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Well, the rule of thumb is that a biogas digester kept at the proper temperature — body heat, which is 105 degrees Fahrenheit … or at least it is for a cow — will produce its own volume in biogas every day. According to this rule, if your digester is a cubic meter, and you keep it properly warm, you will get a cubic meter of biogas from it, every day. (That’s about 35 cubic feet, and comfortably more than most families will need to cook their lunch and dinner, but not a lot more.)
But that rule of thumb comes from experience with manure-fed digesters. That is, if you have a digester and you’re just putting manure in it, then the rule of thumb applies. But the fact is that different substrates produce different amounts of biogas. Remember when we said that food waste makes great biogas? (No? Well, we did….) You can see the difference by looking at the following chart which I produced, using data published by the Bavarian Association for the Promotion of Solar Energy:
Freshly cut grass clippings can ultimately produce better than 1½ cubic feet of biogas per dried pound. By contrast, the same dry weight of cow manure, under the same conditions, will produce less than a quarter of that. If you’re lucky enough to have enough of what the Bavarians call “residual fats,” then the comparable pound will produce 24 times the amount of biogas as the cow manure. So like I said: different substrates produce different amounts of biogas. In spades.
Now, of course, all energy, but most especially small-scale (they call it) alternative energy, biogas included, is situational. It doesn’t matter how fancy/cool some to-be-purchased wind electric system is if you don’t have wind where you are. And you’ll never heat your water with the sun… at night. In the same way, it shouldn’t matter to you how great corn silage is for making biogas if you don’t have any corn silage. And as for “residual fats”— like used cooking oil, maybe?— the biodiesel folks probably have all that stuff snatched up before the fast food places have time to even think about pulling the last dripping French fry out of it.
But these days, almost any of us can get access to wasted food, stuff that gets tossed from places out all over town, restaurants, and cafeterias and grocery stores. Mark my words: A day will come when food waste will be as hard to get as used cooking oil is now. But for the time being, almost any of us can get just about as much as we want.
So again (since I still haven’t told you, right?) how much biogas can you get from your food waste digester?
Well, maybe I did give you the answer: about 4 (or more) times as much per dry pound as you could if you were using cow manure. In general, in other words, take the rule of thumb and multiply by 4.
And how much will you need? Well, what you really need to get detailed answers about your specific situation is more information. For example, to figure out how much biogas you need to heat your house, you need to know a lot about the weather outside, how large the house is, how well insulated, and things like that. That whole process is described in The Complete Biogas Handbook, chapter 28. The book will also tell you how to convert the burners on your stove to run on biogas, the practical details of designing your own digester, how to figure out things like how much hot water you need, how much biogas it will take to heat it, and all the cool stuff you need to know to really make practical use of biogas. Besides, when you visit the site you can find the best free information on the web about how to build any of the four most common home-scale digesters (on the “build” page).
Now, at this juncture, most explanations that I have seen about biogas get a bit coy, and they don’t give you really practical information in a clear form. We’re not going to do that. The chart below is like no other that I have seen in all my years of involvement with this subject.
The chart asumes two critical things: first, that you are digesting food waste, and second, that the digester is at body temp. Given just those two assumptions, it shows you how many 1-gallon buckets of food waste you need to be able to get the outcome you want— and what size of digester you’ll need too. Simple. Clear. Ready? Here it is:
req’d, 1 gal
|Lights, 100 w equiv.||2.5||2 lights, 3 hours in the evening||15||0.5||22|
|Cooking, per burner||20.0||2 burners, 2 hours, 2 meals||80||2.0||120|
|Hot water, per gal||4.5||Assume 30 gal/da for shower, dish washing, etc.||135||3.5||200|
|Engine, 100 HP||1600.0||Small engine (genset?), 4 hours/day||6400||160.0||9,600|
(For hot water, we figured we would need to raise the temperature from
50° to 130°F, @40% efficiency, using biogas @60% methane.)
If you've been reading along with these blog posts, you'll know that in part 2, we mentioned that we were going to answer what is, for biogas, the ‘holy grail’ question: “Can I run my car on biogas?” Well, look at the chart. The answer seems pretty obvious: sure you can; but you need to get a hold of a couple of hundred one gallon buckets of food waste… every day. (That’s based on the thought that most cars have engines that are larger than 100 HP.)
What? Did you think I'd tell you some fairy tale? Cars are Big, Lumbering, Inefficient Energy Hogs. Does it really make sense that you could power one with three or four buckets of food waste?
If you do the math, you'll see that a standard engine requires about 16 ft3 per HP per hour. And that 100 HP engine? The digester needed to provide 4 daily hours of fuel for it (if kept at body temps) would be the size of an above-ground swimming pool: 20 feet across and 4 feet high. And keep looking. The chart can give you a lot more information like that ... And along those lines, notice that except for running your Range Rover, the biogas you need to provide light, cooking and hot water for an averagefamily can be produced if you can find 6 gallons of food waste a day. Is that a lot or a little?
So I think that's good, yeah? Biogas and food waste. And the folks who end up making food waste into biogas will be doing the rest of us a great big favor, because when that food waste gets put into the landfill, it produces methane there too. The difference is that the methane from the landfill goes into the atmosphere, and there…, well, it’s a very powerful greenhouse gas: it has 38 times the negative impact of carbon dioxide. But when we make biogas (and burn the biogas), all that methane is turned into carbon dioxide… and the impact of the food waste is dramatically reduced. Talk about a powerful way to reduce your carbon footprint: Think biogas.
Be a good guy. Make some food waste into biogas, and then burn the biogas, joyfully. (The first time you see that pale, almost invisble blue flame, you’ll be hooked, for sure.)
I think maybe in the next blog series we’ll say a few things about temperature, a really important parameter in biogas production. And, hey, just because I like you, I’m going to give you an Excel spreadsheet to calculate the effect of temperature on the digestion process. Keep reading...
David William House is the author of The Complete Biogas Handbook.