Application of "Next Generation" Ultra-Low NOx Burners in High Temperature Furnaces Richard D. Waibel, PhD - John Zink Co., LLC - Tulsa, OK, USA

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1 Application of "Next Generation" Ultra-Low NOx Burners in High Temperature Furnaces Richard D. Waibel, PhD - John Zink Co., LLC - Tulsa, OK, USA Sommario Nel corso degli anni passati, é stata sviluppata una tecnologia per Bruciatori ultra Low NOx basata sul principio della premiscelazione magra. Tale tecnologia é stata utilizzata in una numerosa serie di applicazioni su forni industriali, con notevole successo nella riduzione delle emissioni di ossidi di azoto. Recentemente questo tipo di bruciatore é stato applicato ai forni ad alta temperatura per la produzione di etilene. Questi forni sono un applicazione particolarmente difficile a causa delle elevate temperature operative nella sezione radiante e a causa delle esigenze di particolari profili del flusso termico nella camera stessa. Infatti, cambiamenti nella distribuzione del flusso termico possono influire sulle rese dei prodotti, come pure sulla presenza di alte temperature localizzate sui tubi,che possono ridurne la durata e la frequenza dei cicli di decoking. L applicazione delle tecnologie ultra Low NOx con premiscelazione magra é stata il frutto di uno sforzo coordinato tra i progettisti dei bruciatori, dei forni e degli utilizzatori finali. La combinazione del bruciatore ultra Low NOx a premiscelazione magra e l analisi tramite CFD ha permesso agli utilizzatori finali di raggiungere risultati per le emissioni che hanno permesso loro di ottemperare alle più recenti normative degli Stati Uniti, senza dover ricorrere a sistemi di riduzione catalitica selettiva e senza sacrificare né la produzione né il funzionamento del forno. I bruciatori a bassa emissione di NOx utilizzati precedentemente in questa difficile applicazione ad alta temperatura, hanno prodotto emissioni in un campo tra 26 e 47 mg di NOx per MJ ( lb per milione di Btu). Prove in campo hanno mostrato che i bruciatori con tecnologia Ultra Low NOx a premiscelazione magra possono raggiungere emissioni comprese tra 13 e 16 mg di NOx per MJ ( lb per milione di Btu). Summary Over the past several years lean-premix ultra-low NOx burners have been developed and utilized in a variety of industrial furnace applications with remarkable success in reducing NOx emissions. Recently this type of burner has been applied to high temperature ethylene furnaces. These furnaces are a particularly difficult application in terms of the high operating temperatures in the furnace radiant section and the prescribed flux profile requirements in this chamber. Changes in the heat flux distribution can lead to effects on product yields as well as localized high tube metal temperatures affecting tube life and reducing run lengths between decoking cylcles. Application of the lean-premix ultra-low NOx burners has been a collaborative effort between the burner designers, furnace designers and the endusers. 1 The combination of ultra-low NOx lean-premix burner design and computational fluid dynamic analysis has allowed the enduser to

