Paper # 000 Topic: Heterogeneous Combustion, Sprays & Droplets

8th

U. S. National Combustion Meeting

Organized by the Western States Section of the Combustion Institute

and hosted by the University of Utah

May 19-22, 2013

Particle Morphology and Composition of Ash from Heavy Fuel Oil

Fired at Atmospheric Conditions with Various Spray Injection

Pressures

Daniel Tovar1, Daniel Ellis

1, Vipperla, Ravi-kumar

2 and Dale R. Tree

1

1Department of Mechanical Engineering, Brigham Young University, Provo UT, 84606

2 General Electric, Energy Division, Greenville, S.C., 29615

Land-based turbine engines are currently used to burn heavy fuel oil (HFO), which is a lower cost fuel. HFO

contains inorganic material that forms deposits on turbine blades reducing output and efficiency. Magnesium based

additives are used to inhibit vanadium pentoxide deposition and reduce the corrosive nature of the gas and deposits in the

hot gas path of the gas turbine. The focus of this study was to determine particle morphology and elemental composition

of ash when firing HFO in an atmospheric combustor at various fuel injector atomization pressures. Prior to firing, the

HFO was washed with water to remove sodium and potassium. A commercially available magnesium based additive was

used to inhibit the vanadium in the HFO. Fuel was injected using an air-blast atomizer at air blast atomization gage

pressures of 117, 186, and 255 kPa. Ash was collected from three locations downstream of combustion: 1) a water-

cooled probe just downstream of the flame and upstream of a cyclone particle separator, 2) a cyclone separator, and 3) an

inline exhaust bag filter downstream of the cyclone. A Philips XL30 environmental Scanning Electron Microscope

(SEM) provided images and weight percent of elements of the ash. Images taken from the SEM clearly show two particle

types: 1) hollow spherical particles, or cenospheres, and 2) submicron agglomerated spherical particles. The cenospheres

contained high carbon concentrations and were found primarily in the cyclone and probe bag filter. Particles collected

downstream of the cyclone were primarily sub-micron in size and inorganic in composition. It is postulated that the

cenospheres are the result of incomplete combustion of fuel oil droplets while the submicron spheres are nucleated

inorganic material that initially evaporated from the liquid droplets. While increasing atomization pressure decreased the

carbon content of the ash at all measurement locations, the atomization had little influence on the inorganic composition

of the particles. The fine condensed phase particles and the larger cenosphere particles both produced similar

compositions of inorganic material.

1. Introduction

Heavy fuel oil (HFO) is the residual product of crude oil that has been refined in distillation towers. HFO is

very viscous, and has a high boiling temperature [1]. Although there are many uses for fuels gleaned from

crude oil, HFO requires special designs due to its tendency to produce ash. Land-based turbine engines are

currently used to burn HFO, which is a lower cost fuel. The ash byproduct reduces gas turbine output and

efficiency as it deposits on the turbine blades [2, 3]. There are many elements found in HFO that are

responsible for ash corrosion, however, the primary corrosive elements are vanadium, sodium, and potassium.

Although HFO ash has been studied in the past [4], the objective of this work was to determine if atmospheric

combustion using low pressure atomization could be used to produce ash particles of similar size and

composition to high pressure gas turbine ash.

This work has been done in an atmospheric reactor at three different atomization pressures. The ash was

collected at three locations: pre-cyclone, cyclone, and post-cyclone in order to determine differences in

composition between the sizes of particles collected.

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2. Method

Two of the inorganic materials (sodium and potassium) found in HFO are water soluble and are therefore

removed in commercial operations before HFO is burned in a gas turbine engine. The same process was

therefore followed in the laboratory tests. Typically, the HFO is mixed with water and then the water is

removed with a centrifuge. However, since HFO has an almost identical specific gravity to that of water, we

were unable to perform the separation with a small centrifuge in the lab. As a result, HFO was mixed with

diesel fuel prior to washing in order to lower the specific gravity and facilitate separation. The separation

process was further facilitated by adding, a small amount of demulsifier (0.02% of total volume) to the HFO.

