Changing of the Guard: Coal-free by 2023

Dong Energy has converted five of its seven Denmark power plants from natural gas or coal to biomass, with the remaining two conversions expected in the next few years.
By Ron Kotrba | June 30, 2017

Dong Energy, the Danish energy conglomerate that formed in 2006 when six energy companies in Denmark merged, announced an ambitious goal in February to stop its use of coal in power generation by 2023. The European energy giant has already made significant strides in slashing coal consumption, cutting it by 73 percent over the past decade.

“We’ve decided to take the final step and phase out the use of coal at all our power stations,” says Henrik Poulsen, Dong Energy CEO. “The future belongs to renewable energy sources, and therefore we’re now converting the last of our coal-fired power stations to sustainable biomass.”

The company achieved its 73 percent drop in coal use through a reduction in the number of power stations it operates, as well as through conversions to biomass. Dong Energy also claims to have constructed more offshore wind production capacity than any other company in the world. In essence, Dong Energy has transformed from being one of the most coal-intensive utilities in Europe to being among the continent’s greenest energy companies.

Dong Energy has used wood chips and pellets in increasing percentages at two of its power plants, Herning and Avedøre (near Copenhagen), since 2002. Last year, the Avedøre and Studstrup (near Aarhus, Denmark) power stations underwent conversions to operate on 100 percent biomass, mostly wood pellets, but some straw as well. This spring, Dong Energy completed another conversion project at its Skærbæk Power Station near Fredericia, Denmark. That facility, originally fueled by natural gas, now runs on 100 percent wood chips.

While elimination of coal is Dong Energy’s 2023 goal, two of its power plants, the Herning and Skærbæk power stations, have been converted from gas to wood chips. “If you want to transform a gas-fired unit to biomass, you will often have to build a new boiler, depending on which biomass fuel you choose,” says Ole Thomsen, senior vice president and chief operating officer in Dong Energy’s bioenergy and thermal power division. “We are commissioning Skærbæk Power Station this autumn, after a comprehensive conversion, where we have constructed two new boilers. The new boilers will be connected to the existing turbine and, in that way, we will be utilizing as much of the existing infrastructure as possible.”

Conversion Upgrades
Thomsen says the conversion from coal to wood pellets is relatively simple. “Most of our power stations have been converted from coal-fired to wood pellet-fired,” he says. “Both coal and wood pellets are dust-fired—meaning we are grinding the fuel and blowing it into the boiler with air. Therefore, we can use the existing boiler, fuel mills and burners where we inject the fuel. But we need to modify [other] existing equipment to make it ready for wood pellets.”

The two main areas in the conversion process from coal to wood pellets, according to Thomsen, are changes in transportation of the fuel to the boiler, and changes in the air temperature and flow. “The differences between coal and wood pellets are quite big,” he says. “Wood pellets are more sensitive to moisture, so we store them inside. The wood pellets are dustier and the calorific value is a bit lower than coal. But, from an environmental perspective, wood pellets are much better for the climate.”

Dust management at coal plants is relatively simple and straightforward, says Morten Reinhold, senior project manager at Dong Energy. “In the flue gas, an electrostatic filter is used,” he says. “And on the fuel logistics side, only intermediate water spray systems are used to minimize dust from the open coal yards.” When these coal plants are converted to use wood pellets, however, Reinhold says there is a big difference in dust management on fuel logistics since pellets are dustier. He adds that the quality of pellets effects how much dust they emit.  “The conveyor system has been redesigned to manage the extra dust in the fuel,” he says. “That means closed storage and conveyor systems with appropriate dust aspiration systems.”

While the coal plants utilized open storage, converting to wood pellets required Dong Energy to build closed silos to house the wood pellets. “The open conveyor system has been modified to a closed conveyor system at dusty hotspots,” Reinhold says, “and a dust aspiration system has been installed at dusty hotspots.” He notes that these changes in the dust management systems were fully implemented even at Dong Energy’s power stations that had previously incorporated wood pellets on a percentage basis.

