Lignite is soft coal anf the most porus of the coals. Slightly less porus but harder is Bituminous coal and finely there is Antracite or hard coal. As far as heating value the harder the coal the more BTU's per unit weight. This is because hard coal has a higher carbon content.
For filtration value the softer the coal the more porous the structure. This is why the soft coals make them superior for filtration. The problem is the soft coals contain far more phosphorus than a hard coal and can leach this into the tank. Therefore soft coals for filtration are often acid washed to remove the phosphates.
Granular activated carbon is "pressure cooked" with a strong base. This vastly increases the pore structure. It is perferred for filtration but the activation process is costly and hence it is fairly expensive. FYI-activated carbon is rarely made from the othe coals but rather things like peanut or coconut shell. The type of base material determine how porus the final product will be.
The cheap, porus coals are often used in the hobby but look for a brand that claims low phosphate content. If you have the bucks, then op for GAC.
Thus endith the sermon.
