5.2 Fossil Fuels and Products of Combustion

In Lesson 3 (Energy Supply and Demand), we explored how the United States and the world source their energy. One key takeaway: fossil fuels—petroleum, natural gas, and coal—still dominate the global energy mix.

Want the latest data? Check out the U.S. Energy Information Administration's U.S. Energy Facts Explained for up-to-date statistics. As of 2024, fossil fuels supply more than 80% of total U.S. energy consumption.

 How Fossil Fuels Release Energy

When we use fossil fuels for energy, we typically burn (combust) them. This chemical reaction releases stored energy—but also emits substances into the atmosphere.

A simplified representation of hydrocarbon combustion looks like this:

$${\text{C}}_{\text{x}}{\text{H}}_{\text{y}}{\text{ + O}}_{\text{2}}{\text{ (from air) → CO}}_{\text{2}}{\text{ + (y/2) H}}_{\text{2}}\text{O + other products}$$

Important note: The "air" in this reaction isn't just oxygen. Earth's atmosphere is ~78% nitrogen (N₂), ~21% oxygen (O₂), and ~1% argon and other gases. At high combustion temperatures, nitrogen can react to form nitrogen oxides (NO_x)—a key pollutant we'll discuss shortly.

What's Actually in Fossil Fuels?

Fossil fuels are primarily made of carbon (C) and hydrogen (H)—the same building blocks found in all living things. That's no coincidence: fossil fuels formed over millions of years from ancient plants and microorganisms that were buried, compressed, and shielded from decay.

In addition to carbon and hydrogen, fossil fuels contain small amounts of:

• Sulfur (S) → can form SO₂ (a contributor to acid rain)
• Nitrogen (N) → contributes to NO_x formation
• Oxygen (O), trace metals, and mineral matter

Clarification: Not everything in the fuel is emitted unchanged. During combustion, chemical bonds break and reform, producing new compounds—some useful (like energy and water vapor), others harmful (like CO₂, NO_x, SO₂, and particulates).

In this unit, we'll examine the major pollutants released from combustion, how they affect human health and the environment, and what strategies exist to reduce their impact.

Text description of the Fossil Fuel Composition image.

Natural gas is composed of carbon, hydrogen, nitrogen, sulfur, and oxygen.

Petroleum is composed of carbon, hydrogen, nitrogen, sulfur, oxygen, and minerals.

Coal is composed of carbon, hydrogen, nitrogen, sulfur, oxygen, and minerals.

Credit: © Penn State is licensed under CC BY-NC-SA 4.0

Instructions: Click on the purple hot spot shown above the piece of coal below to determine what products are formed from each during combustion.

The most common products of combustion we will discuss in this module are as follows.