BFO

BFO+ Water 25%

‘Flame’

12.5 17.5 22.5 27.5 deg.ATDC

Visual Study on Combustion of Low-Grade Fuel Water Emulsion

*Hiroshi Tajima, Koji Takasaki, Masayoshi Nakashima, Keiichiro Kawano Makoto Ohishi*1, Jun Yanagi*1 and Shin-nosuke Osafune*1

Interdisciplinary Graduate School of Engineering Sciences, Kyushu University

6-1 Kasuga-koen, Kasuga-City, Fukuoka 816-8580, Japan

Key words: Visual Study, Low-grade Fuel, Water Emulsion, Flame Temperature, Soot Formation

ABSTRACT

In CIMAC in May 2001, the largest congress for marine and stationary engines in the world, many engine builders have announced that they apply the water injection into the cylinder to reduce the NOx emission. There are three methods of water injection into the cylinder, FWE (Fuel Water Emulsion), SFWI (Stratified Fuel Water Injection) and DWI (Direct Water Injection) [1]. At the last COMODIA in 1998, the authors presented about the effect of stratified fuel water injection to reduce NOx and smoke at the same time [2].

In the present study, to confirm the effect of fuel-water emulsion on combustion, experiments have been carried out using a visual engine and a visual combustion chamber. Figure 1 shows the burning flames in the two cases, (a) pure BFO (Bunker Fuel Oil) and (b) BFO-water emulsion. The combustion system of the visual engine simulates the one for low-speed marine engines.

According to it, the flame of (b) shows the better combustion state, less soot-cloud and less after-burning than the flame of (a). In the presentation, reduction of the flame temperature measured by the two-color method and reduction of the soot formation inside the flame observed by the back diffused laser photo technique applying the fuel-water emulsion are introduced. [1] Jorach, R.W. et al. : MTZ 61 (2000) Nr.12, pp.854-861. [2] Takasaki, K. et al. : Proc. COMODIA 98 (1998), pp.57-62. *1 Mitsubishi Heavy Industries, Ltd.

Improvement of flame combustion using BFO-water emulsion

Combustion Chamber

Diffuser Lens

+Ar Laser Diffuser Plate

High-speedVideo Camera

QuartzWindow

Band PassFilter

INTRODUCTION In CIMAC in May 2001, the largest congress

for marine and stationary engines in the world, many engine builders have announced that they apply the water injection into the cylinder to reduce the NOx emission. At the last COMODIA in 1998, the authors presented about the effect of stratified fuel water injection to reduce NOx and smoke at the same time.

In the present study, to confirm the effect of fuel-water emulsion on combustion, experiments are carried out using a visual engine and a visual combustion chamber. Though the fuel-water emulsion was developed as a measure to reduce NOx emission, its effect to improve the combustion is also noteworthy. MAN B&W Diesel A/S has a record that a low-speed 2-stroke engine with a bore of 900 mm for a stationary power plant has been running using bunker fuel-water emulsion (20% water) without any major trouble for no less than 17 years [1].

The main purpose of this study is not to achieve the drastic reduction of NOx emission but to clarify the improvement of combustion process due to water addition. In this study, to examine the effect of fuel-water emulsion on bunker fuel combustion, the following experiments are carried out.

- Measurement of the emission from a running test

engine - Examination of the combustion process using a visual

test engine - Examination of the spray/flame characteristics using a

visual combustion chamber EXPERIMENTAL PROCEDURE Running Test Engine

A turbocharged 6-cylinder engine with 200 mm bore [2] is used for the test runs. The specification of this engine is given in Table 1.

Visual Engine The visual engine, explained in detail in [3], is

used to take high-speed photos of the combustion process. It actually is a supercharged, 2-stroke-cycle single-cylinder engine with a bore of 190 mm. Pmi and engine speed at the test are 1.6 MPa and 400 rpm respectively Visual Combustion Chamber

The visual combustion chamber shown in Figure 1 is used to examine the combustion of a single spray/flame and the soot formation inside the flame. The single fuel spray is injected into the chamber in which air at a pressure of 2.5 MPa and a temperature of 670°C is charged. Photos of spray/flame are taken through the glass window fitted on the side of the chamber. A special fuel injection system for bunker fuel oil, which is actuated by a hydraulic pressure and controlled electronically, is applied to this chamber. Fuel is injected with a injection pressure of 66 MPa.

