A Wholistic Approach to Enable the Next Era of NOx Reducing Technology in Process Burners

Paper from the AFRC 2018 conference titled A Wholistic Approach to Enable the Next Era of NOx Reducing Technology in Process Burners

Since the development of the "Next Generation Ultra-Low NOx" technology that began nearly 30 years ago, significant reduction of NOx and CO emissions from process heating burner technology has been evasive. Increasing regulatory pressure to achieve even lower combustion emissions has driven the industry to utilize flue gas aftertreatment systems as best available control technology (BACT). Leveraging advanced computing technologies paired with cutting-edge combustion subject matter expertise can enable the next step-change in burner design and performance to provide economic and technical alternatives to the current approach.; A wholistic technical approach identifies a significant gap in the common measurements and controls applied to process heaters today. Global measurements such as total fuel flow to the heater, out-the-stack excess oxygen, and process variables such as product outlet temperature cannot provide enough insight into the combustion region of the heater to optimize emissions.; The new SOLEX™ burner integrates innovative measurement and control strategies with combustion expertise to achieve as low as 5 per million by volume (ppm) NOx emissions regardless of the fuel composition and independent of furnace temperature. The burner system can achieve these low NOx emissions levels from start up to full capacity with near zero CO emissions, regardless of the firebox temperature. Flame lengths are consistently below 1 ft / MMBtu/hr (1 m / MW), solving many issues that Ultra-Low NOx process burner technologies consistently face in the market today.

A Wholistic Approach to Enable the Next Era of NOx Reducing Technology in Process Burners American Flame Research Committee 2018 Industrial Combustion Symposium Presented September 17-18, 2018, Salt Lake City, Utah USA Chad Carroll and Tom Korb John Zink Company, LLC Tulsa, Oklahoma USA Abstract Since the development of the "Next Generation Ultra-Low NOx" technology that began nearly 30 years ago, significant reduction of NOx and CO emissions from process heating burner technology has been evasive. Increasing regulatory pressure to achieve even lower combustion emissions has driven the industry to utilize flue gas aftertreatment systems as best available control technology (BACT). Leveraging advanced computing technologies paired with cuttingedge combustion subject matter expertise can enable the next step-change in burner design and performance to provide economic and technical alternatives to the current approach. A wholistic technical approach identifies a significant gap in the common measurements and controls applied to process heaters today. Global measurements such as total fuel flow to the heater, out-the-stack excess oxygen, and process variables such as product outlet temperature cannot provide enough insight into the combustion region of the heater to optimize emissions. The new SOLEX™ burner integrates innovative measurement and control strategies with combustion expertise to achieve as low as 5 per million by volume (ppm) NOx emissions regardless of the fuel composition and independent of furnace temperature. The burner system can achieve these low NOx emissions levels from start up to full capacity with near zero CO emissions, regardless of the firebox temperature. Flame lengths are consistently below 1 ft / MMBtu/hr (1 m / MW), solving many issues that Ultra-Low NOx process burner technologies consistently face in the market today. Introduction NOx Development History Historical NOx levels from new burner technology in process heater applications was on a significant decline until the 2000s. Figure 1 illustrates this historical trend for the last 60 years. For comparison purposes, emissions data in the figure are based on typical firebox temperatures (1500°F, 815°C) firing typical refinery fuel gases with ambient temperature air at 15% excess air. NOx emissions from the below technologies are very dependent on furnace operating conditions. For example, an ultra-low NOx process burner that achieves 25 ppm in a refining 1 application routinely achieves 40-45 ppm in ethylene applications where the firebox temperatures exceed 2000°F (1093°C) and high hydrogen fuels are commonly utilized. Figure 1 - History of Representative Burner Technology NOx Emissions for Process Heaters Before the 1970s, raw gas or premix burners were applied with NOx emissions commonly greater than 100 ppm. The primary design goal of this technology is to promote combustion stability by turbulent mixing of fuel and air. These burner technologies consistently produce compact flames as there are no NOx reducing techniques applied. Low NOx Burners A major push for emissions reduction by environmental agencies in the 1970-1980s resulted in the development of various "Low NOx" burner technologies. Partial air and fuel staging techniques were introduced that achieve NOx emissions of 80 and 40 ppm, respectively.1 The flame volume of these burner technologies is slightly larger than their predecessors, though not large enough to cause significant flame interaction or flame-to-tube impingement concerns. A rule-of-thumb for Low NOx burner flame length is approximately 1 ft / MMBtu/hr (1 m / MW). Ultra-Low NOx Burners The development of the "Ultra-Low NOx" burner technology in the process heating industry resulted