Article pubs.acs.org/EF
Experimental Investigation of Double-Swirled Non-premixed Syngas Flames by Planar Laser-Induced Fluorescence Bing Ge,* Shusheng Zang, Peiqing Guo, and Yinsheng Tian Turbo Machinery Institute, Shanghai Jiaotong University, Shanghai 200240, People’s Republic of China ABSTRACT: Experimental measurements are carried out on a non-premixed model combustor, equipped with a double-swirled syngas burner. Mixtures of H2, CO, and CO2 with appropriate contents are used to simulate corresponding syngas used in a practical gas turbine combustor. Planar laser-induced fluorescence (PLIF) of OH radical measurement is adopted to identify main reaction zones and burnt gas regions as well. Together with the temperature and emission measurements during the exhaust section, some important characteristics of the syngas flame are investigated overall. In this paper, the effects of the N2 dilution and CO/H2 molar ratio consisting of syngas fuel are investigated. Experimental results show that the syngas flame root near the burner exit demonstrates a double flame front structure. The increase of the CO/H2 ratio alters the shape and position of the flame root and compresses the main reaction zone. The existence of N2 diluents, however, changes the whole flame root structure and merges into a single flame root. The CO emission can be controlled to a desirable level for the case of a high CO/H2 ratio if the flow field is well-organized.
1. INTRODUCTION Today, a consensus has been reached throughout the world on the importance of environmental protection and emission reduction. The development and adoption of clean coal power generation technology is essential for the sustainable development of industry and economy. Among these technologies, the integrated gasification combined cycle (IGCC) system is considered one of the most promising methods for efficient use of energy with low pollution and even cost-effective reduced CO2 emissions with the assistance of fuel decarburization.1−3 Moreover, processes of gasification allow for a wide range of solid combustibles, including coal, biomass, and municipal solid waste (MSW), to be converted into syngas mixtures that will be used in a gas turbine engine to generate electricity. Although IGCC power plants have drawn increased attention recently, only a few experimental data are available for syngas4−11 compared to the extensive research about methane and other hydrocarbon fuels.12−14 The composition of syngas varies from different gasification processes and fuel resources, but it overall consists of H2 and CO as the main combustible components. Various amounts of diluents, such as H2O, CO2, or N2, may also be added before syngas is burnt. Because of the fact that the chemical and physical properties of syngas drastically differ from that of natural gas, the experimental data acquired from methane cannot be adopted directly. Moreover, because the content of H2 and CO that comprises syngas fuel varies in a relatively wide range, the property of syngas fuel itself may also be different. Therefore, the study on the prediction of the combustion behavior fed with syngas is required and necessary. In a typical gas turbine combustion chamber, flame is often stabilized by swirling air and/or fuel, to achieve high efficiency, low emission, and high flame stability.15 The objective of this study lies in the investigation of non-premixed turbulent combustion characteristics fueled with syngas-based mixtures as a function of the CO/H2 ratio and N2 dilution content in a model combustor equipped with a double-swirled sprayer. © 2012 American Chemical Society
The typical syngas fuel is simulated by a synthetic mixture of H2, CO, and CO2, while N2 is used to dilute syngas fuels up to 40% (by volume). Planar laser-induced fluorescence (PLIF) measurement of OH radicals is adopted to yield useful insight into the chemical reaction zones and flame structures in syngas flame.
