Tri-Fuel Cogeneration System Mixes Digester, Landfill and Natural Gas

An award-winning design-build project in Miami-Dade County fuels engine-generators with a mix of digester, landfill and natural gas.
Tri-Fuel Cogeneration System Mixes Digester, Landfill and Natural Gas
The gas conditioning system includes iron sponges and chillers to remove moisture and cool the gases before they enter the cogeneration units. Those engines can use biogas, natural gas, and landfill gas in any combination.

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It’s one thing to expand a cogeneration system. It’s another to do it on a fast track without taking the existing system out of service or disrupting operations.

The Miami-Dade Water and Sewer Department did just that at its 112.5 mgd South District Wastewater Treatment Plant in 2015. It also expanded its fuel supply by adding the capacity to burn methane from a neighboring landfill in a variable combination with the traditional digester gas and natural gas.

The increased generation capacity will save about $900,000 a year for the department, the sixth-largest wastewater utility in the United States. For tapping renewable energy resources, the project received $1.5 million in assistance from the Energy Efficiency and Conservation Program of the U.S. Department of Energy.

It took about 16 months for the $25 million project to go into operation, according to Bob Ortiz, vice president of the Brown and Caldwell engineering and construction firm. His company teamed with general contractor Poole & Kent on the design-build project.

Fuel choices

Ortiz says designing a system that can burn three fuels was one of the biggest challenges. “They had some excess landfill gas available, so that led to the idea of trying to recover it,” he says. “Blending is a challenge, and each source has a different heating value. In addition, the cogen engines were required to meet defined mechanical efficiency and air emissions standards for any given feedstock.”

The old system had three 0.9 MW engines (total capacity of 2.7 MW). The new 8 MW combined heat and power system uses four 2.0 MW Cummins C2000 N6C engine-generators. Despite the three fuel sources, the system requires only one gas-cleanup system. Digester gas accounts for most of the fuel burned by the system.

“Typically, natural gas is used to sweeten the digester gas as needed to produce more energy,” says Ortiz. “The plan was to start with about 200 cfm of landfill gas and slowly increase that to as much as 800 cfm. The quality of the landfill gas varies, so you have to be careful with blending it.”

Methane content is also different: Landfill gas typically contains about 45 percent methane, while digester gas has a methane content of about 60 percent.

Tricky logistics

The new system increases electrical efficiency from 33 percent to 37 percent at full load and meets the nitrogen oxide (NOx) emission standard of 1.0 gram per brake horsepower-hour. The expanded system can provide up to 90 percent of the treatment plant’s power demand and reduce grid-purchased electricity by about 30 percent.

To maintain power generation and digester heating without interruption, the engines had to be installed without disrupting the existing cogeneration system, installed in the early 1990s. The new engines also had to fit into a tight footprint. In a discussion of lessons learned after completion of the project, the participants considered that the biggest accomplishment, according to Ortiz.

Two of the new engine-generators and related systems were installed and put into service before the old engines could be removed. “It was pretty tight,” says Ortiz. “They were jacked up, put on rollers, and pulled out one at a time.”

Poole & Kent had anticipated that contingency when it built the South District plant in 1991. “They thought ahead, and the building was designed to do that,” says Ortiz.

The final two new engines were then installed and put into service before the final old generator could be removed. The design includes room for a fifth generator for future expansion.

The 5 kV switchgear did require a new building, constructed next to the treatment plant. The new cogeneration facility was completed in June 2015.

Careful coordination

Ortiz says close collaboration between the Miami-Dade Water and Sewer Department and the design-build team made it possible to complete a difficult job on schedule and on budget. “A lot of times, owners are at an arm’s length,” he says. “Working very closely with the design-build team really helped and avoided a lot of those little things that can eventually cause problems. Meetings were held every week to talk about outstanding issues, problems and action items.”

Cooperation among all parties also allowed the permitting to be completed in short time. Permit documents were developed in just three months, allowing the contractor to start work quickly.

That teamwork and other attributes earned the project the Best Overall Award in the water/wastewater category from the Florida Region of the Design-Build Institute of America. In addition, the Association of Metropolitan Water Agencies gave the project its Sustainable Water Utility Management Award.

The South District facility is one of three plants Miami-Dade County uses to treat wastewater from its 350,000 retail sewer customers. It serves the unincorporated areas of the county and provides wholesale service to 12 municipalities and the Homestead Air Reserve Base. All of the treatment plant’s effluent is reclaimed through deep-well injection into the underlying aquifer.

With experience from the project in hand, Ortiz says Brown and Caldwell hopes to have another opportunity with Miami-Dade on a similar project at the county’s other wastewater treatment plant, which may also be able to tap a source of landfill gas.



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