Combust . Sci . and Tech ., 1995, Vol . 106, pp . 383—391

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Detailed Measurements in Heavy Oil and OiI/Wate rEmulsion Flames

J. M . BALLESTER, N . FUEYO Laboratorio de Investigación en Tecnologías dela Combustión (LITEC), CSIC María de Luna, 3, 50015-Zaragoza, Spain

C . DOPAZO Fluid Mechanics Group, School of Mechanical Engineering ,María de Luna, 3, 50015-Zaragoza, Spain

(Received November 18, 1994; in final form February 16, 1995)

ABSTRACT—The present work investigates the effect of the addition of water in the form of an emulsion o nthe combustion of heavy oil. Tests burning neat oil and oil/8% water emulsions are conducted in a largelaboratory furnace (0.33 MW thermal input) . The effect of the water addition is analyzed by a detaile dcharacterization of the flames as well as of the gaseous and solid emissions .

Key Words : Water/oil emulsions, heavy oil combustion, particulates, NO X

NOMENCLATURE

D a

Diameter of the combustion air duct (0 .085 m)r

Radial coordinate (m)SMD Sauter mean diameter (µm )Tfo

Oil injection temperature (°C )x

Axial coordinate from the quarl exit (m )20

Spray angle at the exit of the atomizer (° )

1 INTRODUCTIO N

Several works on distillate fuel/water emulsions have found that soot formation i slower than with the pure oils (Nazha and Crookes, 1984; Ahmad and Gollahali, 1993) .However, more drastic effects are obtained when burning heavy fuels where, beside ssoot particles, most of the solid emissions come from the carbonaceous residu egenerated in the liquid-phase pyrolysis reactions . The studies of Sjdgren (1977), Jacque set al . (1977), Cunningham et al. (1983) and Ballester et al . (1993) on the burning o fheavy oil and its emulsions found reductions in particle emissions ranging from 50 t o90% . Among the different hypotheses postulated to explain those effects, the mos twidely accepted is the occurrence of the so-called micro-explosions ; a secondaryatomization of the initial spray is produced as a consequence of the disruptiv eevaporation of the water droplets contained in an oil drop . The combustion of oil/wate r

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emulsions is also found to affect the emission of other pollutants . Kozinski (1994 )demonstrates that the formation of PAC in heavy oil/water emulsion flames is signifi-cantly reduced . Cunningham et al . (1983) measure a decrease of around 10% in theemission of NO X when burning heavy oil/water emulsions . However, Gollahali et al .(1984) encounter no differences in N O ., emissions between heavy oil and emulsion flames .

2 EXPERIMENTAL FACILITY AND TEST CONDITION S

The tests are conducted in an experimental furnace at a thermal input of 0 .33 MW. Thecombustion chamber is cylindrical (length = 3 .2 m, i .d . = 0.61 m) and verticallyoriented, with the flow moving downwards. All the elements forming de furnace arecooled by separate water jackets and the upper half of the combustion chamber i srefractory lined. A swirl-stabilized burner with a 30° semiangle quarl is located at thecenter of the roof. The Swirl Number of the air flow is 0 .66 in all the tests presented here .The combustion air temperature is maintained at 240°C and the flow rate is adjusted i norder to obtain a concentration of oxygen in the flue gases of 1 .2% . The oil gun is placedalong the axis of the air duct with the frontal plane of the atomizer aligned with the edg eof the quarl . A simplex pressure-swirl nozzle giving a hollow cone spray is used .

Heavy oil customarily burnt in power plant boilers is used in the present experi-ments . Distilled water is injected into the oil stream before reaching the pump . Thequality of the emulsion is guaranteed by a static mixer located on the high-pressure line .The observation of emulsion samples through a microscope indicates that all the waterdroplets are smaller than 20 µm, most of them being between 3 and 6µm . The presen tstudy comprises four tests . The oil was injected with and without water addition, and a ttwo different temperatures . Other experimental conditions are maintained the same inall the cases . The test designations and the conditions are specified in Table 1 .

The neat oil data (T100 and T120) are documented from a previous series o fisothermal atomization tests, that provide the spray drop size distribution and theinjection angle . Table 1 displays the spray characteristics obtained for both cases . Theemulsion sprays have not been tested in the atomization rig . The determination of th e

TABLE 1

Experimental conditions, fuel spray characteristics and emissions of CO, UHC, NOx andparticulates for the combustion tests

Run T100 ET100 T120 ET120

Oil temperature (°C) 100 100 120 12 0Water (% by weight) 0 8 0 8SMD (µm) 83 .2 100 .5* 54.3 65 .6 *20(°) 31 .3 23 .1* 51 .2 37 .8 *[CO], ppm 91 40 43 5 0[UHC], ppm 144 58 72 5 5Solid emissions, mg/Nm 3 584 200 310 14 5[NO,,], ppm 314 312 488 375

* Estimates from the neat oil sprays.

