Covanta Energy and the City and County of Honolulu recently completed a $300 million expansion of a 20-year-old waste-to-energy (WTE) facility. The plant is now capable of processing up to 3,000 tons of municipal refuse daily, recycling all the metals, and generating up to 90 MW—enough to supply nearly 10% of Oahu’s electricity.
|Courtesy: Covanta Energy|
Oahu or “The Gathering Place” is the most populated of the islands in the State of Hawaii with just under one million permanent residents. Aptly named, the island hosts up to six million visitors each year who expect fun in the sun and a swim in clear ocean waters. One reason the island remains a tourist paradise—its pristine beauty—can be attributed to the solid waste, recycling, and energy management programs designed by the City and County of Honolulu.
High population density on an island of less than 600 square miles leaves little open space for new development, and setting aside space for a new landfill is out of the question, so the million tons of solid waste produced each year pose a significant concern. The island is also searching for new sources of electricity, because the State of Hawaii has no indigenous fossil fuel resources. Historically, electricity was generated by burning imported liquid fuels from the mainland and elsewhere. Today, there is a rapidly growing biofuels industry plus geothermal, photovoltaic, and wind energy plants offsetting imported liquid fuels. There’s also one of the largest and most flexible waste-to-energy (WTE) plants in the U.S. The Honolulu Program of Waste Energy Recovery, or H-POWER, located on a 28-acre site in Kapolei, is the cornerstone of the City and County of Honolulu’s long-term efforts to reduce fuel oil imports, significantly reduce the volume of landfilled materials, and recycle metals found in solid waste (Figure 1).
|1. Expanded facility. The new Boiler 3 is shown in the center of the photo. The existing refuse-derived fuel Boilers 1 and 2 are behind Boiler 3 in the enclosed building shown in the photo. The two steam turbines are housed in another enclosed building behind Boilers 1 and 2. The plant is capable of processing 3,000 tons per day of municipal solid waste, producing up to 90 MW, and recycling all of the entering ferrous and nonferrous metals. A time-lapse video of the construction can be viewed at www.youtube.com/watch?v=NdxKLu2c-Hg. Courtesy: Covanta Energy|
H-POWER entered service in 1990 and is owned by the City and County of Honolulu (which encompasses the entire island). It has been operated since 1993 by Covanta Energy Corp. On Oct. 9, 2012, the 900-ton-per-day expansion that pushed plant capability up to 90 MW was dedicated. According to City and County of Honolulu Refuse Division Chief Manny Lanuevo, the plant is now capable of processing 85% of Oahu’s post-recycled municipal solid waste and supplying up to10% of the island’s electricity needs (Figure 2).
|2. Outdoor plant. The structural steel in the center of the photo houses the new Boiler 3. Moving from the boiler to the right is the spray dryer absorber (SDA) scrubber, the fabric filter baghouse, and the stack. A continuous emissions monitoring house is located on top of the stack inlet ductwork. The enclosed Boilers 1 and 2 are located behind Boiler 3, and the steam turbine building is located in between. Courtesy: Covanta Energy|
The renewable power produced by the plant is sold to Hawaiian Electric Co. (HECO), offsetting its need to import about one million barrels of oil each year. HECO purchases electricity based upon a time-of-day rate linked to a price index plus a 5 cents per kWh capacity payment. Uniquely, the contract purchase price is lower than HECO’s current avoided cost, and HECO passes the purchased electricity through to its customers.
The original plant consists of two waterwall furnace boilers (Boilers 1 and 2) with reverse-traveling stoker grates that can process up to approximately 2,100 tons per day of refuse-derived fuel (RDF). The two boilers produce steam for a single 57-MW steam turbine (Turbine 1). Boilers 1 and 2 are fitted with a semi-dry flue gas scrubber with lime injection and a fabric filter baghouse.
