Oxygen Normalisation – Video Transcript for the Hearing Impaired
Today we are going to talk about oxygen references. The concept of oxygen references is really important when it comes to stack testing and their purpose is to reverse the effect of pollutant dilution. Let’s think of an example. Say, we have a coal-fired boiler and in this coal-fired boiler stack there is a concentration of particulate matter of 200 milligrams per normal cubic meter (mg/Nm3). If ambient air is introduced into that stack, whether on purpose, or just as a virtue of how the process works, that means that this concentration is going to be diluted by the ambient air. We need a way that we can reverse the effect of this dilution so that we can compare this stack that has pollutant dilution, with another that doesn’t have dilution, and the way we do that is by using oxygen references.
Now to apply an oxygen reference, we multiply the concentration of the pollutant by an oxygen reference factor. That factor is 21% minus your oxygen reference, divided by, 21% minus the oxygen that you measure in the stack. Oxygen references are always specified in the legislation, and for a coal-fired boiler, the specified oxygen reference is 10%. We can insert that 10% into our equation. The bottom of this equation needs the oxygen concentration that is measured in the stack.Now keep in mind that ambient air contains 21% oxygen. 10% is sort of the typical oxygen concentration that you would expect directly downstream of a coal-fired boiler before any ambient air has been added. If ambient air has been added, your concentration of oxygen will be higher than 10%.
Let’s for example say that the concentration that we measure is 18%. So the oxygen reference factor equals 3.6. Now what that means is that every time that a stack has an oxygen concentration of 18%, we are going to multiply the pollutant concentration by this factor of 3.6. So the effect of dilution is going to be reversed because we are going to be multiplying the diluted concentration by this factor that is greater than 1, and basically making the concentration bigger and correcting it back to what it should have been. Oxygen references also work the other way around. So say for some reason the boiler is operating incorrectly and we are only measuring an oxygen concentration of 5%.
What happens to the oxygen reference factor is that it becomes less than one, and so we will then be multiplying the pollutant emissions by a factor of less than one, which will make the emissions smaller – once again correcting them so that they are representative of what is actually happening in the process, so that they can be compared on a like-for-like basis with other stacks.Something that is really important to know is that oxygen references can only be used in situations where combustion is taking place. During combustion, oxygen is consumed and the concentration of oxygen goes from 21% to a number lower than 21%.
If you have a process that is happening at ambient conditions, i.e. no combustion is happening, you will measure an oxygen concentration of 21% downstream of the process. What’s going to happen when you put that 21% in your equation, is that the bottom of your equation becomes zero. We know that we can’t divide by zero.So in summary, oxygen references are used to reverse the effect of pollutant dilution to make results comparable between stacks. But they can only be used when combustion is taking place and they cannot be used in ambient situations.
You can watch the video here.