Once the HFO had been washed it was sent to the lab to verify that sodium and potassium were near the 1

ppm limit typically used in the field. To inhibit the formation and deposition of vanadium pentoxide as well

as reduce the corrosive nature of the gas in the hot gas path of the gas turbine, magnesium based additives

were also injected into the HFO just prior to combustion [4]. The following procedure details the process used

in the lab experiments to simulate washing and additive addition.

HFO Fuel Preparation Procedure

1. 104L of HFO was heated to 43.3°C in a 208L barrel.

2. 5.6L of No. 2 diesel fuel was mixed with 34mL of demulsifier.

3. The diesel fuel and demulsifier mixture was then added to the HFO.

4. 60.5L of deionized water was added to the HFO.

5. The entire mixture was stirred with a mixing pad for approximately ten minutes.

6. The fuel water mixture was allowed to separate for a period of 24 hrs.

7. Water was pumped from the bottom of the barrel.

8. Processes 4 through 7 were repeated two more times.

9. The HFO was pumped through a 3m simplex filter.

Table 1 shows properties for a washed HFO sample representative of the HFO that was utilized in the

experiments.

Table 1: Properties of washed HFO.

Ultimate Analysis Ash Analysis

Carbon, wt% 84 Sodium, ppm 2.2

Hydrogen, wt% 10.90 Magnesium, ppm 481

Nitrogen, wt% .36 Aluminum, ppm 18

Sulfur, wt% 1.890 Silicon, ppm 153

Ash, wt% 0.21 Potassium, ppm 1.4

Oxygen, wt% 2.64 Calcium, ppm 96.6

Moisture, vol% <0.1 Vanadium, ppm 44.2

Iron, ppm 45.7

Physical Data Nickel, ppm 26

Density, kg/m3 @ 15°C <1029

Flash Point, Pensky-

Martens CC >62°C

Viscosity, cSt @ 50°C 211-638

Specific Gravity, 60F/60F 0.988

A down fired, refractory lined reactor called the Burner Flow Reactor (BFR) was utilized to burn the HFO.

The BFR, shown in Figure 1, consists of six cylindrical sections each of 0.4 m in length for a total height of

2.4 m and an inside diameter of 0.75 m. All sections have four access ports which are located at 90 degree

angles from each other. The nominal thermal capacity of the reactor is 160 kWth.

3

Figure 1: Schematic diagram of the experimental apparatus.

Preceding HFO combustion, the BFR was heated with methane until the walls reached a minimum

temperature of 1000 K. The methane tube was then replaced with an oil atomizing fuel line. As can be seen in

Figure 1, a high viscosity pump was used to transfer the HFO from the washed and filtered HFO barrel to the

reactor through electrically heated and temperature controlled (T = 93 oC) tubing. Baker Petrolite KI-200

Vanadium Corrosion Inhibitor, an additive containing magnesium, was injected into the fuel line using a

syringe metering system. The additive flow rate was calculated according to the manufacturer instructions to

produce a ratio of magnesium in the additive to vanadium in the fuel of 3:1. Both the fuel and additive would

then flow through a static mixer before being atomized and burned in the BFR. HFO temperature was

measured right before entering the burner. Inside the burner, the fuel entered an air assisted Airo atomizing

nozzle part No. 30615-15 and adaptor part No. 23034-2 which were purchased from Delavan Spray

Technologies. A flow meter was used to monitor HFO flow into the reactor. The HFO and atomizing air

pressures were monitored by pressure gauges installed on their respective lines.

4

For this experiment, ash was collected at three locations, as labeled in Figure 1. A water-cooled probe was

inserted into the bottom of the BFR through an access port. The probe used an ejector to create suction which

would then direct flue gas into a 1 m bag filter, McMaster-Carr, No.5168k51 and 51635k211. Ash exiting

the BFR entered a vertical water-cooled pipe with a wet-bottom barrel located directly beneath the reactor.

This wet ash barrel is used primarily for coal ash collections where larger particles are produced and for the

collection of reactor wall deposits that fall off during testing. Particles exiting this wet bottom barrel tube

were not collected. Flue gas continued into a cyclone where particles larger than approximately 2μm were

collected (note Figure 1). The final sample location downstream of the cyclone collected particles that were

too small to be collected in the cyclone. These particles were also collected using the same type of bag filter

as was used with the sample probe. Particles were vacuumed off of the bags through a filter. The filter

element was then removed and the particles collected in a vial.

