A thoughtful and kind reader of my last blog, “Let there be light,” wrote to question my methane dream of a free lunch and the “math” I used to obtain it. I normally don’t respond to comments except that other readers of the comment might be discouraged by the markedly reduced gas production our anonymous commentator suggested was more realistic—based on different assumptions and the logical conclusions that follow...and a bias to gasification technology, it seems. We are on the same side in the end, just exchanging clarifying assumptions to support our different dreams.
The mistake in the reader's response/comment was the assumption that Volatile Solids (Vs) in grass is equal to the dry matter portion of the grass. The Chinese biogas manual published in 1976 (which I assume he used as a reference and which I really like and use myself, especially for how to make brick domes--bovidas!) says that 1 ton of dry grass can produce X volume of methane over 30 days. Since wet grass is mostly water, it would seem to make sense that you’d need a lot more wet grass to equal 1 ton of dry matter to get your big lunch of methane. But, it’s more complicated than that.
What happens when you dry grass is that the actual Vs fraction is seriously reduced—maybe by half. And, if the grass was low quality (no protein) to begin with, then you will get even less methane from it. Further, the dry grass equation was typically derived from feeding the dry grass/hay substrate to cattle and using the cow dung as the feedstock for the methane potential. And that is where the formula/critique really runs amok. The Vs potential from cow dung is typically about 1/10th. Strange as it may seem, cow dung is not the best substrate for methane production (1 lb. of poop evolves somewhere between 1.5 to max. 5 ft3 Humanure/sewer sludge produces even less. The reason cow dung (gobar in India) isn’t as productive as fresh grass is that the cellulose (primary methane substrate) in cow dung is significantly reduced due to its loss through digestion by anaerobic bacteria in the rumen.
So that brings us back to grass—fresh, wet fertilized grass, like Coastal Bermuda, or any grass with protein over 10%. What’s the Vs/CH4 potential for such grasses? Not 5 ft3 optimal for cow dung, but 10.7ft3 per pound of fresh, green grass. Now that assumes that all other factors for methane production are ideal—heat, additional water and stirring so the bacteria get everything they need to make gas with every tiny bite they take. You can study the easy to follow design theory by Peter-John Meynell (I think he’s the world’s leading practitioner and thus my authority) and the highly technical studies by CRS press on methane production if you want further confidence on this matter. Pick your poison.
While I’m at it, let me address one other typical “problem” that methane junkies make a big deal out of that need not obtain with grass methanogenesis. If you’ve read the literature at all, you know that the production of methane is a “two-stage” process in which the substrate is initially digested by acidifiers (anaerobic bacteria), which produce an acid substrate that the methanogens love and consume at high rates. They stuff themselves, then go to bed, reproduce and pass gas. (Confirms humans are, in fact, evolved from such lower life forms!) The higher the temperature, the more they reproduce and make gas! Nice life!
Given that process, say the purists, you have to design and run a two-stage (bloody expensive) digester or the Darwinian acidifiers will overwhelm the weaker methanogens and the whole mess will stop until the two sides come to terms. And the two will indeed settle down and get back to normal life and good methane production.
One final note about this two-stage process the purists push on us. Remember the cow stomach model? If you feed the digester like you should feed your cow, then it will not get “acidosis” and overwhelm (inhibit) methanogens from doing their thing. Cows manage this, so I figure we can to. How? Notice your cow. It eats twice a day and lies down to chew the cud. A standing cow is a hungry or an upset cow. We will therefore feed our cow/digester twice a day with 50 pounds in the morning (saved from the previous day’s cutting) and 50 pounds in the evening and always within an hour of the scheduled time—exactly like you feed and milk a cow. The slug of grass goes in, excites the acidifiers who go right to work and then run out of food/grass and the methanogens take over. Before the acidifiers start to die off, you feed them again, and throughout the night they stay happy (the digester will cool down somewhat and slow the process nicely) until breakfast in the morning. You can put the digester “stomach” on a schedule just like a cow—the more regular, the more gas you’ll get. My model digester will always stop making methane whenever I overload it to show off “the big flush” for guests and then slack off loading it, but it will sort itself out and get back to work and, then, suddenly a whole lot of gas bubbles up and we are back in business.
In the end, the numbers (from whatever authority one picks—they’re not on the internet!) are just a guide and give us light to suggest a way forward with hope. What we need is a working, full-scale system that is integrated into a regenerative landscape. Only then will we see what grass can actually do. Watch this Blog for the construction, installation and operation of my design for methane production from cut grass on my off-grid,10-acre homestead. I’ll post photos and a running commentary. Hopefully this digester project will get done this summer while we have grass and heat.
Mine is a methane dream. I know nothing of gasification other than a few desktop designs I’ve done for Africa and some US parks with trash trees to use as sustainable biomass sources. I think the correct order for the use of gasification where I live is: make methane first, dry the sludge, and gasify that. You have only ash to dispose of at that point. Not good for me. I value the sludge for growing more grass as much as the methane it will produce.
I believe that in this bio-region, methane from fresh grass is an under-utilized, regenerative resource for maintaining a modest, self-reliant, rural life and an independent community that such a lifestyle will not only support but require.