The subject of Continuous Particulate emission monitoring to satisfy regulatory requirements is of relatively new interest as a result of recent changes in legislation. Historically regulators were concerned with the visual impact of the discharge from a stack and therefore emission limits were expressed in terms of colour or opacity. However with the advent of emission limits for a process being defined in terms of mass concentration (expressed in mg/ m3) the issue of continuous particulate monitoring has become a new and growing regulatory requirement.
Particulate emission monitoring is a challenging technical field, not only because of the application specific accuracy and performance of particulate monitors, but also due to the harsh environment in which they must continuously operate. By definition the stack environment includes particulate or dust which will test the robustness of any instrument.
At the cornerstone of all absolute measurements (in mg/m 3 ) is isokinetic or gravimetric sampling in which a sample of flue gas is collected and weighed. This testing provides the calibration that gives most in-situ continuous emission monitors the ability to monitor particulate in absolute terms. Most instruments can also provide qualitative indication of changes and trends in dust levels, should they not be calibrated by isokinetic testing. This provides a solution to a growing requirement of environmental legislation for indicative, parametric or qualitative monitoring.
Why particulate emissions are continuously monitored
Operators of industrial stacks use continuous particulate monitoring instrumentation for a variety of process and environmental purposes, whether it is to provide better feedback on a process, to satisfy environmental legislation or provide positive proof that stack emissions are under continuous control. Continuous monitoring of particulate is complementary to isokinetic sampling in that it gives visibility to the dynamics of a process. This dynamic data is crucial in many industrial process or particulate control applications.
Types of continuous particulate monitoring
Particulate emission monitoring is normally divided into 3 distinct areas categorised by the quality and type of information provided.
- Concentration measurement in mg/m3. A subset of this type of monitoring involves the uncertainty of measurement to be assessed in line with the Waste Incineration Directive and Large Combustion Plant EU directives
- Bag leak monitoring (monitoring of particulate trends). This has historically been referred to as Qualitative monitoring
- Filter gross failure detection
At the top end of continuous particulate emission monitoring is concentration measurement. Such an instrument provides an output calibrated to show particulate levels in mg/m 3 on a continuous basis.
The instruments are calibrated by comparison to a reference Isokinetic sampling method e.g. BS EN 13284 part 1 or ISO9096 2003.
Concentration monitors are typically recalibrated at least once a year by reference to isokinetic tests, since this is the only way of checking the calibration in mg/m 3 .
When the uncertainty of measurement must be assessed (to satisfy specific requirements of the Waste Incineration Directive and large Combustion Directive) then it is necessary to:
- Perform a more elaborate calibration procedure (referred to as a QAL2)
- Ensure the instrument has additional quality control features to detect changes in the instrument’s performance by use of surrogate zero and span materials
- Ensure the instrument to be audited against controlled reference materials. Instruments which meet these requirements have a specific approval referred to as a QAL1
Bag leak (Qualitative) monitoring
In many applications, the absolute level of particulate is not the issue of critical importance and does not justify the cost of isokinetic sampling. Of much more relevance is the trend in particulate levels and monitoring changes in levels over time. This is particularly true for bag filter applications with high emission limits (50mg/m 3 ) where provided the bag filter is working properly, emissions will be predictably below the emission limit in the range of 5 – 10mg/m 3 . The monitoring objective therefore is to provide feedback on the performance of the bag filter and a pragmatic, cost effective approach is bag leak or qualitative monitoring. Likewise many process applications, such as measuring particle loss from a drying process, are more driven by a desire to monitor changes rather than know exact levels. In some instances the instruments can be calibrated to offer an indication to the range of dust typically being emitted in mg/m 3 .
Qualitative or Indicative monitoring is the same as concentration measurement with two important exceptions:
There is no regulatory need to calibrate the instrument with isokinetic sampling since the output is in terms of a relative dust output rather than an absolute level. Sometimes approximate calibrations (based on an engineering estimate of the emissions) are applied to qualitative instruments, but this is for approximate referencing rather than to satisfy any desire to make measurements.
Qualitative instruments provide a relative output (4-20mA or serial data) of relative dust levels. Units of measurement are usually a % of full scale or a factor of normal emissions.
To ensure an appropriate level of quality assurance, qualitative instruments include internal zero and reference checks to detect changes in instrument operation.