The RDF is prepared by first preprocessing municipal solid waste through a series of conveyers and shredders for removal of any nonprocessible and nonburnable materials. The preprocessing system uses magnets to pull ferrous metals (tin cans) and uses eddy current separators to extract nonferrous metals (aluminum cans) from the waste stream for recycling. The volume of ash produced by the boilers is one-tenth of the volume of municipal waste volume combusted (Figure 3).
|3. Remote fuel handling. Waste entering the new section of the building is managed by a remote-controlled overhead bridge crane with an electro-hydraulic orange-peel grapple directed by manipulation of joysticks located in the new main control room. The fuel for Boilers 1 and 2 is pushed on the floor to the in-feed conveyors by loaders and bulldozers. Courtesy: Covanta Energy|
A continuous emissions monitoring system (CEMS) measures the stack gas for emissions, such as carbon monoxide, sulfur dioxide, nitrogen oxides, oxygen, and opacity. The CEMS also sends signals to the control room so that operators can continuously monitor flue gas quality and the performance of each boiler. The CEMS data is electronically saved, reviewed, and summarized into routine reports submitted to the Hawaii Department of Health. For the entire plant, exceedances are rare, and the plant is routinely recognized by the State of Hawaii for reliably meeting its emissions limits.
Burn What Comes
Covanta Energy received a contract from the City and County of Honolulu to design, build, and operate the expanded facility in mid-2009. Covanta hired Parsons Corp. as its general contractor in December 2009 for the fast-track project. Parsons was responsible for plant design and engineering, installation, and commissioning—all without interfering with operation of the existing facility. Burns & Roe provided process design services and technical support to Covanta for direct purchase of the plant’s major equipment.
The expansion project added a China-based Anhui Jinding Boiler Co. Ltd. mass-burn boiler (Boiler 3) and a Siemens 33-MW steam turbine (Turbine 2) that increased the plant’s solid waste processing capacity to about 3,000 tons per day and electricity capacity to 90 MW.
Additional equipment installed during the expansion project included a Martin GmbH reverse reciprocating grate system that is integrated with the boiler, a tipping/receiving building, and an AMEREX spray dryer absorber (SDA) and a fabric filter baghouse. The SDA uses a lime/water slurry mixture to neutralize and cool acid gases, such as sulfur dioxide and hydrogen chloride. The fabric filter baghouse removes particulate matter (fly ash) and provides a secondary acid gas neutralization surface on the filter cake. The stack of Boiler 3 is monitored with a CEMS like that used on Boilers 1 and 2. A Jervis B. Webb material-handling system collects the ash produced, ready for transportation to the landfill.
The addition of a mass-burn boiler adjacent to the two existing RDF-fueled boilers provides increased operating flexibility to the plant and is unique among the 40 municipal waste-fired plants that Covanta owns and/or operates. Instead of preprocessing solid wastes prior to burning, the mass-burn boiler burns anything and everything that is placed on the traveling grate. For example, a mattress or tire placed on the grate will burn to completion, and the steel that remains is recycled, not sent to the landfill. Covanta operators are now able to direct large waste items to the new Boiler 3 instead using landfill disposal, as in the past (Figure 4).
|4. Mass-burn boiler added. Boiler 3 is a mass-burn boiler, which means it will consume any combustible objects, even mattresses and tires. Any metal remaining after combustion is recycled. Courtesy: Covanta Energy|
Unique Operations Scheme
Increased operating flexibility is achieved by using a combined steam header system; that is, the steam from all three boilers is piped to a common header from which each of the two steam turbines receives 900 psig, 830F superheated steam. For example, Covanta expedited startup of Boiler 3, placing it in service before the steam turbine system, due to a new power purchase agreement delay. The steam produced by Boiler 3 was added to the common steam header to produce additional electricity from Turbine 1, as the original plant was boiler-limited. Excess steam produced by Boiler 3 was desuperheated and bypassed around the steam turbine directly to a bypass condenser.
