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Apr-2006

Process burner spacing

Burner-to-burner interaction can increase NOx emissions and cause flame impingement on process tubes

Mahmoud Fleifil, Michael Lorra, Charles E Baukal, Jeff A Lewallen and Daniel E Wright
John Zink Company LLC
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Article Summary
Burner-to-burner interaction is a phenomenon that is characterised by the creation of a dense fire cloud, an increase in NOx emissions and an altered heat flux distribution in the furnace. It occurs frequently in multiple-burner heaters, where vertical cylinder heaters have been observed to have the severest interactions. Although this problem has been around for a while, it had not been well understood. Only ad-hoc solutions had been introduced to mitigate it. A comprehensive study was completed, which revealed a dimensionless interaction parameter that could identify conditions that were likely to produce burner-to-burner interaction. Observations of interactions were made on lab-scale, small-scale and full-scale burners to validate the parameter. Two case studies are presented using CFD modelling. The first study shows that interaction was unlikely in a vertical cylindrical furnace where the spacing between burners was decreased. The other study confirmed interaction observed in the field and suggested modifications that were successfully implemented and led to the elimination of the burner-to-burner interaction. Specific rules of thumb, which are based on experience, are given to predict the likelihood of interactions in ethylene-cracking furnaces.

The problem
In response to the increased demand to reduce NOx emissions from industrial heaters, new burner technologies have been developed to meet the more stringent emission regulations.1 Most of these technologies use some type of furnace gas recirculation (Figure 1) to reduce the flame temperature, so that thermal NOx emission can be reduced substantially.2 However, diluting the fuel-air mixture with furnace gases results in lower burning rates, which often makes the flames significantly longer. Both longer flames and increased volumetric heat release density (Btu/hr/ft3 or MW/m3) in the newer multi-burner industrial furnaces have led to frequent occurrences of a phenomenon often referred to as burner-to-burner (BtoB) interaction. The occurrence of this phenomenon is indicated by the formation of a large, bright yellow fire cloud and by an increase in NOx emissions compared to single-burner performance. Although this problem has been observed in all types of multi-burner heaters, it is much more severe in vertical cylindrical heaters compared to other types such as cabin heaters.

It would be desirable to obtain a simple rule of thumb to characterise and predict the onset of BtoB interaction. However, the complexity of the problem limits the application of each rule to the specific case from which it was deduced. Some of the BtoB interaction rules that have been proposed for vertical cylindrical heaters are based on the heat-release rate and the geometrical dimensions of the furnace. The rule usually takes the form of a criterion value that demarcates the absence of interaction from the onset of interaction. Figure 2 shows the dimensions discussed in the following rules. One such rule uses the heat release of a single burner per unit distance between two adjacent burners (Btu/hr/ft or MW/m).A second rule uses the total heat release of the heater per unit area (Btu/hr/ft2 or MW/m2) of the outermost burner circle, which is the circle that passes through the centres of the outermost burners. A third rule uses the total heat release of the heater per unit area (Btu/hr/ft2 or MW/m2) of the tube circle, which is the circle that passes through the centres of the heater process tubes. A fourth rule uses the total heat release per unit volume of the heater (Btu/hr/ft3 or MW/m3).

In extreme cases, when the criterion value is close to zero for no interaction and relatively large for full interaction, these rules can correctly predict the performance of a multi-burner heater. However, predicting the accurate conditions for the onset of interaction is a challenge for any of these rules. In other words, it is easy to predict the sufficient conditions (extreme cases) for interaction, but it is difficult to predict the necessary conditions (onset conditions) for the interaction. These rules fail for two reasons: oversimplification of the problem and failure to account for all significant parameters. For instance, during a research study of this phenomenon, it was observed that BtoB interaction depends on both fuel pressure and fuel composition, while none of the above rules account for such a dependence.

BtoB interaction, sometimes referred to as flame-to-flame interaction (FtoF), is a complex phenomenon that makes the behaviour of the multiple-flame configuration much different from that of an individual flame. Increased flame length, increased NOx emissions, poor mixing, modified heat flux distribution and decreased surrounding gas entrainment are some of the results of FtoF interaction. While aerodynamic interaction of the jets in a multiple-jet system is considered to be the most important factor that governs FtoF interaction, the presence of combustion is also a major factor. In other words, the shape of the flame not only responds to the turbulence in the surrounding medium, but also to the combustion itself, which creates eddies that can affect neighbouring flames.

In vertical cylindrical heaters, the BtoB spacing is more critical than in box-type heaters, because the burner spacing has an impact on the flue gas flow into the centre of the furnace. If the spacing between burners is too small, the interaction of the flames can actually block the flow of flue gases from the circumference of the furnace into the centre. This generates a low-pressure zone in the centre of the furnace that can effectively pull the flames toward the centre of the furnace. This increases the tendency for the flames to merge together and create a large fire cloud that can impinge on process tubes. Another effect is the creation of a reducing (oxygen-deficient) atmosphere in the centre of the furnace. Since the flue gases are blocked, the O2 concentration decreases at the centre, leading to increased CO formation on that side of the flame and creating the potential for increased flame lengths. Longer flames can also alter the design heat flux distribution, as more heat is transferred higher in the furnace. NOx emissions may also increase because there are insufficient flue gases on the inside of the burner circle that can be pulled into the flames. Many low-NOx burners use furnace gas entrainment to reduce the flame temperature, which reduces NOx emissions.

An extensive theoretical and experimental investigation was completed to develop a suitable criterion for identifying interaction in vertical cylindrical heaters. Details of that investigation are given elsewhere.3 The developed criterion depends on both physical and chemical parameters; namely, the geometry of the burners and the fuel composition. Interaction is defined here as the influence of flames on each other that leads to the creation of a yellow flame cloud (due to the formation of soot particles) between burners. A dimensionless number was developed that can be used as a measure for the interaction strength. Both lab-scale and small-scale burners were used in the experimental investigation.

Consider two diffusion flames, as shown in Figure 3, that are adjacent to each other. There is a critical height at which jets interact Zc.The flame height is designated here as Lf . If the flame height is less than the critical height (Lf < Zc), no FtoF interaction will occur. If the flame height is greater than the critical height (Lf > Zc), the flames will interact. FtoF interaction of a group of diffusion flames arranged in a circle, as in a vertical cylindrical furnace, can be characterised in a similar manner.

Experimental study
Twelve lab-scale burners were built. Each was a single-jet burner with its diffusion flame stabilised on a conical bluff body downstream of the fuel jet. The objective of testing with a simple lab-scale burner was to study the FtoF interaction on a fundamental basis and to verify the interaction criterion for configurations with a large number of burners. These burners were tested in a small lab furnace equipped with sensors for measuring fuel pressure, temperature, flow rate, flue gas temperature, furnace draft, wet and dry gas analysis of excess oxygen, carbon monoxide and nitrogen oxides. The following control parameters that affect FtoF interaction were varied: heat release, fuel pressure, excess air, burner circle diameter and number of burners.
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