This article discusses different types of direct-fired gas burners that can be used for high-temperature applications. It will explain the differences in design and how they relate to complexity, cost and efficiency. It will also explain the trade-off between efficiency and NOx emissions and highlight a combustion technology that makes it possible to have the best of both worlds. Finally, it will touch on the economics of high-efficiency gas burners.
Gas Burner Technology
The graph depicted in Figure 1 shows combustion efficiency (based on the lower heating value) as a function of exhaust-gas temperature prior to the heat exchanger (if one exists for a particular burner type). In the case of a direct-fired burner, this is equal to the temperature of the furnace.
The curve labeled e = 0 represents a cold-air burner (i.e., no combustion air preheat). As the name implies, the air is cold (or at approximately ambient temperature) as it enters the combustion chamber. This type of burner has a very simple design, which generally consists of a gas pipe, gas nozzle, air pipe, air diffusor/mixer, combustion chamber and possibly a burner tile. At a temperature of 2350°F (1288°C), the best-possible efficiency for this type of burner is approximately 36%. Many older cold-air burners have an efficiency that is even lower than the theoretical maximum due to a variety of factors such as burner design, lack of maintenance and improper tuning. So while this burner type tends to be very inexpensive, it is also very inefficient (especially at forging temperatures). Figure 2 shows a typical cold-air burner.
The curve labeled e = 0.4 represents a burner equipped with a central heat exchanger. This is also known as a hot-air burner because the combustion air is preheated by means of the heat exchanger before it reaches the combustion chamber. The design is very similar to that of a cold-air burner, but some of the materials may be of a higher grade to withstand the higher air temperatures. In this case, the combustion air is preheated to approximately 40% of the exhaust-gas inlet temperature.
At the reference temperature of 2350°F (1288°C), this burner type has an efficiency in the range of 50-55%. Although this burner type itself is rather inexpensive and more efficient than a cold-air burner, the external heat exchanger required tends to be large, complicated and expensive. Figure 3 shows a typical hot-air burner installation with external heat exchanger.
The curve labeled e = 0.6 represents a self-recuperative burner. With this burner type, the heat exchanger is an integral part of the burner, and it sits directly inside the wall of the furnace. This arrangement helps to minimize heat losses to the ambient and thereby provides increased efficiency. In this case, the combustion-air preheat can range anywhere from 40-65% of the exhaust-gas inlet temperature, depending on the relationship between burner power and heat-exchanger surface area. So at the same reference temperature of 2350°F (1288°C), this burner type can achieve an efficiency ranging from 50-65%. This burner is at least as efficient, if not more so, than a hot-air burner. It also offers a much more compact heat exchanger. While the cost of the burner itself is higher, it eliminates the cost, complexity and space requirements for an external heat exchanger. Figure 4 shows a cross section of a self-recuperative burner.
The curve labeled e = 0.8 on the graph represents a regenerative burner. This burner type is equipped with heat-storage media such as ceramic balls, discs, etc. Heat-storage media is therefore in direct contact with either hot exhaust gas or cold combustion air, depending on the point in the regeneration cycle. In the first half of the cycle, the hot exhaust gas heats the storage media to a very high temperature. Switching valves are then activated, and the flow path is reversed so that cold combustion air now flows over the heat-storage media.
With this arrangement, the combustion air is preheated to approximately 80-85% of the exhaust-gas inlet temperature. At the reference temperature of 2350°F (1288°C), this burner type has an efficiency in the range of 75-80%. Traditional regenerative burners fire in pairs – one burner fires while the other exhausts and vice versa.
A newer type known as the self-regenerative burner integrates all the regenerators and switching valves into one self-contained unit. Each burner contains six passageways, and each passageway contains a row of ceramic honeycomb discs that serve as the heat-storage media. At any point in the cycle, three passageways are exhausting, and the other three passageways are admitting combustion air. After about 10 seconds, the switching valves cycle, and the flow path is reversed.
This burner offers the ultimate in fuel-efficiency. It is certainly more complex and expensive than the other types, but the fuel savings can easily compensate for these drawbacks (especially at forging temperatures). Figure 5 shows a cross section of a self-regenerative burner.
NOx Reduction Techniques for High-Efficiency Burners
Traditionally, there has been a trade-off between combustion efficiency and NOx emissions. In order to achieve high efficiency, it is necessary to preheat the combustion air to high temperatures. These high combustion-air preheat temperatures lead to high peak flame temperatures, which are the primary driver in NOx formation. NOx emissions are an exponential function of peak flame temperature, so they tend to increase rapidly with increasing furnace temperature and increasing combustion-air preheat temperature. There are a number of techniques available to help combat this problem.
