Hydrous pyrolysis of two kinds of low-rank coal for relatively long duration

Brown coal samples were treated with hot water in a stainless steel batch reactor at 623 K for 2–72 h. After this hot hydrous treatment, gas, oils, and residues were recovered. The resulting residues were chemically analyzed in detail to understand the reaction chemistry during hydrous pyrolysis. Oxygen functionalities were analyzed chemically with the titration method and carbon types in the residue were examined by solid-state 13C NMR measurement. Elemental analyses showed that the oxygen atoms in the residue decreased markedly up to 2 h while treatments longer than 48 h were also very effective in removing oxygen functionalities from brown coal. The detailed chemical analyses revealed that alcoholic hydroxyl and carboxyl groups were decomposed in the earlier stages of the treatment, and that ether bonds may be cleaved during the latter stages of the hot hydrous pyrolysis. Experiments using two kinds of brown coal gave very different results. A comparison of the chemical structure of these two coals revealed the origin of the difference; one of them has a greater amount of hydroaromatic moieties than the other, which act as a hydrogen source even during hydrous pyrolysis occurring at temperatures as low as 623 K.