Volume 21, number 3, September 2007

Improve the energy efficiency of industrial furnaces

Industrial furnace chimneys often release large quantities of energy into the atmosphere. One of the ways to improve efficiency is to recover energy from flue gases to preheat furnace burner combustion air.

Several industries in Québec use the heat released in their chimneys to preheat combustion air. Different efficient technologies allowing recovery of large quantities of energy are available to perform this function, such as the chimney flue gas heat recuperator, the radiant tube recuperator and regenerative burners. Table 1 illustrates the percentage energy savings according to the temperature of the furnace and the combustion air.

Table 1. Performance data according to North American Manufacturing Company

Furnace temperature
Combustion air temperature
Combustion products temperature
natural gas saving
1 470 1 220 270 30
1 830 1 560 330 42
2 190 1 900 370 52
2 640 2 280 460 69

Chimney flue gas heat recuperator

Some industrial processes require an indirect energy flow, such as molten lead or zinc tanks. For heating, burners are installed in a double wall around the tank containing the molten metal. The hot flue gases are exhausted through a common chimney (see Figure 1). For this type of application, installation of a flue gas heat recuperator to preheat the combustion air proves to be an effective energy efficiency measure (EEM).

Figure 1. Chimney flue gas heat recuperator

One Québec industry has implemented this EEM. Its process involves boiling zinc ingots in a pot at 1672°F and flue gases arriving in the flue at 1750°F.

The ceramic plate cross-flow exchanger recuperator has three combustion air passes and one flue gas pass. It extracts energy from the flue gases and transfers it to the combustion air. With the temperatures involved, the heat is transferred by radiation from the hot flue gases and by convection from the combustion air. This allows preheating of the combustion air to 912°F, for a 26.7% energy saving. The payback period was 1.8 year.

In addition, because the combustion air is preheated, the flame reaches a higher temperature. The heat transfer in the pot is thus higher because there is more radiation. This leads to a 7% increase in productivity.

Radiant tube recuperator

In controlled atmosphere continuous furnaces, combustion is performed in a U-tube. The tube is heated from within and transmits heat to the furnace by radiation. A furnace can be equipped with several U-tubes to attain the power and temperature required for heat treatment. Each tube is equipped with a burner at one end and the combustion products are exhausted through the other end of the tube to the outdoors. It is possible to integrate a recuperator into the end of the tube that exhausts the hot gases (see Figure 2). This recuperator’s role is to extract the energy contained in combustion products and transfer it to the combustion air.

Figure 2. Radiant tube recuperator

One Québec company has implemented this EEM on its hydrogen-rich controlled atmosphere furnaces required for steel reduction. This heat treatment is performed at a high temperature of 1800°F.

Natural gas provides the required energy intake, and the combustion products, before being exhausted, pass through an indirect recuperator to extract the energy content. The combustion air is thus preheated to 1000°F and the energy saving reaches 31%. The payback period was 3.6 years and includes new tubes and burners, recuperators and improvement of the controls.

This EEM results in other positive aspects for this company. Since the combustion air is preheated, the flame reaches a higher temperature and the heat transfer by radiation within the furnace is higher, allowing a higher temperature to be reached in the furnace. Thus, new products could be processed, expanding the com-pany’s clientele.

Regenerative burners

These burners are installed in glass melting furnaces, aluminium recasting furnaces and steel forging furnaces. These burners operate in pairs and the objective of preheating the combustion air is achieved inside the burners.

The two burners are equipped with a porous ceramic battery for heat recovery from the combustion products and operate in cyclical mode. First, as shown on the left side of the Figure 3, burner 2 is energized, the flame heats the furnace and the combustion products circulate inside the furnace. Before leaving the furnace, the combustion products pass into burner 1, where they release their heat by reheating a porous ceramic bed.

In the second part of the cycle, on the right side of Figure 3, burner 2 is shut down, putting it into recovery mode, and burner 1 is energized. The combustion air in burner 1 is then preheated by passing into the porous ceramic bed. Then the cycle is reversed again. The typical length of a cycle is 20 seconds, varying according to different design parameters.

Figure 3. Regenerative burners

One Québec industry replaced four furnaces, which had become less efficient. One of them was dismantled to make room for a new high production capacity furnace equipped with regenerative burners.

The furnace operating temperature is 2300°F and, with recovery, the combustion air reaches 2000°F. The energy savings from this EEM is 55% and the payback period was 2.2 years.

Other advantages result from this new furnace, such as reduction of the intensity of greenhouse gas emissions. In addition, the ultra-low NOx class burners reduce nitrogen oxide emissions. The new furnace is better insulated and more airtight, allowing maintenance of internal positive pressure. This reduces infiltrations and makes the furnace’s internal temperature more uniform, while having less burners than the former furnaces.


Different technologies improve energy efficiency and help Quebec industries be more productive.

Since natural gas combustion generates clean fumes, unless the industrial process contaminates them, energy recovery to preheat combustion air can obtain very substantial energy savings.

Guy Desrosiers, Eng.
Technical Advisor, Major Corporations