Oil Oil is used primarily in the transportation sector. Compared to coal, very little is used to generate electricity. Thus, it is likely that at some point in the 21 st century, conservation of oil will become crucial. One barrel has an energy equivalent of about 5.8 million Btu. Natural Gas Natural gas is generally regarded as the cleanest of the fossil fossils.
Natural gas is also a fuel for some fuel cells.
Although pollutant emissions from fuel cells are very low compared to heat engines, the problem of CO2 emission remains as long as the fuel cell is fueled by natural gas. Natural gas is mainly used for space heating. Also, it is significantly used by the manufacturing industry. Until recently, its use was discouraged for electricity generation. Coal was the predominant fuel of the early part of the 20 th century, and oil was the predominant fuel of the middle to late part of the 20 th century.