study of enhancing the swirl burner performance_edit fajri

Study of Enhancing the Swirl Burner Performance

On a Small scale Biomass Gasification

Adi Surjosatyo1 and Farid Nasir Ani2

1Department of Mechanical Engineering, Faculty of Engineering

University of Indonesia, 16424 Depok, Indonesia

2Faculty of Mechanical Engineering

Universiti Teknologi Malaysia

Karung Berkunci 791, 81310 Johor Bahru

Johor DT, Malaysia

AbstractThere are many processes for converting biomass into a more useful form of energy. One

of the most popular technological processes is through direct combustion of biomass or in a controlled atmosphere or gasification. The current gasifier that has a maximum heat capacity of combustion system of 15.02 kW, used to burn biomass (oil palm shell) produces gases of low calorific gas through a low calorific swirl gas burner. Some problem appears using low calorific, it causes no circulation and weak swirling flame, and this will be an increase of residence time at high temperature. The gas burner, which is incorporated with the two-stage biomass combustion system, consists of burner tube and swirl-vane. These swirl gas burners that consist of different turning vane positions i.e. 20o, 30o and 40o, are equal to the swirl number of 0.22, 0.356 and 0.508, respectively. The experimental shows that maximum heat release of the gas burner is 5.8 kW at equivalence ratio of 1.21 with the gas flow rate of 1.04 g/hr. The flame temperature of the gas burner reached a range of 590 to 677 oC at the equivalence ratio, of 1.16 to 1.66. In case of swirl flames while increased the swirl number, the flame length decreased significantly with the increasing premixing and flame changed from orange-yellow color to a blue color as the characteristics of a higher level of premixing.

Keywords: Gas burner, Swirl Burner Performance, biomass gasification, swirl flame

1. Introduction

On acquiring the best performance of gas burner, it is necessary to find a design of gas

burner so that the combustion efficiency can be increased. And also, currently, the reduction of

pollutant emissions from practical combustion devices is a major issue in combustion research.

One of the main pollutants is NOx and CO. Therefore, some different methods have been

proposed to reduce these emissions. These include, for example, partially premixed turbulent

Corresponding author: [email protected]

flames [1], rotating matrix swirl burner [2], heat recirculation ceramic burner [3], air swirl burner

[4] , tangential inlet swirl burner [5], air staging [6], reburning and low NOx burners. In general,

these methods try to reduce the residence time in high temperature regions or to avoid high

oxygen concentration in such regions. Some previous study [6,7,8,9] mentioned, gas turbine

combustors and industrial systems utilized a high-swirl type of burner in which the swirling

motion generated by the injector (or burner) is sufficiently high to produce a fully developed

internal recirculation zone at the entrance of the combustor. For conventional non-premixed

combustion, the role of the large recirculation zone, also known as the toroidal vortex core, is to

promote turbulent mixing of fuel and air. In premixed systems, the recirculation zone provides a

stable heat source for continuous ignition of the fresh reactants, as refers to the review of Syred

and Beer [7] for extensive background on the basic processes and practical implementation of

high-swirl combustors7.

But according of some study [10.11], low-swirl combustion is a relatively recent

development, is an excellent tool for laboratory research on flame/turbulent interactions. Its

operating principle exploits the “propagating wave” nature of premixed flames and is not valid

for non-premixed combustion. Premixed flames consume the reactants in the form of self-

sustained reacting waves that propagate at flame speeds controlled by mixture compositions,

thermodynamic conditions, and turbulence intensities. In contrast, non-premixed diffusion

flames do not propagate (i.e., move through the reactants) because burning occurs only at the

mixing zones of the fuel and oxidizer streams. To capture a fast moving turbulent premixed

flame as a “standing wave” that remains stationary, low-swirl combustion exploits a fluid

mechanical phenomenon called a divergent flow. As the name implies, divergent flow is an

expanding flow stream. It is formed when the swirl intensities are deliberately low such that

vortex breakdown, a precursor to the formation of flow reversal and recirculation, does not

occur. Therefore, the Low Swirl Combustion (LSC principle is fundamentally different from the

high-swirl concept of typical Dry Low NOx (DLN) gas turbines, where strong toroidal vortexes

are the essential flow elements to maintain and continuously reignite the flames. The engineering

guideline for the LSB is specified in terms of a range of swirl number (0.4 < S < 0.55).

