Three screw pumps successfully pump palm oil for power generation in Hawaii

Open a magazine or turn on the television today and renewable or green energy will likely be a topic of discussion. The typical subjects that make headlines are solar and wind power. However, biofuels are also garnering a lot of attention.

Ethanol added to gasoline for automobiles is a well-known biofuel that has been in the news. However, a wide range of other plant- and animal-derived fuels are available that can be blended with fuel oil in various concentrations to create a fuel that can be burned in standard oil-fired power plants.

To make biofuels economical, the liquid must be capable of burning in existing power plants without building expensive new facilities. The mixture varies based on the type of material that is mixed with the fuel oil and its affect on plant emissions. The sustainability of some fuels is in question due to environmental effects and the energy required to produce the feedstock. Also, the energy yields can be much lower than standard fuel oil.

However, as more utilities and states promise to lower their carbon footprints, biofuels will become an important option to burning straight fuel oil. To blend these biofuels into a usable mixture, advances in pump designs and materials may be required to handle these new fuels.

Palm Oil in Hawaii

Hawaii is a clear example of an area that is pushing for biofuel use. The state has committed to an aggressive target of generating 40 percent of its power for renewable resources by 2030, up from less than 10 percent currently. At this time, the majority of its power is generated by burning low-sulfur diesel oil. Solar, wind and geothermal will undoubtedly play a part in reaching its goal, but obviously a conversion of the existing fuel oil plants will also be required.

The local power company has embarked on a plan to test a biodiesel made from palm oil to meet its broader goal. Palm oil is a crude palm oil blended with palm stearin, and it is reported to have 3.5 times the energy yield of ethanol. This high energy density makes it a good candidate to be used in high concentrations with the existing fuel oil.

To test the usefulness of palm oil in a power plant, it is necessary to have a system with a pump to handle the existing fuel oil and a separate pump for the palm oil.
The two fuels are then blended downstream of the fuel/palm oil handling pumps in measured concentrations. This blended fuel is then fed into the plant and the resulting power output and emissions can be measured to determine the optimal blend, as well as the overall suitability of the product.

The Right Pump for the Job

Due to the requirements for different flow rates and pressures, three rotor screw pumps were selected for pumping the palm oil.

Three screw pumps have been used for years as liquid fuel injection pumps for gas turbines and steam boilers. These pumps are used quite frequently for fuel transfer and injection in power plants because their output can be controlled by simply changing the pump speed. In addition, their output is not affected by changes in system pressure.

Three screw pump designs for fuel oil are generally capable of flow rates from a few gallons per minute up to several thousand gallons per minute and differential pressures up to 1,700 psi. This pump type consists of three rotors, one power and two driven rotors. The power rotor (coupled to driver) performs the pumping work, while the idlers act to seal off the pumping chambers.

The power rotor drives the idler rotors by a rolling line contact. The pumped fluid creates a barrier between the rotors, preventing metal-to-metal contact of the rotating elements. The liquid film also supports the rotors in the liner, which acts like a large journal bearing, eliminating contact between the rotors and the liner. Direct rolling contact eliminates the need for timing gears, which are required in twin screw timed pumps.

Most three screw pump designs offer single internal (product lubricated) or external (grease or oil lubricated) bearing configurations. In a majority of fuel applications, the bearing is mounted external to the pumped fluid.

Since three screw pumps are hydrodynamically balanced, the bearing simply positions the rotor set within the liner, eliminating the need for thrust bearings or wear plates.

This balancing system also allows the mechanical seal to see only suction pressure, greatly simplifying the seal selection.

As the fuel oil enters a three screw pump, it fills the suction side of the screw set. As the screws turn, oil is conveyed from suction to discharge.

Theoretically, this positive displacement action moves a fixed volume from suction to discharge regardless of discharge pressure, and the delivered flow is only a function of the physical size of the chambers and the speed of the pump.

In practice, there will be some internal slip inside the pump, which will slightly reduce the delivered flow of the pump. The slip is generally minimal in normal fuel oil applications, but it does increase with increasing pressure and decreasing viscosity.

When pumping standard fuel oil, the materials of construction are easy to determine. However when pumping biofuels, particular attention must be paid to the type of product being pumped.

Even if the viscosity of the biofuel is identical to hydrocarbon fuel oil, the lubricity and corrosiveness may be vastly different. Lubricity is critical for a three screw pump because the pumped fluid supports the rotors in the liner. If the lubricity of the product is too low, metal-to-metal contact may occur between the moving parts, which can result in premature wear and possible pump failure.

To combat this, most manufacturers add a coating to the liner bores to enhance the lubricity. This coating may be a material such as Babbitt, bronze or a PTFE-graphite blend.

This liner, as well as the base materials of the pump, must withstand any corrosiveness of the biofuel. Depending on what makes up the feedstock, certain materials can quickly be attacked. Extensive research is currently underway to determine the suitability of different materials when handling these liquids. In most cases, carbon steel and stainless steel have been found to be suitable, while cast iron and non-ferrous materials must be closely examined in conjunction with the exact fuel that is being pumped. Flushing and cleaning is usually recommended when the equipment must sit idle.

The Results

Initial testing in Hawaii has shown promising results in both power output and emissions, even when burning 100 percent palm oil. They were able to meet full power output by burning about 10 percent more palm oil than would be required with standard fuel oil.

Also, emissions of nitrogen oxides and sulfur dioxides were lower (as were visible emissions) when burning palm oil. The testing also shows that three screw pumps can be successfully used to transport this biofuel using carbon steel materials and special liner coatings.

 As the pumps log additional operating hours, additional inspections will be required to verify the long-term suitability of the selected materials.
It appears that biofuels may play a central roll in meeting Hawaii's renewable energy goal.

It is also becoming obvious that as new biofuels are developed, the pump industry will need to continue to adapt its products to meet the challenges of an expanding renewable energy landscape.

Pumps & Systems, May 2011