Human breathing generates a significant amount of carbon dioxide. High level of carbon dioxide can be extremely hazardous. Carbon dioxide mitigation methods such as soda lime carbon dioxide scrubber curtains and purging with high air flows can reduce carbon dioxide levels. This is especially critical for confined spaces such as refuge chambers, also known as refuge alternatives (RA), used in underground mining post-disaster. Mine Safety and Health Administration (MSHA) regulations require RAs in underground coal mines to provide a life-sustaining environment for miners trapped underground when escape is impossible. According to the regulations, the average carbon dioxide concentration in the occupied structure shall not exceed 1.0% over the rated duration, and excursions shall not exceed 2.5%. A 1.0% carbon dioxide atmosphere is the threshold of a serious health risk. The 15-minute short-term exposure limit (STEL) for carbon dioxide set by the National Institute for Occupational Safety and Health (NIOSH) and the American Conference of Governmental Industrial Hygienists (ACGIH) is 3%. Therefore, excursions to 2.5% carbon dioxide must be mitigated quickly.While occupied and without breathable air supply, the RA interior oxygen level will decrease, and the carbon dioxide level will increase quickly due to breathing. For build-in-place (BIP) RAs, one mitigation strategy is to implement a borehole air supply (BAS) to supply fresh from the surface. MSHA regulations require that such a BAS must supply fresh air at 12.5 cfm or more per person to maintain the oxygen and carbon dioxide level within safety range as specified in MSHA regulations. However, the minimal fresh air flow (FAF) rate needed to maintain the 1% CO2 level will depend on multiple factors including the number of the people, the volume of the BIP, and the air temperature/pressure within the BIP. To predict the interior CO2 concentration, a 96-hour test will be needed for each combination of factors as listed above.&nbsp; It requires numerous 96-hour tests which will take tremendous efforts. Given the infinite possibility of the combinations (number of people, size of the BIP), it’s impractical to accomplish all the tests for every case.In this paper, researchers at NIOSH developed a model that can predict how the %CO2 changes with time for breathing air test, given the number of people and the Fresh Air Flow (FAF) rate in a confined space. The model was then compared and validated with test data. The benchmarked model was used predict the %CO2 given the number of people and FAF rate without conducting a 96-hour test. The methodology used in this model can also be used to estimate other gas level within a confined space.

Mathematical Modeling for Carbon Dioxide Level Within Confined Spaces

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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