**4.1 Test fuels**

Five types of kerosene fraction, with different distillation and compositional properties, were used. The physical properties of the test fuels are shown in Table 3. Regarding fuel K, the end point (246.5 °C) and density (0.7547 g/cm3) were lower than those of the other fuels.


Table 3. Physical properties of test fuels used in the deposit accumulation experiment

Five types of kerosene fraction, with different distillation and compositional properties, were used. The physical properties of the test fuels are shown in Table 3. Regarding fuel K, the end point (246.5 °C) and density (0.7547 g/cm3) were lower than those of the other fuels.

Density (15C)[g/cm3] JIS K2249 0.7910 0.7895 0.7884 0.7980 0.7547

5 vol% [C] 166.0 161.0 161.5 158.0 164.5 10 vol% [C] 167.5 165.5 165.0 160.5 165.5 20 vol% [C] 174.5 172.5 171.5 167.0 168.5 30 vol% [C] 181.0 179.5 178.0 174.0 172.5 40 vol% [C] 187.5 187.0 184.5 182.0 177.5 50 vol% [C] 194.5 194.5 192.0 191.5 182.5 60 vol% [C] 202.5 204.0 201.0 201.5 190.5 70 vol% [C] 212.0 214.0 210.5 211.5 201.0 80 vol% [C] 223.0 227.0 225.5 223.0 215.0 90 vol% [C] 236.0 244.5 239.0 238.5 232.5 95 vol% [C] 246.0 259.5 254.0 249.0 242.0 97 vol% [C] 252.0 270.5 264.0 256.0 246.5 End point [C] 256.0 276.5 270.0 259.5 246.5 Percent recovery [vol%] 98.5 98.5 98.5 98.5 98.5 Percent residue [vol%] 1.0 1.0 1.0 1.0 1.0 Percent loss [vol%] 0.5 0.5 0.5 0.5 0.5

JIS K2254

Kinematic viscosity (30C) [mm2/s] JIS K2283 1.347 1.365 1.335 1.373 1.334 Net calorific value [J/g] JIS K2279 43380 43430 43440 43390 44100 Smoke point [mm] JIS K2537 23.0 24.0 24.0 26.0 >50 Freezing point [C] JIS K2276 -48.5 -45.5 -49.0 -68.0 -66.0 Flash point [C] JIS K2265 45.0 43.5 44.0 39.5 46.0

Sulphur [mass ppm] JIS K2541 8 4 2 <1 <1 Nitrogen [mass ppm] JIS K2609 <1 <1 <1 <1 <1

Table 3. Physical properties of test fuels used in the deposit accumulation experiment

CH ratio ― 0.51437 0.51132 0.50953 0.50713 0.46107

Method 85.8 85.9 85.6 85.8 84.6

Method 14.0 14.1 14.1 14.2 15.4

method G H I <sup>J</sup> <sup>K</sup>

153.5 147.0 146.5 145.5 156.0

**4. Effects of fuel properties on deposit accumulation** 

Test item Test

Initial boiling point [C]

Carbon [mass%] JPI

Hydrogen [mass%] JPI

**4.1 Test fuels** 

Distillation characteristics

Elemental analysis

Table 4 shows the results of the composition analyses of the test fuels. The results confirmed that fuel K consisted only of saturated hydrocarbons (i.e., paraffins and naphthenes). In contrast, other fuels contained between 7.6~19.5 vol% aromatic hydrocarbons. Most of the aromatic components of the fuels were one-ring aromatics; two-ring aromatics were very rare. In addition, the following low-stability components are also shown in Table 4: naphtheno benzenes, olefin hydrocarbons (bromine number), dienes (diene value), and organic peroxides (peroxide number). Naphtheno benzenes, compounds that consist of naphthene and aromatic rings, were contained in higher concentrations in fuel G (5.4 vol%); these molecules were not contained in fuel K. The bromine number was largest in fuel J (28.8 mgBr2/100g) and lowest in fuel K (4.4 mgBr2/100g). The diene values of fuels J and K indicated 0.05 gI2/100g and 0.01 gI2/100g, respectively; dienes were not detected in other fuels. The peroxide numbers demonstrate that none of the test fuels contained organic peroxides.


