This article discusses the fundamentals of measuring NOx, including the three common approaches for obtaining a NOx measurement and the importance of maintaining accuracy.
How and Why Should One Measure NOx?
NOx emissions (NO & NO2) must be closely monitored with a gas analyzer to ensure that combustion systems, such as boilers and engines, are running safely and efficiently as well as complying with environmental regulations.
Traditional methods for measuring NOx involve the extraction and analysis of a representative flue gas sample from the stack or exhaust of a combustion source. Common sensor technologies for measuring NOx include electrochemical, non-dispersive infrared (NDIR), and chemiluminescence technologies. Depending on the application and motivation for testing, there are three common approaches for obtaining a NOx measurement:
Three Common Approaches for Measuring NOx
Also known as Calculated NOx, this method involves the direct measurement of Nitric Oxide (NO), and the subsequent calculation of the expected Nitrogen Dioxide (NO2) content using an assumed NO to NO2 ratio (typically 1.05). For example, a direct NO measurement of 50 ppm would be multiplied by 1.05 for a NOx reading of 52.5 ppm.
While this method of measurement may be acceptable for certain applications, it is not the most accurate method because the ratio may vary depending on the fuel type and the combustion process.
This method is significantly more accurate than a Simple NOx reading because it involves the direct measurement of both NO and NO2 using separate high accuracy gas sensors (commonly electrochemical sensors). The Total NOx reading is therefore the sum of the measured NO and the measured NO2. Certain environmental agencies, like the U.S. EPA, may require this more accurate NOx reading.
This is the most accurate method of NOx measurement because it considers the water solubility of NO2. When NO2 comes into contact with condensation, as much as half of the gas concentration can dissolve and significantly affect (reduce) the gas measurement. To account for this, certain sampling techniques can be employed to maintain the integrity of the gas sample and eliminate any water that may accumulate within the sample line.
The most common and effective of these techniques involves a cooling system, like an thermoelectric chiller, which rapidly cools and dries the flue gas sample prior to entering the sample hose and the gas analyzer. This measurement method ensures that the NO and NO2 readings are highly accurate and representative of the NOx emissions being released from the combustion process.
The Importance of Oxygen Reference
Monitoring and reporting NOx emissions levels are critical duties for engineers working in certain industries that utilize fossil fuel-fired combustion systems. In order to prevent facilities from intentionally diluting their emissions readings with O2 to artificially reduce NOx levels, regulatory agencies often require emissions readings to be reported based on standard O2 reference levels.
This process allows regulatory agencies to prevent these attempts to falsely reduce reported NOx emissions levels. Emissions from boilers are commonly calculated to a 3% O2 reference level, and engine emissions are commonly calculated to a 15% O2 reference level.
What is Low NOx?
As we become increasingly aware of the negative impact that fossil fuel emissions have on human and environment health, federal and state environmental agencies are instituting more stringent emissions restrictions and decreasing the permissible emissions levels of certain toxic gases like NOx.
Manufacturers of combustion systems are now required to ensure that their equipment is low NOx compliant, and technicians working in the residential, commercial, and industrial sectors are required to install and maintain equipment that meets the low NOx emissions regulations.
The term “Low NOx” commonly refers to NOx emissions levels below 500 ppm, but certain regulations require emissions to be below 10 ppm. At these low levels, it is imperative to measure True NOx (i.e., NO + NO2) to reduce the error, as a few ppm accounts for a significant portion of the NOx emissions. A resolution of 0.1 ppm is often needed at these low levels to increase accuracy when measuring.
How to Best Achieve High Accuracy Gas Measurements
110 Years of Combustion Analysis Experience
Ultimately, here at Bacharach, we want you to take the right measurements to ensure that your combustion systems are running safely and efficiently as well as complying with environmental regulations. In fact, for over 100 years, Bacharach has been making high-performance combustion analyzers, and therefore we have a wealth of combustion analysis expertise to offer our customers.
Now that we’ve explored the fundamentals of NOx testing, the last point to address is how to best achieve such high accuracy gas measurements. The answer? A combustion analyzer. And, in this case, the Bacharach PCA 400 portable combustion gas and emissions analyzer.
PCA 400 Combustion & Emissions Analyzer
Indeed, the PCA 400 is a powerful and advanced solution providing high accuracy gas measurements for EPA compliance–level monitoring of boilers, engines, and other combustion systems. It’s capable of performing Simple NOx, Total NOx, and True NOx measurements and is available with an optional thermoelectric chiller for high accuracy Low NOx readings.
The instrument also features an oxygen reference setting, a dilution feature to protect gas sensors under high load conditions, and a variety of probe and hose options to satisfy a wide range of applications. Plus, users have the option to save measured data to internal memory, or stream it live via Bluetooth® communications to a PC or smart handheld device for compliance reporting. Finally, emissions reports can be generated easily and quickly to meet compliance requirements.
Undoubtedly, the PCA 400 is the best way for you to achieve high accuracy gas measurements for NOx compliance, and you can learn about what it can do for you today here. ∎
How are you currently measuring your NOx emissions? Let us know by leaving a comment below.