The Influence of Hydrogen on the Combustion Characteristics of Lean Premix Swirl CH 4 /H 2 Flames

Transcription

1 The Influence of Hydrogen on the Combustion Characteristics of Lean Premix Swirl CH 4 /H 2 Flames Liu Xiaopei*, Chen Mingmin, Duan Dongxia, Zhang Hongwu and Enrico Gottardo Shanghai Electric Gas Turbine Co.,Ltd, CHINA. ( liuxp3@shanghai-electric.com, chenmm2@shanghai-electric.com ) Institute of Engineering Thermophysics, Chinese Academy of Sciences, CHINA. ( duandongxia@iet.cn, zhw@iet.cn) Ansaldo Energia S.p.A., ITALY ( Enrico.Gottardo@ansaldoenergia.com) ABSTRACT Combustion characteristics of lean premix swirl CH4/H2 flames have been studied by numerical simulation and experiment in a model combustor which was designed for natural gas combustion. The volume concentration of hydrogen in the mixture was varied from to 2%. The effects of hydrogen on the flow field structure was studied by numerical steady state simulations. A cold condition numerical simulation showed that the variation of hydrogen concentration has no obvious influence on the shape and strength of the CRZ (central recirculation zone). In a hot condition numerical simulation however, the hydrogen concentration changes have an influence on the temperature field, and hence, the hot flow field does show considerable differences. The diameter of the CRZ decreases with the increase of hydrogen concentration, while the recirculation becomes weaker. In addition to that, the flame becomes shorter with the increase of hydrogen concentration, while the NOx emissions increases. Based on the experimental results and frequency spectrum analysis, it was found that thermoacoustic oscillations became stronger with the presence of hydrogen. Yet, the frequencies of the thermoacoustic oscillations were hardly influenced by the hydrogen. Keywords: hydrogen; thermoacoustic; swirl. 1. Introduction Higher combustibility associated with hydrogen has received increased attention as an additive to other traditional hydrocarbon fuels for extending the lean combustion flammability limits and achieving more stable combustion. Alternative fuels such as SNG and biomass syngas are getting more and more attention recently. These fuels typically contain a significant fraction of hydrogen and they could be further mixed with natural gas in the pipelines before the point of use. In such cases the composition of the fuels can be subject to fluctuations especially related to hydrogen concentration. In order to develop an enhanced fuel flexible gas turbine combustor capable to withstand a wider fuel composition range it is important to deeply understand the role of hydrogen in the combustion of such hybrid fuels.

2 There are significant differences in physical and chemical characteristic between traditional hydrocarbon fuel and hydrogen/hydrocarbon hybrid fuel. Unlike the hydrocarbon fuel, most significant effects of hydrogen/hydrocarbon hybrid fuel are more depended on hydrogen, such as flame speed, heat release ratio, adiabatic flame temperature [1][2][3]. With the high reactivity of hydrogen, the burning velocity was improved, and preventing local flame out [4][5]. The change of those micro features resulted in the variations of flame shape, the position of flame center (center of heat release), the area of the flame which will affect the mechanism of thermoacoustic oscillation [6]. In the lean-premixed swirl combustion system, the recirculation play a significant role in flame stability which was influenced by the addition of hydrogen into methane [7][8]. According to the time delay model of thermoacoustic oscillation, the key parameter, the phase between acoustics and combustion, was influenced by turbulent flame speed which was affected by the parameters such as the density of the fuel, consumption ratio and the area of flame. While the composition changing, those parameters become different, finally resulted in the change of the phase between acoustics and combustion which will enhance or weaken the oscillation [9][1]. On the other hand, the phase was also influenced by the convection time and chemical time which was highly depended on the fuel composition [11]. Among the reports and studies on methane flame characteristic, not much is known about the characteristic of hydrogen/hydrocarbon hybrid fuel [12][13][14]. The most issues with hydrogen/hydrocarbon hybrid fuel is associated with the significant variation in their fuel compositions that changes the combustion characteristics such as flame speed, heat release ratio, local fuel consumption rate and flame instability mechanisms. Those variation play important role in macro phenomenon such as flashback, NOX emissions, auto ignition. The objective of the research was to investigate the role of hydrogen addition to methane fuel in lean-premixed swirl flame in a model combustor which was designed for methane fuel. The volume concentration of hydrogen in the mixture was varied from to 2%. The detailed flow field with different amount of hydrogen was analysed by numerical steady state simulation. The role of different hydrogen concentration on the thermoacoustic oscillation and emissions were examined by normal pressure experiment with the same combustor. 2. Model combustor The model combustor was mainly including radial swirler with the swirl number is 1.15, fuel injector, pilot nozzle, premixing section and flame tube as shown in Figure 1. The injected fuel from the injector was added to the combustion air from the annular air inlet at the downstream of the flame tube, and then mixed and swirled in the swirler as it is passed into the combustion zone.