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physical properties of oil/water emulsions indicates an increase of 40% in viscosity withrespect to the neat oil. The spray characteristics for the two emulsion runs can beestimated from the empirical correlations obtained for these atomizers and are

included in Table 1 .Gas composition measurements in the combustion tests are conducted with individ -

ual on-line analyzers for 0 2, CO 2, CO, SO2, NO/NOx an UHC. The gas samplingwithin the flame is carried out using a water-cooled probe . Solids concentrations in theflue gases are determined with a particle-gas sampling probe containing a sintered-bronze filter and operated at isokinetic sampling conditions . Gas temperatures ar eobtained using uncoated bare wire thermocouples (Pt/Pt 10%Rh) with a diameter o f

70 µm. A comparison of the measurements with a 40 µm thermocouple and a suctio npyrometer indicates errors due to radiative cooling lower than 7% for the highes t

temperatures .The furnace is allowed to warm up until stable wall temperatures (1200 ± 100°C) ar e

reached after about 5 h . The repeatability of the reported species concentrations i sestimated between 5 and 10%, depending on the compound, its magnitude and th e

location inside the flame . Gas temperatures are repeatable within less than 4% . Redun-dant measurements of particle emissions display a deviation of 15% about the mean .

3 RESULTS AND DISCUSSIO N

3.1 Run T100 vs ET100

The evaporation and mixing processes are rather slow in flame T100 compared to th erest of the Runs. This behaviour can be explained by the characteristics of the oil spray .Compared with case T120, the spray obtained in Run T100 presents a narrow angle o finjection and a large drop size . An immediate consequence of the larger SMD isa longer evaporation time, which extends the zone of fuel vapor generation over a larg e

area. On the other hand, the drops are highly concentrated around the axis and th e

mixing with the air stream proceeds slowly .The addition of water has a dramatic effect on the flame when the fuel injectio n

temperature is 100°C. The visible flame length is approximately 1 m for Run T100 and

it reduces to about 0 .6 m when burning an oil/8% water emulsion (Run ET100) . Thischange suggests that an enhancement in the evaporation and mixing processes occur sleading to a shorter reaction zone . The diametral profiles obtained for the concentra -tions of UHC and CO at x/Da = 8 .67 confirm these observations (Fig. 1) . Whereas a tthis distance case T100 displays UHC and CO levels aboye 0 .4 and 1.3%, respectively ,the concentrations for Run ET100 are of the order of the ones obtained at the exit of th e

furnace. A similar burnout is only achieved in Run T100 at an axial distance aroun d

x/Da = 13.A similar effect on the flame length is obtained if atomization is improved by heating

the fuel at a higher temperature, as the results from Run T120 indicate (Ballester an d

Dopazo, 1994) . However, as pointed out previously, the addition of water increases th efuel viscosity and leads to a coarser and narrower spray . Hence, the micro-explosio nphenomenon is thought to cause an important reduction in the size distribution of the

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1 . 6

1 .4

-2

r/Da

FIGURE 1 Comparison of the radial profiles of UHC and CO at x/D a = 8.67 for Runs T100 and ET100 .

oil drops. The dramatic effect of the secondary atomization on the oil drop size can b einferred from Plates 1 and 2, where samples of the solid emissions collected for bot hRuns are presented. The particles from Run ET100 display a much smaller siz edistribution and irregular shapes as opposite to the T100 sample, where all the particle sllave a near spherical form. Both differences are indicative of the important modifica-tion of the initial spray by the secondary atomization caused by the micro-explosions .Some spherical compact cenospheres which undergo no secondary atomization arealso present in the sample .

The spatial distribution of temperature obtained for flames T100 and ET100 can b ecompared in Figure 2 . The structure shown in the graphs is similar in both cases, bu tthe level of temperatures is reduced by about 65°C for the combustion of the oil/wate remulsion. The absorption of heat by the additional water can cause an average decreas eof about 25°C. The remaining 40°C can only be attributed to an enhanced loss of energ yby radiation to the walls in the first part of the flame. Some studies indicate the

opposite, i .e., higher temperatures are measured in the emulsion flame, which ar eattributed to a reduced radiation intensity due to the lower concentration of particle s(Gollahali et al., 1984; Ahmad et al ., 1993). The recent work of Kozinski (1994) report sa decrease in temperature in the emulsion flame that exceeds 100°C . The temperaturedifference reported in the present work remained sensibly constant (in the rang e50–80°C) in other four comparisons between heavy oil and emulsion flames . Therefore,

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PLATE 1 SEM photograph of a sample of particles from Run T100 .

