Methods for Monitoring Stack Dust Particulates

Monitoring air quality is essential in reducing pollution in industrial settings. Continuous Emission Monitoring Systems (CEMS) track emissions, including particulate matter (PM) and gaseous pollutants, to help ensure compliance with environmental regulations. These systems provide a range of methods to accurately measure PM levels. They often correlate parameters, such as light scattering with dust concentration in the air.

Types of particulate matter monitoring techniques

Several techniques are available for PM monitoring, each suitable for specific conditions. The two main categories are in-situ systems and extractive systems.

• In-situ systems: These systems, such as point or cross-duct sensors, are used where temperatures are above the dew point.
• Extractive systems: These are applied when water droplets are present in the gas stream, requiring different handling.

In-situ techniques for particulate matter measurement

In-situ systems use light-based methods and probe electrification to measure PM. Here are the main types of in-situ PM measurement techniques:

• Light attenuation (Transmissometry)

  Transmissometry measures the amount of light absorbed by particles in a light beam. Opacity and extinction instruments measure the reduction in light directly. Ratio-metric opacity systems calculate the variation in light intensity to determine PM concentration. This technique is effective for higher dust levels.

• Light scattering

  Light scattering measures how particles scatter light in specific directions. Instruments measure forward, side or back scatter, depending on the setup. Forward scattering, especially, allows detection of dust at very low concentrations, making it highly suitable for environments with minimal particulate matter. The angle and intensity of the scattered light depend on particle size, shape and refractive index, providing a nuanced measurement.

• Probe electrification

  In this method, an electrical current generated by particles interacting with a grounded probe correlates to dust concentration. Probe electrification is sensitive to low concentrations but requires frequent maintenance, as sticky or moist particles can interfere with readings. This method is less suitable for use after electrostatic precipitators (ESPs) unless special shielding is used.

• Optical scintillation

  Optical scintillation differs from transmissometry by using a wide light beam without focusing lenses. It detects particle movement by measuring fluctuations in light frequency (scintillation) caused by particles in the gas stream. The higher the PM concentration, the more the light fluctuates. Optical scintillation is useful for measuring across a stack and provides readings by comparing to gravimetric data.