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Lifted flame property and interchangeability of natural gas on partially premixed gas burners
Case Studies in Thermal Engineering 12 (2018) 333–339
Contents lists available at ScienceDirect
Case Studies in Thermal Engineering journal homepage: www.elsevier.com/locate/csite
Lifted flame property and interchangeability of natural gas on partially premixed gas burners
T
⁎
Zhiguang Chen , Chaokui Qin, Pengfei Duan School of Mechanical Engineering, Tongji University, Shanghai, China
A R T IC LE I N F O
ABS TRA CT
Keywords: Partially premixed gas burners Lifted flames Natural gas AGA lifting index
The relationship between lifted flames and burner head temperature of partially premixed gas burners supplied by various natural gas sources was investigated, and the application of AGA(American Gas Association) lifting limit for Chinese domestic gas burners was studied. Results showed that lifted flames became more seriously with the burner head temperature decreasing, also with the increase of lift height of flames, it would lead to reduce the burner head temperature, eventually the burner head would reach the thermal equilibrium state which maintained stable flames and temperature. The lifting limit curve of partially premixed gas burners was only one under the same gas source, and it only depended on the burner structures and gas source characteristics, but nothing to do with the burner head temperature. The gas source with lower Wobbe index and calorific value made the burner appearing lifted flame more easily. Comparing with the experiment results and AGA lifting index prediction, it suggested that the limits of AGA lifting index IL≤ 1.10 should be change into IL≤ 1.05 using in China.
1. Introduction The study of lifted property of partially premixed flames is very important, because they are one of the most common flames that propagate in stratified mixtures of fuel and oxidant. Lifted flames are using widely in commercial boilers, where the lifted jet flame is utilized to reduce the damage to nozzle material by minimizing contact between the flame and the nozzle. But in domestic gas burners, lifted flames will make the flame unstable including blowout, incomplete combustion and lower combustion temperature, which will affect normal use. In China, with the development of natural gas industry, more and more cities began to supply natural gas instead of manufactured gas into gas network. When manufactured gas is substituted by natural gas, the users of domestic gas burners will be seriously affected by the gas interchangeability problems, especially lifting and incomplete combustion. It is very necessary to study the properties of lifted flames of partially premixed gas burners and the lifting prediction methods for Chinese gas interchangeability. Since the early 1900s, many studies had examined the effects of different fuel source compositions on the combustion characteristics of a burner and focused on developing index values. In 1927 the American Gas Association (AGA) began a 6-year study of interchangeability called the Mixed Gas Research, and approximately 175,000 separate tests and examinations were conducted during this study. The investigation resulted in the development of a general formula, called “AGA C index” which was first made public in 1936 [1]. In 1946 the AGA Laboratories published a series of reports and bulletins of an investigation of gas interchangeability, after a large number of experiments, three interchangeability index were developed for “yellow-tips”, “flashback” and “lifting”, designated IY, IF and IL respectively [2,3]. In 1951 Weaver added the incomplete combustion index to the AGA indices in
⁎
Corresponding author. E-mail address: [email protected] (Z. Chen).
https://doi.org/10.1016/j.csite.2018.05.004 Received 4 April 2018; Received in revised form 5 May 2018; Accepted 6 May 2018 Available online 08 May 2018 2214-157X/ © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Case Studies in Thermal Engineering 12 (2018) 333–339
Z. Chen et al.
order to estimate interchangeability more accurately and provide detailed explanations of the changes in combustion characteristics [4]. While an index calculation approach has often been used in the United States, European countries have tended to use a graphical approach to predict fuel interchangeability, for example the Delbourg diagrammatic developed in France, the Dutton diagrammatic investigated in England and so on [5]. Using these methods, two fuels are considered interchangeable when their index values lie within the same defined range. Recently years with the development of gas industry, multi gas sources have been supplied into a same gas network in many cities, and some new gas interchangeability researches have been studied. In 2005 the Natural Gas Council (NGC+) formed a technical work group to address the hydrocarbon liquid dropout issues specific to domestic supply and another technical work group to address the interchangeability issues associated with high Btu liquefied natural gas (LNG) imports [6]. In order for LNG to be a truly globally traded commodity, BP had developed several in-house models to predict and resolve quality and interchangeability issues in 2010 [7]. Chang-Eon Lee [8–10] studied the stability of a partially premixed flame feed by landfill gas (LFG), liquefied natural gas (LNG), methane, Liquefied Petroleum Gas (LPG)and LPG-mixed fuels, and investigated the interchangeability between LFG and LPG-mixed fuels. Wanneng Dai [11] studied the flame stability limits of biogas flame. It was found that the lifting limits are enhanced with an increase of port diameter or mixture temperature and with a decrease of CO2 concentration. By review the previous investigations about gas interchangeability and flame stability, the main objective of this paper is to study the relationship between lifted flames and burner head temperature of partially premixed gas burners supplied by various natural gas sources, and then to study the application of AGA lifting limit for Chinese domestic gas burners fueled by different natural gases, which is to develop an appropriate lifting prediction method for Chinese gas interchangeability. 