One method of increasing the performance of the burner is using the swirler that can be

inserted inside the burner model, as shown in Figure 1. Swirler means here are guided vane

system in which so positioned that they can deflect the flow direction. Thus in current


construction the swirler positioned in the axial pipe flow which have a fixed set of vanes at a

certain angle to the mainstream direction, which deflects the stream into rotation. This flow

rotation is generating a recirculation bubble which plays an important role in flame stabilization.

There are two main requirements for flame stabilization, first, mixture ratios within flammability

limits and second, velocities low enough to match burning velocities.

Figure 1. The gas burner tube inserted with a 20o vane pack swirler

Swirl significantly influences heat and mass transfer in many natural and technological

flows (Shtern et al., 1998). Swirl is used in vortex burners and chemical reactors to stabilize the

flame front and to increase the surface area across which heat and mass transfer exchange occurs.

In vortex devices, centrifugal acceleration can be high as 104 times the gravity and provides

useful stratification of temperature and density. Hot low-density fluid collects near the axis of

rotation, i.e. away from side-walls, while the near-wall region consists of cold high density fluid.

In waste and fuel burners and within furnaces, swirl is often used to modify flow

characteristics. Because of the intense recirculation patterns in swirling flows (burning gases

travel back towards the burner bringing heat energy and reactive species to promote ignition in

entering fuel-air mixture) rotation is found to shorten the flame (Niessen, 1995).

2. Experimental Procedure


The current swirl burner is the upper part of the complete combustion system that was

fabricated in the combustion laboratory. The fuel-gas in the gas burner is produced by the

gasifier, the lower part of the combustion system.

Mixing gas will be ignited in the secondary chamber, which has dimension of 0.3 m

diameter and 0.35 m length. It was constructed from 1 mm mild steel-plate. Because of the

present gasifier is a pilot scale facility, the chamber was not necessary lined with the refractory

since this facility was not utilized for a long operation. However, the best furnace chamber

should be lined with refractory to prevent excessive heat loss.

The producing gas from the primary chamber was kept constant around 1.04 g/hr. For the

secondary air, its flow rate was varied from 438, 498, 535 and 622 lpm. The operating conditions

for the turbulent premixed flames considered in the present study are summarized in Table 1.

The nominal heat release rate is obtained by multiplying the fuel mass flow rate by its nominal

heating value of 5100 kJ/m3 (White and Plaskett, 1981).

Table 1. Operating Conditions for Turbulent Premixed Flames

Flow Parameter

Swirl-vane angle

20o 30o 40o

Equivalence Ratio, - 1.16 – 1.66

Nominal heat Release, kW 5.8

Gas flowrate, g/hr 1.04

Flame Temperature, oC 560 569 632

Temperature at burner exit, oC 244 253 284

Range of secondary air flowrate, lpm 438 - 622

Combustor pressure, atm Atmospheric pressure

2.1 Instrumentation Set-Up


Figure 2.a and 2.b show the configuration of the combustion system and enlargement of

secondary chamber, respectively. The flowmeter used was ventury type and before utilized for

the air flowrate measurement (primary and secondary air supply), it was calibrated with the

standard flowmeter. Adjusting the air supply consumption, two air valves were mounted on each

air-supply pipes. Measuring the air flowrate can be done directly through reading the

differentiation of water level in the U-tube.

Figure 2. (a) The configuration of the combustion system with, 1: Primarychamber, 2: Cyclonic chamber, 3: Swirl vane, 4: Secondary chamber, 5: Gas ejector.

(b) The enlargement of secondary chamber with, 1: Cyclonic chamber, 2: burner tube, 3: Vane-hub, 4: Swirl vane, 5: Flame zone, 6: Secondary chamber.

2.2 Type of Swirl-Burner

The current swirl-burner has three different types of the swirl-vane angle. The current

swirl burner constructions have an almost similar dimension to the swirl burner model that used

in predicted study. Material used for the current swirl burner construction is mild steel. This kind


of material can resist the temperature below 1000 K in continuously operation. Table 2 presents

the important parameter of the current designed swirl-vane burner.