from another wave of regulator pressure in the early 1990s. This technology is 2 commonly applied today and utilizes significant fuel staging to induce large volumes of firebox flue gas into the flame (see Figure 2). This technology is also known as internal flue gas recirculation technology, InfurNOx™ technology, or staged fuel technology. Figure 2 - Ultra-Low NOx Burner Technology for Process Heating Applications It is important to highlight the process heating industry definition of Next Generation Ultra-Low NOx technology and how it compares to the power industry definition of Next Generation Ultra-Low NOx technology. Generally, a single regulating body services both industries which has resulted in some confusion related to what level of NOx emissions are achievable in each industry. External Flue Gas Recirculation Boiler burner Next Generation Ultra-Low NOx technology commonly utilizes external flue gas recirculation (cooled flue gases drawn from the stack of the boiler) to dilute ambient combustion air and reduce combustion temperatures.2 The use of external flue gas recirculation has historically resulted in boiler burner NOx levels that are less than process heating internal flue gas recirculation technologies by a factor of two or more. The application of external flue gas recirculation has resulted in boiler burner technology that can routinely achieve single digit NOx levels. 3 Figure 3 below shows a plan view of a boiler system that illustrates how external flue gas recirculation "FGR" is typically integrated. 3 Figure 3 - Ultra-Low NOx Boiler System Post-Treatment To achieve boiler level emissions in process heaters, it is common to pair a flue gas aftertreatment system, such as a selective catalytic reduction technology (SCR), with internal flue gas recirculation process burner technology.4 SCR technology can routinely achieve 80-90% NOx reduction, allowing the use of higher NOx internal FGR from the process burner as compared to the lower NOx external FGR from boiler burners. Burner Challenges With the process heater emissions broadly satisfied by the application of SCR technology, the process heating industry has been developing ways to mitigate negative performance characteristics of internal flue gas recirculation process burners. This technology stages greater than 80% of the fuel gas to achieved reduced NOx emissions. This couples with relatively low air-side pressure drop (as most applications are natural draft) and significant internal flue gas entrainment to result in burner flames that are much more voluminous than the partial staged air / fuel predecessors. For reference, Ultra-Low NOx process burners typically produce flame lengths approximately 2 ft / MMBtu/hr (2 m / MW), though flame volume with this technology can vary significantly based on a wide array of operational parameters. When Ultra-Low NOx process burners were originally developed and installed, there were many cases where larger flame volumes resulted in severe flame interaction, flame elongation, and flame-to-tube impingement. Since early 2000, computational fluid dynamics (CFD) has been utilized extensively to study furnace flue gas patterns that drive the flame 4 behavior and ultimately heat flux within multi-burner applications to ensure the heater design requirements are satisfied. Fundamental challenges in the application of Ultra-Low NOx technology are not entirely removed with the use of an SCR and proper burner orientation within the heater. Relative to conventional and Low NOx burner technologies, Ultra-Low NOx process burners often produce higher CO emissions at start up and turndown conditions, or in applications with low furnace flue gas bridge wall temperatures (<1300°F, 704°C). Substantial internal flue gas entrainment has the effect of reducing the peak flame temperatures at normal heater operating cases and firebox temperatures but works "too well" at turndown conditions or in cold furnace applications. This relatively cold flue gas can locally quench portions of the flame causing the elevated CO emissions. Figure 4 illustrates the competing response that NOx and CO exhibit versus firebox temperature. Figure 4 - Ultra-Low NOx Process Burner NOx & CO with Varying Firebox Temperature In the 1990s through 2000s, emissions permits were typically scoped for design / normal firing cases where elevated firebox temperatures are expected. During this time, CO emissions at turndown or start up were not a primary focus. Recently, burner emission guarantees are being requested for the full operating range of the burner. Low temperature conditions at burner turndown cases stretch the capabilities of Ultra-Low NOx burner technology. In practice, at turn down and start up conditions, operators are forced to choose between high excess O2 to reduce CO emissions and low excess O2 to meet NOx requirements. In some cases, due to flame length or turndown emission constraints, a Low NOx burner technology (partially staged air or fuel burner) with shorter flame, lower CO, and higher NOx emissions is paired with an SCR. This approach has allowed some flexibility to target low CO emissions in startup and turndown operating cases while still satisfying NOx requirements at the 5 normal operating conditions. This approach may also be applied in short firebox applications where minimal NOx emissions and short flames are required. Flameless Combustion Because of the inherent difficulties, complexity, and cost in the application of SCR technology, many have studied the use of