Which fuel boils water best?
Boil 1, 2 or 5 litres of water on an LPG stove, a kerosene stove, a coal stove, an open wood fire or a dung-cake chulo. See the heat the water needs and how many grams of each fuel burn to give it.
Core concept
Each fuel gives a fixed heat per gram, its calorific value, but a stove passes only part of it to the pot; grams burnt = heat needed ÷ (calorific value × efficiency), so a good fuel in a good stove burns far less
Topic: Energy and technology
Heating water takes energy: H = m × s × Δθ, with s = 4.2 kJ per kg per °C for water. To bring 1 litre (1 kg) from 25 °C to 100 °C takes 1 × 4.2 × 75 = 315 kJ. Fuels give heat when they burn, and the heat from each gram is the fuel's calorific value: about 50 kJ/g for LPG, 45 for kerosene, 25 for coal, 17 for dry wood and only 8 for dung cakes. But much of the heat escapes round the pot. An LPG stove puts about 60% into the water, a kerosene stove 40%, a coal stove 25%, an open wood fire 15% and a dung-cake chulo 10%. So the grams of fuel burnt = heat needed ÷ (calorific value × efficiency).
What you will learn
- Work out the heat needed to boil water, H = m s Δθ.
- Explain calorific value and stove efficiency.
- Compare how much of each fuel is burnt for the same job.
Before you start
- Know that heat H = m s Δθ and what a fuel is.
Materials
- A kitchen: notice which fuel your family uses to cook
Equipment
- None
Procedure
- Record 1 litre on an LPG stove.
- Record the same on kerosene, coal, wood and dung cakes.
- Boil 5 litres with LPG and record.
How this simulation is simplified
- Water is heated from 25 °C to 100 °C: 4.2 kJ/kg/°C × 75 °C = 315 kJ a litre.
- Calorific value and stove efficiency: LPG 50 kJ/g, 60%; kerosene 45, 40%; coal 25, 25%; dry wood 17, 15% (open fire); dung cakes 8, 10%. Typical values.