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Extinction limits of swirl non-premixed methane flames and CH/CH/ N mixtures: experimental evaluation and thermodynamic calculations 2
Wojciech Jerzak Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01293 • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018
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Abstract
12
In this study, we aimed to evaluate the extinction limits of fuel-rich and fuel-lean flames
13
created in non-premixed coaxial swirl burner of combustion substrates. The experiments were
14
performed with three types of fuels: methane and two mixtures of CH4/CxHy/N2. The analyzed
15
fuels reflected varied composition of the natural gas containing methane, ethane, propane,
16
butane, and nitrogen. The examination of the extinction limits of flames isolated from the
17
surroundings with a quartz tube was based on the limit ratios of equivalence (Φ). Results
18
showed that an increase in the stream of methane directed toward the burner improves
19
blowout limits as determined by the equivalence ratios. However, this pattern was not
20
observed for the other studied fuels. A concurrent increase in the content of CxHy and N2 in
21
natural gas implies worsening of extinction limits of fuel-lean flames and improvement of
22
extinction limits for fuel-rich flames. Also performed thermodynamic calculations using the
23
FactSage 6.3 software package, specifying the adiabatic temperature of flames and the
24
equilibrium composition of exhaust gases in the areas close to the extinction of flames. It was
25
found that it is possible to estimate the improvement or deterioration of the blowout limits of
26
flames resulting from changes in the composition of natural gas based on the equilibrium
27
exhaust composition.
28
1.
Introduction
29
To ensure stable operation of combustion systems in the widest possible range of
30
power regulation, special constructions of nozzles, burners, air swirlers, and bluff bodies (for
31
low-power burners) are used.1-3 These elements increase the effectiveness of mixing of natural
32
gas with the oxidant and thus improve the stability of gas flames. The creation of a stable gas
33
flame is possible only at a certain specific range of speed of combustion substrates flowing
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out of the nozzle. The range of stable combustion is determined by the type and composition
35
of the fuel, the equivalence ratio (Φ ), construction and dimensions of the burner, presence of
36
pilot flame, method of mixing (e.g. swirl production), temperature of the flammable mixture,
37
system pressure, variation of heat release rate, and so on.4-6
38
Extinction of flames can occur for the following reasons: the competition between
39
chain branching and chain terminating reactions; removal of critical amount of heat; flame
40
stretch; excessive fuel flow; excessive air flow and/or swirl flow; the disappearance of
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recirculation structure in swirl flame. 1-11 The closer to the limit of flame extinction, the lower
42
its temperature. The temperature drop affects the reaction rate of the branching reactions more
43
than that of the terminating reactions. Consequently, the concentration of free radicals (e.g.
44
CH, OH, O) is reduced, which might lead to flame extinction. When we approach to a local
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extinction, then the heat released from the flame decreases due to the reduction in the surface
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area of the flame.5 Stretching reduces the flame thickness and increases the rate of substrate
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consumption per unit area of the flame and can lead to extinction.7
48
The extinction limits of fuel-rich and fuel-lean flames is called blowout, if the flame
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extinguishes directly from the burner lip (without lifting-off).8 In turn, the blow-off
50
phenomenon, precedes flame lifting-off from the burner. Feikema et al.
51
the first studies about the blowout rich and lean limits associated with an excessive air or fuel
52
velocity. The addition of swirl to the air stream can cause up to a sixfold improvement in the
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lean blowout limits of non-premixed flames. Dawson et al.1 examined visualization of blow-
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off events in turbulent methane-air lean premixed flames. Close to blow-off, the flame closed
55
at the axis and took an “M” shape. Measurements of OH chemiluminescence, flame
56
tomography and OH-PLIF allowed to quantify the duration of the blow-off transient and the
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reaction zone location.1,2
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published one of
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There is a large body of numerical work focused on understanding a phenomenon of flame
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extinction, among which there are works by Tyliszczak et al.11, Zhang and Mastorakos.12 The
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notable example of successful numeric prediction of blow-off was performed by Tyliszczak et
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al..11 Blow-off was triggered by a sudden increase in the air mass flow rate and LES/CMC
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(Large Eddy Simulation/Conditional Moment Closure) method was shown to be able to
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capture the local flame extinction and the subsequent blow-off process. During the blow-off
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process the total heat release gradually decreases.12 Exceeding the limit of extinction may
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result in an unexpected loss of power of the device, and in addition, burner rebooting is in
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most cases very difficult or even impossible.