flameless technology to reduce combustion emissions in refining applications. Flameless technology can routinely achieve single digit NOx emissions and has been used for years in industries with high furnace temperatures and minimal operational swings, as is observed in the steel industry. However, broad commercial adoption of flameless technology in process heaters has not occurred. This is likely due the relatively low firebox temperatures and relatively large operating swings that plants experience. To enable a flameless operating mode, a start-up burner or start-up operating mode within the flameless burner technology is typically applied. The start-up mode normally achieves NOx and CO emissions levels equal to a Low or Ultra-Low NOx process burner. After a specific furnace temperature permissive is satisfied (firebox temperature greater than the autoignition temperature of the fuel gas), an operational switch is executed to divert fuel from the start-up mode to the flameless operating mode. Ultimately, the required use of a start-up mode results in a turndown window of operation that is unable to meet the increasing pressure for simultaneous performance of SCR level NOx and low CO at all times. SOLEX Burner Technology John Zink has developed a new burner-only technology that can achieve NOx emissions of 5 ppm while alleviating the undesirable tradeoffs associated with Ultra-Low NOx process burners and SCR systems. In the development process, rigorous fundamental evaluations of existing NOx reducing techniques were performed. Each NOx reducing technique was pushed to its limit in physical testing and evaluated with CFD. The insight gained throughout these experiments and evaluations became the building blocks utilized to develop a new burner that can satisfy many of the emissions, performance, and operating constraints in the market today. Figure 5 is a diagram and Figure 6 is a photograph of the SOLEX burner that can achieve industry-best emissions in all operating modes without the use of a SCR. The SOLEX burner consists of two prominent firing zones referred to as AIRmix™ zone and COOLmix™ zone. 6 Figure 5 - SOLEX Burner Combustion Zones Figure 6 - SOLEX Burner Firing Refinery Fuel Gas at 8 MMBtu/hr The AIRmix combustion zone utilizes lean premix to achieve sub 5 ppm NOx emissions, corrected to 3% O2 dry. The NOx performance from the AIRmix zone is fundamentally decoupled from the flue gas environment as the flame is cooled by excess air, not internal or external flue gas dilution as is common in process burner and boiler burner Ultra-Low NOx technologies, respectively. Additionally, the majority of AIRmix zone combustion is completed within the burner tile itself. This patent-pending arrangement further isolates the AIRmix combustion zone from the furnace flue gas environment, resulting in near-zero CO emissions at 7 start-up and turndown conditions. This level of NOx and CO emissions performance in start-up and turndown conditions has not been possible with Ultra-Low NOx process burners. The COOLmix zone is a staged fuel zone comprised of a relatively small fraction of the total fuel in comparison to Ultra-Low NOx process burner technology. This zone is fired to consume the excess air remaining from the AIRmix zone to achieve the target excess air level necessary for fuel efficiency, typically 2-3% excess O2. To maintain single digit NOx performance, the COOLmix zone utilizes John Zink's patented remote fuel staging technology. This staging technique results in minimal levels of NOx, but much like flameless combustion, it requires a firebox volume temperature that is above the fuel autoignition temperature before being introduced to the heater. Figure 7 is a CFD model illustration of multiple SOLEX burners installed within a cabin style process heater. The region depicted as a blue flame in this simulation illustrates the location within the firebox where the CO concentration is greater than 2000 ppm and where temperature is significantly above the firebox flue gas temperature. This CFD post processing technique results in an isosurface that successfully represents the visual flames observed within John Zink's Research, Development, and Test Center located in Tulsa, Oklahoma USA. This model was executed with refinery fuel gases at 3% excess O2 (dry) and the per-burner heat release was approximately 8 MMBtu/hr (2.3 MW). Greater than half of the burner duty is flowing through the AIRmix combustion zone while less than half of the burner duty is flowing through the COOLmix nozzles installed between the burner and tube passes in the floor of the heater. Figure 7 - CFD of Multi-Burner SOLEX® Implementation in a Cabin Heater There are various technical points worth highlighting relative to this combustion CFD illustration. 1. The AIRmix flames are extremely short. The AIRmix zone of combustion routinely results in visual flames with a length of less than 0.5 ft / MMBtu/hr (0.5 m / MW). 8 2. The flames produced in the COOLmix combustion zone are not visible. Utilizing the proprietary remote fuel staging technique results in a combustion zone that is flameless in nature. Various CFD simulations as well as single and multi-burner tests indicate the COOLmix flame oxidizing at temperatures slightly above the firebox flue gas temperature. The AIRmix and COOLmix arrangement places the hottest flame temperature zone (the AIRmix zone) as far away from the process tubes as possible. 