Figure 1 shows how to take the high-peed photos with so-called ‘Back Diffused Laser’ (BDL) technique. With this technique the luminous flame is cut with a band pass filter and only the liquid particles like

Test engine M-200

Engine type 4 Stroke DI, Turbo-charged

Number of cylinders 3 (6) Cylinder bore : D 200 mm Piston stroke : S 270 mm Max. power / cyl. 85 kW Engine speed 1000 rpm max. Pme

(mean effective press.) 1.2 MPa

Fuel inj. nozzle 5 × φ 0.30 mm + 4 × φ 0.26 mm

Figure 1 Visual combustion chamber

Table 1 Specification of running test engine

unevaporated fuel or the solid particles like soot formed inside the flame can be seen. Fuels

Gas oil (GO) and bunker fuel oil (BFO) are used for the running test. Marine diesel oil (MDO) and BFO are tested in the visual engine and the visual combustion chamber. The compositions of GO and MDO have very little difference. On the other hand BFO contains much heavy (residual) portion. Table 2 shows the characteristics of these fuels. When BFO is used it is preheated to a temperature of 100°C (BFO-1) or 80°C (BFO-2) in order to obtain a viscosity of 20 mm2/s.

A motionless mixer made by Toray Engineering Co. is used to emulsify the fuel and water. When BFO is emulsified both the water and BFO are preheated to 80°C and mixed. A small amount, 0.4% of fuel mass, of a surface active agent is added to stabilize the emulsified state. Emulsified states of MDO and BFO are checked using a microscope, and it is confirmed that both the fuels are well emulsified to particles of a few micron-meter in diameter.

In this study, the water % added to fuel is defined as follows.

water % = volume of water / volume of fuel EXPERIMENTAL RESULTS Running Test Result

In such a case that more than 50% of water is added to reduce NOx emission to half, the capacity of injection system must be enlarged. Such the modification leads to higher cost and instable running at low load. In this study water % is limited to 25%, which may be allowable without changing the injection system.

Figure 2 shows the running test result up to about 20% water emulsion. For both GO and BFO NOx emission is about 30% reduced with about 20% water emulsion. Seeing the smoke emission Bosch Smoke Unit (BSU) in case of GO is reduced from 1.0 to 0.3 and that in case of BFO is reduced from 1.3 to 0.7 with about 20% water emulsion. As results NOx and smoke ………………………………………………………….

are successfully reduced without any sacrifice of specific fuel consumption for both the fuels. Test Result with Visual Engine

The visual test engine has the side-injection system combustion chamber same as the above-mentioned record-holding MAN B&W low-speed engine. In this system fuel is injected from the side of combustion chamber through the two injection nozzles with four injection holes each.

Figure 3 shows the comparison of heat release rate in the visual test engine between (a) pure BFO and (b) BFO + 25% water. The mass of BFO is the same in both the cases, which means the injected liquid volume in case (b) is 25% larger than case (a). For that reason the injection duration in case (b) is longer than case (a) as seen in the injection pressure curves in this figure. However comparing the heat release rates, combustion duration of (b) is rather shorter than (a), especially the …

Gas Oil MDO BFO-1 BFO-2

Density (@15 °C) kg/m3 830 843 988 935 Kinematic Viscosity(@50°C) mm2/s 2.0 2.5 381 75

Sulfur wt% 0.15 0.8 2.2 2.8

Nitrogen wt% 0.38 0.29 Water wt% 0.17 <0.05

Ash wt% 0.06 0.03 Residual Carbon wt% 0 0.1 12 12 Lower Calorific Value MJ/ kg 43.0 42.5 40.5 41.0

Figure 2 Running test result Table 2 Fuel properties

* BFO-1: BFO for running test

* BFO-2: BFO for visual test

Water content %

Gas OilBFO-1

0 10 20 30200

250

300

be

g/k

Wh

400

600

800

1000

1200

NO

x p

pm

(13

O )

NO

x p

pm

( 5

O )

%%

20

1.0

2.02

Smok

e B.

