I observe every year in autumn that fallen leaves are piled and left to rot. Assuming there are biogas facilities (say within 10 miles) is there some reason to not ferment the leaves? Is the energy density too low? In cities, the leaves are collected anyway. In forests, could extracting leaves draw too much nutrients from the soil?
First, how do we look at the biogas yield of a suvbstrate? We look at the total solids (TS) and volatile solids (VS), and then measure the gas production per VS. TS is the fraction that remains after thorough drying, while VS is the fraction that is lost after heating the dry stuff to a few hundred °C for an hour or so. VS is a handy stand in for the part of the matter than may be organically degraded.
Yield of leaves is, according to this paper 377 Nl/kg VS Biogas at around 50% Methane. VS is about 82% of TS. The paper gives no numbers, I'd expect leaves when they fall of the tree to have 30%+ TS, but the TS will also vary wildly depending on weather. For comnaprison, good quality corn whole crop silage would be around 660 Nl/kg VS and 65% methane.
The low yield is likely due to to the high ligning content. This paper states a far higher yield for pretreated leaves vs. non-pretreated: Crushing or milling will break up fiber and so make the cellulose inside, that would otherwise be encased in lignin, accessible to the bacteria. However the second paper does not give convenient numbers for gas yield, so I can't do a direct comparison with 377Nl/kg. According to this (german) article one can also prepare a mash and then do a solid liquid separation, the liquid contains an easily gasifiable fraction and the solid fraction would be incinerated. Since this would also work on grass etc. (which are available when leaves are not) such a process could be a worthwhile addition to a biogas plant.
Leaves, when raked of the ground, will be mixed with sand, grit, earth, sticks and other debris. Depending on the biogas plant in question, this can be a problem or not.
AFAIK forest litter plays an important role in a forests nutrient cycle. Gathering leaves from a forest also sounds challenging. so only leaves that would be gathered anyway.
Leaves are 'produced' only in fall. It makes sense to produce biogas from leaves if:
- The leaves are collected anyway, off roadways, parking lots etc.
- there's an existing biogas plant that can take an extra load every fall
- this biogas plant and downstream process can handle the extra grit etc. from the leaves
Yes, you could.
Biogas production is heavily depending on outside factors. If you have to haul the matter a significant distance, you may end up with a net loss. Biogas overall is a way to intercept some energy that would otherwise be lost.
Remember too, that biogas is mostly methane. If your system leaks, then you are replacing the CO2 that the detritivores would produce with methane -- a much more potent (but shorter lived) gas.
Another way to harvest some of the energy: Build a large compost bin, run layers of plastic pipe in it, and use the heat of the pile to preheat water for domestic use. There has been some research setups for this.
If you garden, a better use may be to till the leaves into your garden, or compost them, and distribute the compost to your garden.