| The research objective of this thesis is focused on an attempt to clarify the existing two contradictory views on coal structure i.e. as either a 'porous rock' or a 'pseudo-polymer'. This has been accomplished by studying the sorption of gases and vapors on coal samples, of varying rank, for various kinds of gases and covers over a range of conditions, and solvent/vapor induced swelling phenomena.; The porosity of coals was investigated by measuring true and apparent densities using different displacement fluids, adsorption derived surface areas and pore volumes, and pore size distribution using nitrogen adsorption and mercury porosimetry. The fact that the measured helium densities for all coals are significantly larger than those measured by mercury displacement, indicates quite clearly that all coals are porous, and have interconnected pore networks. Surface areas and pore size distributions determined by using CO{dollar}sb2{dollar}, and N{dollar}sb2{dollar} adsorption, and mercury porosimetry, again, indicate that coals contain pores in the range 3.3A to 1 {dollar}mu{dollar}m range. It must be concluded that their structures are dominated by micropores Given the small surface areas (0 to 54 m{dollar}sp2{dollar}/g) determined by nitrogen and argon areas adsorption at {dollar}-{dollar}196{dollar}spcirc{dollar}C, and calculated by BET equation, it must be concluded that they are not true representations of the total surface areas for coals. Carbon dioxide is probably the best choice for adsorbate since it is small (minimum dimension 3.3A) and its high saturated vapor pressure (64 atm at room temperature) allow for the collection of data that fit well into the range of the D-R equation (P/Po: 10{dollar}sp{lcub}-5{rcub}{dollar} to 0.2) with a conventional gas adsorption apparatus.; The polymeric nature of coals was characterized quantitatively by applying the modified Gaussian network model to the measured equilibrium pyridine swelling data of raw and pyridine-extracted coals. From the data generated by this approach, it is concluded that, at 25{dollar}spcirc{dollar}C, the number average repeating units between crosslinks (N) of pyridine-extracted coals vary from 9 to 13 for low and medium rank coals (73 to 88%C). As rank increases further, then, the value of N decreases to 4.6 for the medium volatile bituminous coal (88%C) and to 2.8 for the anthracite (90%C). These results indicate that while anthracites and high rank bituminous coals have few crosslinks, coals of lower rank (medium and low rank bituminous coals and lignites) exhibit the properties of extensively crosslinked networks. Also, solvent swelling for coals was found to be a function of temperature, coal rank, electron donor number, branched chain, chain length, and polarity of organic solvents.; From the data generated in this thesis it must be concluded that, dependent on rank, the physical structure of coals can conform to both the suggested "porous rock" and "pseudo-polymer" models. It must be stated that all coals are to varying degrees porous. It is the pseudo-polymeric properties that are most rank dependent. One must conclude from these data that the structure of anthracites, and higher rank bituminous coals (medium-volatile), are best described as being entirely consistent with the "porous rock" model, whereas, medium and lower rank bituminous coals, and lignites are best exemplified as having the properties and structure of porous thermosetting polymers. |
