5.2 Fossil Fuels and Products of Combustion
5.2 Fossil Fuels and Products of CombustionIn 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:
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 (NOx)—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 NOx 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, NOx, 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.
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.
5.2.1 Carbon Dioxide
5.2.1 Carbon DioxideCarbon Dioxide (CO2)
Fossil fuels—coal, oil, and natural gas—are rich in carbon (C) and hydrogen (H). When we burn them for energy, carbon combines with oxygen from the air to form carbon dioxide (CO2). In fact, CO2 is the largest chemical product by mass released from fossil fuel combustion.
Fun connection: Yes, humans also exhale CO2 when we breathe! And plants need CO2 for photosynthesis—the process that powers life on Earth. So CO2 itself isn't "bad". The problem is scale and speed:
- Natural carbon cycle: Plants absorb ~120 billion tons of CO2/year; oceans and soils absorb more.
- Human addition: Burning fossil fuels adds ~38 billion tons of extra CO2 per year (as of 2024)—far more than natural systems can absorb quickly.
Use the “our world in data” interactive below to see which countries emit the most carbon emissions total.
5.2.2 Carbon Monoxide
5.2.2 Carbon MonoxideCarbon Monoxide (CO)
Carbon monoxide is a colorless, odorless, and tasteless gas formed when fossil fuels don't burn completely—a process called incomplete combustion. This happens when there isn't enough oxygen, the temperature is too low, or the fuel-air mixture isn't well mixed.
Carbon dioxide & carbon monoxide (Chemistry) (4:04)
Transcript: Carbon dioxide & carbon monoxide (Chemistry) (4:04)
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[Presenter] Most chemical reactions are pretty predictable. If we know what substances we start with, we know what substances will be formed, and we'll get the same result every time those substances react.
But sometimes the very same reactants can yield different products. There are reactions where the temperature can affect which substances are formed, and there are reactions where it matters whether we use a larger or smaller amount of one of the reactants.
That’s the case with the graphite and oxygen example. Usually, when there is plenty of oxygen, carbon and oxygen will form carbon dioxide when the graphite burns. When there is less available oxygen, like when the reaction takes place in an area with limited air supply, another reaction will occur as well, where another product is formed. In this substance, each carbon atom is attached to one oxygen atom instead of two. Instead of carbon dioxide, carbon monoxide is formed, where “mono” means one. The name of the compound is contracted from mono oxide to monoxide.
Carbon dioxide and carbon monoxide. They sound almost the same, and both are made up of carbon and oxygen, but there is an important difference between them. Carbon dioxide is naturally present in the atmosphere. There’s not much—only four hundredths of one percent—but it’s vital for all plant life. The air you exhale contains about a hundred times that amount, about four percent carbon dioxide. If there is a lot of carbon dioxide in a room, the air feels bad, but it is not dangerous to inhale.
Carbon monoxide, on the other hand, is poisonous for humans and animals. If the air you inhale contains as little as one percent carbon monoxide, that’s enough to kill you within minutes. Carbon monoxide is formed not only when pure graphite burns. Other combustible substances that contain carbon, such as petrol, oil, plastic, or wood, can also form carbon monoxide.
It’s called incomplete combustion and occurs as soon as the oxygen level gets too low. In a house fire, the carbon monoxide produced is particularly dangerous. It can make the people in the house unconscious before they have time to get out, or they can even die in their sleep without even noticing there is a fire. In a fire, there is a greater risk of dying from carbon monoxide poisoning than from the flames.
Good job they had a working smoke detector.
It’s a simplification to say that when carbon compounds burn, we get either carbon dioxide or carbon monoxide. In reality, both reactions take place at the same time. Less oxygen results in more carbon monoxide. Chemical reactions can give different products even though we start with the same reactants, and in this case, where carbon reacts with oxygen, this difference can mean life or death.
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Common sources include:
- Vehicles (cars, trucks, buses) — especially in idling or poorly tuned engines
- Home appliances: gas stoves, kerosene heaters, fireplaces, and portable generators
- Industrial processes: boilers, furnaces, and certain manufacturing operations
Why Is CO So Dangerous?
CO is extremely hazardous because it binds to hemoglobin in your blood ~200 times more tightly than oxygen does. When you inhale CO:
- It displaces oxygen in your bloodstream
- Your heart, brain, and other vital organs are starved of oxygen
- Symptoms progress rapidly:
→ Mild exposure: headache, dizziness, nausea, fatigue
→ Moderate exposure: confusion, blurred vision, difficulty breathing
→ Severe exposure: loss of consciousness, organ damage, death
Critical fact: Because CO has no smell or color, you can't detect it without a monitor. Poisoning can happen quickly—and silently.
How to Stay Safe: Prevention Saves Lives
Never run a vehicle inside a closed garage—even with the door open, fumes can accumulate dangerously.
Never use portable generators, grills, or camp stoves indoors (including garages, basements, or near windows).
Ensure proper ventilation for all fuel-burning appliances (heaters, fireplaces, water heaters).