S.U.

200

300

400

500

600

after-burning after 35° ATDC is much less in case (b).Though deterioration of ignitability by adding water is not so clear in this heat release rate, there is 1° crank angle gap in ignition timing between (a) and (b) according to the visual data explained in the next paragraph.

Figure 4 shows the burning flames in the two cases, (a) pure BFO and (b) BFO + 25% water. As mentioned above this engine has originally two injection nozzles. However when the photos of burning flames are taken, only one injection nozzle is set in the combustion chamber (one more is set outside of the combustion chamber) and the flame from one injection nozzle is photographed so as to observe its trajectory clearly without overlapping of the flames.

Figure 4 includes three kinds of color photos. The photos entitled ‘Flame’ show the outward appearance of the flame. Seeing ‘Flame’, the flame of (a) is forming black soot-cloud at 17.5° ATDC in the lower part of visual field where the flame touches the piston surface. On the other hand, the flame of (b) shows almost no soot-cloud there. Such the soot-cloud on the piston surface is usually formed in the case that fuel-rich spray

is cooled down by the piston [3]. It is considered that the reduced formation of soot-cloud by water addition is achieved because air entrainment into the spray is improved, i.e. such the fuel-rich region is reduced.

Seeing ‘Flame’ at 22.5° and 27.5° ATDC, the flame of (a) is spreading anticlockwise to the left side in the upper part of visual field taken by the air swirl. In case of (b), the flame has burned up earlier than (a) already in the right side of visual field.

The processed photos entitled ‘Temp’ in Figure 4 show the distribution of flame temperature computed with the two-color method [4]. In ‘Temp’, red parts represent higher temperature (about 2400 K) than green parts (about 2150 K). Comparing the distribution of red part between (a) and (b), it is clear that the hot spots are drastically reduced by water addition. It proves the reduction of NOx formation.

The processed photos entitled ‘KL’ in Figure 4 express the distribution of density of soot being formed and burning inside the flame. In ‘KL’, more soot are being formed and burning in the white part than in the blue part. Comparing (a) and (b), the white part in (b) is much less than in (a) especially at 22.5° and 27.5° ATDC. That shows the reduction of soot formation by water addition. Test Result with Visual Combustion Chamber

Improvement of combustion by water emulsion is further examined applying the ‘back diffused laser (BDL)’ photo technique to the single flame in the visual combustion chamber. Figure 5 is the photo at the last stage of combustion of (a) pure MDO (Marine Diesel Oil) and of (b) MDO-water emulsion (water 25%) taken from a window of the chamber. The photo entitled ‘Flame’ shows the outward appearance of the flame. ‘BDL’ photo is taken at the same time as ‘Flame’ eliminating the luminous flame by a band-pass filter. The black parts in ‘BDL’ photo represent unevaporated liquid fuel or soot formed inside the flame.

In ‘BDL’ photo of Figure 5 (a), a thin black bar of approx. 40 mm length regarded as the liquid fuel is seen just under the injection nozzle. According to it, it is clear that MDO spray evaporates completely within 40 mm. Seeing both the ‘Flame’ and ‘BDL’ of (a), it is also clear that there is a large black part inside the flame. It might be the soot formed by the combustion of fuel-rich zone in the flame. On the other hand, only small black part can be seen in ‘BDL’ photo of (b). It represents a drastic reduction of the soot formation thanks to MDO-water emulsion.

