## 3 Results and Discussion
### Simulation Results Analysis
Through the above analysis, the corresponding control measures proposed in this paper were that methane gas alarms should be installed in high-frequency working areas of operators and interlocked with emergency exhaust fans. When Z = \(-\)1.59 m, \(-\)0.80 m, 0 m, and 0.80 m, the three ventilation methods had no obvious dilution effect on carbon dioxide and hydrogen sulfide gases in this area, and the equilibrium concentrations of the two gases exceeded the lethal concentration. Due to the high toxicity of hydrogen sulfide and its obvious odor, some studies and accident investigation reports believe that biogas poisoning and suffocation accidents are caused by hydrogen sulfide and, thus, ignore the colorless and odorless carbon dioxide gas which can easily cause suffocation of workers. There are three main reasons why carbon dioxide should not be ignored in biogas poisoning and suffocation accidents: First, from the simulation results, when forced ventilation and emergency exhaust fans worked together, the carbon dioxide concentration in the area 0.8 m and below on the z-axis was as high as 17%, far exceeding its lethal concentration. If operators perform inspection and maintenance in this area, the risk of suffocation would increase. Second, during the fermentation process of the raw materials, carbon dioxide gas was produced first and then hydrogen sulfide gas. Carbon dioxide gas is already produced during the hydrolysis stage of the raw materials, and Carbon dioxide gas is also produced during the subsequent acid production and methane production stages. The production of hydrogen sulfide gas requires sulfate-reducing bacteria to use the intermediate products produced during the hydrolysis and acid production stages as electron donors to reduce sulfate to sulfide which then combines with hydrogen ions to form hydrogen sulfide. When the concentration of hydrogen sulfide reaches the solubility limit of the liquid, it overflows the liquid surface in the form of gas. Third, the production of carbon dioxide by fermentation of raw materials is not easily affected by the contents of specific elements, while the contents of sulfate in the raw materials directly determine the output of hydrogen sulfide gas. The output of hydrogen sulfide gas varies for different raw materials. In a word, the causative factor of biogas poisoning and suffocation accidents cannot be uniformly determined to be hydrogen sulfide gas. The standards and specifications related to biogas plants require the installation of methane gas alarms and hydrogen sulfide gas alarms in places prone to biogas leakage, and only stipulate that the methane gas alarms should be interlocked with the emergency exhaust fans. This shows that the forced ventilation system and emergency exhaust fans required by the standards and specifications are set up to prevent the explosion of combustible gases. However, the content of methane, carbon dioxide and hydrogen sulfide in the biogas produced by the pre-treatment pool workshop was relatively low, and poisoning and suffocation accidents had become the main type of accidents. Therefore, the requirements in the standards and specifications were not reasonable for the pre-treatment pool workshop. Existing measures to prevent and control poisoning and suffocation accidents mostly focus on operators' safety consciousness, such as consciously wearing protective equipment and entering the factory according to procedures. If the safety awareness of operators and the operating procedures cannot be guaranteed, the incidence of poisoning and suffocation accidents would remain high. Therefore, it is necessary to reduce the incidence of poisoning and suffocation accidents from the perspective of controlling the concentration of toxic and harmful gases.