Choose the right NOx Control Technology

The National Ambient Air Quality Standards for Ozone (NAAQS), are not met by most major urban areas of the United States. Atmospheric research has shown that ozone is formed by a complex series of chemical reactions, including volatile organic compounds (VOCs), and nitrogen oxides. These studies show that urban areas with higher VOC/NOx ratios than 15:1 can reduce ambient levels of ozone by reducing NOx emission. To comply with the NAAQS standard, many states have adopted NOx control regulations regarding combustion devices.

This article discusses how to characterize NOx emissions from industrial combustion engines. The article then gives guidance on how to assess the available NOx control technologies, and choose an appropriate control method.

Characterizing Emissions

The NOx emissions from most industrial combustion devices have not yet been measured. These units’ NOx emissions have been simply calculated using a variety of factors. However, recent regulations require that the NOx emissions of affected units be known with certainty. This will allow each unit to be assessed for compliance and will also permit the identification of applicable fee control technologies for units that require modifications in order to comply.

To verify the NOx emissions characteristics of each combustion device, it is important to test them all. To provide timely and relevant information to make decisions about the applicability or non-applicability of NOx control technology, the testing process should be simplified.

It is common to choose one device from a group of units for character testing (NOx CO2, 02, and 2) The unit is tested at three load points, which are the normal operating range. Excess oxygen variation testing is done at each load point. Figure 1 shows the results of typical characterisation tests. The remaining classes are only tested at one load point at full load.

The operational data collected during testing are combined with the NOx, CO and NOx data used to determine the compliance status of each unit as well as the relevant NOx control technologies that need to be modified. This approach allows multiple units to be tested simultaneously and provides the required operational data for the engineer to evaluate any potential NOx control technology.

Fundamental Concepts

RACT (reasonably available control technology) standards for NOx emissions are defined as an emission limit such as 0.2 lbNOx/MMBtu. This is in place of mandating specific NOx control technologies. There may be a variety of control options depending on the fuel used and the design of the combustion engine. It is important to know how NOx emissions are made before you choose the right RACT.

The combustion process produces NOx emissions. This is a function both of the fuel composition and operating mode. Each parameter can have a significant impact on the final level NOx emissions.

NOx formation can be attributed to three distinct mechanisms.

1. Formation of Thermal NOx;

2. No formation, prompt (i.e.. quickly forming);

3. Formation of NOx in fuel