Nuclear to Coal to Hydrogen: Sheldon Station Blazes a Trail

Aerial photo of the Sheldon power plant showing the generating station and its coal yard with rural Nebraska fields extending in the distance.
Photo: NPPD

You’d have to be pretty lost on a road trip through the southeastern part of the Cornhusker state to run across Nebraska Public Power District’s (NPPD’s) Sheldon power plant.

And that would be too bad, because Sheldon may deserve at least a mention in the annals of industrial history. If so, then it’s on track to add to that citation.

The plant was first built between 1958 and 1963 as an experimental nuclear power plant for the U.S. Atomic Energy Commission. That equipment is long gone. In its place is a two-unit coal-fired power plant that for a time tried (without much luck) to use old tires as fuel. It burned Eastern U.S. coal and today takes delivery of trainloads of low-sulfur coal from Wyoming’s Powder River Basin.

That’s set to change as NPPD engineers advance plans to convert Sheldon’s 125 megawatt (MW) Unit 2 from coal to hydrogen. Doing so would make Sheldon the largest hydrogen-fueled electric power station in the United States.

“Contracts are in place to move the project forward,” says John Swanson, generation strategy manager for NPPD. His job is to “look under rocks” for new opportunities. It was NPPD’s work on carbon storage and sequestration methods that led to introductions being made about a possible source of hydrogen for the power plant. 

In 2015, NPPD agreed to work with California-based Monolith, which broke ground last September on the first of a planned two-stage $50 million plant to produce carbon black from natural gas. (A sort of high-surface-area soot, carbon black is formed by the incomplete combustion of petroleum products.) The carbon black would find commercial uses in everything from plastics to car tires. Meanwhile, the hydrogen left behind as a byproduct  would travel a short distance by pipeline to the Sheldon station. There it would fire a new dual-fuel boiler and generate up to 125 MW of electricity for NPPD’s distribution customers.

The first stage of Monolith’s carbon black plant is expected to be mechanically complete in 2018. When it’s up and running, the plant will buy electricity from the Norris Public Power District, local reseller of NPPD’s electric power.

And, by 2020 work could be underway to convert Sheldon to burn hydrogen. Under current plans, by 2022 much of the plant’s power output could help supply electricity to multiple production lines at the Monolith plant, which could employ up to 600 people.

Between now and then, however, a lot of engineering work needs to be done to prep Unit 2 for conversion.

The thinking now, says John Meacham, senior staff engineer at NPPD, is to tear out the coal-fired boiler and use the existing structural framework to hang a new boiler. But because hydrogen’s heat characteristics are different than coal, a larger boiler will be needed to maintain the power plant’s 125 MW generating capacity. To fit, the new larger boiler will extend down into the plant’s basement area, which currently houses coal ash collection equipment. That equipment becomes obsolete after conversion, Meacham says.

The boiler’s burner configuration will be changed from a cyclone pattern to a wall-fired arrangement with a windbox. The new burners will be segmented with one set dedicated to hydrogen and a separate set for natural gas.

NPPD is planning on dual-fuel capability in case the hydrogen supply from Monolith is interrupted for any reason, including business failure.

Meacham says that some structural steel will need to be modified to handle rapid heat change if the hydrogen-fueled fireball goes out. Not only does a hydrogen fireball have a hotter flame front than coal, but, he says, it also goes away faster in an outage. That creates a thermal transient that needs to be planned for in the boiler’s support structure.

Unsurprisingly, Sheldon station’s environmental profile will improve with the hydrogen conversion. NPPD says that about 1 million metric tons of carbon dioxide emissions will be eliminated each year with the end of coal combustion at Unit 2. And the utility’s portfolio of carbon-free generation sources will grow to nearly 50 percent.

Engineers plan to add emission control equipment at Sheldon to deal with nitrogen oxide (NOx) produced when the hydrogen fuel mixes with ambient air during combustion. The plant also may produce a plume of water vapor, the result of hydrogen combustion.

“With the appropriate materials there will be no issue with the vapor,” Meacham says.

For now, Meacham’s biggest worry is how pure the hydrogen stream will be. The gas will be around 95 percent hydrogen “with some cats and dogs” in the stream as well, he says. That means the gas isn’t pure enough for use in fuel cells or food production, but it is good enough for a power plant boiler. The gas stream quality  is likely to vary as Monolith’s production produces different forms of carbon black.

“We’re awaiting a final spec on the fuel from Monolith,” Meacham says.  The quality is unlikely to affect boiler operation, but likely will impact Sheldon’s NOx emission profile.

Both Swanson and Meacham declined to reveal conversion cost estimates. But Meacham says that NPPD looked at several different technologies before concluding that the conversion offered the lowest-cost option.

One technology looked at was a gas-fired combustion turbine. The U.S. Department of Energy estimates that the cost to develop a greenfield power plant with a combustion turbine may range from $680-$1100  per kilowatt. That’s on the order of $8.5 to $13.75 million for a brand-new 125 MW plant. But Sheldon will be a conversion with a lot of equipment already in place, including the expensive turbine and generator. The project to date has received green lights in part because its conversion will not increase the cost of electricity to NPPD’s wholesale customers.

“It’s a cost share with Monolith” and does not include any government funding, Sheldon says, adding “it’s mostly them and a little of us.”

For now, it’s also mostly Monolith on the construction side of things. Aside from Meacham and Swanson, NPPD has not yet fully staffed a project development team.

 But in discussing the planned conversion, Swanson betrays some of the entrepreneurial drive that led NPPD on its journey from nuclear to coal to hydrogen at the out-of-the-way Sheldon station.

“We always have one eye on the future,” he says.

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