Modeling Power Production in a Tubular Carbon Fuel Cell

Introduction

As developing countries consume increasing quantities of fossil fuels to elevate their standard of living, there is growing concern about greenhouse gas emissions, and an increased need for mitigating emissions during power generation. It is clear that in the foreseeable future renewable energy technologies will be unable to eliminate fossil fuel dependence, and hence efforts to reduce the environmental impact of carbon-based fuels are required.

Carbon Fuel Cells (CFCs) offer the double benefit of efficient utilization of carbonaceous fuels such as coal or biomass, and the production of a concentrated stream of CO₂ that can be easily stored or sold as a marketable product.

Previous work in our laboratory has demonstrated solid-oxide based CFC for efficient electricity production from various types of carbons [1-3]. Our CFC utilizes a bed of solid carbon fuel at the anode compartment and air at the cathode compartment, which are separated by a dense yttria-stabilized zirconia electrolyte (YSZ) for selective transport of oxide ions. Carbon dioxide in the anode compartment reacts with the carbon to produce CO via the Boudouard reaction.

C + CO₂ → 2CO                (1) 

As shown in Figure 1, the cell oxidizes CO to generate electricity, producing an outlet stream of CO₂, part of which can be recycled to the anode and the reminder sent for storage. 

Modeling the Tubular Carbon Fuel Cell Geometry

In this presentation we provide a comprehensive operational model that couples heat transfer with chemical and electrochemical processes as well as mass transport in a tubular carbon fuel cell. In the carbon bed, the Boudouard gasification reaction of the solid carbon fuel is endothermic, while CO oxidation at the anode surface is exothermic. As the kinetics of both reactions is temperature dependent, it is important to understand the coupled relationship between reaction rates, heat release and local temperature. Thus, consideration of heat transfer effects is necessary to develop a realistic understanding of the overall cell operation.

The operation of the cell results in an anode exhaust containing largely CO₂ with the remaining balance of unreacted CO. Any CO in the exhaust is carbon fuel still capable of undergoing oxidation to produce electricity at the cell. Thus, fuel utilization improves with increasing CO₂/CO ratio at the exhaust, and this influences the overall conversion efficiency of the cell.

In this work, an operational model for a tubular CFC was developed that takes into account heat transfer and temperature distributions within the cell. The parameters in the model were determined experimentally.  The model was then used to map out the operational space for power density and cell efficiency.  Furthermore, geometrical parameters such as fuel bed height and tubular placement can be tuned to minimize the mole fraction of CO in the exhaust.  The model was implemented for multiple tubular geometries and spacing between tubes in order to determine how these parameters affect overall fuel cell performance.

This presentation will address the dependence of cell efficiency and power density on the cell and carbon bed geometries, which will then aid in the optimization of tubular cell design for the air-carbon fuel cell. Optimal operation conditions are identified for maintaining high efficiencies while also achieving realistic power densities.

References

1.    A. C. Lee, S. Li, R. E. Mitchell, and T. M. Gür. Electrochem. Solid State Lett. 11(2), B20-B23 (2008).

2.    B. R. Alexander, R. E. Mitchell, and T. M. Gür. Proceeding of the Combustion Institute 34, 3445 (2013).

3.    B. R. Alexander, R. E. Mitchell, and T. M. Gür. J. Electrochem. Soc. 159(3), B347-B354 (2012).