Effect of sorbed water on conversion of coal by rapid pyrolysis

The effects of water sorbed in coal on its rapid pyrolysis characteristics were examined for three different low-rank coals (Beulah-Zap (BZ), Yallourn (YL), and South Banko (SB)). The pyrolysis was performed in a Curie-point pyrolyzer at temperatures ranging from 631 to 1193 K, employing raw and completely dried samples of the individual coals and also partially dried samples of BZ. The yield of water formed by pyrolysis, Y-pw, was defined as the difference between the total mass of evolved water and that of the initially sorbed water, f(w), per unit mass of completely dried coal. Y-pw for the raw BZ (f(w) = 47.5%) was 4.4-5.5% at 863-1193 K and appreciably lower than that of the completely dried sample (f(w) = 0%), 10.8-10.9%. It was also found that a higher f(w) results in a lower Y-pw. Similar effects of sorbed water were confirmed for YL and SE. Analyses of the product distributions revealed that the decrease in Y-pw is counterbalanced by increased conversions of oxygen and hydrogen into char as solid residue, CO, H-2, and, in particular, liquids. In pyrolysis at 758-1037 K, greater amounts of hydrogen and oxygen were converted into liquids in the raw BZ than in the completely dried sample by 5-8 mol of H and 2-4 mol of O per 100 mol of C. Considering that 11-23 mol of carbon was converted into liquids per 100 mol of C, the differences mean a significant increase in the O and H contents of liquids induced by sorbed water. The enrichment of hydrogen and oxygen was confirmed by the detection of much higher concentrations of hydroxyls and aliphatic hydrogen in liquids from the raw BZ than from the completely dried sample. These effects of sorbed water cannot be simply explained by a decrease in the heating rate of the coal due to the large heat of vaporization of water, since a decrease generally leads to a higher yield of water and fewer oxygen functional groups in liquid products and char, as reported in the Literature. Therefore, it was concluded that a portion of the sorbed water remains in the pyrolyzing coal/char matrix under the rapid heating conditions and participates in thermochemical reactions taking place therein.