Effects of CO2 on gas evolution and char structure formation during lump coal pyrolysis at elevated pressures

• CO2 atmosphere is favorable to the release of H2O during coal pyrolysis.
• Initial evolution temperature of gas shows a notable dependency on atmosphere and pressure.
• Chars prepared in CO2 atmosphere have more cycloalkane and aliphatic structures.
• CO2 atmosphere is beneficial to the growth of char crystal layer in vertical position.

The gas release properties and changes in the char structure during lump coal pyrolysis were investigated in Ar and CO2 atmosphere. Fixed bed reactor and gas chromatography were employed to generate char at four pressures (0.1–1.5 MPa in 0.5 MPa increments) and measure gas composition from room temperature to 700 °C in 50 °C increments, respectively. The influences of pressure and atmosphere on char structural changes were examined by FT-IR and XRD. The results indicate that CO2 atmosphere is favorable to the release of H2O. The initial evolution temperature of gas species shows a notable dependency on atmosphere and pressure. Especially for CH4, CO2 atmosphere can promote its emission in advance and make the evolution profile subdued within 0.5 MPa but hinder its evolution over the pressure range of 0.5 MPa, and this inhibitive effect is more evident at a higher pressure. The chars prepared in CO2 atmosphere have more cycloalkane and aliphatic structures, and the role of atmosphere is particularly evident when the pressure exceeds 0.5 MPa. There are more content of hydroxyl, secondary hydroxyl, olefinic CC bonds, aromatic CC structure left in chars prepared under CO2 atmosphere compared with those prepared under Ar atmosphere. The interplanar distance d0 0 2 indicates that Ar atmosphere is more helpful to the graphitization of char during pressurized coal pyrolysis, the stacking height Lc of char sample prepared in CO2 is always higher than that prepared in Ar atmosphere within 1.0 MPa, CO2 atmosphere is beneficial to the growth of crystal layer in vertical position.