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Study of Partially Premixed Burners

Bernardo de Mascarenhas Santos Passos de Sousa Instituto Superior Técnico – Universidade de Lisboa

Mechanical Engineering Department – Center IN+ – Lab. of Themofluids and Energy Systems Av.Rovisco Pais, 1049-001 Lisbon, Portugal

Abstract The following thesis centers itself on a detailed study of a rich-lean slit burner. An assessment of the influence of burner’s geometry, secondary air, fuel-rich and fuel-lean premixtures on the stability limits and pollutants emission was carried out. Several geometries were tested in order to determine their impact on methane and propane lean premixtures’ stability. Subsequently, the best methane geometry was chosen to perform rich-lean flame stability tests. With the new limits defined, NOx, CO and HC emissions were assessed. The main conclusions were: 1) The slit burner’s geometry revealed to be determinant in lean premixed flames’ stability for higher flow rates. Different gases showed opposite stability trends, which were attributed to fuels different properties. 2) For the used rich-lean flame configuration, the equivalence ratio gradient and the rich premixture Reynolds number were found to be the major importance factors on stability. The secondary air has proved to reduce flame’s stability. The rich-lean flame is more stable than a corresponding lean premixed flame. 3) Regarding pollutant emissions, an overall equivalence ratio closer to stoichiometry achieved the best compromise. This parameter has shown to be a useful indicator in the prediction of pollutant emissions and on the understanding of these flames’ global dynamics. The secondary air has generally shown advantages except when used in excess. The rich equivalence ratio has shown both advantages and disadvantages, depending on the emitted specie.

Keywords: rich-lean combustion, slit burner, triple flames, pollutant emissions, NOx, methane

1 Introduction Over the last decades, humanity has experienced a general concern over pollutant emissions due to the extensive use of fossil fuels. As a consequence, the related industries have been forced to develop new solutions and overcome the existent technology thresholds. Among these, the domestic gas burners industry is not an exception. With countries like European Union members and United States of America establishing new emission standard limits, this industry is now facing a new challenge, improving and redesigning the currently employed technology. Out of the new breakthroughs, a new rich-lean burner has been demonstrating remarkable results [1]. It relies on burning two different premixed mixtures of the same gas, namely one rich and other lean, in a slit burner. The ignition of these premixed mixtures at the burner’s exit results in a triple flame. Since then, it has sparked the interest of scientific community and engineers for its wide range of applicability and characteristics. Domestic gas burners, lean-burn gas turbines, internal combustion engines and gas-fired furnaces are some of the current practical applications of these flames [2]–[5]. Devices' efficiency improvement and reduction of pollutant emissions are the most prominent advantages of these flames, giving them a key role and making them hard to avoid [5], [6]. It is known that the combination of several factors, such as geometry [7], velocity and equivalence ratio gradients [8], [9], flame structure [8], [9], among others, lead to the successful mitigation of NOx and CO emissions.

Motivated by the new patent’s exceptional results, this study has as its main objective the understanding of its mechanisms and determination of the influence of triple flame combustion’s main parameters, on the flame’s overall stability and emissions. Thus, based on the rich-lean burner, a slit burner was design to enable multiple configurations. Figure 1.1 illustrates one of the flames obtained with such configuration.

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Figure 1.1 – Illustration of the used triple flame configuration

2 Experimental Setup As previously mentioned, all experiments were performed with a slit burner. Figure 2.1 depicts a schematic of the experimental setup developed. Two main streams, one of gas and another of air, feed the system. The gas used may be propane or methane. The air stream passes through an air filter, removing the water and other impurities before continuing its course. Both main streams are split into two branches that connect to four flow meters. The control is done by computer using the flow meters’ program, resorting to an excel sheet developed to calculate each flow rate for a certain desired condition for both rich and lean mixtures. After passing through the flow meters, each gas stream is mixed with the respective air stream in order to create two different premixed mixtures, one rich and other lean. As shown in Figure 2.2, the mixtures are plugged to the burner in separate places and injected by means of tubular wall-perforated injectors. The rich mixture is plugged to a plenum at the bottom of the burner, while the lean mixture enters directly in the burner’s core channel. The rich premixture fills the plenum and supplies the outer fuel-rich slots by flowing through the channels above the plenum’s exit. Aside of these channels, there are two draught forced passages.

