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Catalytic Partial Oxidation of Methane over Rhodium

 

Motivation

In the catalytic-rich / lean combustion process part of the air from the compressor is mixed with the entire fuel stream and undergoes partial oxidation in a catalytic reactor. The hot product gas mixture is then mixed with the remaining air and combusted in a conventional gas-phase burner. Within this group the performance of the Rh-based CPO reactor itself was studied. (Fig. 1)

Fig. 1: Photograph of CPO reactor and schematic of catalytic-rich / lean combustion process
 

Approach

To study the CPO the same combined experimental and numerical approach as described in the CST section was used. Under fuel rich conditions the analysis of gas-phase reactions was performed with formaldehyde-PLIF (instead of OH-PLIF). The 352nm laser sheet was tunable to allow for on- and off-resonance measurements and thus for an increase in signal to noise ratio.

Next to the optically accessible reactor a honeycomb type prototype reactor with short residence times (~8ms) that also enabled transient studies was used.

 

Recent studies

Validation of chemical reaction schemes

Detailed hetero- /homogeneous chemical reaction schemes have been validated. Raman measured major species concentration profiles validated the heterogeneous reaction scheme (Fig. 2), whereas the PLIF has shown that the ignition delay times of the homogeneous ignition were reproduced well at pressures up to 10 bar (Fig. 3).

 

 Fig 2: Comparison of RAMAN measured and computed concentration profiles for five axial reactor positions.

 

Fig. 3: Comparison CH2O-PLIF image and predicted CH2O concentrations.

 

Light-off and hysteresis

The honeycomb reactor shown in figure 1 was modeled with a full elliptic 2D single channel transient code. Light-off temperatures and mode were reproduced, numerical results are shown in figure 4. Large exhaust gas dilution (up to 80% H2O/CO2) did not inhibit light-off but increased methane conversion and synthesis gas selectivities. Figure 5 illustrates the importance of total oxidation reaction during the first part of light-off.

A large extinction hysteresis was observed. After light-off was achieved the reactor inlet temperature could be reduced down to room temperature without extinction of the catalytic reactions. This stable behavior is beneficial for idle or part-load gas-turbine operation.

Fig. 4: Catalyst surface temperatures at different times during light-off.

Fig. 5: 2D major species concentrations and temperature plots for three times during catalyst light-off.