Pyrolysis Characteristics of Low-Rank Coal under a CO-Containing Atmosphere and Properties of the Prepared Coal Chars

Herein, the pyrolysis characteristics of low-rank coal under a CO-containing atmosphere was studied via thermogravimetry coupled with mass spectrometry and Fourier transform infrared analysis. Further, the pore structure, carbon chemical structure, and combustion reactivity of the prepared coal chars were characterized via N2/CO2 adsorption, Raman spectroscopy, and thermal analysis, respectively. The CO-containing atmosphere suppressed coal devolatilization at the rapid pyrolysis stage. During coal pyrolysis, this atmosphere reduced CH4 evolution by suppressing the formation of free radicals such as CH3 • and CH2 •; H2 evolution reduced owing to the inhibition of the formation of hydrogen free radicals and the working of the inverse water–gas reaction; the H2O yield was increased through the inverse water–gas reaction but not by the conversion of the hydroxyl group in coal. At 300–700 °C, the disproportionation reaction of CO produced copious CO2 molecules, significantly raising the CO2 emission intensity over the CO2 released during coal pyrolysis. The pore structure of the chars-CO was suppressed via shingling of the carbon particles generated in the disproportionation reaction and the inverse water–gas reaction, which inhibits the release of volatiles in a CO-containing atmosphere. The carbon chemical structure of the chars-CO was ordered by the interaction between the char’s skeleton structure and the newly formed carbon and the contribution of the carbon particles with an ordered structure adhered on the char. The combustion reactivity of chars-CO was not improved by its higher volatile content, because the underdeveloped pore structure and ordered carbon structure degraded the combustion reactivity of chars-CO. Conversely, the pore structure of chars-CO is more developed than that of chars-N2 due to the release of volatiles in the high-temperature segment before the gasification reaction starts so as to improve the gasification reactivity of chars-CO.