ENERGY-SAVING POSSIBILITIES FOR GAS-FIRED INDUSTRIAL Of the various available nergy efficiency is a top pri- perature. For a system without air ority in the steelmaking and preheat, efficiency decreases with options for increasing heat treating industries. Be- rising exhaust gas temperature. At a the energy efficiency cause hot exhaust gases repre- process temperature of 1000°C E sent the largest source for (1830°F), about 50% of the fuel input in industrial furnaces, losses in most industrial furnaces, will be lost as hot exhaust gas heat. preheating the combustion preheating the combustion air pro- To determine the usefullness of air vides the highest potential for energy preheat, the relative air preheat () air offers the most effective savings. This article discusses the ad- can be defined as: vantages of different strategies to way to increase efficiency    boost energy efficiency and factors  preheat - air ~ preheat =  =  in most furnaces. that must be considered when eval- exhaust - air exhaust uating their suitability for the appli-  Joachim G. Wünning cation. where preheat is air preheat tempera-  WS Inc., Elyria, Ohio ture, exhaust is hot exhaust tempera-  Energy Efficiency ture, and air is air inlet temperature. Related to Gas Losses The air preheat temperature is the Efficiency is usually defined as: temperature supplied to the burner. Energy losses between a central heat Efficiency = benefit exchanger and the burner have to be expenditure considered. The hot exhaust temper- ature is the temperature of the ex- Regarding firing systems for indus- haust gases leaving the , trial furnaces, efficiency or available which, in most cases, is close to the heat, is defined as: process temperature. In radiant tube heated furnaces, this temperature can Efficiency = fuel input - exhaust gas losses be substantially higher than the fur- fuel input nace temperature. The air inlet tem- = 1 - (fuel input - exhaust gas losses) perature is usually ambient air, and, fuel input therefore, the relative air preheat can be expressed as the ratio of preheat Figure 1 shows efficiency as a func- temperature to hot exhaust temper- tion of exhaust gas, or process tem- ature. The relative air preheat value

100

 = 1

75 0.8

0.6 Regenerative burner 0.4 50 Recuperative burner 0.2

Efficiency, % Efficiency,  Central heat exchanger = 0 25 Air preheat temperature Relative air preheat ()  Exhaust temperature

0 0 250 500 750 1000 1250 1500 Exhaust temperature, °C (prior to the heat exchanger) Fig. 1 — Efficiency as a function of exhaust gas of industrial furnace-firing systems. HEAT TREATING PROGRESS • SEPTEMBER/OCTOBER 2007 37 k • A 100 NTU = m • cp

Counterflow heat exchanger where A is heat exchanger surface 80 area, k is coefficient, m

Regenerative burner is mass flow, and cp is specific heat. Figure 2 shows the relative air pre- heat in a simplified diagram for ), %  60 Self-recuperative burner counterflow and coflow heat ex- changers. The savings can be calcu- lated as: Coflow heat exchanger

40 1 - low efficiency Plug-in recuperator Savings = Relative air preheat ( Central recuperator high efficiency which translates to savings of 20% if 20 a system with 68% efficiency is up- graded to 85% efficiency.

Continuous Direct-Fired Furnaces 0 0 2 4 6 8 10 One option (Fig. 3) to lower the ex- NTU (number of transfer unit) haust gas losses is to add an un- Fig. 2 — Heat exchanger performance; i.e., relative air preheat in a simplified diagram for heated section to the furnace where counterflow and coflow heat exchangers. the incoming products are preheated. This is quite effective as long as the flue gas is hot, but very long preheat zones would be necessary to transfer a considerable amount of heat. This method to improve efficiency is Zone 0 Zone 1 Zone 2 Zone 3 common in tunnel furnaces in the ce- ramic industry. Additional useful cooling of ex- haust gases can be done in a central heat exchanger (Fig. 4). The limita- Fig. 3 — One way to lower the exhaust gas losses in a direct-fired continuous furnace is to tion stems from the recuperator de- add an unheated section to the furnace where incoming products are preheated. sign and size, as well as the max- imum temperature for the hot air Chimney Recuperator control valves. Common air preheat temperatures are 300 to 500°C (570 to

