Comparing Different Fuels Chemistry Tutorial

Key Concepts

• A fuel can be defined as a substance that is used for producing energy by combustion.
• Fuels can be compared in terms of their:

  ⚛ energy output

  ⚛ suitability for purpose

  ⚛ nature of products of combustion

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Energy Output of Fuels

The S.I. unit of measurement for energy is the joule, J.¹
The amount of energy that can be obtained by the combustion of a fuel is its energy output.
The energy output of a fuel is measured in joules per amount of fuel combusted.

The amount of energy obtained by combustion of a fuel, the energy output, depends on the conditions of the combustion reaction.
Some systems designed to combust a fuel are more efficient at converting the stored chemical energy in the fuel into energy that we can use to do work.
For this reason, we find tables of heat content, or energy values, for fuels rather than tables of energy output.

The substances that are commonly used as fuels are not pure substances, they are mixtures of different substances.
For example, natural gas contains methane, CH₄, as well as ethane, C₂H₆, propane, C₃H₈, and butane, C₄H₁₀. Petrol (gasoline) contains a number of different hydrocarbons including octane, C₈H₁₈.
For this reason the energy content, or heat content, of these fuels is measured in units of joules per gram of fuel, J g^-1, rather than in joules per mole of fuel.
A substance is useful as a fuel if it releases a large amount of energy per gram, so it is even more useful to express the heat content of fuels in units of kilojoules per gram, kJ g^-1, or even in megajoules per tonne, MJ tonne^-1.

The table below gives values for the heat content of various fuels in kilojoules per gram of fuel combusted, kJ g^-1, and in megajoules per tonne of fuel, MJ tonne^-1:

From the table, the heat content of natural gas is 54 kJ g^-1, that is, the heat content is 54 kJ per gram of natural gas combusted. Combustion of 1 gram of natural gas produces 54 kJ of energy.
Similarly, the combustion of 1 gram of petrol (gasoline) produces 48 kJ of energy. Less energy is released by the combustion of 1 gram of petrol compared to the energy released by the combustion of 1 gram of natural gas.
The combustion of 1 gram of brown coal releases even less energy, just 16 kJ of energy.

This means that if 1 g of natural gas were to undergo combustion, a maximum of 54 kJ of energy could be released.
If 5 grams of natural gas were combusted, 5 × 54 = 270 kJ of energy would be released.
If 0.25 grams of natural gas were combusted, 0.25 × 54 = 13.5 kJ of energy would be released.

We could write a mathematical equation to represent these calculations:

energy released (kJ) = heat content (kJ g^-1) × mass of fuel (g)

For example, how much energy in kJ is released by the combustion of 160 g of diesel?

energy released (kJ) = heat content of diesel (kJ g^-1) × mass of diesel (g)

energy released (kJ) = 45 (kJ g^-1) × 160 (g) = 7 200 kJ

We can rearrange this equation by dividing both sides of the equation by the heat content of the fuel in order to calculate the mass of fuel required to produce a particular amount of energy:

For example, what mass of brown coal is required to produce 800 kJ of energy?

mass of brown coal (g) = energy (kJ) ÷ heat content of brown coal (kJ g^-1)

mass of brown coal (g) = 800 kJ ÷ 16 (kJ g^-1) = 50 g

Suitability of Fuel for Purpose

In the section above we found that, if we combust the same mass of each fuel, then natural gas will provide the most energy.
So why do we use all these other fuels? Why not just use natural gas?

