The Coal chain goes something like this (rather a bit vague and I'm not geology expert): Peat (coal precursor, it is found in the soil layer) then comes Lignite (kind of a compacted Peat found in soil and sedimentary layers) then comes Sub-bituminous (kind of half way between lignite and bituminous), then comes Bituminous (a compacted lignite found in sedimentary layers) then comes Anthracite (a compacted and heated bituminous, was used in some areas in steel making instead of coke, found in metamorphic layers), finely you get Graphite (a compacted and heated anthracite, found in metamorphic layers, hard to ignite raw, can be processed to burn easier (by adding bits of burnable organic matter)).
This is a relatively easy Mod that any on can do to their raw/objects files, however they have to be done to the files before you start your game, otherwise they will not work (see modding section for more details, a further warning these are untested Mods and should be tested to ensure that they work correctly as I'm not sure I have used the syntax correctly, in other words I'm still learning to mod).
This is added to matgloss_stone_soil.txt
[MATGLOSS_STONE:PEAT]
[NAME:peat][COLOR:0:6:1][TILE:176]
[SOIL][AQUIFER]
[ITEM_SYMBOL:15]
[SOLID_DENSITY:1600]
you will note that I did not add [SPEC_HEAT:]
[IGNITE_POINT:]
[MELTING_POINT:]
or [BOILING_POINT:] as I did not want my entire soil layer on some maps burning down to bedrock (think what a map with magma touching peat would be like if I did that, WHOOSH!!!)
this is added to matgloss_stone_mineral.txt
[MATGLOSS_STONE:COAL_ANTHRACITE]
[NAME:anthracite coal][COLOR:0:7:1][TILE:15]
[ENVIRONMENT:METAMORPHIC:VEIN:100]
[ITEM_SYMBOL:15]
[SPEC_HEAT:409]
[IGNITE_POINT:11440]
[MELTING_POINT:NONE]
[BOILING_POINT:16708]
[SOLID_DENSITY:1446]
and then you add this to your reaction_standard.txt
[REACTION:PEAT_TO_COKE]
[NAME:make coke from peat]
[SMELTER]
[REAGENT:2:STONE:NO_SUBTYPE:STONE:PEAT]
[PRODUCT:100:3:BAR:NO_SUBTYPE:COAL:COKE]
[FUEL]
(note 2 peat is burned to make 3 coke -1 fuel = 2 fuel gain)
[REACTION:ANTHRACITE_COAL_TO_COKE]
[NAME:make coke from anthracite coal]
[SMELTER]
[REAGENT:1:STONE:NO_SUBTYPE:STONE:COAL_ANTHRACITE]
[PRODUCT:100:4:BAR:NO_SUBTYPE:COAL:COKE]
[FUEL]
[REACTION:GRAPHITE_TO_COKE]
[NAME:make coke from graphite]
[SMELTER]
[REAGENT:1:STONE:NO_SUBTYPE:STONE:GRAPHITE]
[REAGENT:1:WOOD:NONE:WOOD:NONE]
[PRODUCT:100:6:BAR:NO_SUBTYPE:COAL:COKE]
[FUEL]
(note you must use one graphite and 1 wood to make 6 fuel -1 fuel for a net gain of 5 fuel; it makes 6 fuel, 5 for the graphite and 1 for the wood)