| Coal gasification can usually be divided into two processes including the pyrolysis of coal and the gasification of char. The latter, as a rate control step, directly affects the efficiency of char gasification. There are many factors influencing the reactivity of char, among which char structure is one of the main influence factors. In fact, char structure could evolve as the gasification reaction, which simultaneously affects the migration and transformation of alkali metal and alkaline earth metals. There are very close links between the evolution of char structure and the reactivity of gasification. That is to say, the structure decides the nature, the nature affects the efficiency of coal gasification. So investigating the evolution of char structure during gasification is really essential for a better understanding of gasification mechanism.This study aims to investigate the evolution of char structure and reactivity during the gasification of various chars. The chars made from Xishan bituminous, Ximeng lignite (two typical Chinese coals) and cornstalk separately at temperature of 1000℃ were characterized in terms of their gasification reactivities with the atmosphere of H2O, CO2 or H2O/CO2 mixture (800℃ or 950℃) by fixed bed reactor-online gas analysis system. The chars produced at different gasification conversion levels were further analyzed with N2 adsorption and Raman spectroscopy for the changes of chemical and physical structure. The contents of alkali and alkaline earth metallic species in these chars were quantified by ion chromatography. The links between reactivity and structure evolution during gasification of various chars in different atmosphere, and the synergy effects in the process of co-gasification in H2O/CO2 or coal char/cornstalk char were researched. The main results are as following:(1) In the conditions of H2O atmosphere and 800℃ temperature, the gasification reactivity of cornstalk char is related to its specific surface areas, the contents of potassium and magnesium. However, the gasification intrinsic reactivity of lignite char is related to its chemical structure. In the conditions of H2O atmosphere and 950℃ temperature, the gasification reactivity of bituminous char is related to its specific surface area to some extent.(2) In the conditions of CO2 atmosphere and 800℃ temperature, the gasification reactivity of cornstalk char is related to its specific surface area, the contents of potassium, magnesium and calcium. However, the gasification reactivity of lignite char is related to its specific surface areas and chemical structure. In the conditions of CO2 atmosphere and 950℃ temperature, the gasification intrinsic reactivity of bituminous char is related to its chemical structure.(3) In the conditions of H2O mixed CO2 atmosphere and 800℃ temperature, the gasification reactivity of cornstalk char is related to its specific surface area, the contents of potassium and magnesium. The gasification reactivity of lignite char is related to its chemical structure, and is associated with the contents of sodium, potassium and calcium when carbon conversion is less than thirty percent. In the conditions of H2O mixed CO2 atmosphere and 950℃ temperature, the gasification reactivity of bituminous char is related to its specific surface areas when carbon conversion is Tess than twenty percent, however, the effects of the chemical structure and the contents of minerals are not significant.(4) There existing synergy effects in the co-gasification of H2O/CO2 for different chars, which is linked to the specific surface areas. As to cornstalk char, the synergy effect of intrinsic reaction rate is related to the conditions of potassium, magnesium and calcium. However, the synergy effect of intrinsic reaction rate is related to its chemical structure for lignite char, and is associated with potassium for bituminous char.(5) There existing synergy effects in the co-gasification of coal chars and biomass char. The gasification reactivity of mixed chars is the highest when the ratio of lignite char and cornstalk char is 2:1. The gasification reactivity increases with the increase of the ratio of cornstalk char in the co-gasification of bituminous char and cornstalk char. |
