Emulsion Fuels are mixtures of water with hydrocarbon fuel. While they are generally maintained by a surfactant additive, this project uses advanced mixing technology to achieve a stable pure water and hydrocarbon emulsion. A primary objective of creating an emulsion from a fuel oil is reduced emissions. Emulsions can be made from diesel, the full range of heavy fuel oils, biodiesel and many liquid alternative fuels.
Chemistry
Diesel is a blend of various hydrocarbon compounds, including contaminants which do not contribute to combustion, such as sulfur and heavy metals, in stoichiometric ratios ranging from C10H20 to C14H26 typically treated as: C13H23.
Generally, the chemical equation for stoichiometric incomplete combustion of diesel fuel in air can be expressed as follows:
4C13H23 + 49O2 + 3.76(49)N2=18C + 17CO + 17CO2 + 46H20 + 3.76(49)N2
It is significant that the nitrogen is not reacted in the combustion, but is available to react in the heated exhaust gases; and that the incomplete combustion can leave three distinct carbon products.
The presence of additional water, H2O, in the combustion environment from the emulsion, enhances the oxidation of the carbon. The oxidation of carbon, to carbon monoxide consumes 131 kJoules per mole,
C+H2O=CO+H2-131 kJ
And, further oxidation of carbon monoxide to carbon dioxide adds 283 kJoules per mole.
CO+1/2O2=CO2+283 kJ
The objective of complete combustion is to convert all of the CxHy to CO2, to achieve the maximum heat output. The difference between combustion to CO2 compared to CO is 4,000 BTU per pound. The uncombusted carbon becomes ash which further denigrates the thermal performance.
By assuring sufficient interaction of the fuel and the free carbon with Oxygen, full combustion can be achieved. This maximizes the output energy, and minimizes the emission of noxious chemicals.
In internal combustion engines, the completeness of combustion is constrained by the time the fuel spends in the combustion chamber. In order to maximize combustion, the surface area and dispersion of fuel droplets must be maximized.
The rate of combustion is determined by the surface area of the interface of the fuel and the oxygen. Fuel in the center of a droplet is not available for combustion until it is dispersed into the oxygen environment, or after the surrounding fuel is combusted in the deflagration process. The extremely small water droplet size and corresponding large fuel surface area in the nanoemulsion tremendously increases theoverall rate of combustionby extending the deflagration properties of the combustible material to a larger area. This extracts more energy from the fuel during the time available for combustion, either the internal combustion cycle or the fuel availability in the combustion chamber flow of a an external combustion machine.
Both the energy extraction and avoidance of objectionable combustion products are achieved by minimizing the size water dropletsand increasing the surface area of the fuel. This is the primary accomplishment of the nanoemulsion process. It significantly improves the energy available from a defined quantity of fuel.
Use of proprietary laser-based mixing technology instead of high shear mechanical emulsification dramatically reduces droplet size. So much so, that the emulsion is stable for very long periods, in a wide temperature range, without a surfactant.
Since water and fuel oil do not mix and water is generally deleterious to both engine processes and components, it is necessary to suspend the water in an emulsion. In conventional emulsions, this is accomplished by a surfactant additive. The surfactant includes an ionizing agent which has a hydrophilic moiety that binds with the water molecules and a hydrophobic moiety which creates an ionic surface to the water droplets. Thus, the chemistry of the bulk liquid behaves much like the ingredient fuel oil. The emulsion usually has less than 2% free water, just like the ingredient diesel and conductivity similar to the ingredient fuel oil.
The nanoemulsion in this project created by advanced laser mixing technology has such small water droplet size that the emulsion is stable without a surfactant (and is stable over long periods as discussed in the following sections and referenced material.
The inclusion of water in the fuel oil has two important benefits:
Reduction of the combustion temperature, which dramatically reduces production of NOX.
Providing additional stoichiometric Hydrogen and Oxygen in the combustion and exhaust vapor to scavenge nitrogen, sulfur and carbon freed in the combustion, reducing production of NOx, SOx and soot.
Stability
Conventional emulsion fuels have two serious disadvantages:
The thermal penalty of the latent heat of vaporization of the contained water, which reduces the power output per volume of the emulsion fuel, and The large droplet size of the emulsion leads to settling and agglomeration and dissolution of the emulsion into the original constituents, the emulsion is not stable.
These detriments result in reduced fuel economy and complex operating schemes which cause uneconomical operation.
