4A Fossil fuel air pollution - incomplete combustion, pollution from carbon monoxide, particulates and soot

Anthropogenic means any environmental change or pollution due to human activity

Doc Brown's GCSE/IGCSE/O Level KS4 science-CHEMISTRY Revision Notes

See also

4B Pollution, Accidents and Economic Aspects of the Petrochemical Industry

4C Greenhouse effect, global warming, climate change, carbon footprint from fossil fuel burning

4D Fossil fuel air pollution - effects of sulfur oxides and nitrogen oxides

A local acid rain project!!!

4A Atmospheric pollution: The Incomplete Combustion of hydrocarbons and CO

Carbon dioxide, carbon monoxide, soot, water vapour, sulfur dioxide and oxides of nitrogen are all produced in the combustion of fossil fuels. Carbon dioxide and water vapour are the main 'greenhouse gases'.

INTRODUCTION - burning fossil fuels

• AIR POLLUTION - soot particulates and carbon monoxide

  • There are good reasons why we should do our best to control atmospheric pollution - pollutants build up and create environmental problems for most plants and animals, particularly in the air of many of the world's cities causing ill-health e.g respiratory diseases like asthma and other respiratory conditions.

  • To reduce pollution (and global warming too!) we need to burn less fossil fuels and develop other 'greener energy sources'

  • Sources and pollution problems due to incomplete combustion e.g. soot, particulates and carbon monoxide are dealt below.

  • Pollution and health problems from sulfur oxides and nitrogen oxides are dealt with in section 4D Fossil fuel pollution and effects from sulfur oxides and nitrogen oxides

• Starting with coal combustion, then onto burning hydrocarbons and pollution

• Some organic compounds are used as fuels. Other organic compounds, including plastics, are burned as waste. Burning these organic compounds releases gases into the atmosphere.

  • All organic compounds consist partly of carbon atoms and many contain hydrogen and other atoms such as oxygen and nitrogen. Coal, crude oil, natural gas (methane) and wood contain organic compounds
    • all are used as fuels, either directly like coal or natural gas,
    • or indirectly as coke from coal or petrol from crude oil etc.,
    • and apart from wood, they are finite (limited reserve) fossil (from decayed organic material) fuels.
  • Many hydrocarbons are fuels i.e. a substance burned to release heat energy.
  • When organic compounds are burned in a plentiful supply of air the carbon is oxidised to carbon dioxide and the hydrogen is oxidised to water.
  • In a limited supply of air incomplete combustion occurs forming carbon monoxide and/ or carbon.
    • Carbon monoxide is poisonous because it reduces the capacity of blood to carry oxygen.
  • Combustion equations and tests for combustion products are all on the Oil Notes 3. web page, lots of examples and diagrams too.
  • Each fossil fuel has a different cost, efficiency and cleanliness on burning. Generally speaking in 'cleanliness' the order is methane (natural gas) > alkanes in petrol > heavy oil and from left to right there is also an increase in C/H atom ratio in the molecule so more CO₂ produced too.
  • The combustion of plastics (and other organic compounds) which contain chlorine and nitrogen produce poisonous fumes when burnt e.g. choking hydrogen chloride HCl and toxic hydrogen cyanide HCN respectively. Especially where there is a limited supply of air. The combustion products of carbon (toxic CO and CO₂) and hydrogen (H₂O) are also formed.

• The fuel coal consists mainly of carbon, which, if burned/ignited in excess air, combusts to form carbon dioxide.

  • carbon + oxygen ====> carbon dioxide

  • C_(s) + O_2(g) ====> CO_2(g)

  • 

  • This is what you expect to happen in an open domestic coal fire.

• If not enough air/oxygen is available, coal will only 'half' burn to form the deadly odourless, colourless and toxic gas carbon monoxide.

  • 2C_(s) + O_2(g) ====> 2CO_(g)

  • This can happen if organic material, coal or peat or any combustible material is smouldering underground and is obviously a dangerous situation.

  • If it was formed in a domestic coal fire it will quite happily burn with a pale blue flame to form the 'safe' combustion product carbon dioxide gas.

  • 2CO_(s) + O_2(g) ====> 2CO_2(g)

  • However in house fires, there tends to be a lack of oxygen and fumes of smoke and carbon monoxide are formed, which probably causes more deaths in fires than being burnt,

• When fossil fuels burn efficiently in an excess of air/oxygen the main products are carbon dioxide and water e.g.

