I shoot a .54 Pedersoli Kodiak with large charges, (150 grns), of 2F and I was using some new, pre-lubed pillow ticking patches, 018", and they were doing the same thing, after I fired I could see them on the ground smouldering....
So first I changed to dry pillow ticking with pure Neetsfoot oil as patch lube and that put an end to the burning patches. Next I tried a suggestion from someone on another forum and that was to use Hoppes BP Solvent and Patch lube. That worked great and it was easier to load the next ball. Then I added into the mix a .54 pre-lubed, felt overpowder wad that I saturated with molten SPG bullet wax. That and the Hoppes gave me the best accuracy and ease of ramming the next ball in a fouled barrel.