## 3 Results and Discussion
### Simulation of Biogas Diffusion in Pre-Treatment Pool
#### 3.4.1 Biogas Diffusion Under Free Ventilation  
The ground space size of the workshop was 9 m \(\times\) 5 m \(\times\) 2.6 m. In the middle of the workshop, there was a 3 m \(\times\) 2.5 m \(\times\) 3.2 m pit-type pre-treatment pool, and the liquid level was 1.6 m. The air temperature in the workshop was 33 \({}^{\circ}\)C.  
Establish the pre-treatment pool workshop model. The space above the liquid surface of the pre-treatment pool was defined as the fluid simulation area, and the center of the horizontal plane of the workshop was considered the origin. The X-axis value of the overall fluid domain was between 0 m and 9 m, the Y-axis value was between 0 m and 5 m, and the Z-axis value was between \(-\)1.6 m and 2.6 m. The size of the two windows was 0.85 m \(\times\) 0.85 m, and both were 1 m above the ground. The model is shown in Figure 5.  
Boundary conditions. The manure fermentation in the pre-treatment pool workshop produces biogas. According to the staff working in the biogas plant, the composition and proportion of the biogas produced in the pre-treatment pool were as follows: methane 3-5%, carbon dioxide 10-25%, hydrogen sulfide 0.03-0.5%. For this simulation, the components of biogas produced in the pre-treatment pool were 4% methane, 17.5% carbon dioxide, and 0.5% hydrogen sulfide, and the gas production rate was 0.1 m/s. To simulate the diffusion of biogas under natural ventilation, it is necessary to set the wind speed of the window. This paper used the average wind speed of 2.94 m/s in 2023 in the city where the biogas station was located as the wind speed of the air inlet window (window 1), and the other window (window 2) as the outlet. Note that the air flew through the air inlet window. The biogas diffusion process in the pre-treatment pool workshop was simulated for 1000 s with a data sampling time interval of 1 s.  
Grid-independent verification  
Verification of independent verification in a simulation is an important part of ensuring the accuracy and reliability of numerical simulation results. This study divided three groups of independent grid parameters, and the number of grids (Elements) was 1,613,069, 2,463,506 and 3,173,269, respectively. The average element quality of the three groups meshing was more than 0.8, indicating that the overall quality of the meshing was high. The CH\({}_{4}\) mass fraction at the point (0, 0, 2.40) was selected as the monitoring point, and the simulation values obtained from the three groups meshing were compared. Figure 6 shows the CH\({}_{4}\) mass fraction at the same point in three groups, and the final equilibrium values were relatively close. Therefore, from the perspective of accuracy and efficiency, the number of grids for this simulation was selected as 1,613,069. Figure 5: Fluid simulation area model of the pre-treatment pool workshop. (4) Simulation Results. Methane, carbon dioxide and hydrogen sulfide gases were detected at 6 points, which were (0, 0, \(-\)1.59), (0, 0, \(-\)0.80), (0, 0, 0), (0, 0, 0.80), (0, 0, 1.60) and (0, 0, 2.40). Figure 7 shows the changes in methane gas volume fraction at six monitoring points. The methane gas concentration values at the six monitoring points all reached equilibrium at around 320 s. When Z = \(-\)1.59 m, \(-\)0.80 m and 0 m, the maximum equilibrium value of the volume fraction of methane gas was 3.99%. When Z = 0.80 m, the equilibrium value of the volume fraction of methane gas was 3.39%. Considering that the explosion limit of methane is 5-15%, the methane gas concentrations at the above four points were close to the lower explosion limit. When Z = 1.60 m and 2.40 m, the equilibrium values of the volume fraction of methane gas were 0.0033% and 0.55%, respectively. When Z = 1.60 m, the volume fraction of methane gas was the smallest. This was because there was a 0.85 m \(\times\) 0.85 m window 1 m above the ground. Under natural ventilation (wind speed was 2.94 m/s), there was less methane gas there. The other  
Figure 6: CH\({}_{4}\) mass fractions at 2.4 m for three grids.