Smoke emissions from combustion processes have been an issue globally for many hundreds of years and the introduction of regulations has sought to control and minimise levels of those emissions.
The Ringelmann method, introduced in the 19th century, assessed smoke emissions by visually comparing the colour of smoke to shades of grey printed on a card. Six Channel Particle Counter For Atmospheric Monitoring
Today’s methods of opacity measurement still use the optical characteristics of stack gases to measure the amount of smoke or dust.
Smoke and dust are by-products of all combustion processes, including coal- and oil-fired generating units.
While it is true that the industry is moving away from burning coal to burning alternative sources that are seen as being cleaner, such as natural gas and renewables, coal use in power generation is likely to remain at levels of around 20 to 30 per cent of the total for the foreseeable future.
Since these emissions are damaging to the environment and cause health problems even for people living far from the source, regulators such as the US Environmental Protection Agency (EPA) and the UK Environment Agency regulate emissions by establishing an Emission Limit Value (ELV).
That value is the amount of smoke or dust which can be emitted without incurring legal penalties.
To ensure compliance, power-generation plant operators must measure their emissions and report their measurement results.
Smoke and dust are collectively known as particulate matter, or PM. One of the most obvious signs of PM emissions is a visible plume of smoke leaving the stack.
When light passes through a gas containing smoke and dust, some of the light is lost through scattering, absorption and reflection by the particles. The amount of light lost depends on the number and the size of the particles, so that loss can be used as a measure of the PM concentration in the stack.
Opacity is a measure of light attenuation ” the fraction of light lost in crossing the stack. An opacity monitor measures this value and reports it directly.
A PM monitor measures optical characteristics of the stack gas and uses the value to calculate the PM concentration in mg/m3.
The calculation uses a calibration factor, unique to that specific installation.
In general, opacity and PM measurements are only required on processes that burn coal, oil and waste materials, such as incinerators. Natural gas does not contain dust and ash, so gas-fired processes do not produce PM emissions.
However, many natural gas power generation facilities use oil as a backup fuel and are still required to install a PM or opacity monitor.
The ASTM D6216 standard (Standard Practice for Opacity Monitor Manufacturers to Certify Conformance with Design and Performance Specifications) defines opacity as the degree to which particulate emissions reduce (due to absorption, reflection and scattering) the intensity of transmitted photopic light and obscure the view of an object through ambient air, an effluent gas stream or an optical medium, of a given path length.
These processes are illustrated in Figure 1, indicating light rays passing through a sample while others are scattered, absorbed or reflected.
If the intensity of light entering the sample (left side of Figure 1) is I0 and the intensity leaving the sample (right side of Figure 1) is I, we can express opacity mathematically as: Opacity = (1- I/I0) àƒ– 100 per cent.
If the sample contains no particles, the intensities I and I0 will be the same, so the opacity is 0 per cent.
If the sample has so many particles that none of the light passes all the way through, I = 0 and the opacity is 100 per cent.
Some regulatory bodies set an emission limit value (ELV) in plume opacity at the stack exit, with units of percentage opacity.
Others set the ELV in terms of the mass concentration of PM emitted from the stack and is expressed as a mass concentration, with units of mg/ m3.
The US EPA traditionally has set emission limits in percentage opacity, but recent rules have moved to the use of mass concentration units. European regulators always have set limits in mass concentration units. The choice depends on the compliance requirements of the local regulatory auth