FUTURE FLASH AT THUNDER BAY

Thunder Bay Generating Station would burn steam-treated 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.)

Weatherability was measured on a pellet durability index to compare each pellet's relative quality in terms of mechanical strength and an ability to resist degradation. [2]

Water was thought to induce pellet degradation and dust production, so eight types of torrefied wood pellet and two types of pellet produced with steam explosion were immersed in water. [2]

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 OPG's evaluation tests.

Dust generation was then tested on steam-treated pellets using a rubber-lined tumbler, operating 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.” [2]

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

A review of Thunder Bay Generating Station’s handling systems followed. A small number of areas were highlighted for minor safety improvements which would mitigate any build-up of electrostatic charge. [2]

A slide gate at the mouth of the initial reclaim hopper was extended to reduce the free flow of pellets, limiting the volume of biofuel 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. OPG was then satisfied that all dust and ignition hazards had been properly addressed.

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

STO Les Marshall noted at the time: “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.”

A test burn was carried out next [2] [3]. 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 power plant to prove that low-carbon, steam-treated black pellets could replace coal with little modification to plant facilities.