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Wastewater treatment plants tap power in digester gas

This fuel cell and waste heat recovery unit turns digester gas into electricity to provide 36 percent of the power needs of the Palmdale, Calif., water reclamation plant. (Photo by Los Angeles County Sanitation Districts)

There is more than one way to capture digester gas and turn it into electricity and heat, as the Los Angeles County Sanitation Districts are demonstrating at several water reclamation plants.

The districts generate a total of about 87 MW of biogas power, with 23 MW coming from four self-generation projects. “There are a lot of good reasons for using biogas to generate power, both economic and environmental,” said LACSD Division Engineer Ed Wheless.

Those reasons include reducing emissions that contribute to global warming, increasing plant reliability through the use of waste heat and standby generators and saving money on power purchases. “Both Palmdale and Lancaster will save approximately $150,000 each year on their power bills,” Wheless said, naming the most innovative installations as examples.

The generators at those two water reclamation plants represent firsts for biogas projects. In January 2005, the Palmdale WRP commissioned California ’s first high-efficiency fuel cell to run on gas from a wastewater treatment plant. The Lancaster plant is the first wastewater treatment plant in the western United States to run a large microturbine on its digester gas. “Both units are going to be money makers,” declared Wheless.

Fuel cell offers high efficiency, near zero emissions
The Palmdale plant draws about 36 percent of its power needs from the 250-kW DFC 300A molten carbonate fuel cell built by FuelCell Energy.

The unit uses about 60 percent of the digester gas the plant produces from processing 15 million gallons of wastewater per day. Digester gas contains moisture and silica compounds that can damage power generation equipment. The facility conditions the gas by chilling it to remove the moisture. The gas is then reheated and run through a series of carbon beds to remove contaminants.

Using the waste heat from the fuel cell as process heat boosts its 47-percent electrical efficiency to a total electrical and thermal efficiency of 73 percent. Because there is no actual combustion, there is more heat left over for other plant processes, including space heating.

A grant from the California Public Utilities Commission paid for half the $1.9 million cost of installation. The districts had wanted to test a fuel cell “forever,” explained Wheless. “The funding from the Self-Generation Incentive Program combined with low interest rates made it economical,” he said. “We figured it was time to try.”

Economical microturbine suited to wastewater plants
A CPUC grant also funded about 40 percent of the microturbine installation at the Lancaster WRP. Even without the incentive, Wheless pointed out, the microturbine is very economical. “The payback should only be about three years.”

The 250-kW Ingersoll-Rand generator cost only $720,000 to install and has been more widely used than the fuel cell. That may make the microturbine a better choice for wastewater plants, even with a combined electrical and thermal efficiency of only 51 percent. The unit provides 24 percent of the plant’s electrical demands and uses about 64 percent of the available digester gas.

Ingersoll Rand designed and built the fuel-conditioning system necessary to prepare the gas for combustion. The company is also providing operating and maintenance services for the first three years.

The microturbine burns low-BTU gas efficiently, producing ultra-low nitrogen oxide and carbon emissions. The Palmdale fuel cell system has near zero emissions. “In both cases, the emissions are far less than flaring,” said Wheless.

Districts committed to biogas benefits
The districts’ history of using biogas to produce electricity dates back to 1938. The joint water pollution control plant in Carson, Calif., burned gas from wastewater sludge digesters to provide all its light and power needs. The system was so successful that the facility was disconnected from the local electric utility.

Today, the JWPCP facility is a combined-cycle cogeneration powerplant with three nine-MW gas turbine generators and one three-MW steam turbine generator. Gas from district WRPs fuel internal combustion engines, gas turbines, steam boilers and Capstone microturbines. District landfills sell biogas-generated electricity to Southern California Edison.

Wheless estimates that as many as 19 wastewater treatment facilities statewide could install fuel cell or microturbine systems that qualify for the CPUC incentive. “The smaller plants are ideal candidates for a two- to three- year payback,” he said.

One consideration for a small plant is that the amount of biogas it produces could extend the payback time, Wheless noted. “The unit is going to make money, just not as much or as quickly,” he said.

Still, he added, the technology exists and he would like to see more wastewater plants take advantage of it. With almost 70 years in the business of turning what others think of as waste into water, energy and recyclable material, the Los Angeles County Sanitation Districts are in full agreement.