Coal breaker remaining. The sorting by size is particularly important for anthracite coal. In order to burn efficiently, air must flow evenly around anthracite. Subsequently, most anthracite coal is sold in uniform sizes. In the 1910s, there were six commercial sizes of coal (with the smallest size having three subsets): Coal pieces smaller than in size were considered "culm," and unable to be separated from the impurities (and thus useless). The grade of coal ranged from a low of 5 percent impurities for steam or broken coal to a high of 15 percent for pea-size coal and its subsets. Coal breakers were
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