In message ,
newshound writes
As another poster said, saltpetre (potassium nitrate) is a good
oxidising agent if you can saturate the stump with it before applying
fire. Potassium permanganate is another possibility, perhaps easier to
obtain. When trying to purchase such things in kilogram quantities,
suggest blacking up, put on false beard, and speak with funny accent.
(Not).

I recently had a need to produce lots of smoke and Potassium Nitrate was
easily and cheaply available from suppliers on Ebay.

As far a removing stumps, I drilled lots of 1" dia holes about 6" into a
12" dia stump a few years ago and filled with Pot' Nitrate. I all but
forgot about it for 3 or 4 years and when I went back to it it just
pulled out of the ground , having rotted below the surface.
--
Bill