Nitrogen oxides (NO_x) are known to be immediately dangerous to human and environmental health. Mobile and stationary diesel and gasoline engines are contributing largely to the worldwide NO_x emissions. Emulsification of the fuel with water is a way to reduce the NO_x emissions of engines. The ultrasonic emulsification is an effective means for generating fine-size fuel/water-emulsions.

Cars and trucks, aircraft, electric generators, forklifts, air conditioning units and boilers generate large amounts of particulate matter (PM) and NO_x by the combustion of petroleum products. NO_x refers to mixtures of nitric oxide (NO) and nitric dioxide (NO₂) as well as N₂O, NO₃, N₂O₄ and N₂O₅. Nitric oxide and nitric dioxide contribute to low-level ozone, smog and are hazardous to the environment and humans. Environmental regulation addresses the air pollutant emissions by tightening limits. Engine emissions do also include sulfur dioxide (SO₂) as a result of sulfur compounds in the fuel. This problem is reduced by hydrodesulfurization or ultrasonically assisted desulfurization.

Running on Fuel/Water Emulsion

Within the recent years, much work has been done on the influence of water on NO_x emission levels. Various fuel:water volumetric ratios from 1:1 to 19:1 have been tested for combustion properties. In most cases, 1 to 2 volume percent surfactant was added for emulsion stabilization.

Background on Combustion

The combustion of fuel generates thermal and mechanical energy. The mechanical fraction can be used to drive pistons or turbines for propulsion or electricity generation. In most engines, the thermal energy is not used. This results in a lower thermodynamic efficiency.

Approx. 90% of the NO_x resulting from the fuel combustion process is NO. The NO is primarily formed by the oxidation of atmospheric nitrogen (N₂). Water added to the fuel lowers the combustion temperature due to water evaporation. When the water in the fuel-water emulsion evaporates, the surrounding fuel is vaporized, too. This increases the surface area of the fuel. The lower temperature and the better fuel distribution are leading to a lower formation of NO_x.

Ultrasonic Emulsification

Introducing water into the fuel combustion has been shown in many works to lower the NO_x emissions. The water can be added by forming a fuel/water emulsion in two ways:

• unstabilized: inline emulsification of water into the fuel prior to injection
• stabilized: manufacture of a stable fuel/water emulsion to be used as a drop-in fuel alternative

Canfield (1999) summarizes the NO_x reduction by the use of water and other additives:

• unstabilized emulsion
  • water added vol%: 10 to 80%
  • NO_x reduction by: 4 to 60%
• stabilized emulsion
  • water added vol%: 25 to 50%
  • NO_x reduction by: 22 to 83%

Emulsion

An emulsion is a mixture of generally immiscible liquids (phases), such as oil and water. During the process of emulsification, the disperse phase (e.g. water) is introduced into the liquid phase (e.g. oil). By the application of high shear, the particle size (= droplet size) of the disperse phase is reduced. The smaller the particle size, the more stable is the generated emulsion. Additional stability can be achieved by the introduction of surfactants or stabilizers. Click at the graphic above to see sample results for the ultrasonic emulsification of 10% water in motor oil (Velocite 3, Mobil Oil, Hamburg Germany). This study was conducted by Behrend and Schubert (2000).

Ultrasound

When sonicating liquids at high intensities, the sound waves that propagate into the liquid media result in alternating high-pressure (compression) and low-pressure (rarefaction) cycles, with rates depending on the frequency. During the low-pressure cycle, high-intensity ultrasonic waves create small vacuum bubbles or voids in the liquid. When the bubbles attain a volume at which they can no longer absorb energy, they collapse violently during a high-pressure cycle. This phenomenon is termed cavitation. During the implosion very high temperatures (approx. 5,000K) and pressures (approx. 2,000atm) are reached locally. The implosion of the cavitation bubble also results in liquid jets of up to 280m/s velocity.

Ultrasound has been proven to generate very homogenous emulsions of water in oil (w/o) and oil in water (o/w) by the high cavitational shear. As the parameters of ultrasonication are well controllable, the particle size and distribution is well adjustable and repeatable. Typically, the ultrasound is applied in a flow-cell reactor. Therefore, the emulsion can be made continuously in-line. For this reason, ultrasonication can be used for the making of stabilized and unstabilized emulsions.

The table below shows general processing capacities for various ultrasonic power levels.

Ultrasonic Degassing and Defoaming

Ultrasound does also help to reduce the amount of air bubbles in the emulsion mixture. The picture to the right shows the effect (5sec. progress images from left to right) of ultrasonication on the bubble content. As variations in the bubble content cause fluctuations in the injection timing, a degassing, deaeration and defoaming by ultrasonication improves the engine performance.

Ultrasonic Process Equipment

Hielscher is the leading supplier of high capacity ultrasonic devices, worldwide. As Hielscher makes ultrasonic processors of up to 16kW power per single device, there is no limit in plant size or processing capacity. Clusters of several 16kW systems are being used the manufacture of large volumes of drop-in fuels. Industrial fuel processing does not need much ultrasonic energy. The actual energy requirement can be determined using a 1kW ultrasonic processor in bench-top scale. All results from such bench-top trials can be scaled up easily.

Costs of Ultrasonication

Ultrasonication is an effective processing technology. Ultrasonic processing costs result mainly from the investment
for ultrasonic devices, utility costs and maintenance. The outstanding energy efficiency (see chart) of Hielscher ultrasonic devices helps to reduce the utility costs.

Literature

Behrend, O., Schubert, H. (2000): Influence of continuous phase viscosity on emulsification by ultrasound, in: Ultrasonics Sonochemistry 7 (2000) 77-85.

Canfield, A., C. (1999): Effects of Diesel-Water Emulsion Combustion on Diesel Engine NO_x Emissions, in: Master Thesis presented to the graduate school of the University of Florida, 1999.