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Carbon dioxide emissions follow stoichiometry: carbon oxidised times 44/12, the ratio of molecular masses. Dividing by energy content gives the emission factor in kilograms of carbon dioxide per gigajoule — about 95 for coal, 74 for oil and 56 for natural gas, which is the basis of fuel switching arguments.
Emissions
CO2 (t) = fuel (t) x carbon content x oxidation factor x 44/12
Emission factor
Factor (kg CO2/GJ) = CO2 (t) x 1000 / energy (GJ)
Greenhouse gas reporting must use the emission factors and methodology prescribed by the applicable reporting scheme.
A small fraction of carbon leaves unburned in ash and soot. IPCC default oxidation factors are 98-100% depending on fuel and combustion technology.
Carbon has atomic mass 12 and carbon dioxide molecular mass 44, so each tonne of carbon burnt produces 3.667 tonnes of carbon dioxide.