Figure 2.1 – Schematics of the experimental setup

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A fan coupled to the burner’s bottom forces air entrance through these passages. An Arduino that is connected to the computer controls it. On top there’s a core fuel-lean zone, two adjacent cavities and two outer fuel-rich slots. In order to avoid ambient air entrainment and consequent flame’s equivalence ratio and behavior changes, the burner was confined with a steel bell jar shape chamber. In an attempt to achieve stationary conditions and minimize these thermal effects, a heat exchanger was developed to cool down the burner. In order to establish the desired working conditions, the fan’s flow velocity pattern was characterized. As such six power conditions were selected: 9%, 12.5%, 25%, 50%, 75% and 100%. The velocity profile for each power conditions was determined using a pitot tube. The longitudinal difference between the two sides was slightly smaller than the transversal differences. Seeing that, the velocity average values taken are those listed on total average in Table 2.1.

Table 2.1 – Longitudinal velocity average for each fan power condition. Velocity in m/s.

3 Results and Discussions 3.1 Geometry Selection Analyzing the above-described rich-lean concept, many are the parameters whose impact on flame’s stability and emissions may be relevant. Therefore, it was decided to fix all the dimensions except lean premixture slit’s width (w) and rim thicknesses (t) the only geometrical variables. With only two geometric variables, nine geometries were tested in order to define which is the most stable using solely lean mixtures. These variables influence on the stability limits was tested for both methane and propane. During these tests, the coupled fan was turned off and the burner unconfined, following Sogo and Hase’s [10] procedure.The stability limits were defined for each geometry. The geometry that possesses the broader limit was considered the most stable and was therefore selected for the rich-lean flames tests. This choice was based on the assumption that the most stable geometry (i.e. for the

Figure 2.2 – Burner’s admission system - detailed view

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same fuel flow rate, the geometry that reaches the lowest ϕL) would also be the one with the lowest NOx emissions. Figure 3.1 represents the measured stability limits for methane. The influence of the geometric parameters proves to be slightly dominant for low fuel flow rates in both cases.

Figure 3.1 – Methane's geometries stability limit: lean premixture

However, when fuel flow rate increases their influence became more noticeable, especially when methane was used. For methane, it was also noticed that an increase of fuel flow rate results in a broader stability limit for almost all the geometries tested. On the other hand, propane reveals to have the opposite trend. In all the tested geometries, the increase of fuel flow rate resulted in a decrease of the stability limit. Methane’s best geometry revealed to be far more stable in higher regimes than other tested geometries.

3.2 Triple Flames 3.2.1 Stability Once the most stable geometries were chosen, the fuel-rich mixture was introduced. For this assessment only methane was used as a tested gas. The burner was confined and the new stability limits were defined. Two different assessments were performed, the first with the fan turned off and the other with secondary air correspondent to fan powers of 9% and 25%.

3.2.1.1 Without Assisted Ventilation:

With the purpose of evaluate solely the rich premixtures’ effects on the triple flame’s stability the ventilation was switched off. For the fuel-rich premixed side, two Reynolds numbers were defined (36 and 72). For each Reynolds number, three rich equivalence ratios were set, namely 1.2, 1.5 and 2.0. With this variation one can see the influence of these parameters on the overall stability and respective limit. Three lean-fuel gas flow rates were chosen: 0.35, 0.7 and 1.40 SLPM. With the fuel-rich premixed flames ignited, the lean mixture’s airflow was gradually changed till another blowout situation occurs. Figure 3.2 illustrates the new stability limits without assisted ventilation.

As one can see, the richer the outer flame is, the broader the limit became. It is known that the increase of rich premixed flames’ equivalence ratio leads to an increase of radicals and decrease in flame’s temperature and burning velocity. This stability augmentation was also noticed when the rich flames’ Reynolds number increases. This effect is even bigger when the velocity gradient between rich premixed and lean premixed flames increases (once more due to an increase of radicals diffusion). The effect of these two parameters revealed to be crucial in flame’s stability, especially when used together.

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3.2.1.2 Assisted Ventilation:

Subsequently, the rich-lean stability limits were once more defined, this time with the fan turned on. The secondary air established conditions corresponded to 9% and 25% of fan power. Figure 3.3 depicts the obtained results. The secondary air increase steeply decreased the flame’s stability limit.