Table 4. Composition of test fuels used in the deposit accumulation experiment

### **4.2 Relationship between fuel properties and deposit accumulation**

Tar-like deposits, formed by the thermal decomposition and polycondensation of the fuel, adhered to and accumulated on the upper part of the wick during combustion. To investigate the effects of fuel properties on tar-like deposit accumulation, tar-like deposits were accumulated on the wick by prolonged combustion. The deposit mass was obtained by subtracting the mass of unused wick from the mass of the wick that had accumulated deposits. In this process, if unburnt fuel remained in the wick with deposits, an accurate deposit mass could not be obtained. To eliminate remaining fuel, the wick was pulled up from the pool with a flame, and combustion was continued until all fuel was used. Obtained deposit was visually confirmed as tar-like deposit, but soot particle might have slightly adhered on the wick.

Effects of Fuel Properties on Diffusion Combustion and Deposit Accumulation 13

accumulation ratio of fuel K might be caused by several factors: (1) the amount of lowstability components in fuel K was very low, (2) pyrolysis of saturated hydrocarbons was hard to occur within the range of the internal temperature of the wick, and (3) deposits formation from the pyrolysis products of saturated hydrocarbons took relatively long time.

0 200 400 600 800 <sup>0</sup>

G: 0.00749%

H: 0.00580% I : 0.00574%

J: 0.00343%

K: 0.00064%

Fuel consumption [g]

Aromatic hydrocarbons Naphtheno benzenes

0 0.002 0.004 0.006 0.008 <sup>0</sup>

Deposit accumulation ratio [%]

Fig. 12. Relationship between aromatic hydrocarbons, naphtheno benzenes and deposit

The relationship between aromatic hydrocarbons, naphtheno benzenes and the deposit accumulation ratio is presented in Fig. 12. The deposit accumulation ratio was strongly associated with the content of aromatics and naphtheno benzenes. As naphtheno benzenes are

0

2

4

Naphtheno benzenes [vol

%]

6

8

10

8

16

Aromatic hydrocarbons [vol

accumulation ratio

24

%]

20

30

H

Fig. 11. Relationship between fuel consumption and deposit mass

I

Deposit mass [mg]

40

50

Figure 10 shows temporal change of deposit mass. In this experiment, one wick was sequentially used in the measurement of one test fuel. As seen in the figure, the deposit growth rate decreased after a certain period of time except fuel K. This result implies that the accumulation of deposits proceeded in two stages. Deposits were first rapidly accumulated in voids of the wick, which gradually saturate over time; we called this period the "internal accumulation mode". Once the voids became saturated with deposits, deposits subsequently accumulated on the outer surface of the wick; we called this phase the "surface growth mode". Although the deposit growth rate in the internal accumulation mode varied somewhat due to individual differences of the wick, the growth rate of the surface growth mode had reproducibility. Regarding fuel K, the deposit accumulation would not have attained to the surface growth mode owing to its extremely low deposit growth rate.

Fig. 10. Temporal change of deposit mass

As the deposit growth rate in the internal accumulation mode was influenced by individual differences of the wick, the relationship between deposit mass and fuel consumption was investigated in the surface growth mode. The results are shown in Fig. 11. Note that the results reported here for fuel K are not for the surface growth mode. Straight lines in the figure were obtained by the least-squares method, and the slopes correspond to the deposit accumulation ratio (the percentage of conversion from fuel to deposit). Deposit accumulation ratios are also indicated in the figure.

