IFRF Combustion Journal Article Number , June 2000 ISSN X

Transcription

1 IFRF Combustion Journal Article Number , June 2000 ISSN X SCALING OF LOW NO X FLAMES OF NATURAL GAS S. Orsino and R. Weber Corresponding Author: Stefano Orsino IFRF Research Station B.V. P.O. Box CA IJmuiden The Netherlands Telephone: Fax: stefano.orsino@ifrf.net IFRF - Combustion Journal journal@ifrf.net

2 IFRF Combustion Journal Orsino and Weber ABSTRACT In the process leading to improve burner design, small and semi-industrial scale experiments play an important role providing detailed diagnostics to guide the burner concept evaluation. A methodology of scaling of natural gas flames was developed at IFRF over the last ten years. The methodology was derived using a comprehensive set of experiments, which cover the thermal input span from MW to 12 MW with intermediate scales of 0.3 MW, 1.3 MW and 4 MW. These experiments were carried out within the SCALING-400 project. In the present paper the data from the SCALING 400 study concerning the staged NO X flames are analysed. A quite substantial NO X emission reduction is observed in the highly staged flames. The staged fuel jets immerge into hot combustion products. The mixing of the fuel gas with the combustion air takes place after the original fuel jet has been diluted with combustion products. Scaling rules of staged-low NO X flames should aim to reach these conditions. To achieve similarity in the flow pattern the general conclusions are: The staged fuel-to-central-air momentum ratio must be maintained; The staged Natural Gas (NG) jet momentum and Re must be high enough; The effect of the furnace confinement is secondary for confinement ratio larger than 3 (the ratio between the furnace diameter and the quarl outlet diameter); The geometrical burner similarity should be maintained; The heat extraction in the furnace should be similar. Key Words: Scaling, natural gas flames, Low NO X flames, staged flames

3 IFRF Combustion Journal Orsino and Weber 1. INTRODUCTION Burner development experiments performed either in laboratory scale, or semiindustrial scale, impose limitations on interpretation and direct application of data to the full-industrial size. The justification for small-scale experiments, when developing a new burner is primarily economic in nature. Moreover, it is extremely difficult to perform detailed validation experiments on new or existing burners in full-industrial scale. This is due to reasons of physical size and associated limitations because of operational restrictions and access. Thus, from the point of view of burner design, it is very important to determine the effects of scale on burner performance. Designing and developing combustion equipment requires careful consideration of how the burner, developed either in laboratory or semi-industrial scale, will perform at full-industrial size. The experiments presented in this paper represent the work carried out for a large project examining the scaling of natural gas flames (the SCALING 400 project [5]). 1.1 SCALING CRITERIA Scaling of combustion systems involves consideration of fluid flow and combustion and heat transfer in the near vicinity of the burner and in the furnace. General theoretical considerations of scaling of combustion systems can be found, for example in Spalding [1] and Beér [2]. Salvi&Payne [3] reviewed several scaling methods. More recently, Smart [4] and Weber [6] also reviewed these methods. There seems to be a common perception that in order to achieve similarity in fuel and oxidant distribution, the physical processes must be manipulated. Ideally, from the chemistry point of view, the distribution of the residence times should be similar. This can perhaps be achieved if the combustion system is scaled using a constant residence time approach. The principal of this scaling criterion is to maintain the ratio D 0 /U 0 constant, where D 0 is the characteristic burner diameter and U 0 the inlet fluid velocity (typically the combustion air). The ratio D 0 /U 0 represents the inertial timescale of the flow and it has been demonstrated that for simple burner it represents the flame residence time [6]. Using

4 IFRF Combustion Journal Orsino and Weber this scaling approach the characteristic burner diameter can be evaluated at different thermal input (Q) as: D 0 Q 0.33 In practice the burner manufacturers do not use the constant residence time approach but instead, the constant velocity principal is applied. Using this criterion the characteristic burner diameter at different thermal input is evaluated as follows: D 0 Q 0.5 The constant velocity scaling principle was adopted in the SCALING 400 project. 1.2 RESEARCH APPROACH Five versions of the generic burner have been designed. These correspond to 0.03 MW, 0.3 MW, 1.3 MW, 4 MW and 12 MW thermal inputs, as shown in the table below. Table 1: Original SCALING 400 tests. Thermal input (MW) Location Michigan University 0.3 Michigan University 0.3 BERL a 1.3 IFRF, IJmuiden 4 IFRF, IJmuiden 12 IFRF at John Zink Co., Tulsa, OK a The Burner Engineering Research Laboratory (a joint GRI (Gas Research Institute) project with involvement of BERL/EERC/SANDIA/UCI and the IFRF) Figure 1 shows the research approach of this scaling project. Five of the six experiments listed in Table 1 were planned in such a way that the geometrical burner/furnace similarity prevails. That is, the ratio of the furnace diameter to a characteristic burner diameter is identical. Moreover, the constant velocity criterion is applied for the burner scaling, and the inlet swirling levels are the same for the five thermal inputs considered. This ensures the aerodynamic similarity of the flames generated; i.e., all the flames should be of similar shape and flow pattern.

