pagot et al - afrc-jfrc 2004

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AFRC-JFRC 2004 Joint International Combustion Symposium, Maui, Hawaii, Oct. 10-13, 2004

Some Characteristics of Multi-Jet Flares

P.R. Pagot1, E.W. Grandmaison* and A. Sobiesiak

2

Department of Chemical Engineering

Queen's University

Kingston, ON K7L 3N6CANADA

1Petrobras, Petrleo Brasileiro S.A.

2Department of Mechanical, Automotive & Materials Engineering, University of Windsor,

*Corresponding author:

Department of Chemical Engineering

Queens UniversityKingston, ON K7L 3N6

Canada

TEL: 613-533-2771FAX: 613-533-6637

Email:[email protected]

ABSTRACT

Results of a test program with a multi-jet, high momentum burner simulating many of the

features found in offshore flare systems are described. Previous work (Pagot et al., 2003) hasgiven background on a hot and cold model study for this type of flare. The features of the flare

flames are described in more detail in this paper. The burner consisted of twenty-four nozzles

mounted on eight supply lines with firing rates in the range of 268 523 kW, corresponding to

jet velocities of 39.3 78.5 m/s with a natural gas fuel. The flare model was tested with different

nozzle fittings and jet to cross-flow momentum flux ratios ranging from 65 to 268. The flamestructure was identified through the use of still pictures and CCD imaging techniques. Gas

temperature and composition measurements along the flare trajectory were also obtained. Thismulti-jet configuration provides good flare stability allowing firing rates higher than single jets

in a cross-flow. Over the range of firing rates investigated, the flame could be divided in two

regions, a momentum interaction zone where there is an initial mixing between the fuel jets andthe cross flow, followed by a zone where there was a straight flame trajectory characteristic of a

strongly buoyant flame. Near the transition point between these regions there was a higher gas

temperature, minimum O2 concentration and maximum CO concentration. The flare efficiency,

expressed in terms of the unburned hydrocarbon fuel, was in typically in excess of 99%.

IntroductionFlares are used to burn waste and vent hydrocarbons from petrochemical production and

processing facilities. Research in the problem of flaring technology has increased in recent years

in support of more stringent regulations related to combustion efficiency, pollutant emissions and

radiation levels from flaring operations. An early review of the flaring problem has beenpresented by Brzustowski (1976) and more current topics are discussed by Bandaru and Turns

(2000). These references provide background dealing with a single jet and relatively low

momentum flares.

1
mailto:[email protected]:[email protected]:[email protected]


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The problem of interest in this paper deals with the structure and properties of multi-jet

flares used in offshore oil production systems. The flaring process for these units pose problemssimilar to land-based operations but with added complications due to the rather isolated nature of

these production units. Some of the concerns include emission levels and the radiation levels

incident on the working area of oilrigs at current and proposed increased production rates. High

capacity offshore flare systems able to burn up to 4 10

6

m

3

/day (~1.9 GW) are commonly used.A typical ship-based facility is shown in Fig. 1, with the flare system mounted on a large boom(~80 m long) angled from the main deck of a FPSO (Floating Production Storage and

Offloading) ship. This geometry helps maintain flame radiation at an acceptable level for the

working environment on the ship. A top view of a flare system is shown in Fig. 2, depicting aseries of smaller burners mounted on eight arms emanating from a central fuel supply manifold.

The smaller burners consist of another series of eight arms, each with multiple fuel ports

constituting the multi-jet arrangement commonly used in this flare configuration. This burner

system is intended to provide consistent combustion efficiency and flare performanceindependent of wind conditions. In the present work (Pagot, 2002), a flare model was developed

incorporating three different nozzle attachments to investigate the effect of these mixing-altering

devices on the resulting flare system. An overview of some cold- and hot-model features of thiswork has been presented by Pagot et al. (2003). In the present paper, more details of the flare

performance are described in terms of emissions (O2, CO, unburned hydrocarbons and NOx), gas

temperature and geometrical features of the flame.