Turbine 1 operates on steam header pressure control, but Turbine 2 operates on speed control when synchronized to the HECO grid, through a single electrical interconnection point. This requirement is peculiar for several reasons: the small size of the plant compared to all other HECO resources, the plant is typically operated as a baseload renewable energy resource, and the unique electrical interconnection requirement for HECO’s power distribution system.
There were a few growing pains with the large mass-burn boiler during startup, but the problems were resolved prior to commercial operation. For example, additional reciprocating grate temperature thermocouples and wider-angle air- and water-cooled cameras were added to resolve combustion problems experienced early in the Boiler 3 startup. The reciprocating grate system on Boiler 3 is the largest ever produced by Martin GmbH.
A Long-Term View
The City and County of Honolulu reached agreement with HECO in June 2012 for a revised and expanded 20-year power purchase agreement for electricity produced by the H-POWER plant. Earlier, Covanta had negotiated a new 20-year operating and maintenance agreement with the City and County of Honolulu that became effective in 2012.
H-POWER’s historical performance metrics certainly justified its investment in the expansion project. The numbers are the envy of any fossil fuel–fired power plant manager. According to Facility Manager Robert Webster, the plant operates 24/7 like any baseload utility plant with an average capacity factor in the range of 93% to 94%. The new addition (Boiler 3 and Turbine 2) is predicted to operate at 97.5%. The availability of the older plant averages around 90%, but the new equipment is expected to enable 94% availability. The value of the common steam header design is apparent.
More importantly, power generated by the expanded facility will offset the need to import about one million barrels of oil each year. According to Webster, during its 22 years of operation, H-POWER has already processed 13 million tons of waste, eliminating 15 million barrels of imported oil and saving 500 hundred acres of land that would otherwise be used for landfills. The plant has also recovered 450,000 tons of metals for recycling, about the weight of four aircraft carriers.
Only the problem of ash disposal remains for the City and County of Honolulu to resolve before closing the loop on its entire solid waste and recycling ecosystem. Ash produced from the three boilers currently goes to the local landfill. Covanta and the City have completed some pilot testing to show its viability for use as building product additives, much as the ash produced from coal-fired plants is used in road construction and brick products. Reuse of the ash as a construction material is governed by very strict regulations, even though the ash is “sterilized” and is inert, having been combusted at furnace temperatures near 2,000F. An environmental review of a proposed ash reuse plan presented by Covanta is ongoing, and a positive result is expected, particularly as historical ash toxicity tests have always been negative. The positive economic and environmental rewards make ash reuse a key business objective for both the City and County of Honolulu and Covanta.
H-POWER has been recognized often for its contributions to the business and technology of producing electricity from municipal waste. In 2007, H-POWER was designated as an Occupational Safety and Health Administration Voluntary Protection Program Star Facility, and the following year it was awarded first place for excellence in safety and health by the American Society of Safety Engineers and Hawaii Occupational Safety and Health. The facility was recognized as a Top 250 Hawaiian Business by Hawaii Business Magazine in 2011. Most recently, in late 2012, it received a “Facility of the Year” award from the American Society of Mechanical Engineers for its metals and energy recovery.
Mayor Peter Carlisle summed up the project well in his comments presented at the plant dedication (Figure 5). The project team built a plant that “ranks among the very best waste-to-energy conversion plants in the entire world. This is something we all should be proud of, it’s good for the environment, it’s good for the taxpayers.”
|5. Cutting of the ribbon. On Oct. 9, 2012, Mayor Peter Carlisle dedicated the new facility at the H-POWER plant. From left to right, Director of Environmental Services Tim Steinberger, former Mayor Mufi Hannemann, current Mayor Peter Carlisle, and Covanta Energy COO Seth Myones. Courtesy: State of Hawaii, Governor’s Office|
Ho‘omaika‘i ‘Ana (congratulations) to the City and County of Honolulu and to the staff of H-POWER for a job well done.
— Dr. Robert Peltier, PE is POWER’s editor-in-chief.