Laser based measurement of Dust
BackgroundDust monitoring will always be of great importance for industry world wide. Dust emission levels are decreasing with new and better filtering techniques and decreasing emission limits necessitates increased measurement accuracy. Continuous in situ dust monitoring is one of the most important means to meet these new demands. Furthermore, the ever increasing demand for low level detection limits and high accuracy require manufacturers of dust monitoring equipment to implement new and improved technology.
Meeting these demands has been our main goal during the development of our dust monitors. Norsk Elektro Optikk AS can therefore provide several Dust Monitors for continuous and accurate in situ dust measurements based on a unique technology with superior performance to conventional dust monitoring equipment. These dust monitors open up a whole new range of opportunities for industry in dust monitoring. By combining the best of diode laser technology and optical solutions, with rugged industrial design, the dust monitors can operate with a response time down to 0.05 seconds and a measurement range which can be selected from 0-5 mg/Nm3 up to 0 - 100 g/Nm3
Measurement principle, stack versionThe LaserDust Monitors are optical instruments based on transmitting red light from a diode laser to two detectors diametrically opposite to the transmitter. By its unique optical design, the LaserDust Monitor is not susceptible to interference from dust on the optical windows. This is a common problem with ordinary optical measurement techniques based on absorption. The LaserDust Monitor provides not only reliable measurements, but also reduced need for maintenance, i.e. cleaning of the optical windows.
The LaserDust Monitor is really two instruments in one. Two signals are generated when the laser beam is sent across a stack or duct where dust is present: One signal represents the amount of direct absorption (reduction in intensity) the light undergoes due to the dust present. The second signal is the amount of forward scattered light from the dust particles. The dust particles will scatter the laser light much the same way as particles appear to 'light up' when drifting through a ray of sunlight.
Dust calculations based on forward scattered light are accurate for concentrations down to 0.1 mg/m3, the scattered light dust signal will saturate at approximately 200 mg/m3. Dust calculations based on direct absorption have virtually unlimited dynamic range, but are not very accurate for low dust concentrations (below 100 mg/m3). The LaserDust Monitor has two modes of operation. The "scatter" mode is normally chosen for low dust concentrations, up to 200 mg/Nm3 whereas the "direct" mode is used for higher concentrations. The mode selection is made in software using the service program. This gives our instrument superior performance over most conventional dust monitors on the market i. e.:
An example of the performance with a response time of 0.05 seconds is illustrated in the figure below.
All the information about dust pulses from pulsed bag-filters is obtained i.e. the shape, peak value and inte- grated value of the dust pulse. Dust pulses with a length of up to 20 seconds can be displayed.
Development goals have been low maintenance cost and easy calibrationThe rugged industrial design and the air purging make the LaserDust Monitor easy to maintain. There are no moving parts in the instruments and normal preventative maintenance actions are limited to visual inspection and cleaning of the windows. Experience shows that maintenance intervals exceed 3 months. However, all critical parameters are monitored internally and warning messages are given if maintenance is required beyond the recommended maintenance intervals.
The LaserDust Monitor should be calibrated in situ, against a reliable extractive method. This is necessary to cater for different dust types with different optical properties.
Measurement principles open path versionIn addition to the stack mounted dust monitor NEO has also developed an open path version based on laser technology. The open path version is suitable for dust emission monitoring in primary aluminium smelters, silicon carbide plants etc. The figure below shows the basic optical design of the open path dust monitor.
The figure above shows the basic principle of operation for NEO's open path dust monitors. To the left we can see the transceiver unit and to the right the retro reflector. The design is based on sending a laser beam through a measurement volume, the laser beam being returned to the transceiver unit by a retro reflector. It is a coaxial design with the laser in the centre and the reflected light being collected by a larger donut shaped lens and focused onto a detector. Quantification of the dust concentration is based on measurement of the loss of optical transmission through the measurement volume containing a varying amount of dust.