Solid fuel purposes, such as refining metals. Coal is the largest source of energy for the generation of electricity worldwide, as well as one of the largest worldwide anthropogenic sources of carbon dioxide releases. The extraction of coal, its use in energy production and its byproducts are all associated with environmental and health effects including climate change. Variations such as smokeless coal can be formed naturally in the form of anthracite, a metamorphosed type of coal with a very high carbon content that gives off a smokeless flame when set alight. Coke is a fuel with few impurities and a high carbon
Carbon-neutral fuel can be captured and buried, making fossil fuels carbon-neutral, although not renewable. Carbon capture from exhaust gas can make carbon-neutral fuels carbon negative. Other hydrocarbons can be broken down to produce hydrogen and carbon dioxide which could then be stored while the hydrogen is used for energy or fuel, which would also be carbon-neutral. The most energy-efficient fuel to produce is hydrogen gas, which can be used in hydrogen fuel cell vehicles, and which requires the fewest process steps to produce. Methanol can be made from a chemical reaction of a carbon-dioxide molecule with three hydrogen molecules to produce methanol