Significant changes in the needs and processes of the modern industrial world have forced burner technology to change as well.

New Eclipse TJSR direct-fired, self-recuperative, high-velocity burner


Fig. 1. Premix-type velocity burner, circa 1950s

In the late 1950s, simple hardening and annealing metallurgy was very prevalent. Direct-fired box furnaces were most often used for production metallurgy and in heavy-equipment manufacturing. Atmosphere furnaces were most often used in the fastener industry and for automotive precision-gear manufacturing. These furnaces were made air and atmosphere tight by using metallic tubes as the heat transfer medium. The direct-fired box furnace commonly used premix-type velocity burners (Fig. 1). The concept of these burners was to have the gaseous fuel mixed with air, forcing the resulting combustion flue products out of the burner block at high velocities.

This velocity produced very good temperature uniformity in the furnace and improved heat transfer to the parts, so users could depend on constant quality of the processes and products. There were, however, distinct disadvantages with premix technology – poor fuel efficiency, limited turndown capabilities and a potential for flame flashback. This created the demand for the development of a large number of nozzle-mixing-type burner technologies to overcome these limitations.

Fig. 2. Preheated combustion air supplied by a recuperator

Energy Costs Drive Innovation

In the 1970s, the price of fuel started to climb, driving combustion users to demand newer technologies to save on energy costs. In Great Britain and Germany, in particular, the cost of fuel drove manufacturers to develop very fuel-efficient designs. This was the start of the process of taking known designs, modifying the mix nozzles and adding the element of preheated combustion air.

By the late 1980s, it was learned that by using high-temperature combustion air in a burner, the flame temperature would increase by several hundred degrees. This technique increased the heat transfer to the parts, and fuel savings of up to 35% could be realized. This discovery led designers to further develop and refine direct-fired equipment and tube-firing burners. Soon, efficiencies of 55-65% were being realized by making the recuperative section an integral part of the burner assembly (Fig. 2).

This type of design was now widely accepted by the North American markets, where fuel costs had approached the levels seen earlier in Europe and open Asian markets. So, in the course of only 30 years, there had been a complete shift in the technology and acceptance of these technologies by both the furnace manufacturers and the end users of industrial furnaces.

* Based on HHV

The New Role of Control Technology

It was also in this time period that a new type of control method was being developed. Until the 1980s, the control of burners was predominantly either a high/low-firing operation or a modulating or proportional control. In these systems, the firing of the burners was cycled up or down based on a temperature reading in the furnace in an attempt to match the heat input to the desired temperature. With high/low operation, the burners cycled between two distinct settings of input. With modulating or proportional control, the burners would cycle air and fuel flows to control the temperature to meet the setpoint desired.

With the availability of computerized controls and valves rated for high-cycle use, a new type of burner-control system, or scheme, was developed. This type of control, referred to as pulse firing, incorporated the control elements of earlier firing schemes. It also allowed the burners to be fired independently of each other and in shorter, or longer, controlled bursts of input. This, in turn, allowed for very precise temperature control, making it possible to precisely match firing recipes to more complex product specifications.

With the recuperative technologies reaching refined levels, burner designs were now being developed to accommodate the newer firing methods. This led to the development of burners such as the Eclipse ThermJet-style burner, which uses multiple levels of air staging for desired firing characteristics (Fig. 4).

The advantages of this type burner are:
  • Extremely high exit velocities, which stir the flue gases in the furnace for improved heat transfer
  • Lower emission levels because of lowered peak flame temperatures
  • Extremely wide adjustment ranges of excess air and gas operation to suit temperature and process configuration
The basic design features of this jet technology continue to advance the development of self-recuperative burner designs.

Fig. 4. Cutaway of Eclipse jet-style burner illustrates air staging

Emissions Take the Spotlight

In the mid-1990s, a new concern emerged for combustion equipment designers and users – greenhouse-gas awareness and air-quality regulatory efforts, such as the Kyoto Protocol. In the U.S., regulatory bodies, such as the California South Coast Air Quality Board, began demanding air quality standards similar to those promoted in Europe since the early 1990s. The emerging markets in Asia were following the global mandate for cleaner, more efficient burner designs as well. There was a demand and a design push for lower-emission, higher-output burners.

The easiest technique to lower NOx emission levels from indirect-fired burners was to design the burner as a recirculating device, mixing the flue-gas products back into the stream of gas and combustion air. This was made possible by the advances in superalloys and ceramic-based materials that could withstand increased thermal shock and internal temperatures of the flue-gas streams and higher velocities (Fig. 5).

Fig. 5. New Eclipse SER V5 indirect-fired, self-recuperative burner

New Materials Deliver Higher Efficiencies

The burner, heat exchanger and recirculating device were all designed to be an integral part of the burner. This ingenious internal-recuperator design reduced the factory footprint of the furnace and alleviated the need for large, bulky and remote heat exchangers to retrieve the waste heat from the flue stream and pipe it to the burners. With a continuing demand for higher efficiency and lower maintenance, the newer designs of self-recuperative burners (SRBs) for direct- and indirect-fired applications are still improving.
  • The heat-exchanger design has been optimized by additional surface area and increased turbulence to improve the convection heat transfer.
  • Efficiency in the high 70% range (based on lower heating values) is now becoming common with new burner designs.
  • There are much lower NOx emissions as a result of internal flue-gas recirculation.
  • Internal insulation has been added to improve the working environment.
  • Ceramic is being used more frequently for the heat exchanger and burner nozzle, thereby providing higher temperature capabilities and extended service life.
  • Ceramic is also being used more widely for the inner and outer tubes for indirect-fired applications, increasing energy output density and the service life of these components.
Recently, the concept of flameless oxidation was developed. This technology allows for a very slow and controlled mixing of the fuel and air within the furnace chamber (or radiant tube, if indirect firing is being used). As a result, the peak temperatures of the flame stay below the critical stages where high levels of thermal nitrogen oxides (NOx) are formed. Development continues on this innovative combustion technology.

Burner Designs for the Future

The SRB of today (and those of tomorrow) is a very advanced design. Features include:
  • Use of superalloys and space-age ceramics assuring the hardware life is measured in decades instead of years
  • Use of advanced heat exchangers increasing efficiencies from 25% to 80% or higher
  • Exit velocities tuned to the exact process for which they are being used
  • Operation on almost any control scheme whether high-low, modulating or pulse firing
  • Reduction of emission levels by almost 90% in just 10 years
In conclusion, the burner technology of tomorrow will continue to be refined and will evolve to deliver even better process control, higher efficiencies and lower emissions. With all of these design improvements, perfect combustion certainly seems within reach.

For more information: Contact Jim Roberts, industry manager – metals markets; Eclipse, Inc., 1665 Elmwood Road, Rockford, IL 61103; tel: 815-637-7217; e-mail: jroberts@eclipsenet.com; web: www.eclipsenet.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: direct-fired burner, tube fired, pulse fired, self-recuperative, NOx emissions, indirect firing