2. Experiment setup 2.1. Gas-blending system Test gases were provided by gas-blending sub-system through which methane, ethane, propane, butane, nitrogen and carbon dioxide, were blended to give exactly the same constituents as test gases, also the same Wobbe index and combustion potential were achieved. The gas-blending sub-system includes a 5 m3 storage tank, as shown in Fig. 1. During gas-blending operation, pipeline natural gas (PNG) available in the laboratory was used to purge the tank. Pure components such as methane, ethane, propane etc. were successively piped through a gas meter which has full scale of 10 m3/h and ± 0.2% FS precision to monitor gas flow into the storage tank. Gas pressure gauges and a heating circuit were incorporated to improve precision. A propelling mixer located in the top of storage tank helped mix uniformly the fed constituents. The purities of individual components involved were as follows: methane 99%, ethane 99.5%, propane 99.95%, butane 99.95%, nitrogen 99.999% and carbon dioxide 99.6%. After all the individual constituents were fed into the storage tank, the mixture remained for 3–5 h while propeller was working. Then gas was sampled and analyzed by means of gas chromatography. When the constituents of blended gas fell within permissible limitations compared with test gases listed in Table 1[12], the blended gases could be regarded as identical to test gases. 2.2. Precision test burner system In order to study the relationship between lifted flames and burner head temperature of partially premixed gas burners, a precision partially premixed combustion burner was designed, named precision test burner (PTB), including burner head, low pressure
Fig. 1. The schematic diagram of experimental testing system. 334
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Table 1 Accuracy and repeatability of gas chromatography. Gas component percentage (%)
Permissible error (%)
0–0.1 0.1–1.0 1.0–5.0 5.0–10 ＞10
0.02 0.07 0.1 0.12 0.3
ejector segment and air-fuel mixing chamber, as shown in Fig. 2. The PTB was designed based on the statistical analysis of 65 partially premixed gas burners which are representative in Chinese domestic gas burner market with the data of size, depth, inclination, spacing and port loading. The burner head includes 36 round ports and the designed port loading is 8 W/mm2, diameter 2.5 mm, depth 5 mm, inclination 15° and spacing 5 mm. In order to measure the real-time monitoring of burner head temperature of PTB, three K-type thermocouples which has a ± 0.75% FS precision were used to monitor the temperature of premixed flow, burner head surface and flame with the sampling frequency of 1 Hz. The positions of three K-type thermocouples were shown in Fig. 2, while the measure position of premixed flow within the head was at the inner wall of PTB, 3 mm to ports, the burner head surface temperature measure position at the center circle and the temperature measure point of flame was in the middle of two ports. All the three temperature data were recorded by computer. The PTB test system includes PTB, air flow meter, gas flow meter, and flame shape recording camera, as shown in Fig. 1. The experimental compressed air flowed through a wet gas flow meter with full scale of 6 m3/h and ± 0.1% FS precision into the mixing chamber of PTB. The experimental sampled gas stored in the storage tank was piped through a soap film flow meter with the test range of 0.001–30 L/min and ± 0.1% FS precision to the PTB. The primary air factor was controlled by adjusting the different flow mixing ratio between air-side and gas-side. A digital single lens reflex Camera of Canon EOS50D was used to record the form of the flame, including the length and width of the inner cone of the premix flame. The lifting limit curve and the relationship between lifted flames and burner head temperature of partially premixed gas burners under various gas sources were tested. 3. Results and discussion 3.1. Relationship between lifted flames and temperature In this experiment, PNG and pure N2 were used to mix sampled gases. The subscripts of PNG, such as 20 in PNG20, represent the percentage by volume of N2 in the PNG. The compositions and properties of the gases used are shown in Table 2. The experimental results of three temperature measurement which changed with the lifted flames under two different gas sources were shown in Fig. 3(a). According to the results, the relationship between the burner head temperature and partially premixed lifted flames was causality. With the burner head temperature decreasing, the lifted flames became more seriously. Meanwhile with the increase of the lift height of flames, it would lead to reduce the burner head temperature. But eventually the burner head would reach the thermal equilibrium state which maintained stable flames and temperature. As shown in Fig. 3(b), comparing with PNG0, PNG20 was combusted on a lower temperature, and the lifted flames appeared on lower temperature. With the decrease of burner head temperature, the lifted flames were more serious during PNG20 than PNG0 Domesitc gas burner head
Low pressure ejector
k type thermocouple Expansion chamber for mixed primary air and gas
Air inlet Gas inlet Fig. 2. The schematic diagram of PTB. 335
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Table 2 The compositions and properties of gases tested in PTB. Mole%
CH4
C2H6
C3H8
C4H10
C5H12
CO2
N2
Hs(MJ/m3）
s
Ws(MJ/m3）
PNG0 PNG20
93.45 74.55
3.49 2.79
0.59 0.47
0.17 0.14
0.03 0.03
0.64 0.51
1.62 21.52
38.44 30.65
0.594 0.669
49.90 37.50
Fig. 3. The relationship between lifted flames and temperature of PTB.