2. EXPERIMENTAL SECTION In this study, experiments are performed in an optically accessible combustion chamber, with a length of 200 mm and square section of 100 × 100 mm (Figure 1). Two of three quartz windows are positioned at left and right sides for laser sheet, and the other window is placed on the top of the combustion chamber for signal capture. The chamber is convectively cooled with air-cooling passages around its radial direction. Upstream of the combustion chamber, the sprayer is mounted with a double-swirled structure, as shown in Figure 1. The outer diameter of the sprayer is 42 mm. Air is supplied from a plenum and then delivered through the axial swirler before entering the combustion chamber test section. The air swirler consists of eight channels, with the swirl number on the order of 0.8 based on its geometric dimension. The fuel flow is also swirled by an axial swirler with six 45° vanes, leading to its swirl number on the same order as the air swirler. The syngas components of H2, CO, CO2, and diluting N2 are delivered from cylindrical gas tanks. The content of syngas fuel is adjusted by monitoring and controlling four mass flow controllers for four kinds of gases and is well-mixed in a fuel mixer before going into the sprayer. The air flow rate is controlled by another flow controller upstream of the plenum. During the experiment, the global equivalence ratio is kept at 0.36 for all flames. Two sets of flame parameters are designed: one for the investigation of the influence of the CO/H2 ratio (in volume) in syngas composition and the other for the effect of N2 dilution. During the experiment, PLIF of the OH radical is employed to detect reaction zone dimension and flame front position. The schematic view of the PLIF system is shown in Figure 2. The excitation Received: November 18, 2011 Revised: February 4, 2012 Published: February 13, 2012 1585
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Figure 1. Sketch of the burner with a double-swirled structure. timing delay of the PLIF system is controlled by a pulse delay generator DG535. The gate width or exposure time of the ICCD camera is set to 100 ns to include a complete OH fluorescence for each instantaneous laser shot. A total of 500 single-shot OH PLIF raw images are recorded for post-processing for each flame. The resolution of each image is higher than 0.1 mm/pixel. In addition to OH radical detection by the PLIF technique, the temperature and CO emission at the exhaust section after the combustion chamber are measured by six thermocouples and Siemens gas analyzer (ULTRAMAT 23), respectively. Six thermocouples are disposed at six different radial positions, where the area of each ring is equal. The exhaust mean temperature is obtained by the algebraic average of these six temperatures. The CO emission is converted to the CO concentration at the 15% O2 condition, with the O2 concentration recorded simultaneously, followed by eq 1.
[CO]standard = ([CO]measured (0.21 − [O2 ]standard )) /(0.21 − [O2 ]measured )
(1)
3. RESULTS AND DISCUSSION 3.1. Effect of CO/H2 Ratios on Mid-calorific Syngas Flames. The fuel composition and properties for the investigation of the influence of the CO/H2 ratio (in volume) in syngas composition are summarized in Table 1. Figure 3 shows typical photographs of syngas flames under different CO/H2 ratios in the model combustor with a dual-swirled burner. As shown in Figure 3, there are double flame fronts at the flame root region for all syngas flames. There are possibly three reasons contributing to the phenomenon of double flame fronts. First, there are two shear layers on both sides of the fuel swirling flow: one is the outer layer, between the fuel swirling flow and the air swirling flow, and the other is the inner layer, between the fuel swirling flow and the high-temperature recirculating gas, which can provide a stable ignition source. Second, the main contents of mid-calorific syngas are CO and H2, and the flame speed is high (the flame speeds of H2 and
