Display omitted •Burn out level and unburnt carbon amount influenced by rank and inertinite content.•Organo-petrographic constituents in char is controlled by coal microlithotype.•The intrinsic reactivity of inertinite is low with respect to other macerals.•High-density char may be mainly derived from inertinite dominated microlithotypes.•A new type of char named vitrosphere is reported. The inability to yield heat up to the expected and desired level by a few of the coal fired thermal power plants can primarily be attributed to the ineffectual combustion of coal. In an intensive endeavour to understand the role of petrographic characteristics in combustion behavior, the authors have collected inertinite rich feed coal and fly ash from six different power plants in India. The technological characteristics, petrographic make up (maceral – and microlithotype composition) and vitrinite reflectance of the feed coals were studied in details. The variation in char types were identified in the various fly ashes. The chars from different feed coals were prepared under controlled laboratory conditions. The burn out level and the unburnt carbon amount in fly ash appear to be controlled by rank and inertinite content. The good correlation between the high-density chars and inertinite dominated microlithotype suggests that high-density chars are mainly derived from these related microlithotypes. The presence of a new type of char, named vitrosphere is reported. The fly ash with little unburnt carbon is predominantly comprised of plerospheres.