Note that The heating value of standard coal is about 30,080 kJ/kg 1 L of water has a mass of 1.0 kg
The mass of 15 L of water = 15 kg. The latent heat of vaporization of water is about 2260 kJ/kg, The energy required to boil 15 L of water is (2260 kJ/kg)*(15 kg) = 33900 kJ
The mass of coal required to provide this energy is (33900 kJ)/(30080 kJ/kg) = 1.127 kg
Because 1 kg = 2.205 lb, the mass of coal required is (1.127 kg)*(2.205 lb/kg) = 2.485 lb