Figure 2. Schematic view of experimental facilities with the OH PLIF system. laser derives from a pulsed Nd:YAG laser pumping a tunable dye laser with Rhodamine 6G as the dye solution before going through a frequency double crystal. The output ultraviolet laser beam has the wavelength of 281.46 nm with a pulse duration of 20 ns and is used to excite OH radicals on the R1(9) line of the A2Σ + X2Π (v′ = 1, and v″ = 0) transition.16 The ultraviolet laser beam is expanded by a set of spherical and cylindrical lenses, forming a laser sheet with the thickness less than 500 μm. The laser sheet is guided vertically through the center of the test section in the combustion chamber. The fluorescence is then collected around the wavelength of 310 nm by an intensified charge-coupled device (ICCD) camera placed perpendicular to the laser sheet plane with a Nikon ultraviolet (UV) lens, in front of which a combined UG11 and WG305 interference filter set is installed to suppress scattered laser light and background flame radiation. The
Table 1. Syngas Composition and Test Cases for the Investigation of the Influence of the CO/H2 Ratio syngas composition (by volume) fuel code
H2 (%)
CO (%)
CO2 (%)
S-CH020 S-CH050 S-CH100 S-TJ S-CH200 S-CH300 S-CH400
70.8 56.7 42.5 37.3 28.3 21.3 17.0
14.2 28.3 42.5 47.7 56.7 63.7 68.0
15.0 15.0 15.0 15.0 15.0 15.0 15.0
N2 (%)
M (g/mol)
LHV (MJ N−1 m−3)
CO/H2 ratio
Refuel
Reair
12.0 15.7 19.4 20.7 23.0 24.9 26.0
9.431 9.69 9.95 10.05 10.21 10.34 10.42
0.20 0.50 1.00 1.28 2.00 3.00 4.00
3900 4500 5150 5300 5600 5800 5900
21600
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ones. With the increase of the CO/H2 ratio in syngas fuel, these two flame fronts tend to be merged together, thus forming a stronger flame root at the burner exit, as in the case of SCH400. Meanwhile, the axial length of the unburned fuel near the burner decreases, indicating that the location where the chemical reaction starts moves upstream of the combustor for flames of syngas with higher CO/H2 ratios. The regions with the highest OH intensities stand for the burnt gas at high temperature probably existing near recirculation zones, outside of which there exist the main reaction zones inside the plane of measurement. With a higher CO/H2 ratio, the main reaction zone of the syngas flame expands, especially in the radial direction, causing the distance between the two reaction zones on the upper and lower sides of the centerline and the OH intensity at the centerline increases. As the CO/H2 ratio increases, the content of CO out of the syngas fuel increases, with that of H2 decreasing, leading to a fuel with more weight and less diffusivity. Because of the effect on part of the fuel that just leaves the burner by recirculation flows at high temperature, the local temperature near the exit of the burner increases with syngas at higher CO/H2 ratios, which leads to the movement of the reaction zone toward the burner exit. On the other hand, because the chemical reaction speed of CO is much lower than that of H2, syngas with more CO and less H2 needs more time and space to complete the reaction. As a result, the main reaction zone is stretched. On the basis of the flame front indicated by OH radical distribution, several variables regarding flame structures for further quantitative research are defined. These variables include inner and outer flame angles at the burner exit (αin and αout) and the axial length and radial width of the main reaction zone (W and L). Figure 5 shows the definitions of the aforementioned variables.
Figure 3. Photographs of mid-calorific syngas flames under different CO/H2 ratios in the model combustor with a dual-swirled burner.
CO are 292 and 43 cm/s). Lastly, the mid-calorific syngas has a high flame temperature. Thus, there is double flame fronts in the mid-calorific syngas flame. As shown in Figure 3, the flame is brighter and the structure of double flame fronts is clearer with the increase of the CO/H2 ratio. The ensemble images of mean OH PLIF intensities of syngas flames with various CO/H2 ratios in the model combustor fueled through the burner with a double-swirler structure after post-processing procedures, including the average of 500 instantaneous images and shot-to-shot correction of laser energy, are shown in Figure 4. Air and fuel passages inside the double-
Figure 5. Definition of the flame angle and dimension of the main reaction zone. Figure 4. Averaged OH radical distributions for syngas flame under various CO/H2 ratio conditions.
