He understood a stone or a knot in timber had created a spark which landed in wood shavings.
The fire gutted the hopper but the main factory about 30 metres away was undamaged.
Mr Kennedy said firefighters spent about three hours at the scene, then returned a couple of times last night to check the fire hadn't reignited.
TRT manager John Blair said the blaze could have been much worse.
"We're very lucky the wind was going towards the river - it blew the flames away. It could have got into the wood racks, it just took one building."
He agreed with Mr Kennedy that a rogue spark was probably to blame.
"We are pretty sure it's a spark that got into one of the machines and sucked up into the system. It's just bad luck."
He had no idea of the cost of the damage. The loss of the hopper - which stored wood and shavings from the factory and elsewhere - would have little impact on operations. "We will have to get some sort of system, whether it's bags or whatever," Mr Blair said.
The hopper had originally come from TRT's Romilly Street factory when the company moved to Cobden Street in 1999.