I generated a lot of sawdust when I cut up the Trex-like material for reboa rding my park style front porch bench. I used a product called Ultraboard form Menards.
I filled newspaper delivery plastic bags with the sawdust. Today, I starte d fire (first for the season) in our heatolater fireplace. Once it was bur ning well, I put a newspaper bag of sawdust in the middle of th fire and it burned quite well. No a roaring fire, but definitely more flames that the wood underneath the bag would produce. I could see little rivulates (of p lastic?) running out of the unburned sawdust just before the flames ignited that particular area of the sawdust. I will have to see if there is much resideue left when I take out the ashes tomorrow, but it sure seens like it is burning fairly cleanly, and I am definitely getting additional heat fro m the fireplace with the plasticized sawdust.
The firebox is completely separate airwise from the room air circulating so I don't have to worry about any noxious fumes from the plastic. The only thing is what it might do to the chimney in terms of soot/creosote build-up .