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The Pros and Cons of Particulate Matter Monitors

Most large-scale combustion processes need to measure the amount of particulate matter (PM) leaving their stack. The main exceptions are those burning natural gas, which is assumed to contain no particles. AMETEK Land offers three instruments which measure PM – the 4500 MkIII, 4650-PM and 4750-PM as shown in Figure 1. They all work by measuring the interactions between a beam of light and the particles in the stack gases, as shown in Figure 2. 

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Figure 1 - AMETEK Land PM measurement technology.

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Figure 2 - Interactions between a beam of light and the particles in stack gases

The 4500 MkIII is an opacity monitor which measures the loss of intensity when a beam of light crosses a stack containing particles. It is ideal for processes where the emission limit value (ELV) is set as % opacity rather than mg/m3 or where the stack contains a significant amount of particulate matter. It is especially valuable in processes where there is flow stratification as the light beam crosses the full width of the stack. As a general rule, the 4500 MkIII works well when the ELV is greater than 50 mg/m3 divided by the stack diameter in metres. For a 2 m diameter stack, the 4500 MkIII can be used when the ELV is greater than 25 mg/m3.

Opacity measurement is not suitable for measurements in very clean stacks because it works by measuring the difference between the light intensity when there is no PM present (zero opacity) and when some light is lost through scattering. Small amounts of PM scatter very little light so it is difficult to distinguish changes in intensity which can be 1% or less. Laser light-scattering is more sensitive because there is no signal when there are no particles in the beam. Because of this, the detection limit can be as low as 0.1 mg/m3, and such instruments can be used on processes where the ELV is only 5 mg/m3.

The 4650-PM and 4750-PM both work by measuring the amount of light scattered by particles passing through a red laser beam. The 4650-PM measures the amount of light scattered in a forward direction, a few degrees away from the direction of the laser beam. The 4750-PM uses back-scattering, measuring the amount of light scattered back towards the light source. The different geometries mean the instruments have different characteristics. To see the relative scattering intensity for a particle of fly ash in a stack visit here.  The intensity in the forward direction is substantially greater than that in the backward direction. The underlying physics also shows that the intensity of forward-scattered light is less dependent on particle size than is the back-scattered light.
 
To measure the forward-scattered light, the 4650-PM uses a concave mirror mounted on the end of a probe which is inserted into the stack. The mirror collects the light and focuses it onto a quartz rod which transmits it to a detector mounted outside the stack, as shown in Figure 4. The interaction region, where the PM scatters the laser light, is a relatively small volume, so it is important that the 4650-PM probe is mounted in a location where the PM concentration is representative of that in the stack as a whole. 

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Figure 3 - See how the 4650-PM measures forward-scattered light.

The 4750-PM shines a beam of light into the stack at a slight angle and measures the amount of light scattered back towards the instrument. This geometry means that the interaction region is larger than for a forward-scattering measurement, so the instrument is somewhat less sensitive to variations in PM concentration across the stack. However, there are some drawbacks – the reduced intensity of the back-scattered light means the instrument is less sensitive than the 4650-PM and the physics of light-scattering mean that changes in particle size have a larger effect on the instrument’s reading than for the alternative measuring instruments. It is also important that the laser light hitting the far side of the stack is not reflected back to the detector, as this will cause a large offset in the reading. As a consequence, the minimum stack size for the 4750-PM is around 1.5 m, as shown in Figure 5.

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Figure 4 - Minimum stack size for the 4750-PM

All PM monitors have to be manually audited from time to time, typically once every three months. The different measurement methods and geometries mean that different approaches are needed. For an opacity monitor, an External Zero Device (EZD) is fitted to the front of the transceiver. This simulates a clear-path condition so that the instrument reads 0% opacity. Calibrated neutral-density filters are inserted into the light beam and the readings compared to the calibrated value. This is a fast and easy process which can usually be performed while the instrument is still mounted on the stack. 

Because the 4650-PM probe is inserted into the stack, it must be withdrawn and allowed to cool before the audit can be performed. The audit is performed by inserting a calibrated scattering device is inserted into the interaction region.

Auditing the 4750-PM requires a manual audit unit which scatters some of the laser light back to the detector. As with the 4500 MkIII, this process can be done while the instrument is mounted on the stack.

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