Government incentives help a California treatment plant go green with digester-gas-driven fuel cells, a reliable source of renewable energy
The San Joaquin Valley is known for its stagnant air. Pollution gets trapped between California’s Coastal Range and the Sierra Nevada Mountains, giving the valley the second-worst air quality in the nation, behind only Los Angeles.
In the City of Tulare, schools display air-quality flags, farmers are being paid to switch from diesel to electric pumps, and the Tulare Water Pollution Control Facility has turned to fuel cells to produce electricity.
Director of Public Works Lew Nelson has plenty of biogas available at the city’s two wastewater treatment plants. A 6 mgd biofilter/activated sludge plant treats domestic wastewater from 60,000 residents. Next door is a new 12 mgd sequencing batch reactor following a 4 mgd bulk volume fermenter anaerobic reactor put online in 1995. It treats high-strength wastewater from the dairy processing industry and produces most of the facility’s methane.
The fuel cells provide about 40 percent of the domestic plant’s electricity while also running thirty 60 hp aeration mixers for equalization basins at the industrial plant.
Clean and efficient
Tulare has three 300 kW DFC300 stationary fuel cell units from FuelCell Energy in Danbury, Conn. The 900 kW cogeneration project became operational in September 2007, though fuel cells weren’t in the original plan.
“We asked for internal combustion engines or turbine generator sets,” says Nelson. “We were approached to see if we would accept proposals for fuel cells.” He found plenty of reason to do so. Califor-nia’s self-generation program provided incentives for fuel cells that were four times that for turbines or engines. “Today there is no incentive for turbines or engines, only for fuel cells or wind,” Nelson says.
With an incentive of $4.50 per watt (up to a megawatt), fuel cells were about $10,000 cheaper than an engine, he says. The $7.1 million project received $4.1 million in incentive payments for a final cost of $3 million. A fourth 300 kW fuel cell being installed this fall will get the same incentive for the first 100 kW and $2.25 per watt for the rest — more than $900,000 toward the $2 million cost.
“Fuel cells do not require air pollution permits because they are so clean in NOx, VOCs and particulate emissions,” says Nelson. “We’ve probably reduced those air emissions by 90 percent or more.” That’s because there is no combustion — the electricity is produced through a chemical reaction. Recovered heat is used to preheat sludge for the anaerobic digester, saving even more energy.
The project’s environmental impact will be measured this year so that the plant can get renewable energy credits, a future revenue source. The project has already earned a Clean Air Excellence Award from the U.S. EPA, and the city was named to the EPA Green Power Partnership Top 20 list for its use of onsite power.
Fuel cells require treatment of the methane to avoid damage to the equipment. At first, the plant’s gas processing system experienced trouble, reducing fuel cell availability to about 80 percent. “The gas system was down a lot for little recurring problems, like with a small condensate pump,” says Nelson.
The situation revealed what Nelson admits was his error, and a lesson for others. “I made an erroneous decision that the fuel cells would only be run with biogas instead of using natural gas as a backup,” he says. “The fuel cells have to make electricity to make the investment work.”
In 2008, that decision came back to haunt him again. The fuel cells generated just over 8 million kWh, but then a 15-year-old anaerobic digester had performance problems and had to be taken out of service for seven weeks in early 2009. Two fuel cell stacks also developed internal cracks. “They were caused by the frequent unscheduled cool-downs from failure of the gas processing system, and my decision to not have natural gas backup,” he admits.
Despite the learning curve, the fuel cells provided a savings of $138,000 on the electric bill in 2008. A backup natural gas supply has been added, and the system’s 2010 performance bears out Nelson’s optimism about fuel cell technology.
“I’m going to save $840,000 on my electric bill this year, minus $500,000 for the maintenance agreements,” he says. “So I’ll save $340,000 this year, which is roughly what we projected originally.” Even though the expected payback is now six years rather than four, Nelson considers the technology a good investment.
He’s even planning to add more fuel cells. “We’re in talks to build a 1-million-gallon anaerobic high-rate pretreatment device that will produce a lot of additional gas,” Nelson says. There is also biogas available from the new industrial plant’s anaerobic sludge digesters that is still being flared off.
This time, Nelson plans to add the natural gas capability from the start: “The whole key is to generate electricity, and keep generating.”