Simulation and Validation of Hydrogen Sulfide Removal from Fan Ventilated Confined-space Manure Storages
Open Access
- Author:
- Zhao, Juan
- Graduate Program:
- Agricultural and Biological Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 03, 2006
- Committee Members:
- Harvey Bright Manbeck, Committee Chair/Co-Chair
Dennis J Murphy, Committee Chair/Co-Chair
Virendra Puri, Committee Member
Jelena Srebric, Committee Member - Keywords:
- Confined-space manure storages
ventilation
hydrogen sulfide
CFD modeling protocols
validation
air exchange rate - Abstract:
- Confined-space manure storage entry has been identified as a major safety concern in the agricultural industry. Oxygen-deficient atmospheres, as well as toxic and/or explosive gases (e.g., NH3, H2S, CH4, and CO2), often result from fermentation and accumulation of the manure in confined-space storages. These gases often create very hazardous conditions to farmers who may need to enter these confined-space manure storages to work or perform maintenance. Hydrogen sulfide (H2S), a highly toxic and irritating gas, was the gas of interest to investigate the effectiveness of forced ventilation strategies for eliminating the toxic and oxygen deficient atmospheres in confined-space manure storages. The overall goal of this research was to develop and validate computational fluid dynamics (CFD) modeling protocols to simulate H2S removal from fan ventilated confined-space manure storages. The CFD model was used to predict the H2S decay and the time (Tpel) to reduce H2S concentrations to the Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL) of 10 ppm in a 5.49 m ¡Á 2.74m ¡Á 1.83 m rectangular prismatic confined-space manure storage during forced ventilation. Previously identified best ventilation strategies (fan and outlet locations) for confined-space manure storages with three floor cover types (i.e., solid, fully-slotted, and partially-slotted) and experimentally measured H2S emission rates from this study were key CFD model inputs. The CFD-predicted and experimentally measured Tpel values throughout the entire storage were compared to validate the CFD modeling protocols. An extended validation of the CFD modeling protocols was conducted using an independent on-farm confined-space manure pit. The validated CFD modeling protocols were used to study the effect of air exchange rate and manure storage size on H2S removal. H2S emission rates from manure in a confined-space manure storage facility were identified for incorporation as a boundary condition for the CFD modeling. In this research, the H2S emission rates were measured experimentally under typical forced ventilation airflow conditions in a prismatic manure storage facility for three weather regimes (Cold: t < 13 oC, Mild: 13 oC < t < 18 oC, Hot: t > 18 oC) for two air exchange rates (Low: 3 AC/min, High: 5 AC/min). The measurements were conducted using the same confined-space manure storages used in previously reported research to measured the decay of H2S and Tpel¡¯s from fan ventilated, rectangular prismatic manure storages. An exponential regression model of the form, y = was identified to express the H2S emission rate from the manure. The emission rates were obtained for six combinations of three weather regimes and two air exchange rates. Air temperature and air velocity above the manure surface significantly (P < 0.05) affect H2S emissions from the stored manure surface. The PHOENICS (version 3.6, CHAM, 2005) CFD code was used to simulate H2S decay during forced ventilation in the 5.49 m ¡Á 2.74 m ¡Á 1.83 m rectangular prismatic confined-space manure storage during forced ventilation. Simulations were conducted for storages with solid, partially-slotted and fully-slotted floor covers. Based on the PHOENICS CFD code performance verification, the CFD code satisfactorily described the flow feature of jet flow near the ventilation fan and successfully simulated H2S concentration decay in a fan ventilated confined-space manure storage facility. In addition, computational grid-dependence and time-step sensitivity studies ensured the accuracy of the CFD simulations. Statistical criteria for validation of the CFD modeling protocols for controlled laboratory type studies were used for application to the confined-space manure storage field studies. The validation criteria selected were: (1) Predicted and measured times to reduce gas concentration to OSHA Permissible Exposure Limit (10 ppm for H2S) agreed to within 10 %; (2) The correlation coefficient of regression of simulated versus measured results is 0.9 or greater; (3) The slope of regression of simulated versus measured results is between 0.75 and 1.25; and (4) The intercept of regression of simulated versus measured results is less than or equal to 25 % of the average measured results. The simulated results compared were the time to reduce H2S concentration to 10 ppm (Tpel) and times to reduce H2S concentration to 50% (T50), 25% (T25), and 10% (T10) of initial concentration. The comparisons for the simulated and measured Tpel, T50, T25, and T10 values showed that the CFD model and selected protocols were adequate to simulate H2S concentration decay during forced ventilation in the confined-space manure storages. The selected modeling protocols included incorporation of appropriate inter-contamination ratios in the incoming ventilation air. The inter-contamination ratios used as CFD model inputs were based on field measurements at the prismatic confined-space manure storage facility. Most measured and