Gasification reactivity of charcoal with CO2. Part II: Metal catalysis as a function of conversion
The catalytic influence of major metal species found with waste wood (Na, K, Ca, Mg, Zn, Pb, Cu) was studied during the gasification of nitrate salt impregnated charcoal with CO2 at 800degreesC in the kinetically controlled regime. In contrast with literature, the respective reaction rate data were analysed over the entire carbon conversion (X) range by using extended kinetic relations to quantify catalyst metal-specific reaction rate contributions not accounted for by the original random pore model of Bhatia and Perlmutter. The kinetic analysis provided valuable insights in the underlying mechanisms. With alkali nitrate impregnated charcoal, it is demonstrated that the often found reaction rate maximum around X similar to 0.7 may merely reflect increasing catalytic activity resulting from alkali accumulation in the charcoal, superimposed on structural changes in the charcoal micropore domain. Impregnated earth-alkali nitrates revealed substantial activity as well, but only during the early gasification stage (X < 0.2), hereby, underlining their sintering propensity in combination with the localised deposition of the earth-alkaline nitrate salt in the former wood cells by the impregnation procedure. Added heavy metal nitrates revealed no activity, apart from lowering the charcoal reactivity over the entire conversion range by ca. 15% compared with the untreated charcoal, suggesting inhibition by covering, hence, blocking of otherwise accessible active charcoal surface sites and/or by deactivation of neighbouring indigenous alkali due to immobilisation at the heavy metal oxide surfaces formed during pyrolysis. The extended kinetic relation reproduced all of our reaction rate data well over the entire gasification stage, hereby, supporting the superposition of micropore domain and catalyst specific effects. (C) 2002 Elsevier Science Ltd. All rights reserved.