First, we could consider how easy or difficult it is to obtain the fuel.
Historically, the first fuel used for heating homes and cooking was wood. Since trees were readily available, wood was easy to obtain.
With the invention of the steam engine, the Industrial Revolution saw the use of coal as a fuel increase. Coal is largely composed of carbon and was formed when living matter was subjected to high pressure over a long period of time. Coal is readily available as a solid in coal seams that could be mined, historically, by individuals using a pick to hack lumps of coal out of the seam. Australia has large reserves of coal; black coal is mined in the Sydney Basin in New South Wales and in the Bowen Basin and central Queensland, while the La Trobe Valley in Victoria produces brown coal.
The invention of the internal combustion engine led to the development and proliferation of various modes of transport including cars, trucks, buses. Products based on crude oil such as diesel and petrol (gasoline) became important fuels. While the source of crude oil is also the compression of living matter over long periods of time like coal, these crude oil deposits are not as widespread nor as easy to access as coal seams. Countries of the Middle East are major exporters of crude oil to the rest of the world. Machinery is required to drill down to the crude oil deposit and bring it to the surface. Furthermore, the crude oil itself needs to be refined in order to produce useful fuels.
Natural gas is largely methane mixed with other gases such as ethane, propane and butane, and, like the other fossil fuels such as coal and crude oil, also has its beginning in living matter that is compressed under layers of rock for a long period of time. Natural gas is often, although not always, found with deposits of crude oil. Large reserves of natural gas are available in the Gippsland Basin (Bass Strait) in Victoria, Cooper Basin in South Australia and the Carnarvon Basin (North-West Shelf) of Western Australia. Being a gas, however, makes it even more difficult to trap, transport and store. It can, however, be piped from its source direct to homes for use in heating and cooking, or to electricity generating power plants.
Coal Seam Gas (CGS), also known as coal bed methane, is composed primarily of methane with only few impurities, and is the gas trapped within coal seams, so it also classified as fossil fuel. Historically it is this gas that has caused explosions in coal mines. Recently, technology has been developed to extract this gas from coal seams using water to force the gas to the surface. Australia has large deposits of coal seam gas which are currently being tapped in the Bowen Basin of Queensland and the Surat Basin in New South Wales.

So, one reason why natural gas is not so widely used, even though it has a high heat content, is because it is not as easily accessed as other fossil fuels.

In order to be useful as a fuel, the substance must also be easy to store and transport.²
Solids, such as coal and wood, are relatively easy to store and transport because they can be heaped up into a pile. They do not require any special vessel for storage or transport. Because coal and wood are easy to stockpile, they have been used extensively as a fuel to generate electricity. In the past they have also been stockpiled by households for use as a fuel in home heating and cooking. Historically, the ability to stockpile coal led to its being used to fuel steam engines.
Liquids, and mixtures of liquids, such as petrol (gasoline) and diesel (petrodiesel) are also relatively easy to store and transport in special vessels designed for the purpose. The petrol tank (gas tank) of a motor vehicle is a good example of the ease with which a liquid fuel can be stored and transported.
Gases like natural gas and coal seam gas (CGS) are more difficult to store and transport because the volume of a gas is so much greater than that of a solid or a liquid, and because the gas is much more likely to ignite. For use in home heating and cooking, or to fuel power plants, it is possible to pipe the gas directly from its source to its destination.
For other uses, such as fuels for vehicles, gases are stored under pressure to reduce the volume they occupy in special gas bottles, or gas cylinders.
Ideally, sufficiently high pressure could be used to condense the gas into a liquid. Liquefied petroleum gas (LPG) is a by-product of the processing of natural gas and crude oil in which the propane and butane gases have been sufficiently compressed to form a liquid. Unfortunately, natural gas can't be liquefied at room temperature, making it difficult to transport any way except by gas pipeline.

So, another reason why natural gas is not so widely used, even though it has a high heat content, is because it is more difficult to store and transport than other fossil fuels.

Fossil fuels like coal, petrol, diesel (petrodiesel), natural gas and coal seam gas (CGS), take millions of years to produce and we are using these deposits up at a faster rate than they can be replenished. For this reason these fuels are referred to as "non-renewable" fuels.
A lot of interest, and investment, is in the development of "renewable" fuels, those that can be produced at the same rate as they are consumed. These fuels are often produced from living biological matter (called biomass) and are referred to as biofuels. Bioethanol, produced by the fermentation of sugars found in plants, can be used as fuel, or, it can be added to petrol (gasoline) to extend the lifetime of the non-renewable fuel. Biodiesel, produced from vegetable oils or animal fats, can be used as a fuel, or, added to diesel produced from crude oil (petrodiesel) to extend the lifetime of the non-renewable fuel. Biogas, also known as swamp gas or marsh gas, produced when organic matter rots in the absence of oxygen, can be used as a fuel to generate electricity.

