Optimising the efficiency of dust control equipment — near incident forward scatter particulate monitoring instruments

Phoenix Instrumentation Pty Ltd
By William Averdieck, PCME Ltd
Friday, 25 July, 2008


Environmental regulations and responsibilities are stimulating industrial process operators to minimise particulate emissions from fabric filter and electrostatic precipitator dust arrestment plant by optimised maintenance and operation programs. The online monitoring of particulate emissions plays a fundamental role in this optimisation, providing the evidence and feedback of any improvements.

New particulate monitoring instruments based on near incident forward scatter overcome the limitations of many existing instruments, since calibration (and hence accuracy) is unaffected by changing particle size as the arrestment plant becomes more efficient.

Existing practices for continuous particle emission monitoring

Industrial dust control equipment

Particulate emissions from industrial plant are generally controlled by one of three major control technologies:

  • Fabric filter dust collectors, in which the particles are collected on a filter membrane. These devices are also referred to as baghouses and bag filters.
  • Cyclones, in which the particles are collected by centrifuge.
  • Electrostatic precipitators (ESP), in which particles are electrostatically attracted to a high-voltage plate.

Fabric filter dust collectors have played an increasing role in the industrial market during the past 20 years due to their ability to meet falling emission limits. They have the capability to reduce emissions even below 1 mg/m3, the emission limit for toxic particles (eg, lead from battery factories). Cyclones are used less frequently these days as a final control device since they are only efficient with larger particles (ie, woodchips). Electrostatic precipitators are still used to control emissions with high volumes of flue gas (eg, cement kilns and power plants), although new plants might choose fabric filters due to their higher efficiency. ESPs are also used with wet collectors when gaseous emissions must also be reduced.

Reasons for continuous monitoring

Regulators and process operators continuously monitor the emissions from these types of particulate arrestment plant for two reasons: first to ensure emissions are below legal limits and secondly (and more fundamentally) to ensure the proper operation of the arrestment plant, because if the arrestment plant is working correctly and it is fit for purpose, no emission limit will be exceeded.

It is therefore common practice to monitor emissions continuously, and record and analyse data to assist regulatory compliance and arrestment plant control.

Particle size sensitivity limitations of continuous monitoring technologies

There is a range of proven technologies for continuously monitoring particulate emissions from industrial processes, and the appropriateness of each is dependent on the specific application. The core technologies are:

  • Opacity: In which the amount of light absorbed by particles in the path of a cross stack light beam is measured.
  • Scintillation: In which the amount of flicker caused by particles crossing a cross stack light beam is measured.
  • Light scatter: In which the amount of light scattered (reflected) by the particles from a light beam passing into the stack is measured.
  • Electrodynamic: In which the variation in electrical signal induced by particles that are naturally charged passing a sensor rod in the stack is measured.
  • Triboelectric: In which the amount of current produced by particles colliding with a sensor rod in the stack is measured.

The parameter measured by the instrument can be related to particle concentration, and therefore instruments can in many circumstances be calibrated in absolute terms (mg/m3) by comparison to the results of an isokinetic test. It can therefore be understood that the instrument is not actually measuring dust concentration, but inferring it from the measurement of a parameter that can be correlated to dust concentration (eg, light absorbency, electrical signal). This leads to the ‘Achilles heal’ of all continuous particulate monitors — the measured parameter may also be affected by other changes in process and particle characteristics. The cross sensitivities are specific to measurement technique; however, all in-situ techniques are sensitive to both the type of particle and the size of the particle.

In most industrial applications (with the exception of incineration), cross sensitivity to particle type is not a problem, since the application is fixed and therefore the type of particle remains relatively constant (eg, limestone dust comes from a kaolin plant). However, the issue of particle size sensitivity is of far higher significance. Emissions from industrial plants increase when dust collection equipment becomes less efficient or fails. In these circumstances, both dust concentration and particle size change together, meaning the accuracy of the measured dust concentration (at the time one might be most interested in its validity) is often in error due to particle size cross sensitivities.

Near incident forward light scattering (pro-scattering)

Principle of operation

A ‘near incident’ forward scatter particle emission instrument overcomes 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 (Jones, 1999).

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 light scattered less than 90° is less dependent on particle type. As the scattering angle is reduced towards 0° (ie, small angle with incidence beam) the cross sensitivity to particle size also diminishes significantly.


Figure 1: Types of light scattering

Pro-scattering instruments have been designed with a scattering angle of between 5 and 10°, significantly reducing the effect of changing particle size. This results in added complications in practical implementation, but provides the key characteristic of reduced particle size sensitivity.

Practical implementation

The probe-based design used in the pro-scattering instrument has been used to minimise errors in measurement arising from alignment. A probe permits the incident beam angle and scattered light signal to be maintained at a defined angle with a high level of precision. Changes in this angle which would be likely in a cross stack design due to stack wall movement are eliminated by the use of rigid materials and mechanics.

