Catalytic Combustion of Syngas

Abstract

The catalytic combustion of syngas/air mixtures over Pt has been investigated numerically in a channel-flow configuration using 2D steady and transient computer codes with detailed hetero-/homogeneous chemistry, transport, and heat transfer mechanisms in the solid. Simulations were carried out for syngas compositions with varying H₂ and CO contents, pressures of 1 to 15 bar, and linear velocities relevant to power generation systems. It is shown that the homogeneous (gas-phase) chemistry of both H₂ and CO cannot be neglected at elevated pressures, even at the very large geometrical confinements relevant to practical catalytic reactors. The diffusional imbalance of hydrogen can lead, depending on its content in the syngas, to superadiabatic surface temperatures that may endanger the catalyst and reactor integrity. On the other hand, the presence of gas-phase H₂ combustion moderates the superadiabatic wall temperatures by shielding the catalyst from the hydrogen-rich channel core. Above a transition temperature of about 700 K, which is roughly independent of pressure and syngas composition, the heterogeneous (catalytic) pathways of CO and H₂ are decoupled, while the chemical interactions between the heterogeneous pathway of each individual fuel component with the homogeneous pathway of the other are minimal. Below the aforementioned transition temperature the catalyst is covered predominantly by CO, which in turn inhibits the catalytic conversion of both fuel components. While the addition of carbon monoxide in hydrogen hinders the catalytic ignition of the latter, there is no clear improvement in the ignition characteristics of CO by adding H₂. Strategies for reactor thermal management are finally outlined in light of the attained superadiabatic surface temperatures of hydrogen-rich syngas fuels.

Keywords:

• Catalytic ignition
• Hetero-/homogeneous syngas combustion
• Platinum catalyst
• Syngas catalytic combustion

Support was provided by the Swiss Federal Office of Energy (BFE), the Swiss Commission of Technology and Innovation (KTI) under contract No. 8457.2, and ALSTOM Power of Switzerland.

Notes

^(a)From Deutschmann et al. (2000). In the surface and desorption reactions, the reaction rate coefficient is k = AT ^b exp(− E/RT), A [mole-cm-Kelvin-s] and E [kJ/mol]. In the adsorption reactions, except S2, S3 and S8, A denotes a sticking coefficient (γ). Reactions S1 and S2 are duplicate. Reactions S3 and S13 have a Pt-order of 1 and 2, respectively. The suffix (s) denotes a surface species and θ _i the coverage of surface species i.

^(a)From Warnatz (1996; 2005). Reaction rate k = AT ^b exp(− E/RT), A [mole-cm-Kelvin-s], E[kJ/mol]. Third body efficiencies: ω(H₂O) = 6.5, ω(O₂) = ω(N₂) = 0.4, ω(H₂) = 1.0, ω(CO) = 0.75, ω(CO₂) = 1.5. The reaction pairs (14, 15) and (21, 22) are duplicate. Reactions 8 and 16 are Troe reactions centered at 0.5 (second entries are the low pressure limits).