Topic 6.5 Notes – Fossil Fuels

What Fossil Fuels Are and Why They Release Energy

Fossil fuels are nonrenewable energy resources formed from ancient organic matter over millions of years. They are nonrenewable because we burn them far faster than Earth can remake them.

The three major fossil fuels are:

• Coal
  A carbon-rich solid formed mainly from ancient land plants.
• Petroleum
  A liquid mixture of hydrocarbons. It is drilled out of the ground and then refined into usable fuels like gasoline and diesel.
• Natural gas
  A gaseous fuel, mostly methane $(CH_4)$. It burns more cleanly than coal or petroleum, but it still causes pollution.

What all three have in common is stored chemical potential energy in their carbon-containing bonds. When those bonds are broken and new bonds form during combustion, energy is released.

$$\text{fossil fuel} + O_2 \rightarrow CO_2 + H_2O + \text{energy}$$

Methane is the standard example:

$$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{energy}$$

This is an exothermic reaction, which means it releases energy, mostly as heat.

A common test point is complete vs. incomplete combustion:

• Complete combustion happens with enough oxygen and mainly makes carbon dioxide and water.
• Incomplete combustion happens when oxygen is limited and can make carbon monoxide and unburned hydrocarbons.

From Combustion to Electricity

Most fossil-fuel power plants are thermal power plants. The fuel does not become electricity directly. It becomes heat first.

A coal-fired steam plant follows the sequence shown here:

Steam power plant layout

Here’s the full sequence:

1. Burn fuel
2. Release heat
3. Boil water into steam
4. Steam spins a turbine
5. Turbine spins a generator
6. Generator produces electricity
7. Steam is cooled, condensed, and reused

The energy changes are:

chemical energy → thermal energy → mechanical energy → electrical energy

One detail students mix up all the time:

• Turbine = provides motion
• Generator = turns that motion into electricity

Power plants are not 100% efficient. Some energy leaves as waste heat in exhaust gases and cooling water.

Natural gas can also work a little differently:

• In a gas turbine, hot combustion gases spin the turbine directly.
• In a combined-cycle system, that hot exhaust is used to make steam for a second turbine, which increases efficiency.

How Fossil Fuels Are Extracted

Extraction depends on the fuel.

Coal

Coal is mined by:

• Surface mining
  Includes strip mining, open-pit mining, and mountaintop removal. Major impacts include habitat loss, erosion, sediment runoff, and land damage.
• Subsurface mining
  Uses shafts and tunnels. It can cause land subsidence and acidic drainage when exposed sulfur minerals react with water and oxygen.

Petroleum and natural gas

These are usually obtained by drilling wells on land or offshore. Conventional deposits are often in porous rock beneath an impermeable layer.

Major impacts include:

• habitat disturbance
• leaks and pipeline failures
• contamination from drilling fluids or produced water
• oil spills

Hydraulic fracturing

Fracking is used for oil and natural gas trapped in low-permeability rock such as shale.

The basic process is:

1. Drill vertically
2. Often turn horizontally
3. Add steel casing and cement
4. Inject high-pressure fluid
5. Crack the rock
6. Fuel flows back to the well

Hydraulic fracturing

Fracking fluid contains water, sand, and chemical additives.

Major fracking concerns:

• Groundwater contamination
  • defective casing or cement
  • chemical spills
  • spills of flowback or produced water
  • improper wastewater disposal
  • methane migration
• VOCs released from wells, tanks, compressors, wastewater, and loading operations
  VOCs can be toxic and help form ground-level ozone
• other effects include habitat fragmentation, high water use, methane leakage, and induced seismicity

Environmental Effects of Burning Fossil Fuels

Burning fossil fuels moves carbon from geologic storage into the atmosphere.

• Carbon dioxide $(CO_2)$
  All fossil fuels release it. It strengthens the greenhouse effect and contributes to climate change. Per unit of energy, coal emits the most, petroleum is in the middle, and natural gas emits the least.
• Sulfur dioxide $(SO_2)$
  Mostly comes from sulfur impurities in coal and petroleum. It contributes to respiratory irritation, sulfate particulates, and acid deposition.
• Nitrogen oxides $(NO_x)$
  Form during high-temperature combustion. They contribute to smog, ground-level ozone, acid deposition, respiratory problems, and fine particulates.
• Particulate matter, carbon monoxide, and hazardous substances
  Coal can release soot, fly ash, fine particles, and mercury. Incomplete combustion can release carbon monoxide.
• Solid waste and water impacts
  Coal combustion produces bottom ash and fly ash, which can contaminate soil or water. Thermal power plants use lots of cooling water, and warm discharge can cause thermal pollution and lower dissolved oxygen.

Comparing Coal, Petroleum, and Natural Gas

One rule helps on almost every question here. Keep extraction impacts separate from combustion impacts. A fuel can burn more cleanly at the power plant and still cause serious environmental damage when it is extracted.