## 3 Results and Discussion
### Simulation of Biogas Diffusion in Pre-Treatment Pool
#### 3.4.2 Diffusion of Biogas Under Forced Ventilation (6 Times/h)  
It can be seen from the natural ventilation simulation results that natural ventilation can dilute the concentration to a certain extent, but the disadvantage of natural ventilation is that the wind speed conditions are unstable. If there was no wind or the wind speed was low, higher concentrations of toxic and harmful gases would easily accumulate in the pre-treatment pool workshop. It can be seen from criterion item 13.9 of 'Technical Specifications for Biogas Engineering Part 1: Engineering Design NY/T 1220.1-2019' that when natural ventilation cannot meet the requirements, forced ventilation can be used. 1. Establish the model. In accordance with the requirements for the number and area of vents in the standard specifications, two vents were added to the natural ventilation simulation model. The size of the two windows was 0.4 m \(\times\) 0.4 m. The vent close to the ground was 0.2 m away from the ground, and the other vent close to the roof was 0.2 m away from the roof. The model is shown in Figure 13. 2. Grid meshing. The number of simulation grids was 1,611,141, and the average element quality was 0.84, which indicated that the overall grid quality was high. 3. Boundary conditions. According to criterion item 13.9 of 'Technical Specifications for Biogas Engineering Part 1: Engineering Design NY/T 1220.1-2019', the number of forced ventilation frequencies  
Figure 12: Distribution of hydrogen sulfide gas concentration at different time points under free ventilation. Figure 13: Fluid simulation model of the pre-treatment pool workshop. was set at 6 times/h. According to criterion item 6.3.9 of the 'Design Code for Heating, Ventilation and Air Conditioning of Industrial Buildings GB50019-2015', this simulation discharged 2/3 of the total exhaust volume from the lower vent and 1/3 of the total exhaust volume from the upper vent. Specifically, two windows (window 1 and window 2) were used as air inlets, and the calculated speed was 0.1487 m/s. Window 3 was used as the air outlet, and the exhaust volume accounted for two-thirds of the total exhaust volume. Window 4 was used as the air outlet, and the exhaust volume accounted for one-third of the total exhaust volume. The biogas diffusion rate and composition ratio referred to the natural ventilation simulation parameters. The biogas diffusion process in the pre-treatment pool workshop was simulated for 1500 s with a data sampling time interval of 1 s.  
(4) Simulation results. Figure 14 shows the changes in methane gas volume fraction at six monitoring points. As shown in Figure 14, the methane gas concentration values at the six monitoring points all reached equilibrium at around 700 s. When Z = \(-\)1.59 m, \(-\)0.80 m, 0 m, and 0.80 m, the volume fraction equilibrium value of methane gas was 3.99%, which had reached 80% of the lower explosion limit of methane; when Z = 1.60 m, the volume fraction equilibrium value of methane gas was 3.39%, which had reached 68% of the lower explosion limit of methane; when Z = 2.40 m, the volume fraction equilibrium value of methane gas was 0.99%, which had reached 20% of the lower explosion limit of methane. Figure 15 demonstrates the distribution of methane gas concentration at different time points. As shown in Figure 15, forced ventilation (6 times/h) diluted the methane gas in the area above 1.6 m in the z-axis direction, while the methane gas in the area below was not diluted. Figure 16 exhibits the changes in carbon dioxide gas volume fraction at the six monitoring points. As shown in Figure 16, the carbon dioxide gas concentration values at the six monitoring points all reached equilibrium at around 700 s. When Z = \(-\)1.59 m, \(-\)0.80 m, 0 m, and 0.8 m, the volume fraction equilibrium value of carbon dioxide gas was 17.47%; when Z = 1.60 m, the volume fraction equilibrium value of carbon dioxide gas was 14.82%; when Z = 2.40 m, the volume fraction equilibrium value of carbon dioxide gas was 4.37%.