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Carbon-Based Energy Systems > Advanced Coal

Co-Production of Hydrogen and Electricity in Carbon Fuel Cells

Start Date: September 2012
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Investigators

Reginald Mitchell, Department of Mechanical Engineering, and Turgut Gür, Department of Materials Science and Engineering, Stanford University

Objective

The research team will investigate a novel steam-carbon electrochemical cell and demonstrate its viability to produce carbon-free hydrogen and electricity at reasonable cost using coal or biomass as the primary feedstock. The design combines two fuel cell processes, steam-carbon and air-carbon, to produce hydrogen, electricity and a concentrated stream of carbon dioxide (CO2) that is nearly capture-ready. The team will study the phenomena and material behaviors that govern the electrochemical conversion processes, with plans to advance the technology toward development of a prototype.

Background

Most hydrogen is produced commercially through a process called steam reforming, in which steam is reacted with natural gas in the presence of air to make hydrogen and a dilute CO2 flue stream. The process is efficient but is cost-effective only for centralized large-scale production. An alternative technique is electrolysis of water, which is costly and requires vast amounts of electricity. A novel method for simultaneous production of carbon-free hydrogen and electricity by coupling of a steam-carbon and air-carbon fuel cell is compared Figure 1 to conventional conversion of coal to electricity and fuels. The coupled cell uses coal and steam as feedstocks, where hydrogen is produced from steam (H2O), and electricity is produced from CO fuel that is generated by gasifying the coal bed with CO2.  This scheme offers the simultaneous production of hydrogen and electricity in amounts that can be adjusted based on their relative demands.

Figure 1

Figure 1: Flow diagram of conventional conversion of coal to fuels and electricity versus proposed design.

Approach

The main tasks of the proposed research are: 1) investigate and understand the limitations of rate-determining processes and materials behavior; 2) improve cell performance and durability by optimizing materials, microstructure and operating conditions; 3) assess the efficiency of the coupled cell both experimentally and by modeling; and 4) identify sorbents capable of reducing the sulfur content of the syngas product.  A schematic of how the steam-carbon and air-carbon fuel cells will be coupled is depicted in Figure 2.

Figure 2

Figure 2: Schematic of coupled steam-carbon (left) and air-carbon (right) fuel cells.

Electrochemical cell performance: Experiments will involve chemical and electrochemical measurements as well as gas diagnostics. Compositional analysis correlated with fuel cell performance will reveal the risk factors and possible rate-limiting processes. Cell efficiency will be characterized as a function of hydrogen-production rates and simultaneous power generation.

Enhancing cell activity: Researchers will identify: 1) electrode materials that enhance surface reactions; 2) highly catalytic cathode materials for steam reduction with good electronic conductivity, and good stability; and 3) anode materials that possess tolerance with respect to sulfur and sulfurous compounds. Prospective materials include perovskite-based oxides.

Sulfur removal studies: Sorbent particles will be dispersed on a porous, inert support material. Measurements of sulfur uptake will be made in micro-reactors that simulate the carbon-bed gaseous environment. These kinetic measurements will be correlated to characteristics of the sorbent particles (mean pore size, porosity, etc.) and the extent of dispersion on the support materials.

Coal char conversion rates: The reactivity of chars made of coal and biomass materials will be determined by subjecting the chars to a series of experiments. The measurements and experiments will help determine specific surface-area variations with char conversion and determine mass loss rates as functions of temperature, as well as oxygen, carbon monoxide and CO2 mole fractions.

Modeling: Researchers will develop a model of the steam-coal fuel cell coupled to an air-coal fuel cell that can be used to assess the performance of the system and identify optimum operating conditions. The effects of convection, diffusion and chemical reaction in the bed of coal char particles, as well as mass transport and electrochemistry in both cells will be accounted for. The model can be used to optimize the system by revealing hydrogen-production rates and power generation as a function of operating conditions.