Fuels, energy content and energy density: VCE Chemistry Unit 3

What this dot point is asking

VCAA wants the definition of a fuel, the distinction between fossil fuels and biofuels, and a quantitative comparison of fuels using energy content (kJ per gram) and energy density (kJ per litre), with a comment on renewability.

The answer

A fuel is a substance that releases useful energy when it undergoes combustion (an exothermic reaction with oxygen). The most common fuels in human use are the hydrocarbon fossil fuels and the carbon-based biofuels.

Fossil fuels vs biofuels

Fossil fuels release CO2 that was locked underground for millions of years, adding net carbon to the atmosphere. Biofuels release CO2 that the source crop absorbed during growth in the same season, so the net carbon contribution is close to zero (ignoring transport and processing emissions). This is the "carbon neutral" argument for biofuels.

Energy content vs energy density

Two different quantities, both used to compare fuels.

• Energy content (also called specific energy) is the energy released per unit mass, usually in kJ g^-1 or MJ kg^-1. Best for comparing fuels by weight, for example in aviation.
• Energy density is the energy released per unit volume, usually in kJ L^-1 or MJ L^-1. Best for comparing fuels by tank size, for example in cars.

Typical values (approximate, for the VCE data book era):

Two patterns to lock in:

1. More C-H bonds, more energy. Hydrocarbons with little to no oxygen pack more chemical energy per gram than oxygenated fuels (alcohols, biodiesel). Hydrogen tops the list on a per-gram basis.
2. Liquid fuels usually win on energy density. A gas spreads out; a liquid is hundreds of times denser. Hydrogen has the highest energy content per gram but the worst energy density unless it is compressed, liquefied or stored chemically.

Comparing fuels: the trade-off

The "best" fuel depends on the application.

• Aviation: mass matters most. Aviation kerosene wins (high kJ g^-1, liquid).
• Cars: energy density matters more (limited tank volume). Petrol and diesel win.
• Heating, electricity: cost and infrastructure dominate. Coal, natural gas and (increasingly) biogas compete.
• Carbon footprint: biofuels win in principle, but their land use and processing energy reduce the advantage.
• Sustainability: only biofuels are renewable on a human timescale.

Examples in context

Example 1. Loy Yang brown coal versus Bass Strait natural gas. AGL Loy Yang fires roughly $30 \, \text{Mt}$ of brown coal per year to generate $2200 \, \text{MW}$. Brown coal has energy content $\sim 10 \, \text{MJ/kg}$ (wet, as mined) versus black coal $\sim 25 \, \text{MJ/kg}$ and natural gas $\sim 50 \, \text{MJ/kg}$. Esso's Bass Strait gas, piped to Longford and then to Newport gas-turbine peaker, releases $890 \, \text{kJ}$ per mole on combustion: $\text{CH}_4 + 2 \text{O}_2 \to \text{CO}_2 + 2 \text{H}_2 \text{O}$, $\Delta H = -890 \, \text{kJ/mol}$. Per gram: $890 / 16 = 55.6 \, \text{kJ/g}$. The carbon-to-energy ratio matters too: methane releases $0.018 \, \text{mol}$ $\text{CO}_2$ per kJ; brown coal $0.030$. Gas peakers emit roughly half the $\text{CO}_2$ per kWh of Loy Yang's coal.

Example 2. Sunshot bioethanol versus petrol in flex-fuel vehicles. Sunshot Industries near Mackay produces bioethanol from sugar-cane molasses, supplied to E10 petrol stations across Queensland and NSW. Ethanol combustion: $\text{C}_2 \text{H}_5 \text{OH} + 3 \text{O}_2 \to 2 \text{CO}_2 + 3 \text{H}_2 \text{O}$, $\Delta H = -1367 \, \text{kJ/mol}$. Per gram: $1367 / 46 = 29.7 \, \text{kJ/g}$. Per litre (density $0.789 \, \text{g/mL}$): $29.7 \times 789 = 23{,}440 \, \text{kJ/L}$. Compared with petrol's $34{,}000 \, \text{kJ/L}$ (octane based), ethanol delivers only $69\%$ of the energy per litre, so a vehicle on E100 uses $\sim 45\%$ more fuel for the same distance. The carbon "saved" is recaptured by the next year's cane crop, so net emissions are about a quarter of petrol's.

Try this

Q1. Distinguish between energy content and energy density of a fuel. Give units for each and an example. [3 marks]

• Cue. Energy content: $\text{kJ/g}$. Density: $\text{kJ/L}$. Example: hydrogen has highest content ($142 \, \text{kJ/g}$) but lowest density as gas ($\sim 11 \, \text{kJ/L}$ at STP).

Q2. Methane has $\Delta H_c = -890 \, \text{kJ/mol}$. (a) Calculate the energy content in $\text{kJ/g}$. (b) Calculate the mass of $\text{CH}_4$ needed to heat $200 \, \text{L}$ of water from $15$ to $40^{\circ}\text{C}$ assuming $60\%$ efficiency. [4 marks]

• Cue. (a) $890 / 16.0 = 55.6 \, \text{kJ/g}$. (b) $q = 200{,}000 \times 4.18 \times 25 = 20.9 \times 10^6 \, \text{J} = 20{,}900 \, \text{kJ}$; net energy required $= 20{,}900 / 0.60 = 34{,}833 \, \text{kJ}$; mass $= 34{,}833 / 55.6 = 627 \, \text{g}$.

Q3. Compare brown coal, natural gas and bioethanol. (a) Rank by energy content (kJ/g). (b) Compare $\text{CO}_2$ per kJ. (c) Justify why Victorian power policy is shifting from coal to gas peakers and renewables. [2+2+2 marks]

• Cue. (a) Natural gas ($55$) > bioethanol ($30$) > brown coal ($10$). (b) Coal worst; gas roughly half; bioethanol close to carbon neutral. (c) Reduces emissions per kWh and supports transition while batteries scale.