Biogas Does More Than Power Cogeneration Systems. Now It Powers Vehicles.

A Nebraska city replaces a combined heat and power facility with a gas scrubbing system that yields renewable fuel for vehicles.

Biogas Does More Than Power Cogeneration Systems. Now It Powers Vehicles.

Digesters at the Theresa Street Water Resource Recovery Facility display the Lincoln Renew logo.

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When it came time to replace the biogas-fueled combined heat and power system at the Theresa Street Water Resource Recovery Facility in Nebraska, one number stood out: the price of utility electricity.

“Right now, power in the Midwest, certainly here, is fairly reasonable,” says Steve Crisler, superintendent of water resource recovery facilities for Lincoln Wastewater System. “It’s 5 cents per kilowatt-hour. To spend the capital, operating and maintenance costs to offset 5-cent power was a challenge.”

But the CHP system that had produced about a third of the facility’s power and heated the digesters since 1991 was showing its age. The generators had been rebuilt several times by 2014, when the utility staff began discussing alternative uses for >span class="s2">Theresa Street facility (28 mgd design, 26 mgd average).

“The system in general, at 25 years old, was reaching the end of its useful life, certainly the end of its efficient life,” Crisler says.

Multiple options

HDR Engineering helped evaluate 11 alternatives; the team then pared the list to four. One was selling the biogas, with some sulfur and moisture removed, to the University of Nebraska. That would have relatively low capital costs but a long payback. Reinvesting in CHP required more capital but a shorter payback.

Producing pipeline-quality natural gas and marketing it as a vehicle fuel had the highest initial costs but a short payback of four to five years. That was the option selected. The key to the decision was the federal renewable fuel standard program, which offers credits known as Renewable Identification Numbers.

By scrubbing the biogas to pipeline quality and marketing it for vehicle fuel, the utility can qualify the gas as an advanced biofuel eligible for RINs. The project will cost about $9 million overall, but it can generate about $2 million to $3 million a year from fuel sales and the credits.

The buyers of the credits are fossil fuel suppliers and refiners that are required under federal law to blend renewable fuels into transportation fuels and to buy RINs to meet their obligations under the U.S. EPA Renewable Fuel Standard program volume obligations. The credits create the value of the renewable natural gas purchased by a fuel company.

Wheels in motion

In February, the Lincoln City Council approved a contract with Bluesource, an energy management company, to oversee the sale of the gas, including storage, marketing and the sale of the environmental credits. “The Renewable Fuel program is the game changer,” Crisler says. “That is the source of revenue that pays for the investment rather quickly.”

Although RIN legislation has set up the program only through 2022, Crisler is confident it will stay in place. Even if not, the clean biofuel will have value. “In the future, we can supply gas to our transit system, StarTran, or to our own city fleet,” Crisler says.

“There are a lot of options with pipeline-quality fuel coming out of this facility over and above the federal Renewable Fuel program. It may not have the same return on investment, but it’s certainly a viable option for us. It’s a sustainable project.”

Simple transition

The project will not substantially affect facility operations. Instead of going to the CHP system, the gas will go through a series of steps to reduce moisture, hydrogen sulfide and siloxane and then through a membrane to remove carbon dioxide. Finally, the gas will be chilled to remove more moisture before it is compressed and fed to the pipeline.

After years of maintaining a CHP facility, Crisler thinks the facility staff will be well equipped to maintain the gas scrubbing equipment. “We feel the fuel- cleaning equipment, compressors and membranes are right up our alley,” Crisler says. “We think this will be a little easier to maintain than the CHP facility.”

Gas scrubbing was scheduled to begin in April, and the portal to the pipeline was expected to be operational by June 1. Lincoln expects to produce about 8,000 MMBtu of renewable natural gas per month thereafter. The project is already a point of pride in Lincoln. The egg-shaped digesters now have the Lincoln Renew logo painted on them.

Exporting energy

Advanced biofuel production is hardly the first big sustainability project for Lincoln Wastewater System. In 2014, the utility began sending treated effluent to the 164-acre University of Nebraska Innovation Campus for heating and cooling.

The campus, next to the Theresa Street facility, uses the thermal capacity of the 55- to 75-degree F effluent for heating and cooling, then sends it back to the facility for discharge into Salt Creek. Lincoln’s other water resource recovery facility, Northeast (10 mgd design, 5 mgd average), sends its effluent to the Lincoln Electric System Terry Bundy Generating Station for its cooling tower.

The facilities also use effluent for washing and process water. If purchased from the municipal system, that water would cost about half a million dollars a year.

Lincoln long ago replaced the typical packing on pumps to mechanical seals that stop leaks and allow the pumps to operate at lower amperage, saving 10% to 30% on energy. Later, variable-speed drives and soft-start switches were added to equipment at the facilities.

Ceramic fine-bubble diffusers (Sanitaire - a Xylem Brand) and Turblex blowers with variable guide vanes were installed to save energy on aeration. Recently, Sulzer high-speed turbo blowers were added at the Northeast facility.

Point of pride

The wastewater system has worked with a consultant to quantify net energy usage at both facilities and to establish a baseline from which to measure future energy improvements.

With the CHP facility shut down, the Theresa Street facility will have to buy utility power and fuel, but the facility will be a net energy exporter, considering the fuel that it will produce and the thermal energy it sends to the Innovation Campus and the Lincoln Electric System generating station.

“Now we are using more electricity and natural gas, but look at all the Btus we’re sending outside the fence,” Crisler says. “If you break it down to Btus, we are very close to energy neutral on an annual basis and even positive in the summer season. We’re very proud of that.”  


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