The two power stations originally designed for coal, but now fueled by wood pellets—Studstrup and Avedøre 1 (the Avedøre plant consists of two separate power production units, the original of which was fueled by coal)—utilize the same emissions control systems designed for coal with no changes, Reinhold says. These systems include a variety of elements, Reinhold says, such as low-NOx burners. “The fuel burners have been optimized for minimum generation of nitrogen oxides in the flue gas,” he tells Biomass Magazine. Then, to remove most of the NOx that is still generated from low-NOx combustion, the power stations feature de-NOx units that add ammonia to remove 94 percent of the NOx from the flue gas. In addition, each facility includes desulfurization units, which remove 98 percent of the sulfur dioxide. “This works by adding water and limestone to the flue gas,” Reinhold says. “The byproduct is gypsum, which is then sold for industrial use.” Finally, an electrostatic filter is employed to absorb nearly all—99.9 percent—of the ash particles in the flue gas. “The ash is used in the cement industry,” Reinhold says.

Other major areas of consideration when converting coal power plants to wood pellets are spark detection, fire suppression and general fire safety protocols. “Introducing wood pellets at our power plants has made a large impact on the safety protocol,” Thomsen says. “As wood pellets are a more biologically active product and sensitive to moisture than coal, self-ignition is a bigger issue regarding fire.” He says the greater amount of dust generated from wood pellets during transportation means a higher risk for dust explosion.

Reinhold says Dong Energy has developed and implemented new systems to improve fire safety at the converted power stations that now employ wood pellets. These include spark and fire detection systems; explosion suppression systems; and automatic, semiautomatic and manual firefighting systems. “We’ve also established new administrative procedures, and training in firefighting developed and implemented in close cooperation with the authorities and local fire brigade,” Reinhold says.

Future Conversions, Impacts
Two remaining facilities, the Asnæs and Esbjerg power stations, must now be converted to biomass in order for Dong Energy to accomplish its goal of becoming coal-free by 2023. The company says it’s in dialogue with the heating customers in Kalundborg and Esbjerg to discuss converting the two combined-heat-and-power (CHP) plants to use wood chips instead of coal when their respective heating agreements expire at the end of 2017 and 2019.

“If we reach an agreement with our heating customers to convert Asnæs and Esbjerg to biomass, our biomass consumption in 2023 will be approximately 3 million metric tons in total,” Dong Energy’s lead media advisor Carsten Birkeland Kjær tells Biomass Magazine. She explains the total breaks down to roughly 1.7 million tons of wood pellets, 1.2 million tons of wood chips and 125,000 tons of straw. “In 2023, the company will have reduced its annual carbon dioxide (CO2) emissions by almost 18 million tons compared to 2006 levels,” Birkeland Kjær says. “The total reduction corresponds to the annual emissions of more than 9 million cars. In 2023, the company’s electricity and heating production will emit approximately only a half-million tons of CO2 annually, primarily from gas-fired boilers covering peak loads in the district heating system and situations with lack of power.”

The likelihood of Dong Energy reaching an agreement with its heating customers is high. “Cooperation with our heating customers is good,” CEO Poulsen says. “The large cities have ambitious goals to reduce their CO2 emissions and demand green district heating from our power stations. In cooperation with the municipal heating companies, we’ve already converted a large part of our power plants to using sustainable wood pellets and wood chips as fuel instead of coal and gas.”

Copenhagen Mayor of Technical and Environmental Affairs Morten Kabell, who also chairs the Metropolitan Copenhagen Heating Transmission Company (CTR), says the city has a goal of becoming the world’s first CO2-neutral capital. “Through our heating contracts, CTR has contributed to making the district heating from Avedøre Power Station green,” he says, “and now coal is disappearing altogether. It’s an important element in our strategy that all heating must be CO2-neutral by 2025.”

While the share of coal—the highest CO2-emitting fuel around—in global electricity production is still a major slice of the pie at 40 percent, Denmark is a shining example of environmental stewardship. Over the past decade, the small European country has reduced its total greenhouse gas emissions by 25 million tons of CO2 annually, according to Dong Energy. And for those skeptics who question whether one company with offshore wind power and a handful of converted power plants fueled by biomass can really make a difference, Dong Energy’s contribution to Denmark’s total reduction in GHGs amounts to well over half.

“With the combination of electricity from our offshore wind farms and green district heating as well as flexible green power from our biomass-fired power stations, we are well on our way towards a green, independent and economically sustainable energy system,” Poulsen says.

Author: Ron Kotrba
Senior Editor, Biomass Magazine