There are companies that specialise in cleaning and cutting drums.
Before applying heat to empty drums, specialist companies thoroughly clean and test the inside of the drums with gas monitoring instruments, to ensure that there is no chemical residue or explosive vapour.
These firms also specialise in removing dents from drums ready for re-use. Do not be tempted to remove dents using air pressure without adequate controls - the drums commonly over-fill and burst.
Go to the specialists.