Figure 6 reports the variation of the flame angle near the burner exit with the change of the CO/H2 ratio of syngas fuel. As shown in Figure 6, the inner flame angle αin decreases from
swirler burner are depicted on the left side of each image by the blue and red color, respectively. The size of the burner and its position to the OH PLIF test section are both in accordance with that during the experiment after proper image processing. Because of its symmetry about the centerline of the model combustor along the axial direction, only the upper half of each flame is displayed, and the bottom of each image corresponds to the centerline. The legend is also shown in this figure, indicating the relative intensity of the OH radical fluorescence. The OH intensity distribution shows that syngas is burnt in the shape of a shuttle with the CO/H2 ratio varying from 0.2 to 4.0. In regions close to the burner exit, there exist two areas of very low OH intensity, representing the unburned swirling fuel near the centerline and air on the upper left corner. The edge of the flame root can be recognized as two parts of inner and outer
Figure 6. CO/H2 ratio effects on the flame angle for syngas flame: (a) inner flame angle and (b) outer flame angle. 1587
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exit is relatively low; thus, the additional part does not help elongate main reaction zones. The effect of the CO/H2 ratio on the exhaust mean gas temperature is presented in Figure 8a. It is shown that the
53° for case S-CH020 to 42° for case S-CH300, while the outer flame angle αout also decreases but with a milder range from nearly 50° for case S-CH020 to 42° for case S-CH400. The decrease of the flame angle both inside and outside indicates that the inner and outer flame fronts tend to shrink toward the centerline, resulting in a sharper flame root close to the burner exit. This is probably caused by the fact that the molecular weight of the fuel increases with the increase of the CO/H2 ratio of syngas, leading to less diffusivity of fresh fuel entering the combustor. Without proper mixing among the high-temperature recirculation mixture, fresh air and fuel, the chemical reaction takes place with less chance. It should be noted that, in the case of S-CH400, its inner flame angle αin is abnormally higher than that in the case of S-CH300. As the CO/H2 ratio increases to 4.0, the boundary between inner and outer flame fronts becomes blurry and hard to recognize. In such a case, only the outer flame angle is effective, and this phenomenon should not be overrated. Furthermore, because of the fact that the inner flame front stays closer to swirling syngas from the air swirling passage of the burner, the inner flame angle αin receives more impact from the variation of fuel properties. The axial and radial dimensions of the main reaction zones of syngas flames under conditions of various CO/H2 ratios are shown in Figure 7. These two defined variables reflect to which
Figure 8. Exhaust mean temperatures and lower heating value (LHV) for syngas fuel under various CO/H2 ratio conditions: (a) exhaust mean temperatures and (b) LHV.
exhaust temperature increases almost linearly for the CO/H2 ratio below 2.0 and becomes independent at higher CO/H2 ratios. This result partially reflects the rising low heat values of fuels with an increasing CO/H2 ratio, which can be seen from Figure 8b. Another important reason for the existing discrepancy lies in the fact that, in low CO/H2 ratio cases, the size of the main reaction zone is limited, so that a cooling air effect cannot be neglected downstream of the combustor, leading to a measured temperature lower than its theoretical value. Figure 9 shows exhaust CO emissions for syngas under various CO/H2 ratio conditions. The tendency of the CO emis-
Figure 7. CO/H2 ratio effects on the dimension of the main reaction zone: (a) axial length of the main reaction zone and (b) radial width of the main reaction zone. Figure 9. CO emissions at the exhaust section for syngas under various CO/H2 ratio conditions.
