Pollutants from Burning Fuels

Introduction

A perfectly pure hydrocarbon burning completely would give only carbon dioxide and water. Real fuels and real engines are messier. Coal and crude oil contain impurities, air is mostly nitrogen, and combustion is never perfectly complete. As a result, burning fuels releases a cocktail of other pollutants. Three of the most important are sulfur dioxide, nitrogen oxides and particulates, each with its own origin, effects and solution.

Core explanation

Sulfur dioxide (SO₂). Many fossil fuels, especially coal and heavy oils, contain small amounts of sulfur compounds left over from the organisms they formed from. When the fuel burns, the sulfur is oxidised: S + O₂ → SO₂. Sulfur dioxide is a colourless gas with a choking smell. It irritates the airways and can trigger asthma attacks. In the atmosphere it dissolves in water droplets and is further oxidised, eventually forming sulfuric acid, a major cause of acid rain .

Nitrogen oxides (NOₓ). Nitrogen is normally very unreactive. But in the extremely hot conditions inside engines, power-station furnaces and jet turbines, nitrogen and oxygen from the air combine: N₂ + O₂ → 2NO. Nitrogen monoxide then reacts with more oxygen in the air: 2NO + O₂ → 2NO₂. Together these are called NOₓ. The key point is that the nitrogen comes from the air , not from the fuel, so even a nitrogen-free fuel can produce NOₓ if it burns hot enough. Nitrogen dioxide is a brown, toxic gas that damages the lungs, contributes to acid rain and helps form photochemical smog in sunlight.

Particulates. Incomplete combustion releases tiny particles of carbon (soot) and unburnt hydrocarbons. They are grouped by size: PM₁₀ are smaller than 10 micrometres and PM₂.₅ smaller than 2.5 micrometres. The smallest penetrate deep into the lungs and even into the bloodstream, and are linked to heart and lung disease. Particulates also darken buildings and can reduce sunlight reaching the ground, a phenomenon called global dimming.

Effects of acid rain. Acid rain damages limestone and marble buildings and statues, harms trees by damaging leaves and removing nutrients from soil, and makes lakes and rivers too acidic for fish and other aquatic life.

Reducing the emissions:

- Desulfurisation — sulfur is removed from petrol and diesel at refineries, so modern road fuels are very low in sulfur. - Flue-gas scrubbing — in power stations, exhaust gases pass through a slurry of calcium carbonate or calcium oxide, which reacts with and captures SO₂. - Catalytic converters — in car exhausts, platinum-group metals catalyse reactions such as 2CO + 2NO → 2CO₂ + N₂, removing both CO and NO at once. - Particulate filters — diesel vehicles trap soot in a filter that is periodically burnt clean.

Step-by-step reasoning

To identify the source of a pollutant:

1. Look at which elements it contains. 2. If it contains sulfur, the sulfur must have come from impurities in the fuel. 3. If it contains nitrogen, check whether the fuel contains nitrogen; if not, it came from air at high temperature. 4. If it is carbon or CO, it came from incomplete combustion of the fuel.

Visual explanation

Picture a car exhaust pipe with arrows labelled leaving it: CO₂ and H₂O from complete combustion, CO and soot from incomplete combustion, NOₓ from the hot air inside the cylinders and a trace of SO₂ from sulfur in the fuel. A box labelled "catalytic converter" part-way along the pipe turns the CO and NOₓ arrows into CO₂ and N₂.

Real-world analogy

A kitchen that cooks a meal produces not only the dinner but washing-up, smoke and scraps. Improving the recipe (cleaner fuel), using an extractor fan (scrubbers and filters) and cooking at the right heat (engine design) all reduce the mess without stopping the cooking.

Real-world example

In the 1970s and 1980s, acid rain badly damaged forests and lakes in Scandinavia, central Europe and North America. International agreements to fit scrubbers to power stations and reduce sulfur in fuels cut sulfur dioxide emissions dramatically, and many affected lakes have since slowly recovered.

Why?

Why do nitrogen and oxygen not react in ordinary air? The triple bond in N₂ is very strong, so the reaction has a very high activation energy. Only the extreme temperatures in engines and furnaces provide enough energy for a significant amount of N₂ to react.

Common misconception

"Nitrogen oxides come from nitrogen in the fuel." Petrol contains almost no nitrogen. Most NOₓ from vehicles forms from nitrogen in the air drawn into the hot engine.

Worked example

Question: A power station burns 1000 kg of coal containing 2% sulfur by mass. What mass of sulfur dioxide could form? (S = 32, O = 16)

Reasoning: Mass of sulfur = 2% × 1000 = 20 kg. S + O₂ → SO₂: 32 g of S gives 64 g of SO₂, a ratio of 2 : 1 by mass. So 20 kg of S gives 40 kg of SO₂.

Answer: 40 kg of sulfur dioxide.

Quick check

1. Why do car engines produce nitrogen oxides even though petrol contains no nitrogen? Answer: The high temperature in the engine makes nitrogen and oxygen from the air react together.

Exam focus

Match each pollutant to its source and effect: SO₂ — sulfur impurities — acid rain and breathing problems; NOₓ — air at high temperature — acid rain, smog and lung damage; particulates — incomplete combustion — lung and heart disease, global dimming. Know one control method for each.

Advanced insight

A catalytic converter works best only within a narrow ratio of air to fuel, so modern engines use an oxygen sensor in the exhaust to adjust fuel injection many times a second. There is a trade-off: hotter, leaner combustion reduces CO and soot but tends to increase NOₓ, so engineers must balance competing pollutants.

Summary

Burning fuels releases more than CO₂ and water. Sulfur impurities form SO₂; high temperatures make atmospheric nitrogen and oxygen form NOₓ; incomplete combustion releases particulates. These cause acid rain, smog and lung and heart disease. Desulfurisation, scrubbers, catalytic converters and particulate filters reduce emissions.

Practice questions

1. Write an equation for the formation of sulfur dioxide from sulfur. Answer: S + O₂ → SO₂. 2. Give two environmental effects of acid rain. Answer: For example, it damages limestone buildings and makes lakes too acidic for fish. 3. What two pollutants does the reaction 2CO + 2NO → 2CO₂ + N₂ remove, and where does it take place? Answer: Carbon monoxide and nitrogen monoxide; in a vehicle's catalytic converter. 4. Why are PM₂.₅ particles considered more harmful than larger particles? Answer: They are small enough to travel deep into the lungs and even into the bloodstream. 5. Describe one way sulfur dioxide emissions from power stations are reduced. Answer: Exhaust gases are passed through a calcium carbonate or calcium oxide slurry, which reacts with and removes the SO₂.