5 IFRF Combustion Journal Orsino and Weber Figure 1:The research approach into scaling. For all the experiments placed along "the line of thought" (in Figure 1) there is a perfect burner/furnace geometrical similarity, and efforts were made to fulfil the requirement of thermal similarity. Additional experiments on flames issued from the same burner but in a different confinement and with different heat extraction profile were carried out at the Burner Engineering Research Laboratory. The Burner Engineering Research Laboratory (BERL) experiments provide quantification of the effect of both confinement and the heat extraction profile on the flame properties and NO X flue emissions. In addition to the original six experiments shown in Figure 1 a seventh one, a 1.3 MW test was carried out at British Gas (BG). The BG/GRI (Gas Research Institute) burner version is scaled in the same manner as the original tests of the SCALING 400 project, but the furnace confinement is higher. 2. EXPERIMENTAL METHODS 2.1 GENERIC NATURAL GAS BURNER The generic swirl-stabilized natural gas burner used in the SCALING 400 study is shown in Figure 2.

6 IFRF Combustion Journal Orsino and Weber Figure 2: Generic natural gas burner. 2.2 SCALED VERSIONS OF THE GENERIC BURNER In Figure 2, the dimensions of the burner elements are normalized with respect to the diameter of the combustion air duct (D 0 ). The diameter for the five considered thermal inputs is given in Table 2, together with details of the staging Natural Gas (NG) pipes. The different burner versions are all geometrically similar and the combustion air velocity is 28 m/s (at 15% excess air) for all burners. Table 2: Dimensions of the burners. Input (MW) D 0 (mm) Natural Gas Staging Pipes (number of pipes x internal diameter in mm) x x x x x 0.75

7 IFRF Combustion Journal Orsino and Weber The generic natural gas burner is designed to generate both high-no X (unstaged) and low- NO X flames (staged). The unstaged flames, with all the fuel gas supplied through the central injector, are short and intense with a distinct internal recirculation zone (IRZ) formed in the burner vicinity. The staged flames are longer, with the flame length increasing with the degree of fuel staging. The generic furnace design is shown in Figure 3. Figure 3: General sketch of the experimental furnaces. The effect of furnace size on the burner aerodynamics is mainly governed by the ratio of the furnace diameter (D f ) to the burner characteristic diameter D 0. Furnace length (L f ) and furnace outlet diameter (D c ) were presumed to be of lesser importance. In the five primary tests the furnaces were designed to have geometrical furnace/burner similarity. Table 3: Experimental furnaces geometric data. Input (MW) D 0 (mm) D f (mm) D f /D out 0.03 UM UM BERL IFRF BG/GRI IFRF JZ Co

8 IFRF Combustion Journal Orsino and Weber In Table 3 the main geometric dimensions are reported, further details of the individual furnaces can be found in the literature [7]. The furnace confinement is defined as the ratio between the furnace diameter and the quarl diameter (D out =1.65 D 0 in fig.2). 3. SINGLE TEST DATA ANALYSIS 3.1 IN FLAME DATA Detailed in-flame measurements were conducted in the seven experiments for a highly staged flame. In all the tests the flames used for detailed in-flame measurements have similar inlet swirl, temperature and velocity for the combustion air. The fuel staging is approximately 80 % for all the flames used for in-flame measurements except the 0.3 MW BERL Trial, which was measured under lower staging (60 %). The 12MW, 4 MW, 1.3 MW (IFRF), 0.3 MW (BERL and UM) flames have very similar flame shape and present similar flame characteristics: An internal recirculation zone (IRZ) with an O 2 concentration of approximately 5-6 % volume dry, and a NO X concentration similar to the output NO X. concentration; An extended external recirculation zone (ERZ) with the same O 2 and NO X concentrations as the flue gas; Low in-flame peak temperature compared to the unstaged flames. The 1.3 MW BG/GRI test with higher confinement exhibits different results to the 12 MW test. In the IRZ the O 2 concentration is lower and the temperature higher compared to the measurements in the other tests. Due to the higher confinement the ERZ is very small. This results in a lower entrainment of hot combustion products in the natural gas jet. Combustion products do not dilute the fuel jet, and the combustion is more intense causing higher in-flame temperatures.

