i tried drilling some 3/4" holes in the stumps with an auger bit and an 18 volt cordless drill. it really seemed to drain the batteries fast. cutting some 4-8" grooves in there with a chainsaw and then filling them with a petrochemical would probably be more efficient. make the grooves look like this:

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increasing the surface area of the stump with the cuts should allow for a more rapid and thorough burning.