Along with the production of biogas, the emission of carbon dioxide gas from the product also brings the main concern of how safe the concentration of carbon dioxide gas from the biogas industry can be. In fact, the accidental release of carbon dioxide can cause severe damage and loss during biogas production. The ability to anticipate foreseeable accident scenarios and investigate their consequences is a fundamental aspect in the risk assessment of a process or technology.
Compared to the natural gas, biogas will show higher concentration of carbon dioxide due to the low carbon dioxide content in the natural gas. As the total population of the world increases, so does the demand for the energy. As reported in RenewableEnergyWorld.com, the President of the United States of America announced the addition of three manufacturing centers for the generation of green energy (Williams, 2013).
For biogas, it usually refers to a gas that is formed by the decomposition of organic matter in the absence of oxygen. However, this composition depends greatly on the nature of the raw material waste and the way it is being processed (Naskeo, 2009). The incident like the farmer who died after being trapped in the confined space is always related to the danger of biogas.
Problem Statement
Objective
Scope of Study
Hazard of Biogas
Therefore, it is crucial to always check the value for the explosive limit of methane gas so that its composition in air will never fall within this range. Based on the density value, methane gas is lighter than air and will accumulate at the roof space. At low concentration, methane gas can act as an anesthetic and the victim may not be conscious due to suffocation.
At the high concentration, methane can lead to asphyxiation due to the oxygen being displaced in the atmosphere. According to the Jefferson Lab Policy, an oxygen level of 19.5% in the atmosphere is considered a dangerous oxygen deficiency in the atmosphere, compared to the normal value of 21% (Oxygen Deficiency Hazard, 2008). In addition to methane gas, hydrogen sulfide gas (H2S) can also have dangerous consequences for workers, even if it contains 0% to 3% of the biogas volume.
H2S gas is a flammable gas that can be identified by the smell of rotten eggs at a concentration of 0.03 ppm to 0.15 ppm in air.
So the CO2 gas will not cause any toxic behavior if the concentration is less than 3% in air, but if more than 5%, CO2 gas will irritate the respiratory tract (Thomas, Martinka, 2012). To assess how toxic the CO2 gas concentration effect with exposure time, the Health and Safety Executive (HSE) has constructed the Dangerous Toxic Load (DTL) assessment (Harper, 2011). Under this assessment, there are two assessments which are Specific Level of Toxicity (SLOT) and Significant Likelihood of Death (SLOD) which can be used as a benchmark for examining the CO2 gas toxicity for this project.
The high concentration of CO2 together with the hydrogen sulphide (H2S) can create the highly corrosive environment when not dried (Eekelen, 2011). However, the behavior of CO2 toxicity distribution model of biogas process is not yet available. Even though most of its chemical and physical properties are known, there is still not enough data on how much the concentration of CO2 will spread and move through the atmosphere from certain point of discharge.
Oxygen (O 2 ) Deficiency
In addition to biogas, natural gas is also one of the most important sources of energy in the world. In addition to the way in which natural gas is produced, natural gas also differs in composition from biogas. As mentioned earlier, biogas is mainly produced by the breakdown of organic compounds without the presence of oxygen.
As shown in Table 5, the concentration of CO2 in biogas is higher than in natural gas. This is one of the reasons why biogas was chosen as a research topic for this project. For the early prediction, the different composition between these two gases will give a different result on the dispersion model.
Biogas Behavior
Transport and storage of biogas will be more advantageous in liquid form to save the necessary area and make the transport process easier. There will be two release phases when the pressurized fluid leaked due to the difference between higher pressures in the pipeline compared to the atmosphere. Aerosol will be produced when the liquid evaporates faster and takes energy from itself and the surroundings to cool itself.
If the flow mass is large, the gas will collect and evaporate to produce an exhaust gas that will act as a dense gas. When biogas is released into the atmosphere, it can be dispersed by turbulence due to the fact that the atmosphere is always in motion caused by eddies. According to Schulze, if there is a leak from the pipeline, the maximum concentrations downwind will occur at steady state which means turbulence will be less with very minimal wind.
On the other hand, in an unstable atmosphere with windy conditions, rapid dilution will occur, with increased emissions producing the worst possible concentrations.
Research Methodology
- Computational Fluid Dynamics (CFD)
- Physical Geometry
- Meshing
- Type of gas
- Wind speed
- Obstacles
The simulation model will be developed using Design Modeler provided by ANSYS-FLUENT CFD. The geometry will be solved based on the 2D XY plane for an easier calculation time. The geometry will be a surface of 10 m wide and 5 m high as a symbol of the environment area.