Combustion 930°F), and in some cases, as high as air blower 600°C (1110°F). Air-preheating limitations can be Hot air overcome using regenerative-burner

Exhaust systems for decentralized heat re- covery (Fig. 5) or self-recuperative Fuel burners (Fig. 6). Every burner has its own heat exchanger, which is placed in the furnace wall or close to the burner. Combustion air-control Fig. 4 — Additional useful cooling of exhaust gases in a continuous furnace can be done valves are located on the cold side of in a central recuperator. the heat exchangers. In addition to is a good number to characterize a providing higher efficiency, such a heat exchanger for air preheating. system provides more exact furnace Heat exchanger performance is temperature control because there is evaluated using the NTU (number of no interaction between furnace transfer unit), which is proportional zones. The lack of the costly insulated to the heat exchanger area and in- hot air piping and a preheat zone versely proportional to the heat ca- usually offsets higher burner costs pacity flow through the heat ex- and the expense of the exhaust col- changer. lection system. Energy savings of 10 38 HEAT TREATING PROGRESS • SEPTEMBER/OCTOBER 2007 to 30% compared with systems having central recuperators can be achieved. Even higher air preheat tempera- tures, and, therefore, higher effi- ciency can be achieved using regen- Air erative burners. For larger burner Exhaust capacities, regenerative burner pairs Fuel are common (Fig. 7), where two burners are linked and alter- nately. Exhaust and combustion air are directed over the regenerators, which are made of ceramic balls or honeycombs. Relative air preheat values of 0.8 to 0.9 are achievable, making these systems very effective. Figure 8 shows a regenerative burner. The burner uses air staging as a NOx- Fig. 5 — Limitations for air preheating can be overcome using regenerative burner systems reducing measure. for decentralized heat recovery, or using self-recuperative burners (see Fig. 6). For smaller burner capacities, a self-regenerative burner allows the (Figs.9 and 10). same high efficiency, but with the ad- Fuel savings com- vantage of a single-burner solution. pared with self-recu- There is no need to switch from one perators are in the burner to another; the one burner can range of 10 to 20%, fire continuously just like a recuper- and savings of 50% ative burner. This is possible by inte- or greater have been grating all switching valves and re- achieved compared with Fig. 6 — WS Inc. REKUMAT generators into one compact unit cold air systems. self-recuperative burner.

HEAT TREATING PROGRESS • SEPTEMBER/OCTOBER 2007 39 Closed Open Open Closed Regenerators

Gas Fuel

Exhaust suction blower Combustion air blower Switching valve Fig. 8 — Example of a regenerative burner. Fig. 7 — Regenerative burner pairs are common for larger burner capacities. Courtesy Bloom engineering.

Pneumatic Orifices Furnace switching valves Regenerators wall

Exhaust Combustion air

FLOX Flame

Fuel Flame air

Fig. 10 — WS Inc. REGEMAT self-regen- erative burner.

Radiant Tube-Fired Systems For radiant tubes, decentralized heat recovery is preferable. Central heat exchangers, which are common Fig. 9 — Self-regenerative burner. for large direct-fired furnaces, are not practical for radiant tube fired sys- tems because there is no central ex- haust outlet of the furnace. Hot ex- haust gases would have to be Straight tube Single ended radiant tube transported to the heat exchanger in costly insulated ducts, and then the hot air has to be distributed back to SER single ended radiant tube the individual radiant tubes. For ra- P-tube diant tube heating, a good heat re- covery system is essential because exhaust temperatures are often sub- U-tube stantially higher than the furnace temperature. It is particularly true Double P-tube for ceramic radiant tubes having high heat-release rates. Different radiant tube designs (Fig. 11) require different strategies for W-tube A-tube Fig. 11 — Radiant-tube designs. 40 HEAT TREATING PROGRESS • SEPTEMBER/OCTOBER 2007 heat recovery. In straight-through tubes, heat recovery is very rare. For Fuel U- and W-tubes, the most common way to preheat the combustion air is to use a plug-in recuperator (Fig. 12). To enhance the air preheat, external recuperators are also possible. The limitation for air preheating stems Preheated air from the necessity to guide the hot air from the exhaust leg to the burner and also from the coflow heat ex- Plug in recuperator Exhaust changer design. Higher air preheat temperatures, Fig. 12 — W-tube with plug in recuperator. and, thereby, higher efficiency can be achieved using regenerative burner Fuel systems in U-, W- and A-tubes. Two burners per tube alternate firing (Fig. Air 13). Regenerative systems allow air preheat temperatures close to the fur- nace temperature. Energy savings of more than 20% compared with sys- tems using plug-in recuperators are Exhaust typical. Besides energy savings, tem- perature uniformity of the tubes is much better due to the alternating flow direction in the tube. It is impor- tant to pay attention to NOx forma- Air tion due to the high air preheat and system complexity (i.e., two burners Fuel per tube).