3.2. Lifting limit curve of PTB Experiment had been done to find whether there were different lifting limit curves of a same partially premixed gas burner at different burner head temperatures, and the characteristic lifting limit curves also could be obtained with this method. The experimental process was as follows: igniting burner and preheating 10–15 mins, the primary air was increased at a given constant gas rate until the flames just lift from the ports, and the exact air and gas rate then being determined at this point. This procedure was repeated for a number of different gas rates at a specific burner head temperature. In this experiment, it used PTB as the test sample and PNG0 as the test gas. The flame temperature was named as the reference temperature of burner head. And two lifting limit curves of PTB at 350 °C and 150 °C were observed. The test results were shown in Fig. 4 and we could find that the two lifting limit curves were almost uniform. The lifting limit curve of the partially premixed gas burner was only one under the same gas source, and it only depended on the burner structures and gas source characteristics, but nothing to do with the burner head temperature. It meant that the lifting limit curve of a partially premixed gas burner was defined when the characteristics of gas source were given, but it must maintain the burner head temperature steady to read the lifted flame. This conclusion was different from Wanneng Dai [11] studied results. With the results of above experiment, the lifting limit curve of PTB under PNG0 and PNG20 was tested. The testing results of two gas sources were shown in Fig. 5(a), the lifting limit curve of PNG20 was lower than PNG0 and the no lifted area of PNG20 was also narrower. So the gas source with lower Wobbe index and calorific value made the burner appearing lifted flame more easily. The
Fig. 4. The lifting limit curve of PTB with different head temperature. 336
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Fig. 5. The lifting limit curve of PTB in PNG0 and PNG20.
results were same as Chang-Eon Lee [8–10] studied. When poor natural gas (natural gas with less heavy hydrocarbon and more methane, so the Wobbe index and calorific is lower) interchanged with rich natural gas(natural gas with more heavy hydrocarbon and less methane have a higher Wobbe index and calorific), it would cause serious lifting problems on domestic gas burners which are adjusted by rich natural gas, but not vice-versa. The data of different lifting limit curves were plotted on a semi-logarithmic coordinates, as shown in Fig. 5(b). The mutually parallel straight lines could be gained when the head temperature was maintained a constant but not same value and the relationship could be organized into the following formula [2]:
log q = m⋅α + K q: port loading (W/mm2)，α: ratio of primary air，m: constant of the slope of lifting limit curve(concerned with the burner structures, m was a constant when the burner structures were defined)，K: lifting limit constant(concerned with the gas source characteristics, K was a constant when the components of gas source was given. According to the lifting limit constant with pure gas source which proposed by AGA [2] (list in Table 3), the lifting limit constant K of PNG0 and PNG20 were calculated as 1.2 and 1.1. But if according to the lifting limit curve equation which was obtained from this experiment, the lifting limit constant K of PNG0 and PNG20 could be 1.6 and 1.5 respectively. So it was very necessary to study the application of AGA lifting limit for Chinese domestic gas burners fueled by different natural gases and to develop an appropriate lifting prediction method for Chinese gas interchangeability. 3.3. Application of lifting index When gas interchanges, some domestic gas burners are very easy to appear unsteady flame like lifting, flashback, yellow-tips and so on, and will cause some serious problems. For example, lifted flames will make the burners ignite difficultly and easily be blown off which will affect the normal daily use. In 1946, AGA gave an index to predict lifted flames when gas interchanged, named IL, and the equation is [2,3]:
IL =
Ka fa as fs aa
(
Ks − lg
fa fs
)
IL: index of lifting interchangeability; K: lifting limit constant; a: volume of air theoretically required for complete combustion; f: primary air factor; a and s: subscripts designating adjustment and substitute gases, respectively. In order to test and verify the application of AGA lifting index for Chinese natural gas interchangeability prediction, 17 sets of domestic gas burners covering 11 different brands and three types of ports, namely, round, square and ribbon, were selected as representatives of popular burner structures, and 7 kinds of natural gases covering 3 different types, namely PNG, LNG and offshore natural gas (ONG), were selected as sampled gas sources. The compositions and properties of sampled gas sources were listed in Table 4. Initial adjustments were made for 17 sets of domestic gas burners under adjustment gas (LNG3). The shape and hardness of Table 3 Values of lifting constant K for various simple gases. Gas
CH4
C2H6
C3H8
C4H10
N2
CO2
K
0.67
1.419
1.931
2.55
0.688
1.08
337
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Table 4 The compositions and properties of sampled gas sources. Mole%
CH4
C2H6
C3H8
C4H10
CO2
N2
Ws(MJ/m3）
PNG1 PNG2 PNG3 OSG1 LNG1 LNG2 LNG3
96.00 92.79 98.90 85.99 98.50 97.00 90.70
0.70 4.00 0.20 9.61 0.00 1.90 7.50
0.20 0.30 0.22 0.20 0.30 0.30 0.30
0.10 0.30 0.20 0.00 0.10 0.10 0.10
2.30 1.80 0.00 3.60 0.00 0.00 0.20
0.70 0.80 0.46 0.60 1.10 0.70 1.20
48.3 49.4 50.7 48.8 50.1 50.6 51.1