Features present in more sophisticated bag leak monitors include:
- Alarm delay and averaging to differentiate between dust pulses from bag cleaning and proper failure conditions
- Pulse tracking capability to compare the magnitude of dust pulses resulting from bag cleaning. This permits the development of broken bags to be anticipated and their position to be located
Gross failure detection
Gross failure detection or broken bag detection is the simplest form of particulate monitoring. An alarm is activated should a significant increase of particulate loading be detected, indicating a failure of the pollution arrestment plant (e.g. Filter gross failure or cyclone overflow). Instruments used for filter failure detection do not necessarily need to be accurate, nor have the sensitivity to measure dust levels in normal conditions; of more importance is a relatively repetitive response to an increased particle level.
Features present in more sophisticated gross failure monitors include:
- Dual alarm levels so an early warning alarm can be supplied in addition to gross failure
Technologies for continuous particulate monitoring
There are at least eight different technologies used in particulate emission monitors. No one technology has ideal characteristics in all applications and therefore different technologies are used in different applications depending on exact requirements. The techniques most commonly used for particulate monitoring are as follows:
- Dynamic opacity
- Back/Side scatter
- Oscillating filter (vibrating tapered element)
- Beta attenuation
In the main, the adoption of each technology has been driven by its effectiveness and value in a particular application. However the regulatory approval process has also had an effect on the use of certain technologies. An overview of each technique and its characteristics follows:
Opacity meters or optical transmissometers are in-situ systems which measure the decrease in light intensity due to absorption and scattering as the beam crosses the stack. Opacity systems are insensitive to interference from other flue gas components since the beam spectrum is in the photopic region (400-700 nm) and most other flue gas constituents do not absorb radiation in this spectral region. These instruments have two uses, to measure smoke density in transmission, opacity, Ringelmann units or optical density (extinction) and/or mass concentration of particulate in mg/Nm 3
Opacity measurements are dependent on particle size, composition, shape, colour and refractive index. These properties may change with fuel type and thus calibration may be necessary with variation of process conditions. These monitors are generally only accurate within narrow limits of these parameters and it is necessary for each instrument to be calibrated by isokinetic sampling.
There are two formats for opacity devices – single and dual beam. Single path monitors simply project a beam across a duct to a receiver. In dual beam devices the beam is projected between two transceivers. This enables each transceiver to compensate for gradual window contamination by using clean mirrors inserted periodically into the beam path, furthermore, any errors caused by misalignment of the sensors may be compensated for.
A simpler technique is to use a single transceiver and fold the beam using a retro-reflector, these are known as double pass transmissometers.
An alternative type of cross stack optical dust monitor is the Dynamic Opacity device which is a ratiometric device. As such it is more tolerant to dust contamination and misalignment than opacity instruments, although offsets due to heat haze can restrict its use to applications with dust levels greater than 25mg/m 3 .
Back / side scatter
Light in an angle beam projected into the duct will be scattered (reflected) to a detector if particles are present in the duct.
Scattering instruments in general can measure much lower emissions than opacity instruments and are therefore suitable for processes controlled by highly efficient bagfilters.
There are two types of back scatter devices. The first are particularly suited to in-situ applications in small ducts where low levels of dust are present. They have off-line zero and span checks and are able to compensate for the effects of debris on the optical surfaces. They also possess high efficiency air purges to combat the build up of material on the lenses.
The second type project the beam across the stack and detect light scattered at a much smaller angle (typically 50), and as a consequence are less sensitive to fine particles. They do have the ability to cover a larger and more representative sample volume than the first type. However, none of the existing forms of this technology possess off-line zero and span checks to compensate for any build-up of material on the lenses. Back scatter devices only measure the mass concentration of dust and are unable to measure optical density.
There are three types of forward scatter monitor available currently, the extractive, probe configuration and cross duct. The extractive type draws a sample from the stack via a sampling nozzle and then presents it to a forward scattering photometer. Light is scattered by the particles and is detected by a photo-detector placed at an angle of 150 to the light beam. This system can only measure mass concentration of particulate in mg/Nm 3 after calibration with an isokinetic sample. The advantage of this system is the ability to heat the sampling system, where there are significant amounts of moisture in the stack.
