Emerging Ultra-Low-NOx Burner Technology for the Heat-Treat Industry
The current business climate that producers in the heat-treat market face has put immense pressure on manufacturing facilities throughout our country.
Customers demand higher-quality products at more competitive pricing. Management requires continuous improvement in productivity, quality and overall costs. All the while, government continues to require reductions in NOx emissions with the ever-looming possibility of regulating a manufacturing facility’s “carbon footprint” as well. These demands leave producers continually searching for more effective products that provide improvements for their processes to meet these enormous challenges.
The heat-treat industry is particularly affected by these demands. The exploration for a product that meets as many of these objec-tives as possible at an acceptable amount of investment capital can be extremely challenging and often downright frustrating.
Nearly 80 years in the global metals market provides Bloom Engineering with an understanding of these industry needs. This industry know-how resulted in a recent breakthrough in ultra-low-NOx burner technology – the Bloom SC1500-series burner (Fig. 1). Over the years, the company’s experience has led to continual advancements in ultra-low-NOx burners. Product testing, refinement and development continue at the company’s 10,000-square-foot development facility at its U.S. corporate headquarters in Pittsburgh, Pa. (Fig. 2).
The SC1500-series burner utilizes an ultra-low-NOx design that addresses the escalating government regulations on furnace emissions and the demand for improved heating uniformity for high-quality products. This burner provides NOx emissions of less than 20 ppm. This new technology also provides extremely uniform burner heat-release patterns that are capable of achieving uniformity requirements as low as +/-5?F throughout the furnace.
This type of performance exceeds many of today’s strictest uniformity specifications, which are often required in aerospace, medical, oil/gas and other high-quality product processes. The burner also is designed to operate at near stoichiometric air-to-fuel ratios, thereby ensuring very efficient fuel consumption.
The SC1500 burner is a multi-staged technology burner design evolved from incorporating high-velocity Uni-Heat® burner technology with laboratory breakthroughs to lower the emissions. The new design was developed through significant experimentation in the lab combined with CFD modeling and analysis to expedite the development process (Fig. 3). A multitude of emission-improving technologies were modeled and tested in various combinations to arrive at a final design. The goal of the final design was to significantly improve emissions and maintain the characteristics desired to provide optimum heat-treating combustion performance.
The SC1500 design includes a good mixing, high-velocity combustion pattern. It minimizes NOx by utilizing Bloom’s latest air-staging techniques coupled with high-momentum fuel-nozzle technology. The result is a patented ultra-low-NOx-emitting burner with a uniform heat-release combustion profile.
This uniform, ultra-low NOx flame temperature profile improves upon all previous designs. This design, coupled with proper burner arrangements and control techniques, allows many applications to achieve uniformity surveys of +/-5?F. This was not possible in the past.
This burner design includes a typical single air and single gas connection body design that allows for simple control, piping and even retrofit installations. In the primary stage of combustion, a portion of the burner combustion air mixes with a portion of the fuel, creating a stable flame anchor. This allows a portion of the heat to be released near the burner wall. In the secondary combustion stage, the remainder of the air is introduced through self-contained air-staging ports located in the included burner block/tile to provide a uniform and controlled heat-release profile in the burner flame. This arrangement provides extremely uniform temperatures throughout the furnace chamber (Fig. 4).
The burner body uses a reliable and durable cast iron design with threaded connections. The burner includes a robust cast iron mounting plate and a metal-encased refractory port block. The block material and encasement ensure a long-life design that is suitable for installation in refractory or fiber-walled furnaces.
The standard burner uses a medium-velocity port design (an optional high-velocity design is also available where applications require). The state-of-the-art design makes this high-velocity firing option possible without any detrimental effects on NOx emissions. This allows the customer to “fine tune” his furnace system performance by applying a burner with excellent stirring action that optimizes the batch uniformity for lower-temperature applications.
Ignition, Cold Stability and Safety
The SC1500 burner can be ignited via simple, low-maintenance direct-spark igniters (Fig. 5), small pilot burners or even a hand torch. The burner body also provides flame-supervision ports capable of accommodating a UV sensor or flame-rod. The direct-spark igniter can also serve as a dual-purpose flame-rod/igniter in many applications, which reduces flame-monitoring package costs, mitigates overall system maintenance and increases reliability. This technology provides a reliable flame signal from cold furnace startup through hot operation.
The combustion pattern (flame) is extremely stable. In most cases, the burner can be operated with minimal excess air even in a cold furnace. Furthermore, the burner’s stability characteristics allow cold furnace starts without the need for additional valves/piping or special “startup” modes. This allows for safer, simplified controls and efficient in-ratio firing (Fig. 6).
In some localities, higher NOx levels are recognized during furnace startup periods due to existing startup-mode burner technologies that emit higher NOx. In most cases, however, the SC1500-series burner eliminates this issue because ultra-low-NOx emissions are achieved from cold start all the way through typical hot furnace operating temperatures.
Emissions and Control
Historically, burner performance limited furnace control. This new technology no longer poses that impediment because of its very flexible fuel-air ratio control techniques and burner turndown. The combustion pattern and burner design lend themselves well to pulse firing and/or modulation control. The burner provides reliable ignition at high fire and is also capable of firing at a turndown of 10 to 1 (10% of the nominal burner firing rate) while still maintaining good flame characteristics and ultra-low NOx. The burner mixing is so efficient that it can be used in applications that fire at sub-stoichiometric conditions, in-ratio or with high excess air. The burner typically operates at 5% excess air with mass-flow metering control for optimum efficiency. These combustion parameters allow finite control of the furnace combustion system.
Furthermore, the combustion pattern is so stable and efficient that, even when operating in near-ratio combustion, the burner’s carbon monoxide (CO) emission levels do not typically exceed 20 ppm and often remain less than 10 ppm. Many existing burner designs produce higher CO emissions at furnace temperatures below 1400?F (760?C). This may lead to unsafe OSHA CO levels around a furnace, and it also creates concerns for nauseating aldehyde odors. These safety concerns are avoided with the SC1500 design.
The burner has both cold and hot combustion-air designs for those looking to minimize their fuel usage. Typically, hot combustion air dramatically increases NOx emissions. However, as can be seen on the emissions graph (Fig. 7), this design minimizes that effect. On cold combustion air, the burner provides groundbreaking sub-20-ppm NOx emissions. The hot air emissions are also very low but vary with air preheat and furnace temperature. As mentioned earlier, a high-velocity furnace-stirring burner design is available without negatively impacting the NOx emissions created.
Bloom has raised the bar for achievable NOx emissions in today’s furnaces while also providing a burner uniquely suited for the rigorous needs of heat-treat producers. The Bloom SC1500-series burner provides customers with an efficient product capable of ultra-low NOx emissions while also allowing them to achieve superior furnace uniformities that approach +/-5?F.
These performance characteristics lend themselves to both low- and high-temperature heat-treating applications where tight uniformity specifications and/or local regulations severely limit furnace emissions. These burners can help producers with furnaces operating at temperatures as high as 2500?F (1371?C). This allows SC1500 technology to benefit nearly every heat-treating application on the market today (Fig. 8). Due to its size and simple controls, it is also easily retrofitted onto existing furnaces. These burners have been installed on differing process applications in Texas, California and other states where local regulations have greatly reduced allowable NOx emissions. IH
1U.S. Patent 6,685,463
For more information: Contact Bloom Engineering Company, Inc., 5460 Horning Road, Pittsburgh, PA 15061; tel: 412-653-3500; web: www.bloomeng.com