**3.1 Test fuels**

2 Fossil Fuel and the Environment

A wick flame was formed with a wick combustion burner, as shown in Fig. 1. The burner was equipped with a pool filled with fuel, and a wick was put in the pool. Fuel was supplied from a tank to the pool through a float chamber under the fuel tank. To form a steady flame, the fuel level within the pool was kept constant by the float. The fuel flow rate was derived from the weight loss of the burner, measured by an electronic balance. The pool was made of aluminium. The pool had an outer diameter of 20 mm, an inner diameter of 16 mm, and a depth of 6 mm. The wick, made of sintered bronze metal (39 % porosity), was placed in the centre of the pool. The wick was cylindrical (8 mm diameter, 18 mm length, 6 mm wall thickness) with a flat bottom (8 mm diameter, 2 mm thickness). The wick was put in the pool so that the bottom protruded 7 mm from the pool rim. The distance from the fuel

Flame

Level adjustor

Laser diagnostic techniques are able to probe combustion products nonintrusively. Laserinduced fluorescence (LIF) and laser-induced incandescence (LII) are attractive techniques for combustion diagnostic and can be used to obtain information about PAHs (Hayashida, 2006) and soot (Shaddix, 1996), respectively. We measured the two-dimensional distribution of the PAHs-LIF in diffusion flames, and laser-induced incandescence (LII) was also used to visualize the soot distribution. Figure 2 shows schematic of the optical arrangement. The laser diagnostic system consisted of an Nd: YAG laser (Spectron Laser Systems, SL856G), a dye laser (Lumonics, HD-300B), and a doubling unit (Lumonics, HT-1000). The laser light was formed into a light sheet (0.5 mm×46 mm) by cylindrical lenses and was introduced into a target flame. Laser-induced emissions were detected by an ICCD camera (Andor

Sintered metal

6mm

Wick

Pool

Fuel tank

Float

Electronic balance

**2. Experimental apparatus 2.1 Wick combustion burner** 

surface to top of the wick was 10 mm.

16mm

20mm

**2.2 Laser diagnostic system** 

Fig. 1. Schematic of wick combustion burner

8mm

7mm

Six types of fuel, with different distillation and compositional properties, were used. The physical properties of the test fuels are shown in Table 1. The calorific value of each fuel was same level, because the difference of net calorific value between test fuels was within 2.5%. Regarding fuel F, its smoke point (>50 mm) implies that the sooting tendency of fuel F was much smaller than that of the other fuels. Distillation characteristics indicate that fuel C was light, whereas fuel D was heavy in the test fuels. Fuel F was comparatively light, and fuel A, B and E had similar distillation characteristics. Influence of sulphur and nitrogen compounds on combustion was negligibly-small, because the contents of sulphur and nitrogen were extremely low.

Table 2 shows the results of the composition analyses of the test fuels. Although content of n-paraffin was not much difference between test fuels (28.6~34.4 vol%), there was considerable difference of i-paraffin content such as the fuel F (57.8 vol%) and fuel C (12.1 vol%). Content of naphthene hydrocarbons was low in fuel F (12.4 vol%) and comparatively low in fuel A (23.5 vol%). Aromatic hydrocarbons were much contained in fuel C (23.3 vol%), and were little contained in fuel E (9.9 vol%). Fuel F did not have any aromatics. Most of the aromatic components of the test fuels were one-ring aromatics; two-ring aromatics were very rare.


Effects of Fuel Properties on Diffusion Combustion and Deposit Accumulation 5

Figure 3 shows photographs of the test flames. Flame lengths of fuel C and F were longer than that of the other fuels because of relatively lower distillation temperature. Soot emissions from the flame tip were confirmed in every flame; amount of soot emission of fuel

ABCDE

0 0.4 0.8 1.2

*z* / *Lf*

F

**3.2 Flame temperature and flame luminosity** 

0

0.002

Fig. 4. Axial profiles of flame luminosities

0.006

0.01

D E F

A B C

0.002

Flame luminousity [a.u.]

0.006

0.01

0.014

Fig. 3. Photographs of test flames

20 [mm]

F was very low.


