Evaluation of drying induced changes in the molecular mobility of coal by means of pulsed proton NMR

Drying induced changes in the molecular properties of six different as-received coals with water contents ranging from 8 to 60 wt % of their wet weight were investigated on the basis of the mobility of the coal hydrogen and the distribution of different types of water. When dried at 303 K, a brown coal releases water in the following order: free water identical to bulk water, bound water that froze at around 226 K, finally, nonfreezable water that never froze even at 123 K. According to ¹H NMR criteria, a portion of the coal hydrogen was found to be mobile. The amount of the mobile coal hydrogen (C_MH) varied inversely with the amount of the nonfreezable water, while the release of the free and bound water had little effect on the reduction of C_MH. For coals with water contents of up to 32 wt %, C_MH in the as-received samples agreed well with the hydroxylic hydrogen content, C_DH, which was determined by a hydrogen-deuterium exchange technique. However, in coals with higher water content, C_MH was approximately twice as great as C_DH.