The soot and nitrogen oxide reduction using water/heavy fuel oil emulsion.

Moroianu, Corneliu ; Muntean, Angela ; Samoilescu, Gheorghe 等

1. INTRODUCTION

Every year approximately 10 million tones of nitrogen oxide (N[O.sub.x]) are produced by ships. The world fleet include 55% slow speed diesel, 40% medium speed diesel and 5% other engines. Slow speed diesel engines produce higher N[O.sub.x] emissions. The photochemical smog and acid rain are also produced by N[O.sub.x] emissions. Introducing water into the combustion chamber the temperature of burning is reduced due to the vaporation process. There are two ways to introduce water. The first way is through air intake, using humidification and the latter way is by water/fuel emulsion. The latter way mentioned above reduces smoke, while humidification increases it. Water/fuel emulsion and direct water injection have the maximum effect on N[O.sub.x] reduction. This paper presents the resultates of burning the emulsified and non emulsified fuel droplet on the laboratory condition(simulator). The emulsification of the water with oil was performed by means hydrodynamic liquid generator.

2. THE WATER/FUEL EMULSIONS GENERATION USING ULTRASOUNDS

The high intensity ultrasound generation in liquid is realized using a hydrodynamic whistle. The hydrodynamic whistle for liquids, Fig.1, consists on a tapered nozzle (1), with a segment (2) placed in front of it at 0.3 - 1 mm. The segment is fixed in one or two nodal points (Jinescu, G. 1983).

Passing through the nozzle, the liquid flow hits the segment fixed on one end on the bracket. At about 12-15 bar pressure it resonates at frequency (Popa & Iscrulescu, 1983):

f = 22.4xd/4x[square root of 3] x 1/[I.sup.2] x [square root of E/[rho]] [Hz] (1)

where

l--is the segment length [m]; d--segment thickness [m];

E--Young's modulus [N/[m.sup.2]]; [rho]--density.

Fig.1. The hydrodynamic generator for liquids; 1--snout, 2--flexible segment.

If the resonating segment is made of, the resonance frequency is

f = 5.4 x [10.sup.5] x d/[I.sup.2] [Hz]. (2)

The ultrasound frequency depends on the flow pressure. The hydrodynamic generator irradiates ultrasounds in the working environment as the following relation reveals:

f = v/h x 0.5 [Hz] (3)

where

v--is the flow rate in the nozzle [m/s]; h--the distance between the nozzle and resonating flexible segment [m].

The nozzle and the vibrator segment are situated in a resonant chamber with an acoustic form (Jianu C. 1996).

3. THE EXPERIMENTAL APPROACH

The experimental approach has two stages: in the former stage is the combustion of non-emulsified fuel and the latter one is the combustion process of oil/water emulsion (for the lock of space I didn't write the combustion oscillogram of non-emulsified fuel).The emulsions have different water mass fractions in fuel. The analyses have been performed in special laboratories. The fuel drops bathing at pre-established diameters was realized using a special device. The same temperature and air conditions were used for emulsified and non emulsified fuel drop combustion. The process was realized in free convection (Law, 1997). The remarks were directly made with the naked eye, and the processes were registered using a special camera.

4. THE WATER/FUEL EMULSION FUNDAMENTAL CHARACTERISTICS

In order to state the quality and the steadiness of an ultrasound emulsion its properties will be compared to those of a non emulsified fuel shown in Table 1. The water/fuel emulsion has been stored for 45 days. The determinations revealed a good stability in time. There is no separation of liquid components danger. We also performed tests in order to determine the water/fuel emulsion behavior in heat-proof vessels. Emulsions were stored for 60 and 150 days at 50[degrees], 60[degrees] and 80[degrees]C temperatures. The water drop dimensions and the percentage analyses were also performed.

5. THE WATER/HEAVY FUEL EMULSIONS COMBUSTION OSCILOGRAMS

Fuel combustion graphology is a new technical and scientific field (Ghia 1991). It transposes the fuel combustion processes in a simulator using graphics. It is an easy way to establish the ignition and combustion characteristics and relations between combustion conditions and fuel specifications. The combustion oscillogram is a graphic of combustion process development for a liquid droplet. This diagram shows the time variation t of radiation intensity I of volatile substances caused as a result of combustion changed into an electric signal by means of an optical-electronical system equipped with a photoelectrical cell. On a standard condition, the oscillogram establishes self-ignition delay [[tau].sub.i], the combustion time of volatile matters [[tau].sub.v], the cenosphere combustion time [[tau].sub.c], the maximum radiation intensity for cenosphere combustion [I.sub.c], the maximum radiation intensity for volatile matter combustion [I.sub.v], the energy radiation resulting from cenosphere burning, transformed by photocell into electric energy [E.sub.c], etc The standard condition of the combustion tests are mainly specified by the combustion chamber geometry, the temperature, the pressure inside chamber, the emulsion temperature, the fuel admission system, the air flow conditions around droplet and the initial and average diameters of the droplet.

