After more than 60 years, the last shipment of coal arrived by train at Xcel Energy’s Black Dog power plant in Burnsville. Xcel Energy will stop generating electricity with coal there and replace the coal-fired units with a more efficient, cleaner natural gas-fired unit. (The Minnesota Star Tribune)

At 680 feet tall, the concrete smokestack of Xcel Energy's Black Dog power plant towers over the Minnesota River bottomlands. It's a Burnsville landmark and a monument to a half-century of electricity generated by burning millions of tons of coal.

What comes out of that stack is nearly invisible, but it's potent. Black Dog pumped out 1.9 million tons of carbon dioxide last year, making it Minnesota's fourth-largest carbon polluter among power plants last year, according to state data.

Next week, the big stack will stop exhaling, once the final stockpile of coal is burned. Black Dog will complete its conversion to cleaner-burning natural gas, ahead of a federal order to clean up its operations or shut down.

The muzzling of the Black Dog plant is a small but significant step away from the dirtiest energy source and a key factor in climate change. Coal-burning generators in Hoyt Lakes and Schroeder, Minn., will also shut down within the next month.

On Wednesday, the last coal train screeched into the plant, greeted by news cameras and Xcel Energy executives. Chris Clark, president of Xcel's Minnesota operations, talked about his company's plan to get 63 percent of its electricity from carbon-free sources by 2030. "We do see a transition away from coal," he said.

Some of the Xcel execs admitted feeling a bit sentimental about the change. After all, the Black Dog plant has burned coal since the 1950s. At that time, the coal arrived by barges traveling up the Mississippi and Minnesota rivers.

They were replaced in the 1980s by rail cars. The trains arrived once or twice each week, hopper cars brimming with low-sulfur coal scraped from the prairies of Wyoming's Powder River Basin.

Black Dog's coal units provide enough electricity for 200,000 homes. But its enormous smokestack lacks any scrubbers, meaning there's no filter for the toxic by-products of coal combustion.

Our dependence on coal has environmental implications all along the chain, from mountaintop destruction and deadly mine disasters to mercury contamination of fish, acid rain and greenhouse gases that heat up the planet. Wastewater lagoons where coal ash is dumped can spill into rivers. The fine soot particles from coal combustion can cause heart attacks.

A federal regulation on mercury and air toxics ultimately spelled the end of coal at Black Dog, which produced 73 pounds of mercury pollution in 2011, said Anne Jackson, principal engineer in the air policy unit of the Minnesota Pollution Control Agency. Nationwide, the emission reductions from that rule are expected to have enormous health benefits, including from 4,200 to 11,000 fewer premature deaths, according to the U.S. Environmental Protection Agency.

While two units were converted to natural gas in 2002, Black Dog's units 3 and 4 will keep running on coal up to the federal deadline, but the mountain of coal behind the plant has dwindled to nothing. The state has already approved a plan to cap the coal ash ponds, said Brian Behm, the plant director. Xcel estimates the coal "decommissioning" will wrap up in 2021.

Black Dog is small enough that the strip mines of Wyoming will barely notice the lost customer. Although natural gas burns much cleaner, it also produces carbon pollution. Still, what happened at Black Dog is one more step toward a more sustainable energy future.

Burnsville Mayor Elizabeth Kautz also showed up Wednesday to watch the last train roll in to the power plant. Headlights flashing, the blue-and-white locomotive crossed a bridge over Black Dog Lake and stopped at the entrance to the dumper building. Its 125 cars would offload 12,500 tons of coal. By next week, the plant will have burned through all of it.

Kautz remembered when the power plant was just about the only business in Burnsville, so it has a special meaning for the city.

"It's going to be cleaner facility that we can be proud of in the 21st century," Kautz said. Kautz talked about the recreational trail that will replace Black Dog Road as another part of the area's environmental renaissance.

Once the big smokestack comes down, though, Kautz has a lingering concern: "How do I find Burnsville when I'm flying in?"

Contact James Eli Shiffer at james.shiffer@startribune.com or 612-673-4116. Read his blog at startribune.com/fulldisclosure.

about the writer

James Eli Shiffer

Topic Team Leader

James Eli Shiffer is the topics team leader for the Minnesota Star Tribune, supervising coverage of climate and the environment as well as human services. Previously he was the cities team leader, watchdog and data editor and wrote the Full Disclosure and Whistleblower columns.