A cogeneration wastewater facility takes on the challenges of hydrogen sulfide.
By Ed Ritchie
The trend of burning digester gas in wastewater cogeneration systems is growing, but equipment manufacturers are finding that the unruly gas has more than its fair share of challenges.
Burning digester gas (methane) in two Caterpillar gas-fueled generator sets seemed like the ideal solution to utilizing excess methane at the City of Lethbridge’s wastewater treatment utility in Alberta, Canada. The plant serves 77,000 residents and 2,000 businesses that create a typical flow rate of 10.4 million gallons per day. The idea began in 2000 with a feasibility study to determine the benefits of making better use of methane from the anaerobic digesters. The study painted a picture of blue skies for the project, but it didn’t fully anticipate one variable: the impact of the environment on the quality of the gas.
Much of the problem came from the hydrogen sulfide content of the digester gas, according to Doug Winter, plant manager at Lethbridge. The system depends on a set of two iron sponge scrubbers using iron oxide–impregnated wood chips to remove moisture and reduce the concentration of hydrogen sulfide levels to 100 ppm, a considerable reduction from untreated levels that ranged from 2,000 ppm to 4,000 ppm.
“We went through some growing pains,” Winter recalls. “We expected 300 days of use from the scrubbers, but initially they weren’t lasting longer than four to eight weeks.”
The trouble was consistent with the plant’s history in using the digester gas in boilers that heated the anaerobic digesters to 37°C (99°F) for maximum digestion efficiency.
Before cogeneration, the plant used about 30% of the methane to fire its boilers and flared the excess. “The problem was that hydrogen sulfide condenses and becomes an acid that’s very corrosive to the boilers,” says Winter. “Every six months we had to replace boiler tubes. It was a high-maintenance operation, and a rebuild was about $3,000. But it still paid for itself compared to the cost of natural gas.”
High maintenance wasn’t part of the plan for the new scrubbers—or for the engine-mounted TecJet valves that modulate natural gas flow to deliver the optimum fuel blend for the two Cat G3516, 16-cylinder, lean-burn engines operating at 1,200 rpm. Nonetheless, the Edmonton, AB–based design team of Lockerbie & Hole Contracting Ltd., along with Calgary, AB–based Cat dealer Finning Power Systems, had to stop the hydrogen sulfide from overwhelming the system.
“It’s a big process to remove the hydrogen sulfide, and we anticipated a certain amount, but there turned out to be quite a bit more,” says Daniel Tuan, Cat dealer and service supervisor for Finning. “At times, with digester plants, you may have very good quality gas or very poor quality. The amount of water and sewage can affect it. On a very rainy day or certain times of the year, the gas quality is poor.”
Natural gas supplements the engines to maintain electrical power for the plant during times of poor quality. The system uses a methane sensor that adds natural gas when quality drops below a certain point. “There are a lot of gas wells in Alberta and a lot of sour gas sites that are still in the ground,” notes Tuan. “The groundwater gets into the digester system, and that’s why the scrubbers are needed. Otherwise, the hydrogen sulfide would wreak havoc on the engines.”
According to Winter, problems from the scrubber failures stressed the TecJets and the oxygen sensors. Even though the engines supply power to the plant, Lethbridge still buys power from the grid, so when the engines became unstable the kilowatt generation fluctuated up and down erratically. “We were getting close to our export level, which we aren’t allowed to do,” says Winter. “The engines shut down and caused a blackout for the plant. That happened a few times, and it was very frustrating. Anyone that uses digester gas—such as landfills and other plants out there that have oxygen sensors and systems—will see that it’s not that unique to have these problems.”
Winter’s assessment coincided with a report from a county landfill in the White and Oxford townships of New Jersey. The site experienced a similar problem in mid-December 2006, when a new cogeneration scrubber system failed to remove sulfur or hydrogen sulfide from the landfill gas. WC Landfill Energy LLC of West Atlantic City, NJ, is trying to resolve the issue.
Finning and Lockerbie have made significant strides in resolving the problem at Lethbridge, and the plant hasn’t experienced a blackout in many months. In fact, Winter is confident that the system is ready to operate in a manner that moves beyond just supplementing the gas and electricity once purchased from utility companies.
Originally, Finning installed the generator sets and accompanying switchgear to operate in parallel with the local utility grid. Both units produce 1,100 kW combined, enough to fulfill about 70% of the plant’s electrical load. Winter would like to see the engines produce enough power to export electricity back to the grid for sale to the utility.
Such a goal would seem infeasible, because running both engines depends upon two variables: the quality and the quantity of gas generated in the biological process. When there isn’t enough gas in the digester, the system balances generator operation times so the running hours will be equal. It’s good for maintaining the Cats but compromises the amount of power that would be needed to export to the grid. Then, too, at maximum output the engines are still supplying only 70% of the plant’s power needs. Where would the balance come from?
According to Winter, an upgrade to the blowers would improve the system enough to shift the balance of power. “The old units use a lot of power, and we’re installing new units that have fine air filters and lower energy requirements,” he says. “That could give us the potential to sell back to the grid.”
Tuan says the upgrade could work because the blowers are a key component in cycling the operating hours between the two engines. He likens their performance to an engine’s fuel pump. “If they pull too hard on the digester field and there is not enough gas to use, it’s like adding water to the fuel,” explains Tuan. “You have to regulate the blowers to keep the fuel at a certain Btu value.”
Ultimately, Winter believes the new blowers will allow for a more reliable BTU value. “I’m hoping that within the next six months to a year we are able to make more power by different programming of the generators and blower economics,” Winter says. “It would earn money for us and make the project more worthwhile in the long run.”
Meanwhile, the engines have undergone their 25,000-hour mainframe overhaul, and Tuan says the Cat design has proved to be a consistent performer. What remains inconsistent is the nature of digester gas.
“Every site is different,” Tuan notes. “A lot of it has to do with the region, and then you’ve got the nature of the waste and the decay process in the area. When it comes to landfills and digester gas, you can never do enough research.”
Ed Ritchie specializes in energy, transportation, and communication technologies.
DE - March/April 2007
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