Swelling behavior of several bituminous coals and their thermally treated coals

The swelling behavior in different solvents of 4 bituminous coals with different ranks and their residues from extraction by CS2/NMP mixed solvent(1∶1 in volume) were measured.The change in swelling property of the four coals thermally treated at different temperature was observed.The results show that the swelling ratio decreases with increasing rank of coal.For lower rank bituminous coals the swelling ratios in polar solvent are higher than those in non-polar solvent,and this difference decreases with increasing rank.The cross-linking densities of the four residues decrease,and the swelling ratios increase compared with those of raw coals.The swelling ratios of the four thermally treated coals under 150 ℃ in CS2 increase,suggesting the decrease in cross-linking density of them.When the thermal treatment temperature increases to 240 ℃,the swelling rations of the other three coals in NMP and CS2 increase again except gas coal,demonstrating the further decrease in cross-linking density.This result is coincident with the extraction yield change in the mixed solvent of the thermally treated coal.For example,the extraction yield of lean coal treated at 240 ℃ increases from 6.9% to 17.3%. FT-IR results show the removal of oxygen group of the thermally treated coals. This may explain the increase in swelling ratio and extraction yield in the mixed solvent of coal after thermal treatment.The cross-linking density of the thermally treated coal decreases because of the break of hydrogen bonds due to removal of C=O and-OH oxygen groups during the thermal treatment,resulting in the increases of swelling ratio and extraction yield in the mixed solvent of thermally treated coal compared with those of raw coal.