Table 1. Physical properties of test fuels used in the diffusion combustion experiment


Table 2. Composition of test fuels used in the diffusion combustion experiment

### **3.2 Flame temperature and flame luminosity**

4 Fossil Fuel and the Environment

Distillation characteristics

Elemental

analysis

Kinematic viscosity (30C)

Mono-naphtheno

Fuel name A B C D E F Density (15C) [g/cm3] 0.7912 0.8007 0.8011 0.8000 0.7945 0.7547

Initial boiling point [C] 149.0 156.5 148.5 149.5 154.5 157.0 5 vol% [C] 163.5 169.5 160.5 165.5 168.5 165.5 10 vol% [C] 166.0 173.5 161.0 169.5 172.5 166.0 20 vol% [C] 173.0 179.5 165.5 178.0 179.5 169.5 30 vol% [C] 180.0 186.0 168.5 185.5 184.5 173.0 40 vol% [C] 187.0 193.0 172.0 194.0 191.0 177.5 50 vol% [C] 195.5 201.0 175.0 203.0 198.0 183.0 60 vol% [C] 204.5 210.5 179.0 213.0 204.5 190.5 70 vol% [C] 215.0 221.0 183.5 225.0 212.5 200.5 80 vol% [C] 227.5 233.5 189.5 237.0 222.5 214.5 90 vol% [C] 243.0 247.0 198.0 252.0 236.0 231.0 95 vol% [C] 253.0 257.5 205.5 262.0 247.0 240.0 97 vol% [C] 259.0 262.5 209.5 268.0 253.0 244.0 End point [C] 267.0 271.5 221.5 273.5 260.0 246.5 Percent recovery [vol%] 98.5 98.5 98.5 98.5 98.5 98.5 Percent residue [vol%] 1.0 1.0 1.0 1.0 1.0 1.0 Percent loss [vol%] 0.5 0.5 0.5 0.5 0.5 0.5

[mm2/s] 1.361 1.475 1.096 1.501 1.455 1.322

A B C D E F

Sulphur [mass ppm] 6 7 21 32 3 <1 Nitrogen [mass ppm] <1 <1 <1 <1 <1 <1 Carbon [mass%] 86.0 86.1 86.3 86.0 85.7 84.5 Hydrogen [mass%] 14.0 13.9 13.4 13.9 14.2 15.2

Net calorific value [J/g] 43380 43280 43000 43310 43440 44100 Smoke point [mm] 23.0 23 21.5 22.0 28.0 >50 Freezing point [C] -45.0 -45.0 -70.5 -42.5 -49.5 < -70.0 Flash point [C] 43.5 47.0 40.5 45.5 46.5 46.0

Table 1. Physical properties of test fuels used in the diffusion combustion experiment

Component Composition [vol%]

n-paraffins 34.4 32.0 28.6 30.4 29.5 29.8 i-paraffins 22.8 20.0 12.1 15.5 29.0 57.8 Mono-naphthenes 18.7 21.8 30.7 27.4 24.7 10.1 Di-naphthenes 3.9 5.2 4.1 6.6 5.4 2.3 Poly-naphthenes 0.9 1.2 1.2 1.2 1.4 0.0 Alkylbenzenes 12.7 10.8 19.9 10.6 6.1 0.0

benzenes 5.3 6.5 1.6 6.1 3.3 0.0 Di-naphtheno benzenes 0.4 0.6 0.0 0.6 0.2 0.0 Poly-aromatics 1.0 1.9 1.8 1.5 0.3 0.0

Table 2. Composition of test fuels used in the diffusion combustion experiment

Figure 3 shows photographs of the test flames. Flame lengths of fuel C and F were longer than that of the other fuels because of relatively lower distillation temperature. Soot emissions from the flame tip were confirmed in every flame; amount of soot emission of fuel F was very low.

Fig. 3. Photographs of test flames

Fig. 4. Axial profiles of flame luminosities

Effects of Fuel Properties on Diffusion Combustion and Deposit Accumulation 7

0 0.4 0.8 1.2

D E F

A B C

*z* / *Lf*

PAHs are considered to be precursors of soot particles formed in a flame, because it bridges the mass gap between fuel molecules and soot particles (Hepp, 1995). Formation and growth of PAHs arise from chemical reactions beginning with pyrolysis of fuel, and then inception of soot particles occurs by coagulation of grown PAHs. To estimate the effect of fuel properties on the soot emission and the soot generation characteristics, concentration

Figure 7 shows planar images of LII and PAHs-LIF obtained from the test flames. Since the concentrations of fuel F was significantly lower than the other fuels, another color scale was supplied in the figure. Except for fuel F, high concentration of LII at outer edge of the flame reveals that the soot was presence cylindrically in the flame. Regarding the fuel F, although the presence of soot was cylindrically in the lower part of the flame, upper part was not such distribution. LII distributions indicate that soot particles quickly formed in the case of fuel C and D. Since these fuels contain relatively much two-ring and poly-aromatics, soot particles

In each flame, PAHs-LIF was detected just after the wick, and the intensity decreases with increasing the distance from the wick. Regarding the fuel F, PAHs-LIF appeared comparatively strong between *z*=10~20 mm. A very low PAHs-LIF intensity of fuel F implies a very low PAHs concentration. It was confirmed that PAHs-LIF intensity became

Figure 8 shows the intensity profiles of PAHs-LIF and LII on the flame axis. Note that the PAHs-LIF intensity of fuel F was indicated as multiply the original data by 5. The peak of

1000

Fig. 6. Axial temperature profiles of the test flames

**3.3 Concentration distributions of PAHs and soot** 

distributions of soot and PAHs was investigated.

might be promptly formed from those aromatics.

stronger with the aromatic contents increases.

