Regardless of the motive, the first step undertaken by a furnace operator is to investigate the various technologies available. Two of the most intriguing technologies are regenerative burners and oxy-fuel burners.
The most commonly used fuel combusted in furnace applications is natural gas, which is comprised of over 90% methane (CH4). Air, comprised of about 20% oxygen (O2) and the balance nitrogen (N2), is usually used as the oxidizer. Equation 1 expresses the typical combustion reaction, firing with 10% excess air.
CH4 + 2.2 O2 + 8.27 N2 ® CO2 + 2H4O + 0.2 O2 + 8.27 N2 (1)
1 Fuel + 10.47 Air ® 8.7% CO2 +17.4% H2O + 1.7% O2 + 72.1% N2
The hydrocarbon fuel chemically changes to a collection of exhaust gases: carbon dioxide (CO2) and water vapor (H2O). The N2 in the exhaust gas came directly from the combustion air.
As shown, the process starts with chemical energy in the fuel. There is also some sensible energy in the oxidizer, proportional to its temperature. From this heat supply, process heat is used to heat the product as well as the various system losses (refractories, water, etc.). Finally, a significant amount of heat is carried out of the furnace in the form of sensible heat in the exhaust gases. This energy balance is summarized in equation 2.
Chemical energy (fuel) + Sensible heat (oxidizer) – Sensible heat (exhaust) = Process heat (useful heat) (2)
Based on equation 2, we calculate the combustion efficiency as the useful process heat divided by the chemical energy in the fuel:
Efficiency = Process heat / Chemical energy in fuel (3)