Sub-bituminous coal, also known as black lignite, falls between lignite and bituminous coal, as per the classification system used in the United States and Canada. Geologically, it is a young coal, having formed anywhere from 251 million years ago to the present. When dry and free of ash, sub-bituminous coal contains 42- to 52-percent carbon; its calorific value ranges from 19 to 26 megajoules per kilogram, the Encyclopaedia Britannica states. In terms of appearance, it is dark brown to black and is brighter than lignite, which often has a woody structure rather than a compact shine. Some sub-bituminous coal looks exactly like bituminous coal to the naked eye. One advantage of sub-bituminous coal is that it contains less water than lignite and is therefore harder, a characteristic that makes it more suitable for transportation and storage. However, sub-bituminous coal's sulfur content is sometimes lower than 1 percent, well below the level found in bituminous coal. That means more sub-bituminous coal than bituminous coal must be burned to create the same amount of energy. Even so, many power plants have switched to sub-bituminous coal as bituminous coal's high sulfur content is environmentally problematic. Sub-bituminous coal production Nearly half of the world's proven coal reserves are sub-bituminous coal and lignite. The five countries with the largest proven reserves of these types of coal are Romania, Australia, the United Kingdom, Turkey and France, NationMaster states. More specifically, Romania has 107,922 million tonnes, Australia has 52,300 million tonnes, the UK has 21,944 million tonnes, Turkey has 17,879 million tonnes and France has 6,556 million tonnes. In contrast, proven reserves in US, which produces a fair amount of sub-bituminous coal, sit at just 3,107 million tonnes.