2 achieve emission results that meet the emission reduction requirements mandated by the latest regulations without the need for selective catalytic NOx reduction methods and without sacrificing production rates or furnace operation. Previous low NOx burners utilized in this difficult, high temperature application have produced emissions in the range of 26 to 47 mg of NOx per MJ (0.06 to 0.11 lb per million Btu). Field tests have shown that the new lean-premix ultra-low NOx burners can achieve emissions in the range of 13 to 16 mg of NOx per MJ (0.03 to lb per million Btu). Introduction New environmental regulations for the Houston-Galveston area of Texas in the USA originally required that NOx emissions from process heaters in refineries and chemical plants be reduced to 90% of year 2000 levels. 2 The regulation was later modified to require an 80% reduction rather than 90%. The revised regulation specifies that the net emissions from all the process heaters in ethylene plants is limited to 15 mg of NOx per MJ (0.035 pounds of NOx per million Btu) fired. The regulation is based on a summation or of all the emission sources at a location, sometimes called a bubble, so it is possible to control some sources to a lower emission level than other sources in order for the ovall level to be met. Low NOx technology has always been technology driven by regulatory requirement and this new Texas regulation along with tightening NOx emission limits in other locations have spurred a new round of low NOx burner development. However, many process heaters utilize natural draft burners which limits design options and thus NOx reduction options. In addition these applications burn wide ranging fuel compositions, often including high percentages of hydrogen. Ethylene furnaces are a particularly difficult application for low NOx burners in terms of the high operating temperatures in the furnace radiant section and the prescribed flux profile requirements in this chamber. Changes in the heat flux distribution can lead to effects on product yields as well as localized high tube metal temperatures affecting tube life and reducing run lengths between decoking cylcles. Previous low NOx burners for ethylene applications have utilized staged fuel combustion and internal furnace gas recirculation. Although it has been possible to achieve NOx emissions on the order of 11 to 13 mg/mj (0.025 to 0.03 pounds per million Btu) with this type of burner in process heaters in refineries, the emissions levels achieved in ethylene cracking furnaces have been in the range of 25 to 30 mg/mj (0.057 to lb/mmbtu) due to the high operating temperatures in these furnaces. As a result next generation, ultra-low NOx burners were required to meet the new regulations. The application of next generation ultra-low NOx burners to an ethylene furnace was achieved through an intensive, collaborative effort that included single burner testing and development, CFD modeling of multiple burners in an industrial furnace, multi-burner testing in a test furnace and finally testing in a full-scale, industrial ethylene furnace. The CFD and multi-burner test facility testing was done to make certain that the ethylene furnace performance would not be impacted by the burner retrofit.

3 Next Generation Ultra-low NOx Burners The latest next generation burners utilize combinations of enhanced fuel staging, quasiflameless combustion, enhanced internal furnace gas recirculation and lean premix combustion to achieve emissions on the order of 15 mg/mj (0.035 lb/mmbtu). NOx emissions from previous generations of low NOx burners can be improved by optimizing techniques used previously and by employing new techniques. Fuel staging and internal furnace gas recirculation have been enhanced in the latest burners by optimizing and using multiple fuel injection zones to improve furnace gas entrainment. Additional benefit has been obtained by adding lean-premix combustion and quasi-flameless combustion techniques. The lean premix combustion technique produces low NOx levels by reducing both thermal and prompt NOx. Figure 1 shows the relative NOx emissions for both premix and diffusion or raw gas flames under fuel rich and fuel lean conditions. The figure shows that premix flames near the lean flammability limit result in very low NOx emissions. Lean-premix flames produce very low NOx emissions because the intimately mixed excess air reduces local flame temperatures reducing thermal NOx and the lean combustion conditions suppress prompt NOx. Although lean-premix combustion can produce low NOx emissions, used exclusively it is not practical because, by it s very nature, it produces combustion products with very high excess air levels. However, combusting part of the fuel under lean premix conditions reduces overall NOx and lean premix combustion is an excellent source for the high temperature, dilute oxygen, combustion products required for quasi-flameless combustion. Figure 1 Relative NOx Emissions for Diffusion and Premix Flames

4 Flameless combustion results when the fuel and air are each injected so they mix with hightemperature furnace gases prior to mixing with each other. As the furnace gases mix with the fuel and air they both heat and dilute the fuel and air streams. If the furnace gases are hot enough, these mixtures will be above the ignition temperature. Thus when the high-temperature fuel mixture and air mixture mix with each other, simple oxidation reactions occur with only modest increases in the local temperature resulting in low NOx emissions. Quasi-flameless combustion occurs when the staged fuel mixes with large quantities of hightemperature furnace gases prior to mixing with the high temperature lean-premix combustion products. Computational Fluid Dynamic(CFD) Modeling The CFD modeling by the furnace designers began with a base case model of the existing furnace. The original industrial furnace had six hearth burners and six rows of nine radiant wall burners firing on each sidewalls with the six pass radiant coil situated along the centerline of the furnace. The design heat input is 264 GJ (250 MMBtu/hr). The modeling results showed flame shapes, heat flux profiles and tube metal temperatures that agreed well with the baseline furnace performance. Initially 12 lean premix burners were installed in place of the existing 12 hearth burners. Since the ultra-low NOx burners would have longer flames than the existing burners, the bottom two rows of wall mounted burners were deleted from the model to give more separation between the hearth burners and the wall burners. The heat release from the missing wall burners was added to the hearth burners. The results of the model suggested that this configuration would not be acceptable. The model showed that the flame produced by the higher heat release ultra-low NOx burners would impinge on the process coils. This resulted in excessive tube metal temperatures that would accelerate coke formation and provide unacceptably short run lengths between coking cycles. The model was then modified to include 24 lower heat release hearth burners and the original six rows of wall burners. This configuration limited the spacing between hearth burners. The results for this configuration also showed flames curling over into the tubes. An evaluation of the two models suggested that eighteen hearth burners, nine per side, with strategically placed gaps between burners could minimize the flame impingement problems. This configuration was modeled and the results were encouraging. The model predicted no flame impingement with heat flux profiles and tube metal temperatures similar to the base case. Burner Testing The initial development work involved designing and building prototype ultra-low NOx lean premix burners in conjunction with the modeling effort. Both the prototype hearth burner and wall burner were individually tested with simulated fuels and under furnace conditions simulating the operating conditions in an ethylene furnace. Each burner performed well with acceptable flame patterns and NOx emissions well below the required emission limits. The NOx emissions from each single burner test were about 9 to 12 mg/mj (0.02 to lb/mmbtu).

