How cremations are helping to heat homes in Denmark

A clean, white casket trundles calmly into the mouth of an incinerator. The distance is roughly the length of an average person, head to foot. Once the door closes, temperatures will rise up to 1,800 degrees Fahrenheit. After about 75 minutes or so, the crematorium staff will come to collect what remains.

On an average day at the municipal crematorium in Aalborg, Denmark, this scene repeats about 14 times, according to technician Jerrick Park Bisgaard. And as part of a wider national effort to reuse energy, the heat from cremations also helps warm nearby homes.

The bone fragments – what many know as “the ashes” – are just a fraction of what the deceased leaves behind. In a room behind the incinerator, Bisgaard opens a drawer at the workstation and pulls out a bin. The contents — a clutter of scorched pieces of metal — are what he calls “the spare parts.”

“All the parts that’s inside the bodies,” said Bisgaard, pointing out replacement hips, knees and shoulders. “It’s titanium, and that is really, really expensive metal. So we, of course, also reuse that.”

Cremation does not reduce the body — it’s not gone without a trace. Rather, it separates it into parts of that previous form. And those are sorted, each with a place to go.

The ashes go to the next of kin, the titanium goes to a company that makes new surgical implants, and the rest of the body rises up the chimney as gases.

But “instead of just putting it out through the chimney,” Bisgaard said, “we actually use it in a sustainable way.”

The high temperatures of the incinerator turn most of the body — over 90% — into water vapor, carbon dioxide and other “flue gases.” Before they’re allowed to escape, the crematorium’s filtration system “scrubs” the gas of certain chemicals, like chlorine and mercury and heavy metals like lead.

Down in the basement, Bisgaard explained that these blazing-hot gases must be cooled first. That’s the job of two humming boilers directly below the incinerators.

“These boilers, they exchange heat from the flue gases and heat up the return water [from] the district heating system,” Bisgaard said.

Since 2010, this heated water has been piped back into the city’s network, where it circulates through residential radiators. The crematorium can generate enough heat for about 45 homes, according to Bisgaard, who monitors the system for the city.

It’s a fraction of the heating needs for a city of just over 100,000 residents, but in waste-not-want-not Denmark, every bit counts. 

‘Even ethical concerns are not written in stone’

With about 85% of the deceased cremated each year, Denmark has one of the highest rates in the world. It’s a growing trend across much of Europe, but the practice became particularly common in Nordic countries like Denmark after the Second World War. 

But few have followed the country’s lead on reusing the heat from cremations to warm nearby homes — a town in Finland being one example.

In 2015, a similar proposal by a crematorium in Oslo, Norway, was rejected by the local energy company Hafslund Varme. “The issue is that this crematorium is too far away from the existing district heating network, so it’s not economical to connect it,” the company’s spokesman Truls Jemtland told local media. “There are ethical issues which have to be addressed,” he added. 

Jemtland told The World by email that “the question of using excess heat from crematoriums has not been on the agenda since 2015” and declined to comment further.

When the idea was first discussed in Denmark about two decades ago, the ethical questions centered on whether this system would use the dead as fuel.

Those concerns were brought before the Danish Council of Ethics, an independent body that deliberates such issues to help inform public policy. In 2006, the council concluded that the practice was acceptable, noting that “there are good reasons, including environmental reasons, for doing so.”

One of the major reasons cited was that the generation of heat was not the primary function of a cremation, but rather a byproduct.

One of the biggest opponents of the practice was the International Cremation Federation, a Hague-based group promoting the interests of the industry. But times have changed, says Tom Wustenberghs, a Belgian representative of the federation. 

“Even ethical concerns are not written in stone,” he told The World in a video call. “And we think that striving for a good sustainability is as valuable as working on ethical principles. So, we considered that … for sustainable reasons, we could accept the reuse of the heat.”

‘The potential is huge’

Reusing heat from crematoria is one of the more unusual examples of a broader sustainability practice known as “waste heat recovery,” also referred to as recapture, which involves recovering heat generated by industrial processes and channeling it into nearby heating systems.

This excess heat from sources like supermarket refrigeration or cremations can warm dozens of homes, but larger industries can supply far more. A nearby cement plant, for example, provides as much as 25% of Aalborg’s heating needs.

“The potential is huge,” said Peter Sorknaes, who researches energy systems at Aalborg University.

Data centers, for one, are a growing source of heat for communities.

“Every data center has a large share of excess heat that it’s sending out in the cooling towers that might as well be used for heating,” Sorknaes said.

Sorknaes said waste heat recovery is not without challenges: It can be difficult to build the infrastructure needed to move excess heat to consumers, and long-term agreements with industries or data centers could leave communities hanging.

“Once you have the heating network, you need to make an agreement with the data center or the industry,” he said. “And that agreement is important to make and difficult to make because you don’t know if that industry or data center [will be] there in three or four years.”

But geopolitical pressures are also driving interest in waste heat recapture. Russia’s war in Ukraine — and more recent disruptions to global energy markets linked to the war with Iran — have pushed countries like Denmark to reduce their reliance on imported fossil fuels.

“Such systems would reduce the need for fuels for heating buildings and thereby reduce the purchase of natural gas and oil,” Sorknaes wrote in a follow-up email.

What remains

Even with heat recapture and the filtration system, cremations still take an environmental toll.

A single cremation requires around 250 to 285 kilowatt-hours of propane gas — roughly comparable to several days of heating for an average household — and emits CO2 into the air through the chimney.

In the future, Bisgaard expects Danish crematoria will move toward electric or hybrid ovens to reduce reliance on fossil fuels, and eventually use carbon capture technology to limit emissions.

Another pressing concern is capacity. 

Bisgaard said that the cremation rate is likely to only increase.

“And that’s why, right now, we are thinking about building a whole new crematorium with at least three incinerators instead of two,” he said. “Because in a few years, we won’t have the capacity.”

As a faint grey plume drifted from the crematorium’s chimney overhead, Bisgaard looked out over the cemetery.

In Danish cemeteries like this one, there’s a different capacity issue: With fewer and fewer bodies to bury, there’s more and more green space.

“Because more and more people choose to be cremated,” he said, pointing to the green space surrounding the crematorium. “We need smaller and smaller graves out here, because that urn is so small.”

For his own part, Bisgaard, 59, would like to be cremated someday.

But some still choose a traditional burial. Those were the wishes of his mother, who died last month at age 90. She is buried with his father in the town where he grew up, he said, “after a good, long life.”

Whether cremation or burial, he believes it’s important to have a place for the living to visit the dead.

After all, Bisgaard said, a person remains so long as people remain to remember them.

“When the last person who remembers someone is gone,” he said, that’s when there is truly nothing left.