The probe forward scatter instrument has a measurement volume at the tip of a probe which is inserted in the stack. The instrument measures the light scattered at a forward angle to the incident beam (typically coming from a laser diode). These instruments include zero and reference materials and therefore provided the instrument is located so the measurement volume is in a representative position these instruments can provide high accuracy measurement in a variety of low and high dust applications
The cross duct forward scatter instrument has a transmitter and receiver opposite each other on the stack as with a conventional single pass opaciometer. A diode laser is used to project a beam of light into the stack here some of the beam is attenuated and some is scattered by the particulate. The receiver has a large lens behind which are two photo-detectors, the nearer lens detects a transmission signal and the further, the scattered component.
It is claimed that this allows the instrument to have the best of all worlds, for heavy concentrations of dust the instrument uses the transmissometer system and for low levels of particulate (<200 mg/ Nm 3 ) the scattered light system. The scattered light system has an advantage in that its sample volume extends across the duct.
Oscillating filter (vibrating tapered element)
A sample of particulate is collected onto a filter mounted on the end of a resonating or oscillating tapered element. The change in frequency of oscillation is measured since this depends on the mass of the combined filter and collected particulate. This technique is currently semi-continuous since the filter must be periodically replaced, but has the advantage of being mass dependent.
β-gauge samplers are the only systems which continuously measure the mass concentration of particulate by extraction. The particles are collected isokinetically on a filter tape and the change in transmission of β-particles from a radioactive source is monitored. The particulate laden gas is extracted via a small nozzle from the duct, the extraction rate being controlled by a duct flow sensing system. The captured material is placed on a constantly moving sticky tape and then presented to a β gauge to measure the mass. Because of problems with representative particle sampling this system is used more for ambient air applications than in stacks 1 .
Beta systems do not provide short term dynamic monitoring of particulates and a single point measurement may not always be representative. The heated isokinetic sampling train is prone to maintenance problems. Measurements are made against a reference measurement already on the tape in mg/m 3 .
These instruments have an advantage to other technologies in that they are not affected by chemical composition, size or colour changes in the particles and the use of a heated probe obviates water effects. As a result they do not require calibration by a manual reference method. Beta monitors have a range of 2-2000 mg/m 3 depending on sampling rates, sampling frequency and integrating.
Triboelectric devices, or particle impingement probes, detect three separate effects when particulate strikes or passes close to a conductor placed in a particle laden gas stream. Firstly, when a particle strikes the conductor, a charge transfer takes place between particle and conductor: secondly, as the particle strikes the conductor it rubs on the surface and causes a frictional charge. The amount of charge generated by these two effects depends on the velocity of the particle, its mass and the charge history of the particle. The third effect is an inductive charge; as charged particles pass close to the conductor they induce a charge of equal and opposite magnitude in the conductor. The size of the charge is dependent on the proximity of the particle to the conductor and the charge history of the particle.
Like other dust monitors, this system has to be calibrated against an extractive method at each individual site since the electrical effect is dependent on the particle size distribution and composition. Since the response of the probe is sensitive to gas velocity, these systems are most suited to situations where the gas flow is fairly constant.
Tribo-electric monitors are very sensitive to low levels of particulate concentration and are capable of measuring down to 0.01 mg/m3 under suitable conditions. They work best where the particulate material is non-conductive and the conductor is as long as possible.
In such a system the sensing probe is installed across part of the stack and the dc current produced by particle collisions is eliminated by ac filtering techniques. The instrument measures the remaining alternating signal produced by charged particles inducing charge flow in the sensor rod as they pass it. Since the signal is not dependent on particle collisions (unlike triboelectric) the related problems of rod contamination and velocity dependence are minimised.
In applications where the particle charge, particle size and particle distribution remain constant the resulting alternating current is proportional to dust concentration. These instruments can be calibrated in mg/m3 by comparison to the results of isokinetic tests.
Positioning a particulate monitor on a chimney stack
The location of the instrument is of prime importance to achieve representative sampling especially if it is for quantitive measurement i.e. calibrated using an isokinetic method. The relevant standards for isokinetic sampling e.g. BS EN13284 part1 and 2 and BS ISO 9096:2003 and also the Environment Agency technical guidance notes M1 and M2. Representative sample location is generally achieved when the stack profile meets the following requirement for the isokinetic tests;
- Angle of gas flow less than 15° with regard to duct axis
- No local negative flow
- Minimum velocity depending on the flow rate measuring method used (for Pitot tubes a differential pressure larger than 5 Pascal)
- Ratio of the highest to lowest local gas velocities less than 3:1
1 Clarke, 1996
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Published: 10th Jun 2007 in AWE International