1. Adopting Slow Ash Method, make the sulfides in coal sample fully oxidated and discharge before the carbonate decomposition to avoid the calcium sulfate formation.
2. Keep a good ventilation condition in the ashing process to make sure the sulfur oxides can discharge in time. Therefore , the muffle furnace is required to be equipped with chimney. The vent in the furnace door or a slit kept while closing the furnace door are used to ensure a natural air circulation in the furnace.
3. Flatten the coal sample in the crucible to avoid the sunburner. On the one hand, it can ensure the fully combustion; on the other hand, it can prevent the SO2 formed by sulfide from being decomposed by carbonate and then avoid generating calcium oxide.
4. The coal sample is required to be burnt in high temperature over 800℃for enough time to ensure the complete decomposition of carbonate and the fully discharge of SO2.