1200

1400

1000

Temperature [

℃]

1200

1400

1600

Luminosity of the test flames at the centreline was measured by a CdS cell (wavelength sensitivity 400~800 nm, peak sensitivity 580 nm) through lens and pinhole. Obtained results were shown in Fig. 4. Note that measurement position was indicated as *z*/*Lf*. Here, *z* is distance from the wick and *Lf* is flame length from the wick; *Lf* is defined as flame luminosity disappearance position. Since electric resistance of CdS cell decreased with increasing the flame luminosity, the luminosity was expressed by inverse of CdS resistance. Flame luminosity of fuel F was particularly high, whereas luminosity of fuel C was lower than other fuels. The peak luminosity of fuel A, B, D and E was high in order of fuel E, D, A and B, and this order was corresponding to descending order of aromatic contents. According to the theory of black-body radiation, intensity of radiating body (i.e. soot particles) increases with temperature; thus the obtained luminosity would be reflected by flame temperature. Since soot is produced by the incomplete combustion of hydrocarbon fuel, it seems that the high luminosity of fuel F, which was lowest soot emission, was due to its high flame temperature resulting from the least incomplete combustion.

Figure 5 shows the temperature distributions of the test flames obtained by a two-color thermometer (Mitsui optronics, Thermera-seen). Here, two-color thermometer based on the two-color method (Zhao, 1998) measures the radiated energy of soot particles between two narrow wavelength bands, and calculates the ratio of the two energies, which is a function of the temperature. As for the black region in the figure, soot did not exist, or temperature and radiated energy of soot might be below the detection limit. Obtained result reveals that the temperature of flame F was particularly high.

Fig. 5. Temperature distributions of the test flames

Figure 6 shows temperature profiles on centreline of the flame indicated in Fig. 5. The flame temperature of fuel F which did not contain aromatic compounds was the highest. Flame temperature was tendency to decrease in order of increasing aromatic components contained in the fuel, because soot generation within the flame might be increased with increasing the content of aromatics in the fuel.

Luminosity of the test flames at the centreline was measured by a CdS cell (wavelength sensitivity 400~800 nm, peak sensitivity 580 nm) through lens and pinhole. Obtained results were shown in Fig. 4. Note that measurement position was indicated as *z*/*Lf*. Here, *z* is distance from the wick and *Lf* is flame length from the wick; *Lf* is defined as flame luminosity disappearance position. Since electric resistance of CdS cell decreased with increasing the flame luminosity, the luminosity was expressed by inverse of CdS resistance. Flame luminosity of fuel F was particularly high, whereas luminosity of fuel C was lower than other fuels. The peak luminosity of fuel A, B, D and E was high in order of fuel E, D, A and B, and this order was corresponding to descending order of aromatic contents. According to the theory of black-body radiation, intensity of radiating body (i.e. soot particles) increases with temperature; thus the obtained luminosity would be reflected by flame temperature. Since soot is produced by the incomplete combustion of hydrocarbon fuel, it seems that the high luminosity of fuel F, which was lowest soot emission, was due to

Figure 5 shows the temperature distributions of the test flames obtained by a two-color thermometer (Mitsui optronics, Thermera-seen). Here, two-color thermometer based on the two-color method (Zhao, 1998) measures the radiated energy of soot particles between two narrow wavelength bands, and calculates the ratio of the two energies, which is a function of the temperature. As for the black region in the figure, soot did not exist, or temperature and radiated energy of soot might be below the detection limit. Obtained result reveals that

ABCDE

F

1100℃ 1600℃

Figure 6 shows temperature profiles on centreline of the flame indicated in Fig. 5. The flame temperature of fuel F which did not contain aromatic compounds was the highest. Flame temperature was tendency to decrease in order of increasing aromatic components contained in the fuel, because soot generation within the flame might be increased with

its high flame temperature resulting from the least incomplete combustion.

the temperature of flame F was particularly high.

0

Wick

Fig. 5. Temperature distributions of the test flames

increasing the content of aromatics in the fuel.

20 [mm]

Fig. 6. Axial temperature profiles of the test flames
