Norsk Elektro Optikk (NEO) has been part of a research project where the goal was to design a compact analyzer for DeNOx processes based on mid-infrared lasers. Most of the technology already exists in the LaserGas™ Q and the applications can already be solved using this family of analyzers. Below you can read an article written by Dr. Peter Geiser explaining how the LaserGas™ Q NO and NO2 may be the next step to further optimize your DeNOx processes.
Optimization of DeNOx processes
All combustion processes generate emissions containing gases that are potentially harmful for the environment. The formation of acid rain is a crucial and well-known consequence of these emissions. In general, nitrogen oxides (NOx) and sulphur dioxide (SO2) are considered to be the main precursors of acid rain, therefore their emission must be limited or even avoided. Governmental bodies are defining new regulations lowering the emission limits on a regular basis. Consequently, the industry has to increase their efforts to improve their processes and emission control systems.
The majority of industrial users has accepted near-infrared tunable laser absorption spectroscopy (NIR-TLAS) as the best available technique for in-situ emission and process control measurements due to easy installation, fast response time, high accuracy and low maintenance requirements of the sensors compared to extractive systems.
In recent years analyzers using mid-infrared tunable laser absorption spectroscopy (MIR-TLAS), either based on Interband Cascade Lasers (ICL) or Quantum Cascade Lasers (QCL), have emerged in the market. While having the same benefits as NIR-TLAS analyzers, MIR-TLAS utilizes another wavelength region which enables new applications that earlier were considered impossible due to lack of absorption lines in the established wavelength regions or due to strong interference from other gases.
A “DeNOx-process” can be used to reduce NOx emissions from combustion processes. As shown in Figure 1, in a DeNOx-process – also referred to as Selective Catalytic Reduction (SCR) or Selective Non-Catalytic Reduction (SNCR) – either gaseous ammonia (NH3) or urea is used to convert NOx and NH3 into water vapor (H2O) and nitrogen (N2).
The amount of ammonia needed depends on the NOx content of the process gas. Especially in a dynamic process, where the fuel composition changes rapidly, the variation of the NO concentration can be quite high. To regulate the “DeNOx-process” and thus the amount of ammonia added to the process gas, the NOx content has to be measured in-situ, very accurately, and with a fast response time as close as possible to the combustion zone. While extractive systems cannot be used due to the slow response time, near-Infrared NO sensors suffer from strong interference from water vapor and low sensitivity at higher temperatures. Therefore, currently, the remaining ammonia is measured (‘ammonia slip’) to control the DeNOx-process, and NO emissions are merely monitored.
However, it is desirable to use a direct NOx measurement as the control signal early in the process. By measuring NOx before and after the catalytic reduction, full control over the NOx concentration is available and important information about the efficiency in the catalytic reduction process is provided. In this way, the plant operator has the possibility to adjust the NH3 or urea injection accordingly with a much higher precision.
In Table 1, typical process data of a DeNOx application are listed. The information is based on multiple inquiries for this kind of application. As can be seen, the target concentration level of NO is in the ppm range while the main gas components are water vapor and carbon dioxide, typically in the percent range.
Traditionally the NOx is calculated by multiplying the NO value by 1.1 to account for the additional NO2. This might be considered accurate enough for reporting but it is considered to be too inaccurate for DeNOx optimization. The ratio between NO and NO2 may also change rapidly based on variations in the combustion process. This will further decrease the accuracy of the calculated total NOx concentration.
With the new MIR-TLAS analyzers both NO and NO2 can be monitored with high accuracy (< 1 ppm) even with water concentrations up to more than 30%. In other words a “true” NOx value is measured. As long as the measurements are done in-situ the response time can be kept at a minimum, ensuring fast and accurate process optimization.
Figure 2 is depicting an SNCR system for NOx control in a boiler. This is only an example, but illustrates the principle. There are many variations, depending on the industry, leading to different operating conditions (gas temperature and pressure) and gas compositions.
An accurate measurement of NO and NO2 will help the plant operators to control and monitor the injection of NH3, and at the same time, optimizing the SCR or SNCR systems, respectively. Furthermore, the plant operators can predict more precisely, when catalysts in an SCR system have to be replaced, since an optimized DeNOx-process reduces deposition, plugging and potential corrosion. For example: With an ammonia slip of less than 2 ppm, air heaters can run six months to one year before washing is required. In case the ammonia slip is above 10 ppm, washing is required every two weeks to three months.
Therefore, an optimized DeNOx-process has a benefit for both, the end user and the environment.