Table 2 shows the three operating conditions used to produce HFO ash. The objective was to hold all

operating conditions constant except for the atomizing air pressure which was increased from 117 to 255 kPa

(17.0 – 37.1 psig). The air to fuel ratio was held constant such that the wet exhaust oxygen concentration was

held at 4%. In order to facilitate atomization, the HFO was heated to a temperature of 93.3C.

Table 2: Test matrix

Exhaust

O2 (%)

Secondary

Air (kg/hr)

Atomizing Air

Pressure, kPa

(psig)

Fuel Flow

(LPH)

Oil Temp.

(oC)

Swirl

#

Additive

Flow Rate

(ml/hr)

Condition

1

4 200 117 (17.0) 14.4 93.3 .4 8.28

Condition

2

4 200 186 (27.1) 14.4 93.3 .4 8.28

Condition

3

4 200 255 (37.1) 14.4 93.3 .4 8.28

Once the ash was collected from the three collection points, it was analyzed using a Philips XL 30

Environmental Scanning Electron Microscope-Field Emission Electron Gun (ESEM-FEG). The ESEM

provided backscatter micrographs of the ash and Energy Dispersive Spectrometry (EDS) was also employed

to determine elemental composition (wt. %).

Loss on ignition (LOI) measurements, as defined by Eq. 1, were performed on the ash samples using the

ASTM D7348 standard test method. In Eq. 1, is the weight of the ash after all the moisture has been

removed, the weight of the ash after heating to 950oC, or the weight of the carbon free ash. Carbon

burnout was estimated from the LOI measurement according to Eq. 2 where Yash is the as received weight

percent of ash in the HFO. In Eq. 2 it is assumed that all of the mass loss in the ash is due to carbon. Because

some of the LOI mass loss results from elements other than carbon, for example sulfur, Eq. 2 will tend to

underestimate carbon burnout.

(1)

(2)

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10. Results and Discussion

Backscatter micrographs of ash collected in the reactor sample probe upstream of the cyclone for the three

different atomization pressures can be found in Figure 2. Micrographs in Figure 2a-c were taken at

magnifications of approximately 300x. Contrast and brightness are, however, not constant as the micrographs

were taken at different times. The micrographs in Figure 2 show that the ash is composed of two types of

particles: larger darker grey spherical particles or cenospheres most of which are greater than 10μm, and very

fine spherical particles, most of which are on the order of 5μm and smaller. These fine particles are often

agglomerated into chains or clumps of particles. Although Figure 2b appears to contain particles which are

much larger than at the other two pressures, further inspection of these larger particles at a higher

magnification show they are just agglomerations of smaller cenosphere particles. In backscatter imaging, the

intensity of light in the images is proportional to the molecular mass of the elements being imaged. The larger

particles are generally darker while the fine particles are brighter. This darker color suggests more carbon.

Elements of intermediate molar mass are silicon, aluminum, and magnesium while the heaviest elements are

iron, calcium, and vanadium. The large cenosphere particles are therefore indicative of HFO droplets that

have not completely burned out leaving the more stable hydrocarbons and inorganic content in the shape of

the original droplet. The smaller, fine particles indicate condensed phase particles that were formed from

evaporated fuel evolved from the burning HFO droplets. Figure 2 shows that as the atomization pressure is

increased, the cenospheres appear less frequent. This trend suggests that the higher atomization pressures

produce less cenospheres, and better burnout. This is consistent with the likelihood that increased atomization

air pressure decreases droplet size and decreases the number of droplets that are large enough to survive the

combustion process.

(a) (b) (c)

Figure 2: Pre-Cyclone ash at (a) 117kPa, (b) 186kPa, and (c) 255kPa. All micrographs have a scale bar of 100

μm.

Figure 3 shows backscatter micrographs of ash particles collected in the cyclone for each of the three air

atomization pressures. Each micrograph was taken at the same magnification of approximately 300x. Contrast

and brightness are, however, not constant as the micrographs were taken at different times. As was pointed

out in Figure 2, the cyclone ash shows a mixture of cenospheres and nucleated particles at each operating

condition. Although these two types of particles appear in all three micrographs; as the atomization pressure

is increased the cenospheres appear less frequently and the fine particles become more concentrated. This

trend is similar to that of the pre-cyclone ash.

