In Denmark, a smoldering fire in a pellet storage silo at energy utility Ørsted's Studstrup combined heat and power (CHP) plant is nearing extinction thanks to the safe and coordinated efforts by the company and authorities. That said, an arduous yet critical investigation into the how, where, why, who, and when remains.
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According to Ørsted, the smoldering was first discovered when the CHP plant started firing on wood pellets for the heating season, and a heat build-up in the affected storage silo was first detected on September 22, 2022.
The silo is estimated to have contained close to 55 000 tonnes of wood pellets, with some of them glowing and smoldering, resulting in a build-up of smoke within the silo.
We are very aware that the smoke from the fire at the Studstrup plant is bothering our neighbors a lot, and we are very sorry for that. We are in direct dialogue with our nearest neighbors about the inconvenience they are experiencing, and in collaboration with the East Jutland Fire Service, we have put all our efforts into stopping the fire as quickly as we can, an initial statement from Ørsted said, adding that the East Jutland Police provides daily updates on progress with extinguishing the smoldering and anticipated smoke alerts depending on the wind direction.
Located north of Aarhus, the city to which the plant supplies district heating, the initial statement also said that despite the fire, the Studstrup Power Station will have enough pellets available to be able to supply district heating throughout the winter.
Empty the silo without causing a surface fire
According to an East Jutland Police update on September 28, 2022, apart from the smoke, the smoldering posed no immediate danger to human life, and therefore extinguishing efforts have been concentrated on limiting the damage to the silo as much as possible.
As a silo smoldering, a key concern in the contingency process is to avoid introducing oxygen into the silo or using water inside the silo.
The former would lead to an extensive surface fire as hot combustible gases ignite while the latter would lead to total destruction of the silo as the large volume of pellets expand and crumble.
The smoldering in the silo was contained by injecting nitrogen from the top and bottom of the silo while extracting pellets out of the silo to extinguish the smoldering material outside of the silo – a time-consuming process.
Nitrogen has a cooling effect and maintains a low-oxygen environment within the silo suppressing the fire. The preservation of the silo is important because it is an essential part of the heat supply.
On October 7, 2022, the East Jutland Police update noted that the amount of smoldering pellets had been reduced so much so that the fire brigade by the following weekend, could begin extinguishing the fire with water inside the silo itself and that the extinguishing work from then on would proceed relatively quickly.
If you spray water on wooden pellets, they expand violently, which is why we have not previously been able to use that solution, as we risked that the pellets would expand so much that the silo could collapse. Now we have the opportunity to spray water into the silo with water cannons, and now we are putting everything into getting the work done so that citizens can soon be rid the smoke and smell nuisance that the fire has caused, said Kasper Sønderdahl, Emergency Manager in East Jutland Fire Service.
However, attempts by the fire brigade to extinguish the smoldering inside the silo by using water cannons proved unsuccessful.
It turned out that the water simply bounces off the burnt slag of wood pellets in the silo, and that we therefore cannot use water cannons in the extinguishing work as planned, said Kasper Sønderdahl in the October 10, 2022 update.
Instead, a remotely operated bulldozer from the Danish Armed Forces was called into service to access the remaining pellets.
We are in a situation now where we have difficulty removing several wood pellets from the outside with the remote-controlled bulldozer, and we need to be able to drive all the way into the silo to grab the rest. But before we do it, we must be absolutely sure that it is safe, because we take no chances when it comes to the life and health of the crew. Therefore, we are now awaiting an assessment of the risk of the roof falling down. If it is too dangerous, then we are ready to remove the roof completely, so that we can get into the silo and get the job done, commented Kasper Sønderdahl in the October 12, 2022 update.
The fire service removed several parts of the roof to provide better visibility and make a proper risk assessment of its condition.
A total of 13 roof elements were removed to create better ventilation.
The assessment is that it is too risky to have people inside the silo and that the solution of removing the entire roof will take too long. That is why we use, among other things, remote-controlled vehicles, and right now we are making such good progress that we have decided to continue that strategy. We still have a hope that we can finish the work next week, but we don’t know for sure, said Kasper Sønderdahl in the October 14, 2002 update.
With assistance from a remote-controlled bulldozer from the Danish Armed Forces, a private trucker’s truck with a 50-metre-long grab, and within the next few days, a remote-controlled wheeled loader, the remaining smoldering pellets will be out of the silo and extinguished.
An arduous yet critical task remains
With the silo inoperable over the immediate heating season, there is another effect that the Danish financial journal “Finans” pointed out in a recent online article – up to 600 000 tonnes of industrial-grade pellets which the facility would normally use over a heating season will need to find new end-users.
We are talking about a lot of wood pellets, which we now have to cancel orders for or sell back to those we bought from. At least we won’t be using wood pellets this winter, Carsten Birkeland Kjær, a spokesman for Ørsted is to have said to Finans.
Given the current circumstances in Europe, Ørsted’s trading desk ought not to have any great difficulty in doing so. And while this extra volume is no doubt welcome in the industrial marketplace, it will have little impact on the tight residential pellet market, in Denmark and elsewhere in Europe.
What remains after the clean-up operation is a thorough independent third-party investigation into the actual or possible cause(s) or source(s) of the heat build-up in the silo in the first place – an arduous yet critical task and one that, irrespective of the findings, is of benefit for the entire industry value chain.
Lest one forgets, the most important metric for the entire smoldering fire incident thus far is that, thankfully, there have been no reported injuries, or worse, fatalities.
And that is most likely down to the cumulative lessons learned in the sector over the years from previous pellet storage- and handling-related incidents and accidents, several of which had far less fortunate outcomes.