Yellow plume from Portlands part of start up

Joe Reid, an official with Ontario Power Generation, assured passers-by that the foul-looking emissions were part of a start-up test that only lasted about 20 minutes. He said the gas-fired power plant, which has been operating in a single-cycle mode, is preparing to operate in a more efficient combined-cycle mode early next year and was having one of its generators tested.

"The first 20 minutes or so will give you visual on nitrous oxide emissions which are the result of the low operating level required for the commissioning activity," he said. "Once we get past our initial start-up the yellow plume will go away."

Reid said it's similar to starting up a car in the morning and getting lots of smoke. "Over time it dissipates."

A single-cycle plant, which burns natural gas that turns a gas turbine, operates at 35 per cent efficiency. A combined-cycle plant captures waste heat from the first cycle to generate steam that turns a second turbine, allowing the facility to operate at 56 per cent efficiency - meaning 50 per cent more electricity will be generated by the same amount of natural gas.

In combined-cycle mode the plant can produce 550 megawatts of power, up from 250 megawatts in single-cycle mode.