One such technique is known as air staging. With this technique, a portion of the combustion air is mixed with all of the fuel to generate a partial reaction and release some heat. Then the rest of the combustion air is introduced a bit further downstream to complete the reaction and release some more heat. In this way, the reaction is spread out rather than concentrated at one point. This serves to reduce peak flame temperature and thereby decrease NOx emissions.
High-velocity combustion also serves as a NOx reduction technique. Mixing exhaust gases thoroughly inside the furnace or radiant tube has a temperature averaging effect. Therefore, peak flame temperatures are reduced, and NOx emissions are decreased accordingly.
Likewise, flue-gas recirculation can also serve as a NOx reduction technique. Exhaust gases are very hot but not as hot as a flame. So pulling a portion of the inert exhaust gases back into the flame front actually produces a cooling effect. This effect serves to lower peak flame temperatures and hence NOx emissions.
All of these techniques are quite effective under normal conditions. However, when combustion-air preheat temperatures reach very high levels, as in the case where self-recuperative or (self-) regenerative burners are utilized, the techniques are frequently not enough to reduce NOx emissions to acceptable levels. This problem, of course, becomes even more relevant for furnaces operating at very high temperatures.
Fortunately, a revolutionary combustion technology has been developed to resolve this problem. This technology is known as FLOX® combustion, or FLameless OXidation.[1,3] With this special technique, fuel and air are mixed with recirculated exhaust gases, and a spontaneous combustion reaction, which produces no visible flame, takes places. By eliminating the flame from the combustion reaction, peak temperatures are reduced dramatically. This suppresses NOx emissions to a fraction of the level achievable with traditional NOx-reduction techniques.
This process only occurs above the auto-ignition temperature, and some safety factor is required. The FLOX transition temperature is typically set at 1550°F (850°C). Below this temperature, the burner operates in a normal mode of combustion with a flame. Once the FLOX transition temperature is reached, the gas is injected in a fashion that produces a more favorable mixing/recirculation pattern and prevents flame formation and attachment. If the temperature drops below 1550°F (850°C), the burner automatically reverts to “flame” mode.
Economics of High-Efficiency Gas Burners
The investment in high-efficiency gas burners makes sense economically even when gas prices are low, especially at high operating temperatures. For example, assume that a forging furnace operating at 2350°F (1288°C) has a net input requirement of 1 MMBtu/hour. If cold-air burners are utilized, the gross input requirement is approximately 2.8 MMBtu/hour. If hot-air burners are selected, the gross input requirement drops to approximately 2 MMBtu/hour.
By opting for self-recuperative burners, the gross input requirement may be reduced to as little as 1.5 MM Btu/hour. Finally, by going “all in” with (self-) regenerative burners, the gross input requirement is slashed to approximately 1.3 MMBtu/hour. Assuming a natural gas cost of $3/MCF, an annual operating cycle of 8,000 hours per year and an average burner load factor of 70%, the estimated annual fuel cost savings accrued by selecting a (self-) regenerative burner system over a cold-air burner system is approximately $25,000. These savings could pay for the additional cost of the (self-) regenerative burner system in less than two years.
There are a variety of burner types available for high-temperature applications with varying levels of complexity, cost and efficiency. Most high-efficiency gas burners preheat the combustion air to increase the combustion efficiency. Traditionally, there is a trade-off between efficiency and NOx emissions. FLOX® combustion, however, makes it possible to have the best of both worlds.
As demonstrated, the investment in high-efficiency gas burners makes sense economically even when gas prices are low. Our economic analysis of a simple example clearly shows that the higher the operating temperature of a furnace, the more important it is to consider an investment in fuel-saving technology.
For more information: Contact Steven R. Mickey, sales engineer, WS Thermal Process Technology Inc., 8301 West Erie Ave., Lorain, OH 44053; tel: 440-385-6829; fax: 440-960-5454; e-mail: email@example.com; web: www.flox.com.
1. Joachim G. Wünning, Ambrogio Milani: Handbook of Burner Technology for Industrial Furnaces, 2nd Edition, Vulkan Verlag, 2015
2. Image provided by WS Wärmeprozesstechnik GmbH, Dornierstr. 14, 71272 Renningen, Germany
3. Joachim G. Wünning: “Flameless Oxidation,” 6th HiTACG Symposium, Essen, Germany, 2005 (https://flox.com/documents/05_HTACG_FLOX.pdf)