Figure 1. An offshore FPSO ship with a flare boom shown in the left picture and an operatingflare shown in the right picture.

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25.4m

Figure 2. Top view of a typical multi-jet flare system showing forty-one individual burnersmounted on a primary manifold and eight arms supplying fuel for the burners.

Experimental

The flare model consisted of a series of eight arms attached to a central manifold with

three jet nozzles on each arm as shown in Fig. 3. The diameter of each nozzle was dj = 3.0 mm

and the spacing between the nozzles on each arm was S = 48 mm. This separation distance was

selected in order to minimize interference between the jets in the near field region of the jets(Menon, 1984; Menon and Gollahalli, 1988). This burner configuration with 24 plain nozzles

constituted the reference case for comparison with three other cases examined in this work. Thethree additional cases, Fig. 4, had different mixing altering devices positioned above the nozzleports (i) cone attachments simulating a bluff body stabilizer, (ii) a sudden expansion nozzle

creating a precessing jet and (iii) a ring attachment mounted above the nozzle ports.

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87

10.4 376

332

634848

48

Nozzle

48 3

29.1

Blind capNozzles

GasSupply

Figure 3. Side view (left diagram) of the burner model, all dimensions in mm the 87 mm

dimension corresponds to the elevation for the model above the wind tunnel floor incold model studies (described by Pagot et al., 2003). Top view (centre diagram)

showing the position of the jet nozzles in the eight arms originating from the supplymanifold for the model. Partial top view (right diagram) showing dimensions for theposition of the jet nozzles.

Figure 4. Nozzle configurations employed in this experimental study: (i) cone attachments

(top), precessing jet nozzle (middle) and (iii) ring attachments (bottom).

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For the hot model studies, a special test facility was constructed outside the main building

of the CAGCT (Centre for Advanced Gas Combustion Technology, Queens University). Theflare model was mounted on a fixed base with probes for temperature and gas composition

measurements on a three-dimensional traverse system downstream from an open wind tunnel

shown schematically in Fig. 5. This facility provided a mean cross flow velocity of 3.9 m/s and

an average turbulence intensity of ~13% just upstream from the flare model. Commercial naturalgas was supplied to the flare through a series of regulators, needle valves and an orifice meter at

five different flow rates as shown in Table 1. Gas temperatures were measured in the flare

flames with thermocouples manufactured from 50 m diameter Pt-10%Rh (type S) thermocouplewire; a sampling probe (Turnbull, 1995) was used to withdraw and monitor gas samples from theflame for concentrations of O2, CO, NOx and CH4. The mean radiation flux was measured with a

calibrated thermopile radiometer with a water-cooled aluminum block. These quantitative

measurements were supplemented by flame-image recordings with CCD, infrared, VHS and

single shot still cameras. Tests with the hot model system were only performed during relativelycalm night time conditions.

xProbe traverse

stand

Cross flowair supply

450

1200 z800

1000

Natural gas

supply

y150

Figure 5. Side and front view of the hot model test facility showing the wind tunnel supply for

the cross-flow, mounting assembly for the flare model and the three-dimensionaltraverse system for flame probing.

Table 1. Hot model (combustion) experimental conditions.

Nozzle conditions Flaring conditions Cross flow conditions

Test conditions uj, m/s NRe fm , kg/h& Heat release, kW U, m/s2 2

f fu u

A 78.5 16,800 35.8 523 3.9 261B 70.9 15,100 32.3 471 3.9 213

C 55.0 11,700 25.0 384 3.9 128

D 39.3 8,400 17.9 268 3.9 65.4

E 17.8 3,800 8.1 140 3.9 13.4

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ResultsHot model tests were performed with the flare model fitted with the three burner

attachments shown in Fig. 4 and the reference case nozzles. The bulk of the tests were

performed at flow conditions A D with each of the burner attachments and flow condition E for

the reference and ring attachments. Flame images were obtained in CCD and still picture format

to provide an indication of the flame shape and trajectory. Gas temperature and speciesconcentration were measured along the trajectory of the flames for these operating conditions.

