AP Syllabus focus:
‘Burning coal releases pollutants such as carbon dioxide, sulfur dioxide, toxic metals, and particulate matter.’
Coal-fired power and industrial boilers generate electricity and heat by burning coal, but the fuel’s carbon, sulfur, mineral ash, and trace elements produce several major air pollutants with important environmental and health impacts.
What makes coal a high-pollution fuel?
Coal is a carbon-rich rock that commonly contains:
Carbon (forms CO2 when oxidised)
Sulfur impurities (form SO2 when oxidised)
Mineral matter (“ash”) (becomes airborne particles when entrained in exhaust)
Trace toxic elements (metals and metalloids that can volatilise or attach to particles)
Carbon dioxide (CO2)
Carbon dioxide (CO2) is produced when coal’s carbon is fully oxidised during combustion. CO2 is the dominant emission by mass from coal burning because coal is primarily carbon.
Environmental significance: CO2 is a greenhouse gas that increases radiative forcing, contributing to global climate change.
Coal tends to have high CO2 emissions per unit electricity because producing the same energy often requires burning more carbon compared with many other fuels.
Sulfur dioxide (SO2)
Sulfur in coal oxidises during combustion to form sulfur dioxide (SO2), a pungent, reactive gas that can damage living tissues and materials.
Sulfur dioxide (SO2): A sulfur-containing gas produced mainly by burning sulfur-bearing fuels; it irritates the respiratory system and can be transformed in air into sulfur-containing particles and acids.
SO2 is especially important because it can:
Irritate airways and worsen asthma and other respiratory conditions
Contribute to visibility reduction (haze) when converted to sulfate particles
Increase ecosystem stress when deposited to land or water as acidic or sulfate-containing compounds
Toxic metals and other hazardous elements
Coal contains small amounts of toxic metals (and related elements) that can be released in flue gas or bound to particles, including:
Mercury (Hg) (can travel long distances before depositing)
Lead (Pb), arsenic (As), cadmium (Cd), chromium (Cr) (often associated with fine particles)
Other trace elements depending on coal source
These pollutants matter because:
Many are persistent (do not break down) and can cycle between air, soil, and water
Some can bioaccumulate in organisms and biomagnify through food webs after deposition (notably mercury in aquatic systems)
Exposure risks include neurological, developmental, and cardiovascular effects, depending on the metal and dose
Particulate matter (PM)
Coal combustion generates particulate matter from ash, soot-like carbon particles, and particles formed from gases in the plume.
This diagram compares particle sizes relevant to air pollution, showing that PM (≤10 μm) and PM (≤2.5 μm) are far smaller than familiar references like a human hair and fine sand. The figure also labels common sources/composition, reinforcing why combustion-generated PM is a major health concern due to its ability to penetrate deep into the respiratory system. Source
Smaller particles are generally more harmful because they penetrate deeper into the lungs.
Particulate matter (PM): A mixture of solid particles and liquid droplets suspended in air (often reported as PM10 and PM2.5 based on particle diameter), which can harm health and reduce visibility.
Key characteristics of coal-related PM:
Often contains fly ash (mineral particles) plus adsorbed metals and other compounds
Fine PM (PM2.5) is strongly linked to cardiopulmonary health effects because it can reach the alveoli
Causes haze by scattering and absorbing light, reducing visibility in urban and regional airsheds
Why these pollutants often occur together
A single coal plume can contain CO2 (from carbon), SO2 (from sulfur), metals (from trace elements), and PM (from ash and combustion by-products) simultaneously because they originate from different components of the same fuel and the high-temperature combustion process.
Core impacts students should associate with coal combustion pollutants
CO2: climate warming via the greenhouse effect
SO2: respiratory irritation and transformation into sulfate particles and acidic compounds
Toxic metals: long-range transport and deposition; persistence and potential food-web contamination
PM: respiratory and cardiovascular harm; reduced visibility; particle-bound delivery of toxic substances
FAQ
Lower-rank coals often have higher moisture and variable sulfur/ash, which can raise emissions per unit electricity. Higher-rank coals generally have more carbon per mass, but actual emissions depend strongly on sulfur and ash content of the specific seam.
Some emitted mercury is in gaseous forms that remain airborne longer before oxidising and depositing. This enables regional to global transport, with later deposition into waters where it can be converted into biologically available forms.
PM10 includes coarse particles (up to 10 µm) that tend to deposit in the upper airways. PM2.5 is finer (up to 2.5 µm), penetrates deeper into lungs, and often carries more adsorbed toxic compounds per mass.
No. Volatile elements may exit as gases and later condense, while less volatile metals are more likely to remain bound to fly ash. Chemical form affects transport distance, deposition, and toxicity.
Dry handling, storage piles, and spills can generate windblown dust containing fine particles and trace metals. Covering, wetting, and secure containment reduce re-suspension into the air.
Practice Questions
Identify two pollutants released by burning coal and state one harmful effect of each. (2 marks)
1 mark: Correctly identifies any two of: , , toxic metals (e.g. mercury/lead), particulate matter.
1 mark: States one valid harm for each identified pollutant (e.g. → climate warming; → respiratory irritation; metals → toxic/persistent deposition; PM → lung/cardiovascular harm or reduced visibility).
Explain how coal composition leads to the emission of (i) , (ii) , (iii) toxic metals, and (iv) particulate matter, and describe one environmental or health impact for any two of these pollutant categories. (6 marks)
1 mark: Coal carbon oxidises → .
1 mark: Coal sulfur oxidises → .
1 mark: Trace elements in coal released/volatilised or attach to particles → toxic metals in emissions.
1 mark: Mineral ash/combustion by-products become airborne → particulate matter.
1 mark: One correct impact linked to one named category (e.g. PM → cardiopulmonary harm; → warming).
1 mark: Second correct impact linked to a different named category (e.g. metals → bioaccumulation risk after deposition; → respiratory irritation/haze).