3.2.2 Pollutant Emissions Being pollutants emissions what most motivated this work, a more detailed analysis concerning this matter was carried out for CO, HC and NOx. To perform this analysis, methane was used as the single tested gas. The employed geometry was the selected in the methane's lean stability limits analysis. The studied parameters were the secondary air velocity (also referred as fan power), the rich and lean mixtures’ equivalence ratios and the lean mixture Reynolds number. The rich mixtures’ Reynolds number was fixed in 72, similarly with the rich-lean stability limits tests carried with the fan turned on. All the tested rich-lean conditions were compared with a correspondent lean flame, whose emissions tests were performed without ventilation (0% of fan power).In order to investigate the effects of the several studied parameters in these pollutants emission, six analyses were conducted: Constant Lean Equivalence Ratio; Constant Lean Reynolds Number; Overall Equivalence Ratio; Cooling Water Effect; Fan’s Inlet Sealed; Geometry’s New Arrangement. Only the first three are herein described.

3.2.2.1 Constant Lean Equivalence Ratio

Figure 3.4 schematically represents the measured conditions. CO & HC measurements: During the carried tests, CO and HC trends were very similar. Figure 3.5 represents the obtained results for CO. The differences in certain aspects were small, which will be particularly identified further on. Analyzing the results for ϕL=0.6 in both CO and HC cases, the essayed rich-lean flames revealed that they emit less than the lean flame reference (less than half in the majority of the cases). The exception was the condition of (𝑅𝑒! = 30,𝐹𝑎𝑛  𝑃𝑜𝑤𝑒𝑟 = 25%), which [HC] stood somewhat over the lean reference. On the other hand, for ϕL=0.9 the situation is reversed and the lean flame's reference emissions, that were previously above the rich-lean flames' values, are now below. Concerning the influence of the secondary air, in a first approach it reveals to be inconclusive. The secondary airflow has also positive and negative effects on pollutants emission. The worst situations in these pollutants emission corresponded to the maximum fan power. The best results were obtained for secondary air stream velocity of the order of flame’s velocity, which corresponds to 9% of fan power (around 1.4 m/s).

NOx measurements: Throughout the essayed conditions, NOx emissions reveal to be small. The difference between the rich-lean flames’ emissions with the respective reference’s lean flame increase as the lean equivalence ratio tends to stoichiometry. Nevertheless, along the rich-lean flame’s measurements the effect of the lean Reynolds number in NOx emissions seems to barely change this pollutant emission. Excessive amount of secondary air have increased NOx emissions. Concerning the rich equivalence ratios’ measurements, the registered differences between rich conditions were somehow negligible.

Figure 3.2 – Methane’s triple flames stability limits -

0% of Fan Power

Figure 3.3 – Methane’s triple flames stability limits - 9% and 25% of Fan Power (ReR=72)

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Figure 3.4 – Pollutants emissions at constant lean equivalence ratio - Selected conditions

Figure 3.5 – CO emissions with constant lean equivalence ratio (ReR=72)

3.2.2.2 Constant Lean Reynolds Number

Figure 3.6 depicts the conditions chosen to perform this assessment.

CO and HC measurements: Similarly with the previous assessment, in this constant Reynolds number assessment the CO and HC trends were nearly the same. Figure 3.7 represents the achieved results for CO emissions. In both ReL conditions, to about ϕL<0.75 the rich-lean flames’ CO emissions were lower than the correspondent lean reference. However, for ϕL>0.75 the trend reverses and the lean

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reference emissions turns to be lower. The same trend was registered for HC, with the turning point around ϕL=0.70. Rich-lean flame’s short equivalence ratio gradient eventually leads to a more incomplete combustion, despite being close to stoichiometry and with higher flame’s temperatures.

Figure 3.6 – Pollutants emissions at constant lean Reynolds number - Selected conditions

Between ReL conditions only a slight increase in these pollutants emission is noticed. ϕR=2.0 tends to have higher emissions than the other rich condition, again due to incomplete combustions issues. This difference, as well as the overall emissions, tends to decrease with secondary air velocity increase.

Figure 3.7 – CO emissions with constant Reynolds number (ReR=72)

NOx measurements: Similarly to the previous assessment, NOx emissions remained in general low and with an almost flat trend. Figure 3.8 represents the obtained results for NOx emissions. Nonetheless, few exceptions were registered, being their values sometimes much higher than the others. The rich-lean emissions reveal to be low (of the level of lean flame's lowest emissions) and fairly constant throughout the measurements. For 𝜙! < 0.8 the rich-lean emissions are actually slightly above the lean flame reference. The variation of rich equivalence ratios as well as lean Reynolds numbers followed the previous assessment trend, being the registered differences between rich-lean conditions negligible. The exceptions were the conditions with (𝜙! = 1.0; 𝜙! = 1.2).