Table 2. Parameter or dimension design the burner

No.Vane angle

()

Number of vane

Dimension of the swirl-vane

(l,w,th,d, dh ) in mm Swirler Number

(S)le w th d dh

1 20

6

53 35 1 83 19 0.22

2 30 50.6 33 1 83 19 0.356

3 40 38 31 1 83 19 0.508

Where le is equal length of vane, w is width of vane, th is thickness of vane, d is cirle diameter of

vane and dh is hub diameter of vane.

Figure 3 shows the fabricated different types of vanes used for the experimental purposes.

(a) 20o swirl-vane (b) 30o swirl-vane (a) 40o swirl-vane

Figure 3. The difference fabricated swirl-vane for the experimental

Temperature distributions were detected by Chromel-Alumel Thermocouple K-type

assembly. There are six thermocouple-probes located inside the primary-chamber. They are

placed inside the reactor in such a way that their tip remains along the axis of the chamber. The

distance between the thermocouples was determined to ensure the best description of the


temperature field along the axis and to allow for an accurate determination of the propagation

velocity of the combustion front. Another thermocouple-probe in the cyclonic chamber and three

probes in the secondary chamber. All these thermocouple-probes connected with a system data-

logger from Data Taker 605 and all the data by the computer displayed online.

A Rosemount Series 500 gas-analyzer allows following discontinuously the

concentration of O2, CO, NOx, CO2, Excess-air, and Combustion Efficiency. When measured all

the data the gas-probe tip was inserted into the secondary chamber.

2.3 Methods of starting-up

The starting –up process of the combustion system comprise of the following four steps,

i.e. preparation, ignition the producer gas, measurement work and completing the experiment at

work. Detail of each step will be explained as follows:

2.3.1 Preparation before combustion process

Before burning the palm shell waste, it is necessary to do some preparation such as

drying the solid waste until the maximum moisture percentage reached 9% of weight, clearing

the primary combustion chamber from the ash, checking the condition of the instrumentation

such as thermocouples, data logger and the PC, observe the primary chamber cover to prevent

the possibility of gas leakage and check the instrumentation of the portable gas analyzer.

It should be noted that the purpose of burning the waste in this gasification system is to

produce combustible gas. At first, creates the combustion zone by feeding 2 kg the palm shell

into the primary chamber through the screw-feeder. Then, a small amount of kerosene is mixed

with the palm shell inside this chamber. After igniting this mixture, a thin white smoke appears.

Another 5 kg of palm shell was fed to raise the quantity of combustible smoke from the primary

chamber. Leave this condition of around one hour until the T1 (combustion zone) reached 800-

850 oC and T2 (reduction zone) of 500-600 oC.

2.3.2 Ignition of the producer gas

With the correct gasification temperatures, the white smoke-gas is ready to be ignited

with a gas-torch. Based on the experiences, the best condition to produce the flame, when the

combustion zone reaches the temperature, T1 between 950-1200oC.


The ignition was conducted manually using the gas-torch. It was suggested that the

igniter be placed close enough to the burner-tube, as seen in Figure 4. Before ignite the thick

smoke, the air mass flowrate from both primary and secondary air, is adjusted in proportional

ratio so that, the mixed gas can be ignited easily and a sustainable flame occurs outside the

cyclone chamber. The sustainable flame can be maintained successfully at the primary air

flowrate of 125 lpm and the minimum secondary air flowrate of 438 lpm and maximum of 622

lpm.

Figure 4. Ignition area using gas torch (Experimental work)

2.3.3 Measurement work

The scope of measurement activity during combustion process involved recording the

temperature at combustion zone (TC1), reduction zone (TC2), pyrolysis zone, drying zone (TC3,

TC4 and TC5), cyclonic chamber (TC6), exiting flame (TC7 and TC8), emission level at EP1

and EP2, primary and secondary air flowrate (AF1 and AF2), solid fuel flowrate (Q f). To

achieve a reliable result on measurements, all instrumentation is always periodically calibrated,

such as thermocouple, gas analyzer and air flowmeter. All measurements will be conducted in a

steady state condition. The measurements should be started when the flame in sustainable and

stable condition. The solid feeding occurs every 30 minutes and each fuel feeding; it needed 6

minutes at constant flowrate of 0.339 kg/min. This interval of solid feeding was chosen to ensure

the gasification process in primary chamber can support the gas burner to produce the flame

continuously.


