The New Stacks in Town
Not far from the Dublin Port shoreline, perhaps resembling a modern office building more than a power facility, Dublin Waste-to-Energy stands ready to fulfill its purpose.
There, the labor of upward of 1,300 people over the past two years will finally be realized. Adjacent to the new, gleaming white waste-to-energy facility rests the shuttered Poolbeg Power Station, referred to by locals as the Pigeon House, an area landmark that, next to Dublin WTE, is now much less noticeable from a bird’s-eye view. That is, aside from its famous, red-and-white-striped twin stacks, which rise far above most structures in Ireland, even those of its new neighbor. At 100 meters high, Dublin WTE’s stacks are roughly half the height of Poolbeg Power Station’s, which was decommissioned in 2010, though the site still hosts an operational power station.
Commissioning of Dublin WTE has begun, and it won’t be long before it is converting the city’s waste into electricity. Director of Operations Thomas Ericksen is overseeing plant operations, and says the crew is walking down systems and preparing for first fire, which will be in March. Ericksen, who relocated from Orlando, Florida, to Dublin to oversee the project, has been part of Covanta’s team since 1986, working at a number of the company’s U.S. waste-to-energy plants over the length of his career, which he says “has gone fast.”
Currently finishing up getting his staff of 54 in place, Erickson will continue his role at Covanta Dublin for a few more years, before turning it over to his No. 2 man, the chief engineer.
In the meantime, he’ll make sure startup and operations run as smooth as possible, and that the two core functions of the plant are achieved—reducing waste, and providing power to around 80,000 homes. And, down the road, the facility will also provide district heat to 50,000 homes.
Waste In, Power Out
Covanta will operate and maintain the facility, which has secured a 25-year tip fee arrangement with four Dublin local authorities, through the Dublin City Council, to provide disposal services for a minimum of 320,000 tons of waste annually, representing over half of the facility's capacity. The rest has been contracted through the marketplace.
In a nutshell, waste is hauled in to Dublin WTE via truck, that waste is burned to generate heat, the heat is converted to steam, and the steam drives a turbine generator to create power, says Ericksen, who is able to explain the process in great detail without hesitation. “Waste coming in is reduced by about 90 percent by volume due to the combustion process, so you only have about 10 percent of the original volume of waste that you need to dispose of,” he says.
Due to a lack of treatment facilities and active landfills—only six remain open today, according to the Ireland Environmental Protection Agency—the country has relied heavily on waste exports, resulting in outbound shipments totaling several hundred thousand tons of waste annually. In recent years, a push to change waste management practices in Ireland has focused alternate waste disposal and reduction methods, including organic waste separation and collection, and development of anaerobic digestion facilities and treatment plants such as Dublin WTE.
Daily, up to 1,800 tons of waste will be trucked to Dublin WTE by haulers, who will first go over a weigh bridge to, upon their exit when they are reweighed sans waste, calculate how much was brought in. “Trucks then come off the scale and receive to a tipping floor, a rather large bunker that is 75 meters long, 25 meters wide, and about 8 meters deep,” Ericksen says. The majority of the waste is dumped directly into the bunker, and loads are inspected to ensure nothing unacceptable to the facility’s waste license, or anything potentially harmful to employees or the facility, is mixed in with the waste. “If we do find something, we have a quarantine area where we’ll push it over to, and it will be picked up and disposed of properly,” Ericksen says.
The remaining waste is dumped into the pit and picked up by two large, mechanical cranes and fed into the hoppers, where it is sent down a feed chute into the four-pass, horizontal boiler. “The burning takes place on a reciprocating grate, and we have underfire air that comes in off the tipping floor to keep it under negative pressure, which prevents any odors from escaping the facility—so we’re sucking air from the outside in—and we use that air for combustion in the furnace,” Ericksen says. “The burning of the waste takes place on grates, and the heat generated there is used to create steam in the boiler. Water wall tubes transfer the heat energy into steam, which is then piped over to the turbine generator to create power.”
While the power generation process itself ends there, treatment of gasses, emissions and end products is just beginning. “At the end of the system, the flue gas carried through the boiler has to be cleaned up before it’s sent out into the atmosphere,” Ericksen explains. The first step occurs in the furnace, where aqueous ammonia is added to neutralize NOx emissions into nitrogen and water. “The next step is in the back of the boiler—once you have recovered all of the heat you economically can out of the boiler system, the gas is sent through a semidry scrubber. We inject lime slurry and carbon—the lime is used to neutralize acid gases, and the activated carbon is used absorb metals that may be coming in with the waste.”
The next step is a baghouse filtration system, which filters out dust, dirt and particulate matter, followed by a polishing wet scrubber. “We circulate a sodium hydroxide solution through a packed bed scrubber,” Erickson says. “We bring the flue gas through that, and it neutralizes any additional gases that may have come through the system, and absorbs other fine particulate matter. From there, we finally go up and out the stack.”
The facility is also equipped with a continuous emissions monitoring (CEM) system that monitors a number of parameters required by permit, as well as a backup CEM.
Residues from the combustion process will be transferred to an off-site processing plant for recovery of ferrous and nonferrous metals, which are recycled, and production of aggregate materials that can be used for various construction applications, or to secure the voids of depleted salt mines.
The plant is wastewater free, and the majority of its process water supply will come from rain water and a neighboring waste water treatment plant.
Down the road, the two-unit, 600,000-ton-per-year plant will provide district heating to around 50,000 homes. “It was designed to include district heating, but what’s needed now is the infrastructure throughout Dublin to interconnect,” Ericksen says.
For now, Ericksen continues to work finalizing and training the plant’s crew, which he says has exceeded his expectations. “I was a little concerned about staffing the operational side of the plant, as there is only one other waste-to-energy plant in Ireland, and it’s only been here a few years,” he adds. “But I’ve been very pleasantly surprised at the quality of people I have been able to employ, and I look forward to a very successful startup and long-term operation of the facility with this type of expertise. We have a great bunch of guys.”
Author: Anna Simet
Managing Editor, Biomass Magazine
Dublin Waste-to-Energy Fast Facts
Poolbeg, Dublin, Ireland
Covanta Energy (Developer)
Hitachi Zosen Inova (EPC contractor)
PM Group (Civil engineering)
Nominal superheated steamgeneration - 125.0 t/h
Nominal superheated steam temperature - 443.0 degrees C
Nominal outlet steam pressure - 62.0 bara
Duro Dakovic steam boiler
600,000 tons annually
Ireland’s renewable energy feed-in tariff
Power capacity: 58 MW