Iron Coals introduces tiered levels of coal in Minecraft allowing you to drastically increase the smelting rate. The lowest tier right above standard coal is iron coal. Simply surround 8 pieces of coal/charcoal around a block of iron and you have iron coal. It will burn 1.5x the rate of coal.
The same recipe is used for diamond coal by surrounding a block of diamond with gold coal. Diamond coal burns at 6x the rate of coal.
This mod also adds a very costly but immensely valuable piece of coal that will burn forever, so choose wisely where you want to use it. The Aeon coal requires 4 emerald coals, 2 hearts of the sea, 2 nether stars, and 1 block of netherite.
And there are also blocks of each of these that significantly boost the burn rate from 15x to 120x that of coal as well as nuggets that burn fractional rates ranging from 0.2x to 1.2x. The nuggets can also be crafted together to get a single piece of their representative coal.