Undiluted 2-4,D poured into a few holes drilled into the stump works.
We burn mostly cottonwood in our woodstove because that's what we have here. Trees 12"-24" in diameter are actually pretty solid and split nicely if they are dry. One or two good sized splits every hour or two keep the house warm on cold days. Hardwood is rare here so I save my elm, ash and fruitwood for overnighters. The 2002-03 drought killed probably 50 trees on our place, so I keep busy just cutting up the blown down dead ones.
We have two cottonwoods in the yard that are over 4' in diameter, and one behind the garage that is almost 8'. We had one problem branch cut off the big one a few years ago that was 40" in diameter and 125 years old.