Obtained results indicate that the deposit mass of each fuel increased proportionally with increasing fuel consumption. Fuel G displayed the highest deposit accumulation ratio (0.00749 %), followed by fuel H (0.00580 %) and I (0.00574 %). Fuel K (0.00064 %) exhibited an extremely small accumulation ratio. Compared with the fuel compositions shown in Table 4, fuel G, with its high deposit accumulation ratio, was found to contain higher levels of aromatics and naphtheno benzenes; conversely, fuel K, with its low deposit accumulation ratio, contained no aromatics or naphtheno benzenes. Note that there was no correlation between the bromine number, diene value, and accumulation ratio. The extremely low

Figure 10 shows temporal change of deposit mass. In this experiment, one wick was sequentially used in the measurement of one test fuel. As seen in the figure, the deposit growth rate decreased after a certain period of time except fuel K. This result implies that the accumulation of deposits proceeded in two stages. Deposits were first rapidly accumulated in voids of the wick, which gradually saturate over time; we called this period the "internal accumulation mode". Once the voids became saturated with deposits, deposits subsequently accumulated on the outer surface of the wick; we called this phase the "surface growth mode". Although the deposit growth rate in the internal accumulation mode varied somewhat due to individual differences of the wick, the growth rate of the surface growth mode had reproducibility. Regarding fuel K, the deposit accumulation would not have

attained to the surface growth mode owing to its extremely low deposit growth rate.

H

0

accumulation ratios are also indicated in the figure.

16

32

Deposit mass [mg]

Fig. 10. Temporal change of deposit mass

48

I

G

K

0 8 16 24 32

Time [hr]

As the deposit growth rate in the internal accumulation mode was influenced by individual differences of the wick, the relationship between deposit mass and fuel consumption was investigated in the surface growth mode. The results are shown in Fig. 11. Note that the results reported here for fuel K are not for the surface growth mode. Straight lines in the figure were obtained by the least-squares method, and the slopes correspond to the deposit accumulation ratio (the percentage of conversion from fuel to deposit). Deposit

Obtained results indicate that the deposit mass of each fuel increased proportionally with increasing fuel consumption. Fuel G displayed the highest deposit accumulation ratio (0.00749 %), followed by fuel H (0.00580 %) and I (0.00574 %). Fuel K (0.00064 %) exhibited an extremely small accumulation ratio. Compared with the fuel compositions shown in Table 4, fuel G, with its high deposit accumulation ratio, was found to contain higher levels of aromatics and naphtheno benzenes; conversely, fuel K, with its low deposit accumulation ratio, contained no aromatics or naphtheno benzenes. Note that there was no correlation between the bromine number, diene value, and accumulation ratio. The extremely low

J

accumulation ratio of fuel K might be caused by several factors: (1) the amount of lowstability components in fuel K was very low, (2) pyrolysis of saturated hydrocarbons was hard to occur within the range of the internal temperature of the wick, and (3) deposits formation from the pyrolysis products of saturated hydrocarbons took relatively long time.

Fig. 11. Relationship between fuel consumption and deposit mass

Fig. 12. Relationship between aromatic hydrocarbons, naphtheno benzenes and deposit accumulation ratio

The relationship between aromatic hydrocarbons, naphtheno benzenes and the deposit accumulation ratio is presented in Fig. 12. The deposit accumulation ratio was strongly associated with the content of aromatics and naphtheno benzenes. As naphtheno benzenes are

Effects of Fuel Properties on Diffusion Combustion and Deposit Accumulation 15

is probably due to an increase in the carbonized deposit formed at the wick surface and/or soot adhesion on wick. Additionally, some sulphur compounds might be contained in the residue because these fuels contain sulphur, as indicated in Table 3. In contrast, the deposits of fuel K contained no residue, and the mass loss in the range of 250~550 °C was relatively high. This fact demonstrates the very low deposit growth rate of fuel K, as described previously. The deposits of fuel J, with its very high bromine number and diene value, displayed a relatively large mass loss of the heavy polycondensation product; this observation may be due to the contribution of a tar-like deposit that originated from olefins

In this chapter, effects of fuel properties on diffusion combustion and deposit accumulation were studied experimentally. Obtained results could contribute to a design of fuel properties for reduction of the pollutant emissions from diffusion combustion of fossil fuels,

Laminar diffusion flames of the test fuels were formed by using the wick combustion burner. Flame temperature and flame luminosity of each flame were measured. Furthermore, to investigate the effect on soot formation of fuel properties, concentration distributions of PAHs and soot were measured by LIF and LII. The main results are

