This joint research project is performed together with two institutions from the Fraun-hofer society, namely the IFF in Magdeburg and the IKTS in Dresden. It was started in 2007 with financial support from the innovation fund of the president of the Max Planck Society. The scientific aim of PROBIO is to develop an integrated process which combines biomass gasification for hydrogen generation in a fluidized bed with electricity production by hydrogen oxidation in fuel cells (PEMFC or SOFC). The product gas from biomass gasification contains many impurities which have to be removed in a number of primary (dust, tar, etc.) and secondary cleaning steps (carbon monoxide), as illustrated in Figure 1. The main contributions of the PCP group to this project are model-based system design, experimental and theoretical investigation of process alternatives for secondary gas cleaning, and analysis of the CO-tolerance of high-temperature PEMFCs.
With regard to secondary gas cleaning, the PCP group is currently analyzing two innovative process units which are able to reduce the CO content of the gas mixture very efficiently, namely a cyclic water gas shift reactor (CWGSR) and an electro-chemical reactor for preferential CO-oxidation (EPrOx). Our research activities on these two reactors are briefly highlighted in the following.

Figure 1: Generalized flow sheet of the PROBIO process illustrating alternative steps for gas cleaning.

CWGSR: Cyclic Water Gas Shift Reactor

The CWGSR is based upon the periodic reduction of a fixed bed of iron oxide by use of the pre-cleaned product gas coming from biomass gasification which contains CO and H2, and the subsequent re-oxidation with steam by which very pure hydrogen can be produced for the PEMFC (Fig. 2). As fixed bed material, the mixed oxide Fe2O3-Ce0.5Zr0.5O2 has been developed which combines high activity with high oxygen storage capacity and low degradation rates. This material was characterized in detail using TGA, XRD, TPR, TPD, REM, C18O isotope exchange and FT-IR techniques (Galvita et al., 2007a,b; 2008a). The hydrogen generated during the re-oxidation phase contains a very low concentration of CO, and is directly applicable to the PEMFC (Galvita et al., 2008b). At very deep reduction of iron oxide in the CWGSR, some carbon can be deposited due to the Boudouard reaction. Thus, CO traces in the hydrogen gas can be observed during the re-oxidation phase (Galvita et al., 2008c). Our current research activities focus on the quantitative description of the reduction/re-oxidation mechanisms, on the formulation of a suitable reaction kinetic model (Heidebrecht et al., 2008a) and on model-based reactor design (Heidebrecht et al., 2008b,c).

Figure 2: Principle of cyclic water gas shift reactor (left); predicted fuel utilization and H2 concentration (right).

EPrOx: Electrochemical Preferential Oxidation of CO

Classical preferential oxidation of CO (PrOx) in a hydrogen-rich gas down to a level of 10 - 20 ppm is carried out in a process unit which typically constitutes up to 15% of the overall volume of a fuel cell system. Furthermore, the undesired oxidation of hydrogen reduces the overall process efficiency. Recently, Zhang and Datta (JES 152, 2005, A1180) suggested a novel electrochemical preferential oxidation process (EPrOx) which might have the potential to replace PrOx. The working principle of the EPrOx reactor is similar to the PEM fuel cell. But unlike that type of cell, a bimetallic catalyst (PtRu/C) is used at the anode which accelerates the selective electro-oxidation of CO. The EPrOx reactor can exhibit autonomous potential oscillations in the galvanostatic operating mode (Zhang and Datta, JES 149, 2002, A1423). As an extension of the work of these authors, our group investigates coupled EPrOx reactors. Cascading of two or more of these reactors becomes meaningful when the amount of CO to be oxidized is increasing (i.e. at increasing flow rate of the fuel gas or increasing CO concentration at the inlet of the EPrOx device). With the help of a mathematical model, we predicted (Hanke-Rauschenbach et al., 2008b) and experimentally proved (Lu et al., 2008a,b) the crucial importance of the configuration of the electrical connection of the cells.
While two EPrOx reactors in electrical parallel connection exhibit almost the same CO conversion as a single one, a series connection yields an increase of up to 20%. Thus, for EPrOx scale-up, electrical stacking of the reactors will be more promising than increasing the active area of a single cell. The reason for this behavior is due to the fact that the oscillation period of CO oxidation adjusts to the CO level in the feed gas. A parallel electrical connection of two reactors forces them to oscillate at identical frequency which is always a compromise between the optimal frequencies of the two reactors. It turns out that the downstream reactor enslaves the upstream reactor. But in an electrical series connection, each of the reactors can adjust its frequency independently.

Figure 3: Coupled EPrOx reactors in series and parallel connection (left); oscillations of voltage (middle) and CO concentration (right) for series connection.

---

Subprojects:

• Conceptual design of biomass-fed fuel cell systems
• CWGSR: Cyclic water gas shift reactor for H2 purification
• Electrochemical reactor for preferential oxidation of CO
• High-temperature PEM fuel cells

---

(last changed 2009/01/13, JGD)