The Reduction of CO₂ Emissions Via CO₂ Capture and Solid Oxide Fuel Cells

The increase in CO₂ emissions over past decades are the result of a growing dependence on fossil fuels. Examination of CO₂ emission sources revealed that more than 33% of global CO₂ emissions result from coal-fired power plants, which represent the largest stationary source of CO₂. Two proposed approaches for reduction of CO₂ emissions: (i) a short term (i.e. 7-10 years) capture of CO₂ from coal-fired power plants and (ii) a long term (i.e. 10-15 years) approach is the replacement of coal-fired power plants by coal-based fuel cells. These approaches purify CO₂ for sequestration. Carbon capture from existing power plants could be accomplished by passing the flue gas through a sorbent. The sorbent captures the CO₂ from the flue gas then regenerated producing purified CO₂. Direct coal fuel cells directly convert coal to electricity through the electrochemical oxidation of carbon. The mixing of air and coal does not occur in the fuel cell, leading to highly concentrated CO₂ effluent for sequestration.

CO₂ capture was investigated by transient flow, bed temperature measurement, and temperature programmed CO₂ desorption coupled with IR effluent measurement of seventeen sorbents, which had SiO2, carbon, or beta zeolite as a support. The heat released during the exothermic adsorption of CO₂ onto amine resulted in a bed temperature rise. The heat generated could be dissipated with a smaller particle size and greater thermal conductivity. The heat released was used to verify the capture capacity using a thermal camera and high throughput adsorber that screened thirteen sorbents simultaneously. The carbon initially investigated produced an ammonia odor and had a low capture capacity. The ammonia odor was the result of acid-base interaction between the support and amine groups. The use of a neutral carbon increased the capture capacity to 2.8 mmol CO₂/g-sorbent. Beta zeolite, which captures 1.8 mmol CO₂/g-sorbent, was found to contain acid sites that lowered the capture capacity. Molecular probing with benzene indicated a reduction of acidic sites with basic NH₃ treatment and the reduction of surface –OH groups with basic NH₄OH treatment. Beat zeolite treatment with NH₃ and NH₄OH resulted in a capture capacity of 2.0 and 2.2 mmol CO₂/g-sorbent, respectively. Further DRIFTS IR investigation showed the amine interacted with the –OH groups of beta zeolite. Adsorption of CO₂ formed carbonates, which may utilized the O atom from the interaction of the amine and support. The carbonate formation profile was parallel to H-bonding indicating adsorbed CO₂ had a dual-interaction where a carbonate and H-bond was formed. This dual interaction may have inhibited gas and adsorbed phase CO₂ exchange observed on metal surfaces.

LSCF was investigated as an anode material for a direct CH₄ solid oxide fuel cell (SOFC) through unsteady state response coupled with mass spectrometer analysis. Comparison of a Ni anode and LSCF/Ni anode was done to determine if LSCF promoted the electrochemical oxidation of carbon. The introduction of 50% CH₄ into the LSCF/Ni anode SOFC produced a greater amount of CO than the Ni anode, indicating the LSCF increased the initial intrinsic rate of carbon oxidation. The H₂ and CO profile produced by the LSCF/Ni anode lacked a parallel structure indicating different reaction pathways. Current-voltage measurement over LSCF/Ni during 50% CH₄ led to a higher formation of CO than that of the Ni anode, confirming a high intrinsic rate of formation. Removal of CH₄ from the Ni anode resulted in a rapid drop in current; removal of CH₄ from the LSCF/Ni anode resulted in a slow decrease in current and the formation of CO and CO₂. The formation of CO₂ on the LSCF/Ni anode suggests the presences H₂ and CH₄ inhibit the electrochemical oxidation of carbon to CO₂. The formation of CO₂ over the LSCF/Ni anode indicates LSCF ability to completely electrochemically oxidize carbon, which was not observed on the Ni anode. Structural degradation led to failure the Ni anode cell after 0.5 hours of pure CH₄ operation and after 2 hours on the LSCF/Ni anode. These results suggest LSCF promotes the electrochemical oxidation of carbon resulting in a lower intrinsic rate of formation of coke in the Ni/LSCF SOFC.