Emission monitoring with DURAG's dust monitoring solutions
Dust is an air pollutant from industrial emissions that must be monitored. Dust monitors from DURAG GROUP determine the dust concentration in dry and humid flue gases reliably, accurately and continuously. They are robust, durable and low-maintenance, certified for regulatory emission monitoring and represent the best available technology. In addition, monitoring the dust concentration in raw gases enables the optimization of combustion processes.
Industrial Dust Monitoring Equipment for Every Requirement
Depending on the requirements and measuring task, we offer various dust measuring systems that use different physical measuring principles. The measuring devices are installed in the flue gas duct, where they measure the dust concentration in milligrams per cubic meter. The measuring device passes this information on to the plant control room for process control. From there, the data is sent to the relevant authority.
Components of dust measurement systems
A measuring system for measuring dust concentration consists of a measuring device, a supply unit and various other components depending on the product solution. For example, we supply various solutions for the purge air supply and, where required, an operating unit with a display.
Transmission principle
DURAG dust and opacity monitors operate on the basis of the transmission principle. The measurement is performed continuously in the flue gas stream above the dew point without contact and without extracting a sample. The measuring head (sender and receiver) and the reflector are fitted opposite each another. By using the auto collimation principle, the measuring light crosses the measuring path twice and the measurement sensitivity is doubled.
Scattered light principle: Forward scattering
This type of DURAG dust monitor is based on the forward scattering principle and is suitable for monitoring small to medium dust concentrations. It is mounted one-sided at vertical or horizontal oriented stacks, where the probe is in direct contact with the flue gas.
A red laser irradiates the dust particles in the measurement volume of the flue gas duct with modulated light. The light scattered by the dust particles in the forward direction is collected by a lens and then guided to a highly sensitive detector by an optical waveguide. The detector measures the scatter on the basis of the reflection and refraction of the light by the dust particles. If the wavelength of the incident light is much larger than the diameter of the particle, so-called Rayleigh scattering occurs. If the wavelength is about the same size as the diameter of the particles, Mie scattering will occur. The greater the particle size, the more light is scattered in the forward direction.
Scattered light principle: Backward scattering
This type of dust monitor is suitable for monitoring low to medium dust concentrations. The measurement is performed continuously above the dew point in the flue gas stream without contact and without extracting a sample. The light from a red laser diode illuminates the dust particles in the stack and is scattered by the particles. The light scattered backwards by the dust particles is detected using a highly sensitive photodiode.
The ratio of the measured scattered light intensity and the emitted light intensity correlates to the particle density in the measurement volume and is measured by the monitor as a scattered light unit. The measurement volume is defined through the intersection of the laser beam sent and the receiving aperture of the detector. The intensity of the scattered light is a function of the scatter angle, the particle size, the wavelength of the incoming light and the optical properties of the particle, such as color and shape or refraction index.
Extractive measurement
Continuous dust monitoring in wet or humid flue gases requires a special dust monitoring system. The sample needs to be prepared to remove liquids which also scatter light but do not correlate to dust concentration. In extractive measurement, the probe is located inside the stack. The sample is heated up and can be diluted. The dry sample then passes through a measuring cell where the forward scattering principle is applied. The signal resulting from the scattered light detector is a measure of the dust content of the flue gas. The sample is finally transferred back into the stack.
