Recently, a child threw firecrackers into the manhole cover in Dazhou, Sichuan, causing an explosion and a row of manhole covers were blown up. The scene was thrilling, but fortunately no one was injured.
This is not the first time a similar accident has happened. In fact, it happens several times a year that children set off firecrackers to cause sewer explosions. Some children are seriously injured, while others lose their lives.
Many people know that methane exploded. Biogas is a combustible gas mixture, because about 70% of it is methane, which can be burned in the air and is recognized as a clean energy source.
The methane molecule consists of 1 carbon atom and 4 hydrogen atoms. When it burns, it will react with oxygen in the air to produce water and carbon dioxide, and release a lot of heat:
CH？ + 2 O？ CO？ + 2 H？ O+ heat
In general, when we ignite methane in the air, combustion only occurs in a thin layer at the interface between two gases. But if methane and air are mixed in proportion, the situation will change: the chemical reaction will be completed instantly, the heat will be released instantly, and the gas will expand violently. This is what we often call an explosion.
Biogas is mainly produced by microbial decomposition and fermentation of organic matter, and its main components are methane and carbon dioxide, as well as a small amount of nitrogen, hydrogen and a small amount of hydrogen sulfide and oxygen. Methane is colorless and tasteless, and the smell of biogas is mainly caused by hydrogen sulfide.
If you pay attention to the community, it is not difficult to find that there are many round cast iron manhole covers on the ground, some are entrances for power line maintenance, some are wellheads for rainwater drainage, and some are written with "sewage" or "dirt", which are generally manhole covers for septic tanks. In the residential area where many people live, there are usually some huge septic tanks underground, which are built according to the number of residents. There are prescribed standards for the design of septic tanks. If a high-rise building has 1 000 people, then the septic tank below it must have at least 250 cubic meters.
A large amount of excrement is collected in septic tanks, and bacteria slowly decompose in anaerobic environment. After more than one month's fermentation, the solid feces were precipitated, and the liquid was filtered twice and then flowed away through the sewage pipe. Because septic tank is generally a closed space, a large amount of methane gas produced by microorganisms during fermentation will gather in the upper space of septic tank and form dangerous mixed gas with the air brought in by the drainage pipeline above.
Not all septic tanks will explode when they meet Mars. On the periphery of our solar system, there is a huge planet called Uranus, which is full of flammable and explosive methane and hydrogen, but even if you throw a ton of firecrackers on it, it won't be blown up because there is no oxygen.
Compared with hydrogen, the explosion concentration range of methane is very narrow. The air is mixed with 5%~ 15% methane gas, which will explode when it meets Mars, even if it is low or high.
Suppose a 250 cubic meter septic tank contains half of wastewater and feces, and there is a mixed air of125m3, which contains 15% methane, so we can get18.75m3 methane gas. Assuming that 10 cubic meter of methane participates in the explosion reaction, let's calculate its explosion equivalent:
At standard atmospheric pressure and 25℃, the mass of 1 m3 methane is about 0.657kg, so 6.57kg methane in septic tank will explode. The combustion heat of methane is 55.5MJ/kg, and 6.57 kg of methane will produce 365 mj of energy when it explodes.
1 kg TNT explosive produces about 4. 184 MJ of energy, so if this septic tank explodes, it is equivalent to 87 kg TNT explosive!
According to the above calculation results, even if the septic tank capacity is reduced by 10 times, its explosive power is at least equivalent to 8 kilograms of TNT equivalent, so it is not a problem to throw a cast iron manhole cover weighing tens of kilograms to tens of meters high.
From the monitoring video of another methane explosion accident, we can see that a girl stuffed a firecracker into a small hole in the manhole cover, which caused the whole road to be blown up, three people were seriously injured, and 1 child died unfortunately. The lesson is extremely painful!
It can be seen from many surveillance video images of sewer explosions that some children know that it is dangerous to throw firecrackers into manhole covers. Most of them will escape at the first time, and only a few will be in the same place when the explosion occurs. As parents, we should not only make children aware of the harm of doing so, but also discipline children and strictly restrain their behavior. Even if passers-by see children stuffing firecrackers into the sewer, they should stop it in time.
Children are always curious and often don't realize the seriousness of the consequences. Community workers should always monitor the accumulation of biogas in septic tanks, pump away waste water and dirt in time, and eliminate harmful gases in time. The Spring Festival is the peak season for firecrackers sales, and Xiong Haizi is pestering us again. We should take precautions to avoid tragedies.
# The children threw firecrackers into the sewer and blew up the manhole cover #