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Industrial installations in which combustion of the natural gas takes place, such as gas
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turbines (GTs), high temperature air combustion chambers (HiTACs), metallurgical furnaces
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for heat treatment with swirl burners, are designed for a predetermined chemical composition
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of the natural gas. The composition of unrefined natural gas primarily depends on the place of
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its extraction,
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hydrocarbons are the primary constituents of natural gas: CH4 (70–90%), C2H6, C3H8, and
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C4H10 up to 20%.15 According Hashemi et al.17, ethane is the most important component of
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natural gas (except methane), because its varying content in natural gas clearly shifts the
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fuel’s ignition limits. The flammability limit is determined by the concentration range of the
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fuel (% by volume) in the gas mixture with the oxidant. For individual combustible species,
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the upper and the lower flammable limits (UFLs and LFLs, respectively) are distinguished.
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The flammability limit is closely related to the flame extinction phenomena.
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13-15
and to a small extent from the season of the year.16 Following
Using the classical definition for the equivalence ratio, it is possible to express the
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equations of equivalence ratio for lean blowout (ΦLBO) and rich blowout (ΦRBO) later referred
81
to as the limit ratios of equivalence:
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=
/ /
= =
( )
( )
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(1)
(2)
(3)
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Observation of flame extinction (blowout) is the goal of research in this work. Several
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reasons motivated the author of this article to define limits for the extinction of diffusion
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flames—expressed as limit equivalence ratios. First, to check the effect of components of
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natural gas and the stream of flowing fuel on the limiting equivalent ratios at which the flame
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created in the non-premixed coaxial swirl burner disappears. Measuring the temperature at
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different distances from the burner and predicting the equilibrium exhaust composition for the
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areas close to the limit ratios of equivalence were the other important reasons for the conduct
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of this study.
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2.
Materials and Methods
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2.1
Fuel
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As mentioned in the Introduction section, the composition of natural gas can be highly
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diversified depending on its location of extraction and the process of refining. In connection
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with this, the subject of this study was natural gas with variable proportions of its primary
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components. In this study, three gas fuels were tested, with the following letter designations:
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A, B, and C; Table 1 shows their composition. Two gas cylinders procured from the Polish
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distributor Air Liquide were used in the experiments. The first cylinder contained pure
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methane (99.999%), whereas the second cylinder contained a mixture of gases with a weight
99
certificate marked as C. Fuel B was produced by mixing fuel A with C at a proportion of
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54%/46%.
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Table 1. The composition and characteristics of fuel blend used in the study
M
∆
LHV
(g/mol)
(kJ/mol)
(MJ/kg)
Fuel blend components, (mol %)
Fuel designation
CH4
C2H6
C3 H 8
C4H10
N2
0
0
0
0
16.043
-802.3
50.01
A
100.00
B
91.78
5.98
1.38
0.46
0.46
17.527
-862.2
49.19
C
82.00
13.00
3.00
1.00
1.00
19.248
-931.4
48.39
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Table 1, in addition to the composition, shows the lower heating value (LHV) of the fuels
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tested, which was calculated based on the enthalpy of the following combustion reactions: )
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+ 2 → + 2 , ∆ = −802 317 (*+)
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, + 3.5 → 2 + 3 , ∆ = −1 427 861.8 (*+ )
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1 2 + 5 → 3 + 4 , ∆ = −2 044 055.1 (*+ )
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34 + 6.5 → 4 + 5 , ∆ = −2 658 574.8 (
(4) )
)
) ) *+
(5)
(6)
(7)
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Enthalpy values were calculated in FactSage 6.3 software in the Reaction module. The
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reference temperature (T0) was 25 °C and the pressure was 0.1 MPa.
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2.2
Test stand
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Experiments were performed on a stand shown schematically in Fig. 1. The main
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element of the stand is a non-premixed coaxial swirl burner. These types of burners are of
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interest to researchers, as evidenced by the numerous studies conducted recently, inter alia by
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Santhosh and Basu8, Chouaieb et al. 18, Khandelwal et al. 19, Merlo et al. 20, Rashwan et al. 21,
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Torkzadeh et al.22 and Zaidaoui et al..23 The process of combustion of fuels A, B, and C in
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the air atmosphere takes place in a quartz tube with a diameter of 60 mm and a wall thickness
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of 2 mm. Fuel is supplied to the burner by a central nozzle, which it leaves via four radial
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nozzles located at the outlet of the substrates from the burner. Fuel flowing from the injector
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is surrounded by an external oxidant swirl. The injector comprises of 4 holes with 1 mm
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diameter. Flow rates, and the bulk velocities of the fuels and air are shown in Table 2.
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Table 2. Flow rates range, and the bulk velocities of the tested fuels and air.
V6 (781 ⁄ℎ)
V6; (8⁄?)
