When originally commissioned by Ontario Power Generation (OPG) in 1985, the single-unit 205 MW Atikokan coal-fired power station in Atikokan, Canada, was the most modern thermal plant in the OPG portfolio. Back then it represented the state-of-the-art in pulverised coal combustion (PCC). Today, 30 years on it has been transformed into North America’s largest wood pellet fuelled power plant.
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In the large-scale pellet-to-energy world, it is the European heat and power utilities that have pioneered biomass co-firing and coal-to-wood pellet conversions. Utilities such as Fortum and Öresundskraft in Sweden, Dong in Denmark, Essent in the Netherlands, and Drax in the UK to mention a few, with the latter having emerged in a league of its own in terms of pellet volumes consumed.
In North America, power utilities have been slow to embrace the concept of coal-to-wood pellet conversions. In part perhaps due to a lack of coherent policy, a poor economic differential over coal, public skepticism about doing so in the first place, and no North American proof of concept to show for it.
Now there is. Last September the Canadian energy utility Ontario Power Generation (OPG) officially reopened its Atikokan Generating Station (AGS) as the largest coal-to-wood pellet power plant conversion in North America.
Furthermore, OPG has also modified its Thunder Bay Generating Station (TBGS) to run on advanced biomass pellets. Last month TBGS officially entered service making it the world’s first and largest such plant. These two Canadian plant conversions have suddenly put the Province of Ontario and OPG at the forefront of coal-to-pellet power plant conversions.
The adoption of the Ontario coal phase-out regulation in 2007 was the starting point as it meant that AGS would either close or be fully converted to run on biomass. Like Thunder Bay, the AGS was built to fire high-moisture lignite coal making it, on paper, well-suited for biomass firing. Once again the fate of Atikokan as a community seemed to be on the line.
A CA$4 million government-funded Biomass Research Center was set up at AGS and a test program for utilizing wood pellets was put together. During 2008 a series of tests were carried out to assess the ability of the Atikokan boiler to handle pelletized biomass via the dedicated milling concept and if so, figure out what equipment changes or modifications would be necessary as well as any changes in operation.
We pooled together the experiences from trials previously carried out at the Nanticoke plant along with what we had learned from visiting converted facilities in Sweden, Denmark, and the Netherlands, said Atikokan Production Manager, Darcey Bailey.
The 2008 test program also included full operational trials, a noteworthy achievement for an unmodified pulverized coal (PC)-fired boiler. In November 2010 it was announced in the Long-Term Energy Plan for Ontario that AGS was to be converted to use biomass and operate as a peak-demand plant. In September 2012 Atikokan stopped using coal and the plant conversion process was initiated.
The project had three components; administrative, fuel supply, and technical modifications to the fuel handling and powerhouse equipment. The administrative part included securing a 10-year power purchase agreement (PPA) with Ontario Power Authority (OPA).
Pellet off-take contracts were awarded to Resolute Forest Products in Thunder Bay and to Rentech in Atikokan. Each is to supply 45 000 tonnes per annum and both have commissioned new pellet production facilities on account of the OPG deal.
– Of course, wood pellets are entirely different from moist lignite coal. The fundamental difference is the need to keep moisture out and to safely manage wood dust and off-gassing throughout the entire operational process from the moment a truck arrives to when the wood particle enters the furnace and the ash exits the site, said Bailey.
New fuel receiving and storage
In contrast to the TBGS conversion, none of the existing systems for coal receiving, storage, handling, and transfer to the powerhouse could be used. When AGS ran on coal the fuel was supplied by railcar.
Although the railroad siding and connections have been kept, with 90 000 tonnes per annum or about nine unit trains in total, pellet delivery by covered bottom-release railcar is currently not a feasible option. Besides, the contracted pellets are within the 200 km trucking distance.
Actually, we had railcar delivery in 2008 during the plant optimization testing. That particular test series involved replacing coal on a single-burner row, roughly 20 percent co-firing. As we had no pellet receiving or storage, covered grain cars doubled as storage while the volumes needed for testing could be metered out from the railcar. The option is still there to develop should there be a change in the future, said Bailey.
Instead, a new dedicated truck receiving unit, two 5 000 tonne pellet storage silos, and a transfer tower were built. The plant receives around 350 tonnes of pellets per day, five days a week. The single truck receiving unit is built so that trucks reverse up an incline into a covered self-off-loading bay.
– By doing this we avoid getting any external moisture, such as rain or snow run-off from the truck or the outside, into contact with the pellet load, a big risk otherwise with a truck-tip system or a tipper truck, commented Bailey.
The driver self off-loads the 35-tonne payload onto an upward belt feeding storage where the pellets are screened to remove any fines from the load. There is a load rejection and recirculation capability at the silos to cool pellets as well as the possibility to bypass the storage silos and direct feed the boiler house.
Under normal circumstances, the pellets are transferred from the receiving unit to the top of one of the pellet silos. The concrete silos are filled using a central spiral chute reducing the pellet drop.
Each silo has multi-level and location temperature monitoring and trending along with aeration and inert gas injection capabilities. Explosion panels and dust control systems are located around the top of each silo. The silos have hopper bottoms enabling first in first out of the pellets and are gravity fed to a belt conveyor fuel infeed to the boiler house.
As you notice the emphasis is on soft handling to avoid generating fines and dust. The conveyor inclines and fed speed is such to avoid pellet movement during transit, commented Bailey.
Inside the boiler house, the fuel is distributed into one of five existing in-plant bunkers that have been converted to surge bins. The surge bins each have the capacity to hold 45 tonnes instead of 750 tonnes reducing the overall volume of in-plant fuel. Each bin has ports to enable the injection of firefighting media.
From the surge bin, the pellets are fed into the five existing MPS 75G roll-race pulverizers which have undergone significant modification, including adding a second inner wall and rotating throat nozzle rings, to increase velocity and reduce classification.
Isolation valves are located above and below the pulverizers to prevent any event migration. The surge bins, pulverizers, and pulverized fuel lines are equipped with automatic dry-chemical explosion suppression systems.
The boiler, a single Babcock & Wilcox natural circulation boiler of the opposed-fired design, had 15 dual-register low-NOx burners with flue-gas recirculation to facilitate low-load steam temperature control. These have been replaced by new modified Mark III burners with no changes to boiler openings and full use of existing gas recirculation.
The boiler was equipped with a single regenerative secondary air heater and a dedicated primary air heater. The primary air heater has been changed to a primary air cooler.
On account of the dry wood particle compared to the moist lignite dust, we need to cool the primary air to the required temperature and avoid any pre-ignition. The upside is that we can reduce heat losses, improve efficiency and avoid back-end expansion problems by heat transfer to the feedwater system, highlighted Bailey.
The ash handling has also been modified to reduce fire risk. New drag-chain conveyors replaced the old pneumatic conveying system. Two existing cold-side electrostatic precipitators (ESPs) provide particulate capture. In the control room, a whole new system was installed.
The original coal-fired Atikokan plant cost almost CA$800 million to build back in the 1980s. Initial project estimates announced by OPG put the cost of conversion at CA$170 million. The financial basis for the Atikokan conversion is the 10-year power purchase agreement (PPA) with the Ontario Power Authority (OPA), which essentially covers conversion and pellet supply costs.
In that context, with the original plant and now the conversion along with the indirect benefits such as the new Rentech pellet plant, it seems to have been a good investment decision for OPG and Atikokan.
Essentially Atikokan represented the pinnacle of our pulverized coal combustion know-how. Now OPG is instead at the international forefront of using biomass and it is very stimulating to be a part of that position transition, ended Darcey Bailey.