Nature of Products of Combustion of Fuels

All the fuels, both renewable and non-renewable, that we have been discussing contain carbon.
When a fuel containing carbon combusts in the presence of excess oxygen it will produce carbon dioxide gas (CO_2(g)).
Another way of expressing this is to say that there is a low fuel to oxygen ratio, or, fuel:oxygen is low.

Coal is mostly made up of carbon.
When carbon (C_(s)) combusts in excess oxygen (O_2(g)) the product is carbon dioxide (CO_2(g)).
The combustion of 1 mole of carbon, (C_(s)), releases 393 kJ of energy.
We say that the enthalpy of combustion of carbon is -393 kJ mol^-1 (ΔH = -393 kJ mol^-1)

Petrol (gasoline) is made up of a number of different compounds including octane (C₈H₁₈).
Octane will combust in excess oxygen to produce carbon dioxide gas (CO_2(g)) and water vapor (H₂O_(g)).
Complete combustion of 1 mole of octane releases 5464 kJ of energy.
The enthalpy of combustion of octane is -5464 kJ mol^-1 (ΔH = -5464 kJ mol^-1)

Natural gas or coal seam gas is made up largely of methane (CH_4(g)).
Methane combusts in excess oxygen to produce carbon dioxide gas (CO_2(g)) and water vapor (H₂O_(g)).
1 mole of methane undergoes complete combustion to release 888 kJ of energy.
The enthalpy of combustion of methane is -888 kJ mol^-1 (ΔH = -888 kJ mol^-1).

Carbon dioxide is known as a Greenhouse Gas, one of the gases that contributes towards keeping the earth warm as a result of the Greenhouse Effect. Unfortunately, it appears that our increased use of carbon-containing fuels is increasing the amount of carbon dioxide in the atmosphere, contributing to a rise in global temperatures and adversely affecting the earth's climate.
We would prefer to use fuels that generate less carbon dioxide gas.
Fossil fuels are a store of carbon from ancient times. When combusted this "age-old" carbon is released into the atmosphere increasing the level of atmospheric carbon dioxide.
Renewable carbon-based fuels such as biodiesel and biogas are a recent store of carbon. So, when these fuels combust we are only putting back the same amount of carbon as we took out, and this carbon in the form of atmospheric carbon dioxide can be used to grow more plant matter to be used to generate more biofuel. Such fuels are commonly referred to as "carbon-neutral".

When there is insufficient oxygen gas for the complete combustion of a carbon-based fuel, toxic carbon monoxide gas (CO_(g)) and air-polluting soot (C_(s)) can be produced.
This can occur when the fuel to oxygen ratio in an internal combustion engine is too high.
The chemical equation below shows an example of the incomplete combustion of the octane, C₈H_18(l), found in petrol (gasoline):

C₈H_18(l) + 9O_2(g) → 2C_(s) + 3CO_(g) + 3CO_2(g) + 9H₂O_(l)

Fossils fuels have another disadvantage. Because they are mixtures they can contain substances which can form harmful products when combusted. One such impurity present in fossil fuels is sulfur.
When sulfur (S_(s)) burns in excess oxygen it produces sulfur dioxide gas (SO_2(g)):

S_(s) + O_2(g) → SO_2(g)

Sulfur dioxide gas (SO_2(g)) reacts with water in the atmosphere (H₂O_(g)) to produce sulfurous acid (H₂SO_3(aq)) and sulfuric acid (H₂SO_4(aq))
These acids are a major contributor to the increased acidity of rainwater, known as acid rain which is having a detrimental impact on living things on land and in water.

