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 com
pletely dried samples of the individual coals and also partially dried samp
les 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 initial
ly 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 tha
n 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 conversi
ons of oxygen and hydrogen into char as solid residue, CO, H-2, and, in par
ticular, liquids. In pyrolysis at 758-1037 K, greater amounts of hydrogen a
nd 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 confirme
d 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 oxy
gen functional groups in liquid products and char, as reported in the Liter
ature. Therefore, it was concluded that a portion of the sorbed water remai
ns in the pyrolyzing coal/char matrix under the rapid heating conditions an
d participates in thermochemical reactions taking place therein.