DIY





Complete Biogas: Temperature and Biogas, Part 2


| 10/30/2014 9:54:00 AM



So basically, the thought we left you with last time (here, part 1) was that the US climate is not “biogas-friendly.” Recall? Even at the very best “biogas weather” places in the continental US, to attain anything close to optimal temperature for better than 150 days a year, we have to heat the digester. 

Even here, in semi-tropical Oregon, with the highest happiness index in the US (well actually… I just made that up), I cannot escape certain realities. For me, it’s 335 days of heating the digester, and based on what I see outside my window and the weather for the last week, I’m starting that countdown….

Now, some might ask: isn’t it true that you can produce biogas at colder temperatures? And, yes, sure, not only is it possible, but millions of people around the world do just that, every day. (This is not a hyperbole. It’s really, truly millions in India, China, Nepal and other countries. I can prove it to you if you like: You should always ask for proof.) And the vast majority of those folks have underground digesters, which are seldom very warm at all. (Think of lying directly on the ground to sleep. Even if it was dry, you’d get pretty cold, most of the year. Maybe even in Tahiti….)

So how come millions do it… Yet I’m saying that’s not the way to go, huh?

Well, in a biogas digester, as the previous post said, the rate of digestion is dramatically affected by the temperature of the digester. Colder is slower, so a given volume of slurry will produce less biogas for a given period when it is colder. Right? So if you know that, then it seems obvious that the way all those millions compensate for the colder digesters is to make them bigger. Much bigger. Five or ten times bigger.



And bigger, all else being equal, is more expensive, right? (Of course, again, yes.) But in developing countries, larger digesters are necessary because adding a solar heating system and providing good insulation would be (well… might be) more expensive than building that larger digester.



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