The propulsion systems for ships and the steam generators fitted up on the oil tankers run with residual heavy fuel RME 25, RMF 25, RMG 35 range according to ISO 8217/1995. Fig.2 shows the combustion oscillogram for a water emulsified RMG 35. Fuel drop having the following characteristics: --water content 3.4%; lower thermal power 39950 kJ/kg.

The results for a drop with a D0=1.9 mm diameter ignited at [T.sub.fo]=1023 K temperature, [T.sub.a]=293 K ambient temperature, [T.sub.c]=300 K fuel temperature and Re=135 are: --self-ignition delay [[tau].sub.I] = 260 ms; volatile matters burning time [[tau].sub.v]=1070 ms; cenosphere burning time [[tau].sub.c]=1150 ms; volatile matters radiated power [E.sub.v]=700620 [u.c..sup.*]; cenosphere radiated power [E.sub.c]=33870 [u.c..sup.*] ([sup.*]u.c-conventional units)

Fig. 3 prezents the combustion oscillogram for a water emulsified droplet RMG 35having the following characteristics:

--water content 7.5 %; lower thermal power 39050 kJ/kg.

The results for a drop with a [D.sub.0]=1.9 mm diameter ignited at [T.sub.fo]=1023 K temperature, [T.sub.a]=293 K ambient temperature, [T.sub.c]=300 K fuel temperature and Re=135 are:

--self-ignition delay [[tau].sub.I] = 288 ms;--volatile matters burning time [[tau].sub.v]=720 ms;--cenosphere burning time [[tau].sub.c]=732 ms;--volatile matters radiated power [E.sub.v]=549000 [u.c..sup.*];--cenosphere radiated power [E.sub.c]=2530 [u.c..sup.*].

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

In both cases the curve [I.sub.v] = f([tau]) has a gap in the middle part. It is due to water vaporization wich leads to the decrease of combustion temperature. (a potential factor of N[O.sub.x] reduction). Testing the 3.4 % emulsified fuel the [I.sub.v] = f([tau]) volatile matters radiated power is bigger than [E.sub.c] cenosphere radiating power.

By testing the 7.5 % emulsified fuel the radiated power [I.sub.v] = f([tau]) of volatile matters is a little smaller and the radiating power Ec of cenosphere is much reduced. The reason is the secondary vaporization. The explanation relies on the fast heating of water dispersed inside individual fuel droplets. The internal water droplets undergo a spontaneous vaporization of water, causing a violent change of water droplets into steam. On the other side, the vaporization causes a rapid expansion of surrounding oil droplets, bathing the oil into a large number of small fuel droplets. This process is secondary atomization.

Experiments show that a 7.5% water/residual RMG 35 fuel emulsion has better combustion properties than a 3.4% water/residual RMG 35 fuel emulsion. The explanation relies on secondary atomization effect.

6. CONCLUSIONS

The initial droplet strain subjected to water vapors action breaks into smaller droplets. The [I.sub.c] low values of [E.sub.c] and [[tau].sub.c] lead to reduced unburn carbon losses and to decreasing of carbon black (soot) quantity. The increasing of cenosphere burning performance as result of secondary atomization. The introduction of water into the combustion chamber reduces the combustion temperature due to the absortion of energy for vaporization. Humidification can reduce the N[O.sub.x] emissions.

7. REFERENCES

Ghia V. (1991). Combustion Graphology of Fuel Oil, Sci. Tech. Electrotehnica Et Energ., Tome 36, p 379-396, Bucuresti.

Jinescu, G. (1983). The hydrodynamic process and special equipments, Editura Didactica si Pedagogica, Bucuresti

Law, C. K. (1997). Combustion Science and Technology, Vol. 17, p.29-38.

Jianu C. (1996). The combustion of fuels in sound field, Ed. U.P., Bucuresti.

Popa, B. & Iscrulescu, V. (1983). The combustion processes in sound field, Editura Academiei, Bucuresti.

Tab. 1. Fuel characteristics.

 Non mulsified Water/fuel
 Fuel characteristics fuel emulsion

Lower thermal power (kJ/kg). 38650 [+ or -] 845 36920 [+ or -] 845

Water content (%) 0-1 7.5-10.5

Sulphur content (%) 2.5-3.5 2.2-3.3

Conradson coke content (%) 12-20 11.5-19

Density (kg/[m.sup.3]). 960-980 960-985

Viscosity at 80[degrees]C 14-26 14-25.5
([degrees]E)

Average diameter of -- 3-4.4
water drops ([micro]m)

Water drops with smaller -- 80-90.6
than 4.8 [micro]m
diameter content

Viscosity at injection 2.5-3 3.2-5.8
nozles ([degrees]E)