9 IFRF Combustion Journal Orsino and Weber The 0.03 MW test reveals a different flame shape. In the near zone burner where the flames measured in the other experiments show an IRZ with a low oxygen concentration, and a high temperature of approximately 1300 Celsius, a very low temperature (500 Celsius) and high O 2 (up to 20% volume dry) are measured. The inflame and flue gas temperatures are higher than in the other tests, and the combustion appears to be more intense with the peak temperature as high as 1600 Celsius. This is probably due to the fact that the staged NG jet is largely affected by the central swirled air jet and is mixed rapidly with the central air. 3.2 SINGLE TEST RESULTS In the seven experiments input-output measurements were performed to investigate the response of the burner to variations in operation parameters: Fuel staging (the amount of fuel supplied to the staged NG guns, see Figure 2); Inlet swirl level; Combustion air pre-heat; Turn down; Excess air. In this article only the fuel staging will be investigated, in Figures 4 and 5 the NO X versus staging ratio at two different combustion air pre-heat temperatures is shown for the 12 MW and 0.03 MW UM tests. In the figures the staging ratio is defined as the ratio of the fuel supplied through the staged guns versus the total fuel input (staging ratio 0, all the fuel is supplied into the central natural gas channel, staging ratio 100 all the fuel is supplied into the staged guns).

10 IFRF Combustion Journal Orsino and Weber Figure 4: 12 MW Test: Effect of staging ratio. The 12 MW, 0.3 MW, 1.3 MW IFRF and 4 MW show similar trends, with the NO X emissions decreases with the fuel staging. The tests demonstrate that the fuel staging is effective in decreases the NO X emissions only when 20% or more of the fuel is supplied through the secondary injectors. Furthermore at high staging conditions the emission NO X is less dependent on the combustion air preheat than during unstaged (and low staged) conditions. Figure 5: 0.03 MW UM Test: Effect of staging ratio.

11 IFRF Combustion Journal Orsino and Weber The 1.3 MW BG and the 0.03 MW tests show an increase in the NO X emissions with an increase in the staging ratio, the difference from the other tests was expected since both of these tests have different flame shape and in-flame conditions to the other test. 3.3 GENERAL RESULTS Figures 6 shows the dependence of the NO X emissions on the thermal input at 80% gas staging. Figure 6 NO X versus thermal input at 80% staging With the exception of the 0.03 MW test the NO X emissions at 80% staging show an increase from the 0.3 MW to the 1.3 and 4 MW tests and than a decrease in the 12 MW test. The five primary scale tests of the SCALING 400 project should have geometric and thermal similarity, but it can be seen from the data that there are significant variations in the NO X levels. The differences cannot be explained only with a different residence time in the furnace. Using the constant velocity scaling approach and assuming that the flame residence time is proportional to the ratio D 0 /U 0, the residence time in the flame is proportional to the thermal input. Therefore the NO X emissions should increase increasing the thermal input, and that is not the case as it can be seen in Figure 6. More

12 IFRF Combustion Journal Orsino and Weber accurate consideration on the air velocity, staged NG jet momentum and confinement may be relevant to understand the differences in the NO X emissions. The burners are scaled down from the 12 MW test to the 0.03 MW test in order to have the same velocity (constant velocity criterion). The central NG velocity and staged NG velocity are listed in Table 4 for 80% staging. The Froude (Fr) and Reynolds (Re) number for the staged NG jet are also listed in Table 4. Table 4: Staged NG and central NG velocities, Fr and Re numbers for the staged NG jet Test Staged NG velocity Central NG velocity m/s Re a Fr a 12 MW MW MW MW MW u a Re and Fr ( Fr = gd ) are calculated based on the staged natural gas jet There are no large variations in the central natural gas injection between the different scale tests, except for the 0.03 MW Test where the NG velocity is slightly higher. That could be relevant in explaining the different flame shape in the 0.03 MW Test. In the small-scale test the NG tends to penetrate the airflow more and to exit the quarl in the outer region. The central NG is burning more in the outer region close to the staged NG injection. This results in lower temperatures and O 2 concentration is the IRZ. The staged NG velocities reported in Table 4 show that in the 4 MW Test the staged NG velocity is lower than in the other experiments. The staged natural gas injector diameter is too large resulting in a lower momentum jet. In the original design the diameter for the staged NG injector was 9 mm, but this was changed in 12.6 mm.

