As waste-to-energy (WTE) facilities across the country get older, maintenance and upgrades are required to keep them in tip-top shape. Wasatch Integrated Waste Management’s Davis Energy Recovery Facility will be 30 years old next year. But the mass-burn WTE facility is defying the aging process.
The Layton, Utah-based facility has figured out a way to improve its performance and provide a steady supply of steam to neighboring Hill Air Force Base.
Wasatch Integrated Waste Management District was formed in 1984 to provide solid waste disposal services to Davis and Morgan counties. Since 1992, the district has operated the Davis Energy Recovery Facility and the Davis Landfill.
The Davis Energy Recovery Facility, which opened in 1987, is a 420-ton-per-day mass-burn municipal waste combustor with two boiler units. When both units are online, the facility can provide 90 kilopounds per hour of 500-degree Fahrenheit steam to the Air Force base. The facility itself can also be powered from the energy it produces, and it sells about 200 kilowatt hours back to utility grid.
According to Nathan Rich, executive director of Wasatch Integrated Waste Management District, steam is the best use for the energy.
“Energy is worth a lot more as steam,” he explains. “Every time you convert a form of energy, you lose about half of it.”
With no renewable portfolio standards or renewable energy requirements in Utah, the only incentive for the utility to use the steam from the district is avoided cost, Rich explains. The district brings in about $80,000 per year for the electricity produced from the steam, while it makes about $5.1 million per year from the steam it sells to the Air Force.
FULL STEAM AHEAD
Wasatch is beginning the third year of a 10-year fixed-price contract with Hill Air Force Base for its steam. The steam is delivered to the base through a 6,000-foot-long underground pipeline that ties into the base’s main boiler building. The energy recovery facility is located just to the east of the base. The Air Force also has back-up steam-generating capability and operates its own steam loop on the base.
During colder months, the facility provides about 60 percent of the base’s peak demand and can ship as much as 60,000 kilopounds of steam per month, totaling between 450,000 and 500,000 kilopounds per year.
Between the steam from the facility and a landfill-gas-to-energy project at the Davis Landfill, the Hill Air Force Base has been able to exceed a 25 percent renewable energy goal set by the Air Force, says Rich.
Renewing the district’s contract with the Air Force base was not easy. A few one-year contract extensions were given, but it took two years of meetings to get a long-term contract. The contract helped give the district the security it needed to reinvest about $14 million in facility upgrades.
“It is always a struggle to contract with the federal government, and for a number of years we had been operating on one-year contracts,” Rich recalls. “We were to a point where we needed to make that major reinvestment in the facility, so through a series of basically two years of meetings with the Air Force, they agreed to give us that 10-year contract. That has allowed us to reinvest in the facility to ensure that it would be here for another 20 years for the Air Force base.”
OUT WITH THE OLD
Wasatch Integrated Waste Management District has made several improvements to the Davis Energy Recovery Facility over the last two years. Some major equipment replacement was needed due to the age of the facility. This included some of the original steel in the boiler section of the facility.
In addition to replacing old equipment and components, new front-end processing equipment was added in March with the goal of removing green waste from the municipal solid waste (MSW) feedstock.
Green waste contains a high level of moisture, which isn’t ideal for the efficiency of the combustion process. And despite curbside green waste collection in more than half of the communities in Davis and Morgan counties, grass and tree clippings still make up a high percentage of the waste entering the facility.
“We have this really particular thing in Utah,” Rich says, “When our Mormon pioneers settled in the Salt Lake Valley, they were instructed to make the desert bloom.”
More than 150 years later, residents are still turning desert into vegetation. Bags of trash entering the energy recover facility are often 40 to 50 percent grass, according to Rich, and that is a problem.
“It doesn’t burn well. It causes us real problems at the waste-to-energy facility,” he says. “It is wet and it causes excess nitrous oxide (N2O) emissions.
“Energy is worth a lot more as steam. Every time you convert a form of energy, you lose about half of it.” – Nathan RichTo make the situation better, Machinex, Plessisville, Quebec, completed a 50-ton-per-hour mixed waste processing system for the WTE facility to separate out the wet organic fraction from the incoming material. The equipment supplier says the equipment resulted in a weight reduction of up to 50 percent and better calorific value for the WTE facility.
Rich describes the system as the front end of a mixed-waste processing facility consisting of infeed conveyors, modest picking stations and 2-inch trommels. “It has really improved the quality of the fuel and changed the way the waste-to-energy facility has processed in some expected ways and in some unexpected ways.”
Rich knew operating the preprocessing equipment would mean additional effort and cost, but he didn’t anticipate just how much material would be diverted from the WTE facility combustion units. Before the equipment was installed, the WTE facility was only able to process about half of the residential waste the district received. The other half was landfilled. Now all residential waste can be processed the WTE facility.
And prior to the preprocessing step, Rich says, “When the pit got full at the waste-to-energy facility, we would divert the residential waste to the landfill because it was the waste with all the grass in it that wasn’t burning well.”
Now, he says, the converse is true. “When the WTE facility is reaching capacity, we start diverting commercial waste [to the landfill].”
Another change Rich has noticed is natural gas consumption. Prior to the preprocessing equipment, the facility would run a natural gas unit on the back of the burner when there was a problem with combustion.
Without the green waste in the feedstock, Rich says, “My natural gas consumption has dropped 85 percent.” The facility has not had to put a burner in a unit other than to bring it up from a cold status, he notes.
Like most WTE facilities, the Davis Energy Recovery Facility uses lime to control sulfur dioxide (SO2) emissions. Rich says lime consumption is down by about 40 percent.
Another difference is in the metals that are recovered from the combustion ash at the landfill. Recovery has dropped by about half, according to Rich. “The good news is not putting it through the energy recovery facility is not wearing out combustion grates.” But from the revenue and recycling side, less metals are being recovered.
The preprocessing has only been operating for a few months, but overall, Rich says operations have been smoother, and the facility is saving money on reagents and maintenance. He expects to have more data once more time with the system is logged.
The next step, Rich says, is going after more materials on the front end. “We are proposing to the board to add some additional materials recovery equipment and make it into a full blown mixed waste recycling facility.”
Additional equipment on the front end would be used to pull out ferrous and nonferrous metals, cardboard and Nos. 1 and 2 plastics.
Rich says he plans to go to the 19-member district board to make the case for more preprocessing. “I’m just telling the board, even if you have a curbside recycling program and you want to continue to run that, there are still recyclables in the waste, and we will process that for you.”
Rich says an additional $2 per month per household for the service should offset the cost of additional recovery equipment.
Rich says as a government entity, the district has a responsibility to its board and residents. “We provide our board with more information than they need and include them at every step of a project. We strive to provide services that our ratepayers will find valuable and facilities they can be proud of.”