Figure 6 is the photo of (a) pure BFO and (b) of BFO-water emulsion (water 25%) in the visual combustion chamber. Different from MFO, the black part exists in whole length of the flame in ‘BDL’ of Figure 6 (a). BFO cannot evaporate so quickly as MDO because BFO contains much residual portion. In such a case, unevaporated liquid fuel and soot formed inside the flame cannot be distinguished by ‘BDL’ technique. However it is believed with confidence that the clear difference of size of the black part in ‘BDL’ between (a) and (b) in Figure 6 is due to the reduction of soot

-20 0 20 40 600

0.5

1.0

1.5

2.0

d /

dM

PakJ

/deg

.θ

Rate

of h

eat r

elea

seIn

ject

ion

pres

sure

Pinj

θ

(a)(b)

Q

-20 0 20 40 600

40

80

120

Figure 3 Heat release rates in visual engine

D= 190

1728.5

φ

Swirl14

8φ

Injection

BFO-2 BFO-2 + Water 25%

Crank angle deg.ATDC

12.5 17.5 22.5 27.5 deg.ATDC

(a) BFO

(b) BFO+ Water 25%

(a) BFO

(b) BFO+ Water 25%

‘Flame’

K

‘Temp.’

(a) BFO

(b) BFO+ Water 25%

‘KL’

12.5 17.5 22.5 27.5 deg.ATDC

12.5 17.5 22.5 27.5 deg.ATDC

formation thanks to BFO-water emulsion.

2550 2500 2450 2400 2350 2300 2250 2200 2150 2100 2050 2000 1950 1900 1850

Swirl14

8φ

1.501.41 1.31 1.22 1.13 1.03 0.94 0.85 0.75 0.66 0.57 0.47 0.38 0.29 0.19 0.10

Figure 4 Data of BFO-water emulsion in visual test engine

(b) MDO+ Water 25%

(a) MDO

24 25 26 27 ms

‘Flame’ ‘BDL’ (Soot in

Flame)

24 31 32 33 ms Inj. End

‘Flame’ ‘BDL’ (Soot in

Flame)

(b) BFO+ Water 25%

(a) BFO

‘Flame’

‘BDL’ (Soot in

Flame)

‘Flame’

‘BDL’ (Soot in

Flame)

24 25 26 27 28 29 msInj. End

24 31 32 33 34 35 ms Inj. End

Inj. End

CONCLUSION

In order to clarify the improvement in combustion process of low-grade fuel applying fuel-water emulsion, running test and visual study of spray combustion have been carried out.

The following conclusions are drawn. 1. Experiment using a running test engine shows that

NOx and smoke can be successfully reduced without any sacrifice of specific fuel consumption for both gas oil and bunker fuel oil applying about 20% water emulsion.

2. According to the data from a visual test engine, it is confirmed that both combustion temperature and soot formation in the flame of bunker fuel oil can be at the same time restrained applying 25% water emulsion.

3. According to the data from a visual combustion chamber, it is confirmed that soot formation inside the flame of both marine diesel oil and bunker fuel oil can be clearly reduced applying 25% water emulsion.

The improvement of combustion after the end of fuel injection can be estimated by the air/fuel ratio in the spray calculated using the momentum theory [5]. An example of the calculation result is shown in Figure 7. The spray-cones formed at the end of injection are compared between the two cases, i.e. 0% and 25% water addition. As further air entrainment into the spray after the end of injection is impossible, the air/fuel ratio at the end of injection has a substantial effect on combustion after that.

In this study, 40% water emulsion has been also tried. The deterioration of ignition quality becomes a problem when such large amount of water is added, as the ignition delay with 40% water becomes twice of pure BFO.

REFE RENCES

[1] Eilts, P. and Borchsenius, H-J.: Proc. 23rd CIMAC (2001) Vol.3, pp.422-429.

[2] Takasaki, K. et al.: MTZ 60 (1999) Nr.1, pp.62-67. [3] Takasaki, K.: MTZ 59 (1998) Nr.4, pp.276-284. [4] Matsui, Y. et al.: SAE Transaction Vol. 88 (1979)...

pp. 1808. [5] Wakuri, Y. et al.: Bul. of JSME Vol. 3 No. 9 (1960)...

pp. 123

160

Figure 7 Improvement of air-fuel ratio in spray by water addition

Figure 6 BFO-water emulsion in visual combustion chamber

X* Air: Ga*

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251.* ××××==== injinj tt

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