atmospheric flame depends mainly upon primary air for a fixed burner head. By monitoring the primary air shutters of domestic gas burners to change the equivalence ratio, inner cones will become from soft to distinct when the primary air rate raise and the color of flame will change from yellow to clear blue. The primary air shutters of 17 sets of domestic gas burners were so adjusted that resulting flames fell between class − 2 to + 2 shown in Table 5 [13]. Such flames were considered to be most flexible according to previous researches. 17 sets of domestic gas burners were observed to give satisfactory performance under LNG3, namely no lift, no flash-back, no yellow-tips occurred and CO emission remained below specified limit. Afterwards the air shutters remained unchanged, the performance under the other 6 gases were measured in sequence. The measurement procedure of lifted flames was carried out in this way which was igniting the main burner while maintaining pressure at the inlet of gas burner at 2kPa, and if lift could be observed for more than 1/3 of burner ports by visual inspection, after ignition, it should be determined as “lifting” [14]. Testing lifted flames of the 17 kinds of domestic gas burners with different substitute gases, and the statistics results of lifting were shown in Table 6. The prediction results of AGA lifting index were also listed in Table 6. From Table 6, we could conclude that the rate of lifting samples would be over 15% during PNG1, PNG2, OSG1 and LNG1 as the substitute gas. And according to the definition of gas interchangeability by NGC+ [6], it was reasonable to believe that if the rate of lifting samples was under 15%, it will not cause materially changing performance on lifting, and it was like to consider that the substitute gases can be interchangeable. The limit of AGA lifting index IL is IL≤ 1.10, and all of the prediction results were in the range. It meant that AGA lifting index predicted all of the substitute gases would not cause lifting problem. Comparing with the prediction results of lifting and the experiment results, it could conclude: (1) AGA lifting index can’t be totally suitable when it's used to predict lifting for Chinese urban natural gas interchangeability. (2) The predicting results will basically consistent with the experiments’, if the limit of AGA lifting index IL is changed into IL≤ 1.05. 4. Conclusion According to the experimental results, it can conclude that: (1) The relationship between burner head temperature and partially premixed lifted flame was causality. With the burner head temperature decreasing, the lifted flames became more seriously. Meanwhile with the increase of the lift height of flames, it would lead to reduce the burner head temperature. But eventually the burner head would reach the thermal equilibrium state which maintained stable flames and temperature. (2) The lifting limit curve of the partially premixed gas burners was only one under the same gas source, and it only depended on the burner structures and gas source characteristics, but nothing to do with the burner head temperature. It meant that the lifting limit curve of a partially premixed gas burners was defined when the characteristics of gas source were given, but it must maintain the burner head temperature steady to read the lifted flame. (3) Poor natural gas was more easily to cause lifted flames than rich natural gas, and combusted on a lower temperature. When poor natural gas interchanged with rich natural gas, it would cause serious lifting problems on domestic gas burners which were adjusted by rich natural gas, but not vice-versa. (4) AGA lifting index can’t be totally suitable when it's used to predict lifting for Chinese urban natural gas interchangeability.
Table 5 AGA flame code for describing flame characteristics [13]. Code
Flame description
+3 +2 +1 0 −1 −2 −3
Short inner cone, flame may be noisy Inner cones distinct and pointed Inner cones and tips distinct Inner cones rounded, soft tips Inner cones visible, very soft tips Faint inner cones Inner cones broken at top, lazy wavering flames
338
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Table 6 The statistics of lifting samples and AGA lifting index prediction. Lifting
Appearing
Percentage %
IL ≤ 1.10
PNG1 OSG1 LNG1 PNG2 PNG3 LNG2 LNG3
3 3 3 4 2 2 0
17.6 17.6 17.6 23.5 11.8 11.8 0.0
1.0808 1.0661 1.0348 1.0546 1.0212 1.0239 1.0000
Comparing with the experiment results and AGA lifting index prediction, it can conclude that according to China it suggests the limits of AGA lifting index IL≤ 1.10 change into IL≤ 1.05. Conflict of interest The authors declare that there is no conflict of interest regarding the publication of this paper. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
Mixed Gas Research. American Gas Association Reports 689, 1933. Interchangeability of Other Fuel Gases with Natural Gas. American Gas Association Research Bulletin No. 36, 1946. Interchangeability of Various Fuel Gases with Manufactured Gas. American Gas Association Research Bulletin No. 60, 1950. Elmer R. Weaver, Formulas and graphs for Representing the Interchangeability of fuel gases, J. Res. Natl. Bur. Stand. 46 (3) (1951). P.T. Harsha, R.B. Edelman, D.H. France, Catalogue of Existing Interchangeability Prediction Methods, Final Report – Phase II, GRI-80/0021, Gas Research Institute, Chicago, Illinois, US, 1980. NGC+ Interchangeability Work Group, White Paper on Natural Gas Interchangeability and Non-Combustion End Use. 2005. 02. International Gas Union and BP. Guidebook to Gas Interchangeability and Gas Quality. 08, 2010. Chang-Eon Lee, Cheol-Hong Hwang, An experimental study on the flame stability of LFG and LFG-mixed fuels, Fuel 86 (2007) 649–655. Chang-Eon Lee, Cheol-Hong Hwang, Hyun-Yong Lee, A study on the interchangeability of LFG–LPG mixed fuels with LFG quality in domestic combustion appliances, Fuel 87 (2008) 297–303. Chang-Eon Lee, Cheol-Hong Hwang, Sung-Chang Hong, Proposal and validation of a new type of flame stability diagram for partially premixed flames, Fuel 87 (2008) 3687–3693. Wanneng Dai, Chaokui Qin, Zhiguang Chen, Chao Tong, Pengjun Liu, Experimental studies of flame stability limits of biogas flame, Energy Convers. Manag. 63 (2012) 157–161. General Administration of Quality Supervision, Inspection and Quarantine of the People’s Republic of China (AQSIQ). GBT13610−2003 Gas Composition Analysis by Gas Chromatography, China Standard Press, Beijing, 2003 (in Chinese). Gas Technology Institute. Gas Interchangeability Tests: Evaluating the Range of Interchangeability of Vaporized LNG and Natural Gas. 04, 2003. General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China (AQSIQ).GB 16410-2007 Domestic Gas Stove, China Standard Press, Beijing, 2007 (in Chinese).
