<P> Low ash, sulfur, and carbonate coals are prized for power generation because they do not produce much boiler slag and they do not require as much effort to scrub the flue gases to remove particulate matter . Carbonates are deleterious as they readily stick to the boiler apparatus . </P> <P> Smithing coal is a type of high - quality bituminous coal ideally suited for use in a coal forge . It is as free from ash, sulfur, and other impurities as possible . The constituents of the coal should be as follows: </P> <Table> <Tr> <Th> Constituent </Th> <Th> Percentage </Th> </Tr> <Tr> <Td> Sulfur </Td> <Td> Not over 1% </Td> </Tr> <Tr> <Td> Ash </Td> <Td> Not over 7% </Td> </Tr> <Tr> <Td> Carbon </Td> <Td> Not less than 70% </Td> </Tr> <Tr> <Td> Moisture </Td> <Td> Not over 12% </Td> </Tr> </Table> <Tr> <Th> Constituent </Th> <Th> Percentage </Th> </Tr>

What type of rocks are lignite and bituminous coal