6.5 Fossil Fuels

Most power plants, whether they run on coal, gas, oil, nuclear fuel or geothermal heat, make electricity the same basic way: heat water into high-pressure steam, use the steam to spin a turbine, and let the turbine turn a generator.

Follow the energy: the fuel's chemical energy becomes heat when it burns (combustion). The heat turns water into steam. The moving steam's kinetic energy pushes the blades of a turbine, a wheel of fan-like blades on a shaft. The spinning shaft turns a generator, where magnets rotate past coils of wire to produce electricity. The steam is then cooled back into water, often using water from a river, lake or cooling tower, and reused.

At each step some energy is lost as heat, so only about a third of the fuel's energy becomes electricity in a typical coal plant. Natural gas combined-cycle plants, which use the hot exhaust a second time, do better.

Burning any fossil fuel releases carbon dioxide (CO₂), the main greenhouse gas behind climate change, along with water vapor. For example, methane burns as CH₄ + 2O₂ → CO₂ + 2H₂O.

Coal is the dirtiest. It contains sulfur, which becomes sulfur dioxide (SO₂), a cause of acid rain. It also releases nitrogen oxides (NOₓ), particulate matter, mercury and other metals, and leaves behind coal ash. Burning oil products in vehicles releases CO₂, NOₓ, carbon monoxide and volatile organic compounds. Natural gas releases the least of these pollutants, but still releases CO₂.

Hydraulic fracturing, or fracking, gets natural gas and oil out of shale, a dense rock whose tiny pores hold the fuel tightly. Drillers bore down, then turn and drill horizontally through the shale. They pump in water mixed with sand and chemicals at very high pressure, which cracks the rock. The sand props the cracks open so gas and oil can flow up the well.

Fracking made the United States a top producer of natural gas and oil. Its environmental costs include:

• Groundwater contamination if the well casing leaks or wastewater is spilled, letting methane or fracking fluid reach aquifers.
• Release of volatile organic compounds (VOCs) and methane into the air at well sites, which harms local air quality and adds to ground-level ozone.
• Huge water use, often millions of gallons per well, and large volumes of salty, chemical-laden wastewater.
• Small earthquakes linked to pumping wastewater deep underground for disposal.
• Habitat fragmentation from well pads, roads and pipelines.

Tar sands (oil sands) hold bitumen, oil too thick to pump. It is either strip-mined and washed with hot water, or melted underground by injecting steam. Both methods use a lot of water and energy, so oil from tar sands has a higher carbon footprint than conventional crude. Large deposits in Alberta, Canada, have left behind open-pit mines and ponds of toxic tailings (leftover waste).

Other extraction costs include coal mining that strips land and pollutes streams (topic 5.9), oil spills from wells, pipelines and tankers (topic 8.2), and habitat disruption from drilling.