I grow mushrooms at home, and so I have lots of spent grow bags with decayed substrate (wood, millet and soy hulls) in them.

They are great for composting, and make very good soil, but nobody wants them and I used as much as I could for my plants.

Right now, I throw them into the industrial compost bin because of that.

But I think they could be used for something way more useful than just compost.

For example, I thought about chopping them up and dehydrating them, so I can use them as cat litter. Mushrooms are mostly water and the hyphae have lots of surface area, so they could act as a sponge.

Or, maybe they burn fine and make good coals for barbecues or heating?

Do you have any other ideas?