• examples of complete combustion burning are for example natural gas and petrol

  • Methane + oxygen ====> water + carbon dioxide

    • CH_4(g) + 2O_2(g) ===> CO_2(g) + 2H₂O_(l)

    • 

  • pentane + oxygen ====> water + carbon dioxide

    • C₅H_12(g) + 8O_2(g) ====> 5CO_2(g) + 6H₂O_(l)

  • See 3. ALKANES - saturated hydrocarbons and combustion

• Power stations burn huge quantities of fossil fuels in the form of mainly coal, but significant quantities of gas and some oil. The heat from the very exothermic combustion is used turn water into steam and steam driven turbines power the electrical generators (very big dynamos!).

CARBON MONOXIDE, soot and hydrocarbon particulates and incomplete combustion

• Carbon monoxide is a toxic gas and unfortunately it is colourless and odourless and so not easily detected.

  • Carbon monoxide combines with haemoglobin in the blood, reducing its capacity to carry oxygen.

  • CO molecules will displace O₂ oxygen molecules from haemoglobin depriving cells of the essential oxidant O₂ !

  • Lack of oxygen can lead to feeling weak, fainting and reduction of brain function. If you breath in more than traces over a period of time you can go into a coma and death - CO is pretty toxic and lethal dose isn't that high!

• If there is not enough oxygen present to completely burn the fuel to carbon dioxide and water other products may form causing pollution and fuel inefficiency.

  • This is referred to as incomplete combustion.

  • Visually, blue flames indicate complete combustion releasing lots of heat energy, but smokey yellow flames indicate incomplete combustion releasing less energy.

• The most common partially burned products are likely to be carbon C (soot) and deadly carbon monoxide CO.

  • A simplified word equation covering most possibilities is ...

    • hydrocarbon + oxygen (air) ==> carbon (soot) + carbon monoxide + carbon dioxide + water

  • Carbon-soot, a fine black powder-dust is potentially harmful and readily formed in fires i.e. its classically produced by smoky yellow flames and inefficient motor vehicle engines.

    • The soot, like any fine solid 'dust' is harmful when absorbed on the sensitive tissue of the linings of the nose, throat and lungs.

    • Black sooty marks indicate incomplete combustion and soot deposits cause coughing and sore throat and are ejected from your body through sneezing, coughing, and nose blowing. Respiratory problems can follow.

    • Coarse solid particles (10 microns in diameter) are inhaled into your windpipe and settle there, causing irritation and more coughing and ultimately lung damage.

    • Soot particles also are 'carriers' of polycyclic aromatic hydrocarbons (PAH's) on it which are carcinogenic molecules (cause DNA damage - cancer forming) of general formula C_xH_y, where x and y are at least 6.

      • These are examples of unburned hydrocarbons which are emitted from vehicle exhausts as particulates, especially from diesel engines (much more so than emissions from petrol engines).

      • These fine particles of soot and hydrocarbons are called particulates, they reduce air quality and are known to cause health problems, particularly with our respiratory system.

      • Particulates are also believed to cause a phenomena known as 'global dimming', because they scatter sunlight and cause a cooling effect.

  • Even very low concentrations of carbon monoxide can be fatal. Why?

    • Oxygen is carried around the body by a complicated protein molecule in red blood cells called haemoglobin.

    • The bonding between oxygen and haemoglobin is quite weak to allow easy oxygen transfer for cell respiration.

    • Unfortunately, the bonding between carbon monoxide and haemoglobin is stronger, so oxygen is replaced by carbon monoxide and blocks normal cell respiration.

    • The consequences are reduced blood oxygen concentration, particularly affecting the brain, and can lead to fainting-unconsciousness, a coma and eventually death!

    • This is why long road tunnels are ventilated and you should never run a car engine in a closed garage!

• It would appear that the hydrogen in the fuel molecules is more easily burned (oxidised) and usually forms water so the equations for incomplete combustion below show the formation of carbon-soot and 'deadly' carbon monoxide when there is a lack of oxygen for complete combustion.

  • Particulate carbon doesn't get the headlines like acid rain, but it is a serious problem that you get from burning coal and diesel fuel too.

  • As mentioned already, soot is obviously a 'dirty' pollutant coating any surface (including your lungs!) that the soot particles settle on - dark colouration on buildings from the Industrial Revolution the emergence of steam powered Victorian technology.

  • Soot particles of carbon also contain unburned carcinogenic hydrocarbons AND carbon monoxide is involved in the chemistry of photochemical smogs - so all in all, inefficient combustion of fossil-hydrocarbon fuels is very undesirable!

• Incomplete combustion is still exothermic, but there is also less heat released in incomplete combustion compared to complete combustion since not all the carbon atoms of the fuel are fully combined with the maximum amount of oxygen.

  • e.g. for the incomplete combustion of methane possible word and symbol equations are as follows ...

