making compost the traditional way seems to be, at least in part, a way to convert biomass to nitrogen fertilizer. If we want to get the most nitrogen fertilizer, does it make sense to produce biogas methane for use in Haber-Bosch process and also use what remains from methane production as a sort of compost? Or will we get the most fertilizer just by regular composting without methane part?
I guess a related and more specific question would be, does methane production destroy the nitrogen compounds in the biomass? Presumably even if does, the answer to the above question might still be “methane is good” if that’s how energy balance works out, but in such a case it may make sense to try solve the problem and salvage those already present nitrogen compounds.
I don’t get it. Why are you talking about using Methane (CH[sub]4[/sub]) to make nitrogen fertilizer using the Haber-Bosch Process, which utilizes atmospheric Nitrogen to make Ammonia (NH[sub]3[/sub]) to form Nitrates and Nitrites?
Methane is only utilized as a source of the Hydrogen needed in the conversion of N[sub]2[/sub] to Ammonia. Composting to create Methane on the scale required to produce industrial quantites of Ammonia seems to be somewhat inefficient, since Natural Gas is quite easily accessible.
Biologically available Nitrogen is the bottleneck in fertilizer. Although the atmosphere is 78% Nitrogen, it is not in a form accessible to plants and is, for all practical purposes, inert. Certain bacteria have the ability to take N[sub]2[/sub] from the air and convert it into a form usable by plants, called “fixing” the Nitrogen. While beneficial, this is impractical to ramp up to an industrial scale, hence the Haber-Bosch Process and other schemes to convert N[sub]2[/sub].
Regular composting produces methane whether you want it to or not - it’s just that when composting/digestion is done for the purpose of biogas production, there is a capture process in place.
So wherever feasible, it makes sense to capture the methane from composting, because the solid/liquid products of the process will be usable compost either way.
so is biogas production composting exactly the same process as just regular composting, just with the gas being captured? They didn’t hack the process to significantly increase gas output?
Almost certainly they did. If there’s plenty of oxygen, the microbes breaking down the biomass will oxidize it all the way to carbon dioxide (and water); they’ll generally only make methane if there’s no oxygen (air) available. So if you’re making methane, you’re going to keep air out of the tank, and probably monitor things closely to get the right environment for happy little methane-makers.
Conversely, composters don’t really care about methane, they just want things to go quickly, so they’ll try and get oxygen into the mix (that’s part of why you turn the compost pile in your backyard), so the microbes can grow and digest as fast as possible.
Correct - I mean, there are other hacks such as inoculating the mix with the best-of-breed methane producers, grinding up the waste and adding water to optimise the process, etc. But excluding oxygen is almost a side-effect of the containment necessary to capture the methane. Compost in a closed container and the emitted gases will quickly displace the oxygen anyway.