Install battery-backed CO detectors on every level of your home and near sleeping areas. Test them monthly!
Schedule annual maintenance for furnaces, chimneys, and gas appliances to ensure clean, complete combustion.
While CO is primarily a local air quality and safety issue (unlike CO2, which affects global climate), it highlights an important principle in energy engineering: complete, efficient combustion isn't just about performance—it's about protecting human health. Modern engines, power plants, and appliances use advanced controls, catalytic converters, and sensors specifically to minimize CO emissions.
5.2.3 Sulfur Dioxide
5.2.3 Sulfur DioxideSulfur Dioxide (SO2)
Sulfur dioxide is a colorless gas with a sharp, irritating odor. It forms when sulfur (S) present in fossil fuels reacts with oxygen during combustion:
Why do coal and petroleum contain sulfur?
Fossil fuels formed from ancient organic matter that absorbed sulfur from seawater, sediments, and volcanic activity over millions of years. As a result:
- Coal: Often contains 0.5–5% sulfur by weight (varies by mine location)
- Petroleum: Contains sulfur compounds that are partially removed during refining ("sweet" vs. "sour" crude)
- Natural gas: Typically very low in sulfur (mostly removed before distribution)
The Chemistry of Acid Formation
SO₂ is highly soluble in water. When it mixes with atmospheric moisture, a cascade of reactions occurs:
- SO2 + H2O → H2SO3 (sulfurous acid — weak, but irritating)
- H2SO3 + ½O2 → H2SO4 (sulfuric acid — strong acid, major component of acid rain)
- SO2 also reacts with ammonia, metals, and other pollutants to form sulfate particles (aerosols)
These fine particles can:
- Remain suspended in air for days to weeks
- Travel hundreds of miles from the original source
- Scatter light → reduced visibility ("haze")
- Penetrate deep into lungs when inhaled
Global Context & Progress
- Historical peak: U.S. SO₂ emissions peaked in the 1970s (~26 million tons/year), largely from coal-fired power plants.
- Policy success: The 1990 Clean Air Act Amendments created a cap-and-trade program for SO2. Result? U.S. SO2 emissions dropped ~94% since 1990—one of environmental policy's biggest wins.
- Current challenges: SO2 remains a major issue in regions with heavy coal use and fewer emissions controls (e.g., parts of Asia, Eastern Europe). Satellite data now helps track global SO2 hotspots in near real-time.
How Do We Reduce SO2 Emissions?
| Strategy | How It Works | Example |
|---|---|---|
| Flue Gas Desulfurization (FGD) | "Scrubbers" spray limestone slurry into exhaust; SO2 reacts to form gypsum (usable in drywall) | >90% of U.S. coal plants now use scrubbers |
| Fuel Switching | Use low-sulfur coal, natural gas, or renewables instead of high-sulfur coal | U.S. shift from coal → gas cut SO2 dramatically |
| Fuel Desulfurization | Remove sulfur from petroleum during refining (hydrodesulfurization) | Ultra-low-sulfur diesel (ULSD) now standard in vehicles |
| Policy & Monitoring | Emissions caps, continuous monitoring, international agreements | Acid Rain Program; WHO air quality guidelines |
5.2.4 Nitrogen Oxides
5.2.4 Nitrogen OxidesNitrogen Oxides (NOx)
Nitrogen oxides is a generic term for a group of highly reactive gases containing nitrogen and oxygen. The two most important for air quality are:
- Nitric oxide (NO): Colorless, odorless, formed first during combustion
- Nitrogen dioxide (NO2): Reddish-brown gas with a sharp, biting odor; forms when NO reacts with oxygen in the air
How Is NOx Formed?
NOx forms primarily through high-temperature combustion—when nitrogen (N2) and oxygen (O2) from the air react under intense heat. This is called thermal NOx formation:
Key insight: Even if a fuel contains no nitrogen, NOx can still form because air itself is 78% nitrogen. The hotter the flame and the longer gases stay at high temperature, the more NOx is produced.
| Source Category | Examples | Why It Matters |
|---|---|---|
| Transportation | Cars, trucks, buses, ships, aircraft | High-temperature engines; major source in urban areas |
| Electric Power Generation | Coal, oil, and natural gas power plants | Large, continuous combustion sources |
| Industrial/Commercial | Boilers, furnaces, cement kilns, refineries | Often located near communities |
| Residential | Gas stoves, water heaters, fireplaces | Indoor air quality concern; cumulative urban impact |
Why You Can Sometimes See NOx
While NO and many NOx compounds are invisible, nitrogen dioxide (NO2) has a distinctive reddish-brown color. When mixed with other pollutants (like volatile organic compounds and fine particles), it contributes to:
- Photochemical smog: The hazy, brownish layer over cities on sunny days
- Urban haze: Reduced visibility in metropolitan areas and national parks
Text description of the Sunrise towards a smog ridden Los Angeles downtown image.
