(617f) Design and Demonstration of an Ethanol Fuel Processor for HT-PEM Fuel Cell Applications

Introduction and Aim

The aim of this work was the design and demonstration of a compact ethanol fuel processor for small scale fuel cell systems. Promising applications are the independent power supply for stationary off grid applications and backup power as well as leisure and security markets.

Ethanol serves as CO₂neutral fuel as it can be produced from renewable sources. Ethanol furthermore shows technical advantages in terms of high energy density, portability and storage capability so that it is a promising fuel for the mentioned stationary and mobile small power output fuel cell applications.

The fuel cell type to be considered within this development is a high temperature PEM-fuel cell (HT-PEMFC), which shows good CO-tolerance due to the high operation temperature of around 160 °C and therefore requires less complex fuel processing. Further advantages of the higher operation temperature are the potential of effective stack cooling even with media like ambient air and a less complex cathode air supply as no humidification is required [1].

The rated electrical power output of the system is set to be P_el = 200 – 500 W, which means that the fuel processor has an nominal hydrogen output of P_H2,th = 600 – 1500 W. Other requirements for the fuel processor are fast start-up, high robustness with stable operation during planned or undesired parameter changes (e.g. ambient temperature, components and catalyst wearing …), the potential of low cost manufacturing and a low pressure drop to keep the electrical power consumption small. Issues as overall efficiency, maximal heat integration and dynamic operation are not focussed at the beginning of this development.

The autothermal (ATR) or oxidative steam reforming is the preferred reforming principle as it shows a high potential to meet most of the requirements especially robust operation and fast start-up. In addition ATR operation strongly reduces the risk of coke formation and the energy demand for the reforming process [2] [3]. The heat balance can actively be adjusted by varying the oxygen feed.

Main components of the fuel processor are the reformer reactor, the shift-converter and a catalytic burner combined with heat exchangers in order to meet the temperature demands of the different reaction stages. Special attention turns on the evaporation of the liquid media ethanol and water for the reformer and burner, respectively.

Approach and Results of the Work

In order to evaluate the most appropriate parameter set-up for the fuel processor a theoretical analysis of the complete fuel cell system has been carried out with Aspen Plus^®. As a result of sensitivity analyses a basic operation point was defined with a good compromise between high hydrogen yield, heat balance and low CO-concentration. The main operation parameters namely steam to carbon ratio (S/C), oxygen to carbon ratio (O₂/C), reforming temperature (T_Ref) and shift temperature (T_Shift) were set to 2.75, 0.40, 700 °C and 300 °C, respectively.

Furthermore a heat integration analysis by means of the pinch point method has been performed. As result the heat demand for the evaporation and superheating of the reactants can completely be met with the energy content of the anodic off gas assuming a fuel utilization of 0.70 in the fuel cell. No further heat integration is necessary to keep the process running. Therefore the degree of freedom can be kept high, e.g. the shift temperature can be adapted without influencing other streams or parameters.

Due to the presence of oxygen in the reactants precious metal catalysts are compulsory. Furthermore for the autothermal reformer these catalysts have the potential to suppress the formation of the undesired by-product ethylene which serves as a coke precursor [4].

As one experimental part of the work a catalyst screening of reforming catalysts has been carried out at the beginning. Two commercially available catalyst formulations deposited on ceramic monoliths show similar performance. A slight advantage of the chosen catalyst is given concerning high hydrogen yield at high gas hourly space velocities (GHSV) of about 50000 1/h.

The choice of shift and burner catalysts as well as the design of these reactors was based on former experimental work at the ZBT [5] [6]. Good experiences exist with commercial precious metal catalysts which are deposited on ceramic monoliths for those stages as well. Possible GHSV in the range of 20000 1/h up to 50000 1/h resulted in compact shift-converter and burner reactors.

The developed burner contains an electrically heated zone equipped with a metal fibre element upstream of the catalyst for the ethanol to be vaporized homogeneously and the catalyst to be heated during start-up. About five minutes after cold start complete evaporation of the inlet ethanol air mixture and full conversion in the burner catalyst can be reached. For this start-up process energy of about 25 Wh is required. The switchover to anodic off gas (AOG) can easily be performed without interruption. Stable combustion could be achieved for both fuel mixtures.

The evaporation and superheating of the reactants ethanol, water and air takes place in an integrated unit supplied with heat from the catalytic burner. A metal foam structure serves as evaporation support and provides independence of the system position. The integrated unit had to be designed precisely in order to avoid thermal decomposition of ethanol. A proof of concept could be demonstrated operated with the burner. Outlet temperatures up to 350 °C can be reached. The pressure surges are in the range of several millibars which indicates homogeneous evaporation.

Finally all units were assembled to a complete fuel processor, which could be qualified with various operation parameter set-ups. The theoretical defined basic operation point could be confirmed as suitable. The overall start-up time to receive a reformat with appropriate quality to feed a HT-PEMFC (x_CO < 2 %) takes around 30 minutes. At steady state operation the hydrogen power output is around 900 W with H₂and CO fractions of 41.2 % and 1.5 %, respectively.

Literature

[1] Bandlamudi, G.: Systematic characterization of HT PEMFCs Containing PBI/H3PO4 systems. Thermodynamic analysis and Experimental investigations. PhD Thesis, University of Duisburg-Essen, Logos, Berlin, 2011, ISBN 978-3-8325-2962-8

[2] Rabenstein, G.; Hacker, V.: Hydrogen for fuel cells from ethanol by steam-reforming, partial-oxidation and combined auto-thermal reforming: A thermodynamic analysis. Journal of Power Sources 185 (2008), p. 1293 – 1304

[3] Cavallaro, S.; Chiodo, V.; Vita, A.; Freni, S.: Hydrogen production by auto-thermal reforming of ethanol on Rh/Al2O3 catalyst. Journal of Power Sources 123 (2003),
p. 10 - 16

[4] Shekhawat, D.; Spivey, J.J; Berry, D.A.: Fuel Cells: Technologies for fuel processing, Elsevier B.V. 2011

[5] Spitta, C.; Dokupil, M.; Mathiak, J.: Compact propane fuel processor for auxiliary power unit application. Journal of Power Sources 157 (2006), S. 909 – 913

[6] Spitta, C.; Dokupil, M.; Mathiak, J.; Heinzel, A.: Coupling of a small scale hydrogen generator and a PEM fuel cell. Fuel Cells 7, 3 (2007), S. 197-203