysis could promote the subsequent char reactivity.Based on the study of the behaviors of ion-exchangeable Na and Ca during pyrolysis,the chars prepared from the pyrolysis at 900°C were used as the experimental samples.The gasification of the char samples was conducted at different conditions using a microfluidized bed reaction analyzer(MFBRA).In the steam gasification,the char reactivity of Na-Char and Ca-Char was higher than that of H-Char non-catalytic gasification reactivity by 1~2 orders of magnitude.This result showed the excellent catalytic activity of ion-exchangeable Na and Ca.The reactivity profiles of Na-Char and Ca-Char during the whole gasification were studied,and the key factors for controlling the char reactivity were deeply analyzed.The loss of Na catalyst was the main reason for the sharp decrease in the reactivity during the initial gasification stage.By contrast,the gradual change of Ca dispersion contributed to the deactivation of the Ca catalyst.It was also demonstrated that the catalysis of Ca species exhibited better persistence than that of Na species.The reactivity of Na-Char was thus higher than that of Ca-Char during the initial gasification stage,and then lower than that of Ca-Char.In addition,the H2 and syngas production from the Na-Char and Ca-Char catalytic steam gasification was studied.It is observed that Na and Ca species not only accelerated the gas release rate during gasification but also significantly increased H2 production.Variations in the product gases during NaChar and Ca-Char gasification were completely different,which were associated with the different deactivation pathways and catalytic reaction mechanisms of Na and Ca catalyst.The H2 production from steam gasification was directly related to the catalytic activity of Na and Ca.In addition,the high quality syngas with desired H2/CO could be achieved through adjusting the gasification temperature and reacting gas concentration.The above results provided the foundation for the development of high efficient staged gasification and the preparation of H2 and the desired H2/CO syngas.Based on the catalytic steam gasification by Na or Ca alone,the catalytic behaviors of Na/Ca binary catalyst during steam gasification were investigated.The results indicated that a cooperative effect between Na and Ca on the gasification reactivity occurred.The good performance of the Ca catalyst while interacting with the mineral matter enhanced both the Na/Ca-Char reactivity and the resistance of Na to deactivation.The Na additive not only increased the gasification reactivity but also ensured the persistence of the Ca catalyst to deactivation,due to the formation of low-temperature melting eutectic catalyst.Na-Char and Ca-Char in the 30%:70% mixed ratio was an optimum for promoting char reactivity under the current conditions.Additionally,H2 production from Na/Ca-Char mixture gasification could be further increased comparing with the Na-Char or Ca-Char alone.In addition to the study of steam gasification,the catalytic and non-catalytic CO2 gasification was investigated.The results indicated that the reactivity profiles of Na-Char and Ca-Char CO2 gasification were different from the steam gasification.For the CO2 gasification,the catalytic activity of Ca was much higher than Na catalyst,which contributed to the higher reactivity of Ca-Char than that of Na-Char.The loss of Na catalyst was still the main reason for the decrease in the reactivity,but the rate of Na loss was slower than that of steam gasification.The catalytic activity of Na in CO2 gasification had a better persistence than that of steam gasification.The deactivation of Ca catalyst was attributed to its loss and poor dispersion.Na and Ca had an inhibiting effect on char reactivity during Na-/Ca-Char gasification.With increase of the amount of Ca-Char,the inhibiting effect was gradually enhanced.The reactivity of non-catalytic steam gasification was around three times than that of non-catalytic CO2 gasification.But,the orders of Na-Char and Ca-Char reactivity for steam gasification and CO2 gasification would change with increasing carbon conversion,which closely related to the deactivation pathways of Na and Ca at different atmosphere.Based on the study of steam gasification and CO2 gasification,the reaction mechanism between CO2 and H2 O was investigated during the char gasification with CO2/H2 O mixed atmosphere.For the noncatalytic gasification,CO2 and H2 O competed for common active sites on the char surface.There was an inhibiting effect on char reactivity between H2 O and CO2 gasification.In the case of Na catalytic gasification,the similar effect on char reactivity between H2 O and CO2 occurred.Through the gasification experiments with the change of H2 O and CO2,it was demonstrated that the H2 O molecule would consume or occupy lots of catalytic active sites during CO2/H2 O co-gasification.In the case of Ca catalytic gasification with CO2/H2 O mixed atmosphere,the synergic effect between CO2 and H2 O occurred during the initial gasification stage,and there was an inhibiting effect on char reactivity between CO2 and H2 O within the later gasification stage.The change of reaction mechanism between CO2 and H2 O was greatly associated with the role of Ca catalyst during CO2/H2 O co-gasification.Based on the studies of pyrolysis and gasification,the effects of volatile-char interaction on char reactivity were investigated using the fluidized-bed/fixed-bed combination reactor.The results indicated the migration of volatile AAEM species onto the char matrix occurred during volatile-char interaction.The volatile AAEM species had better catalytic activity than residual AAEM species in the original char.The reactivity of the gasified char(50% conversion level)was obviously enhanced after the interaction of volatile and char.It is showed that volatile AAEM species were important to char reactivity and appeared to be essential to achieve positive effects on char gasification from volatile–char interactions.This provides theoretical support for the development of new gasification process to enhance the reactivity of coal char. |
