Generating energy from fossil fuels (College Board AP® Environmental Science): Study Guide

The combustion of fossil fuels

• Combustion is a quick thermochemical reaction that requires:

  • Fuel such as hydrocarbons and alcohols

  • Oxygen

  • An ignition or trigger such as a spark, flame, a buildup of heat or exposure to oxygen

    • Coal, when exposed to high levels of oxygen, will begin to react and heat up

    • Where there is poor air flow, coal will form hot spots, which can then spontaneously ignite as heat builds up

• Combustion generates heat and light in the form of a flame

  • The release of heat means combustion is an exothermic reaction

• The burning of fossil fuels releases the gases carbon dioxide, carbon monoxide, oxides of nitrogen and oxides of sulfur

• When the fuel is a hydrocarbon (e.g. crude oil), then water (H₂O) and carbon dioxide (CO₂) are released

• Hydrocarbon compounds undergo complete and incomplete combustion

  • Complete combustion occurs when there is excess oxygen

  • Incomplete combustion occurs when there is insufficient oxygen to burn and gives rise to unburned hydrocarbons and carbon particulates

• The products of these reactions are unburnt fuel (soot), carbon monoxide and water

• Coal produces more CO₂ than any other fossil fuel when burned to generate electricity

  • The particulate matter of coal irritates respiratory tracts of humans and animals

  • Produces toxic ash with lead, mercury, and arsenic

  • Coal waste is taken and stored in ash ponds in landfills, which can leak into the ground, surface water, or the soil

  • Burned coal releases sulfur and nitrogen oxide, which is an irritant and contributes to smog and acid rain

• The continued burning of fossil fuels is the main contributor to the rise of carbon dioxide levels in the atmosphere

Steam turbine

• Fossil fuels, such as gas and coal, are used to produce energy on demand when energy is needed

  • The fossil fuel is burned (combusted) and used to heat water to produce steam

    • This changes the chemical energy in the fuel into thermal energy

  • The steam turns a turbine

    • The turning of the rotor turns thermal energy into mechanical energy

  • The turbine turns a generator, which turns mechanical energy into electricity (electrical energy)

  • The current produced is transmitted via a power grid

  • The steam is condensed into water to be used again

heat → water→ steam → steam turns a turbine → turbine powers a generator → generator produces electricity

• Coal is approximately 30% efficient as a fuel sources, whereas natural gas is roughly 60% efficient

  • This means that 60% of the available energy in gas is released as heat and is more efficient at generating electricity than coal when burned

The extraction of fossil fuels

• Since fossil fuels are hidden deep underground, they can be reached in a number of ways

  • surface mining

  • underground mining

  • vertical drilling

  • horizontal drilling

  • hydraulic fracturing (fracking)

• Mining is used to extract solid fuels, such as coal and uranium

• Drilling is used to extract gaseous or liquid fuels, such as natural gas and crude oil

  • Vertical drilling is the most common form of drilling

  • Horizontal drilling uses a steerable drill bit and reduces the number of wells

  • Hydraulic drilling is the main method to crush the rocks and extract unconventional natural gas

  • Acidizing dissolves rocks that block the path of the fuel

Extracting oil

• Oil extraction can be multi-staged

• Stage one is drilling down and pumping oil from the reservoir at normal pressure

  • Roughly 15% of a reservoir's oil can be extracted during this stage

• Stage two involves injecting hot water into the reservoir around the well

  • This water pushes the leftover oil toward the well for recovery

• Stage three is when steam, carbon dioxide, or nitrogen gas are pumped into the reservoir to drive any leftover oil toward the well head

• This is very expensive and can cost half the oil extracted

• This technology sequesters carbon dioxide into the deep reservoir, reducing its greenhouse effect

Extracting coal

• Coal is mostly mined from near-surface deposits using strip mining techniques

• Strip-mining causes considerable environmental damage in the forms of erosion and habitat destruction

• Sub-surface mining of coal is less damaging to the surface environment but is much more hazardous for the miners due to tunnel collapses and gas explosions

• Mountaintop mining is a surface mining practice involving the removal of mountaintops to expose coal seams and disposing of the associated mining waste in adjacent valleys known as valley fills

  • This approach is especially harmful to the environment

Extracting natural gas

• Extracting natural gas is done in two ways

  • Conventionally, by drilling

  • Unconventionally through hydraulic fracturing

• Conventional natural gas can be reached because it is trapped under a layer of rock that can be drilled into

  • Usually found together with oil reservoir deposits but sometimes it forms a floating layer on top of the oil

• Unconventional natural gas sits below 4,500 meters, which makes extraction very hard and sometimes economically unviable

• Deep natural gas is hard to extract for different reasons

  • Shale is a fine-grained, layered sedimentary rock that is hard to drill into, making it a costly process

  • Tight gas can only be extracted by fracking and acidizing because it is trapped underground in an impermeable rock formation