Most of the world's energy still comes from fossil fuels. There are strong reasons to replace them:
they are non-renewable: crude oil, coal and natural gas took millions of years to form and will run out;
burning them releases carbon dioxide, a greenhouse gas linked to climate change;
they produce pollutants such as CO, NOx, SO2 and particulates.Main alternativesBiofuels are made from plants or other recently living material, for example bioethanol and biodiesel. They are renewable because new crops can be grown each year (see the next lessons).Hydrogen burns to give water only:
2H2 + O2 → 2H2O
It can be burned in engines or used in fuel cells to generate electricity. However, hydrogen is a gas at room temperature, so it must be compressed or liquefied to store it, and it is highly flammable. It is usually made from natural gas, or by electrolysis of water, which needs electrical energy. Hydrogen is only a low-carbon fuel if that energy comes from renewable sources.Electricity for battery vehicles produces no pollution where it is used, but it is only as clean as the power stations and renewable sources (wind, solar, hydroelectric) that generate it.Worked example: how much oxygen is needed to burn 4 mol of hydrogen? The ratio H2 : O2 is 2 : 1, so 2 mol of oxygen is needed, forming 4 mol of water.When judging any fuel, consider where it comes from, the energy needed to make it, what it releases and how it is stored.
they are non-renewable: crude oil, coal and natural gas took millions of years to form and will run out;
burning them releases carbon dioxide, a greenhouse gas linked to climate change;
they produce pollutants such as CO, NOx, SO2 and particulates.Main alternativesBiofuels are made from plants or other recently living material, for example bioethanol and biodiesel. They are renewable because new crops can be grown each year (see the next lessons).Hydrogen burns to give water only:
It can be burned in engines or used in fuel cells to generate electricity. However, hydrogen is a gas at room temperature, so it must be compressed or liquefied to store it, and it is highly flammable. It is usually made from natural gas, or by electrolysis of water, which needs electrical energy. Hydrogen is only a low-carbon fuel if that energy comes from renewable sources.Electricity for battery vehicles produces no pollution where it is used, but it is only as clean as the power stations and renewable sources (wind, solar, hydroelectric) that generate it.Worked example: how much oxygen is needed to burn 4 mol of hydrogen? The ratio H2 : O2 is 2 : 1, so 2 mol of oxygen is needed, forming 4 mol of water.When judging any fuel, consider where it comes from, the energy needed to make it, what it releases and how it is stored.