Product characterization and pore structure evolution of a tar-rich coal following pyrolysis under nitrogen, steam/nitrogen, and oxygen/nitrogen atmospheres

Steam and an oxidative medium could be the most promising atmosphere for the in-situ pyrolysis of tar-rich coal. This study investigated the influence of these two atmospheres by comparing product yields, tar properties, and pore evolution characteristics of a tar-rich coal following pyrolysis under nitrogen (N2), steam/nitrogen (steam/N2), and oxygen/nitrogen (O2/N2). Compared with N2 pyrolysis, steam/N2 pyrolysis increased the tar yield by 0.54–1.33 wt.% at 350–600 °C; further, O2/N2 pyrolysis increased the tar yield from 0.57 wt.% to 3.53 wt.% at 350 °C while decreasing this value by 0.31–1.83 wt.% at 400–600 °C. At 450 °C and 500 °C, steam addition suppressed the formation of oxygenated compounds, thereby decreasing the O/C ratio of tar from 0.099 and 0.088 to 0.073 and 0.074, respectively. Conversely, O2 introduction presented the oxygen addition of tar by oxidation reactions. Meanwhile, steam inhibited the aromatization of alkanes and the condensation of monocyclic aromatic hydrocarbons (MAHs). By contrast, O2 was able to promote the condensation of phenols and MAHs. At 450 °C, steam/N2 was thought to assist in the cracking of the plastic mass by providing a H2-rich atmosphere, which decreased the pitch content of tar and increased the contents of light fractions. O2/N2 was also beneficial for the formation of light fractions by intensifying the thermal cracking effect. Micro-CT and SEM analysis showed that H2O could promote the formation of micro-sized pores within the coal matrix, decreasing the average pore diameter (APD) from 150.95 μm to 102.67 μm and increasing the average coordination number (ACN) of pores from 6.27 to 8.59. O2 was inclined to enlarge the pyrolysis-induced pore network, which imposed a contrasting effect on APD and ACN parameters. Steam is recommended as the preferred atmosphere for the in-situ pyrolysis of tar-rich coal.