Fuels: combustion and emissions

In a nutshell

Hydrocarbons can undergo combustion. However, a consequence of combustion is the production of toxic emissions such as carbon monoxide. Fossil fuels are a non-renewable source of energy. Alternatives, such as biofuels are being looked into and used but they also pose another set of problems.

Equations

These are the general word equations. You will need to be able to use these to write equations with specific substances.

​$$alkane + oxygen \rightarrow​ carbon \space dioxide + water \space (+ \space e^*) \\ \ \\alkane + oxygen \rightarrow​ carbon \space particulates + carbon \space monoxide + water \space (+ \ e^{**}) \\ \ \\ or \\ \ \\alkane + oxygen \rightarrow​ carbon \space monoxide + carbon \space dioxide + water \space(+ \ e^{**})$$

​$$e^*$$ represents energy

​$$e^{**}$$ represents small amount of energy

Combustion

Combustion is the reaction between a hydrocarbon and oxygen. It is an exothermic reaction.

Complete Combustion

General equation

​ $$alkane + oxygen \rightarrow​ carbon \space dioxide + water \space (+ \space e^*)$$​

​$$e^*$$ represents energy

Example

​$$ethane \space + \space oxygen \rightarrow carbon \space dioxide \space + \space water \\ \ \\2\,C_2H_6 \, + \, 7\, O_2 \rightarrow 4 \,CO_2 \, + \, 6\, H_2O$$​​

​As combustion occurs in the gaseous state, volume can be used to calculate combustion equations. This is because gases have the same molar volume when at the same temperature and pressure. 

Example

What is the molecular formula of $$X$$​, given that $$60cm^3$$ of $$X$$ combusts completely with $$480cm^3$$ of oxygen? $$300cm^3$$ of carbon dioxide is produced.

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Firstly, write out the equation using the volumes provided.

$$60X \, + \, 480 \, O_2 \rightarrow 300 \,CO_2 \,+ \, N \,H_2O$$

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Next, simplify the equation. For this example you can divide by 60.

$$X \, + \, 8 \, O_2 \rightarrow 5 \,CO_2 \,+ \, nH_2O$$

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Any oxygen atoms that are not in carbon dioxide must be in water. This enables you to find the number of water molecules present. Therefore,

$$n \, = (8 \times\,2) - (5 \times\,2) = 6$$

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This implies that there are six molecules of water. So the balanced equation is,

$$X \, + \, 8 \, O_2 \rightarrow 5 \,CO_2 \,+ \, 6 \,H_2O$$

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The equation can be used to identify $$X$$​ as all the carbon atoms will be from molecule $$X$$ to give carbon dioxide and the hydrogen atoms from $$X$$ is found in water. This tells us that there are $$5$$​ carbon atoms and $$12$$ hydrogen atoms in $$X$$. 

Therefore the molecular formula of $$\underline {X \space is \space C_5H_{12}}.$$

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Incomplete combustion

General equation

​

$$alkane + oxygen \rightarrow​ carbon \space particulates + carbon \space monoxide + water \space (+ \ e^{**}) \\ \ \\ or \\ \ \\alkane + oxygen \rightarrow​ carbon \space monoxide + carbon \space dioxide + water \space(+ \ e^{**})$$​​

​

​$$e^{**}$$ represents small amount of energy​

Example

$$ethane \space + \space oxygen \rightarrow carbon \space monoxide \space + \space water \\ \ \\2\,C_2H_6 \, + \, 5\, O_2 \rightarrow 4 \,CO \, + \, 6\, H_2O$$​​​​

Toxic emissions

Carbon monoxide

Carbon monoxide is a poisonous gas that can be fatal in high concentrations. It has a greater affinity for haemoglobin in the bloodstream than oxygen. This leads to a reduction in oxygen transported in the body therefore depriving the cells of oxygen. Fortunately, catalytic converters in vehicles remove carbon monoxide from exhaust fumes.

Sulfur dioxide 

Sulfur is found in some fossil fuels. When burnt they are turned into sulfur dioxide. Once in the atmosphere sulfur dioxide dissolves in the moisture and turns into sulfuric acid - this is acid rain. Acid rain corrodes buildings and structures as well as harming vegetation and aquatic life.

Power stations use an alkaline slurry to remove sulfur dioxide from the flue gases. An alkaline slurry is made by mixing powdered calcium carbonate or calcium oxide with water. When flue gases mix with the alkaline slurry, sulfur dioxide reacts with the calcium compound to form calcium sulfite - which is harmless.

Oxides of nitrogen $$(NO_x)$$​​

Oxides of nitrogen are formed due to the high temperature and pressure in a vehicle engine, leading to a reaction between nitrogen and oxygen from the air. This can go on to further react in the presence of sunlight to produce ground level ozone. 

Ground level ozone exacerbates respiratory illnesses and can cause irritation to the eye. As with carbon monoxide, catalytic converters can remove them from exhaust fumes.

Example

An example of a possible reaction that may occur in a catalytic converter.

​$$nitrogen \space monoxide \,+ \, carbon \space monoxide\rightarrow nitrogen \space \,+\, carbon \space dioxide \\ \ \\2NO(g) \,+ \,2CO (g) \rightarrow N_2 (g)\,+\,2CO_2 (g)$$​​

Fossil fuels

Carbon dioxide is a greenhouse gas which is good at absorbing infrared energy. Some of the energy is emitted towards the Earth, keeping the atmosphere warm. This is known as the greenhouse effect. A greater concentration of carbon dioxide leads to the Earth's temperature rising, which is known as global warming.

Fossil fuels are non-renewable, producing a large amount of energy when burnt. As it is a finite resource it is not sustainable to use it as a source of fuel. 

Biofuels

Biofuels are renewable and are classed as carbon neutral. This is because the carbon dioxide they emit was absorbed by the plants before being used as biofuels. They include fuels such as bioethanol, biodiesel and biogas.

Some of the issues faced when transitioning to the use of biofuels is the land required to grow the plants for fuel. Developing countries which have land may have a source of income to meet the demand of fuels desired by developed countries. However, this will mean less space to grow crops for food, potentially leading to starvation.