>;1. In case of insufficient air volume in the swirl generator, the value of the
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swirl number will drop. Then, the effect of flame extinction was studied by gradually
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changing the stream of the air. Fig. 2(a)–(c) shows that the extinction limits of lean flames are
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the most favorable for fuel A, with Φ LBO limit values in the range of 0.368 to 0.507. The
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leaner the flame is in the fuel, lower will be its height and its width, as visible in Fig. 3(a)–(c).
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In addition, the Φ decline is evidently accompanied by flame expansion, just above the exit
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plane of the burner. The flame in these combustion conditions is compact, axially
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symmetrical, blue, and its shape resembles a tulip flower. It should be mentioned here that no
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lift-of effect of swirl flame over the burner was observed in experiments.
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If the content of CxHy and N2 simultaneously increases in fuel (as it is the case of fuels
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B→C), we are dealing with a gradual deterioration of extinction limits of lean flames.
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Visually, the appearance of lean flame for fuels A, B, and C is identical; therefore, the images
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of flames of the aforementioned fuels were not compared in this study. Deterioration of the
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limits of extinction of lean flames as a result of an increase in the stream of tested fuels can be
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observed in Fig. 2(a)–(c). The reason for an increase in the limit equivalence ratios with an
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increasing fuel stream may be an increased heat flow toward the wall of the quartz tube.
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However, toroidal gas recirculation zones are present in the swirl flame—external
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recirculation zone (ERZ) and internal recirculation zone (IRZ)—marked in Fig. 4(a), which
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improve the stability of the flames. The existence of an ERZ certifies that the flame is “rolled”
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down by the wall of the quartz tube exposed with red dotted line as shown in Fig. 4 (b). For
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the highest tested value of the fuel stream (Fig. 4(c)), the IRZ is less clear, which may indicate
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the dispersion of the zone of intense reactions. An increase in the height of the flame is also
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visually noticeable.
175
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Fig. 2. Effect of the stream of tested fuels on the limit zone of flames determined by Φ.
177 178
Fig. 3. Effect of Φ on changes in the appearance of lean flames of fuel A for conditions close
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to extinction (a)–(c)
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Recirculation of hot combustion gases improves the mixing of reagents in the quartz
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chamber and thereby improving the efficiency of the process of combustion. The occurrence
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of the blue color of the flame (zone of intense reaction with the highest temperature) in Figs. 3
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and 4 is due to the radiation caused by the excited CH radicals.26
184 185
Fig. 4. Effect of fuel C stream on the shape of lean flames under conditions close to
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extinction.
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According to the results of the limits of extinction of fuel-rich flames, we can see that
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fuel C is the most favorable among the three tested fuels. Improved extinction limits with an
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increase in flowing fuel stream was also found for the analyzed fuels A, B, and C. In case of
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fuel A, the limit values ΦRBO were found to be in the range of , whereas in case
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of fuel C, it was . Improvement in the extinction limits of fuel-rich flame in
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case of fuel C was found to be relatively low than that of fuel A. Before the flame
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extinguishes, the burner is heated to the temperature that is determined by the combustion
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conditions (Φ ). The temperature at the outlet’s surface from the burner affects the limits of
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flame extinction.
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Fig. 5 (a) - (c) show the results of temperature measurements in the flame axis for three
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equivalence ratios, successively: 0.56, 1.0 and 1.52. The presented results include the
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temperature correction for radiative heat losses. Calculation of temperature corrections was
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made using the following equation: 27 T = T +
200 201
UVW WG XW
(10)
The heat transfer coefficient (ht) at the wire surface was determined based on equation (11): 36 ℎ =
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YZ[ \W
(11)
203
The temperature measurement error based on the second part of equation (10) ranged from 41
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to 99 °C.
