It turns out that PG&E was testing their backup fuel for Units I and II. Normally the power plant runs off of natural gas, but in the case of a gas shortage the plant is capable of running off of fuel oil. Periodically they test that capability.
PG&E spokesperson Jana Morris explained that the opacity test, designed to check for particulate in the plant's backup oil, is routine and required by the air quality board. The test began on Tuesday with Unit I. What happened yesterday was an "operation error," Morris said, a valve was out of position in Unit II. There were two emissions of black smoke from the plant, according to Morris -- one in the early morning and one in the afternoon. The second occurred when tuning was taking place on the out-of-position valve, she said.
Lloyd Green, a field officer with the NCUAQMD, said that the air quality management district was aware of both emissions. The first occurred at 6:19 a.m. and lasted three minutes. The second started at 12:19 and lasted two minutes. According to Green PG&E is allowed up to three minutes of similar emission every hour, but the black smoke that could be seen yesterday was not a normal occurrence. "It threw a bit more particulate out there than was normal," Green said, "but for the short term I wouldn’t be worrying about it."
Image from www.ear.eu.int . (It's not the Eureka plant.)