The work reported in this thesis is a study of the changes in porous texture during steam gasification of a model char derived from a PVDC/PAN copolymer and a char derived from a highly volatile, bituminous coal (Markham Main). The techniques employed to analyse the complete pore structure of the chars were helium pycnometry, mercury porosimetry and gas adsorption. Adsorption measurements were made using Ar at 77 K, CO2 at 195 K and CO2 at 273 K in an attempt to analyse as much of the pore structure as possible. Total surface area (TSA) and micropore volume were shown to increase with bum-off for both chars, reaching a maximum value at about 70% bum-off. The proportion of non-microporous surface area, obtained using the Isotherm Subtraction method, increased with progressive gasification up to about 60% burn-off, confirming quantitatively what was expected from mercury porosimetry measurements. All results indicate that the model char is more microporous than the coal char. A new dynamic method is described for measuring the active surface area (ASA) of carbons and chars, which involves oxygen chemisorption followed by a temperature-programmed desorption of CO2 and CO; the method gives similar results to the standard volumetric technique. ASA, measured by this technique, increases with burn-off for both chars, reaching a maximum value at 70-80% burn-off. Reactivities of the two chars were measured using a fixed-bed reactor and a thermogravimetric balance; results from the two techniques are in good agreement. Gasification conditions were chosen such that the rate was chemically controlled. Maximum reactivity was observed at about 70% bum-off for both chars. When reactivity is expressed on a weight basis (gC/g sec) the model char is more reactive than the coal char. However, when the reactivity is expressed per unit ASA the coal char appears to be the most reactive of the two. It is concluded from this survey that ASA is a more reliable reactivity normalisation parameter than TSA.
|Date of Award||1985|