The Sierra Club might be surprised to learn that some of these emissions aren't soot

Environmental organizations, EPA and many state agencies have taken to suing over opacity violations in recent years because it is low hanging fruit, not because it is a substantive air pollution issue. Opacity is usually measured by a continuous opacity monitor so it is very unlikely that the Sierra Club lawsuit has anything to do with steam which is not counted as opacity by the monitors. So the suggestions in these posts that the Sierra Club does not understand that point are probably unwarranted.
That is not to say that this still isn’t an unwarranted lawsuit. I spent too much time over the last 15 years of my career dealing with opacity and ultimately found that it did not matter whether there was a real air pollution issue or simply an aesthetic problem, the harassing lawyers eventually prevailed. If you want to know more about opacity than you probably need to know then read on.
According to the AP, the Sierra Club maintains Minnesota Power has violated clean air standards more than 12,000 times over the last five years at three coal-fired power plants. That sounds like a big number but the opacity standard is for a six-minute average. There are three plants with six stacks: Boswell has three units with one common stack and a second stack that serves the fourth unit, Laskin has two units with one common stack and Taconite Harbor has three stacks for its three units. Each year there are 87,600 six minute periods times five year times six stacks so there were 12,000 excursions over 2,628,000 possible six-minute periods or 0.46% of the time.
So what is the opacity clean air standard excursion? The opacity limit goes back to the very beginning of air quality enforcement when it was used to determine whether or not a facility was operating efficiently. Opacity in a smoke stack is caused by the blocking and scattering of light by particulates from the incombustible ash components of the fuel or by carbon caused by incomplete combustion. Incomplete combustion suggests that the operator is not running his unit as well as he should except in the case of startup, shutdown and malfunction where unless you can void the laws of physics incomplete combustion is going to happen. Early on the limit was determined by “reading” smoke, a process in which someone gets certified as a smoke reader then runs out and checks the plume. I assume the MN standard is similar to other states where I have worked which would be that you are allowed one six minute period of opacity greater than 20% but less than 27% per hour. Any six-minute period greater than 27% and anything more than one per hour greater than 20% counts as one of the excursions in the 12,000 number. Of course, no smoke reader ever worked 24/7 and there are specific requirements when you can read smoke so these numbers are based on continuous opacity monitors (COMS). Those monitors basically measures the amount of light passing through the stack to determine the opacity.
Before the compliance lawyers and the environmental organizations glommed on to this as one more way to harass coal-fired power plants, EPA had realistic guidance. The EPA Office of Enforcement and Compliance Assurance published guidance entitled “The Timely and Appropriate Enforcement Response to High Priority Violations” in June 1999. The policy was designed to direct scrutiny to those violations that are most important. It included a specific case study to determine if opacity would be considered a high priority violation. The most stringent policy defined a high priority violation for opacity when measured with COMS as one that exceeds 30% opacity greater than 3% of the operating time during a quarterly reporting period, excluding exemptions for startup, shutdown and malfunction. Given the 0.46% number over five years I doubt that these plants exceeded that limit.
The bottom line is that any realistic evaluation of health or welfare impacts associated with opacity would conclude that the impacts are very small. Local problems associated with smoke stacks (e.g. acid smut) are not necessarily correlated to opacity violations. Fine particulate matter measurements indicate a small fraction associated with directly emitted power plant particulate matter. In other words the fine particulate matter that the environmental organizations are so worried about is not associated with opacity. Nonetheless, the big number quoted without any context, the impression that coal-fired power plants are evil, and implication that the noble lawyers at the Sierra Club are watching out for our best interests will likely result in more added costs to produce power in the way of a fine or additional control equipment requirements.

If you can see it, it is neither steam nor water vapor. Technically, what we see coming out of the smokestacks are clouds, made up of tiny water droplets. But the term clouds is reserved for the meteorological phenomenon. The trouble is, no name has been given, although I suppose it could just be called “condensate”.

@rogercaiazza says:
March 26, 2014 at 1:46 pm
First, thanks for helping me fulfill my daily goal of “learn something new every day.” Next, are the 6-minute periods serial or on a rolling basis? Is it 12:00 – 12:06, 12:06 – 12:12, 12:12 -12:18, etc, or 12:00 – 12:06, 12:01 – 12:07, 12:02 – 12:08 with some kind of averaging?