烟煤
Smokeless fuels generally have a high calorific value, with that of anthracite being greater than dry wood for example, and many smokeless briquettes are made from this type of coal. Thus anthracite has a calorific value of 32.50 MJ/kg compared with that of dry wood of about 21 MJ/kg. Lignite or brown coal is even worse with a heat of combustion of only 15.00 MJ/kg owing to the presence of non-combustible impurities. Bituminous coal has a value lower than anthracite, but neither lignite nor bituminous coal are smokeless owing to volatile content.