Volker Schmitz from PCME Ltd reviews established methods of measuring dust concentration in industrial installations and highlights the benefits of PCME's electrodynamic techniques.
Established dust analysis methods
Gravimetric techniques: Gravimetric sampling involves taking an isokinetic sample (ie: one in which the velocity of the sampling nozzle is the same as the stack) through a pre-weighed filter and then re-weighing the filter after a given time. One also measures the flow and can thus obtain the concentration of the dust in mg/m³.
It is the only method that gives the user a real concentration. All other methods are relative methods and must be calibrated accordingly. This means that to read concentration values using any other dust monitor, it first has to be calibrated using a gravimetric sample.
Dynamic opacity: In the past 10 years dynamic opacity has become a modern, widely-used monitoring technique. Such systems offer a tenfold increase in resolution and far less stringent maintenance requirements than conventional opacity systems.
Like opacity monitors, dynamic opacity monitors are based on a light beam passing across the airflow. The essential difference is that they measure not the beam intensity (as in opacity techniques), but the temporal variation (scintillation) in its intensity. This scintillation results from the statistical variations in the distribution of particles in the airstream. The higher the concentration of particles, the greater the range of variation.
The scintillation is substantially independent of the absolute value of the light intensity, down to very low light levels. Scintillation measurements are both reliable and repeatable. The monitor can be calibrated by an isokinetic sample test to give concentrations in mg/m³
The essential proviso is that the empirical constant, k, must not vary. Its value is a function of the installation, the type of dust and the particle size and is effectively constant in a wide range of large stack and combustion applications including incineration plants, electrostatic precipitator and other arrestment plants. In principle dynamic opacity is an ideal alternative to traditional opacity techniques for emission monitoring.
Light scattering: Another optical measurement technology is light scattering. This is a new 'near–incident' forward scatter particle emission instrument that has recently been developed to overcome the cross-sensitivity to particle size discussed above. The theory behind light scattering is well understood in that the total amount of scattered light from a particle cloud is dependent on the angle between the incident beam and the measured scattered angle.
The scattered light between 90° and 180° (referred to as back-scatter) is sensitive to particle refractive index and particle size and shape while the scattered light less than 90° is less dependent on particle type. As the scattering angle is reduced towards 0° (i.e. small angle with incidence beam) the cross sensitivity to particle size also diminishes significantly.
Electrodynamic measurement
A completely different, non-optical technology is electrodynamic sensing. Electrodynamic systems do not measure particles that collide with the sensor, but instead measure the natural charge of the particles that arises from collisions with other particles, the duct wall, filters and other components of the system in which they are contained.
The PCME system uses the interaction between the particles and the sensor which is a kind of induction of the charge, the result of which is a stable AC current. This removes two major problems. Firstly electrodynamic systems do not depend on the velocity of the particles because the natural charge of the particle is the same whether it approaches the sensor slowly or rapidly. This has been tested by an independent institute in Germany.
The second and more important point is that the induction of the charge of the particle is not affected by contamination. The induction happens even if there is a layer of dust on the sensor rod. Despite this, PCME recommends checking for build-up every six months depending on the system used. The system can be fitted with in-built contamination checks so the instrument itself tells the user when it needs to be cleaned.
Other advantages are that it is easy to install and requires minimal maintenance. Crucially, in comparison to gravimetric sampling, which gives the user a rough estimate for one point of the system at a given time, electrodynamic systems give a continuous output and continuous monitoring.
Benefits of electrodynamic techniques
Monitoring enables the user to assess the efficiency of their filter systems. In any process requiring them, filters are essential. Without a filter, there can be no clean product and therefore there is a strong need to control filter efficiency. If an operator carries out a gravimetric sample just to satisfy regulations once a year, they get no process information.
Bag filters are used in many industries, but over a period of time 'cake' builds up on the filter element and the differential pressure increases. Cleaning the filter elements with pulsed air releases the dust, but sooner or later the filters will start to leak due to mechanical stress. This means that the user loses product and hence money.
If a dust monitor is sensitive and dynamic enough during the cleaning cycle, the user will observe what are known as cleaning peaks, as shown in Figure 3. It shows a three chamber filter that shows slightly higher peaks in the third chamber than the others. This means that the chamber has started to leak. Without a monitoring system, the user would likely have to wait until catastrophic failure, but knowing in advance the user can schedule maintenance in advance and can also judge the amount of product lost. The user can also avoid costly unplanned shutdowns.
Figure 3 indicates to the trained eye that the cleaning system is not working, which means the other filter compartments may experience problems and may start to leak sooner than normal due to the extra mechanical stress. It is possible to get all of this information from the instrument without even calibrating because the absolute dust concentration is unimportant. The important thing is that the filter performance is seen to be poor and that means that sooner or later the emission limit will be reached. If the filter is allowed to perform properly, this will never be reached.
Gypsum industry
The gypsum industry is bound by increasing national and international legislation. Therefore it is interesting to see a dust monitor not only as an environmental issue, but also ask 'can you save money with a dust monitor?
The emphatic answer is 'Yes!' This is another reason to have good control of the filter efficiency. Controlling filter efficiency gives process information without additional cost for calibration. In summary installation of a dust control system is about scheduled maintenance, avoidance of product loss and also reduction of operating costs, played off against the cost of the instrumentation.
Another point is that because the operator knows, for example in this case, that the problem is in the third compartment, they do not have to have a large number of filter bags in stock and the average lifetime of the bags can be expanded. This is because the user can use them until they are actually broken and not simply replace them as a matter of routine when they still have a substantial service-time still ahead of them.
Potential savings
It is fairly simple to calculate potential savings by only changing one compartment of the filter system rather than every single filter bag. Installation of the dust monitor costs around US$9000 so the cost benefits of installation can be calculated on a site-by-site basis.
