Fuel cells are widely considered as a clean and energy-efficient technology that shows promise as a future potential replacement for the internal-combustion (IC) engines in vehicles, IC engines and gas turbines in stationary power generation, and for batteries in portable power applications. Proton-exchange membrane (PEM) fuel cells are particularly promising as they are compact and lightweight units that work at comparatively lower temperatures and pressures, which makes them good candidates for use in mobile and small-scale, distributed stationary power generation applications.
Hydrogen (H2), that PEM fuel cells need to operate, can be produced from different renewable and non-renewable feed-stocks through various processes. Due to its higher efficiency, currently steam methane reforming (SMR) is the primary industrial process that is used for H2 production. Carbon monoxide (CO) that is simultaneously produced as an undesirable by-product of SMR is known to be particularly detrimental for PEM fuel cell operation. CO severely poisons the electro-active Pt surface in the anodes, thus preventing H2 oxidation. The highest concentration of CO that PEM fuel cells are tolerant to depends on the material used for preparing the anodes, reportedly ranging from 10 to 100 ppm. For the H2 produced from SMR to be utilized in PEM fuel cells, care must, therefore, be taken towards decreasing its CO content prior to entering the fuel cell stack. Typically, the first step involves utilizing the water gas shift (WGS) reaction in tandem with the SMR in order to react the CO in the reformate mixture, and to produce additional H2. Conventional WGS reactors can attain high CO conversions, but not to the degree required to prevent CO-induced PEM fuel cell stack performance degradation. This, then, means that following the H2/CO2 separation step (e.g., via pressure swing adsorption (PSA)) an additional CO removal step must be implemented involving either the preferential oxidation of CO (PROX) or methanation in order to reduce the CO level to the acceptable fuel cell operational range.
The above conventional SMR+WGS+PSA+PROX (or methanation) system is one of significant complexity; to add to this complexity, since the WGS reaction is equilibrium-limited, usually two reactors are utilized, one operating at high temperatures (high-temperature shift or HTS) and the other at low temperatures (low-temperature shift or LTS) in order to maximize the CO conversion.
In this study, instead, a realistic size, high performance Palladium (Pd) membrane (in terms of its H2 permeability and selectivity) is used along with a LTS catalyst in a membrane reactor system (Pd-MR) for pure hydrogen production from a feed with a simulated reformate composition through the use of the WGS reaction.
Prior to its use in the reactor experiments, the membrane is characterized through single-gas permeation measurements. The pressure dependency of the H2 permeation rate through the membrane is examined experimentally, and the effect of the presence of CO on membrane hydrogen permeability is investigated. The effect of feed pressure and flow rate and sweep ratio on membrane performance during the WGS experiments is experimentally studied, and the results are compared with the predictions of a mathematical model. The model is further used to study the design aspects of the process. It is shown that the Pd-MR system under study is capable of attaining almost complete CO conversion and a full hydrogen recovery at realistic experimental conditions akin to those utilized in industrial applications. During our studies, both the membrane and the catalyst proved to be stable, and showed robust performance throughout the membrane reactor experiments which lasted for more than a month. The hydrogen product purity is investigated for each experiment, with the CO content found to always be bellow 100 ppm. A packed bed methanation reactor is further utilized after the WGS-MR to completely eliminate the CO present in the hydrogen stream.
The Pd-MR system proved to provide many advantages over the conventional technology. By removing hydrogen from the reactor, almost complete conversion was achieved with less steam being utilized. Using highly permselective Pd membranes, a high H2 recovery and purity was achieved. CO2 separated in the reject side of the membrane at high pressures, and the hydrogen loss was minimized during the subsequent methanation step. Even the use of inexpensive, low-pressure steam as a sweep for the permeate side of the Pd-MR provided an advantage, as it generated a pre-humidified H2 product as a feed, which is a requirement for many of the PEM fuel cell stack applications.