2.3.4 Completing the experimental work

Switching off the air blower and the solid fuel feeder causes the flame to extinguish

immediately. Then, the temperature in the primary chamber decreases slowly to around 100 oC

(373 K), so that the gasification process could not produce enough combustible gas.

3. Results and Discussion

3.1 Flammability limits

For combustion to be effective, such as to maintain a low NOx formation and to keep of a

low CO emission, the should be varied in a properly range. This can be done through vary the

secondary-air flowrate. The increasing this flowrate from 438 to 622 lpm has reduced the from

1.66 to 1.16. For 20o and 30o swirl-vane angle burner, the increasing of air flowrate until 622 lpm

or = 1.16, has increased the flame strength. Thus, since the velocity of premixed flow is

slightly below than the flame velocity, the increment of secondary-air flowrate is adjusted

proportional, so that it can encourage the flame strength. For 40o swirl angle, even though the

increase of air flowrate gives the same trend of flame strength, at <1.33 occurs a flashback

flame into the burner, then it caused the flame to extinguish immediately. It has a possibility, that

the reducing of causes the rising of total flow velocity that is higher than 20o and 30o swirl-vane

burner. As consequence, it effects to the increasing of burning velocity. The flame velocity

magnitude for 40o swirl-vane is much higher than the others. Furthermore, this burning velocity

is far beyond the premixed flow velocity and it causes a flashback flame.


1.2 1.25 1.3 1.35 1.4 1.45 1.5 1.55 1.6 1.65 1.70.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

Sw

irl n

um

be

r

EQR

Figure 5. Effect of the swirl number to the flammability

Figure 5 shows the working area of the current study related to the increasing of swirl-

vane. The graphs indicate that the increasing of swirl number may reduce the equivalent ratio1,6

3.2 Flame Appearance

Swirl burner with an appropriate strength is found to increase the air-fuel mixing,

increase the combustion efficiency, reduce the flame length, decrease the residence time at high

temperature and decrease NOx emissions. In order to have this effect, the swirl must be strong

enough so that the vorticity can diffuse to the centerline and form a circulation zone.

The current experiments is performed the photographs of combustion flame of different

swirl-vane angle burner, i.e 20o, 30o and 40o as shown in Figure 6. (a) The photographs in Figure

6. (b) show a clear different of flame shape and color. The 20o swirl-vane burner (left) dominate

the yellow and red color flame and a small quantity of blue color flame (unfortunately not clear

in the photo) that anchored at the end of burner rim. The blue flame near the burner rim is due to

swirling-induced recirculation and enhancement of local premixing. The flame appearance is

rather in a poor condition, due to a lower burning velocity. The yellow and red color is due to the

appearance of soot particle. The 30o swirl-vane burner (middle) shows a better flame appearance.

As expected, it shows a stronger radial and vortex velocity around the burner center. The lower


Richer mixture

Leaner mixture

turbulence intensity gives a higher burning velocity than the previous swirl-burner. It can be

noticed also, since it occurs a higher radial velocity, the vane hub at the burner center, can be

seen easily. Around this hub the yellow flame color appears. It indicates this zone occurs a better

mixing. The reaction was ideally stabilized in that yellow patches periodically appeared. The

dominated blue color flame appears at 40o swirl-vane burner (right). It shows that swirl effect is

more active and creates a higher momentum flux ratio (Gupta, 1995 and Cheng et al., 2001).

This encourages more bubble-like recirculation zone near the the burner rim. The red color flame

appears in a thin flame form as the effect lower velocity and thin soot formation. The strong

radial and vortex velocity occurs in this burner type since the burning velocity has higher

velocity. It can be noticed, that the flame lengths tend to decrease for higher degree of swirl-vane

angle. It means the combustion reaction is more stable and efficient. However, effect of higher

aerodynamic effect, increasing the swirling air-fuel mixture and decreasing momentum flux

ratio6.

(a) (b) (c)

Figure 6. Photographs that show different flame of 20o (a), 30o (b) and 40o (c) swirl-vane burner at = 1.33.

