Fuels and combustion
A fuel is something that burns to give heat. Fuels can be solid (wood, coal), liquid (diesel, fuel oil) or gas (LPG, natural gas). Electric furnaces use electricity instead of burning.
The calorific value (CV) is the heat given by 1 kg of fuel when it burns fully. Rough values: wood about 15 MJ/kg, coal about 25 to 30 MJ/kg, LPG and natural gas about 46 to 50 MJ/kg. Gas gives more heat per kg and burns cleaner.
Combustion is fuel joining oxygen. For methane: CH4 + 2 O2 → CO2 + 2 H2O + heat. With enough air the burning is complete. With too little air it is incomplete and makes carbon monoxide (CO, poisonous) and soot, with less heat.
The air in a kiln is called the atmosphere. Extra air gives an oxidizing atmosphere (iron in clay turns red). Little air gives a reducing atmosphere (iron turns grey or black).
Furnaces and kilns
A furnace or kiln is an insulated chamber for heating ware. Walls are made of refractory bricks or ceramic fibre that stand high heat and hold the heat in.
- Batch (intermittent) kiln: load, fire, cool, unload. Simple, flexible, uses more fuel.
- Tunnel kiln: ware rides on cars through hot zones: preheat, fire, cool. Continuous and saves fuel because hot exhaust warms the incoming ware.
- Electric furnace: heating elements (like silicon carbide rods). Clean and easy to control.
Firing curve: heat slowly, hold (soak) at the top temperature, cool slowly. Too fast heating or cooling cracks the ware. At about 1000 to 1400 °C the grains sinter: they join and pores shrink, so the ware becomes hard and strong without fully melting.
Melting
Melting turns a solid into liquid. A glass batch (silica sand, soda ash, limestone) melts at about 1500 °C in a glass furnace or crucible. Then bubbles rise out (fining) and the liquid is cooled to be shaped.
Many ceramics have very high melting points. Alumina melts at about 2070 °C. Factories do not melt them; they sinter the powder instead.
Heat needed to raise the temperature: Q = m × c × ΔT. Melting itself needs extra heat, the latent heat. Real furnaces lose heat through walls and exhaust, so efficiency = useful heat ÷ heat supplied × 100.
Key formulas and definitions
- Heat from fuel = mass × calorific value
- Q = m × c × ΔT (heat to warm a mass)
- Efficiency % = useful heat ÷ heat supplied × 100
- T (K) = T (°C) + 273
- CH4 + 2 O2 → CO2 + 2 H2O + heat
Worked examples
1. How much heat comes from burning 10 kg of LPG with CV 46 MJ/kg?
Heat = 10 × 46 = 460 MJ.
2. Convert 1200 °C to kelvin.
T = 1200 + 273 = 1473 K.
3. Find the heat to warm 200 kg of ware from 20 °C to 1020 °C. Take c = 1.0 kJ/kg·K.
ΔT = 1000 K. Q = 200 × 1.0 × 1000 = 200,000 kJ = 200 MJ.
4. The kiln has 50% efficiency. How much LPG (CV 46 MJ/kg) is needed to give 200 MJ of useful heat?
Heat supplied = 200 ÷ 0.50 = 400 MJ. Mass of LPG = 400 ÷ 46 = 8.7 kg.
5. A kiln is supplied 500 MJ and 200 MJ heats the ware. Find its efficiency.
Efficiency = 200 ÷ 500 × 100 = 40%.
6. For CH4 + 2 O2 → CO2 + 2 H2O, how many grams of CO2 form from 16 g of methane? (C = 12, O = 16, H = 1)
CH4 = 16 g/mol. CO2 = 44 g/mol, 1 mol each. So 16 g CH4 gives 44 g CO2.
Common mistakes
- Thinking electricity is a fuel that burns. It heats elements; nothing burns.
- Mixing up Celsius and kelvin in heat sums. ΔT is the same in both, but T itself is not.
- Heating ware very fast. The outside heats first and cracks the piece.
- Thinking sintering means melting. In sintering the grains join but the piece stays solid.