Peat is a very primitive source of fuel found in many wetlands. It's decaying plant matter that is often dug and set to dry to be burned later as fuel.
This could be implemented as an alternative for those looking for high quality fuel without high enough survival to make a decent bone saw. One would dig in a swamp and would get peat dependent on the soil quality. This would give swamps more use and could even lead to some becoming claimed. The peat would then be left to dry on a herbalist table and act just like coal afterwards.
So, in short this would:
Make high Q swamps valuable
Give a decent fuel source to those without a dedicated survivalist/carpenter/tree farmer
Give a decent fuel source to those without clay
Give another use for the shovel
The dried peat would be soft capped by the herbalist tables so high Q boards are still needed, but it would be an initial investment instead of a long term commitment. As a consequence, one must wait an extended period of time for the peat to dry.