3.3 Effect of swirl-vane burner on emission

Figure 7 shows the variation of flame temperature due to the increasing of equivalent

ratio (). The graph shows that the burner work on rich mixture, above the stoichiometric

equivalent ratio ( >1). The highest temperature that is reached by 40o swirl-vane burner is 665 oC at of 1.33. Other burner configurations, such as 30o and 20o swirl-vane burner give a lower

flame temperature i.e. 590 and 580 oC at of 1.17 respectively. On these burners show a


decreasing of diffusion flame when the rises to the condition of fuel rich mixing. This causes

by the reducing of reaction kinetic rate or decrease in conversion efficiency.

The graphs show also the effect of different swirl-vane angle on the variation of flame

temperature. It indicates an aerodynamic of swirl vane contributes or causes the increase mixing

quality of the premixed fuel. Higher swirl-vane angle determines significantly the uniformity of

the premixed fuel, furthermore, this encourages the increasing of the reaction kinetic rates. The

variation of flame temperature is dependence on this kinetics rate.

1.1 1.2 1.3 1.4 1.5 1.6 1.7

560

580

600

620

640

660

680

20 deg

Polynomial (20 deg)

30 deg

Polynomial (30 deg)

40 deg

Polynomial (40 deg)

Te

mp

era

ture

(o

C)

EQR

Figure 7. Effect of increasing the EQR- equivalent ratio () on different swirl- vane to the flame temperature

Figure 8 shows the increasing of oxygen consumption and production of carbon dioxide

for the increasing of swirl-vane angle. To increase a higher flame temperature, it needs a higher

air or oxygen consumption. As already explained on the previous paragraph, that the 40o swirl-

vane gives a better diffusion rate, so that the oxygen reacts with the premixed fuel faster than

other swirl-vane. As the consequence, the excess air at 40o swirl-vane is lower than other swirl-

vane as shown in Figure 10. It seems that the consumption of oxygen is high enough and

possibly the mixing process is smoother, so that the flame temperature at 40o swirl-vane


increases much higher than other swirl-vane. At = 1.66 for 20o swirl angle, the O2 consumption

is 11.6 % with the flame temperature of 565oC and with the same for 40o swirl angle, the O2

consumption is 9.1 % at 632oC.

550 575 600 625 650 6754

6

8

10

12

14

16

CO2

O2

20 deg

30 deg 40 deg

20 deg

30 deg

40 deg

Em

issi

on

of C

O2 a

nd

O2 (

%)

Temperature (oC)

Figure 8. Variation of CO2 and O2 against the flame temperature on different

swirl-vane angle

Other important combustion parameters are the effect increasing of to the variation of

CO and NOx. Figure 9 shows the increment of CO and decreasing of NOx emission for all swirl-

vane burners on increasing from 1.16 to 1.66. The highest CO emission of 391 ppm occurs at

=1.66 for 30o swirl angle, however, in average is reached 311.25 ppm for 20o swirl angle. The

lowest is achieved 65 ppm at =1.33 for 40o swirl angle. The CO formation is dependence on the

mixed-quality between the premixed fuel and the oxygen. As already discussed, 40 o swirl angle

gives a smoother diffusion between the fuel and oxygen, and encourages a higher PVC formation

and RFZ structure that can increase the burning time or combustion residence time. A longer

residence time produces more oxidation process to form CO2. Therefore, it allows reducing the

uncompleted reaction.


1.1 1.2 1.3 1.4 1.5 1.6 1.7

0

100

200

300

400

500

600

CO

Polynomial (CO )

NOx

Polynomial (NOx )

20 deg

30 deg

40 deg

20 deg

30 deg

40 deg

EQR

Em

iss

ion

of

CO

an

d N

Ox

(pp

m)

Figure 9. Effect of the increasing of EQR- equivalent ratio () on different swirl-

vane to the variation of CO and NOx emission level

Current study work in the range of equivalent ratio 1.16<<1.66 or higher than the

stoichiometric value ( > 1). The graphs show that higher causes to a condition of a higher fuel

composition rather than oxygen composition. As consequence, it causes a slower reaction rate

and the flame temperature is not high, lower than 1000oC. Thus, this condition has an advantage

to reduce the NOx formation rate. Based on this combustion condition, the NOx forming of the

current study can be categorized in Prompt NO or Fenimore reaction mechanism. In this

mechanism, NO is rapidly produced in the flame zone of laminar premixed flames long before

the NO forming by the thermal mechanism. Turn (1996) and Strahle (1993) have described the

