Modeling of Methanol-Fueled SOFC with Anode Offgas Recycling for Simplified System Design

Abstract

Objective :Methanol is a fuel that can be synthesized from either fossil fuel or biomass. Use of methanol as a fuel in SOFCs offers several advantages. A major advantage lies in its ease of storage, handling, distribution, and input, being found in liquid form under ambient conditions. Compared to most other fuels, methanol is rather easily produced, and is known to have a lesser tendency for carbon deposition when vaporized and heated at high temperature.In this work, we conducted modeling and simulations of a methanol-fed SOFC system with anode gas recycling. Anode gas recycling provides water vapor in the fuel inlet, thus decreasing the tendency of the feed gas to form carbon deposits in the fuel cell stacks. This strategy also helps reduce thermal stresses in the stacks that would arise if the reforming was attempted in-situ with 100% water vapor originating from outside the SOFC system. The recycling also helps increase the fuel utilization. A thermodynamic analysis was conducted to calculate the energy generated, absorbed, or exchanged in the various blocks of the system, the expected system energy output, as well as conditions under which carbon deposition is prevented. Results:The model is based on an integrated energy process with anode gas recycling, a rated output electrical power of 100 kW, and an electrical efficiency of 50% to initiate simulations. Using the CHEMCAD software, equilibrium gaseous compositions were calculated. The oxygen flow rate and the cell temperature were set at five times the stoichiometric value and 800°C, respectively. Thermodynamic calculations show that the gas mixture at the anode inlet will not lead to carbon deposition above 575°C. The energy generated in the cell by electrooxidation is calculated at 144.6 kW. 44.6 kW of thermal energy is thus generated at the stack level. This heat is removed by the anode and cathode outlet flows, as well as minor heat losses through the system walls. A mass recycle ratio of 1.47 was obtained. Energy involved in each of 9 system blocks was calculated. Overall system and electrical efficiencies of 75% and 57% were obtained from simulations based on lower heating value. A voltage ranging from 0.91 to 0.94 V across the cell stacks was obtained at a fuel utilization factor of 80% and a temperature of 800°C. Finally, simulation iterations lead to an output fuel cell power of 98.8 kW, which is close to the power value used as input at the start of simulations.Conclusion:The recycle ratio of 1.47 contributes to system efficiency. By contrast, it was reported that for propane, a fuel utilized in Northern Canada, a recycle ratio greater than 5 is required for reliable functioning of SOFC systems. Modeling and simulation results clearly showed that methanol has very little tendency to form carbon deposits during cracking in situ when used in a simple anode gas recycling system. A very good electrical efficiency is thus obtained.