The radiation heat flux was also measured for each nozzle fixture at firing rates up to 523 kW.The flare model produce stable, lifted flames as shown by the photographs (1/30s

exposure) in Fig. 6. The left picture for each case shows the side view of the flame and the right

picture shows the front-view observed from a downstream position. The individual jets from

each nozzle remained distinct and separate up to a distance of about 2S (S denotes the distancebetween adjacent jets in the flare model). Subsequently, the flames merge into a luminous

region that is more evident on the downstream side of the flare. The visible flames for the flare

models with cone attachments and precessing jets were larger than the reference case and thering attachment nozzles. The visible flames for the ring attachments nozzles were the smallest of

the four cases.CCD images of the flames were also recorded at 1/500s and 300 samples were used togenerate black and white images (100s time period) to estimate the flame trajectory, length, area

and volume. Images for the four flare cases are shown in Fig. 7 for test condition A. The top

photographs for each flare case show the black and white rendering of the average image for theflame and the bottom graphs illustrate the extent of the flame in terms of a 50% intermittent

region for the edge of the flame. More detailed intermittency profiles (10, 50 and 90%) based on

the side-view of the flames for the reference case and the cone attachments are also shown in

Fig. 8. These data indicate that the general shape of the flames are quite similar at the differentintermittency levels, but with a broader distribution on the downstream side of the flare model.

Based on these observations the 50% intermittency level was chosen to provide an estimate ofthe flame length, flame volume, and more importantly, the buoyant flame volume at the different

operating conditions. The flame trajectories depicted by the solid symbols in Fig. 7 were

estimated from the midpoint of the flame wide (50% intermittency) at any elevation, z, above theflare source.

The front-view of the flames (right photographs in Fig. 6 and right pictures and graphs in

Fig. 7) shows a good vertical alignment of the flare models. The front-view also shows arelatively smooth ellipsoidal shape for the reference case and ring attachments, but a more

bulbous shape in the lower part of the flare for the cone and precessing jet attachments. The

flare trajectories for the side view images in Fig. 7 follow a smooth trend through the initialregion of the flame followed by a relatively straight trajectory further downstream. An

explanation for this behaviour is shown in Fig. 9 where the side-view 50% intermittency profile

is shown for the reference case at flow condition A. Superimposed on this profile is the

trajectory for a 3 mm diameter single jet flame in a cross flow (for the flow conditions used inthe present study, over a trajectory distance of 300mm) based on the correlation of Brzustowski

(1976, 1980). Two regions are also shown in this figure Zone 1, where there is a strong

jet/cross-flow interaction and Zone 2, where there is strong evidence of buoyancy for thecombined system of jets. The correlation of Brzustowski (1976, 1980) should show good

agreement with the lead jet in the flare model since this nozzle fluid is not strongly affected by

the downstream jets in the initial jet/cross-flow interaction region. The present results appear to

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Reference case:1.00

1 0

0

Cone attachments:

Precessing jet attachments:

Ring attachments:

Fig. 6. Side and front-view images (left and right photographs, respectively) of the flare flames

under test conditions A, based on still pictures at 1/30s shutter speed.

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Reference case

-400 0 400 800 1200-200

0

200

400

600

800

1000

1200

-800 -400 0 400 800

z,mm

x mm x, mm

Cone attachments

-400 0 400 800 1200-200

0

200

400

600

800

1000

1200

-800 -400 0 400 800

z,mm

x, mm x, mm

Figure 7. contd

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Precessing jet attachments

-400 0 400 800 1200-200

0

200

400

600

800

1000

1200

-800 -400 0 400 800

z,mm

x, mm x, mm Ring attachments

-400 0 400 800 1200-200

0

200

400

600

800

1000

1200

-800 -400 0 400 800

z,mm

x, mm x, mm Figure 7. Time averaged (100 s) CCD images of the side and front view of the flames

operating at flow condition A (top picture for each flow fixture) and 50%intermittency profiles of the side and front view of the flame (graph images for each

flow fixture). The flame trajectory deduced from the intermittency profiles is shownby the solid circle symbols.

