Quantitative Microscopic Characterization Of The Structure And The Combustion Reactivity Of The Char Prepared In N₂and CO₂Atmospheres

The reduction of CO₂emissions is essential to the mitigation of global warming.O₂/CO₂combustion has been suggested as one of the important technologies for carboncapture and storage （CCS） in coal-fired power plants. Compared to conventional aircombustion, O₂/CO₂combustion affects pulverized coal combustion characteristics. As oneof the important characteristics, char reactivity has significant impacts on coal ignition,burnout and boiler efficiency. Although lots of researchers have made efforts to study themechanisms, there are still arguments on the effects of O₂/CO₂combustion on charreactivity. The structure of coal chars is one of the important factors that influence the charreactivity, and there are some limitations of traditional methods in the characterization ofchar structure. The quantitative method based on microscopy can not only measure theclosed pores, but also can obtain parameters such as char particle diameter, circularity, wallthickness and porosity, which can be input into mathematical models and assist indeveloping a better understanding of char combustion. In this work, a software namedImageJ was used to quantitatively characterize the structure of chars generated in differentpyrolysis atmospheres （N₂and CO₂）, and the results were used to explain the differences inthe combustion reactivity of N₂-char and CO₂-char in various atmospheres.First, the low-density fraction （<1.4g/cm3） of a bituminous coal was prepared andused to generate N₂-char and CO₂-char, respectively, on a drop tube furnace. Then theImageJ software was used to quantitatively characterize the structure of N₂-char andCO₂-char. The results showed that, the porosity, diameter, perimeter and feretdiameter ofN₂-char were all larger than CO₂-char, indicating the char swelled more extensively and hada more porous structure in N₂atmosphere. Quantitative characterization data showed that,the N₂atmosphere generated more Group I chars and Group II chars than the CO₂atmosphere, which generated more Group III chars. There were no significant differences inchar wall thickness and crystalline structure between CO₂-char and N₂-char, indicating thatpyrolysis atmosphere did not influence the char wall thickness and crystalline structuresignificantly for the studied coal. Due to gasification, the weight loss determined using ash tracer method for CO₂-char was higher than that for N₂-char.Second, a non-isothermal thermo-gravimetric analysis （TGA） method was used toinvestigate the combustion reactivity of N₂-char and CO₂-char in various atmospheres.TGA data showed that, in the same combustion mode （O₂/N₂or O₂/CO₂）, char reactivityincreased with increasing O₂concentration. At the same O₂level, char reactivity was higherin O₂/N₂than in O₂/CO₂. Due to the more porous structure, N₂-char had a higher reactivitythan CO₂-char in the same combustion mode and at the same O₂level.