- (Disambiguation: In Dwarf Fortress, "coal" is not the same as bituminous coal - see that article if desired.)
Refined coal, coal fuel, and just coal are three in-game terms that refer the same thing, any unit of charcoal or coke, the two fuels used to power high-temperature industries, namely metalworking and glassworking. One unit of either of these fuels is required for every task at a conventional (non-magma) smelter, forge, kiln, or glass furnace. Magma versions of these facilities do not need fuel, and in fact cannot use conventional fuel to power them if their magma sources fail. Refined coal is also needed as part of the pig iron and steel reactions (see below).
(Note that Bituminous coal is not "refined coal", "coal fuel", or "coal", though it is directly related.)
Though not seen in-game, the single word "fuel" is sometimes used informally in the forums and in various wiki articles in place of the in-game terms "refined coal", "coal fuel" or just "coal". Since some understandable confusion can arise over the usage of the various terms "refined coal", "coal fuel", "coal", "charcoal" and "coke" (and also "bituminous coal", which is not a fuel), the simple umbrella term "fuel" will be used in the explanations in this article.
The two sub-types of fuel are charcoal and coke, and they are nearly identical for in-game purposes, even if they come from different sources and have different in-game names. Whenever you see "coal fuel" or "refined coal" or "coal" in-game, those refer to either charcoal or coke - they are interchangeable, and either one will serve the same purpose, as fuel for the desired activity. (Bituminous coal is not the same as "coal", and does not refer to fuel, but can be used to create fuel at a smelter.)
Both charcoal and coke come in bars, and are stockpiled in Bars/Block stockpiles if the "coal" entry is selected within the "Bars" section. There is currently no way (or need) to distinguish between the two when designating stockpiles.
At the risk of repetition, but to be perfectly clear - despite the different names, there is no distinction between charcoal and coke for any workshop-related activities. Both are "refined coal" or "coal fuel" or "coal", or just "fuel". The only actual difference between charcoal and coke has to do with Elven traders - selling them charcoal will offend them, but coke will not.
Charcoal and coke are not fire-safe building materials.
Bituminous coal blocks and lignite blocks (sometimes offered by traders) can neither be used as fuel nor converted into fuel or back into stones in any way. They can only be used for building purposes and moody dwarves may sometimes use them in artifacts and they can also be utilized for certain forms of fun.
 In-game usage
At a conventional (non-magma) smelter, if you try to add a smelting task and have no fuel, you will see the message:
"Must have coal fuel"
You will not be allowed to add any tasks to any conventional smelter until you have at least one unit of either charcoal or coke.
If you have queued up several tasks and then run out of fuel, you will get an announcement similar to:
"Urist McFuelUser cancels job: Needs refined coal."
Note that a forge or glass furnace will let you add tasks if you have no fuel, and only once a dwarf arrives and finds they cannot complete that task will you get the announcement to that effect. You will get one announcement for each un-fueled task that is unable to be completed. Jobs are canceled when there is no fuel.
When you get either of these messages, you need more fuel - either charcoal or coke will serve equally well.
To place bars of fuel in a stockpile, create a Bars/Blocks stockpile, or a custom stockpile with nabled and permit "coal" (found in the "Bars" sub-category).
 The two sub-types of fuel
There are 2 sub-types of conventional fuel - charcoal and coke. Charcoal and coke are identical in use - a single unit of either one powers one activity at a conventional smelter, forge, kiln or glass furnace, or can serve as the ingredient to make pig iron or steel. They are completely interchangeable, the only difference is their source.
 Charcoal, from wood
- Charcoal is created at a wood furnace using one wood log by a dwarf with the Wood burning labor enabled. Skill levels in the Wood burner skill are not required but reduce the time required for this activity.
 Coke, from bituminous coal or lignite
- Coke is a type of refined coal, and is identical to charcoal for all game purposes. It is created at a smelter using one unit of fuel by a dwarf with the Furnace Operating labor enabled. Skill levels in the Furnace operator skill are not required but reduce the time required for this activity.
- Coke can be created from bituminous coal, producing 3 coke (net gain of 2 fuel), or lignite, producing 2 coke (net gain of 1 fuel). Magma smelters do not require conventional fuel for this process, and so the net yield for each is effectively +1 greater.
 Magma facilities
Magma replaces conventional fuel for magma smelters, magma forges, magma kilns and magma glass furnaces. No fuel is needed, nor can it be used, for activities at these facilities. Note that you will still need charcoal or coke for smelting bars of pig iron and steel. (And see next section.)
 Pig iron and steel production
Note that when smelting pig iron or steel at any location, one unit of either charcoal or coke is required in the reaction. Again, there is no distinction - both charcoal and coke serve equally well in the production of bars of either. That same one unit can simultaneously serve as the fuel for the activity at conventional (non-magma) smelters, so there is no savings at magma smelters in producing these two materials - one (and only one) charcoal or coke is consumed each time, either way. See those articles for more information.