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Technology for Continuous Emissions Monitoring - Light Opacity

Continuous Emissions Monitoring with Light Opacity and Modulation Technology – the Pros and Cons

Opacity Technology

Dust monitoring equipment using light absorption technology as a means to measure particulate densities (generally known as opacity monitors) is generally regarded as being less accurate then that using light scatter technology at low level of dust. The accuracy and reliability of these continuous emissions monitors is affected by the following:

  • They measure the light absorption in a narrow beam of light across the stack (typically less than 1% of its cross-sectional area). The accuracy of this dust monitoring equipment is therefore vulnerable to uneven particle distribution and turbulence (see electro-dynamic and tribo-electric monitor)
  • As moisture absorbs light, opacity monitors will measure humidity in the gas stream as the presence of particulates
  • Light lost as a result of scattering is treated as if absorbed, with resulting errors in measurement
  • Changes in particle shape and composition will change the apparent absorption rate leading to measurement errors
  • Changes in the ambient light levels affect the perceived absorption
  • Installation is complicated by the needs to access at two points directly opposite one another on the stack, and for critically precise alignment
  • Deterioration of accuracy arising from fouling of the optics is an on-going threat to accuracy

Light Modulation Technology

Continuous emissions monitoring equipment using light modulation technology measures the signal variations arising from the momentary blockage of light as a particle crosses the beam. They are therefore less affected by fouling and misalignment than opacity instruments, but share two disadvantages:

  • They measure the light modulation within in a narrow beam of light across the stack (typically less than 1% of its cross-sectional area), and their accuracy is hence vulnerable to uneven particle distribution and turbulence
  • As moisture absorbs light, a modulation instrument will measure humidity in the gas stream as the presence of particulates