The evolution of coal char structure under the oxy-fuel combustion containing high H2O

•H2O-gasification is higher than CO2-gasification in O2/CO2/H2O atmosphere.•Porosity of char from a containing-H2O atmosphere was significantly enhanced.•Amorphous carbon ratio decreased in an atmosphere containing-H2O.•Carbon reactivity exerts a more impact on combustion than specific surface area.

Oxy-fuel wet recycle system with a higher energy efficiency exhibits more attractive. Compared to conventional pulverized coal combustion, influences of gasification in oxy-fuel combustion is more complicated, especially in wet recycle system. Meanwhile, gasification has a significant impact on char structure, which can directly affect the combustion characteristic of char. The evolution of coal char structure under the oxy-fuel combustion containing high H2O was investigated. Char samples were prepared in CO2/H2O atmosphere using a drop-tube furnace (DTF), N2, CO2 and N2/H2O atmosphere as the control group. Carbon content, surface morphology, pore structure, carbon structure and characteristics temperature of the coal char were analyzed. Experimental results show that carbon burnout rate increases with the content of volatile matter increase. Char particles from a containing-H2O atmosphere formed around less fine particles than the char particles under no-H2O atmosphere, because fine coal particles in the atmosphere containing-H2O are easy reacted and taken away from the airflow. The remarkable reduction of char pores under an atmosphere containing CO2 or H2O caused by carbon consumption during gasification reaction, which can not only produce a more porous structure but also diminish the proportion of large pores. Amorphous carbon in coal could be quickly reacted in a containing-H2O atmosphere. Atmosphere containing H2O dramatically reduces char characteristics temperature at 21% O2 concentrations. In conclusion, char combustion reactivity affected by carbon reactivity largely than the specific surface area in oxy-fuel atmosphere containing high H2O.