Sandwiched between an oil refinery and a coal-fired power plant in Campbell Industrial park sits Hawaii’s largest producer of electricity from renewable energy.

The often overlooked facility doesn’t have any solar panels or wind turbines. A steady stream of garbage trucks lining up to disgorge their loads reveals the facility’s fuel source: trash.

The city’s HPOWER waste-to-energy project converts more than half of Oahu’s municipal solid waste into electricity, reduces the pressure on the island’s crowded landfills and cuts the amount of imported fossil fuel needed to keep the lights on in the nation’s most oil-dependent state.

The plant took a step forward in 2012 when the city expanded the capacity and upgraded the equipment at HPOWER, making it one of the nation’s leaders in waste-to-energy.

HPOWER is a key piece of Honolulu’s "integrated solid waste program" designed to keep the island’s flow of trash under control, said Manny Lanuevo, chief of the city’s Refuse Division.

"HPOWER and our recycling program compliment each other to make sure we minimize, if not eliminate, things going to the landfill," he said.

The plant is run by a subsidiary of New Jersey-based Covanta Energy, which operates more than 40 waste-to-energy facilities around the world.

The biggest criticism of HPOWER is its emissions, which are regulated by the Environmental Protection Agency and the state Department of Health.

But the benefits of waste-to-energy far outweigh the downside of emissions from the plant’s stacks, said Covanta’s Gail Godenzi, business manager for the plant.

"The benefits are you reduce what goes to the landfill, you recover metal and you reduce the amount of greenhouse gasses by not producing methane, which is what is given off when municipal solid waste decomposes at a landfill," she said.

The carbon dioxide emitted by burning waste is less harmful to the environment than the methane that would have been produced otherwise, Godenzi said, adding that the plant has never been cited for an emissions violation since it began operating in 1990.

From the perspective of electricity utility ratepayers, Hawaiian Electric Co. is able to buy electricity produced by HPOWER at a price below what the utility pays for power generated from wind energy projects and by burning oil. And the fact that HPOWER produces consistent power gives it an advantage over wind and solar, which suffer from issues of intermittency.

HPOWER, which is recognized as a renewable energy source under the Hawaii Clean Energy Initiative, last year generated enough electricity to power 50,000 homes at an average price of 17 cents a kilowatt-hour. A revised power purchase agreement that went into effect in January 2013 resulted in a reduction from the average price of 21.5 cents a kilowatt-hour paid by HECO in 2012.

The HPOWER pricing compares favorably with the 20.8 cents a kilowatt-hour HECO pays for electricity from the Kawailoa Wind project on Oahu’s North Shore and the roughly 22 cents a kilowatt-hour it pays to generate electricity from low sulfur fuel oil at current prices. Planned utility-scale solar projects will produced power for an estimated 16 cents a kilowatt-hour.

Coal provides the cheapest power, but the state has prohibited additional coal plants because of concern over emissions. The AES Hawaii coal-fired plant across the street from HPOWER, sells power to HECO for 3.1 cents a kilowatt-hour.

"HPOWER provides firm, renewable electricity to the Oahu grid that is lower cost than oil-fired generation and makes a significant contribution to meeting our renewable energy goals," said Scott Seu, HECO’s vice president for energy resources and operations. "It has the added benefit for our community in keeping hundreds of thousands of tons of waste out of our landfills for use in providing a significant part of the essential electricity we need every day."

HECO pays the city from 6 to 16 cents a kilowatt-hour for HPOWER electricity depending on the time of day and amount of power provided, plus a 5 cents per kilowatt-hour "capacity payment" during the peak hours from 7 a.m. to 9 p.m. HECO could bring down the average price it pays to HPOWER if it bought more of the electricity during off-peak hours when the price is lower, Godenzi said.

The HPOWER expansion completed in 2012 boosted its net generating capacity to 73 megawatts from 46 mega-watts. By adding a third boiler and a second turbine, the facility was able to boost its maximum intake of municipal solid waste to 3,000 tons a day from 2,100.

But more than just adding capacity, the upgrade included a new "mass burn" boiler that allowed HPOWER to increase the range of waste it can accept to include bulky items, such as mattresses, furniture and carpets. Those items were previously hauled to the Waimanalo Gulch Sanitary Landfill because the original HPOWER facility could not handle them. The addition of the mass burn boiler has resulted in the diversion of about 200 tons of bulky items a day from the landfill.

The trash used in the boilers in the original HPOWER plant required extensive processing and separating on the front end before the material could be burned. The resulting "refuse-derived fuel" is very combustible, but the amount of processing required makes for a lower efficiency than what can be achieved with the new mass burn unit, Godenzi said.

Major appliances that contain refrigerant refrigerators, freezers and air conditioners are still handled separately by a specialty recycling company that removes the chemicals and electronics and sells the remaining steel for scrap.

A powerful shredder installed as part of the 2012 upgrade quickly turns the bulky items into small pieces that can be easily fed into the furnace, where they are incinerated at a temperature range of 1,800 to 2,000 degrees Fahrenheit. The heat is used to create steam, which turns a turbine to generate electricity.

When the trash is brought to the mass burn boiler, it is dumped on the floor of a warehouselike building and pushed into a pile by bulldozers and front-end loaders. Crews remove large pieces of steel and put them in a separate bin for transport to a recycling center.

Two crane operators sitting in a control room overlooking the floor use 7,500-pound grapples to grab giant-size fists of trash for processing. With the flick of their joy sticks, the operators pick up the bulky items and drop them into the shredder.

"You never know what you’re going to see," said Kawika Alama, one of the operators. He uses his grapple to drop wooden pallets, plastic lawn chairs and mattresses into the shredder, which are unrecognizable when they exit the device. The shredded items are mixed back in with the rest of the trash and dropped into the furnace by the grapple operators.

The operators also use the grapples to mix up the trash and give it an even consistency before it is burned.

"Mixing the fuel makes sure we have constant flow of steam through the turbines," said Jamison Zrust, a Covanta control room operator. "HECO wants a steady electricity supply."

Zrust also monitors an array of computer screens with information about levels of various emissions, including sulfur dioxide, nitrous oxide and carbon monoxide.

Any metal, dirt, glass and other noncombustible items that survive the furnace are recovered at the end of the process. The resulting ash, which is about 20 percent of the volume of the original waste, is hauled to the landfill. Any metal pieces are transported to a commercial recycler, Godenzi said.

Another advantage of the mass burn unit is that it can accommodate tires. The plant began burning small amounts of tires last fall, while closely monitoring any changes in the level of emissions. It is estimated that HPOWER can take as many as 65,000 tires a year without exceeding it emissions standards, she said.

The next category of waste in HPOWER’s sights is treated sewage sludge. The city plans to build a special handling facility at HPOWER to accept the sludge and feed it into the mass burn furnace, according to Godenzi. That would represent another 20,000 tons of material a year that could be diverted from Waimanalo Gulch, she said.