Hawaiian Electric advances the use of biodiesel as a gas-turbine fuel

HECO’s Campbell gas turbine is behind the parking lot now equipped with PV panels that both shade employee vehicles and produce 128 kW for internal use

HECO’s Campbell gas turbine is behind the parking lot now equipped with PV panels that both shade employee vehicles and produce 128 kW for internal use

State-mandated renewable portfolio standards are designed to shift the production of electricity from fossil fuels and nuclear power to wind, solar, biomass, and other green alternatives. The standards differ among the three-dozen states that have established RPSs with regard to the percentage of kilowatt-hours that must be produced from renewables by a given year. Hawaii has the most challenging rules in the nation, requiring its utilities to produce 40% of their net electricity sales from renewable resources by the end of 2030 (25% by the end of 2020).

Hawaiian Electric Co (HECO), the primary electric supplier to homes and businesses on the island of Oahu, is investing heavily in the development of wind and solar generation to meet this requirement. But of greater importance to owner/operators of gas turbines, the utility is believed to be the global leader in the use of biodiesel, burning B100 (100% biodiesel) in a nominal 110-MW, oil-only, simple-cycle peaking W501D5A at its Campbell Industrial Park Generating Station (CIP) to back up intermittent renewables.

The industrial park also is home to the utility’s Barbers Point tank farm and adjacent to the Kalaeloa Cogeneration Plant and the HPower waste-to-energy facility, both independent power producers that sell electricity to HECO.

The production and combustion of liquid and gaseous fuels from biomass, refuse, coal, and other solids is not new. In fact, diesel engines had operated on vegetable oils long before the first gas turbine (GT) appeared. According to a report on Wikipedia, August 10 is the 120th anniversary of Rudolf Diesel’s prime model, a single-cylinder engine with a flywheel at its base, debuting and running on peanut oil.

Importantly, all biofuels are not equal in the eyes of environmentalists. HECO’s work in the field began more than five years ago, Cecily Barnes, the utility’s fuels manager told the editors. Some early tests involved the use of palm oil, which is a suitable fuel, but the utility encountered environmental concerns surrounding its use because of the deforestation associated with at least some palm plantations. So, the utility partnered with the Natural Resources Defense Council to develop environmental guidelines for the sustainable production and use of biofuels.

Renewable Energy Group Inc, Ames, Iowa, recently signed its third contract with HECO to supply Campbell Industrial Park between 3- and 7-million gal/yr of high-quality biodiesel processed from used cooking oil (known as yellow grease) and waste animal fats. The contract runs from mid-2012 to mid-2015. REG says its North American processing facilities are capable of producing more than 200-million gal/yr of biodiesel.

There is no indigenous natural gas in Hawaii and HECO has relied heavily on fuel oil for power generation, making the state’s residential electricity the most expensive in the nation. Three quarters of Oahu’s electricity was produced from the combustion of fuel oil in 2012. The premium the company pays for biodiesel shipped from the Lower 48 to Hawaii is partially offset by a tax incentive of up to $1/gal awarded by the IRS to certified blenders of petroleum diesel and biodiesel. The maximum incentive is earned for a mixture containing 99.9% biodiesel and 0.1% petroleum diesel, which is what HECO burns. The fuel is transported to Hawaii by ship in 6340-gal ISO tank containers supplied by Agmark Logistics Inc.  

The black-start Campbell D5A, equipped with a water-injected DF42 combustion system, was purchased “new” on the grey market. When HECO bought the unit it had not committed to burning biodiesel, but Siemens warranted the engine for operation on that fuel. The OEM modified the gas turbine before shipment to Hawaii, to minimize coking of biodiesel on fuel nozzles during shutdown.

Siemens and HECO collaborated on the testing of biodiesel prior to installation of the D5A to assure operational success. Tests were conducted in October 2008 at ENEL’s respected Sesta Laboratory under the OEM’s direction. The test rig was arranged in a DF42 configuration and emissions targets were achieved with water injection when firing B100 fuel up to full-power conditions.

One of the findings of the Sesta tests, later confirmed in the field, was that biodiesel cokes at a lower temperature than petroleum diesel. This suggested implementing two changes to the engine before first operation:

• Make the candlesticks and their tips removable. The D5A has one candlestick in each of its 14 combustors.

• Install an effective system to purge fuel from the nozzles on shutdown. The purge system installed is not 100% effective and nozzles do experience coking, which gradually increases based on starts. Both NOx and CO levels remain steady until the amount of coking hits a tipping point. Eventually, the candlesticks must be cleaned in an ultrasonic sink; this requires a day-long outage. HECO engineers are exploring alternatives to the ineffective water purge—possibly a long air purge.

Campbell’s new peaker was commissioned on No. 2 petroleum distillate and met all of Siemens’ performance guarantees, which were based on that fuel. Operation on biodiesel began before the end of 2009 and the unit only has burned the alternative fuel since. Generally speaking, said Robert Isler, PE, manager of generation project development, biodiesel is a “drop-in” fuel for this engine.