1. The peak luminosity of flame was high in order of decreasing aromatic components

2. Flame temperature tended to decrease with increasing aromatic components contained

3. In each flame, the PAHs-LIF intensity rapidly decreases with distance from the wick. The disappearance location of the PAHs-LIF coincides with the location where the LII

4. Soot emission increased with increasing content of aromatic hydrocarbons in the fuel.

Deposit accumulation processes were investigated by using the wick combustion burner. Deposit accumulation rate was estimated from the mass of deposit which accumulated on wick and the fuel consumption, and components of the deposit were estimated from the TG-

1. Except for fuel K, deposit accumulation per unit time (deposit growth rate) decreased

2. The conversion percentage from fuel to deposit (deposit accumulation ratio) increased

3. The primary components of deposits were heavy polycondensation products (50~60 wt%). Heavy kerosene components, light polycondensation products (about 20 wt%),

with greater contents of aromatics and naphtheno benzenes in the fuels.

and kerosene fractions (about 15 wt%) were also identified in the deposits.

and for suppression of the deposit accumulation within a combustion device.

**5.1 Effects of fuel properties on diffusion combustion** 

**5.2 Effects of fuel properties on deposit accumulation** 

DTA analysis. The main results are summarized as follows:

and dienes (Zanier, 1998).

summarized as follows:

in the fuel.

contained in the fuel.

intensity rapidly increases.

after a certain time after ignition.

**5. Conclusions** 

one of the aromatics, more naphtheno benzenes may be contained in the aromatic-rich fuel. It is well known that naphtheno benzenes are more susceptible to thermal decomposition at lower temperatures than other hydrocarbons; thus, pyrolysis and polycondensation of naphtheno benzenes would be performed within the wick. Finally, naphtheno benzenes were transformed into tar-like deposits. Conversely, because the pyrolysis of aromatics was difficult to perform due to their high thermal stability, initial aromatics contained in the fuel transformed into heavy molecules and tar-like deposit without pyrolysis. Note that the sooting tendency became stronger with increasing aromatic content in the fuel; soot adherence to the wick possibly affected the results shown in Fig. 12. Although the deposits did not necessarily originate from aromatics and naphtheno benzenes, the deposit accumulation ratio of fuel K was extremely low and no correlation was found between the bromine number, diene value, and deposit accumulation ratio. These results suggested that most of the deposits originated from the naphtheno benzenes and/or aromatic hydrocarbons.

#### **4.3 Deposit analysis**

Deposits that accumulated on the wick were extracted by diethyl ether and chloroform solutions and dried after evaporation of the solutions. The collected deposits were then analyzed with a simultaneous thermogravimetry/differential thermal analysis (TG-DTA) instrument (TA Instruments, SDT2960). Analysis was performed by the following steps:


[heating rate; 50°C/min]

Table 5 shows the mass loss of the deposits in each step. Considering the boiling point of hydrocarbons, the mass loss of each step corresponds to (1) the kerosene fraction, (2) the heavy kerosene component and light polycondensation products of low molecular weight, and (3) the heavy polycondensation product. The residue was considered to be a carbonized deposit and soot particles.


#### Table 5. Results of TG-DTA analysis

The results of TG-DTA analysis suggested that most of the deposit components were heavy polycondensation products formed by thermal decomposition and polycondensation of the fuel within the wick. Residues of fuel G, H and I, which contained higher amounts of aromatics and naphtheno benzenes, were higher than that of the other two fuels; this result is probably due to an increase in the carbonized deposit formed at the wick surface and/or soot adhesion on wick. Additionally, some sulphur compounds might be contained in the residue because these fuels contain sulphur, as indicated in Table 3. In contrast, the deposits of fuel K contained no residue, and the mass loss in the range of 250~550 °C was relatively high. This fact demonstrates the very low deposit growth rate of fuel K, as described previously. The deposits of fuel J, with its very high bromine number and diene value, displayed a relatively large mass loss of the heavy polycondensation product; this observation may be due to the contribution of a tar-like deposit that originated from olefins and dienes (Zanier, 1998).
