Kinetic study of methane combustion over perovskite catalysts

(eng) The vast amount o f pollutions emitted irom various industrial and domestic sources causesviolation o f the equilibrium o f worldwide ecological Systems nowadays. Elimination of volatileorganic compounds (VOC's), which participate in greenhouse problem and can provoke adversehealth effects, by means of catalytic combustion is a very important issue in environmentalprotection.This work deals with a laboratory-scale kinetic study of catalytic combustion o f methanepresented in a complex reaction environment, namely in the gaseous émissions ffom the steel-making industries. A typical composition o f these émissions contains a variety o f VOC's, H2,H20 and COx in addition to nitrogen- and sulfur-containing molécules.The goal o f the présent research is to fin d a kinetic model, which is able to predict the total VOCconversion over a selected robust catalyst as a function o f operating variables, i.e. the partialpressure o f reactants and inhibitor molécules, the space-time and the température.The optimization o f the composition o f perovskite-type oxide catalysts ascribed by the generalformula o f Lai_xCexCoyMni_y03 with respect to activity and résistance to sulfur poisoning leads tothe sélection of the species Lao^Ceo 1C0O3. However, S 02 and H2S irreversibly poison thecatalysts thus they should be removed ffom the feed stream in order to ensure a reasonablecatalyst lifetime.Combustion o f methane over Lao.gCeo 1C0O3 was proven to détermine the total VOC conversionsince among them it has the lowest combustion activity and the température range o f itsconversion is only slightly affected by the other hydrocarbons. Methane combustion is stronglyinhibited by H20 and weakly by C 0 2. The other inorganic molécules, i.e. NO, NH3 and CO causeneither inhibition nor deactivation.The kinetics of methane combustion, considering the influence o f the reactants, reflects veryaccurately a steady-state oxidation mechanism. Three first-order elementary steps control theoverall reaction rate, viz. (a) the adsorption of oxygen which forms active surface oxygen by fastreaction, (b) the reaction between surface oxygen and methane from the gas phase to form anintermediary that is transformed to H20 and C 02 by fast reaction and (c) the desorption o f theproducts. The corresponding kinetic model was discriminated ffom 23 plausible models on thebasis o f experimental data ffom an intégral fixed bed reactor and a Micro-Berty reactor.Under real combustion conditions, when the partial pressure of water in the feed is significantlyhigher than the one produced by methane combustion, the desorption of H20 becomes anequilibrium process and the overall rate is controlled by only two elementary steps, i.e. theadsorption o f oxygen and the reaction.