339
Contents lists available at ScienceDirect
Case Studies in Thermal Engineering journal homepage: www.elsevier.com/locate/csite
Lifted flame property and interchangeability of natural gas on partially premixed gas burners
T
⁎
Zhiguang Chen , Chaokui Qin, Pengfei Duan School of Mechanical Engineering, Tongji University, Shanghai, China
A R T IC LE I N F O
ABS TRA CT
Keywords: Partially premixed gas burners Lifted flames Natural gas AGA lifting index
The relationship between lifted flames and burner head temperature of partially premixed gas burners supplied by various natural gas sources was investigated, and the application of AGA(American Gas Association) lifting limit for Chinese domestic gas burners was studied. Results showed that lifted flames became more seriously with the burner head temperature decreasing, also with the increase of lift height of flames, it would lead to reduce the burner head temperature, eventually the burner head would reach the thermal equilibrium state which maintained stable flames and temperature. The lifting limit curve of partially premixed gas burners was only one under the same gas source, and it only depended on the burner structures and gas source characteristics, but nothing to do with the burner head temperature. The gas source with lower Wobbe index and calorific value made the burner appearing lifted flame more easily. Comparing with the experiment results and AGA lifting index prediction, it suggested that the limits of AGA lifting index IL≤ 1.10 should be change into IL≤ 1.05 using in China.
1. Introduction The study of lifted property of partially premixed flames is very important, because they are one of the most common flames that propagate in stratified mixtures of fuel and oxidant. Lifted flames are using widely in commercial boilers, where the lifted jet flame is utilized to reduce the damage to nozzle material by minimizing contact between the flame and the nozzle. But in domestic gas burners, lifted flames will make the flame unstable including blowout, incomplete combustion and lower combustion temperature, which will affect normal use. In China, with the development of natural gas industry, more and more cities began to supply natural gas instead of manufactured gas into gas network. When manufactured gas is substituted by natural gas, the users of domestic gas burners will be seriously affected by the gas interchangeability problems, especially lifting and incomplete combustion. It is very necessary to study the properties of lifted flames of partially premixed gas burners and the lifting prediction methods for Chinese gas interchangeability. Since the early 1900s, many studies had examined the effects of different fuel source compositions on the combustion characteristics of a burner and focused on developing index values. In 1927 the American Gas Association (AGA) began a 6-year study of interchangeability called the Mixed Gas Research, and approximately 175,000 separate tests and examinations were conducted during this study. The investigation resulted in the development of a general formula, called “AGA C index” which was first made public in 1936 [1]. In 1946 the AGA Laboratories published a series of reports and bulletins of an investigation of gas interchangeability, after a large number of experiments, three interchangeability index were developed for “yellow-tips”, “flashback” and “lifting”, designated IY, IF and IL respectively [2,3]. In 1951 Weaver added the incomplete combustion index to the AGA indices in
⁎
Corresponding author. E-mail address: [email protected] (Z. Chen).
https://doi.org/10.1016/j.csite.2018.05.004 Received 4 April 2018; Received in revised form 5 May 2018; Accepted 6 May 2018 Available online 08 May 2018 2214-157X/ © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Case Studies in Thermal Engineering 12 (2018) 333–339
