When the Biomass plant, a lynchpin of Middlebury College’s carbon neutrality goal, shuts down, it is a massive undertaking. Starting the night of Thursday, Oct. 10, the plant’s input of wood chips was stalled, the burners died down and steam pumping through the College’s pipes was heated by oil instead of the gasification process.
Kelly Boe, central heating plant manager, said the longer the plant runs, the greater the effort required to clean it.
“This thing has about an eight to 10 week cycle on it before we have to shut it down and clean it out,” Boe said. “Since it started up five years ago we’ve tried to increase that cycle so we can run longer and have more of the work for the campus be done by biomass as much with oil. We just went 16 weeks, which is the farthest we’ve ever gone.”
Understandably, an operation as large as the Biomass plant does not shut down without planning ahead of time.
“The real issue with this guy is you don’t just shut it off for an hour,” Boe said. “It’s like a fireplace. The way you shut it down is you just shut the wood off and starve it. And if it goes out then you have to restart it. It’s not like pushing a button and it starts for you.”
The run-up to the shutdown meant letting the gasifier burn through the remaining supply of wood chips that arrive at three tall garage doors at the back of the plant. Ninety tons a day of wood chips arrive daily at the plant.
“The bunker holds a one-day supply,” Heating Plant Operator Myron Selleck said, gesturing to the pit.
Although the plant is a highly visible part of Middlebury’s campus, most students probably do not grasp the extent of the process that starts with semis pulling up to plant and ends with hot water and heated buildings. In another difference from a wood stove, the Biomass gasifier does not simply burn wood chips.
“The way the whole system works is we put wood in the [gasifier], and we restrict the amount of oxygen that we let it see,” Boe said. “The wood won’t actually combust. It will give up a gas which is a flammable gas that is primarily hydrogen and carbon monoxide, which both have energy value. We maintain the gas at about 3 percent oxygen. It goes over to the boiler, and just before it gets to the boiler, we hit it with another shot of oxygen, and that’s when it catches on fire, and it’ll actually burn at that point.”
Burning the wood-gas rather than the wood itself makes the plant heat up water to create steam more efficiently.
September was an ideal time to run the biomass for the last leg of its 16-week marathon.
“In the month of September, 23 out of the 30 days were oil-free,” Boe said.
Having temperate weather meant the demand for steam from the plant was not as great as it is in summer, when the Biomass plant sends it to Bicentennial Hall.
“We use steam to cool the building,” Boe said. “The steam will regenerate the refrigerant.”
The plant has been a financial boon to the College on top of being a highly visible reminder of Middlebury’s commitment to sustainability.
“It was put in place because of the carbon-neutral goal the College had for 2016, but there is a great financial windfall that comes with that when it runs and you don’t burn oil,” Boe said.
Despite a glitch-free 16 weeks of operation, the success of the plant was never certain, especially when the plant was first installed.
“There’s not many of these out there, and this one was one of the first put in of its size,” Boe said. “So even the manufacturer was in uncharted waters. We had to learn our way around it and figure out how we needed to run it full capacity and how to get from four weeks to six weeks to eight weeks to 16 weeks.”
Despite the numerous steps in the Biomass process, Boe said the plant can be run by one person. Nevertheless, the 10-hour shifts that the heating plant operators work overlap by several hours just in case.
The plant operators continue to strive for greater efficiency for the plant and less reliance on oil when the plant is not running. Boe said that the goal of maximizing efficiency leads to some discussion.
“Going to 16 weeks gets a little bit of a debate going because there are seven or eight of us, so we all have an opinion about how the next thing should work,” he said.
One theory that is moving forward has to do with supplementing the plant with bio-methane purchased from a supplier south of Middlebury. That proposal, however, is not without controversy and complicated engineering.
“It’s all tied into the natural gas pipeline that is getting some attention now, as to whether or not that’s the way we convey it to the boiler,” Boe said. “And as for the burned [methane], we will have to reconfigure the boilers.”
A misconception students and faculty have about the plant, Boe said, is that the Biomass facility is configuring heating for individual classrooms and dorms.
“If you go to turn your thermostat up and it doesn’t heat up, you don’t necessarily call here and say, ‘Hey heating plant, can you turn our heat on?” Boe said. “But we’ll get a call every now and then from a professor saying, it’s a hundred degrees in this building, can you guys shut this thing off? And we say, ‘Well, the steam is in the line but something else is not allowing it to shut off.’”
Even so, Boe said the plant operators attempt to navigate the peak times for steam demand, which are, according to Boe, in the morning, lunchtime, when the cafeteria dishwashers start up, and in the evenings.
While it takes meticulous planning to configure, Boe said from an engineering perspective that he prefers working with the biomass plant than flipping a switch to turn on the oil-burning side of the plant.
“In addition to being expensive and bad for the environment and everything else, the oil boiler is relatively boring,” he said.