3 Figure 1.The model combustor 3. Numerical simulation 3.1 Method and process of the simulation The focus of the simulation is the detailed flow field in the flame tube, meanwhile the combustor is a periodical symmetrical structure, so the calculation region was simplified for reducing the computation load as shown in Figure 2. The angle of the fan-shaped calculation region is 6, moreover the mesh of fuel injector, swirler and the pilot nozzle is refined. Figure 2.The mesh of the calculation region The size of the grid is significant important for the result of the simulation as if the mesh is too coarse may result in an inaccurate outcome, if the size of the mesh is too small will make the results difficult to converge and a waste of time. There are two sets of mesh, 3.5 million and 7million, to be selected as the suitable mesh for simulation. The axial velocity distribution along the radial direction of the two sets of mesh at different axial position (the detail position as shown in Figure 3) was compares as shown in Figure 4.

4 Figure 3.Detail position The axial velocity along the radial direction of the two sets mesh keep almost the same. From the perspective of time and computing resource, it is better to choose the 3.5 million mesh as the final mesh for simulation million mesh-l1 7. million mesh-l million mesh-l2 7. million mesh-l r(m) r(m) (a)l1 (b) L million mesh-l3 7. million mesh-l r(m) (c)l3 Figure 4.The comparison of axial velocity of the two sets mesh According to the applicable conditions and calculation precision of different turbulent model and combustion model, finally the realized k-ε model was chosen as the turbulence model, the combustion model was finite-rate/eddy-dissipation model for prevention of the immediately ignition when the fuel mixed with the air. 3.2 The result and discussion of the numerical simulation In this section results from numerical simulation of different amount of hydrogen flow field characteristics are presented. The intention of the simulation is to examine the

5 role of hydrogen on the flow field and velocity profile. In the simulation, the equivalent ratio is kept constant at.583 and, the air mass flowrate is kept constant at 266g/s. To change the composition of the fuel the concentration of hydrogen is changed As shown in Figure 5 for different hydrogen concentration, it can be observed that the profile of the axial velocity along the radial direction at different axial position in cold condition, at different hydrogen concentration, is almost the same, except for a little difference at position L2. which is the outlet of the premix section, where the axial velocity increase with the increase of hydrogen concentration. Those results can be attributed to the increase of fuel volume flowrate due to the change of fuel composition. When the air mass flowrate is 266g/s, and the equivalence ratio is the.583, increase the hydrogen volume concentration by 1%, the total volume flowrate will increase about.4%. According to that, it can be found the variation in volume flowrate is relatively small due to the change of fuel composition in the examined conditions that can t make significant difference in axial velocity profile =% =1% =2% =% =1% =2% (a)l1 (b)l =% =1% =2% =% =1% =2% (c)l3 (d)l4