    • (a) methane + oxygen ====> carbon (soot) + water

      • CH_4(g) + O_2(g) ====> C_(s) + 2H₂O_(l)

      • 

      • for soot (mainly carbon) formation, smokey yellow flame

    • and

    • (b) methane + oxygen ====> carbon monoxide + water

      • 2CH_4(g) + 3O_2(g) ====> 2CO_(g) + 4H₂O_(l)

      • 

      • for the formation of carbon monoxide

    • For larger hydrocarbon molecules all sorts of equations can be written showing the formation of carbon-soot particles and carbon monoxide molecules e.g.

    • A liquid alkane in petrol:

      • (i) C₇H₁₆(l)  +  4O₂(g)  ===>  7C(s)  +  8H₂O(l)

      • (ii) C₇H₁₆(l)  +  7½O₂(g)  ===>  7CO(g)  +  8H₂O(l)

      • Some of these hydrocarbons can pass through the engine without being oxidised.

      • Unburned hydrocarbons belong to a group of chemicals known as volatile organic compounds (VOCs).

      • In the atmosphere the VOCs react with nitrogen oxides (e.g. NO₂) in sunlight to produce a photochemical smog haze - a very unpleasant and harmful concoction!

        • Photochemical smog contains many chemicals that can irritate the eyes, throat and lungs.

        • (more on catalytic converters in section 4D)

    • These incomplete combustion reactions cause the formation of soot particulates and carbon monoxide e.g. from a car exhaust where the catalytic converter doesn't work.

• Therefore it is extremely important that any combustion system is as efficient as possible e.g. gas heaters, furnaces etc. must all have excellent ventilation for complete combustion to harmless water and carbon dioxide.

• If there is any smell of gas, make sure (i) all appliances are turned off, (ii) all sources of ignition are absent, and (iii) ring the gas board!

• Faulty gas appliances have led to tragic deaths.

  • Carbon monoxide is colourless and odourless and even low concentrations in the air can be fatal.

  • It also accounts for why long road tunnels need to be well ventilated too to avoid any toxic effects.

  • Hot water heaters, gas fires and central heating boilers, should all be regularly serviced to ensure they burn efficiently and not produce any potentially harmful levels of toxic carbon monoxide.

  • All gas fired appliances

CARBON MONOXIDE - catalytic removal (more on catalytic converters in section 4D)

• Carbon monoxide is unfortunately emitted by all car exhausts, though catalytic converters help reduce this by converting nitrogen monoxide (another pollutant, See oxides of nitrogen pollution) and carbon monoxide into harmless nitrogen and carbon dioxide.

  •  2NO_(g) + 2CO_(g) ====> N_2(g) + 2CO_2(g)

  • Transition metals like platinum and rhodium are used in the catalytic converter which can remove over 90% of the harmful gases.

    • For more on this see catalytic converters in section 4D

  • Nitrogen monoxide, NO, is formed by the combination of nitrogen and oxygen at high temperature (up to 1000^oC) in automobile engines (cars, lorries, buses etc. - its all the same!)

  • N_2(g) + O_2(g) ====> 2NO_(g)

  • Nitrogen monoxide readily forms nitrogen dioxide (nitrogen(IV) oxide) by combining with oxygen in air on exit from the engine exhaust.

  • 2NO_(g) + O_2(g) ====> 2NO_2(g)

    • Note: Nitrogen monoxide is also known as 'nitric oxide' or 'nitrogen(II) oxide'

  • Nitrogen dioxide is a lung and eye irritant, and, along with nitrogen monoxide, it is involved in the complex chemistry of photochemical smogs which can also produce ozone and other harmful chemicals in the air.

  • The reduction of fossil fuel burning is the only way to reduce photochemical smog e.g. using photovoltaic cells to harness solar energy to produce electricity.

  • Using solar power indirectly in this way to run electric cars is potentially a good partial solution to the problem.

For other notes on nitrogen oxides see Fossil fuel air pollution - effects of sulfur oxides and nitrogen oxides notes

See also pollution problems of burning plastics

PLEASE READ CAREFULLY - from an article by George Monbiot "What car fumes do to our children"

Other useful pages

Multiple Choice Quizzes and Worksheets

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (easier-foundation-level)

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (harder-higher-level)

KS4 Science GCSE/IGCSE m/c QUIZ on other aspects of Organic Chemistry

and 3 linked easy Oil Products gap-fill quiz worksheets

ALSO gap-fill ('word-fill') exercises originally written for ...

... Ex AQA GCSE Science Useful products from crude oil AND Oil, Hydrocarbons & Cracking etc.

... Ex OCR 21st C GCSE Science Worksheet gap-fill C1.1c Air pollutants etc ...

... Edexcel GCSE Science Crude Oil and its Fractional distillation etc ...

... each set are interlinked, so clicking on one of the above leads to a sequence of several quizzes