Combustion and the Environment

Introduction

Burning fuels heats our homes, powers most of our transport and generates much of our electricity. But the products of combustion do not simply disappear: they enter the atmosphere, where they can harm health, damage buildings and ecosystems and change the climate. This page brings together what you know about complete and incomplete combustion to explain the main pollutants from burning and the ways chemists and engineers reduce them.

Core explanation

Where the products come from. Every product of burning can be traced to the atoms present in the fuel or the air:

Pollutant Source Main effect --- --- --- Carbon dioxide, CO₂ Carbon in any fuel (complete combustion) Greenhouse gas; climate change; ocean acidification Carbon monoxide, CO Incomplete combustion Toxic Soot (particulates) Incomplete combustion Lung and heart disease; blackens buildings; global dimming Sulfur dioxide, SO₂ Sulfur impurities in coal and some oil Acid rain; breathing problems Nitrogen oxides, NOₓ Nitrogen and oxygen from the air reacting at high temperature in engines Acid rain; smog; breathing problems Unburnt hydrocarbons Fuel that escapes burning Contribute to smog

Carbon dioxide and climate. Carbon dioxide is not toxic at normal levels, but it is a greenhouse gas . It absorbs infrared radiation given off by the warm Earth, trapping heat in the atmosphere. Burning fossil fuels has raised atmospheric CO₂ from about 280 parts per million before the Industrial Revolution to over 420 parts per million today, contributing to global warming. Dissolved CO₂ also makes seawater slightly more acidic.

Acid rain. Sulfur dioxide and nitrogen oxides dissolve in rainwater to form acids. Acid rain damages trees, makes lakes too acidic for fish, and corrodes limestone buildings and metal structures — linking back to the corrosion ideas in this unit.

Nitrogen oxides are special. They do not come from the fuel at all. At the very high temperatures inside an engine, nitrogen and oxygen from the air react together. Hotter burning can therefore increase NOₓ even while it reduces soot.

Cleaner burning. Several strategies reduce harm:

- Plenty of air ensures complete combustion, cutting carbon monoxide and soot. - Removing sulfur from fuels before use, or removing sulfur dioxide from power-station chimneys (flue gas desulfurisation, often with calcium carbonate). - Catalytic converters in vehicles convert carbon monoxide, nitrogen oxides and unburnt hydrocarbons into carbon dioxide, nitrogen and water. - Particulate filters trap soot from diesel engines. - Using less fuel and switching fuels: insulation, efficient engines, renewable electricity and, where possible, fuels such as hydrogen whose combustion produces only water.

Step-by-step reasoning

To explain where a pollutant comes from:

1. Identify the atoms in the pollutant. 2. Decide whether they came from the fuel (carbon, hydrogen, sulfur) or from the air (nitrogen, oxygen). 3. Decide whether the pollutant forms in complete or incomplete combustion, or from high temperature. 4. Link it to its effect and a way of reducing it.

Visual explanation

Picture a car exhaust pipe with arrows streaming out, each labelled with a molecule: CO₂, H₂O, CO, NO₂, C (soot). Then add a honeycomb-shaped catalytic converter partway along the pipe: on the far side the arrows now show mostly CO₂, N₂ and H₂O, with far less CO and NO₂.

Real-world analogy

The atmosphere is like a shared room where everyone's exhaust ends up. A single candle makes no noticeable difference, but billions of engines, boilers and power stations add up, just as many people each dropping a little litter can fill a park.

Real-world example

In the mid-twentieth century, burning coal in homes and factories caused severe smogs in cities such as London; the 1952 Great Smog is estimated to have killed thousands of people. Clean air laws that restricted smoky fuels, followed later by removing sulfur from fuels and fitting catalytic converters, greatly reduced soot and sulfur dioxide levels.

Why?

Why does burning a "clean" fuel such as natural gas still add to climate change? Even complete combustion turns all the carbon in the fuel into carbon dioxide. Cleaner burning reduces carbon monoxide and soot, but only using less carbon-based fuel reduces carbon dioxide emissions.

Common misconception

"Carbon dioxide causes acid rain and the ozone hole." Acid rain is mainly caused by sulfur dioxide and nitrogen oxides, and the ozone hole was caused mainly by CFCs. Carbon dioxide's main effect is as a greenhouse gas driving climate change.

Worked example

Question: Explain why low-sulfur diesel reduces acid rain but does not reduce carbon dioxide emissions.

Reasoning: Sulfur dioxide comes from sulfur impurities; carbon dioxide comes from the carbon in the hydrocarbons themselves.

Answer: With less sulfur in the fuel, less sulfur dioxide forms, so less acid rain. The fuel still contains the same carbon, which burns to carbon dioxide.

Quick check

1. Which pollutant is produced when nitrogen from the air reacts in a hot engine? Answer: Nitrogen oxides (NOₓ).

Exam focus

Be precise about each pollutant's source and effect; mixing them up loses marks. Key pairs: CO₂ → greenhouse effect; CO → toxic; SO₂ and NOₓ → acid rain; particulates → breathing problems and global dimming. Know how a catalytic converter helps.

Advanced insight

A catalytic converter uses metals such as platinum, palladium and rhodium spread thinly over a ceramic honeycomb. Reactions include 2CO + 2NO → 2CO₂ + N₂. The catalyst is not used up, but it works properly only when hot, so most pollution from a car comes in the first few minutes after a cold start.

Summary

Burning fuels releases carbon dioxide, a greenhouse gas, and can produce carbon monoxide, soot, sulfur dioxide, nitrogen oxides and unburnt hydrocarbons. These cause climate change, toxic effects, acid rain and smog. Plenty of air, desulfurisation, catalytic converters, filters and using less fossil fuel all make combustion cleaner.

Practice questions

1. Why does burning coal containing sulfur contribute to acid rain? Answer: Sulfur burns to form sulfur dioxide, which dissolves in rainwater to form an acidic solution. 2. Name the pollutant that forms only when there is too little oxygen and is toxic. Answer: Carbon monoxide. 3. State one environmental effect of increased carbon dioxide in the atmosphere. Answer: Global warming and climate change (or ocean acidification). 4. What does a catalytic converter do? Answer: It converts harmful exhaust gases such as carbon monoxide and nitrogen oxides into less harmful ones such as carbon dioxide and nitrogen.