Hetero-/Homogeneous Combustion of Fuel-Lean CH₄/O₂/N₂ Mixtures over PdO at Elevated Pressures

Abstract

The heterogeneous and homogeneous combustion of fuel-lean CH₄/O₂/N₂ mixtures over PdO was investigated experimentally and numerically at equivalence ratios φ=0.27-0.44, pressures 1-12 bar and surface temperatures 710-1075 K. In situ Raman measurements of major gas-phase species concentrations across the boundary layer of a channel-flow catalytic reactor assessed the heterogeneous reactivity, while planar laser induced fluorescence (LIF) of the OH radical monitored homogeneous combustion. Simulations were performed using a 2-D code with detailed heterogeneous and homogeneous reaction mechanisms. Comparisons between Raman-measured and predicted transverse profiles of major species mole fractions attested the atmospheric-pressure suitability of a detailed surface mechanism and allowed for the construction of a global catalytic step valid in the range 1-12 bar. The methane catalytic reaction rate exhibited an overall pressure dependence ~p1-n where the exponent n was itself a monotonically increasing function of pressure, rising from 0.58 at 3 bar to 1.02 at 12 bar. This resulted in a non-monotonic pressure dependence of the catalytic reaction rate in the range 1-12 bar, a behavior in stark contrast to other noble metals (Pt and Rh) where the methane reaction rates always increased with rising pressure. Surface temperatures remained well-below the PdO decomposition temperature at each corresponding pressure, owning largely to the "self-regulating" temperature effect of PdO, and this in turn mitigated homogeneous ignition. Simulations using the PdO decomposition temperatures as boundary conditions for the wall temperatures were further performed for practical CH₄/air catalytic reactors in power generation systems. It was shown that for p < 7 bar (a range relevant to microreactors) homogeneous ignition was altogether suppressed. For higher pressures relevant to gas-turbine burners, however, gaseous combustion ought to be considered in the reactor design.

Department(s)

Mechanical and Aerospace Engineering

Keywords and Phrases

Catalytic combustion of CH4 on PdO; High-pressure CH4 combustion; Homogeneous ignition of CH4 over PdO; In situ Raman and LIF; Pressure-dependence of CH4 catalytic reactivity on PdO

International Standard Serial Number (ISSN)

1540-7489

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2019 Elsevier Ltd, All rights reserved.

Publication Date

01 Apr 2019

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