Discussion of Water-in-Oil Emulsion Combustion
Water-in-oil emulsions have existed since the turn of the century. However, only recently have companies such as Petroferm been able to control the stability and quality of the emulsion to make it advantageous for industrial and power-generating uses. Petroferm's water-in-oil emulsion system and products are now used in a variety of boilers and furnaces along the East Coast. The emulsions are engineered to provide reduced carbon particulate, lower opacity and lower nitrogen oxide levels. In many cases, increased thermal efficiency, as well as reduced fireside fouling result. By understanding how this technology works the advantages of PEP-99 are exposed.
Petroferm's PEP-99 Emulsion
PEP-99 emulsion is a water-in-oil emulsion fuel consisting of from 5 percent to 10 percent water dispersed as droplets in a continuous oil phase. The key to achieving maximum combustion with this fuel is producing water droplets in the range of 5 microns to 20 microns in diameter within the oil. This is accomplished by introducing a small amount of PEP-99 surfactant to control water droplet size and prevent coalescence. Proper mixing and proportioning of the water, oil and PEP-99 surfactant creates a very stable emulsion that is ready to burn. Figure I compares the size of a typical fuel oil droplet sprayed into the combustion chamber with a PEP-99 emulsified fuel droplet containing many water droplets of 5 microns to 20 microns in size.
FIGURE 1: Comparison of Emulsified Fuel and Residual Fuel Oil
A typical burner atomizer produces a spray of fuel oil droplets around 100 microns to 200 microns in diameter, depending on fuel quality and atomizer design. Typically, the larger fuel droplets do not completely burn, leaving unburned carbon to collect on heat transfer surfaces and escape as particulate matter in the exhaust gases. This reduces overall thermal efficiency.
FIGURE 2: Typical Fuel Oil Combustion Flame
Benefits of Secondary Atomization
In the combustion of a water-in-oil emulsion, the primary spray fuel droplets are further divided as a result of the explosive vaporization caused by rapid heating of the water dispersed within the individual fuel droplets. The internal water droplets undergo spontaneous nucleation of steam bubbles at a temperature well above 212 F, causing a violent conversion of the water droplet to steam. The vaporization, in turn, produces a rapid expansion of the surrounding oil droplets, fragmenting the oil into a vast number of smaller fuel droplets. The name for this process is secondary atomization. A National Science Foundation (NSF) Study at Adelphi University verified this phenomenon with high speed cinematography analysis. The pictures show that droplets of emulsified fuel, 200 microns in size, fragmented into a multitude of 1 to 10 micron-sized fuel droplets.
In order for secondary atomization to be most effective in a combustion process, very small droplets with a well-controlled size distribution are necessary. If the number of water droplets is too small (1 micron or less), insufficient energy will be produced to cause secondary atomization. On the other hand, larger droplets (10 microns or larger) reduce the number of droplets for explosion and tend to produce less violent explosions within the oil droplets because of nucleation taking place at lower temperatures.
Petroferm's PEP-99 emulsion fuel was developed to meet these important requirements. The product is recognized for its ability to produce properly sized and uniformly distributed water-in-oil droplets with unusual stability.
How Secondary Atomization Works in a Boiler
FIGURE 3: Effect of Secondary Atomization on a Flame Due to Increased Surface Area
The previous illustration shows the effect of secondary atomization in a boiler. This process greatly increases the number and surface area of the fuel droplets in the flame zone. Since the combustion of fuel is a surface reaction, the greater the surface area, the less time it takes to burn out the carbon. This results in overall shorter flame length which reduces the possibility of flame impinging on the back wall of the boiler. This shorter flame length most likely creates the condition favorable to reduced fireside fouling. The vanadium/sodium/magnesium complexes formed in the combustion process go through a "tacky" state as they cool. If they are at or above this tacky state temperature, the complexes will adhere to boiler surfaces upon impingement. Shorter flame length allows for a radiant "cool down" period prior to impingement on boiler surfaces, and therefore, theoretically, less adherence. Improved atomization creates smaller particle size complexes which in turn improves the radiant cooling capability.