While fossil fuels have served us well, and biofuels will help provide combustible fuels into the future, there is general agreement that we need to find alternative energy sources that do not rely on the combustion of carbon-containing compounds if we are to maintain our standard of living while reducing the damage we do to our environment.
One focus of attention has been on the use of hydrogen as fuel.
When hydrogen (H_2(g)) combusts in excess oxygen (O_2(g)), the product is water (H₂O_(g)):

H_2(g) + ½O_2(g) → H₂O_(g)

At atmospheric pressure, and at temperatures below the boiling point of water (≈ 100°C) water will condense to liquid water (H₂O_(l)). If we can find a way to economically produce hydrogen gas from water, then it should be possible to re-use this water to produce hydrogen which could then be used as a fuel. And hydrogen is an energy dense fuel, 1 gram of hydrogen could produce 120 kJ of energy.
This is what is meant when we refer to the "hydrogen economy".

Worked Example of Comparing Fuels

Question : The table below gives the enthalpy of combustion of various substances that can be used as fuels:

Fuel Name	Fuel Formula	ΔH kJ mol-1
methane gas	CH4(g)	-888
butane gas	C4H10(g)	-2877
liquid octane	C8H18(l)	-5470

You have been asked to choose one of these fuels as the energy source for a new outdoor BBQ.
The complete combustion of the fuel must produce the most energy per mole of carbon dioxide gas produced.
Which fuel will you choose?

Solution:

(Based on the StoPGoPS approach to problem solving.)

1. What is the question asking you to do?

   Choose the fuel that releases the most energy per mole of CO_2(g) produced.

2. What data (information) have you been given in the question?

   Extract the data from the question: write the complete combustion reaction for each fuel using the data in the table given

   (a) CH_4(g) + 2O_2(g) → CO_2(g) + 2H₂O_(g)     ΔH = -888 kJ mol^-1

   (b) C₄H_10(g) + ¹³/₂O_2(g) → 4CO_2(g) + 5H₂O_(g)     ΔH = -2877 kJ mol^-1

   (c) C₈H_18(g) + ²⁵/₂O_2(g) → 8CO_2(g) + 9H₂O_(g)     ΔH = -5470 kJ mol^-1

3. What is the relationship between what you know and what you need to find out?

   Energy released by the complete combustion of 1 mole of each fuel equals the value of ΔH

   Energy released per mole of CO_2(g) produced = ΔH ÷ moles of CO_2(g)

   (a) Complete combustion of 1 mole of CH_4(g): energy released = 888 kJ ÷ 1 mol CO₂ = 888 kJ

   (b) Complete combustion of 1 mole of C₄H_10(g): energy released = 2877 kJ ÷ 4 mol CO₂ = 719 kJ

   (c) Complete combustion of 1 mole of C₈H_18(g): energy released = 5470 kJ ÷ 8 mol CO₂ = 684 kJ

4. Decide which fuel releases the most energy per mole of CO_2(g) produced

   Combustion of 1 mole of methane releases more energy, 888 kJ, per mole of CO_2(g) produced.

5. Is your answer plausible?

   Re-write each combustion equation for 1 mole of CO_2(g):

   (a) CH_4(g) + 2O_2(g) → CO_2(g) + 2H₂O_(g)     ΔH = -888 kJ mol^-1 (of CO_2(g))

   (b) ¹/₄C₄H_10(g) + ¹³/₈O_2(g) → CO_2(g) + ⁵/₄H₂O_(g)     ΔH = -2877/4 = -719 kJ mol^-1 (of CO_2(g))

   (c) ¹/₈C₈H_18(g) + ²⁵/₁₆O_2(g) → CO_2(g) + ⁹/₈H₂O_(g)     ΔH = -5470/8 = -684 kJ mol^-1 (of CO_2(g))

   Complete combustion of methane, CH_4(g), produces the most energy per mole of CO_2(g) produced.
   Since this is the same answer we got above using a different set of calculations, we are reasonably confident that our answer is plausible.

6. State your solution to the problem "choose the fuel that produces the most energy per mole of CO_2(g) produced":

   Methane, CH_4(g), is the best fuel to maximise the amount of energy and minimise the amount of carbon dioxide produced.