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RE: Taking Care of Business – Methane Digesters
Thank you for pointing out these important aspects to methane digesters. How can fires be avoided and is there a way Hydrogene Sulfide levels can be regulated?
Haven't heard about the ones in China in the 70's. I wonder why that went out of practice. Was that during the Great Leap Forward? I can see how a great idea could be ditched because it was introduced together with something that could not work, like backyard furnaces. Or maybe I'm completely wrong, but would love to hear it still.
It sounds like you've had seen or maybe even worked with some digesters. Would love to know more about it, what aspects you liked, what not, and why. :-)
The easiest way to manage Hydrogen Sulfide is by introducing some sacrificial iron which will react with the sulfur. High surface area is best. And if you have the means H2S levels should be monitored, or personal H2S monitors can be worn. A small amount of oxygen will also help to reduce sulfur but can also be dangerous.
I am guessing China got away from the digesters as farms got larger, and more of the population became urbanized. As I understand, they still employ digesters there but they are larger and fewer now. When the population was largely rural, more people relied on digester gas for cooking at home.
The lower explosion limit range for methane is 5 to 15 % approximately. Digester gas can range for 45 to 80% methane, so dilution of the gas to the explosive limit is what you need to watch for. This can happen outside or inside the digester through a leak depending on if the gas is positive or negative pressure.
I have been to digesters all around the US. I used to work for a company that would treat the gas prior to use in a boiler or an engine. Great technology, but 50% of digesters at US municipal treatment plants have failed due largely to poor operation or trouble managing sulfur etc. Digesters are often not a primary treatment thus they are an afterthought for many operators.
Sulfur will react with a wide variety of metal including parts of your engine if you have one. It is something you have to be conscious of cause all metal parts will wear when exposed to gas. Plastic is often preferred.
Wow, thanks for this detailed info, @n8msic! It really does sound like digesters are merely an afterthought, as you said, whether in China or in the US. Too bad, but that also means there is a huge potential to develop them, big and small, for animals and humans (and modify the restrictive regulations).