Portable CEMS - the search for Accuracy
“Portable” Continuous Emissions Monitoring System (CEMS)
The search for accuracy
When it comes to portable gas analyzers for measuring emissions, not all units are created equal. When the selection of your portable matters most though is when you are using it as a CEMS (Continuous Emissions Monitoring System) – this means using it for some extended period of time (1-12 hours non-stop for instance). Whether you already own a CEMS or just need some detailed emissions data for an exhaust stream, the type of portable you use can make a tremendous difference in the results you obtain.
Checking a CEMS with a Portable Gas Analyzer
A significant mistake people make is using a portable gas analyzer to verify or check their CEMS readings. Many a repeat Stack Test was mistakenly performed (at great expense) because a portable’s readings didn’t match up with a CEMS readings.
Having a CEMS means you have an accurate and precise way to measure low concentrations of exhaust gases in hot and wet sample streams. Having a portable can often mean you simply have a way to measure “approximate” concentrations in hot-wet exhaust streams – and the higher the concentration the better.
Why don’t portable measure as accurately as CEMS?
Technology: One of the most frequently and least expensive technologies that portables utilize is electrochemical. An electrochemical cell is a device capable of generating electrical energy from chemical reactions. These chemical reactions produce a current that is directly proportional to gas concentration.
To their credit, these sensors can be very sensitive to target gas and can measure down to ppm levels. They are truly suitable and ideal for “fixed” gas detection (i.e. ambient monitoring to alert when leaks take place). They are also good for a quick generalized reading (spot check) in a stack or process. Where they fall short is when they are used for “continuous” detection for several reasons:
- As the chemical catalyst in the cells are consumed, the properties of the cell change. Constant recalibration may be required to maintain accurate readings.
- Humidity and moisture levels can affect the chemical catalyst. As the cell dries out, or conversely, absorbs moisture, again the properties change potentially requiring calibration tweaks.
- Operation in low oxygen environments alters sensor performance in sensors in which oxidation of the target gas takes place at the sensing electrode. Examples are certain ammonia, carbon monoxide, and hydrogen sulfide electrochemical cells.
- Reading very low levels (i.e. sub 20 ppm) accurately are not typically possible for these types of cells. Most portables will have CO ranges only down to 0-1,000 ppm. Reading 10 ppm on a scale of 0-1,000 is problematic as typically less than 10% of scale can be in the noise area of an analyzer and much less accurate or reliable.
- Another reason for inaccuracy with portables is that the calibration bottles typically used to check them are ±10% accuracy themselves. If a 0-1,000 ppm scale is off by up to 100 ppm, imagine how accurate a 9 ppm reading is going to be.
Application: The biggest detriment to running a portable with electrochemical technology continuously is usually the gas stream being measured. Most emissions sources (and many process sources) run at elevated temperatures and contain moisture. If a sample is being extracted, it will cool and condense leaving lots of moisture behind in the cells. This not only changes the measurement properties but can eventually saturate the cells rendering them non-operable. Short measurement times are usually recommended for this reason.
Many portable units come with some kind of hygroscopic moisture removal device, but they are usually tiny and again not meant for long measurement. Often too much moisture can clog the filters and/or overwhelm an internal pump.
Another issue is the gas stream itself. If corrosive gases are present or other gases that react with the cell chemistry, it can affect readings and the instruments’ ability to operate properly.
Bottom Line for Electrochemical Portables:
- Inexpensive (relatively)
- Good for quick measurements in an exhaust gas stream
- Good for high level concentrations
- Not ideal for longer term sampling (studies, monitoring changing conditions)
- Not ideal for low level concentrations
- Not ideal for high accuracy and precision
The SOLUTION – a true Portable CEMS
So, what if you want to do longer term measurements of an exhaust or process stream and you want accurate and reliable results? If a manufacturer is calling their portable a “CEMS” then it needs to address the issues above with both the hot-wet sample handling and the measurement technology. Here’s what to look for and how these issues are addressed:
Probe & Umblical: Even though it’s “Portable”, a Portable CEMS will still need a robust front-end sampling system. The best unit will have a heated probe and heated umbilical (sample tubing) for delivering the sample to the analyzer portion of the instrument. This will act to keep the sample above the dewpoint and prevent it from condensing. Besides preventing formation of unwanted condensibles and clogging, it also prevents premature removal of target gas during condensate formation.
Sample Conditioner: next in line before the analyzer is particle and moisture removal and cooling of the sample. In the best-case scenario this is accomplished with a bunch of different components:
- Multiple water scrubbers
- Charcoal Scrubber
- Inline Fine Particulate Filter (0.3µm) and Microfine Dust Filter (0.2µm)
- Thermoelectric Chiller using Peltier Cooling with Peristaltic Pump for Continuous Moisture Removal
- Diaphragmtype Sample Pump with Temperature Alarm
- Flowmeter for accurate control of sample
All the above components work together to deliver a clean and dry sample to the Portable Analyzer. They also ensure the ability to sample “continuously” without moisture saturation, plugging, and other issues.
The best portable for low level continuous gas measurement will not utilize electrochemical cells (although Oxygen cells are still used sometimes). Technologies that measure accurately and precisely will be utilized and in turn will offer much higher reliability and minimal maintenance. There will also be multiple technologies utilized depending on the target gas as different technologies are more specific and ideal for different gases.
NDIR – non-dispersive infrared technology is commonly used for NOx (Nitric Oxides), CO (Carbon Monoxide) and CO2 (Carbon Dioxide). In the simplest sense, using an infrared light source, the detector measures absorption of light at wavelengths specific to the target gas.
These analyzers are accurate, precise, and repeatable. They are also very stable and do not change characteristics over their run time. Since the gas going through the detector has been cleaned and dry, there is very little affect on the optics with regard to dirt and corrosion (thereby reducing maintenance requirements).
The key is the ability to measure accurately across many more, and lower, ranges. Here are some typical ranges:
SO2 (Sulfur Dioxide) is measured with NDUV (non-dispersive ultraviolet) technology. This works in a similar fashion to the NDIR except that ultraviolet waves are absorbed instead of IR light. Again, the advantage is in the ranges achievable. For example:
For Oxygen, electrochemical cells are often used as they are the most stable of these types of cells and the measurements are almost always in Percent (%) ranges where high precision is not as critical. Another technology called Paramagnetic can be utilized as well for even better measurement. Unlike electrochemical sensing technologies, a Paramagnetic cell never needs changing and its performance never deteriorates over time, reducing ongoing maintenance requirements and delivering a long operational life. The most common Oxygen range is 0-25% but ranges of 0-5% and 0-10% are also available.
For difficult gases like HCl (Hydrogen Chloride), NH3 (Ammonia) & CH4 (Methane), Tunable-Diode Laser (TDL) is a technology that offer uncompromised measurement performance. In this case the analyzer measure absorption of specific laser light wavelengths.
There are portable and temporary ways to measure exhaust and process gases available in the market. Using good sampling techniques and the proper measurement technologies, it’s possible to sample for long periods of time (hours) and obtain accurate and reliable results. The typical simple combustion analyzer, while having it’s advantages, is simply not the device for this type of service.
One option Monitoring Solutions offers is the ETG 7550P which offers all the advantages and performance of a true portable CEMS. With a wide range of gases (include gases like NOx, CO, CO2, O2, NH3, CH4, HCl and more) the 7550P offers reliable and accurate measurements in a complete cost-effective package.
Contact us today to learn more and request a quote.