Chemiluminescence has long been recognized as the best practical analytical method for NOx detection in a wide variety of applications. The technology is easily capable of making PPT (parts per trillion) measurements in research settings. Chemiluminescence analyzers are recognized as the only practical choice for ambient air quality monitoring stations, where thousands of units are deployed worldwide to measure PPB (Parts Per Billion) ambient concentrations. Chemiluminescence analyzers are also the overwhelmingly dominant choice in regulatory combustion monitoring systems, especially as emissions regulations become stricter and permissible levels get lower each year. Single-digit PPM NOx permits, almost unimaginable a decade ago, are now standard in many areas, and these are easily addressed with good "chemi" analyzers, a regime where other technologies used in traditional coal-fired applications cannot perform.
While ultimate performance has never been an issue with chemiluminescence-based NOx analyzers, reliability and maintenance records have been far from perfect. Delicate photomultiplier tubes and high voltage power supplies, thermally sensitive electronics, unreliable ozone generators, finicky NO2 converters, and many other problematic components have plagued many implementations of this technology over the years. To make matters worse, many manufacturers attempted to work around these problems by adding multiple diagnostics and adjustable parameters (and often confusing interfaces to manage all these), resulting in analyzers that were actually less reliable and harder to maintain. Brand-Gaus has developed a revolutionary chemilumenescence-based NOx detection architecture that removes nearly all the reliability issues associated with traditional NOx analyzers.
While there are some important differences in our implementation of this technology, at the most fundamental level Brand-Gaus chemiluminescence analyzers use the same detection method as other traditional analyzers.
Chemiluminescence literally means a chemical reaction that produces light. When an NO (nitric oxide) molecule reacts with ozone, it is oxidized to NO2, in an excited state. A small fraction of the molecules in this excited state decay by emitting a photon (i.e. giving off light) in the near infrared portion of the spectrum. Thus, if one mixes a gas sample with ozone and measures the amount of light emitted, the concentration of NOx in the sample may be determined. This technique provides an extraordinarily sensitive, selective, and linear measurement of NOx, precisely why chemiluminescence has become the standard high-performance NOx measurement technology. Any NO2 (nitrogen dioxide, the other component of NOx) in the sample may be converted to NO for measurement purposes, as discussed below.
In general, NOx exists as some mix of NO and NO2 together... with the relative fractions of these species depending upon a number of factors. NO is the thermodynamically favored species at high temperatures, which is why it is the dominant species of NOx in combustion exhausts. NO2 is the thermodynamically favored species at low temperatures, making it the dominant species in ambient air. But, the conversion between the species happens extremely slowly at room temperature...while most chemical reactions occur in a small fraction of a second, this conversion takes up to a day under normal atmospheric conditions.
Conversion of NO2 to NO, necessary to get an accurate reading of total NOx since only NO can be detected, can be accomplished in a number of ways. At elevated temperatures, the reaction between NO and NO2 occurs very quickly, and if the temperature is high enough essentially all NO2 in a sample can be converted to NO. As the sample cools, the NOx is temporarily "frozen" as NO since the timescale for conversion back to NO2 is so long. This is the basis for high temperature thermal conversion.
Other converters either try to exploit this basic technique, using catalysts for lower temperature operation, or use a reducing agent that becomes oxidized by stripping one oxygen atom away from NO2 to convert it to NO. While these strategies lower the required operating temperature of the converter, they introduce catalytic or consumptive materials that are often poisoned, depleted, or reacted unpredictably.
As mentioned above, traditional implementations of this chemiluminescence suffer from a number of practical problems, mostly related to reliability and maintainability. In 1999, Brand-Gaus embarked on a project to develop more reliable NOx detection by examining the most problematic aspects of NOx detection technology and eliminating these problems while preserving or enhancing performance. After several years of research, we were able develop an architecture and engineer components that are extraordinarily rugged and reliable. Below is a partial list of the innovations employed in our NOx analyzers and monitors.
Now, with our next-generation chemiluminescence technology, owning and operating a state-of-the-art NOx analyzer no longer requires extensive babysitting, a storeroom full of spare parts, or special expertise to keep it running. Just like your favorite DVM, it simply works, and you don't have to think about it. And best of all, because the old paradigms of extensive "tribal knowledge" and on-site spares requirements are virtually eliminated, there is almost no switching cost to move to the most reliable NOx detection equipment available!