5 However, experience has shown that both flame patterns and NOx emissions could be impacted by burner/burner interactions. For this reason a multi-burner test was organized in the burner test facility. This test involved two hearth burners and ten radiant wall burners in 5 rows of 2 burners per row firing against one wall of the test furnace. The purpose of this test was to evaluate the effect that interaction between the hearth burners and the bottom row of radiant wall burners, as well as the interaction between adjacent radiant wall burners had on emissions and flame patterns. Test results confirmed that flame patterns would be acceptable and the data provided an estimate of the impact of burner interactions on NOx emissions. Flame patterns were evaluated by making gas sampling traverses at various furnace elevations with a water cooled probe. The results of the traverse measurements showed that flame impingement would not be a problem. NOx emissions of about 13 mg/mj (0.03 lb/mmbtu) were higher than the single burner tests, however, they met the goals of the project. The multi-burner test setup also permitted subsequent testing of modifications designed to minimize the burner interactions that increase NOx emissions. As a result, by considering the burners and the furnace as a system, staged fuel injection techniques were developed that allowed NOx emissions to be further decreased by about 20%. These modifications were developed after the decision was made to go forward with the industrial application. Industrial Application Based on the results of the burner testing and CFD the decision was made to install the burners in one ethylene furnace in the field. As noted this full-scale furnace is a typical furnace in an ethylene production facility in the Houston-Galveston area of Texas. The furnace was taken out of service and the 18 new hearth burners and 108 wall burners were installed. As noted the initial installation did not utilize the system modifications that resulted in lower emissions. The results for the field application have been very encouraging in terms of furnace performance. The plant has been able to achieve full production rate and the furnace run lengths have actually increased compared to the previous performance. Initial NOx results were not so encouraging in terms of the goals of the project. NOx emissions ranged from 13 to 24 mg/mj (0.03 to lb/mmbtu) depending on anbient conditions. However, the plant has been very willing to work together with the burner designers and furnace designers to achieve the project goals. As a result the additional modifications developed in the test facility were implemented and tested in the industrial application. The final design now meets the project goals not only as an annual average, but even under the cool, dry winter conditions that are typically the toughest as shown in Figure 2.

6 Initial Design NOx, lb/mmbtu(hhv) Modified Design Humidity, gr/lb dry air Figure 2 NOx emissions versus Ambient Humidity Conclusions The success of this project is due to the combination of : Innovative burner design, CFD modeling of the burners, furnace chamber and process, Multi-burner testing in the test facility, Testing and refinement in the industrial application and Close collaboration of the end user with the burner and furnace designers. As a result of the performance of the initial furnace the end user has committed to installing these burners in the remaining furnaces in this site and at another larger site. Bibliography 1. R. J. Gartside, P. R. Ponzi, F. D. McCarthy, S. G. Chellappan, P.J. Chapman and R. T. Waibel: Commercialization of Ultra-low NOx Burners for Ethylene Heaters, AIChE Spring National Meeting - Ethylene Producers Conference, 2004, New Orleans 2. R. T. Waibel, D. Venizelos, R. Hayes, R. Poe and M. Zimola: Next Generation Ulta-low NOx Burners for Refinery and Petrochemical Process Heaters, International Symposium on Energy and Environment 2002, Capri

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