Derek Stuart looks at the issue of measuring stack dust emission levels through effective opacity monitoring
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.
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.
Opacity and emission limit value
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.
|Figure 1. Loss through scattering, absorption and reflection|
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) × 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.
|Figure 2. Sample with 0% opacity|
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 authorities.
|Figure 3. Sample with 100 per cent opacity|
Opacity monitors must operate 24/7, so reliability is of critical importance. Often used in extreme conditions, the monitors are mounted outdoors at an elevated position on a stack, on which they are exposed to temperature extremes. Given the size of stack structures, there also is minor movement and the monitor must accommodate this displacement.
ASTM D6216 standard requires that a monitor must meet defined performance standards and earn a certificate of conformity. Each opacity monitor must be configured and calibrated for the specific installation where it is used.
Set up requires that the following information be available and recorded:
• The installation path length, that is the distance between the transceiver and retro mounting flanges.
• The US EPA requires the opacity to be expressed as the value seen at the stack exit. This requires a knowledge of the internal stack dimensions at the measurement point and at the stack exit in order to calculate the path length correction factor (PLCF).
Measurement path lengths are the inside dimension of the stack or duct at the measuring point. Since standpipes are purged, the measurement path length is the distance the light beam travels through the dust-laden stack gases.
Exit path length is the inside dimension of the stack or duct at the point where gases are discharged to the atmosphere.
|Figure 4. Path lengths required to configure an opacity monitor|
When there is a straight stack, for example, the PLCF equals 1.0. Tapered stacks have PLCF < 1.0 and installations where monitors are installed on a narrow duct leading to the stack, have a PLCF > 1.0. When the PLCF is greater than 1.0, any error effects are magnified.
Most modern opacity monitors, including the AMETEK Land 4500 MkIII, use a double-pass design, wherein a transceiver projects a beam of light across the stack to a reflector, which returns the light to a detector mounted in the transceiver.
Advantages of the double pass design method include:
• Low-level sensitivity is increased since the light passes through the stack gases twice.
• No power is required at the reflector.
• A simulated zero condition is achieved by placing a reflector in the beam at the transceiver, short-circuiting the stack.
The solution also offers a standard wide temperature range to -40 °C, flexible configuration, easy maintenance and servicing, and a lifetime warranty on the light source.
It is important to note that purge air must be used in opacity monitoring to protect sensitive instruments from hot, corrosive stack gases, by keeping the optics clean and preventing corrosion and contamination of the mounting tubes (See Figure 5).
|Figure 5. Air purge protects delicate optical surfaces from hot, dusty and corrosive stack gases|
The trend globally is to push emission limits even lower.
Even though the US EPA is currently under political pressure and there may be a slowing of regulations going forward, existing regulations are there to protect the environment, and it seems certain that they will be enforced.
Elsewhere globally, requirements for opacity monitoring are on the rise. The more sensitive the measurement, the more complex the monitor.
Having effective and reliable opacity monitoring equipment is therefore vitally important to ensure that a power plant is operating within permitted levels.
As regulations become increasingly stringent accurate opacity monitoring will become even more important to ensure a plant remains compliant.
Derek Stuart is Global Product Manager for Power at AMETEK Land. www.landinst.com.