Z. Chen et al.
order to estimate interchangeability more accurately and provide detailed explanations of the changes in combustion characteristics [4]. While an index calculation approach has often been used in the United States, European countries have tended to use a graphical approach to predict fuel interchangeability, for example the Delbourg diagrammatic developed in France, the Dutton diagrammatic investigated in England and so on [5]. Using these methods, two fuels are considered interchangeable when their index values lie within the same defined range. Recently years with the development of gas industry, multi gas sources have been supplied into a same gas network in many cities, and some new gas interchangeability researches have been studied. In 2005 the Natural Gas Council (NGC+) formed a technical work group to address the hydrocarbon liquid dropout issues specific to domestic supply and another technical work group to address the interchangeability issues associated with high Btu liquefied natural gas (LNG) imports [6]. In order for LNG to be a truly globally traded commodity, BP had developed several in-house models to predict and resolve quality and interchangeability issues in 2010 [7]. Chang-Eon Lee [8–10] studied the stability of a partially premixed flame feed by landfill gas (LFG), liquefied natural gas (LNG), methane, Liquefied Petroleum Gas (LPG)and LPG-mixed fuels, and investigated the interchangeability between LFG and LPG-mixed fuels. Wanneng Dai [11] studied the flame stability limits of biogas flame. It was found that the lifting limits are enhanced with an increase of port diameter or mixture temperature and with a decrease of CO2 concentration. By review the previous investigations about gas interchangeability and flame stability, the main objective of this paper is to study the relationship between lifted flames and burner head temperature of partially premixed gas burners supplied by various natural gas sources, and then to study the application of AGA lifting limit for Chinese domestic gas burners fueled by different natural gases, which is to develop an appropriate lifting prediction method for Chinese gas interchangeability. 2. Experiment setup 2.1. Gas-blending system Test gases were provided by gas-blending sub-system through which methane, ethane, propane, butane, nitrogen and carbon dioxide, were blended to give exactly the same constituents as test gases, also the same Wobbe index and combustion potential were achieved. The gas-blending sub-system includes a 5 m3 storage tank, as shown in Fig. 1. During gas-blending operation, pipeline natural gas (PNG) available in the laboratory was used to purge the tank. Pure components such as methane, ethane, propane etc. were successively piped through a gas meter which has full scale of 10 m3/h and ± 0.2% FS precision to monitor gas flow into the storage tank. Gas pressure gauges and a heating circuit were incorporated to improve precision. A propelling mixer located in the top of storage tank helped mix uniformly the fed constituents. The purities of individual components involved were as follows: methane 99%, ethane 99.5%, propane 99.95%, butane 99.95%, nitrogen 99.999% and carbon dioxide 99.6%. After all the individual constituents were fed into the storage tank, the mixture remained for 3–5 h while propeller was working. Then gas was sampled and analyzed by means of gas chromatography. When the constituents of blended gas fell within permissible limitations compared with test gases listed in Table 1[12], the blended gases could be regarded as identical to test gases. 2.2. Precision test burner system In order to study the relationship between lifted flames and burner head temperature of partially premixed gas burners, a precision partially premixed combustion burner was designed, named precision test burner (PTB), including burner head, low pressure
Fig. 1. The schematic diagram of experimental testing system. 334
Case Studies in Thermal Engineering 12 (2018) 333–339
Z. Chen et al.
Table 1 Accuracy and repeatability of gas chromatography. Gas component percentage (%)
Permissible error (%)
0–0.1 0.1–1.0 1.0–5.0 5.0–10 ＞10
0.02 0.07 0.1 0.12 0.3
ejector segment and air-fuel mixing chamber, as shown in Fig. 2. The PTB was designed based on the statistical analysis of 65 partially premixed gas burners which are representative in Chinese domestic gas burner market with the data of size, depth, inclination, spacing and port loading. The burner head includes 36 round ports and the designed port loading is 8 W/mm2, diameter 2.5 mm, depth 5 mm, inclination 15° and spacing 5 mm. In order to measure the real-time monitoring of burner head temperature of PTB, three K-type thermocouples which has a ± 0.75% FS precision were used to monitor the temperature of premixed flow, burner head surface and flame with the sampling frequency of 1 Hz. The positions of three K-type thermocouples were shown in Fig. 2, while the measure position of premixed flow within the head was at the inner wall of PTB, 3 mm to ports, the burner head surface temperature measure position at the center circle and the temperature measure point of flame was in the middle of two ports. All the three temperature data were recorded by computer. The PTB test system includes PTB, air flow meter, gas flow meter, and flame shape recording camera, as shown in Fig. 1. The experimental compressed air flowed through a wet gas flow meter with full scale of 6 m3/h and ± 0.1% FS precision into the mixing chamber of PTB. The experimental sampled gas stored in the storage tank was piped through a soap film flow meter with the test range of 0.001–30 L/min and ± 0.1% FS precision to the PTB. The primary air factor was controlled by adjusting the different flow mixing ratio between air-side and gas-side. A digital single lens reflex Camera of Canon EOS50D was used to record the form of the flame, including the length and width of the inner cone of the premix flame. The lifting limit curve and the relationship between lifted flames and burner head temperature of partially premixed gas burners under various gas sources were tested. 3. Results and discussion 3.1. Relationship between lifted flames and temperature In this experiment, PNG and pure N2 were used to mix sampled gases. The subscripts of PNG, such as 20 in PNG20, represent the percentage by volume of N2 in the PNG. The compositions and properties of the gases used are shown in Table 2. The experimental results of three temperature measurement which changed with the lifted flames under two different gas sources were shown in Fig. 3(a). According to the results, the relationship between the burner head temperature and partially premixed lifted flames was causality. With the burner head temperature decreasing, the lifted flames became more seriously. Meanwhile with the increase of the lift height of flames, it would lead to reduce the burner head temperature. But eventually the burner head would reach the thermal equilibrium state which maintained stable flames and temperature. As shown in Fig. 3(b), comparing with PNG0, PNG20 was combusted on a lower temperature, and the lifted flames appeared on lower temperature. With the decrease of burner head temperature, the lifted flames were more serious during PNG20 than PNG0 Domesitc gas burner head
Low pressure ejector
k type thermocouple Expansion chamber for mixed primary air and gas
Air inlet Gas inlet Fig. 2. The schematic diagram of PTB. 335
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Table 2 The compositions and properties of gases tested in PTB. Mole%
CH4
C2H6
C3H8
C4H10
C5H12
CO2
N2
Hs(MJ/m3）
s
Ws(MJ/m3）
PNG0 PNG20
93.45 74.55
3.49 2.79
0.59 0.47
0.17 0.14
0.03 0.03
0.64 0.51
1.62 21.52
38.44 30.65
0.594 0.669
49.90 37.50
Fig. 3. The relationship between lifted flames and temperature of PTB.
