Facts about exhaust gas sampling
The sampling system design is the most important element in all CEMS.
The most important element
Even “In-Situ / Cross Stack” CEMS suppliers are meet with the harsh environment in the exhaust gas, they are however often trying to high-light the advantages of no sampling system, which often come out as a disadvantage due to harsh environment being mounted directly onto the exhaust duct which challenge the sensible optics, mirrors, light source and electronic components with problems such as broken light source, fiber optics and mirrors misalignment, due to vibrations, shocks and temperature changes. Another challenge is to convince yourselves if the system is measuring correctly or not, as there is not always a possibility to perform a certified gas calibration on such systems.
As being a CEMS service engineer for more than 30 years and with my hands-on experiences with following methods; “In-situ / Cross Stack”, heated direct sampling, heated sampling with cooler, Nafion tube drying, and dilution extractive sampling system, there is no doubt in my mind that the dilution sampling CEMS is the least maintenance requiring systems available, and the systems providing the best data capture overall time. – Carsten Hansen
Before purchasing a SO2/CO2 CEMS, verify that its detection limits are low enough, as the SO2 content in the exhaust gas is only around 17 ppm for fuel containing 0.1% of sulfur.
Corroborated by experience
My opinion seems corroborated, in particular, by the 2012 figures for the industry, which indicate that the vast majority of fossil-fired utility generating units as well as many industrial sources in the USA have installed and certified continuous emission monitoring systems (CEMS) based on dilution sampling under EPA’s 40 CFR Part 75 CEM Rule, as required under the Acid Rain Program as well as the NOX Budget Program and Clean Air Interstate Rule (CAIR). The 2012 data show that 92.9% of SO2 and 93% of CO2 continuous emission monitoring systems use dilution probe sampling systems.
We see minimum ranges of 0 to 200 ppm and even ranges up to 0 to 1000 ppm SO2. With a precision of ±2% FS (±4 ppm & ±20 ppm) will these systems have difficulties establishing if you are in compliance or not!
The reason why dilution probe sampling CEMS are superior to all other methods is the fact that they measure virtually clean air, sometimes cleaner than the air we breathe in our cities.
This is made possible because of the use of ambient air quality gas sensors which are designed to measure very low concentrations in the ppb and ppt ranges, which thereby give us the benefit of high dilution rates – even at a dilution rate of 100: 1, we still have a lowest detectable limit (LDL) of less than 100 ppb = 0.1 ppm. No other CEMS provides such dynamic ranges.
Make sure that the complete CEMS is being calibrated/verified with calibration gas from the probe tip to the analyzer output, as it is an established fact that the sampling and sample conditioning systems are the main sources of operational failures!