PLATE 2 SEM photograph of a sample of particles from Run ET100 .

the possibility of an experimental error or that the grid of measurements may not b e

representative are ruled out, and the reduction of about 65°C is considered an accurat e

estimate. A possible explanation is that radiative heat transfer is increased due t o

a higher particle concentration caused by a larger number of drops after the secondary

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d0 0

125 0

-4

-3

-2

-1

0

1

2

3

4

r/Da-4

-3

-2

-1

0

1

2

3

4

r/D a

x/ Da

FIGURE 2 Spatial distribution of gas temperature (°C) for Runs T100 and ET100 .

atomization; that can be more important than the soot reduction reported in othe rstudies . In fact, Mattiello et al . (1992) report a significant increase in particle numbe rdensity in the first part of the flame .

Pollutant emissions for the four llames investigated are summarized in Table 1 . Thelevels of CO and UHC emitted in Run ET100 are lower than for Run T100, which i scoherent with the previous discussion . The particle burden in the flue gases i ssignificantly reduced by the addition of water . Some works (Jacques et al ., 1977;Gollahali et al., 1984) postulate that the presence of water can reduce the formation o fcoke by modifying the pyrolysis reactions . However, the works of Marrone et al . (1984 )and Urban and Dryer (1990) conclude that the initial formation of the carbonaceou sresidue from an oil drop is only a function of the oil composition and is independent o fthe drop size or its temperature history . According to that assumption, the changes i nparticulate emissions for a given oil can only be caused by differences in the burnout o fthe initially formed cenospheres . The sample from the emulsion test (Plate 2) contain sa large fraction of cenospheres showing an open structure with the holes separated b ythin membranes . Those characteristics indicate a much more advanced burnout stagethan the particles from Run T100, which are mainly characterized by a spherical an dcompact appearance . These differences are a consequence of the higher arca exposedfor the oxidation due to their smaller size, and also of the improved mixing, tha tincreases the oxygen availability along the particle paths .

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3 .2 Run T120 vs ET12 0

Figure 3 represents the spatial distribution of UHC concentration for the neat oil an demulsion tests of Tfo = 120°C. Differences between Runs T120 and ET120 are muchless important than at Tfo = 100°C. The measurements near the burner (x/Da = 0.36 )are very similar, indicating approximately the same progress in the combustion. Thatoccurs in spite of the important differences expected between the two initial spray s(Table 1) . Moreover, the zones of high CO and UHC concentrations appear slightl yreduced for Run ET120.

The SEM photographs of the solid samples display differences in the morphology of •the particles that are similar to those observed between Runs T100 and ET100 . Theconcentration of solids in the flue gases decreases from 310 to 145 mg/Nm 3 . The gastemperature distributions in the flame show also a decrease of about 65°C in th eemulsion flame .

The NO x emissions display a remarkable difference between the neat oil and th eemulsion combustion tests . The results are 488 ppm for Run T120 and 375 ppm for Ru nET120. Two different possible explanations have been found in the literature for th ereduction of NO x in emulsion flames . Cunningham et al. (1983) attribute a 10 %reduction in NOx to a decrease in flame temperature . A chemical effect is als omentioned in the review of Dryer (1977) . The increase of OH radicals due to the

Run ET12 0

-3

-2

-1

0

1

2

3

4

r/D a-3

-2

-1

0

1

2

3

4

r/Da

FIGURE 3 Spatial distribution of UHC (methane-equivalent % vol, dry basis) for Runs T120 and ET120 .

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presence of the additional water can reduce the formation of NO x by lowering th econcentration of O atoms . The spatial distribution of N Ox inside the furnace can hel pto evaluate the importance of those effects in this study .

The results obtained in the inner core of the flame (Fig . 4) are very similar in bot hcases (see, for example, the zone inside the fine of 300 ppm). On the contrary, significan tdifferences appear downstream . The graph for Run T120 displays a relatively rapidincrease in NO x concentration from 300 ppm until a level close to the exit valu ex/Da = 4.5. In the emulsion flame the increase in NOx for the same distance is onl yabout 70 ppm .

The region where NOx is under 300 ppm is a fuel-rich area, as indicated by the low O Zconcentration and the high levels of CO and UHC . Hence, the main source of NO x inthis region is expected to be the conversion of the fuel-bound nitrogen . Since the gascomposition is very similar in Runs T120 and ET120, it seems logical to obtain almos tthe same results of NOx . Ori the other hand, the largest increase in water concentratio ndue to the addition of water in the emulsion flame occurs in the flame core where th econcentration of combustion products is low . Therefore, the chemical effect throug hthe reduction of O atoms should be most marked in this region . Since the results arealmost identical in both flames, this effect is not considered to play a relevant role in th eformation of NOx .

o

- 2

.4.

