When it comes to portable gas analyzers for measuring emissions, not all units are created equal. The selection of your portable gas analyzer matters most 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. This guide will cover the important issues you need to understand regarding portable CEMS.

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 the readings of a CEMS.  

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 Doesn’t a Portable Measure as Accurately as CEMS?

Technology: One of the most frequently used 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 the gas concentration.  

To their credit, these sensors can be very sensitive at targeting gas and can measure down to parts per million (ppm). They are ideally suited for “fixed” gas detection (i.e., ambient monitoring to alert when leaks occur). 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. That is for several reasons:

• As the chemical catalyst in the cells is consumed, the properties of the cell change.  Constant re-calibration 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, the properties change, potentially requiring calibration tweaks.

• Operation in low oxygen environments alters sensor performance when oxidation of the target gas takes place at the sensing electrode. That can occur for certain ammonia, carbon monoxide, and hydrogen sulfide electrochemical cells.

• Reading very low levels (i.e., sub 20 ppm) accurately is not typically possible for these types of cells. Most portables will have CO ranges only down to 0-1,000 ppm. Reading 10-20 ppm on a scale of 0-1,000 is problematic. Typically less than 10% of the scale can be in the noise area of an analyzer, and it is much less accurate or reliable than a non-portable CEMS.

• Another reason for inaccuracy with portables is that the calibration bottles typically used to check them have a ±10% accuracy.  If a 0-1,000 ppm scale is off by up to 100 ppm, imagine how accurate a 9 ppm reading will be.

Application: The biggest detriment to continuously running a portable with electrochemical technology is usually the accuracy of readings of 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. That process leaves lots of moisture behind in the cells. The condensation 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 a 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 or other gases that react with the cell chemistry are present, it can affect readings and the instruments’ ability to operate properly.

Bottom Line for Electrochemical Portables:

1. Inexpensive (relatively)

2. Good for quick measurements in an exhaust gas stream

3. Good for high-level concentrations

4. Not ideal for longer-term sampling (studies, monitoring changing conditions)

5. Not ideal for low-level concentrations

6. Not ideal for high accuracy and precision

The SOLUTION–A true Portable CEMS

So, how do you conduct longer-term measurements of an exhaust or process stream when you need accurate and reliable results? If a manufacturer calls their portable a “CEMS”, 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:

Sample Handling:

Probe & Umbilical: 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.

The probe and umbilical will act to keep the sample above the dewpoint and prevent condensation. Besides preventing the formation of unwanted condensables 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 various components:

• Multiple water scrubbers

• Charcoal Scrubber

• Inline Fine Particulate Filter (0.3µm) and Micro-fine Dust Filter (0.2µm)

• Thermo-electric Chiller using Peltier Cooling with Peristaltic Pump for Continuous Moisture Removal

• Diaphragm-type 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.

Measurement Technologies:

The best portable for low-level continuous gas measurement will not utilize electrochemical cells (although Oxygen cells are sometimes used). Technologies that measure accurately and precisely will be utilized and will offer much higher reliability and minimal maintenance. Multiple technologies will also be 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 the 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 impact 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:

NOx (NDIR)	0-100/250/500/1,000/2,500 ppm
CO (NDIR)	0-300/500/1,000/2,000/5,000 ppm
CO2 (NDIR)	0-1/10/20/30 vol%
SO2 (NDUV)	0-50/100/200/500/1,000/3,000 ppm

 

SO2 (Sulfur Dioxide) is measured with NDUV (non-dispersive ultraviolet) technology. This works similarly 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. The measurements are almost always in Percent (%) ranges where high precision is not as critical. Another technology called Paramagnetic can be utilized for more accurate measurements.

Unlike electrochemical sensing technologies, a Paramagnetic cell never needs changing. 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 offers uncompromised measurement performance. In this case, the analyzer measures the absorption of specific laser light wavelengths.

CONCLUSION:

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 its advantages, is simply not the device for this type of service.

Monitoring Solutions offers the ETG 7550P, which offers all the advantages and performance of a true portable CEMS. With a wide range of gases (including 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.