Experimental Studies and Optimal Design for a Small-Scale Autonomous Power System Based on Methanol Reforming and a PEM Fuel Cell

Martha Ouzounidou1, Dimitris Ipsakis2, Spyros S. Voutetakis1, Simira A. Papadopoulou3, and Panos Seferlis4. (1) Chemical Process Engineering Research Institute, Centre for Research and Technology - Hellas, 6th km Charilaou-Thermi Road, P.O. Box 60361, Thermi-Thessaloniki, 57001, Greece, (2) Department of Chemical Engineering, Aristotle University of Thessaloniki, P.O. Box 1517, Thessaloniki, Greece, (3) Department of Automation, Alexander Technological Educational Institute of Thessaloniki, P.O. Box 14561, Thessaloniki, 54101, Greece, (4) Mechanical Engineering Department, Aristotle University of Thessaloniki, P.O. Box 424, Thessaloniki, 54124, Greece

A small-scale autonomous power system with a capacity of 1-10 kW methanol fuel processor has been designed, constructed and installed at Chemical Process Engineering Research Institute (CPERI). The methanol fuel processor is a combination of methanol autothermal reformer and preferential oxidation reactor for the production of hydrogen that is subsequently used in a proton exchange membrane (PEM) fuel cell for the production of electrical energy. Applications of such a system involve the installation of power systems in remote areas without access to other power sources for commercial and domestic uses.

This work aims at the experimental study of the methanol processing stage and the optimal design of a fully integrated power system. Hydrogen production via autothermal reforming of methanol comprises the pre-heater, the autothermal reformer, and the preferential oxidation reactor. Methanol and water in liquid phase are converted in gaseous form in the pre-heater and are then introduced to the reformer reactor. In parallel, air is used for the partial oxidation of methanol in order to achieve autothermal reforming conditions in the reactor. One of the most important advantages of the proposed method is that the process is taking place at lower temperatures compared with the reforming temperatures of other organic compounds like methane. The produced hydrogen along with other gaseous products (CO2 and CO) are fed to the preferential oxidation reactor for the removal of CO at levels of 20-50 ppm, as it is well known that higher quantities of CO can poison the anodic electrode of the fuel cell and degrade its performance. The effluent stream of the preferential oxidation reactor consists of a gas mixture with a composition of 55-65% H2, 15-25% CO2, 15-20% N2, 20-50 ppm CO in dry basis, which is introduced to the PEM fuel cell anode chamber.

The present work investigates experimentally the effect of the inlet H2O/CH3OH and O2/CH3OH ratios and the reforming temperature on methanol conversion and CO selectivity. Experimental results verify that higher temperatures lead to higher H2 concentrations but with a simultaneous increase to the CO level in the gas stream. The increase of the H2O/CH3OH ratio reduces CO level due to the water gas reaction and enhances slightly the H2 production rate.

The autonomous nature of the application requires that the system is self-sustained in terms of thermal energy and water supply. Furthermore, the system should be able to meet load specifications and compensate for process disturbances and parameter variations in a timely fashion. Therefore, the design of the combined methanol processing and fuel cell system should both satisfy economic and operational criteria. Operational criteria include efficient start-up and shut-down procedures, load changes and disturbance rejection. Achievable control performance considerations are explicitly taken into account in the evaluation and assessment of the different design alternatives. The optimal design is performed within an optimization framework utilizing a rigorous mathematical model for the entire system, validated from experimental data, with flowsheet topology and operating conditions being the decision variables for the economic and operational performance.