1 0

x/Da

-3

-2

-1

0

1

r/D a

-3

-2

-1

0

1

2

3

4

r/Da

3

4

FIGURE 4 Spatial distribution of NO x (ppmv, dry basis) for Runs T120 and ET120 .

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The differences in the NO x formation appear in a region where the combustio nprocess is near to completion, as indicated by the low concentrations of 0 2, CO andUHC. The maximum flame temperatures are located in this area. Therefore, th egeneration of NO x through the thermal route can be significant . The 65°C difference i nflame temperatures can explain the reduced NO x formation in the emulsion case, a sa result of the strong dependence of the thermal-NO x mechanism on the gas tempera-

ture. The contribution of the fuel-bound nitrogen is probably small in this region ,because the fuel vapor is practically depleted .

The NOx emissions obtained for Runs T100 and ET100 are 314 and 312 ppm ,respectively. This result is thought to be the outcome of two competing effects . First, the

flame temperature measurements indicate a difference identical to that observed in

Runs T120 and ET120, and a reduction in thermal-NO formation should take place .On the other hand, the optimization of the mixing and combustion processes obtaine d

with the emulsion tends to increase the formation of NO in the flame . The result of th etwo opposite effects is, accidentally, a negligible variation in the final NO x in these twocases .

REFERENCE S

Ahmad, I . and Gollahali, S . R. (1993) . Combustion of Microemulsion Sprays. 31st Aerospace SciencesMeeting and Exhibit, Paper AIAA 93-0131, Reno, NV.

Ballester, J . M., Dopazo, C., Vidal, P . J . and Ojeda, L . (1993). Large-scale Laboratory Experiments o nPollutant Emissions in Heavy Oil Combustion . 2nd International Conference on Combustion Technolo-gies for a Clean Environment, 19–22 July, Lisbon.

Ballester, J . M . and Dopazo, C . (1994) . Experimental Study of the hnfluence of Atomization Characteristicson the Combustion of Heavy Oil . Combust . Sci . Tech ., 103, 1–6, 235 .

Cunningham, A. T . S ., Gliddon, B . J. and Squires, R . T. (1983) . Water-in-oil Combustion as a Technique fo rBurning Extra Heavy Fuel Oils in Large Power Station Boilers . Inst . of Mechanical Engineers ofConference on Combustion in Engineering, Paper C76/83, pp . 147–154 .

Dryer, F . L . (1977). Water Addition to Practical Combustion Systems-Concepts and Applications . 16thSymposium (International) on Combustion, The Combustion Institute, Pittsburgh, pp . 279–295 .

Gollahali, S . R ., Nasrullah, M . K. and Bhashi, J. H. (1984) . Combustion and Emission Characteristics o fBurning Sprays of a Residual Oil and its Emulsions with Water . Combust . Flame, 55, 93–103 .

Jacques, M . T., Jordon, J . B., Williams, A . and Hadley-Coates, L . (1977). The Combustion of Water-in-oi lEmulsions and the Influence of Asphaltene Content . 16th Symposium (International) on Combustion ,The Combustion Institute, Pittsburgh, pp . 307—319 .

Kozinski, J . A. (1994) . PAC's Formation and Interaction in Semipractical Flames of Liquid Fuels .Combustion and Flame, 96, 249—260 .

Marrone, N . J ., Kennedy, I . M. and Dryer, F . L . (1984). Coke Formation in the Combustion of Isolate dHeavy Fuel Oil Droplets . Combust. Sci. Tech ., 36, 149—170 .

Mattiello, M ., Cosmai, L., Pistone, L., Beretta, F. and Massoli, P. (1992). Experimental Evidence fo rMicroexplosions in Water/Fuel Oil Emulsion Flames Inferred by Laser Light Scattering . 24th Sympo -sium (International) on Combustion, The Combustion Institute, Pittsburgh, pp . 1573–1578 .

Nazha, M . A . A. and Crookes, R . J . (1984) . Effect of Water Content on Pollutant Formation in a Burnin gSpray of Water-in-diesel Fuel Emulsion . 20th Symposium (International) on Combustion, The Combus -tion Institute, Pittsburgh, pp . 2001-2010 .

Sjógren, A . (1977) . Burning of Water-in-oil Emulsions . 16th Symposium (International) on Combustion, Th eCombustion Institute, Pittsburgh, pp. 297–305 .

Urban, D. L . and Dryer, F. L. (1990). New Results on Coke Formation in the Combustion of Heavy Fue lDroplets . 23rd Symposium (International) on Combustion, The Combustion Institute, Pittsburgh ,pp . 1437–1443 .

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