Utility challenged by consultant over lack of clarity on soot build-up
When a leading British environmental consultancy firm suggested in 2021 that the cause of soot fallout was flakes accumulating on surfaces inside the North Power Station stack, Belco rejected the theory.
The consultant Ricardo Energy & Environment, commissioned by the Regulatory Authority on behalf of the Department of Environment and Natural Resources to provide an independent assessment of the polluting events, said that if there was such accumulation the use of heavy fuel oil is to blame and that a cleaner fuel would be the “most effective” way to mitigate the problem [see separate story].
The consultancy firm questioned Belco’s rejection of the theory, saying: “Belco indicates a view that inspection or cleaning is not necessary and accumulation of material is not occurring. However, it is unclear what this is based on.
“Belco should clarify the basis of the statement that ‘accumulation of soot is not occurring’. Is this statement based on inspection [carried out in June 2020] of the exhaust ducts? It is difficult to verify this, as there is no report on the inspections.
“The flakes described earlier are consistent with deposition of material on the duct internal walls, and ruling this out as a possible cause seems premature based on the limited information provided in this regard.”
Ricardo said in its review dated June 8, 2021: “It seems likely that the flakes are caused by deposit of particulate-forming materials inside the flue [chimney], although this explanation has been rejected by Belco. If this theory is correct, it is caused at least in part by the use of HFO as a fuel.
“The most effective way to mitigate this problem would be to use a lower-polluting fuel.”
The DENR has since told The Royal Gazette that inorganic components were predominant in the soot, “which is to be expected after the condensate is baked on the sides of the flues at [about] 350C for days at a time.”
A spokesman said: “When engines have been off for a period of planned maintenance, significantly reduced moisture in the exhaust causes the hydrated and largely inorganic material to dry up. This coating then peels away from the flue and erupts from the exhaust when the engine starts up again.
“The expansion and contraction of the metal flues due to turning the engines on and off may also disrupt the bond between the soot and the exhaust flues.”
The DENR outlined other potential causes of the soot. The spokesman added: “Other events, such as water washing [a maintenance measure], may lead to soot events because of pressure pulses created when the water rapidly expands and forms steam in the hot stack.
“Similar shocks to the exhaust ductwork could also occur with planned additions of walnut shells [another maintenance measure].
“Finally, the amount of condensate forming over time and adding to previous layers may reach a critical level where the attachment to the flue is overcome by the exhaust stack velocity that causes a soot event.
“DENR considers that any/all of these potential causes listed cannot be ruled out at this time.”
Consultant Ricardo Energy & Environment questioned Belco’s decision not to inspect and clean the North Power Station’s engine ducts and stack more frequently in order to address soot fallouts.
In a 2021 assessment of a Belco report on the emissions, Ricardo Energy & Environment described the decision as “unusual”, adding that such measures could reduce the incidents.
Both Belco and the Department of Environment and Natural Resources have now told The Royal Gazette that cleaning the stacks is too problematic owing to physical and technical constraints.
Fallouts from the stacks have been dumping soot on the properties of Belco’s neighbours and causing health concerns, most noticeably since the commissioning of the NPS. Belco has been working on numerous mitigation and remedial measures, including a multimillion-dollar retrofit of the station.
Belco said in a report dated January 29, 2021, conducted after an investigation by lead NPS contractor Burmeister & Wain Scandinavian Contractor and discussions with MAN Energy Solutions, the engine manufacturer: “Ductwork will not be inspected because the focus for Belco is to make the necessary hardware changes to eliminate the formation of soot completely.”
These included a multimillion-dollar equipment installation of shims and piston crowns – instruments designed to improve fuel combustion. However, the fallouts continue.
Asked whether the company still maintains cleaning is not necessary, Belco told The Royal Gazette: “The exhaust sections are large components of our generation system and the engines are stopped and restarted sometimes multiple times a day, such that it is impossible to clean the entire exhaust system or the engines between stoppage and start-ups.
“The actions are not logistically practical because there are physical and technical constraints to cleaning the flues.”
The DENR told us: “DENR is unaware of how Belco would consider ‘cleaning’ the inside of the complex turbocharger, silencer and the 48-inch diameter metal flue [chimney within the stack] that rises to 65 metres above grade. It is noted that each flue, while offline, would be impacted by the heat from adjacent flues within the stack that are operating at [about] 350C.”
It added: “The DENR/Environmental Authority [the emissions regulator] has not instructed Belco to clean the inside of the stacks. DENR considers that the current measures being taken by Belco to limit the formation of the condensate in the stack – ie, engine operational parameters and abatement systems – should be concluded before stack-cleaning measures are considered.”
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