Isler added that he would not be overly concerned about the feedstock from a technical standpoint, provided the fuel complies with ASTM D6751, “Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels,” as well as the OEM’s requirement that the amount of sodium and potassium contained in the fuel not exceed 0.5 ppm. Sodium + potassium content bears watching, at least in HECO’s case, because those elements are used in the reaction process to make its biodiesel.

Cloud point is another parameter of concern, but not in Hawaii. Recall that the cloud point of a fluid is the temperature at which dissolved solids (wax, in particular) are no longer completely soluble, giving the fluid a cloudy appearance. This metric indicates the tendency of oil to plug filters or small orifices at cold operating temperatures.

The Campbell site is relatively simple in arrangement. The generating station includes the 501D5A with a 210-ft exhaust stack, three-story control building, water-treatment building, two 1.8-million-gal fuel tanks, and four water tanks. Non-potable recycled water is used for all plant operations. Project also included an additional 138-kV transmission line of about two miles in length to deliver power from new and existing units to the grid more reliably. Recently, a 128-kW photovoltaic panel system was installed to both shade the parking lot and produce power for internal use (photo). The system includes two chargers for electric vehicles.

Biodiesel is transferred from the ISO shipping containers to the storage tanks as delivery trucks arrive at the site. This relatively simple operation typically takes less than about an hour per container: Truck enters the plant, lines up the transfer system, pumps out the container, and exits. Isler said the composition of the biodiesel is relatively consistent, even when switching from one feedstock to another.

Stored biodiesel remains in good condition, he added. While published information suggests this fuel should not be in stagnant storage for more than six months, HECO has not experienced any problems associated with fuel degradation at Campbell. The plant always is mixing new fuel with old, so some fuel in the tanks is really old by oil standards.

No fuel recirculation system is installed, Isler continued. But the tanks, designed for naphtha and ethanol, have floating roofs and they limit the amount of oxygen that comes in contact with the biofuel. The tanks were cleaned when the unit was switched for petroleum diesel to biodiesel, but not since. The fuel forwarding skid has pumps and filters (no heater); a second set of dual filters protects the primary fuel pumps.

There is only one liquid fuel system, but it can be used to burn both biodiesel and No. 2, as demonstrated during the switch from petroleum distillate to biodiesel following commissioning activities. The two fuels could be blended as well, in any combination, and that mixture burned. However, any fuel change requires, at a minimum, re-tuning of the combustion system and adjustment of water flow for NOx control. Note that about 10% less water is required for biodiesel than for petroleum distillate to meet the plant’s 42-ppm NOx limit. Also, the practical maximum output is lower when firing biodiesel than conventional No. 2.

The 501D5A typically operates as spinning reserve at minimum load (40 to 50 MW) to protect against a trip of the largest unit in service. This is the most cost-effective way to operate the system, but not the unit. Fuel consumption at minimum load is about one ISO container an hour.

In peaking service, Campbell averages about 100 starts annually and operates a total of about 500 hours. It’s important to check combustion dynamics at full load when burning biodiesel.

Isler said the use of biodiesel has been a positive experience. Compared to alternative biofuels, such as ethanol, it has a higher heat content and is less volatile. This means more power can be produced by a given engine, biodiesel is safer to handle than ethanol, lubricity is better, etc. Results of a preliminary inspection prior to the first combustion inspection were satisfactory, as expected. Opacity is a fact of life during starts and the regulations reflect that need. Initiation of water injection and a clear stack must be accomplished during the allowed startup period of one hour.

Others are adding to the biofuels experience base, although not in the Lower 48 at this time because of the very favorable gas prices. Here are briefs on their experience:

Kalealoa Cogeneration Plant, Campbell’s neighbor, which burns low-sulfur fuel oil from an adjacent refinery, conducted a test of about four hours on one of its two gas turbines, burning roughly 17,000 gal of palm biodiesel in place of the LSFO. Automatic changeover from one fuel to the other worked flawlessly. Results: “Overall, the biodiesel test burn. . .successfully demonstrated the plant’s ability to burn biodiesel as an alternative fuel. The units produced about 1% more power with 0.4% increased efficiency (compared to burning LSFO).

“Preliminary data suggest that increased fuel flow due to the lower heating value of biodiesel contributed to the increase in the output of the unit. . . .Overall pulsations level did not change with biodiesel. NOx, SOx, and particulate emissions significantly decreased. It is believed that the sulfur content in the residual diesel fuel left in the storage tank contributed to the SOx emissions. CO emissions did not show any significant change.”

• In a paper at the 2010 triennial World Energy Congress in Montreal, three GE experts had this to say about the challenges presented by the use of biofuels in aeroderivative gas turbines: “All of the current liquid biofuels used or being considered for the GE aeroderivative product line exhibit a lower heating value less than standard diesel No. 2; thereby requiring a step increase in fuel flow of from 10% to 50% to deliver the same power output.

“Other characteristics of liquid biofuels present challenges in terms of fuel handling that are not an issue for standard diesel No. 2—such as low lubricity, affinity for water, and being an effective solvent. As a result, in some instances the material composition of sealants and gaskets and valve types in the fuel system need to be altered to be compatible with the fuel.”

 

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