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Figure 3.8 – NOx emissions for constant lean Reynolds numbers (ReR=72)

Nevertheless, even with a stoichiometric equivalence ratio in the lean side, the rich-lean flames have emitted much less (three to four times) than the correspondent lean flame reference. This exceptional evidence might be the result of this triple flame structure and the synergic mechanisms between non-premixed and premixed branches, which may reduce NOx emissions due to sharing of heat and intermediate radicals. Increasing ventilation barely influenced the NOx emissions along the measurements. This reduction may be a result of temperature decrease due to excessive secondary airflow, which possibly mitigated the NO thermal mechanism, leading to a significant reduction of NOx emission [11].

3.2.2.3 Overall Equivalence Ratio

From the previous analysis results, it is evident that triple flames’ equivalence ratio gradient is of major importance not only on the flame’s stability, but also on pollutant emissions. With the purpose of evaluating the pollutant emissions’ global picture, a new assessment in terms of overall equivalence ratio was carried out. The use of the overall equivalence ratio gives an idea of the total proportion between gas and air from both premixtures, embodying the contribution of each premixture’s equivalence ratio and Reynolds number. Figure 3.9, Figure 3.10 and Figure 3.11 collectively represent all obtained results for CO, HC and NOx emissions, respectively.

From the obtained results, one can notice a decreasing global trend with the increase of the overall equivalence ratio, especially in the case of NOx. For CO and HC, it is noticed that the pollutants’ emission reaches a minimum around the stoichiometric overall equivalence ratio. These are interesting results, once they suggest that the best compromise corresponds to a global equivalence ratio that is generally avoided locally. As one can see, for most of the verified situations it is possible to identify a pattern along the initial dispersion, making an assessment of these parameters’ global effect easier. Therefore, analysing their effects one can conclude that: 1)The secondary air should be used but not in excess. The best compromise seems to be achieved when the secondary air velocity is of same order of greatness as the flame’s velocity. 2) The increase of the rich equivalence ratio worsens CO and HC emissions in the majority of the cases, while in NOx‘s case there’s a slight decrease. Therefore, the rich equivalence ratio should probably lie somewhere between 1.2 and 2.0 with an overall equivalence ratio around stoichiometry.

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Figure 3.9 – CO emissions versus overall equivalence ratio: a) Fan power dispersion b) Rich equivalence

ratio dispersion

Figure 3.10 – HC emissions versus overall equivalence ratio: a) Fan power dispersion b) Rich equivalence

ratio dispersion

Figure 3.11 – NOx emissions versus overall equivalence ratio: a) Fan power dispersion b) Rich

equivalence ratio dispersion

4 Conclusions The main conclusions can be summarized as follows:

1) Where lean premixed flames’ stability is concerned, the influence of the slit burner’s geometry revealed to be determinant in higher flow rates regimes (especially in methane’s case). The use of different gases led to opposite stability trends, i.e. as fuel flow rate increases methane’s stability limit generally increases and propane’s stability limit decreases. This was attributed to these fuels different properties, in particular the Lewis number.

2) For the used rich-lean flame configuration, the equivalence ratio gradient and the rich premixtures’ Reynolds number were found to be the major importance factors in flame stability. The best results were obtained with the cumulative effect of these two factors. The secondary air has proved to reduce the flame’s stability. Furthermore, it was also concluded that a rich-lean flame is more stable than a corresponding lean premixed flame.

3) Regarding pollutant emissions the best compromise relies on using overall equivalence ratios closer to stoichiometry. This parameter has shown to be a useful indicator in pollutant emission predictions and on the understanding of these flames’ global dynamics.

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4) The use of secondary air has generally shown advantages in reducing pollutant emissions. Nevertheless, its excessive use leads to negative effects. Thus, the best compromised was considered for a secondary air velocity of the order of the flame’s velocity.

5) The rich equivalence ratio has shown both advantages and disadvantages on pollutant emissions, depending on the emitted species. The differences were also closely correlated with the amount of secondary air.

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