Carbonic acid | EPFL Graph Search

In chemistry, carbonic acid is an organic compound with the chemical formula . The molecule rapidly converts to water and carbon dioxide in the presence of water. However, in the absence of water, it is (contrary to popular belief) quite stable at room temperature. The interconversion of carbon dioxide and carbonic acid is related to the breathing cycle of animals and the acidification of natural waters. In biochemistry and physiology, the name "carbonic acid" is sometimes incorrectly applied to aqueous solutions of carbon dioxide. These chemical species play an important role in the bicarbonate buffer system, used to maintain acid–base homeostasis. Terminology in biochemical literature In chemistry, the term "carbonic acid" strictly refers to the chemical compound with the formula H2CO3. Some biochemistry literature effaces the distinction between carbonic acid and carbon dioxide dissolved in extracellular fluid. In physiology, carbon dioxide excr

Air Side Contamination in SOFC Stack Testing

Christian Gehrig, Christian Ludwig, Josef Andreas Schuler, Jan Van Herle, Zacharie Wuillemin

Solid Oxide Fuel Cells (SOFCs) are likely candidates for electricity production with high efficiency and promise reasonable fuel tolerance. While fuel impurities are known anode poisons, less attention is brought to air-purity, especially to air-side contaminants from exogenous sources, which are studied here. This work aims to quantify pollutants and identify their sources during SOFC testing in stack configuration. Post-analyses of a long-term test have shown that performance degradation was mainly due to cathode pollutants originated upstream of the cell, therefore their source identification is crucial. The compressed air system, feeding the airflow to the cathode, was investigated by filtering and subsequent chemical analysis of the filters. Hot-air-sampling was redone in situ at the cathode air entry during a new test run to assess the contaminant concentrations in air in SOFC test conditions. In addition, the behavior of SOFC proximal system components, i.e. alloy oxidation, was characterized separately. The study focused on chromium contamination from Cr-containing high temperature alloys used in Balance-of-Plant (BoP) components, i.e. heat-exchangers, tubing and flanges in direct contact with the airflow at high temperature. Concentrations of volatile Cr-species in air under SOFC testing conditions were compared to Cr-accumulation on the tested cell as well as to Cr-evaporation rates from BoP alloys which were individually characterized regarding oxidation behavior. Evaporated Cr quantities were found to saturate the air with Cr-vapors at the cathode air-inlet, as confirmed by the in-situ measurement of volatile species in the hot airflow. Moreover, these amounts are well correlated to accumulated Cr in the cell after long term testing. Si-containing sub-oxide scale layers of heat-resistant BoP alloys were identified as silicon contamination source besides sub-micron dust from environmental air. The analyses of laboratory air quality revealed sulfur in sufficient quantity to act as a cathode pollutant. The results of this study suggest guidelines to reduce air side contamination of SOFC stacks from exogenous sources.

2010

Air Side Contamination in Solid Oxide Fuel Cell Stack Testing

Christian Gehrig, Aïcha Hessler-Wyser, Christian Ludwig, Josef Andreas Schuler, Jan Van Herle, Zacharie Wuillemin

Elsevier2011

Density effects on nanoparticle transport in the hyporheic zone

David Andrew Barry, Qihao Jiang

A carbon solution composed of nanoparticles was used in experiments designed to explore nanoparticle transport characteristics within the hyporheic zone of a riverbed. Experiments and numerical simulations demonstrated that nanoparticle transport in the hyporheic zone is affected by hydraulic head gradients due to river flow-bedform interactions as well as density gradients associated with the nano-carbon solution. Differences with similar flow/transport situations were examined, and it was found that particulate-enhanced density can change hyporheic transport appreciably. In addition to density, particle settling enhances downward movement of the nano-carbon plume in the riverbed. While nanoparticle transport in the upper hyporheic zone is largely controlled by advection due to flow driven by head gradients at the bed surface, density gradients and particle settling influence the transport process significantly in the lower hyporheic zone. During the transport process, nanoparticles become deposited due to attachment to sand particles and filtration by small pores in the bed. Compared with transport where density variations are minimal, the particulate-induced density gradient induces downward transport of nanoparticles and entrained liquids, leading to deposition/accumulation at the base of the bed.

2018