EvaporationâCombustion of Fuels - American Chemical Society

16 Evaluation of Oil/Water Emulsions for Gas Turbine Engines L. J. SPADACCINI and R. PELMAS

Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0166.ch016

United Technologies Research Center, East Hartford, CT 06108 The feasibility of the dispersion fuels concept for application to gas turbine power plants is evaluated from dispersion fuels formulation studies, from the results of single droplet tests directed toward demonstration of the droplet shattering process, and from the results of initial burner tests of dispersion fuels. Results demonstrate the existence of the "microexplosion" phenomenon in single-droplet combustion experiments. Gas turbine combustor tests indicate that fuel emulsification may alter favorably the efficiency of a practical gas turbine combustor without adversely affecting the turbine inlet temperature profile or NO , CO, and smoke emissions. x

The relatively high cost of light fuel oils continues to affect the economy of this nation adversely because of the large quantity of energy produced by combustion devices that currently require relatively low-viscosity, high-volatility fuels to operate efficiently. The growth rate of power-production-related expenses could be reduced appreciably if such combustion devices could use heavy residual quality oils, and petroleumbased fuels could be conserved if these devices could use coal-derived fuel oils. However, conversion from light to heavy fuel cannot be accomplished without regard for the deleterious effect the use of a residual quality or coal-derived fuel oil would have on combustion efficiency, pollutant emissions, and engine operational costs. That is, the extent of fuel atomization required for efficient combustion in gas turbine and similar combustion devices cannot be achieved if a high-viscosity heavy fuel oil is simply substituted for a lighter oil. Furthermore, the required atomization cannot be realized easily by modifying the configuration of the fuel injector because injectors compatible with current or currently 0-8412-0383-0/78/33-166-232$05.00/0 © American Chemical Society In Evaporation—Combustion of Fuels; Zung, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1978.


16.

SPADACCINI AND PELMAS

Oil/Water Emulsions for Gas Engines 233

foreseen combustors do not appear to offer the capability to generate sprays in which most of the mass of a heavy oil would be associated with small size droplets. However, such sprays may be able to be generated by using emulsified fuels in which the dispersed phase (e.g., another fuel or water) exhibits a much higher vapor pressure characteristic than the primary fuel ( I ) . Injection of such a dispersion creates droplets in which the primary fuel envelopes a number of droplets of the dispersed phase. As the droplets are heated by combustor inlet air or in the initial stage of combustion, the temperature of the droplet passes through the boiling point level of the dispersed phase, whereupon the volume occupied by the dispersed phase increases by several orders of magnitude. Such volume change induces catastrophic shattering (sometimes referred to as the "microexplosion" phenomenon) of the primary fuel droplet into a number of smaller droplets. This chapter describes the results of research on evaluating dispersion fuels for application in gas turbine engines and includes studies of emulsion formulation, single-droplet combustion, and gas turbine combustor tests. Dispersion Fuel Formulation Studies

Dispersions have been formulated using No. 2 and No. 6 (residual) fuel oils as the continuous phase. Dispersions using No. 6 fuel oil as the continuous phase and water as the dispersed phase were prepared using water-to-fuel oil concentration ratios in the range of 2.5-50%. In all cases the dispersion appeared stable for more than several days. Photomicrographic analyses, such as those shown in Figure 1, indicated that the water was dispersed in the fuel oil in droplets having diameters of approximately 1-5 /A. Photomicrographs of neat (0% water) No. 6 fuel oil are presented for comparison. One notable feature of the residual oil dispersions was that the viscosity increased with increased water concentration. Dispersions containing up to 20% water by weight exhibited a pourability characteristic similar to that of neat No. 6 fuel. However, dispersions containing 50% water by weight were marked by a significant increase in viscosity. Since residual oils are normally preheated prior to atomization, dispersions containing high concentrations of water may require a slightly increased amount of preheating to obtain the equivalent viscosity. Care must be taken that the fuel temperature and pressure are consistent with the vapor pressure characteristics of the dispersed phase. However, as described below, microexplosions have been observed using emulsions with water concentrations considerably lower than 50%. Dispersions using No. 2 fuel as the continuous phase were also prepared and, in contrast to residual oil dispersions, were inherently unstable, requiring the use of an emulsifying agent(s). A series of stable disper-

In Evaporation—Combustion of Fuels; Zung, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1978.


234

EVAPORATION-COMBUSTION OF FUELS

Figure 1. No. 6 fuel oil emulsions

Figure 2. No. 2 fuel oil emulsions


16.

SPADACCINI A N D P E L M A S


sions containing up to 10% water in No. 2 fuel oil was prepared by adding approximately *4-l% surfactant. The surfactant was a sorbitan fatty acid ester formed by a mixture of 75% Span 80 and 25% Tween 85 (Atlas Chemical Ind., Wilmington, DE). Samples of these dispersions are shown in Figure 2. These dispersions were white and cloudy and exhibited no noticeable change in viscosity from the neat No. 2 oil. Single ^Droplet Combustion Studies


Tests comprising the heating and combustion of single droplets of neat No. 6 oil and water/No. 6 oil emulsions were performed to investigate the microexplosion phenomenon. Figure 3 shows the test apparatus

Figure 3.

Single droplet test apparatus

to investigate the heating and combustion of single droplets of an emulsified fuel. Fuel droplets were dispensed from a precision microsyringe, photographed, heated by a helical-shaped xenonflashlamp,and photographed again at theflashtubeexit. Theflash-lampwas triggered by the output of a photodetector which sensed the falling droplet. Comparisons were made between dispersion fuels and neat fuels undergoing identical test conditions. Droplet heating rates were varied by regulating the energy discharge level of the flashlamp. Photographs of the heating and combustion of single droplets of neat No. 6 oil and a No. 6 oil-20% water dispersion are shown in the next series of figures. Figure 4 is a progression of photographs of droplets.



