Almost every Continuous Emissions Monitoring System (CEMS) has either an Oxygen Analyzer or a Carbon Dioxide (CO₂) Analyzer.  If you were to look in most air permits that call for the requirement of a CEMS you would find gases listed like NOx (Nitric Oxides), CO (Carbon Monoxide), SO2 (Sulfur Dioxide), Hg (Mercury), and/or THC (Total Hydrocarbons).  But in most cases, you wouldn’t see an Oxygen Analyzer or Carbon Dioxide Analyzer called out specifically.  So why do 99% of all CEMS have one of these two analyzers?

The answer is that they are used as “diluent” analyzers.  In the US EPA CFR40 Part 60 and CFR40 Part 75 regulations you’ll find the requirement for a diluent analyzer spelled out.  You may also find it called out in the way an air permit limit is expressed.  It’s common to see a NOx limit expressed for instance as “10 ppm corrected to 7% O₂”.  These references are where the diluent analyzer requirement comes from even when they are not specified in an air permit.

Here is a technical definition:

A diluent gas is a constituent of flue gas that is measured by the analyzer system, not because it is a pollutant, but because its measurement can be used to provide values used to calculate heat input and pollutant levels.  It is important because we know that its ratio to total stack flow is proportional to heat input.

Diluent gases include O₂ and CO₂.  Although CO₂ is also a pollutant, in this instance It is measured in its role as a diluent gas.  Diluent gas values are measured (via a gas analyzer) to establish the data used to compute emission rates and to calculate Heat Input (which is important because it is often used in pollution emission mass calculations).

Here’s another purpose of the diluent analyzer; a CEMS measures a gas from a sample point near the outlet of a stack.  If a measurement of 10 ppm was present, it would be simple to reduce this number by opening a large service door at the bottom of the stack for instance and drawing in more oxygen.  The extra oxygen would dilute the sample being read by the CEMS while the process would still be putting out the 10 ppm.  This would certainly be beneficial to someone who didn’t want to report too high a level of a measured pollutant.

From the EPA’s standpoint it would be pretty easy to manipulate bad emissions numbers by finding a way to add air into the process after the combustion takes place.  Besides a nefarious method to introduce air, there are many more non-malicious ways that air can enter a process affecting the emissions measured.  Upstream of the CEMS are all kinds of different equipment - an SCR (selective-catalytic-reduction, an ESP (electro-static precipitator), a baghouse, an ID fan (induced-draft) and more.  Anyone of these sources and all the related piping and ducting connecting them create a potential source for leaks and air infiltration.

So, the EPA came up with the requirement for a diluent analyzer to measure oxygen so that the readings can be normalized to a specific percentage (typically 3%, 7% or 15%).  This way even if there is air entering the process, the affects will be accounted for and the pollutant level that the process is generating will still be measured, calculated and reported properly.

So why are both Carbon Dioxide and Oxygen used for diluent analyzers?  Within the EPA regulations one will find formulas for using CO₂ as a surrogate for Oxygen.  Using these formulas, a user can calculate oxygen percentages directly from a CO₂ measurement.  Therefore, if you have a CO₂ analyzer you will not need to add an oxygen analyzer for diluent calculations.  However, either analyzer can also be called out in a permit.  If none are called out though you still need one of them.

The good news is that Oxygen analyzers are extremely reliable and use extremely accurate and robust technologies.  The two main technologies are Paramagnetic and Zirconia Oxide.  Both require little to no maintenance, last a long time, and are extremely simple to maintain.  The zirconia oxide analyzer uses the same technology that’s in your car (when was the last time you calibrated that analyzer?).  It’s also about 1/3 to ½ the cost of the Paramagnetic analyzer and delivers the same performance for a CEMS application.

The CO₂ analyzers typically use NDIR (non-dispersive infrared) technology.  This is also a very robust and very reliable analyzer that will last up to 20 years or more.  They require very little maintenance and provide accurate results in a CEMS application.  The technology has been around for over 40 years and is completely refined.  Relative to other analyzers (like NOx or SO₂) its also an inexpensive analyzer (easily ½ the price).

Whatever the analyzer, Monitoring Solutions can assist with any of your measurement needs.  We sell, service, and support a wide range of Continuous Emissions Monitoring System (CEMS) Analyzers from manufacturers including Thermo, CAI, Servomex, and Brand-Gaus.  We sell our analyzers at the same price as the manufacturer and provide a 24/7 one year phone support contract for the first year at no charge (this has proven useful to customers as they get used to their new analyzers), We also offer free installation when buying three or more analyzers. 

Simply happy to help.
Monitoring Solutions is here to help! Call 908-500-4010 or email jnowak@monsol.com for a competitive quote.

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