Computational Fluid Dynamics Simulations of Gas Evacuation and O2 Recovery Times for Fan-Ventilated Confined-Space Manure Pits
Fatalities associated with entry into on-farm confined-space manure storage facilities occur each year. The fatalities are due to asphyxiation or poisoning by exposure to high concentrations of hydrogen sulfide, methane, carbon dioxide, or oxygen deficiency. Forced ventilation has been identified previously as an effective way to reduce concentrations of these noxious gases to levels that are safe for human entry into these facilities. Previously validated computational fluid dynamics (CFD) modeling protocols were used to identify the influence of several key initial conditions and modeling techniques on gas evacuation or oxygen recovery times for fan-ventilated confined-space manure pits. This article includes an extensive literature review to define the maximum expected initial concentrations and emission rates of primary manure gases in manure pits. The effect of the initial condition, the maximum initial concentrations of contaminant gases, on simulated contaminant gas evacuation time is explored. The influence of boundary conditions (i.e., emission rate (ER) and inter-contamination (INC), the process by which a portion of exhausted contaminant gas re-enters a ventilated confined airspace through the fresh air intake) on CFD outcomes is also explored. Simulation results showed that evacuation times increased as INC-strength (the ratio of contaminant concentration at the fan intake to the concentration in the air exhausted from the manure pit) increased from 0 to 0.40; however, no further increase in evacuation times was predicted for INC-strengths above 0.40. Simulations on oxygen recovery (from 0% to 20.0% by volume) in the confined airspace initially filled completely with a contaminant gas (i.e., H2S, NH3, CH4, and CO2) showed little difference (<5%) in recovery time by gas. Additionally, for a confined airspace (domain) filled with a gas mixture of contaminant gases, simulations showed that time to reduce H2S concentration from a documented high level (10,000 ppm) to the OSHA PEL level (10 ppm) was equal to or greater than evacuation times for other gases from their documented highest initial levels to their safe exposure levels. Also, an initial gaseous mixture including H2S and CO2 at their highest documented concentrations was the critical initial atmosphere in manure pits for performing CFD simulations.