For the point of discharge, pipe location scenario is chosen for this project and will take place at ground level which is on axis X of the aircraft. In general, the leaking pipe size is affected by many factors such as mechanism, stress level and the material properties. However, 10 mm leak size will be chosen based on the IP Model Code (2005) which was used as a reference. The purpose of mesh is to indicate and balance the quality of the mesh and computation time.
To determine which is the good mesh, several simulations will be performed with a variation of the mesh and compared with theoretical results produced by other researchers and standard. The latest result looks more reliable, as the concentration of the biogas will decrease as it flows upwards, because it is diluted with ambient air. This model will be suitable for this project because it provides the gas dispersion that was related to release to the atmosphere.
The problem setup will be done by entering the scattering mode that will be simulated. One of the standards is to compare the result from this simulation project with the experimental data obtained from the Kit Fox experiment. Initial assumption will be made by the state that the gas phase will be gas phase rather than multiphase.
The CO2 content in natural gas is less than 1% and this is one of the reasons why biogas has been chosen as the subject for this project. The wind speed in the surrounding atmosphere will be affected by the intensity of atmospheric turbulence. Logically, with higher atmospheric turbulence, it will dilute the concentration of biogas and reduce the probability of danger.
For this project, the obstacle will be placed at a distance of 3m and 5m from the release point (origin) with dimensions of 1m x 1m.
Gantt Chart
Submission of Progress Report PAST Software Advance Learning
Oral Presentation Submission of Technical Paper
Submission of Dissertation Submission oh Hard Bound
- Effect on Wind Speed
- Effect on Discharge Rate
- Recommendation
- Conclusion
Unlike Figure 14, the 6 m/s wind speed condition begins to show the effect of high turbulence on the CO2 gas concentration. In order to find a safe distance between biogas leakage, the data was plotted against the concentration of CO2 gas from the biogas process. Plots of each simulation for CO2 gas concentration versus distance are shown in Appendices 1, 2, 3 and 4.
From Table 9, the highest CO2 gas concentration at the leak point is 110,000 ppm, which is under wind conditions of 3 m/s and 4 m/s. The highest CO2 gas concentration after a distance of 2 m from the release point is when the wind condition is 3 m/s. Based on Table 3, the minimum concentration of CO2 gas is 63,000 ppm for each inhalation exposure time.
The comparison in terms of CO2 gas concentration and distance for each wind condition can be seen in Figure 15 below. Both results show the large vapor cloud of CO2 gas that is created during the simulation. At a discharge rate of 2 kg/s the vapor cloud of CO2 gas is much larger than a discharge rate of 0.7 kg/s.
The safe distance for each case is determined by drawing a graph of the CO2 gas concentration within a distance of 10 m. Based on Table 11, the maximum concentration of CO2 gas at the discharge point is 110,000 ppm with a discharge rate of 2 kg/s. So, to find the safe distance for this condition, the data is tabulated based on CO2 gas concentration and distance as shown in Table 12.
From the result, a safe distance can be determined by looking at the CO2 gas concentration that will decrease away from the leak site. The concentration of CO2 gas falls to 60, 100 ppm, which can be considered as a standard safe concentration of CO2 gas. The trend of CO2 gas concentration within 10 m of the discharge point for each discharge level can be observed based on the graph in Figure 20.
The formation of a vapor cloud of CO2 gas is also influenced by the rate of biogas release.
Available at: http://www.renewableenergyworld.com. 2011) Modeling and simulation of dense cloud dispersion in urban areas using computational fluid dynamics. 2012) CFD Analysis of Dense Gas Dispersion in Indoor Spaces for Risk Assessment and Mitigation, Hazard Mater.
APPENDIX
Graph of CO2 gas concentration versus distance (3 m/s)
Graph of CO2 gas concentration versus distance (4m/s)
Graph of CO2 gas concentration versus distance (5m/s)
Graph of CO2 gas concentration versus distance (6m/s)
Graph of CO2 gas concentration versus distance (0.1 kg/s) APPENDIX 5: Graph of CO2 gas concentration versus distance (0.06 kg/s)
Graph of CO2 gas concentration versus distance (0.7 kg/s)
Graph of CO2 gas concentration versus distance (2 kg/s)