simulated Tpel¡¯s agreed to within 10 %. In addition, all simulations satisfied the statistical criteria for satisfactory performance of the CFD model and protocols for the three floor cover types considered. Thus, the CFD modeling protocols were successfully validated. The validated CFD modeling protocols were used to perform simulations of H2S decay during forced ventilation for an independent on-farm confined-space manure storage facility located adjacent to a Nebraska-type barn at the Pennsylvania State University Swine Research Center. H2S decay during the selected forced ventilation strategy was measured and recorded during field experiments to identify corresponding Tpel, T50, T25, and T10 values for H2S evacuation. Measured and simulated times were compared at three locations within the confined-space storage. Most simulated and measured Tpel values agreed to within 10 % and all agreed to within 15 %. Comparisons of all the simulated and measured times satisfied the statistical criteria for acceptance of the CFD model and the modeling protocols. Therefore, the CFD modeling protocols were further validated using a different on-farm manure pit. The validated CFD modeling protocols were used to perform simulations for several typical on-farm rectangular and circular confined-space manure storages for several floor cover types (fully-slotted and partially-slotted floor types for rectangular footprint; solid floor type for storages with a circular footprint). Three air exchange rates (1 AC/min, 3 AC/min, and 5 AC/min) were investigated to identify the effect of the air exchange rate on the gas removal. For the rectangular geometry with the two floor cover types, three different lengths were used (5.49m, 12.2m, and 18.3m), while holding the width and depth constant. At the same air exchange rate (i.e., 3 AC/min), the rate of evacuation of H2S from the confined space decreased as the length of the rectangular manure storage increased. As the length of the storage increased to 12.2 m, zones of higher gas concentration (less effectively ventilated zones) developed in the regions closest to the end-walls. The most effectively ventilated zones identified for three lengths for the two floor cover types extended approximately 2.25 m (L = 5.49 m), 3 m (L = 12.2 m), and 5 m (L = 18.3 m) on either side of the centrally located fan. Therefore, a single ventilation fan effectively ventilated the entire confined space for the 5.49 m long manure storage at the air exchange rate of 3 AC/min. However, at storage length of 12.2 m and longer, the most effectively ventilated portion of the confined space extended approximate 50 % of the storage length at the air exchange rate of 3 AC/min. As the AC rates increased in the rectangular confined-space manure storages with the two floor cover types, the rate of evacuation of H2S in the least effectively ventilated region of the storage increased. The relationship between maximum Tpel and AC rate was non-linear and of the form . However, maximum Tpel values decreased at a decreasing rate with AC rate. At AC rates above 3 to 4 AC/min, maximum Tpel values decreased by less than 20 % for each 1.0 change in AC rate. In addition, the effects of gas emission rate and the inter-contamination strength on gas decay in one of the rectangular manure storages were identified. The inter-contamination strength had a more pronounced effect on the time to reduce H2S concentration to OSHA PEL levels than did the gas emission rate from the manure. Decreasing H2S emission rate from ER(t)=0.65 ¡Á exp (-0.009 ¡Á t) (mg/s) to 0 mg/s reduced maximum Tpel by only 7%. Decreasing inter-contamination strength from 28% to 0% reduced maximum Tpel by 19%. Including non-zero gas emission rates and non-zero inter-contamination ratios in the CFD modeling protocols always yield conservative times to reduce gas concentration to OSHA PEL¡¯s in the confined-space manure storage. For the circular geometry, two diameters were used (4.4m and 6.5m), while holding the depth constant. The rate of evacuation of H2S from the storage decreased as the diameter of the storage increased for the same air exchange rate (i.e., 3 AC/min). As the AC rates increased in the circular confined-space manure storages, the rate of evacuation of H2S increased. The relationship between air exchange rate and Tpel was nearly linear for the forced ventilation configuration and circular confined-space manure storages considered. The key findings of this research are: (1) Measured H2S emission rates from stored and agitated manure in forced ventilated manure storage, (2) Validated CFD code and modeling protocols for simulating H2S concentration decay within fan ventilated confined-space manure storages, (3) Identification of relationships between evacuation times (i.e., Tpel) of H2S and air exchange rate for rectangular (non-linear) and circular (nearly linear) confined-space manure storages, (4) Identification of the effect of manure storage length on gas evacuation, and (5) Identification of the effects of gas emission rate from the stored manure and inter-contamination strength in incoming ventilation air on the gas evacuation time from the confined-space manure storages. Based on these findings, a useful simulation protocol was developed to replace measurements of noxious gas concentration decay in fan ventilated confined-space manure storages. These protocols are now available to develop engineering design aids and recommendations for effectively ventilating a wide range of confined-space manure storages.