Optical dust measurements

All dust particles will absorb and scatter incident light. The degree of absorption and scattering will depend on the dust concentration. Both phenomena are used for quantification of dust in our monitors. The attenuation of the incident light is unidirectional (in the direction of the beam). Light scattering will happen in all directions and one distinguishes first of all between forward scattering (also referred to as wave scattering) and back-scattering (or reflection) of light. The wavelength of the incident light will determine the minimum detectable dust particle size. Our instruments apply 670 nm laser light and dust particles of > 1 micron will be resolved. A determination of the particle size or of concentration as a function of particle size is NOT possible with our technique.

Optical dust measurement is not suitable in processes where the flue gas temperature is very close to or below dew point. This is due to the fact that water drops and aerosols will reflect and scatter the light just like dust particles do. This will lead to erroneous measurements.

Transmission measurements

The absorption of the incident light will be detected as transmission change. The light source (a laser diode for our instruments) and the detector have to be mounted diametrically opposite each other. The light intensity inside the dust volume is reduced according to the Beer-Lambert law:

I = Io exp (CL / k)

where Io is the incident light intensity, C is the dust concentration, L is the optical path length, and k is a dust dependent calibration constant.
The transmission T may be expressed as

T = I / Io

and the dust concentration as

C = k / L ln(T)

The measurement signal corresponds to the average dust concentration over the entire optical path (stack). Opacity may be measured as alternative to dust concentration and transmission. This method is very well suited for the measurement of high dust concentrations with a virtually unlimited dynamic range (up to 10 g/m3 for our instruments).
Unfortunately, dust calculations based on light absorption are not very accurate for low dust concentrations (below 10 mg/m3) and they are sensitive to dust deposition on the optical windows of the instruments.

Scattered light measurements

Dust particles will scatter the laser light in much the same way particles appear to 'light up' when drifting through a ray of sunlight. The amount of scattered light is directly proportional to the dust concentration and may be expressed as follows:

C = A Isca

where A is a linear calibration constant, Isca is the intensity of the scattered light and C is the dust concentration.

When using forward scattered light - like in our instruments - the light source and the detector have to be mounted diametrically opposite each other. The directly transmitted light must be blocked in front of the detector.

Dust calculations based on forward scattered light are very accurate for low concentrations (can detect down to approximately 0.1 mg/m3). As our instruments measure light absorption (transmission) and light scattering at the same time (see instrument set-up below), it is possible to normalise the measured scattered value with the absorption signal. Any interference from dust on the optical windows is then eliminated. The disadvantage of this method is the low dynamic range (the signal will typically saturate at around 200 – 500 mg/m3)

Instrument set up

A typical set-up of our LaserDust instrument mounted on a stack is shown in the figure below (click on figure to enlarge).

The LaserDust can be operated in both a/m measurement modes, because the receiver incorporates two separate detectors. The detector closest to the stack ("direct light") measures the absorption of the laser light, whereas the second detector ("scattered light") measures the forward scattered light generated by the dust particles. The "direct light" detector also blocks the directly transmitted light for the scattered light detector.

When operated in the scattered light mode the geometry given by the lens, an aperture in front of the detector, and the distance between the optical parts will determine the extension of the area that is probed in the stack. Depending on the instrument version and optical configuration this so-called active area is typically 0.5 – 5 m measured from the receiver window. Due to the limitation of the active area, dust settling on the instrument windows will not contribute to the scattered light signal. However, dust settling on the windows will attenuate the scattered light signal. As this effect will be the same for the directly transmitted light signal, the direct light signal may be used to normalise the scattered light signal. As a consequence operation in scattered light mode is virtually unaffected by dust deposition on the instrument windows.