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.
Energy Costs Drive InnovationIn 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.
The New Role of Control TechnologyIt 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
Emissions Take the SpotlightIn 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).
New Materials Deliver Higher EfficienciesThe 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.
Burner Designs for the FutureThe 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
For more information: Contact Jim Roberts, industry manager – metals markets; Eclipse, Inc., 1665 Elmwood Road, Rockford, IL 61103; tel: 815-637-7217; e-mail: firstname.lastname@example.org; 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