Selective CO Methanation in H2-Rich Gas for Household Fuel Cell Applications

Polymer electrolyte membrane fuel cells (PEMFCs) are often used for household applications, utilizing hydrogen produced from natural gas from the gas grid. The hydrogen is thereby produced by steam reforming of natural gas followed by a water gas shift (WGS) unit. The H2-rich gas contains besides CO2 small amounts of CO, which deactivates the catalyst used in the PEMFCs. Preferential oxidation has so far been a reliable process to reduce this concentration but valuable H2 is also partly converted. Selective CO methanation considered as an attractive alternative. However, CO2 methanation consuming the valuable H2 has to be minimized. The modelling of selective CO methanation in a household fuel cell system is presented. The simulation was conducted for single and two-stage adiabatic fixed bed reactors (in the latter case with intermediate cooling), and the best operating conditions to achieve the required residual CO content (100 ppm) were calculated. This was done by varying the gas inlet temperature as well as the mass of the catalyst. The feed gas represented a reformate gas downstream of a typical WGS reaction unit (0.5%–1% CO, 10%–25% CO2, and 5%–20% H2O (rest H2)).

CO and CO2 Methanation Over Ni/γ-Al2O3 Prepared by Deposition-Precipitation Method

Various Ni catalysts supported on γ-Al2O3 were prepared by a wet impregnation (WI) method and deposition-precipitation (DP) method with different precipitants and applied to CO and CO2 methanation. The prepared catalysts were characterized by various techniques including nitrogen physisorption, X-ray diffraction (XRD), temperature-programmed reduction with H2 (H2-TPR), H2 chemisorption, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Irrespective of kinds of precipitant, the Ni/γ-Al2O3 catalysts prepared with a DP method showed a remarkable enhanced catalytic performance in CO and CO2 methanation compared with the Ni/γ-Al2O3 catalyst prepared with a WI method owing to the higher catalytic active surface area (CASA). In the case of Ni/γ-Al2O3 catalysts prepared with a DP method, the high calcination temperatures are not favorable for the high catalytic activity due to the decreased reduction degree of Ni oxide species and CASA. The reduction degree of Ni oxide species can be increased with reduction temperature. However, the higher reduction temperature above 500 °C is not desirable to achieve the high catalytic activity because of the decreased CASA. The selective CO methanation was also accomplished at lower temperatures over the Ni/γ-Al2O3 catalyst prepared with a DP method than over the Ni/γ-Al2O3 catalyst prepared with a WI method.

Kinetics and Reactor Design Aspects of Selective Methanation of CO over a Ru/γ-Al2O3 Catalyst in CO2/H2 Rich Gases

Polymer electrolyte membrane fuel cells (PEMFCs) for household applications utilize H2 produced from natural gas via steam reforming followed by a water gas shift (WGS) unit. The H2-rich gas contains CO2 and small amounts of CO, which is a poison for PEMFCs. Today, CO is mostly converted by addition of O2 and preferential oxidation, but H2 is then also partly oxidized. An alternative is selective CO methanation, studied in this work. CO2 methanation is then a highly unwanted reaction, consuming additional H2. The kinetics of CO methanation in CO2/H2 rich gases were studied with a home-made Ru catalyst in a fixed bed reactor at 1 bar and 160–240 °C. Both CO and CO2 methanation can be well described by a Langmuir Hinshelwood approach. The rate of CO2 methanation is slow compared to CO. CO2 is directly converted to methane, i.e., the indirect route via reverse water gas shift (WGS) and subsequent CO methanation could be excluded by the experimental data and in combination with kinetic considerations. Pore diffusion may affect the CO conversion (>200 °C). The kinetic equations were applied to model an adiabatic fixed bed methanation reactor of a fuel cell appliance.