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Definitely higher temperatures in the burner’s axis were noted for a fuel-rich flame (Φ = 1.52;
206
Fig. 5(c)) than that of a fuel-lean flame (Φ =0.56; Fig. 5(a)). In case of fuel A, both fuel-rich
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and fuel-lean flames were characterized by a higher temperature value at a distance nearest to
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the burner (up to approximately 4 cm), whereas in case of fuel C, higher temperature was
209
observed 6 cm above the burner. This may suggest an increase in the length of the flame
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resulting from the oxidation of hydrocarbons as per their hierarchy, which is as follows: CH4;
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C2H6; C3H8, and C4H10. However, as already mentioned, changes in the length of flame
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resulting from the combustion of fuels A, B, and C were not visually perceived. For the sake
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of clarity with respect to Fig. 4, the author have not presented the temperature measurements
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for fuel B, which contained within the limits of fuels A and C. The highest values of
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temperatures in the flame axis were recorded for stoichiometric conditions in accordance with
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Fig. 5(b). The actual flame temperature was found to be lower than that of the adiabatic flame
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temperature. The actual value of the flame’s temperature depends on the temperature of the
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walls of the combustion chamber (the lower the temperature of the walls, the more radiation it
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will absorb). The lowest temperature was found to be in the axis of non-mixed swirl flames
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with the central fuel nozzle, which was confirmed by the measurements of temperatures,26 the
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concentrations of OH free radical,20 and numerical simulations.28 The low concentration of
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OH radical in the area of the burner’s axis suggests a slowdown in the oxidation of
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hydrocarbons, since free radicals such as OH are the driving force during the process of
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oxidation. Fig. 6 shows the visual appearance of the rich flame. It has two distinct zones: the
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blue at the exit plane of the burner and the yellow–orange at the upper part. The yellow–
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orange zone is anchored in the flame’s axis, as shown in Fig. 6. The yellow-orange color
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indicates an excess of flammable gas and a significant deficiency in the oxygen content,
228
which is necessary for combustion. The decomposition of hydrocarbon molecules occurs in
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this part of the flame, and the emitted, strongly heated solid carbon particles shine—resulting
230
in a bright flame. This type of effect in the literature is referred to as yellow tipping of the
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flame. A luminous flame usually has a lower temperature than that of a blue non-luminous
232
flame.
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Fig. 5. Temperature distribution in the axis of the burner for combustible mixtures: (a) lean,
235
(b) stoichiometric, and (c) rich.
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236 237
Fig. 6. Effect of stream of fuel C on the shape of rich flames in conditions close to extinction.
238
2.2
239
In this study, the calculations of thermodynamic equilibrium based on Gibbs technique of
240
minimizing total free energy has been used. Thermodynamic prediction of the equilibrium
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phases emerging during the combustion of fuels A, B, and C was performed using the
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FactSage 6.3 software package. The “Equilib” module together with the FactPS database was
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used in this study. The input data to the “Equilib” module were fuel and air compositions at T
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= 25°C and p = 0.1 MPa. The following air composition expressed in mole% was assumed for
245
the thermodynamic calculations: 78.084 N2; 20.946 O2; 0.934 Ar and 0.036 CO2.
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2.2.1
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Adiabatic flame temperature (AFT) is theoretically the highest value of the propagating
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flame’s temperature, in which there are no losses of heat to the environment. AFT is
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calculated for constant volume or pressure conditions. AFT belongs to the key parameters
250
determining the safe operation of combustion chambers. The knowledge of AFT makes it
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possible to choose the right chamber material and the cooling method. Recent research
252
indicates that the limits of blowout and flashback of premixed swirl flames correlate with a
Thermodynamic calculations
Adiabatic flame temperature
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constant value of AFT, for the variable composition of the oxidizing atmosphere, while
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maintaining the constant inlet velocity of the substrates.29 Li et al.30 recommends the extended
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method of AFT to predict the lower limits of flammability of organic substances containing C,
256
H, O, and N.
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In this study, AFT for the fuels tested was found to be in the range of 0.37 < Φ 1. It may also be concluded
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from Fig. 7 that AFT for fuels A, B, and C near extinction limits is always much higher than
262
that of a rich flame. The limit areas expressed by ΦLBO for fuel-lean flames A
263
and by Φ RBO for fuel-rich flames are shaded with a green line in Fig. 7. The
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ranges of AFT and correspond to the limit areas. The
265
temperature difference marked in Fig. 7 is 317.1°C, which is in the range of equivalence
266
ratios of .
267 268 269
Fig. 7. Adiabatic flame temperature for the tested gas fuels 3.2.2 Wet exhaust gas composition
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The combustion process in a non-premixed flame, also called as a diffusion flame, is
271
controlled by the diffusion of the reagents. In this type of flames, the transport processes are
272
slower than that of a typical combustion reaction; therefore, the chemical kinetics of the
273
reaction plays a small role. Then, it is justifiable to predict the composition of the exhaust
274
gases based on the thermodynamic equilibrium. Fig. 8 shows the equilibrium composition of
275
wet exhaust gases for fuels A and C in conditions close to the extinction of the flame.
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Nitrogen is always the constituent element of the exhaust gas, regardless of Φ. The next
277
formed combustion products with the highest molar fractions near the limits of extinction are
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the following: O2, H2O, and CO2—for fuel-lean flames and H2O, H2, and CO—for fuel-rich
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flames. The inevitable exhaust gas component is argon coming from the air used in the
280
process of combustion. The components of exhaust gases with lower share include NO and
281
OH—for fuel-lean flames and H radicals for fuel-rich flames. Therefore, additionally, the
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fuel-rich flame differs from the fuel-lean flames in the species of dominant radicals. Fig. 8(a)
283
and (b) shows that under low flame conditions, the predominance of oxygen over water vapor
284
takes place when Φ
