IMPORTANCE OF THE REACTING MEDIUM IN ARTIFICIAL MATURATION OF A COAL BY CONFINED PYROLYSIS .2. WATER AND POLAR COMPOUNDS

The role of water in the mechanisms of generation of hydrocarbons and the thermal breakdown of kerogen is still controversial. In some cases, the addition of excess water seems to be necessary to properly simulate the maturation of organic matter whereas in other pyrolysis experiments added water is not required to obtain results comparable to the natural maturation of organic matter. In order to understand the role of water in the confined pyrolysis medium and the organic matter-water interactions, a successive pyrolysis-solvent extraction protocol was performed and excess water was added at each pyrolysis step. The results showed that when polar compounds and water coexist in the reacting medium, the solid residue evolves in the same way as that described by a reference series performed in ''classical'' confined pyrolysis conditions. However, when polars are absent in the reacting medium or when the temperature is higher than 350 degrees C, the role of water as a hydrogenating agent is inhibited. Thus, the aromatization and recombination reactions are enhanced and the generation of hydrocarbons (and especially n-alkanes) sharply decreases. These data suggest that the chemical role of water is effective when intermediates located in the polar fraction and promoting its dissociation are present in the pyrolysis medium. Moreover, the comparison of these results (added water) with those obtained from successive pyrolysis-extraction performed without added water suggests that water cannot completely substitute for the hydrocarbons in the reacting medium. However, as the temperature increases above 350 degrees C the role of water is inhibited as a hydrogenating agent because the competitive role of ionic chemistry is diminished and dehydrogenation chemistry is faster at higher temperatures.