These handicaps can be mitigated by dramatic reduction in droplet size. A number of schemes have been applied to this goal, for example: addition of noncombustible particles, including nanoparticles, to precipitate small droplet size; and agitation by ultrasound, to achieve very fine mixing. These have not been achieved on a production scale.
Stability is influenced by droplet size in two ways: settling and agglomeration.
The interface of immiscible liquids at the surface of droplets in suspension requires energy to create and is subject to surface tension. Depending on the chemistry and physical distribution, the surface tension can be overcome by adhesion, or thermodynamic contribution; permitting larger droplets, and destruction of the emulsion.
Settling rate is defined by Stokes Law, which defines the settling velocity for a very small sphere (droplet) in a viscous liquid, defines the settling of the disbursed droplets in the liquid volume:
v_s = 2/9 ((ρ_p- ρ_f))/η gr^2
where:
vs is the particles’ settling velocity (m/s) (vertically downwards if ρp > ρf, upwards if ρp < ρf ),
g is the gravitational acceleration (m/s2),
η is the viscosity of the fluid,
ρp is the mass density of the particles (kg/m3),
ρf is the mass density of the fluid (kg/m3), and
r is the droplet radius.
The proprietary microemulsion used for operational demonstration in Phase I has a settling rateover 1000 times slower than conventional emulsions, because the droplets (average 300 μm diameter) are 1/33 the size of conventional emulsions. In the proprietary microemulsion, chemical stability of the ionic solution is compatible with the slow settling rate, permitting stable storage for up to a year. However the nanoemulsion generated by advanced laser mixing technology demonstrated in Phase I and implemented in Phase II, produces 1.2μm diameter droplets, thus has a settling rate about 1,000,000 times slower, making the pure water/oil emulsion viable at 14 months with 7.7μm diameter droplets.
The very small droplet size in the microemulsionor nanoemulsion is critically important to its combustion performance. This is because the small droplet size increased the surface area of the fuel – Oxygen interface. The fuel nozzle or injector emits fuel, emulsion fuel or microemulsion droplets of about 100 µm diameter. The droplet size, number and surface are compared in Figure 2 for a typical fuel injector droplet of fuel oil, the proprietary microemulsion and the nanoemulsion. The very small microemulsion droplets yield about 200 times the surface area of a fuel droplet and reduce the oil film thickness compared to conventional emulsion fuel. The nanoemulsion droplet size is dramatically reduced, increasing the reaction surface by a factor of 25,000 that of conventional fuel initially and about 4000 times after 1.2 years settling.. The film thickness for nanoemulsion is only about 30 dieselmolecule diameters at 1.2 years settling time and about 5 diameters from creation to over 8 months.
Combustion performance is not expected to be linear with the increased surface area. The surface area for reaction is increased by the differences in boiling point of the water and the fuel oil:The water vaporizes at 100 ˚C, expanding to about 1700 times its liquid volume, while the oil vaporizes and ignites at 300 ˚C. Thus the water droplets burst to vapor in the combustion environment, and further disburse the combustible fuel. The Phase II project will analyze the correlation to film thickness
Figure 4. Fuel droplet and reaction surface area for diesel, micro- and nanoemulsion.
Performance
Emulsion fuels have been tested in numerous and diverse projects for nearly three decades. A vast majority of experiments have demonstrated reduced noxious emissions. However, developmental implementations suffered from reduced engine performance and detriments of emulsion fuel instability. The microemulsion fuel for this project has been independently demonstrated to sustain operating performance without compromise, and achieve targeted efficiency and noxious emissions performance in operational environments. The proposed nanoemulsion has been demonstrated in the laboratory to significantly outperform the benchmark microemulsion.
A client for the microemulsion fuel,Brisbane Municipality, documented comparison of microemulsion performance compared to standard diesel for municipal vehicles driven by municipal operators and tested by the independent motor vehicle department. The microemulsion vehicles were not modified, and are interchangeable with the conventional fuel vehicles. These demonstrate that microemulsion delivers direct reduction in emissions:
- SOx– decreased to between 10 – 15 PPM overall
- Heavy Metals – decreased tobetween 10 – 15 PPM overall
- NOx – reduced by over 80% for legacy engines
- CO – reduced by over 80%
- CO2–increasedbetween 2% to 4% by volume. (This is a product of increased burn efficiency, reflecting the power extracted by volume relative to the exhaust.)
- C and Particulate Matter – reduced by 90%