extent the main reaction zone of the flame is stretched along axial and radial directions, respectively. As shown in this figure, for cases of CO/H2 ratios below 2.0, the axial length of the main reaction zone drops with the increase of the CO/H2 ratio; however, the CO/H2 ratio has a limited effect on it when the CO/H2 ratio further increases. The radial width of the main reaction zone on the other hand increases almost monotonically with the increase of the CO/H2 ratio. As CO/H2 increases, the scope of the increase in width is larger than that of the decrease in length of the main reaction zone, indicating that the dimension of the main reaction zone grows with more CO and less H2 in syngas fuel. At a lower CO/H2 ratio, syngas tends to diffuse more easily to the high-temperature burnt mixtures; therefore, the main reaction at the outer flame front side is longer at lower CO/H2 ratios. After CO/H2 exceeds 2.0, more and more unburned fuels flow back upstream by the effect of the recirculation zone and the length of the main reaction zone is determined by that near the inner recirculation zone. This can also be the explanation of the increase of the radial width of the main reaction zone. It is noteworthy that, although the chemical reaction starts earlier for syngas with a higher CO/H2 ratio along the axial direction, the intensity of OH radicals in the reaction zone near the burner
sion is very similar to that of the exhaust gas temperature reported in Figure 8a. Within the range during experiments, the maximum CO emission is below 25 ppm (at 15% O2), indicating that the combustion efficiency does not drop quickly with the increase of the CO content in syngas fuel. At a higher level of the CO/H2 ratio, the overall speed of the chemical reaction is lower because of the increase of CO consisting of syngas. On the other hand, if the fuel is completely burnt, additional heat released from syngas can also promote the oxidation of CO, thus reducing the unburned CO in exhaust gas. As a result, well organization of flows inside the combustor, especially near the burner exit and recirculation zones, is essential to control the level of CO emission for syngas with a relatively high CO/H2 ratio. 3.2. Effect of N2 Dilution on Mid-calorific Syngas Flames. In addition to the investigation of the impact on flame characteristics by the CO/H2 ratio of a chosen mid-calorific syngas, a series of experiments using the same syngas as the main fuel are carried out to study the effect of N2 dilution. The fuel composition and properties for the effect of N2 dilution in 1588
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Table 2. Fuel Composition and Test Cases for the Study on the N2 Dilution Effects syngas composition (by volume) fuel code
H2 (%)
CO (%)
CO2 (%)
N2 (%)
LHV (MJ N−1 m−3)
M (g/mol)
S-TJ S9N1 S8N2 S7N3 S6N4 H H9N1 H8N2 H7N3 H6N4 H5N5 C C9N1
37.3 33.6 29.8 26.1 22.4 100 90 80 70 60 50
47.7 42.9 38.2 33.4 28.6
15.0 13.5 12.0 10.5 9.0
0 10 20 30 40 0 10 20 30 40 50 0 10
10.05 9.04 8.04 7.03 6.02 10.79 9.71 8.63 7.55 6.47 5.39 12.63 11.37
20.7 21.4 22.2 22.9 23.6 2.0 4.6 7.2 9.8 12.4 15.0 28.0 28.0
100 90
CO/H2 ratio
equivalence ratio (Φ)
1.28
0.36
0.36
0.36
syngas composition are summarized in Table 2. Figure 10 shows typical photographs of syngas flames with different
Figure 11. Averaged OH radical distributions for the syngas flame under various N2 contents diluted.
altered to a single flame front by the addition of N2 in fuel. Furthermore, the location of the flame root moves upstream with the increase of N2. Meanwhile, at the centerline near the burner exit, there appears a narrow area with a very low OH fluorescence intensity. As 10% of total fuel in volume is added, the reaction zone is obviously expanded. The concentration of OH radicals decreases with the increase of the N2 content in syngas fuel. It is well-known that additional diluents, such as N2, in fuels reduce the content of combustible components. In the case where syngas is used as the main fuel, N2 increases the molecular weight as well. The central low OH region is thus caused by the poor diffusivity of the fuel just entering the combustor. At the same time, although N2 does not participate in the chemical reaction between syngas and oxygen directly, it absorbs heat with the flow of fuel, leading to a lower temperature near the reaction zone, finally causing the expansion of the main reaction zone. Because the inner and outer flame fronts merge into a single one with the addition of N2 in syngas fuel, only the outer flame angle defined in Figure 5 is valid. Besides, CO and H2 are adopted here as the reference main fuels, with N2 dilution at the same content for diluting syngas. As shown in Figure 12, the outer flame angle αout increases with the increase of the N2 content in fuel when the flame is fueled with CO and syngas. On the other hand, the H2 flame angle drops quickly from pure H2 to H2 with 10% (in volume) N2 diluted and then fluctuates in a limited range when the N2 dilution content increases to 50% (in volume). The outer flame angle αout reflects and is also
Figure 10. Photographs of syngas flames with different amounts of N2 dilution in the model combustor.