6 =% =1% =2% 1-1 =% =1% =2% (e)l5 (f)l6 Figure 5.The axial velocity along radial direction at different axial position in cold condition Sometimes, it could use some parameters that related to recirculation such as the maximum diameter of the CRZ (the central recirculation zone, the area enclosed by axial velocity equal to zero), bmax, the length of the CRZ, L, the distance between the axial position of bmax and swirler, lmax, and the quantity of recirculation of high temperature product gas to characterize the flow characteristics in the flame tube. Furthermore, the CRZ is very important for flame stability. The CRZ position in cold condition is shown in Figure 6. From this figure, it can be seen that there is a little difference in the bmax which is decrease with the increase of hydrogen concentration. The phenomenon can be attributed to the small increase in volume flowrate due to the change of fuel composition that make the increase of axial velocity which can inhibited the expand of the CRZ..5 Radial direction position(m) = =1% =2% Axial direction position(m) Figure 6.The CRZ position for different hydrogen concentration in cold condition The axial velocity along the radial direction in hot condition at different axial position is shown in Figure 7. The axial injection velocity increase with hydrogen addition, but the back flow velocity decrease with hydrogen addition. And also, it can be seen the difference between the and 1% of hydrogen volume concentration is smaller than the difference between the 1% and 2% of hydrogen volume concentration.

7 = =1% =2% 4 2 = =1% =2% (a)l1 (b)l = =1% =2% 4 2 = =1% =2% (c)l3 (d)l = =1% =2% = =1% =2% (e)l5 (f)l6 Figure 7. The axial velocity along radial direction at different axial position in hot condition The Figure 8 shows the CRZ position for different hydrogen concentration. The hydrogen addition shifts the axial position of bmax which is indicated by the dotted line in the figure to downstream. The bmax, which is nondimensionalized by dividing the diameter of the flame tube, is.3812,.383,.3792 respectively, at hydrogen concentration, 1%, 2%. The bmax is decreased with the increase of hydrogen concentration. The difference of the diameter in other position is more obvious. The

8 length of the CRZ increase with the increase of hydrogen concentration. The temperature distribution in the combustion zone is shown in Figure 9, as the temperature shown a little increase with the increase of hydrogen concentration. The increase of the temperature makes the increase of axial injection velocity, consequently the expansion of the CRZ is inhibited by the higher and higher axial injection velocity. Due to the decrease of the back flow velocity and higher temperature results in the reduction of the recirculation flow = =1% =2% Axial direction position(m) Figure 8.The CRZ position for different concentration at different axial position Temperature(K) = =1% =2% Temperature(K) = =1% =2% (a)l (b)l5 Figure 9.Temperature along the radial direction at different position 4. Experiment study 4.1 Introduction of the test rig An effective simulation method is still lacked for the thermoacoustic oscillation especially for the intensity of oscillation which is a main problem that gas turbine faced. In order to examine the role hydrogen in thermoacoustic oscillation, the experiment was carried out. As shown in Figure 1, the experimental station mainly consists of air system which was supplied by the air compressors, fuel system which was supplied by gas cylinders,

9 exhaust system, cooling system, measurement system and the model combustor. The fuels that from the different gas cylinders were depressurized, go through the mass flow controller, mixing with each other in the mixer, finally, the hybrid fuel was formed. There are two fuel lines in the head of the burner, the pilot fuel line that used for ignition, the premix fuel line that supply the hybrid fuel as shown in Figure 11. The mix process between the combustion air and hybrid fuel is consistent with that described in the numerical simulation section. The model combustor used in simulation and experiment is the same. Figure 1.Atmospheric experimental station Two dynamic pressure measuring points were arranged in the model combustor, onein the flame tube and the other in the annular air inlet, and used for monitor the pressure during the experiment process to analysis the characteristics of the thermoacoustic oscillation. The type of dynamic pressure sensor is Kulite XCS-19(M)-15D. The flue gas analyzer is Testo 35. A camera was put at the end of the combustor to monitor the combustion process in the flame tube. All the measurement signals were integrated in the collection cabinet for storage. Figure 11.The model combustor in the station

