## 3 Results and Discussion
### Simulation of Biogas Diffusion in Pre-Treatment Pool
#### 3.4.3 Biogas Diffusion Under the Combination of Forced Ventilation and Emergency Exhaust Fan  
From the simulation results of forced ventilation (6 times/h), it can be seen that the methane gas concentration in the pre-treatment pool workshop was close to the lower explosion limit, the carbon dioxide concentration was far beyond the lethal concentration except at the top area of the workshop, and the hydrogen sulfide concentration exceeded the lethal concentration in the entire workshop, so forced ventilation (6 times/h) did not meet the requirements. According to criterion item 4.8.1 of the 'Technical Specifications for Large and Medium-sized biogas plants GB51063-2014', when the methane concentration in the air is detected to reach 20% (volume ratio) of the lower explosion limit, the emergency exhaust fan should be able to open automatically. Considering that the ventilation frequency of the emergency exhaust fan should not be less than 6 times/h, this section is simulated based on the forced ventilation (6 times/h) in the previous section and added the emergency exhaust fan. We assumed that the emergency exhaust fan had a ventilation frequency of 6 times/h. Two windows (window 1 and window 2) were used as air inlets, and the wind speed was set to 0.2974 m/s. In addition, 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 component ratio referred to the simulation parameters in the previous section. We simulated the biogas diffusion process in the pre-treatment pool workshop for 1500 s with a data sampling time interval of 1 s.  
Figure 20 indicates the changes in methane gas volume fraction at six monitoring points. As shown in Figure 20, the methane gas concentration values at the six monitoring points all reached equilibrium at around 300 s. When \(Z=-1.59\) m, \(-0.80\) m, 0 m, and 0  
the z-axis direction was close to the lower explosion limit, and the methane concentration in the area above had not yet reached 20% of the lower explosion limit. Figure 21 displays the distribution of methane gas concentration at different time points. As shown in Figure 21, the combined effect of forced ventilation and emergency exhaust fans diluted the methane gas in the area 1 m and above on the z-axis direction, while the methane gas in the area below 1 m was not diluted. Figure 22 revealed the changes in the volume fraction of carbon dioxide gas at the six monitoring points. According to Figure 22, the carbon dioxide gas concentration values at the six monitoring points all reached equilibrium at around 300 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 about 17%. The carbon dioxide concentration at the above four points exceeded the lethal concentration of carbon dioxide. When Z = 1.60 m, the volume fraction equilibrium value of carbon dioxide gas was 1.49%; when Z = 2.40 m, the volume fraction equilibrium value of carbon dioxide gas reached 0.10%. Figure 21: Distribution of methane gas concentration at different time points under the combination of forced ventilation and emergency exhaust fan. Figure 22: Changes in carbon dioxide gas volume fraction at six monitoring points under the combination of forced ventilation and emergency exhaust fan. Figure 23 demonstrates the distribution of carbon dioxide gas concentration at different time points. As shown in Figure 23, the combined action of forced ventilation and the emergency exhaust fan diluted the carbon dioxide gas in the area above 1 m in the z-axis direction, while the carbon dioxide gas in the area below 1 m was not diluted. Figure 24 displays the changes in the volume fraction of hydrogen sulfide gas at the six monitoring points.