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(a) (b) (c)

Figure 3: Cyclone ash at (a) 117kPa, (b) 186kPa, and (c) 255kPa. All micrographs have a scale bar of 100 μm.

Figure 4 displays the ash collected in the exhaust bag filter located downstream of the cyclone at the three

different atomization pressures. As in Figures 2 and 3, the micrographs were taken at equal magnifications of

approximately 300x, and varying contrasts and brightness. When comparing these micrographs with the

micrographs from the cyclone, it is noticed that the exhaust bag collects a higher density of nucleated particles

than cenospheres. No apparent trends arise from examining the post-cyclone micrographs at different

pressures.

Elemental Analyses can be found in Figures 5-7. Carbon and oxygen have been removed from the analyses

with the remaining elemental mass fractions equal to 1.0. Carbon data will be discussed later with relation to

carbon burnout. The error bars shown on the plots are a representation of plus and minus one standard

deviation of the data. Due to the nature EDS, results must be considered semi-quantitative and therefore

absolute values are of greater uncertainty than the error bars indicate, but these data were all obtained with the

same EDS system and software. Therefore, relative comparisons of composition should be valid with within

the uncertainty indicated.

Figure 5 shows the EDS results of pre-cyclone ash. As noted in Figure 2, this ash contained both cenospheres

and condensed phase particles. The largest components in the ash are magnesium, sulfur and vanadium.

Differences in the amount of each element appear to be relatively independent of the air atomization pressure

even though higher pressure was shown to produce a higher concentration of condensed phase particles.

There does appear to be slightly higher concentrations of Magnesium and Vanadium and a lower

concentration of sulfur at the two higher atomization pressures. This would be consistent with a greater

amount of vanadium being released from smaller droplets and a smaller amount of unburned HFO being

produced which particles tend to contain sulfur.

(a) (b) (c)

Figure 4: Post-Cyclone ash at (a) 117kPa, (b) 186kPa, and (c) 255kPa. All micrographs have a scale bar of

100 μm.

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Figure 5: Concentrations of the inorganic material obtained by EDS in the pre-cyclone ash.

Figure 6 shows the elemental compositions obtained with EDS of the average of 2-3 ash samples at each of

the three operating pressures collected in the cyclone. The error bars represent plus or minus one standard

deviation. As shown in Figure 3, the cyclone collects the larger particles as well as a fraction of the smaller

particles that are either attached to the large particles or have agglomerated. As with the pre-cyclone particles,

the concentrations are highest for magnesium, sulfur, and vanadium. Not shown in the composition data is the

fact that the amount of ash collected in the cyclone decreased dramatically with increasing atomization

pressure. At 255 kPa the amount of ash collected was 50 times less at 255 kPa than at 117 kPa indicating a

much smaller amount of the larger unburned carbon particles. As the amount of ash collected decreased, the

variability of the ash composition increased, presumably due to the small sample size and the possibility of

contamination from deposit particles added to the flue gas between the reactor exit and the cyclone. As the

atomization pressure increased, magnesium, sulfur and vanadium appear to have decreased while calcium and

iron increased. This would appear to be caused by contamination from coal ash that is higher in iron, silicon,

aluminum, and calcium and lower in sulfur, magnesium and vanadium. The differences between compositions

at the three atomization pressures are nevertheless small.

Compositions based on the average of 2 to 3 samples collected from the post-cyclone position are shown in

Figure 7. Once again data are shown for each of the three operating pressures. These particles consisted

primarily of condensed phase particles that were small enough to pass through the cyclone. As with the other

collection locations, these particles contain higher concentrations of magnesium, sulfur, and vanadium. These

particles also contain higher iron concentrations than the other two locations. There are no clear trends of

changing particle concentrations with a change in atomization pressure. This result is consistent with the

observation from the backscatter micrograph that the particles at this location were of the same size and shape

with varying atomization pressure. When comparing the primarily condensed phase particle compositions in

the post-cyclone ash shown in Figure 7 with the cyclone ash from Figure 6, it can be seen that the condensed

phase particles are slightly lower in sulfur and magnesium but higher in vanadium, and iron. The differences

are however surprisingly small considering the large differences in particle shape and size at the two

locations.

