(79f) Kinetic Modeling of Oxidation Methane Conversion in Regime of Filtration Combustion with Superadiabatic Heating

In this presentation we consider modeling of conversion of methane-oxygen-steam mixture to syngas performed in a flow within inert porous solid. Gas conversion in superadiabatic regime of filtration combustion is a promising means to improve energy efficiency of the conversion. If realized such regime allows one to combine high combustion temperature (hence high yield of hydrogen and carbon monoxide) with low temperature of the gaseous products (i.e. low energy cost of the process). Preliminary assessment of the process was performed under the following assumptions: (1) the reaction zone comprises a stationary wave traveling through the porous medium in the direction of gas flow; (2) gas reactions proceed within a narrow reaction zone; (3) equilibrium composition of reaction products is attained; (4) heat and mass transfer are controlled by gas flow and gas-solid heat exchange; pressure drop on the reactor is negligible [1-2]. The model showed that superadiabatic heating of the gas can be achieved, the combustion temperature depending on both net adiabatic heat effect and apparent ignition temperature, the latter being controlled by the detailed kinetics of gas reactions. The higher is apparent activation energy for gas ignition, the higher superadiabatic heating can be attained, so the high combustion temperature is attained at lower heat effect. The parametric domain for the gas compositions (oxygen/methane/steam ratios) that provide desirable low net heat effect and favorable equilibrium composition of syngas has been estimated.

To provide a detailed description for the process, a non-stationary computational model has been developed. The model is based on the following assumptions. The flow of the reaction mixture is considered as unidimensional (no distribution of temperature or concentrations in the direction lateral to the reactor axis). Pressure drop on the reactor is negligible. The solid porous medium and gas flow are considered as interpenetrating continua, each characterized by local temperature and the rate of interphase heat exchange.

The equations set includes conservation equations for both solid and gas with interphase heat exchange and heat release rate of chemical reactions; for each gas species a balance equation is written with convection and diffusion flows and kinetic source terms. The equations set is closed with the equation of state for ideal gas, boundary conditions at the reactor ends, and proper initial concentration and temperature distributions related to ignition conditions. The detailed kinetic scheme of the process comprises 30 species and 121 elementary reactions. It is quite similar to the well-known GRI-Mech-3 kinetic scheme [3]. The rate constants for the reactions are those published in database [4].

The kinetic scheme accounts for the formation and reactions of formaldehyde and active oxymethyl radicals in direct oxidation reactions of methyl radicals. Additionally to that, the scheme considers the peroxide cycle. The peroxide radicals are formed via oxygen addition to methyl radicals and further react to form the branching methyl hydroperoxide. Major active intermediates, methanol and formaldehyde, are formed in this cycle. The peroxide cycle plays important role on the initial low-temperature stage of the partial methane oxidation. For higher temperature, the initial stage of partial oxidation proceeds via competing formaldehyde cycle, which included transformations of formaldehyde as a source of hydrogen. The direct oxidation of formaldehyde (branching) and two reactions of linear decomposition, to hydrogen atom and formyl radicals (branching) and the hydrogen and monoxide molecules (decay) proceed only at high temperature.

For various gas compositions and varied initial temperature preliminary calculations were performed using complete kinetic scheme disregarding spatial distribution. The calculations showed the time evolution of concentrations of products and intermediates and temperature. For initial temperature of 1000 K the reaction time (typical time for reaching maximum temperature) is on the order of 1 s. For mixtures richer in methane, the reaction time is longer, the maximum temperature is lower. The composition of products at maximum temperature is far from established one, the concentration of intermediates remains high. The typical reaction time for water conversion, however, is order of magnitude longer than the reaction time.

The modeling of gas mixture conversion in the regime of filtration combustion proved that a traveling wave regimes with superadiabatic heating for conversion of ultra-rich mixtures are attainable.

References

1. Svetlana S. Kostenko, Eugene V. Polianczyk, Anna A. Karnaukh, Avigeya N. Ivanova, George B. Manelis (2006), «Model of Oxidative-Steam Methane Conversion in regime of Filtration Combustion», Khim. Fiz., vol.25, no.5, pp. 53-63 (In Russian)

2. Svetlana S. Kostenko, Eugene V. Polianczyk, Anna A. Karnaukh, Avigeya N. Ivanova, George B. Manelis (2005), «Macrokinetics of Methane Conversion at Superadiabatic Filtration Combustion», in The Nonequilibrium Processes, vol.1 Combustion and Detonation, Gabriel D.Roy, Serge N. Frolov, and Alexander N. Starik, eds., TORUS PRESS, Moscow, pp. 223 - 229

3. Gregory P. Smith, David M. Golden, Michael Frenklach, Nigel W. Moriarty, Boris Eiteneer, Mikhail Goldenberg, C. Thomas Bowman, Ronald K. Hanson, Soonho Song, William C. Gardiner, Jr., Vitali V. Lissianski, and Zhiwei Qin (2000), 'Gri-Mech 3' http://www.me.berkeley.edu/gri_mech/

4. NIST Standard Reference Database 17 (version 6.0) (1994), F. Westky, J. T. Herron, R.F.Hampson, W.G.Mallard, ed., Gaithersburg MD