## 3 Results and Discussion
### Simulation Results Analysis  
According to on-site investigations in biogas plants, some plants usually choose natural ventilation for pre-treatment pool workshop. However, due to the unstable wind speed of natural ventilation, natural ventilation obviously cannot satisfy the requirements for the pre-treatment pool workshop, a place with a high incidence of poisoning and suffocation accidents. This reflected the importance and urgency of setting up forced ventilation and emergency exhaust. It can be seen from the simulation results that setting up forced ventilation and emergency exhaust can dilute the concentrations of methane, carbon dioxide and hydrogen sulfide to a certain extent. Assuming that the height of the operators is between 1.6 m and 1.8 m, under the condition of forced ventilation and emergency ventilation in the pre-treatment pool workshop, when operators enter the workshop, the concentration of methane gas and carbon dioxide gas in the area 1.6 m and above on the z-axis direction would not bring explosion and suffocation risks. However, under the same conditions, there existed risks of methane gas explosion and carbon dioxide suffocation through forced ventilation alone. For hydrogen sulfide, under the combined effect of forced ventilation and emergency exhaust, the gas concentration in the area 1.6 m and above on the z-axis direction was greatly reduced, but it would still cause harm to the lives of workers. The reason for the above situation was that the proportion of hydrogen sulfide in biogas is generally 0.03-0.5%, the ANSYS Fluent simulation determined its proportion based on the maximum value considering that hydrogen sulfide itself is a highly toxic substance. Therefore, the concentration of hydrogen sulfide gas in the simulation result was relatively high. In addition, the ventilation times in this simulation were determined according to the minimum requirements in the standard specifications, aiming to explore the diffusion law of biogas and prove the necessity of setting up forced ventilation and emergency exhaust fan. At the same time, enterprises are also encouraged to use numerical simulation methods to determine ventilation types and optimal air exchange times that are more in line with actual conditions. Criterion item 4.8.1 of the 'Technical Specifications for Large and Medium-sized biogas plants GBT51063-2014' stipulates that the emergency exhaust fan should be automatically turned on when the methane concentration reaches 20% of the lower explosion limit (methane concentration (volume ratio) reaches 1%). According to the relevant provisions of the industry standard 'Technical Regulations for Urban Gas Alarm Control Systems CJJ/T146', the installation location of the methane concentration alarm should be no more  
Figure 25: Distribution of hydrogen sulfide gas concentration at different time points under the combination of forced ventilation and emergency exhaust fan. than 0.3 m away from the roof. According to the above installation location standards, the installation height of the methane concentration alarm in the pre-treatment pool workshop should be between 2.3-2.6 m. We assumed that the methane alarm in the pre-treatment pool workshop was installed at position Z = 2.4 m. From the simulation results, we can see that the methane gas concentration was between 3% and 4% when Z = \(-\)1.59 m, \(-\)0.80 m, 0 m, and 0.80 m, and this concentration range was close to the lower explosion limit of methane gas. However, the gas concentration at the installation location of the methane alarm (Z = 2.4 m) was less than or close to 1%. If operators work near the pre-treatment pool, the methane gas concentration is close to the lower explosion limit, so there is a risk of explosion. However, the concentration monitored by the methane gas alarm had not yet reached the standard for initiating emergency exhaust ventilation. This reflected the problem of unreasonable installation of methane alarm.