At the beginning of the twenty-first century, most liquid and gaseous fuels used in the world are obtained from oil and natural gas: compressed natural gas (CNG), liquefied petroleum gas (LPG), gasoline (petrol), kerosene, diesel-oil, fuel-oil, aviation fuel, etc. However, petroleum and natural gas are non-renewable energy sources, since they were originated by geological processes during millions of years, enormously slower than their present consumption rate. Although new oil and gas fields are still being discovered, it is clear that easily accessible reserves will run out in a few decades, and many countries are already facing serious energy shortages.