0

5

10

15

20

25

30

35

40

45

50W

t %

Elements

117 kPa

186 kPa

255 kPa

8

Figure 6: Concentrations of the inorganic material obtained by EDS in the cyclone ash.

Figure 7: Concentrations of the inorganic material obtained by EDS in the post-cyclone ash.

Table 3 reports the loss on ignition (LOI) and burnout of the three sample locations at two of the three

atomization pressures. The cyclone ash has the highest LOI and lowest burnout of the three collection points.

This is as expected because the cyclone collects only the larger fraction of particles which are the least likely

0

5

10

15

20

25

30

35

40

45

50W

t %

Elements

117 kPa

186 kPa

255 kPa

0

5

10

15

20

25

30

35

40

45

50

Wt

%

Elements

117 kPa

186 kPa

255 kPa

9

to be burned out. Burnout is very high in the HFO but because of the low ash content of HFO, the carbon

fraction remaining in the ash is still significant. The pre-cyclone ash contains all sizes of ash but because the

majority of the mass is contained in the larger particles, the pre-cyclone ash is similar in LOI and burnout to

that collected by the cyclone. The post cyclone ash does not contain large particles and therefore has a

significantly lower LOI. The low LOI content in the post-cyclone ash is consistent with the micrographs

showing smaller particles and an absence of cenospheres. The cyclone ash contained the most cenospheres.

There are no burnout data available for the 255 kPa operating condition because of the small amount of ash

available.

Table 3: LOI and Burnout for two atomization pressures.

Atomization

Pressure

(kPa)

117 kPa (17 psig) 186 kPa (27 psig)

Sample

Location

Pre

Cyclone

Cyclone Post

Cyclone

Pre

Cyclone

Cyclone Post

Cylcone

LOI (%) 83.91 85.28 38.13 54.68 68.59 34.94

Burnout

(%)

98.89 98.77 99.87 99.74 99.53 99.88

11. Summary and Conclusions

Heavy fuel oil was washed and burned in an atmospheric reactor at three atomization pressures. The ash

produced was sampled at three locations: immediately following combustion (pre-cyclone), from a cyclone

separator, and post-cyclone. The particles were found to be of two distinct types: larger (mostly greater than

10 m), hollow spherical particles and smaller (mostly 2 m and smaller) solid spherical particles. The

smaller particles were often agglomerated into larger chains or aggregates. The larger particles contained high

carbon content (above 30%) and looked porous indicating fuel droplets that were not completely burned out.

These particles became less numerous in comparison to the smaller particles as the atomization pressure

increased. After subtracting out the carbon and oxygen concentrations, all of the particles had similar

concentrations of inorganic material although the smaller micron and submicron particles appeared to contain

slightly higher concentrations of vanadium and iron and lower concentrations of sulfur and magnesium. The

larger particles are presumed to be the result of incomplete combustion of HFO droplets. As the atomization

pressure increased, the size of these particles decreased and the number and mass of these particles decreased

and improved burnout. The smaller micron and submicron particles are assumed to be derived from nucleated

condensed volatile matter produced at high combustion temperatures and condensed upon cooling. The

atomization pressure was found to have a dramatic effect on the amount of carbon and on the number of

larger particles but only a modest impact on the composition of the inorganic material. The composition of the

smaller particles was not impacted by atomization pressure.

Acknowledgements This research was funded by GE Power and Water. We acknowledge the assistance of Paul Glaser of GE

Global Research who consulted on chemistry issues related to the HFO. We also acknowledge Josh

Thornock, Steven Owen, and Trevor Blanc who helped with running of the experiment.

References

[1] C. Soares, (2008). Gas Turbines: A Handbook of Air, Land, and Sea Applications. pp. 251-252. San

Diego: Butterworth-Heinemann.

[2] P.F. Schmidt, (1985). Fuel Oil Manual. pp. 95. New York: Industrial Press INC.

[3] C.H. Jones, (1985). Marine Fuels: A Symposium. pp. 158. Ann Arbor: American Society for Testing and

Materials.

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[4] A.D. Foster, H.E. von Doering, and M.B. Hilt, (1983). “Fuels Flexibility in Heavy-Duty Gas Turbines,”

pp. 27-29. Schenectady, New York: GE Company.