It’s important that people who are tasked with making decisions involving this key measurement (mercury) to understand fundamental issues, limitations, and problems that may be included when measuring it.
So why are mercury measurements ESPECIALLY challenging?
Without getting into a highly technical discussion, there are 5 main points to consider:
- Low Concentrations
- Interferences
- Testing Location
- Sample Transport
- Equipment Limitations
1. Low Concentrations
The first question to ask is “what kind of levels are we measuring?” When it comes to oxygen and CO2 for example, we’re talking about measuring percent levels. With SOx and NOx we are talking about parts per million (ppm) measurements. All of this done with very basic instrumentation that is relatively inexpensive and has been around for dozens of years.
On the other hand, when we are talking about mercury, we’re measuring it on a part per billion (ppb) or even part per trillion (ppt) level!
To give a better idea of how low this concentration truly is, that would be like trying to find one drop of oil in a supertanker that contains ten million gallons of oil.
2. Interferences
The next issue is interferences. This becomes especially important when measuring such low levels because some interferences are present in much higher concentrations than the substance being measured. Taking mercury as an example again, SO2 and NOx are both interferences and can blind an instrument or a measurement method if not accounted for.
To give another analogy, this is like looking at a dense forest and trying to see a single bird’s nest through all the trees. Oh, and the bird’s nest is the size of a thimble.
3. Testing Location
The testing location is also a key consideration. Testing in the stack is relatively simple when compared to the inlet of a control technology where you may need to account for high acid gas concentrations, high particulate levels, and high temperatures. There are ways to mitigate all of these challenges, but it takes an extra level of expertise and experience to make sure the right equipment and technique is being used for each testing location.
4. Sample Transport
Next is sample transport. Not only are we looking for very low levels, but mercury is also especially reactive and tends to stick to or react with many of the materials and substances it comes in contact with. Unfortunately, the vast majority of instruments and EPA methods involve some kind of sample transport. The flue gas needs to be extracted from the stack or duct and then transported through glassware or sample lines to wherever the sample is measured or collected. Sometimes the transport distances can reach hundreds of feet. And many times, this includes taking the sample through filters, scrubbers, dilution systems, or other components. It is very important to understand what may be happening with the analyte while it is being extracted and transported.
Are there certain chemical reactions taking place?
How are the temperature variations affecting it?
Are there leaks in the system?
Is part of the analyte being lost in the system due to scrubbing on the various surfaces that it encounters such as tubing, titanium filters, or chemical scrubbers?
The longer the umbilical the more difficult this transport becomes for mercury specifically.
This is because there are certain Limitations in CEM Equipment that affect their ability to measure mercury. Click here to read on.