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Reference case Cone attachments

-200 0 200 400 600 800 1000

0

200

400

600

800

1000

1200

z,mm

x, mm

Flame intermittency:

10%

50%

90%

-200 0 200 400 600 800 1000

x, mm

Flame intermittency:

10%

50%

90%

Figure 8. Flame intermittency profiles for the side-view of the flare (condition A, Table 1)

for the reference case and cone attachments.

Single jet flame trajectoryin a cross flow

Brzustowski (1976, 1980)

Flaretrajectory

Flare boundary basedon 50% intermittency

Zone 1

Zone 2

Figure 9. Side view of the reference case flare at flow condition A (Table 1) showing 50%

flame intermittency contour and flare trajectory. The trajectory for single jet flame ina cross flow (correlation of Brzustowski, 1976, 1980) at the conditions used in this

work is shown originating from the lead jet in the flare system. Zone 1 depicts the jet

and cross-flow interaction region; Zone 2 depicts the buoyant region of the flareflame.

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support this observation with a strong jet/cross flow interaction region in Zone 1 followed by thebuoyant region in Zone 2.

In addition to the change in flame trajectory due to the impact of buoyancy, there is an

expectation that the side-view trajectories should change with firing rate when the cross flow

momentum flux is constant. An increase in the angle of declination from the vertical withdecreasing firing rate was indeed evident for the reference case and ring attachments as shown

by the side-view profiles (50% intermittency) shown in Fig. 10. Estimates of the side-viewtrajectory for each flare system at different firing rates are also shown in Fig. 11. The reference

case and ring attachments showed consistent behaviour with a decrease in slope as the firing rate

was reduced. The results for the cone and precessing jet attachments exhibited more complex

behaviour with modest changes in slope and a shift in the effective or apparent origin for theflame source. These two cases also exhibited more bulbous flame shape behaviour in the near

field, Fig. 7, and this effect may lead to a more complex aerodynamic resistance to the cross-

flow.Gas temperature measurements were obtained along the flame trajectory for most of the

conditions noted in the conditions in Table 1. An example of these results for test condition Aare shown in Fig. 12 where the gas temperature is plotted as a function of = /L, where is theflame trajectory measured from the top and centre of the flare and L is the curvilinear flame

length estimated from the flame trajectory data. These results show that there is a maximum gas

temperature in the region 0.2 --- 0.4. The solid line in this graph is a second order

polynomial fit for all the data at this flow condition. The data for the reference case and coneattachments are randomly scattered about this curve. On the other hand, the results for the ring

attachments are typically higher while the precessing jet data fall below this curve these

observations can be related to lower NOx emissions described later for the precessing jets. The

maximum centreline temperature of the single, free-jet vertical propane diffusion flame islocated at about 60% of the visible flame length (Becker and Yamazaki, 1978). Under the

influence of a cross flow, the temperature peak observed in the present work moves closer to thenozzle - Botros and Brzustowski (1978) also observed this behaviour with the turbulent diffusionflame in a cross flow. Temperature profiles at other firing rates followed a similar behaviour

with slightly lower temperatures observed at lower firing rates (Pagot, 2002).Gas composition measurements were obtained for O2, CO, CH4, and NOx at test

conditions A-D, Table 1. Data obtained along the flare trajectory at flow condition A is shown

in Fig. 13. The O2 composition typically exhibited minimum values and the CO maximum

values (both about 2 8 %, by vol.) in the range, 0.2 0.5, corresponding to the peak

temperatures in these flames. The CH4 and CO compositions exhibited wide variability for

pagot et al - afrc-jfrc 2004

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