amounts of N2 dilution in the model combustor. As shown in Figure 10a, there are double flame fronts at the flame root region for the case of S-TJ. The color of double flames is sky blue. With the increase of the N2 content, the outer flame front moves toward the downstream and the flame is brighter. Furthermore, the region of the blue flame reduces continuously. When the content of N2 dilution reaches 30%, the double flame front structure at the flame root region is altered to a single flame front, which is similar to the CO flame, as shown in panels d−f of Figure 10. The main reasons for double flame fronts altering to a single flame front are as follows. When midcalorific syngas is diluted by N2, the flame speed and flame temperature reduce. However, the fuel exit velocity increases. Thus, the outer flame front cannot be stable near the fuel sprayer exit, and double flame fronts alter to a single flame front. Figure 11 shows the mean OH radical distribution of syngas flames in the model combustor with a double-swirled burner under conditions of various contents of N2 in syngas fuel, embedded with the actual size and position of the burner represented on the left side of each image. As shown in Figure 11, the double flame front structure at the flame root region is 1589
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combustion efficiency decreases along with an increasing N2 dilution. It is mainly caused by the temperature declination in reaction zones, which is desirable for prompt NOx formation and against CO oxidation. Therefore, further measures should be adopted to balance the emission of CO and NOx under conditions of N2 dilution.
4. CONCLUSION In this paper, experimental results of a series of test conditions regarding simulated mid-caloric syngas in a model combustor with a double-swirled structure burner are presented. The effect of the CO/H2 ratio on flame characteristics is investigated, while the dependence of N2 dilution is also assessed by means of OH PLIF, temperature, and emission measurements. It is demonstrated that the syngas flame exhibits inner and outer flame front structures near the double-swirler burner exit. During the increase of the CO/H2 ratio consisting of syngas fuel, the flame root gradually shrinks with the movement upstream. At the same time, the main reaction zone of the syngas flame is compressed to the centerline, leading to an increase in both the exhaust mean gas temperature and CO emission. A relatively low CO emission can be achieved for high CO/H2 ratio syngas, as long as the fuel is organized to be completely burnt inside high-temperature regions. On the other hand, N2 dilution of syngas fuel leads to an unreacted region on the centerline near the burner exit, probably compromising the flame root stability. Furthermore, the existence of N2 expands the flame opening angle and enlarges the main reaction zone; however, it may lead to high CO emission in exhaust gas, thus requiring other techniques to control NOx and CO emissions at the same time.
Figure 12. N2 dilution effects on the outer flame angle for syngas, H2, and CO flames.
largely dependent upon the structure of the flame root. In H2 flame, there exists a double flame front structure throughout the dilution conditions, while CO and syngas flames are different. If CO is part of the main fuel, the chemical reaction rate is relatively low and further reduced by diluents. The amount of unburned fuel increases consequently, flowing back with the recirculation mixture near the shear stress layer between the central recirculation zone boundary and swirling air. The chemical reaction taken place in these regions smoothes the flame root, leading to a wider flame angle. Axial and radial dimensions of main reaction zones for fuels of CO, H2, and syngas with various amounts of N2 dilution are plotted in Figure 13. It is shown that the radial width increases
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AUTHOR INFORMATION
Corresponding Author
*Fax: +86-21-34206103. E-mail:
[email protected]. Notes
Figure 13. N2 dilution effects on the dimension of main reaction zones fueled with syngas and other reference gas: (a) axial length of the main reaction zone and (b) radial width of the main reaction zone.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support to this study by the National Basic Research Program of China (Grant 2007CB210102).
with the dilution degrees for all three main fuels, which is caused by recirculated combustible mixtures. The axial length also expands for CO and syngas flames, indicating that the main fuel requires more space to complete the reaction. However, for H2 flame, it decreases with an increase of the N2 content. The increase of the maximum recirculation velocity with additional N2 may help shorten the axial span of the reaction zone. The level of CO emission in the exhaust section of the test facility is reported in Figure 14 for syngas with various dilution
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Figure 14. CO emissions at the exhaust section for syngas under various N2 dilution contents.
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