13 IFRF Combustion Journal Orsino and Weber The effect of the jet momentum is relevant to explain the different NO X levels in the seven tests. Figure 7 shows the NO X emissions for the seven tests plotted against the ratio of the central air and staged NG momentum. Figure 7: NO X emissions versus the momentum ratio. The 0.3 MW (BERL and UM) and 12 MW and 1.3 MW IFRF tests show the same emission levels at low momentum ratio (high staged NG momentum - high staging). The 4 MW test has a higher momentum ratio and higher NO X emissions since the staged NG velocity was lower in the 4 MW test than in the other tests. The 1.3 MW BG/GRI and the 0.03 MW Tests show, as was expected, a different trend. The NO X emissions increase with the staging ratio and thus decrease with increasing momentum ratio between staged NG and central air. When the fuel staging decreases the staged natural gas jet is more affected by the central air, the jet entrainment of hot combustion products is lower and the flame shape is different. Figure 8 shows the flame shape in the two different situations: At high staging where the staged NG jet is strong, less affected by the air; At low staging where the staged NG jet is largely affected by the central air jet.

14 IFRF Combustion Journal Orsino and Weber Figure 8: High and low staging flame shape. 4. FINAL COMMENTS In this paper the data from the SCALING 400 project concerning the staged low-no X flames has been analysed. A quite substantial NO X emissions reduction is observed in the highly staged flames. The mixing of the fuel gas with air has to take place after the original fuel jet has been diluted with combustion products. To make this possible it is necessary to have a large external recirculation zone with temperature and gas composition similar to the outlet. Scaling rules of staged-low NO X flames should aim to reach these conditions. To achieve similarity in the flow pattern the general conclusions are:

15 IFRF Combustion Journal Orsino and Weber The staged fuel-to-central-air momentum ratio must be maintained; The staged NG jet momentum and Re must be high enough; The effect of confinement is secondary for a confinement ratio larger than 3; The geometrical burner similarity should be maintained; The heat extraction in the furnace should be similar. The first two considerations are important to have a fuel gas jet entraining a large amount of combustion products. The confinement effect is essential to insure a large external recirculation zone. The geometrical similarities are crucial to have the same mixing and interaction between the central jet and the staged jet. Having a sufficiently large Re number and similar Fr number can satisfy these general indications. Under these conditions the staged jet is like a free jet and not significantly affected by the central jet. The combustion is mainly mixing controlled, and entraining air and hot combustion products burns the gas. In the small scale tests (0.03 MW) these similarities cannot be reached simply by scaling down the burner dimensions, since the resulting Reynolds number seems to be too low (see Table 4). These considerations could be extended to other similar systems where the air and the fuel are injected separately to reach very low emissions.

16 IFRF Combustion Journal Orsino and Weber REFERENCE 1. D. B. SPALDING, The art of partial modelling. The 9th Symposium (International) on Combustion, The Combustion Institute, pp , J. M. BEÉR, Phenomenological models for flames in furnaces. US DOE Workshop on modelling of combustion in practical systems, G. SALVI and R. PAYNE, Investigation into the scaling of combustion systems. IFRF Doc. No., F 31/a/5, J. P. SMART, On the effect of burner scale and coal quality on low-no X burner performance. Ph.D. Thesis, University of London, London, R. WEBER, J. F. DRISCOLL, W. J. A. DAHM and R.T. WAIBEL, Scaling Characteristics of the Aerodynamics and Low-NO X Properties of Industrial Natural Gas Burners. SCALING 400-STUDY. Part I: Test Plan. IFRF Doc. No F40/y/ R.WEBER, Scaling characteristics of aerodynamics, heat transfer, and pollutant emissions in industrial flames, The 26th Symposium (International) on Combustion, The Combustion Institute, pp , A. HSIEH, W. J. A. DAHM, and J. F. DRISCOLL, Scaling laws for NO X emissions performance of burners and furnaces from 0.03 MW to 12 MW, Combustion and Flame 114 pp