Our previous article discussed key considerations that people who are tasked with making decisions around mercury measurements should be aware of. If you missed it, you can find it here.
This article will delve deeper into one of those challenges or specifically, the equipment which is being tasked with making those Hg measurements continuously.
Let’s recap the challenges:
- Low Concentrations: In flue gas Hg is present in ppb or even ppt levels.
- Interferences: Other constituents in the flue gas such as NOx and SOx can blind a mercury analyzer and are present in much higher concentrations than Hg.
- Testing Location: Testing at the stack is much easier than testing at the inlet of an Hg control technology. But, what if you want to evaluate its efficiency?
- Sample Transport: Traditional CEMs are extractive systems that require long umbilical lines to transport the sample to where it is analyzed. But, Hg is incredibly reactive so transport is its own can of worms.
- Equipment Limitations : Discussed shortly…
These challenges are big obstacles for traditional mercury (Hg) CEMs!
To overcome some of these issues, all surfaces coming in contact with the sample must be inert or kept at extremely hot temperatures. Any cold spots in the system can lead to biased measurements because mercury will react with the condensable material that builds up on these surfaces. These temperatures are also especially difficult to maintain when they have to be consistently maintained in a heated sample line that is 100-500 ft long. Moreover, because of these high temperatures, heated umbilical lines are prone to burning and at such extreme lengths, they can be very expensive to replace.
To deal with moisture and acid gases, which can damage internal components of the monitors and/or interfere with the measurements, most mercury CEMS in the USA use dilution probes. Given the low Hg concentrations these monitors are already tasked with measuring, this requires the analyzers to be able to accurately detect mercury at the parts-per-trillion (ppt) level. That’s incredible sensitivity! Unfortunately, most monitors are just not that sensitive. In fact, the University of North Dakota even conducted a study by the Electric Energy Research Center to determine the variability of continuous mercury monitors at low mercury concentrations. They evaluated PS 12A compliant CEMs that exist on the market (Tekran and Thermo) and concluded that the precision and accuracy was close to ± 0.5 ug/dscm. This still passes a RATA, but is often not enough sensitivity for mercury control optimization where the objective is cost savings. (See the study here.)
In flue gas, mercury also exists in both elemental form and oxidized species such as mercuric chloride. The problem is that mercury analyzers can only detect the elemental form, meaning that all oxidized species must first be converted to elemental mercury prior to entering the analyzer. If re-oxidation occurs, it won’t be detected by the equipment. This leads to a need for an extremely reliable thermal converter. Unfortunately, converters are a common point of failure in these types of monitors and aside from long lead times, they can be expensive to replace.
The analyzers used in these systems are typically rack-mounted and thus need to be housed in temperature-controlled rooms, buildings, or large custom-built enclosures. Though these are common in the USA at power plants, that is not necessarily the case for the cement industry, incinerators, or smaller industrial boilers adding to the expense of these monitors.
All of this makes for large complicated systems. These systems necessitate stocking spare parts in order to deal with the extreme lead times that currently exist in the supply chain. But more importantly, these systems require sophisticated expertise to service and maintain them. Unfortunately, turnover of these sophisticated technicians can also be high while the learning curve is lengthy and steep for new technicians.
Still, for many industries, there are requirements in the U.S. (and other parts of the world) to measure mercury continuously.
But is there another option?
THERE IS!! Click here to read on.
