Sub-bituminous coal is a type of coal that falls between lignite and bituminous coal in terms of carbon content and energy output. It typically contains 35-45% carbon and has a lower heating value compared to higher-grade coals. This type of coal is often used for electricity generation and in industrial processes. Sub-bituminous coal is primarily found in the United States, particularly in the Powder River Basin of Wyoming and Montana. It is characterized by its dark brown color and relatively high moisture content, which can affect its combustion efficiency.