Steam Coal for heating purpose
Category Long-Flame Coal (D) and sub-category Long-Flame Gaseous Coal (DG) refer to base quality of average heating value, mainly usable in stoves and boilers. It is highly inflammable but quickly burns out and so proves to be good only if combusted uninterruptedly in large quantities. These base & cheap coals D & DG burn most as exuded volatile matter 35-50% and least as hard fuel coke residue; i.e. they steam by up to 50% in process of combustion.
Steam Coal for metallurgical use
Next rising degrees of coalification apply to Gaseous Coals (G) and Fat Coals (ZH), burning only 30-45% in exuded volatile matter; as well as Coking Coals (K) and Caking Coals (OC), burning only 15-30% in volatiles. These more calorific & more expensive coals G, ZH, K & OC are more saturated with hard fuel coke residue and become good for use in metallurgical and chemical-recovery industries for making ultimate Iron & Steel melts. I.e. they steam twice as little and contain enough coke residue to be regarded as dual purpose coals, fuel material and coking reagent.