To improve the overall efficiency of industrial combustion systems or to increase the flame temperature, flue gases can be used to preheat the incoming combustion air by recirculating them back from the exhaust through a heat exchanger. Heat exchangers are commonly used to improve the overall system efficiency or to increase the flame temperature. Recuperators and regenerators are the two most common types of heat exchangers used in industrial combustion systems and are covered in other sections. Unlike the combustion air, which may be preheated, the fuel is not typically preheated. An exception is for liquid fuels that may need some level of preheating to lower their viscosity for proper atomization. Fuels are not normally preheated for three reasons. The first is that their mass flow rate is much lower compared to the air flow so it is more difficult to transfer the energy in a heat exchanger to the smaller mass flow. The second reason is there are some safety concerns with heating fuels to temperatures about auto-ignition. If the preheated fuel were to leak, it could be easily ignited on contact with air without the need for an external ignition source. A third reason why the fuel is not normally preheated is because the carbon in the fuel can form coke that adheres to the piping and plugs fuel injectors. Another reason to recycle combustion products back through the flame is to reduce nitrogen oxides (NO_x) because the mostly inert flue gases are cooler than the flame and help moderate the flame temperature (Baukal, 2004c). Since thermal NO_x is highly temperature dependent, gas recirculation is an effective and proven technique for reducing NO_x used in industrial combustors, i.e., furnace and flue gas. These are briefly considered next.

A. Flue Gas Recirculation

Flue gas recirculation is schematically shown in Figure 33. In this process, the exhaust products are taken out of the flue and recirculated back into the furnace, often through the burners. The recirculation requires an external, high temperature fan to move the gases and the associated insulated duct work to transport the gases. There are two common reasons for employing this type of recirculation. In the past, the major reason was heat recovery. The hot combustion exhaust products were given a second chance to transfer their energy to a lower temperature heat load. A more recent reason to use this technology is to reduce NO_x emissions (Baukal, 2004c). Although the recirculated gases may be hot, they are not nearly as hot as the flame gases, so the recirculated gases actually cool the flame. The reduced flame temperature reduces NO_x emissions, which are highly temperature dependent.

Figure 33 Diagram of flue gas recirculation (Courtesy of CRC Press; Baukal, 2000).

This type of recirculation can have a significant impact on the heat transfer in the furnace. One primary effect is that the forced convection heat transfer is often increased due to the added mass of gases flowing through the furnace and the higher overall gas velocities. However, the forced convection may be reduced by the lower overall gas temperature by diluting the hot flame products with colder flue gases. The thermal efficiency of most combustion processes increases when employing flue gas recirculation because additional heat is extracted from the flue gases before they are sent out the stack. Therefore, the overall effect when using flue gas recirculation is to increase on convection heat transfer.