District Energy St. Paul, the utility that heats 500 commercial buildings and single-family homes in and around downtown, held an unusual retirement party on Friday. Without a glimmer of nostalgia, CEO Ken Smith gathered with workers and executives to bid farewell to coal.

In 2015, the utility, which was almost entirely coal-powered at its founding 36 years ago, pledged to go coal free by 2020-21. “That would be our last winter of using coal, and we were able to accomplish that in 2019, two years ahead of goal,” Smith said.

The unlikely going-away party reflects changing demand for the fossil fuel, which was once considered the most economical energy source around. Given growing efficiency in harvesting other energy sources, coal now ranks as one of the most expensive ways to produce energy. It’s also received a chilly reception from health and environmental advocates, who have long linked coal to air pollution, asthma and even cancer.

To heat and cool downtown-area properties, District Energy obtains roughly half its raw material from waste wood such as tree trimmings, trees impacted by Emerald Ash Borer and trees felled by storms. Natural gas is the utility’s second-largest fuel source, with coal in recent years coming in third, making up about 20 percent of the fuel mix. To make up for the loss of coal, Smith said District Energy would be “integrating additional natural gas capacity” as a “bridge to the future,” while exploring alternative energy sources.