 


Figure 2: A near-incident forward scatter instrument

 

It is desirable to have as large a measurement volume as possible, as well as use a small scattering angle and therefore it was decided to use a design where all the concentric scattered light in a particular range is used by putting the axis of the receiver lens on the same axis as the incident beam. This introduces the added challenge of blocking the incident beam from being monitored by the detector since the incident beam is several orders of magnitude greater than the scattered signal. This is accomplished by a periscope that deflects the incident beam after scattering and careful beam dumps.

Critical factors that a business can control in an environmentally responsible way include waste management, infrastructure management (buildings, facilities and their energy efficiency), emissions management and asset management.

To ensure the instrument is rugged enough to operate continuously in an aggressive stack environment all optical surfaces are recessed well away from the stack gases and air purges are used to provide positive displacement away from these surfaces.

The instrument must be usable at elevated stack temperatures (up to 400°C) so all electronic components must be near the stack wall to ensure adequate cooling. This required a design in which the diode laser beam is reflected towards the measurement chamber and receiver by a retro reflector at the far end of the tube. Optical fibres were not practical due to light coupling efficiencies.

It was an operational and regulatory requirement to be able to automatically measure and compensate for errors caused by contamination and any instrument drift. A zero and span was therefore incorporated in the instrument.

Operating characteristics

The pro-scatter instrument is proving itself to be suited for operation in a stack environment. The key performance criteria are:

  • Dust concentration range: 0.1–100 mg/m3
  • Max temperature: 0–400°C (optional)
  • Length of measurement cell: 300 mm
  • Probe length into stack: 800 mm

The sensor connects directly to a control unit that provides user interface, data display, plant outputs and recording.

Of key importance to bag filter and other dust arrestment plant activities is the instrument’s response to particle size. The pro-scatter instrument is proving to have a relatively unchanged response to particle size in the range 0.3 to 5 μm, the typical particle size found after bag filters. At larger particle sizes the response still shows a cross sensitivity to particle size.


Figure 3: Response to particle size (near-incident forward scatter)

These results compare very favourably with the particle size response of a forward scattering instrument using a larger scatter angle of 15°. This instrument has a peak in response for particles of approx 1 μm (the wavelength of light used) and then a drop-off in response with increasing particle size. It is this response that makes the calibration unreliable in bag filter applications since the particle size increases at the time of filter failure.


Figure 4: Comparison of response to particle size (15° forward scatter and near-incident forward scatter)

An additional benefit of the near incident forward particle size technique is that response to different particle types is relatively unchanged, making it suitable for incinerator applications where the type of dust can be dependent on different fuel sources from one day to another.


Figure 5: Comparison of response to SiC and AL2O3 (near-incident forward scatter)

Operational benefits of using particle size insensitive instruments

Benefits in bag filter applications

Bag filters comprise a number of long tube filter bags hanging in rows from a clean air manifold. Air passes from the dirty side of the process, through the filter tube to the clean air manifold and then on via a fan to the emission duct or stack.

Dust removal occurs as the dirty air passes through the filter bag. The particles that pass through the filter in the initial stages are those that are smaller than the pore size of the filter media. However, as the filter becomes loaded with particulate, the effective pore size is reduced since gaps on the filter media are filled by particulate that plugs the pores. Therefore the filter becomes more efficient and the size of particles passing through the filter reduces. In high-filtration applications the bags are often pre-coated with limestone to ensure adequate particle entrapment.

Periodically, the filter is cleaned to remove caking particulate from the filter media, otherwise the pressure drop across the filter would increase too much and the filter would be damaged. Therefore, the emissions from a bag filter are dynamic with the dust loading and particle size changing depending on the stage of cleaning. In a periodically cleaned pulse jet collector this variation is regular and predictable.

Particulate is abrasive and therefore filters fail for one of two main causes:

  • Rupture of a bag filter, in which case the increased particles passing through the hole have the increased size of the particles entering into the process. The particle size distribution of the emission is bimodal, made up of both the filtered and input particle size distribution.
  • Wear of the filter media (especially seen with abrasive dusts such as in cement processes), in which the filter pore size gradually becomes larger and hence the total particle size range increases.

The benefits of using an instrument to monitor particle emissions that is not particle size sensitive is to eliminate the errors in measurement which arise from particle size dependence. These errors can be significant under start-up and failure conditions and to date have limited the benefits of continuous particle emission monitoring.

Benefits in electrostatic precipitator applications

The benefits of insensitivity to particle size for particle emission monitors are not restricted to bag filter applications. Similar arguments are also relevant to ESP control equipment where the particle size can change depending on the voltage on the ESP plates and the migration velocity through the ESP. In addition, during soot blowing there can be significantly increased particle size.

Phoenix Instrumentation Pty Ltd
www.phoenix-inst.com.au

Reference

  • Jones, A R (1999), Light Scattering for Particle Characterisation, Progress in Energy and Combustion Science 25, pp 1–53
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