236

EVAPORATION-COMBUSTION

O F FUELS

exiting theflashlampafter they had been heated by discharges having energies up to 8000 J. Results for droplets of neat No. 6 fuel oil are shown in the upper series of photographs, and the results for droplets of a dispersion of 20% water in No. 6 oil are shown below for comparison. The diameter of the unheated droplets was approximately 1900 In the upper series of photographs, a distinct vapor train emanating from the neat fuel droplet is evident throughout the testing range. The droplet enlarged slightly at 6000 J. The appearance of the neat fuel droplet remained essentially unchanged and showed no signs of shattering. In contrast, the lower series of photographs, which depict the heating of a dispersion fuel, shows the droplet inflating and boiling on its surface at approximately 2000 J. The droplet size increased approximately five to six times its original volume. In addition, the small satellite drops in the photographs indicate that droplet shattering occurred. Additional satellite drops may have gone undetected because the camera lens used had a narrow depth of field. Also, the combustion time required to consume the dispersion fuel droplet was less than was required to burn the neat fuel. Although these photographs present evidence of droplet shattering, there is no clear indication of microexplosions. Therefore, a high-speed motion picture camera was used to photograph the heating and com-

Figure 4.

Effect of droplet heating



16.



Figure 5. Fuel droplet combustion bustion of fuel droplets within the flashlamp. The event was viewed indirectly by means of a mirror located on the axis of theflashlamp.The camera was operated at 8000 frames/sec, and the discharge energy of theflashlampwas 1800 J. The results are shown in Figure 5. The upper sequence of photographs, corresponding to neat No. 6 fuel oil, shows the initiation of a flame front and its gradual propagation around the droplet. Again the neat fuel showed no signs of shattering. The results of tests conducted at identical conditions using a dispersion fuel are shown in the lower sequence in Figure 5. Here the film clearly shows the initial stages of droplet boiling and the formation of small satellite droplets. The process continues and results in catastrophic shattering of the primary droplet and combustion of some of the smaller droplets. Finally, a still photograph of the event, representing an integration of the preceding series of frames over the 25-msecflashduration, is shown in Figure 6. Gas Turbine Burner Studies Experimental evaluations of fuel combustion and handling characteristics are most conclusive when they are conducted using hardware and test conditions representative of operational combustors. However, full-scale testing of a large industrial gas turbine combustor was beyond the scope of the effort described here. Therefore, the tests were conducted at conditions representative of an FT4 industrial gas turbine



238

EVAPORATION-COMBUSTION

OF FUELS

Figure 6. Dispersion fuel combustion. No. 6 oil/20% H 0. 2

engine (40,000 hp) using a smaller but generically similar burner to forecast changes of a corresponding nature in the operational FT4 combustor. The test combustor consisted of an FT12 burner can which was modified to operate with inlet air and fuel conditions representative of an FT4 combustor. No attempt was made to optimize the performance of the resultant burner assembly. Five homogenized fuels were tested, each of them based on Redwood 650 oil (see Table I) and containing nominal concentrations of 0, 4, 5, 7.5, and 10% water, respectively. No emulsifying agent was required. The primary performance parameters evaluated were combustion efficiency: actual temperature rise ideal temperature rise '

V c

exhaust temperature pattern factor: Table I. Specific gravity at 298 K H/C Wt% N Wt% S Viscosity (cs)

Properties of Fuel Oils

No. 2

No. 6

0.845 1.74 0.0102 0.2960 5.3 at 298 K

0.9 1.772 0.106 0.3828 122 at 298 K

Redwood 650 0.935 1.22 0.11 1.10 200 at 310 K


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Oil/Water Emulsions for Gas Engines

Burst disc

Probe drive

Heater outlet

lExhaust

Water-cooled throttle valve

locations


Figure 7. Combustor test assembly P.F.=

(Tmax — r y ) exhaust a

8

average temperature rise across burner'

and exhaust smoke. In addition, measurement of the gaseous pollutant emissions were included to corroborate other measurement techniques and to elucidate the effects of fuel emulsification. The combustor test assembly, shown in Figure 7, was comprised of: (1) An electrical resistance-type heater, (2) An inlet diffuser, (3) A cylindrical duct in which the burner can was mounted, (4) A water-cooled instrumentation section, and (5) A remotely operated throttle valve, located in the exhaust ducting. The combustor inlet air pressure wasfixedat 10 atm and the temperature at 530 K. The fuel temperature at injection was maintained between 340 and 360 K. The burner was operated at an overall fuel-air mixture ratio of 0.014, corresponding to normal power (40,000 hp) operation of the FT4 gas turbine. Traversing stainless steel, water-cooled probes were

Figure 8. Modified FT-12 combustor


239

240

EVAPORATION-COMBUSTION OF FUELS

N

2

—Er

H 0 2

Air

Mixer

F —tXJ—mmR


(31 OK)

0 @ H) $3 [2 E CD ®-

Manual valve Filter Regulator Flowmeter Solenoid valve Check valve Pump Accumulator Relief valve

Heater

(340K) Homogenizer

No.2 Oil

Figure 9.

EvaporationâCombustion of Fuels - American Chemical Society

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EvaporationâCombustion of Fuels - American Chemical Society