Fuel Types and Uses

The energy ladder sorts fuels by how clean and efficient they burn: biomass (wood, dung, crop waste) at the bottom, then coal, then natural gas, then electricity at the top. Moving up takes money and infrastructure. A gas stove is useless without a pipeline or delivery truck bringing propane, and without the steady income to keep buying fuel instead of gathering it for free. That is why roughly a billion people still cook over wood or dung even where gas stoves sit in stores nearby: the fuel is cheap or free and the infrastructure to switch is not there, even though it is more polluting and less efficient.

Energy density, the energy released per unit mass or volume, decides which fuel fits which job. Fuels with high energy density per kilogram, like jet fuel and gasoline, pack enormous energy into little weight, which matters when every kilogram has to be lifted into the air or carried in a tank. Coal is energy-dense too but heavier and dirtier to burn per unit of energy, so it stays put in power plants and factories, where weight does not matter and a steady, cheap supply does.

This connects forward to the combustion and efficiency math of the rest of the unit: comparing fuels means converting their energy densities onto the same basis before you can say which one delivers more usable energy, and the ladder explains why switching fuels is an economic and infrastructure problem, not just a technical one.