The image showcases a panoramic view of Los Angeles, dominated by skyscrapers in the background. The skyline is shrouded in a hazy smog. The foreground features a major highway filled with vehicles, curving through the city landscape.
| Strategy | How It Works | Real-World Example |
|---|---|---|
| Catalytic Converters | Use platinum/palladium to convert NOx→ N2 + O2 in vehicle exhaust | Required on all U.S. gasoline vehicles since 1975 |
| Low-NOx Burners | Stage fuel/air injection to lower flame temperature and limit NOx formation | Standard in modern power plants and industrial boilers |
| Selective Catalytic Reduction (SCR) | Inject ammonia/urea into flue gas; catalyst converts NOx to harmless N2 + H2O | Used in >80% of U.S. coal plants and many diesel trucks |
| Electrification & Efficiency | Reduce combustion overall by switching to electric vehicles, heat pumps, renewables | Transportation electrification is the fastest-growing NOx reduction strategy |
| Policy Tools | Emissions standards, cap-and-trade programs, urban low-emission zones | California's Advanced Clean Cars program; EU Euro emissions standards |
5.2.5 Particulate Matter
5.2.5 Particulate MatterParticulate Matter (PM)
Particulate matter (PM) is a mixture of tiny solid particles and liquid droplets suspended in the air. Think of it as a "soup" of microscopic materials—from dust and soot to sulfates, nitrates, and organic compounds.
| Particle Type | Diameter | Visual Comparison | Can Reach... |
|---|---|---|---|
| Coarse PM (PM2.5-10) | 2.5–10 µm | Pollen, mold spores | Nose, throat, upper airways |
| Fine PM (PM2.5) | ≤ 2.5 µm | Smoke, bacteria | Deep lungs (alveoli), bloodstream |
| Ultrafine PM (PM0.1) | ≤ 0.1 µm | Viruses, combustion nanoparticles | Alveoli, potentially cross into blood |
Scale check: A human hair is ~70 µm wide. PM2.5 is 30× smaller—small enough to bypass your body's natural defenses.
Text description of the Size comparisons for PM particles image.
The image visually compares the size of different particles relative to a human hair. It features a large, detailed depiction of a human hair, shown horizontally in a gray hue with visible texture. Below the hair, there are three irregularly shaped particles labeled as "Fine Beach Sand" in various shades of green, orange, and yellow, with a diameter marked as 90 microns. Above and to the right of the hair, there are smaller, round particles in two groups. One group consists of pink spheres labeled "PM2.5," representing combustion particles, organic compounds, and metals, each less than 2.5 microns in diameter. The second group consists of blue spheres labeled "PM10," representing dust, pollen, mold, and other substances, each less than 10 microns in diameter. Arrows point from these groups to indicate their respective sizes relative to the human hair.
Where Does Particulate Matter Come From?
PM forms through two main pathways:
| Type | How It Forms | Common Energy-Related Sources |
|---|---|---|
| Primary PM | Emitted directly during combustion or physical processes | • Soot from diesel engines • Fly ash from coal plants • Dust from mining, construction, unpaved roads |
| Secondary PM | Forms in the atmosphere when gases react | • SO2 → sulfate particles • NOx + VOCs → nitrate particles + organic aerosols • Ammonia (from agriculture) + acids → ammonium salts |
Key insight: Even if a power plant installs filters to catch primary PM, it may still contribute to secondary PM downwind through gas emissions. This is why controlling SO2 and NOx also reduces particulate pollution.
How PM Affects Your Body: It's All About Size
Your respiratory system has natural filters—but PM can bypass them:
| Particle Size | Where It Deposits | Why It Matters |
|---|---|---|
| > 10 µm | Nasal passages, throat | Usually trapped and cleared by mucus/cilia |
| 2.5–10 µm | Upper airways, bronchi | Can irritate airways; trigger coughing, asthma |
| 0.1–2.5 µm | Deep lungs (alveoli) | Most dangerous: can cause inflammation, enter bloodstream |
| < 0.1 µm | Alveoli; may cross into blood | Emerging research links to cardiovascular effects |
| Strategy | How It Works | Real-World Example |
|---|---|---|
| Electrostatic Precipitators (ESPs) | Charge particles electrically; collect them on plates | Used in >90% of U.S. coal plants; >99% efficient for fly ash |
| Fabric Filters (Baghouses) | Force exhaust through fine fabric that traps particles | Common in cement plants, biomass facilities |
| Diesel Particulate Filters (DPFs) | Trap soot in vehicle exhaust; periodically burn it off | Required on modern diesel cars/trucks in U.S., EU |
| Fuel Switching & Efficiency | Reduce combustion overall: renewables, electrification, efficiency | Replacing coal with wind/solar cuts PM at the source |
| Policy & Monitoring | Air quality standards, emissions limits, public alerts | EPA's National Ambient Air Quality Standards (NAAQS) for PM2.5/PM10 |
Particulate matter (PM) is the general term used to describe a mixture of solid particles and liquid droplets found in the air. Some particles are large enough to be seen as dust or dirt. Others are so small they can be detected only with an electron microscope.