3.2. Lifting limit curve of PTB Experiment had been done to find whether there were different lifting limit curves of a same partially premixed gas burner at different burner head temperatures, and the characteristic lifting limit curves also could be obtained with this method. The experimental process was as follows: igniting burner and preheating 10–15 mins, the primary air was increased at a given constant gas rate until the flames just lift from the ports, and the exact air and gas rate then being determined at this point. This procedure was repeated for a number of different gas rates at a specific burner head temperature. In this experiment, it used PTB as the test sample and PNG0 as the test gas. The flame temperature was named as the reference temperature of burner head. And two lifting limit curves of PTB at 350 °C and 150 °C were observed. The test results were shown in Fig. 4 and we could find that the two lifting limit curves were almost uniform. The lifting limit curve of the partially premixed gas burner was only one under the same gas source, and it only depended on the burner structures and gas source characteristics, but nothing to do with the burner head temperature. It meant that the lifting limit curve of a partially premixed gas burner was defined when the characteristics of gas source were given, but it must maintain the burner head temperature steady to read the lifted flame. This conclusion was different from Wanneng Dai [11] studied results. With the results of above experiment, the lifting limit curve of PTB under PNG0 and PNG20 was tested. The testing results of two gas sources were shown in Fig. 5(a), the lifting limit curve of PNG20 was lower than PNG0 and the no lifted area of PNG20 was also narrower. So the gas source with lower Wobbe index and calorific value made the burner appearing lifted flame more easily. The
Fig. 4. The lifting limit curve of PTB with different head temperature. 336
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Fig. 5. The lifting limit curve of PTB in PNG0 and PNG20.
results were same as Chang-Eon Lee [8–10] studied. When poor natural gas (natural gas with less heavy hydrocarbon and more methane, so the Wobbe index and calorific is lower) interchanged with rich natural gas(natural gas with more heavy hydrocarbon and less methane have a higher Wobbe index and calorific), it would cause serious lifting problems on domestic gas burners which are adjusted by rich natural gas, but not vice-versa. The data of different lifting limit curves were plotted on a semi-logarithmic coordinates, as shown in Fig. 5(b). The mutually parallel straight lines could be gained when the head temperature was maintained a constant but not same value and the relationship could be organized into the following formula [2]:
log q = m⋅α + K q: port loading (W/mm2)，α: ratio of primary air，m: constant of the slope of lifting limit curve(concerned with the burner structures, m was a constant when the burner structures were defined)，K: lifting limit constant(concerned with the gas source characteristics, K was a constant when the components of gas source was given. According to the lifting limit constant with pure gas source which proposed by AGA [2] (list in Table 3), the lifting limit constant K of PNG0 and PNG20 were calculated as 1.2 and 1.1. But if according to the lifting limit curve equation which was obtained from this experiment, the lifting limit constant K of PNG0 and PNG20 could be 1.6 and 1.5 respectively. So it was very necessary to study the application of AGA lifting limit for Chinese domestic gas burners fueled by different natural gases and to develop an appropriate lifting prediction method for Chinese gas interchangeability. 3.3. Application of lifting index When gas interchanges, some domestic gas burners are very easy to appear unsteady flame like lifting, flashback, yellow-tips and so on, and will cause some serious problems. For example, lifted flames will make the burners ignite difficultly and easily be blown off which will affect the normal daily use. In 1946, AGA gave an index to predict lifted flames when gas interchanged, named IL, and the equation is [2,3]:
IL =
Ka fa as fs aa
(
Ks − lg
fa fs
)
IL: index of lifting interchangeability; K: lifting limit constant; a: volume of air theoretically required for complete combustion; f: primary air factor; a and s: subscripts designating adjustment and substitute gases, respectively. In order to test and verify the application of AGA lifting index for Chinese natural gas interchangeability prediction, 17 sets of domestic gas burners covering 11 different brands and three types of ports, namely, round, square and ribbon, were selected as representatives of popular burner structures, and 7 kinds of natural gases covering 3 different types, namely PNG, LNG and offshore natural gas (ONG), were selected as sampled gas sources. The compositions and properties of sampled gas sources were listed in Table 4. Initial adjustments were made for 17 sets of domestic gas burners under adjustment gas (LNG3). The shape and hardness of Table 3 Values of lifting constant K for various simple gases. Gas
CH4
C2H6
C3H8
C4H10
N2
CO2
K
0.67
1.419
1.931
2.55
0.688
1.08
