A kinetic model for H2 production by plasmolysis of water vapours at atmospheric pressure in a dielectric barrier discharge microchannel reactor

A kinetic model has been developed for water vapour dissociation flowing at atmospheric pressure under nonthermal plasma conditions and analysed for different pathways of water dissociation, reaction time scales and key kinetic steps for H2 production. A stream of pure water vapour was considered. Different pathways of decomposition such as, dissociation reactions, electron dissociative attachment, dissociative ionisation reactions, ionisation reactions and dissociative excitation reactions were included along with other pathways for different species produced and consumed with in plasmolysis. Dissociation reaction was found to dominate the water vapour plasmolysis followed by electron dissociative attachment reaction being the other important kinetic step. Reaction time scales and concentration were found to be in direct relation with electron density and reached their respective highest values (concentration of H2, C H 2 = 16 moles/m 3 and τreaction ≅ 10−2 s) at maximum electron density (1021 m−3). The time scale for reaching 99% of hydrogen produced (steady state concentration of H2) was found to be around 10−2 s which showed H2 could be produced utilising very little power in micro-plasma reactors, in agreement with published theory (Lozano-Parada JH, Zimmerman WB. The role of kinetics in the design of plasma microreactors. Chemical Engineering Science 2010; 65(17):4925–30). When the model was run as dissociative attachment pathway (Reaction (8)) being the primary reaction to break water vapour, concentration of H2 was reduced from 16 moles/m3 to 4 moles/m3. However, it additionally introduces the H− radicals into the system which subsequently control the production of H by electron detachment. A detailed analysis was done to find out the major steps influencing the overall kinetics of water vapour plasmolysis. Recombination reaction of H–H atoms was not found to be significant for the production of H2. The key step for the generation of H2 was the reaction between H and HO2. Finally, a mechanism consistent with the kinetically important steps is proposed.