FUTURE FLASH AT THUNDER BAY

Steam-treated black pellets are shaping up for a lead role in the energy transition, as the most viable low-emission replacement for coal.

Though their potential to ease manmade climate change was first realised more than a decade ago – on the shores of Lake Superior in Ontario, Canada.

Thunder Bay Generating Station became the world’s first power plant to use advanced black pellets, after Ontario banned the use of coal as fuel.

Instead of decommissioning Thunder Bay Generating Station, Ontario Power Generation (OPG) pioneered a conversion process that is only now being duplicated the world over.

OPG had recently spent CAD$170M converting its coal-fired Atikokan Generating Station to burn white wood pellets.

Atikokan needed new covered storage to protect white wood pellets from the elements. It integrated dedicated receiving and handling systems to mitigate dust generation, and subsequent explosion and fire risks. New milling technology was also required to pulverize the white pellets. All at great cost.

Thunder Bay Generating Station was a peaking plant, meaning it ran only when demand for electricity was exceptionally high. And Ontario Power Generation was insistent it didn’t incur a similar-sized expense.

So, in 2014, it instructed Thunder Bay’s management to make a “low capital cost” switch from coal to biomass at the power plant the following year.

OPG and station chiefs – tasked with making only minor modifications to store, handle, pulverize and combust a suitable replacement for coal – had to think outside the box. So, they returned to previous research by Ontario’s Ministry of Energy and OPG into all possible ways to decarbonize power stations.

In doing so they came to consider, as potential fuel sources, both torrefied black wood pellets and a brand new, previously untested type of advanced black pellet made by steam explosion. And they set out to compare their weatherability, dust generation and milling needs.

Thunder Bay Generating Station would burn advanced black pellets, and so cost CAD$167M less to convert than Atikokan Generating Station.
(Credit: Screenshot from 'Thunder Bay Generating Station' by Concrete Pictures Inc. via youtube.com/watch?v=vL2z2nhAoRA&t=23s, 2:45.)

Biofuel Evaluations

Weatherability was measured on a pellet durability index to compare their relative quality in terms of mechanical strength and ability to resist degradation.

Water was found to induce pellet degradation and dust production, so eight types of torrefied wood pellet and two types produced via steam explosion were immersed in water.

OPG’s Senior Technical Officer Les Marshall recorded: “The project team sought to identify pellets with outstanding performance in this area. This work confirmed the trend for steam exploded pellets to perform significantly better than torrefied fuels when exposed to water.

“Immersion of the torrefied pellets in water has had a clear and significant negative impact on the integrity of the pellets. Steam treated pellets were seen to have a clear advantage.”

Torrefied wood pellets were subsequently discounted from the remainder of the evaluation tests.

Dust generation was then tested on steam-treated pellets using a rubber-lined tumbler, operated for 48 hours. It resulted in only a “relatively small volume of very fine dust” generated, something STO Marshall recorded as “particularly important” in reducing “risks associated with airborne dust generated during handling.”

The milling tests showed only minor changes were needed to the physical mill and the operation of plant pulverizers.

Plant Modifications

A review of Thunder Bay Generating Station’s handling systems followed. A small number of areas were highlighted for minor safety improvements that would mitigate the build-up of electrostatic charge.

A slide gate at the mouth of the initial reclaim hopper was extended to reduce the free flow of pellets, limiting the volume of fuel fed to the conveyor.

The existing conveyor and bunker systems were found to be well grounded but, nevertheless, additional protection was installed on dust collectors along the fuel path.

The project team introduced humidifiers and humidity meters into metal loading bunkers used for pellet burns. It was then satisfied all dust and ignition hazards had been properly addressed.

Two equipment modification were made to the plant’s pulverizers. Increased air flow helped fuel enter the mills. Discharge skirts were removed to allow fuel to exit them more quickly for burning.

STO Les Marshall noted: “The durability of the pellets is essentially constant throughout the handling system and appears to be unaffected by the falls through the transfer points.”

He added: “The airborne dust monitoring results were very encouraging, superior to those for the baseline case handling coal. The personal breathing zone results were all well below the Ontario Occupational Exposure Limits for softwood dust.”

And he concluded: “Taken together, these results indicate the very real potential to employ second generation wood pellets in coal handling systems with relatively minor modifications.”

Test Burn Results

A test burn was carried out next. In his project notes, STO Les Marshall wrote: “When called upon to operate at full load, the unit has demonstrated the capability to easily generate at the full (original coal) nameplate capacity.”

His report continued: “CO emissions have been very good. Auditing very low pollutant levels has become problematic when operating with low pollutant levels. CO production was actually increased during this run to allow for a better check of the continuous emission monitoring system.

“The fine particulate results are also very favourable, and the product of a low ash fuel, good combustion and particulate collection performance with the existing equipment.”

And with that, in February 2015, Thunder Bay Generating Station became the world’s first to prove that low-emission, steam-treated black pellets could replace coal with little modification to plant facilities.

Thunder Bay Generating Station switched from coal to burn advanced black pellets for three years and experienced “very low pollutant levels”
(Credit: Screenshot from 'Thunder Bay Generating Station' by Concrete Pictures Inc. via youtube.com/watch?v=vL2z2nhAoRA&t=23s, 3:46).

Thunder Bay was reconfigured to burn advanced black pellets for a cost of just CAD$3M. The conversion immediately made the CAD$170M spent on converting Atikokan Generating Station – to burn high-emission, less efficient white wood pellets – seem profligate.

The moment heralded the dawn of a new era for fired power stations which would otherwise be taken out of service.

Mr Marshall summarized his report with the words: “The conversion of Thunder Bay Unit 3 from coal to advanced wood pellet firing is the first such project in the world.

“This project confirms the ability to execute a low capital cost conversion project by leveraging the unique properties of these new second generation biomass fuels.

“The Thunder Bay case study has demonstrated the potential for utilities to execute similar projects to repurpose existing coal assets using advanced biomass fuels, as a means of increasing their portfolio of dispatchable, renewable power.”

Thunder Bay Generating Station remained as a peaking plant to supply excess electricity demand until 2018, operating on just steam exploded black biofuel pellets on every occasion it was needed.

Due to its intermittent use, the power plant built in 1963 developed deterioration in areas of its infrastructure which ultimately proved uneconomic to repair.

Thunder Bay Generating Station was decommissioned in 2018 and demolished in 2023 (as documented by Concrete Pictures Inc.), but its legacy as a totem for energy transition remains.

Numerous power stations worldwide followed the pioneering trail it blazed in 2015 and more continue to turn to its blueprint every year to adhere to emissions regulations.