3. COOLmix nozzle placement can be decoupled from the perimeter of the burner tile, resulting in more burner-to-burner spacing flexibility. With Ultra-Low NOx process burners, significant portions of staged fuel around the annulus of the air discharging from the burner tile results in burner arrangements where staged fuel from one burner may be directly adjacent to another burner. If staged gas tips of adjacent burners are too close to one another, elevated NOx emissions, burner-to-burner flame interaction, and undesirable firebox flue gas patterns can result. The SOLEX burner technology eliminates this problem. 4. The COOLmix nozzles can be strategically placed to control heat flux patterns. Some applications, such as ethylene, require the heat flux to be distributed over a long distance with a specified distribution. The additional degree of freedom in COOLmix nozzle location can be beneficial when compared to a traditional process burner technology where the fuel can only be introduced adjacent to the burner itself. This may also prove to be beneficial in retrofit applications when trying to correct adverse flue gas patterns resulting from sub-optimal placement of the original burners. The SOLEX burner can meet industry-leading emissions throughout a large range of operating conditions and fuel gases. The two-fuel zone approach enables the fraction of fuel diverted to the COOLmix zone to be varied to achieve a constant performance of NOx emissions, regardless of the operating conditions. As an example, shown in Figure 8, when firing natural gas with the proper AIRmix to COOLmix duty splits, NOx emissions (left hand vertical axis) of ~ 5 ppm, corrected to 3% O2, can be achieved. If the gas composition is transitioned to a highhydrogen fuel without modifying the split of AIRmix to COOLmix fuel flow, the resulting NOx emission could increase to 8-9 ppm (near 100% increase). However, by automatically increasing the fraction of total fuel being diverted to the COOLmix zone, the AIRmix zone will burn at a leaner state (higher excess air) and 5 ppm NOx emissions can again be achieved. 9 Figure 8: AIRmix Combustion Zone Emissions with Varying Fuel and Excess Air The CO emissions response (right hand vertical axis) to fuel and excess air can also be observed in the above figure. Choosing single-burner emissions boundaries of sub 5 ppm NOx and sub 50 ppm CO, an operating window of excess air from the AIRmix zone is created. Understanding the need for emissions margin in an operating facility relative to single and multiburner research and development test results, emissions guarantees from multi-burner field installations could likely be sub 10 ppm NOx and sub 50 ppm CO throughout the full operating range of a heater. Smart Combustion™ Solutions From Figure 8, it can clearly be seen that global measurements, such as total fuel flow to the heater, out-the-stack excess oxygen, and process variables such as product outlet temperature cannot provide enough insight or control into the combustion region of the heater or burner. A wholistic and integrated controls approach has been developed by implementing elements of the John Zink Smart Combustion solution. Integrating measurement devices, combustion expertise, and real-time calculation and control concepts have resulted in a new burner technology and integrated control approach that is able to achieve single-digit NOx emissions regardless of the fuel composition and relatively independent of furnace temperature. The Smart Combustion approach leverages John Zink fundamental NOx, air flow, and fuel flow engineering models and integrates them into the 10 control system, enabling the SOLEX burner to achieve combustion performance previously not possible by burner-only process heating solutions. Regarding integration of burner safety systems, there are several key points to consider. First, in contrast to completely flameless combustion systems where the start-up burner is shut off to transition to flameless mode, the AIRmix zone is always in operation. This provides a continuous location that is ideal for flame sensing. Second, pilots in flameless systems are often shut off to ensure the flameless mode is sustained properly. With the SOLEX burner technology, a pilot can be integrated within the AIRmix zone to provide a continuous source of ignition as an independent primary combustion zone. Conclusions The SOLEX burner can achieve single-digit NOx emissions and near-zero CO emissions throughout a wide range of operations and applications without the use of an SCR. Integrating elements of John Zink Smart Combustion enables the effective management of AIRmix fuel-toair ratio by real-time modulation of gas to the AIRmix zone and the COOLmix zone. The new burner technology, applied with improved resolution measurement and combustion control, will minimize emissions, alleviate constraints, reduce costs, and minimize reliability risks associated with Ultra-Low NOx process burners combined with SCR units in the market today. References 1 Charles E. Baukal, Industrial Combustion Pollution and Control, Marcel Dekker, New York, 2004. Charles E. Baukal and Wes Bussman, "NOx Emissions," Chapter 15 in The John Zink Hamworthy Combustion Handbook, Vol. 1: Fundamentals, CRC Press, Boca Raton, FL, 2013. 2 Tim Webster and Steve Bortz, "Ultra-Low NOx Burners Can Get Even Better," Power Engineering, November 1st, 2008. 3 4 Charles E. Baukal, I-Ping Chung, Stephen B. Londerville, James G. Seebold, and Richard T. Waibel, "Pollutant Emissions," Chapter 10 in The Coen & Hamworthy Combustion Handbook, CRC Press, Boca Raton, FL, 2013. 11