PYROLYSIS KINETICS AND MATURATION OF COALS FROM THE SAN-JUAN BASIN

Temperature-programmed pyrolysis was used to analyze 18 coals and coaly shales from the San Juan Basin of the southwestern U.S., giving temperatures of maximum evolution rate (T(max) and pyrolysis yields. T(max) values increased with maturity (as measured by vitrinite reflectance [%R(m)]). The pyrolysis yields remained constant or increased slightly with increasing maturity until a value for % R(m) of approximately 1.0 was reached, after which the yields declined rapidly. Pyrolysis-gas chromatography and pyrolysis-FTIR showed that alkane and alkene yields followed similar trends. A subgroup of coals from the Fruitland seam of the San Juan Basin was also analyzed by micropyrolysis at several constant heating rates to determine laboratory pyrolysis kinetics. The kinetic calculations yielded the energy of activation by the approximate method (E(approx)) and the principal energy of activation by the discrete method (principal E(discrete)) in the range 55-57 kcal/mol for the coals in the % R(m) range 0.4-0.9. However, the coal with the highest % R(m) (1.30) had E(approx) and principal E(discrete) values around 63 kcal/mol. These Fruitland seam coals were also extracted with organic solvents, and the kinetic parameters were found to be within experimental error of their unextracted counterparts. Application of the multiple parallel reaction model to the kinetic parameters determined for the four Fruitland seam coals showed only fair agreement between calculated and measured values for T(max) and yield relationships with maturity. This inability of the model to adequately describe the measured data is attributed to the pyrolysis mechanism changing with increasing maturity.