Over the past 15 years, emulsified fuel oil has been shown to significantly reduce unburned carbon, shown in Table I. In one test, Petroferm's PEP-99 emulsion system was measured against No. 6 fuel oil and was shown to reduce total particulate by a remarkable 69 percent.
Another advantage of the secondary atomization produced by emulsified fuel is a reduction in the air required for combustion because of more thorough mixing of the fragmented fuel droplets and combustion air. Reduced excess air reduces the conversion of fuel sulfur to S03.
Reduction in S03 conversion also reduces low temperature corrosion and inhibits the formation of acid mist. Furthermore, decreased excess air inhibits the oxidation of vanadium pentoxide, which reduces corrosion, and the deposition of vanadium complexes on the fireside of the boiler. Flame length and lower excess air should be the key contributor leading to lower fireside fouling.
Other potential benefits of emulsified fuel are:
In summary, emulsified fuels solve a major problem confronting utilities today--the need to reduce unburned carbon and 02 to produce more complete combustion minimizing particulate emissions and fireside fouling.
Petroferms Static Emulsion System
Petroferm employs the most reliable, efficient and cost-effective emulsifier on the market today. The state-of-the-art equipment is designed to produce emulsions at a constant flow rate. Constant rate emulsification provides a much more uniform emulsion fuel with the control of water content and droplet size required to obtain the maximum effect of secondary atomization.
The machine is a low pressure drop, dual stage mixer comprised of a premixing chamber and a static mixing chamber. As the fuel flows through the system at a constant rate and pressure, water is sprayed into the fuel flow at a pressure of 40 psig above that of the fuel. This initial premixing stage serves to atomize and disperse the water into the fuel oil. The premixed liquid then enters the static mixing chamber where shear forces produce additional refinement and controlled water droplet size as the fuel flows between the plates of the static mixer. Flow velocity, residence time, and dispersed phase fraction all contribute to the quality of the emulsion.
The surfactant is introduced into the upstream side of the water pump by a variable speed, D.C. motor, positive displacement metering pump. The surfactant and water are mixed in the centrifugal water pump and fed to the premixing chamber at a controlled constant rate.
Summary of Potential Benefits Using PEP-99 Emulsion Fuel
Each of these benefits has potential economic savings. However, a specific cost savings picture cannot be calculated without thorough research and analysis of equipment. Cost benefits are proportional to the severity of problems found in the existing equipment.
Petroferms Resources And Personnel
Petroferm Inc. is a private, research-oriented specialty chemical company with locations in Atlanta (GA), Skokie (IL) and Fernandina Beach (FL). The personnel are highly skilled scientists, engineers and technical marketing specialists with long experience in the preparation and combustion of emulsified fuels. This strong background has led to numerous patents in the development of fuels, burners, combustion equipment and fuel systems.
Petroferm provides technology, specialty chemicals and technical services in the following areas:
Petroferm possesses in-house engineering expertise in the development and design of processes and process equipment for the production of a variety of emulsions, including emulsified fuels made from heavy crude oils and refinery by-products, as well as emulsified residual fuel oil. The company has developed technologies and equipment for the emulsification of many materials that had not been previously emulsified. This experience and expertise enables Petroferm to develop low-cost, low shear process equipment for producing water-in-oil emulsions at a high level of reliability and control.
Experience with Electric Power Generating Utilities
Petroferm has experience with public utilities, ranging from the development of emulsified fuels, through the design of process equipment and operating installations, to the servicing of operating units. The following are a few of the highlights:
Petroferm' s research laboratory has modern equipment and instrumentation for chemical synthesis, analytical chemistry, and the development and evaluation of products and processes for petroleum applications. The research organization conducts extensive studies with hydrocarbons, including the control of interfacial behavior between oil and water. The group routinely selects combinations of surfactant molecules to achieve desired chemical and physical properties and optimum performance.
In addition to equipment and instrument systems normally found in modern research laboratories, the facility has a variety of specialized equipment, such as:
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