10 4.2 Results and discussion of the experiment The CRZ which acts as a source to stabilize the flame was influenced by the hydrogen concentration. Some fundamental characteristics such flame speed, ignition delay time were all changed due to the addition of hydrogen to methane. Those parameters are all important for the thermoacoustic oscillation. The objective of the experiment was to examine the role of hydrogen on thermoacoustic oscillation and NOX emissions. The combustion air was supplied at 25,the mass flowrate was 16g/s. The equivalence ratio was.583. The air conditions and equivalence ratio keep unchanged during the experiment. The hydrogen volume concentration was varied from to 2%. The intensity of the oscillation as affected by different amount of hydrogen addition to the methane fuel is shown in Figure 12. A key parameters that decided the characteristics of thermoacoustic oscillation is the phase between the pressure oscillation and heat release rate oscillation. While changing the composition of the hybrid fuel, the flow field as described in simulation section, the shape of the flame, and the local dynamic of the flame were all changed, those changing can led to the variation of the phase which can make the couple between the pressure oscillation and heat release rate oscillation more and more strengthen results in the intensity in the flame tube increase with the addition of hydrogen. It can be seen that the intensity of oscillation in both flame tube and annular air inlet increase with the increase of hydrogen concentration. Theoretically, the intensity of the oscillation in the annular air inlet should keep unchanged because of the air condition was keep unchanged. The strengthen phenomenon can be attributed to the oscillation in the annular air inlet was influenced by the oscillation in flame tube. The pressures oscillations in the flame tube could spread upstream across the unchocked fuel nozzles flame tube annular air inlet oscillation intensity(%) hydrogen volume concentration(%) Figure 12.Hydrogen addition effects on intensity of the oscillation

11 The frequency in the flame tube and annular air inlet remain almost the same with the change of hydrogen concentration as shown in Figure 13. The dynamics pressure power spectrum with different hydrogen concentration is shown in Figure 14. There are three evident peaks in the spectrum, the frequency of the peaks are 17Hz, 5Hz, 75Hz respectively. Those are the three first modal of the oscillation. The modal of oscillation with different hydrogen concentration keep almost the same, because of the frequency of the oscillation with all hydrogen concentration is around 17Hz. The addition of hydrogen to the methane fuel has little influence on the frequency of the oscillation. 2 oscillation frequency(hz) flame tube annular air inlet hydrogen volume concentration(%) Figure 13.Hydrogen addition effects on frequency of oscillation Dynamic pressure power spectrum/db re 2μPa frequency(hz) flame tube annular air inlet Dynamic pressure power spectrum/db re 2μPa frequency(hz) flame tube annular air inlet Dynamic pressure power spectrum/db re 2μPa frequency(hz) flame tube annular air inlet Figure 14.Dynamic pressure power spectrum with hydrogen concentration, 1%, 2%

12 respectively The Figure 15 shows hydrogen addition effects on the emissions. The NOx emissions increase with the increase of hydrogen concentration. It can be attributed to the increase of temperature of the combustion zone induced by the addition of hydrogen. The emission of CO remain very low during the experiment conditions NO X (ppm@15% O 2 ) NOX CO CO(ppm@15% O 2 ) hydrogen volume concentration(%) Figure 15.Hydrogen addition effects on emissions 5. Conclusions The results of the simulation and experiment show: In hot condition, with the addition of hydrogen the flow field became different, with the increase, the diameter of the CRZ decrease, the length of the CRZ increase, the axial injection velocity increase and the back flow velocity decrease. The net dominate result is the quantity of the recirculation decrease and the intensity of the recirculation was weakened. According to the results of the experiment, the addition of hydrogen made the intensity of the oscillation enhanced which influenced the oscillation in the annular air inlet. The frequency of the oscillation is almost not influenced by the addition of hydrogen. Because of the increase of temperature of the combustion zone induced by the addition of hydrogen, the NOX emission was increased. The move from % to 1% of hydrogen is less effective than the move from 1% to 2% indicating a nonlinear behavior.

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