5 Misconception on biogas and methane and why you should know
Although methane is the main composition of biogas that have existed on earth since 2.5 billion years ago, not many people have correct understanding about it. In this article, we shall correct 5 typical misconceptions about biogas. Once you have the correct understanding about the biogas, it will make your lives easier and safer.
- Methane has a fouling odor
This is absolutely not true. Methane (CH4), in fact, has neither scent nor odor. Why? It is just happen to be like that. This is a natural characteristic of methane. If someboday said biogas is stinky. It is true, but it is not from methane. It is from hydrogen sulfide gas (H2S) which smell like rotten eggs.
- Biogas is heavier than air
When I talked to clients and they say that biogas could flow on the ground and cause explosion, then I would have to spend some time with him/her to explain the truth about this idea. To get the correct understanding, we need to look at each composition of biogas separately. First, methane has a density of 0.66 kg/m³, while air has a density of 1.2 kg/m3. Hence, methane is lighter than air and will rise swiftly to the sky when the atmosphere is open.
Ten years ago in Thailand, there was a terrible accident where biogas is pushed 100 m along the ground to the nearby boiler room and cause explosion during the thunder storm. This is a highly unusual case when the atmosphere is close.
Another main composition of biogas is carbon dioxide (CO2) with the density of around 1.98 kg/m3. This gas is heavier than air and stay on the ground. However, it cannot cause fire or explosion. In fact, carbon dioxide can help to distinguish fire.
Both methane and carbon dioxide can cause climate change which causes the earth to get hotter. Hence, we have to be careful when using biogas.
- Biogas is 100% methane
Some people may think that biogas contains 100% methane, since methane is the gas that provide heat and energy, while carbon dioxide (CO2) does not. In fact, microorganisms generate equal amount of both gases. Please read this article “What is biogas? Where does it come from? The science behind biogas.” for more details. However, carbon dioxide can dissolve into water much more than methane. Hence, biogas usually contains more methane than carbon dioxide. The typical biogas composition is as follows:
Carbon dioxide 30-50%
Other gaseous 0-1%
Hydrogen sulfide 100-20,000 ppmV
- Biogas can be compressed into cyclindical tank and use just like LPG
This is another classic question that I hear on a monthly basis, particularly during the period when gas and LPG price are expensive. I can say it is similar but not totally the same. Do not forget that biogas has a unique composition. If you can’t remember, please go back to read the previous paragraph. How about LPG composition? Do you know? The answer is propane and butane. You can see that the name is totally different. Then you have realize that methane has only 1 carbon atom. Propane and butane has 3 and 4 carbon atoms, respectively. While LPG has a density of 1.9 kg/m3whcih is heavier than air and biogas.
Another issue is the heating value. In biogas, it contains only maximum 70% methane content. When we can compress LPG to around 100 psi or around 7 barG into the LPG cylinder, we can only compress biogas to the maximum only around 50 psi or 3-4 barG. The heating value of LPG is up to 12,000 kcal/kg, while biogas with 70% methane content has the heating value of only around 7,200 kcal/kg which is 40% less than LPG. I hope the readers will not get too confused with so much figures.
In short, we can compress biogas into a cyclinder, but we get less gas mass and less energy. On a practical side, when we use biogas for cooking, the gas would run out in oly 1-2 weeks. If we want to really use biogas for cooking, we need a bigger storage vessel and chaning of cooking stove. If you really want to use biogas for cooking, I would recommend to upgrade it to CBG (Compressed Biomethane Gas) and compress it to 250 barG. This will be more worthwhile and practical.
- Biogas can dissolve in water
This is another fact that people misunderstood. In fact, carbon dioxide can easily dissolve in water, but methane can hardly dissolve in water. This is why we see more methane composition in biogas than carbon dioxide. Furthermore, engineers can design a degas system to remove CO2 to maximise methane content and save energy in biogas transmission.
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