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Table 4 The compositions and properties of sampled gas sources. Mole%
CH4
C2H6
C3H8
C4H10
CO2
N2
Ws(MJ/m3）
PNG1 PNG2 PNG3 OSG1 LNG1 LNG2 LNG3
96.00 92.79 98.90 85.99 98.50 97.00 90.70
0.70 4.00 0.20 9.61 0.00 1.90 7.50
0.20 0.30 0.22 0.20 0.30 0.30 0.30
0.10 0.30 0.20 0.00 0.10 0.10 0.10
2.30 1.80 0.00 3.60 0.00 0.00 0.20
0.70 0.80 0.46 0.60 1.10 0.70 1.20
48.3 49.4 50.7 48.8 50.1 50.6 51.1
atmospheric flame depends mainly upon primary air for a fixed burner head. By monitoring the primary air shutters of domestic gas burners to change the equivalence ratio, inner cones will become from soft to distinct when the primary air rate raise and the color of flame will change from yellow to clear blue. The primary air shutters of 17 sets of domestic gas burners were so adjusted that resulting flames fell between class − 2 to + 2 shown in Table 5 [13]. Such flames were considered to be most flexible according to previous researches. 17 sets of domestic gas burners were observed to give satisfactory performance under LNG3, namely no lift, no flash-back, no yellow-tips occurred and CO emission remained below specified limit. Afterwards the air shutters remained unchanged, the performance under the other 6 gases were measured in sequence. The measurement procedure of lifted flames was carried out in this way which was igniting the main burner while maintaining pressure at the inlet of gas burner at 2kPa, and if lift could be observed for more than 1/3 of burner ports by visual inspection, after ignition, it should be determined as “lifting” [14]. Testing lifted flames of the 17 kinds of domestic gas burners with different substitute gases, and the statistics results of lifting were shown in Table 6. The prediction results of AGA lifting index were also listed in Table 6. From Table 6, we could conclude that the rate of lifting samples would be over 15% during PNG1, PNG2, OSG1 and LNG1 as the substitute gas. And according to the definition of gas interchangeability by NGC+ [6], it was reasonable to believe that if the rate of lifting samples was under 15%, it will not cause materially changing performance on lifting, and it was like to consider that the substitute gases can be interchangeable. The limit of AGA lifting index IL is IL≤ 1.10, and all of the prediction results were in the range. It meant that AGA lifting index predicted all of the substitute gases would not cause lifting problem. Comparing with the prediction results of lifting and the experiment results, it could conclude: (1) AGA lifting index can’t be totally suitable when it's used to predict lifting for Chinese urban natural gas interchangeability. (2) The predicting results will basically consistent with the experiments’, if the limit of AGA lifting index IL is changed into IL≤ 1.05. 4. Conclusion According to the experimental results, it can conclude that: (1) The relationship between burner head temperature and partially premixed lifted flame was causality. With the burner head temperature decreasing, the lifted flames became more seriously. Meanwhile with the increase of the lift height of flames, it would lead to reduce the burner head temperature. But eventually the burner head would reach the thermal equilibrium state which maintained stable flames and temperature. (2) The lifting limit curve of the partially premixed gas burners was only one under the same gas source, and it only depended on the burner structures and gas source characteristics, but nothing to do with the burner head temperature. It meant that the lifting limit curve of a partially premixed gas burners was defined when the characteristics of gas source were given, but it must maintain the burner head temperature steady to read the lifted flame. (3) Poor natural gas was more easily to cause lifted flames than rich natural gas, and combusted on a lower temperature. When poor natural gas interchanged with rich natural gas, it would cause serious lifting problems on domestic gas burners which were adjusted by rich natural gas, but not vice-versa. (4) AGA lifting index can’t be totally suitable when it's used to predict lifting for Chinese urban natural gas interchangeability.
Table 5 AGA flame code for describing flame characteristics [13]. Code
Flame description
+3 +2 +1 0 −1 −2 −3
Short inner cone, flame may be noisy Inner cones distinct and pointed Inner cones and tips distinct Inner cones rounded, soft tips Inner cones visible, very soft tips Faint inner cones Inner cones broken at top, lazy wavering flames
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Table 6 The statistics of lifting samples and AGA lifting index prediction. Lifting
Appearing
Percentage %
IL ≤ 1.10
PNG1 OSG1 LNG1 PNG2 PNG3 LNG2 LNG3
3 3 3 4 2 2 0
17.6 17.6 17.6 23.5 11.8 11.8 0.0
1.0808 1.0661 1.0348 1.0546 1.0212 1.0239 1.0000
Comparing with the experiment results and AGA lifting index prediction, it can conclude that according to China it suggests the limits of AGA lifting index IL≤ 1.10 change into IL≤ 1.05. Conflict of interest The authors declare that there is no conflict of interest regarding the publication of this paper. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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