Single-ended P- and Double-P Exhaust tubes are usually fired using self-re- Fig. 13 — Regenerative-fired W-tube. cuperative burners. The counterflow heat exchanger, which is placed in- side the furnace wall, allows high air Hot exhaust preheat temperatures, and there is no hot air piping required outside the furnace. For high temperatures, self- Burner nozzle recuperative burners with ceramic heat exchangers (see Fig. 6) are avail- able. Air preheat temperatures in the range of 500 to 700°C (930 to 1290°F) are typical. Figure 14 and 15 show a High velocity flame double-P tube with a self-recupera- tive burner. High-velocity combus- tion provides good temperature uni- Recirculation formity, and internal recirculation allows the application of flameless Fig. 14 — WS Inc. double-P-tube with self-recuperative burner. oxidation, or FLOX, as an effective method to reduce thermal NOx for- Exhaust mation. Self-recuperative burners are widely used because they combine good performance with a high effi- ciency. A self-regenerative burner for ra- diant tubes was developed to com- bine the advantages of regenerative Fuel systems and self-recuperative burn- Air ers (Fig. 16). Figure 17 shows a self- Recuperator regenerative burner that could be Fig. 15 — Double-P-tube with self-recuperative burner. HEAT TREATING PROGRESS • SEPTEMBER/OCTOBER 2007 41 Regenerators Switching valves

Fig. 17 — WS Inc. REGEMAT 250 self-regenerative radiant Fuel Air tube burner.

used for direct firing and to heat re- circulating radiant tubes. The self-re- Fig. 16 — Double-P-tube with self-regenerative burner. generative burner is used in combi- nation with a pulse-firing system; the burner is on/off controlled. All the logic for regenerative switching, flame safety, ignition, and valve op- eration is handled by a local burner- control unit, making installation, start up, and maintenance as easy as with self-recuperative burners. Tube tem- perature uniformity is excellent be- cause of the internal recirculation, and NOx emissions are low due to flameless oxidation. Larger diameter radiant tubes should be used to keep the number of radiant tubes and burners, and, thereby costs, at a minimum. Using double-P tubes, it is possible to heat a furnace using a fraction of the number of burners used in a system with small diameter straight tubes.

Conclusions There are many options for in- creasing the energy efficiency, and preheating combustion air is the most effective way to increase efficiency in most furnaces. The challenges of rising energy costs and stricter envi- ronmental regulations can be met through close cooperation of the end user, the furnace builder, and the burner manufacturer to choose the best possible configuration with re- spect to performance, energy effi- ciency, low emissions, and low main- tenance, while minimizing investments costs.

FLOX, REKUMAT, and REGEMAT are trademarks of WS Inc. For more information: Dr. Joachim G. Wuenning is the president of WS Thermal Process Technology Inc., 719 Sugar Ln., Elyria, OH 44035 ; tel : 440- 365-8029 ; fax : 440-365-9452 ; e-mail; j.g. [email protected]; Web site: www. flox.com. 42 HEAT TREATING PROGRESS • SEPTEMBER/OCTOBER 2007