I did quite a bit of research years ago trying to get together a business plan for a combined fuel plant from waste materials. This was when waste veg oil was just being discussed and a few homebrew fuel producers were trying biodiesel - which needs the addition of methanol.
So the plant would take in veg oil, new timber, old timber, cardboard, jiffy bags (that have plastic inside), mixed paper waste, weed cut round bales and round straw bales, and fresh cut grass from the council.
The plan was to shred the timber, and all cellulose materials, put it through high/low pressure system to explode the cell walls, then ferment it in a two stage process, which first produced methanol and then biobutanol.
The fibres were then compressed, dried and combined with the plastic/paper, with the addition of the waste veg carbon residue and made into pellets to be burnt as fuel. The methanol gets used to produce biodiesel, and the only waste is ash from the boilers that are burning the straw bales, so that goes to forestry as a dressing.
One of the bits of research was the filter material used to separate out the biobutanol from the methanol water mix, and yes it is a membrane on a molecular level...which is expensive and gets clogged easily which was a stumbling block.
Another block was that the EPA & SEPA didn't like plastics going into the pellet mix to be burned, or the print on the paper which may be toxic, and the Fuel pellet standards for this country are so high they can't even include bark in the mix, plus uptake for pellet heating is so slow we would need to start another company giving away the burners just to increase demand for pellets on a contract basis. (Free burner if you sign up to buy pellets from us for 12 months). That was a clusterfck of problems so I shelved the plan.
At the moment the UK buys it's ECO wood pellets from Norway....They have a lot of trees and not much else.