Fenimore mechanism, that the hydrocarbon radicals are believed to react with molecular nitrogen

to form amines or cyano compounds, which are then converted to intermediate compounds that

ultimately form NO. In the equation form can be written as following:


CH + N2 HCN + N (a)

CH2 + N2 HCN + NH (b)

C + N2 CN + N (c)

N + O2 NO + O (d)

HCN + OH CN + H2O (e)

CN + O2 NO + CO (f)

At 30o and 40o swirl angle, the increasing from 1.16 to 1.66 does not give any

significant changes of NOx reduction of 9 and 1.14 % respectively, but for 20o swirl angle shows

a significant reduction at lower of 34.92 %. The maximum NOx formation is reached of 315

ppm at = 1.16 for 20o swirl angle and the minimum is achieved of 69 ppm at the same for 40o

swirl angle. It shows NOx reduction at smaller swirl angle gives more significant value. A strong

possibility is the combustion turbulent plays an important role. At higher swirl-vane angle, since

the turbulent intensity is lower, the mixing process between air and fuel occurs is good enough to

produce a high kinetic rate. For a lower swirl-vane angle, oxygen distribution is needed to

control the NOx formation if it compares with the temperature distribution as seen on Figure 7

the NOx formation shows a proportional curve distribution with the temperature. At higher

temperature, the NOx formation is more significant. Thus, thermal NO mechanism is more

significant at higher temperature.

For all swirl-vane angle, the increasing of gives the decreasing of combustion

efficiency and increasing of Excess Air (EA) as shown in Figure 10. Combustion Efficiency is

attributed to the degree of complete combustion of the hydrocarbon with the air mixing.

Unburned hydrocarbon (UHC) is the presence of incomplete combustion, which is dominating

the emission pollutant at the burner exit (Correy, 1969 and Niessen, 1995).


1.1 1.2 1.3 1.4 1.5 1.6 1.7

40

50

60

70

80

90

100

110

EA

CE

Co

mb

us

tion

Eff

icie

nc

y (

CE

) a

nd

Ex

ce

ss

Air

(E

A)

%

EQR

40 deg

40 deg

30 deg

20 deg

20 deg

30 deg

Figure 10. Effect of the increasing the EQR- equivalent ratio () at different swirl-

vane to the variation of Combustion Efficiency (CE) and Excess air

(EA) level

The composition of UHC is dependence of the degree of fuel-air mixing. Increasing of

EQR means that there is reducing the oxygen supply that cause a slow of reaction kinetic rate. As

consequence, the EA increases and the UHC composition increases also.

The result of measurement was carried out at the exit of ax symmetric combustor over the

significant range of operating conditions, i.e. the present (EQR) range. The results shows that

the maximum CE reaches 95.1 % at = 1.33 for 40o swirl-vane burner. In this condition the EA

reaches the lowest value of 41 %. For 30o and 20o swirl-vane burner reach the average CE of

75.82 and 73.6 % at EA of 48.75 and 89.25 % respectively. Furthermore, the variation of CE can

be associated with the CO emission distribution. As happened at CO emission, the effect of

aerodynamic of swirl-vane, such as increasing of swirl-vane angle from 20 to 40o, the turbulence

intensity near the swirl-vane reduces. This causes a smoother diffusion between fuel and oxygen.

Furthermore, it produces a better combustion process, reduces CO and UHC emission level.


4. Conclusions

Measurements and predictions of the 20o, 30o and 40o swirl-vane burner flow field and

the chemistry in an air staged combustion system has been performed. The result of

measurement was carried out at the exit of ax symmetric combustor over the significant range of

operating conditions, i.e. the present (EQR) range. The results shows that the maximum CE

reaches 95.1 % at = 1.33 for 40o swirl-vane burner. For 30o and 20o swirl-vane burner reach the

average CE of 75.82 and 73.6 % at EA of 48.75 and 89.25 % respectively. The effect of

aerodynamic of swirl-vane, such as increasing of swirl-vane angle from 20 to 40o, the turbulence

intensity near the swirl-vane reduces. This causes a smoother diffusion between fuel and oxygen.

ACKNOLEDGMENT

The authors are grateful tso the Ministry of Science, Technology and Environment under

the IRPA Research Program for research grant awarded and Combustion Laboratory Universiti

Teknologi Malaysia for the support during carry out the research work.

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