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 H 2 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 H 2 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 H 2 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 H 2 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 H 2 . Strategies for reactor thermal management are finally outlined in light of the attained superadiabatic surface temperatures of hydrogen-rich syngas fuels.

Effects of heterogeneous–homogeneous interaction on the homogeneous ignition in hydrogen-fueled catalytic microreactors

“…Under these conditions, the Reynolds number was less than 150 and the laminar flow assumption was valid. The solid thermal conductivity l s ¼ 16 W/(m K) was considered, corresponding to FeCralloy that is a common material for catalytic honeycomb reactors in power generation [9,37] and is well-suited for catalytic microreactors in particular [17].…”

“…A surface emissivity 3 ¼ 0.6 was used for all coated differential interior surface elements, while the inlet and outlet sections and the exterior surfaces of channel wall were treated as black bodies ( 3 ¼ 1.0) [9,17].…”

“…In general, homogeneous combustion can be desirable or detrimental, depending on the specific application and conditions of the system [15]. On the one hand, homogeneous chemistry have positive impact, for example, moderation of the superadiabatic wall temperatures [9], feasibility of operating at higher velocities [16], extension of blow-out and extinction limits [17], and making chemicals in oxidative coupling and oxidative dehydrogenation reactions [18,19]; in these cases, HH reactors are designed to appropriately promote homogeneous combustion. On the other hand, the homogeneous chemistry is an undesired feature because its onset results in flames associated with uncontrolled process temperatures, with severe implications for safety, reactor runaway, device failure, catalyst lifetime, or even explosive behavior; higher reactor temperatures in the presence of homogeneous combustion have been observed experimentally [12] and numerically [20].…”

Hetero‐/Homogeneous Combustion

“…4 bar can be crucial in reactor design. It has been shown [17,64] that the presence of a flame in H 2 -fueled catalytic systems is beneficial in moderating the reactor temperature, as will also be discussed in Section 8.5.…”

“…The second application refers to hetero-/homogeneous combustion of fuellean hydrogen/air mixtures on Pt [64]. The modeled geometry considers a honeycomb channel as an equivalent cylindrical tube of length 75 mm, internal radius 0.6 mm, and a solid wall thickness of 50 mm.…”