Complex sentence: By time-being, the combustion zones moved downward, and the char (semi-coke) produced as a solid residue of pyrolysis, ignited to burn as additional fuel for combustion.
Split into simpler sentences: By time-being, the combustion zones moved downward. As the combustion zone moved down, the char (semi-coke) produced as a solid residue of pyrolysis, was burned as additional fuel for combustion.