Instrumental in approach

David Walker, director of Environmental Monitoring Solutions examines the regulatory requirements for selecting a continuous emissions monitor for particulate measurement, and explores the choices available to industry.

The European Union (EU) has drawn up a number of directives, which set out objectives to be achieved in order to control pollution. The requirements of the directives must be adhered to by each member state of the EU, who is in turn responsible for incorporating the directives into the framework of their national legislation. It is to this end we see many varying interpretations of the directives throughout Europe creating much confusion, even at the level of the authorities, and this then gives rise to poor advice and industries either doing too much or too little to keep on the right side of the law.

The Air Framework Directive 84/360/EEC, Municipal Solid Waste Directive 88/369/EEC, existing Municipal Waste Directive 89/429/EEC, Asbestos Emissions Directive 87/217/EC and Hazardous Waste Incinerator Directive 94/67/EC are just a few of the many directives currently in force within the EU.

The Pollution Prevention and Control (PPC) Regulations 2000 create the UK national framework for implementing the European Community directive on integrated pollution prevention and control (IPPC). All installations currently subject to IPC and local authority air pollution control (LAAPC) are expected to be regulated under PPC, together with a significant number of installations coming under control for the first time.

IPC was incorporated into part one of the Environmental Protection Act 1990 (EPA90). This act established a requirement to control industrial installations involving substances most damaging to the environment. After wading through the endless amounts of legislation and understanding the elements of the granted permit which has to be read in detail, one is faced with the monitoring of emissions to air.

Choosing a continuous emissions monitor (CEM) is in many ways more difficult than negotiating the legislation. The numerous guidance documents on CEMs provided by the Environment Agency and local government all state a CEM must have just one requirement, that of being 'fit-for-purpose'. With the many different technologies available for monitoring emissions-to-air, it is not always a straightforward exercise to know if the chosen CEM is fit for purpose or not.

To explain just one such exercise we can look at the confusion an operator faces when choosing a CEM for particulate measurement.

Particulate measurement is a general term formed around two measurement principles, smoke density measurement and dust concentration measurement. The following example explains the fundamental difference between these two principle measurements.

Smoke density meters merely deliver information on the quality of the dust emission since the measuring covers the transmission property only. Measuring the transmission property alone, smoke density meters only offer a reproducible correlation between the displayed value and the grey scale value of the exhaust gas plume. Therefore no relationship can be found between the measured signal and actual dust concentration. Such devices are commonly referred to as opacity instruments and usually comprise of a halogen bulb as a light source with an optical receiver at the opposite side of the stack usually comprising of a light dependant resistor and a bit of circuitry. The relationship between the measured radiation intensity and emitted radiation intensity provides a measurement of transmission that can be calculated as a percentage.

Dust concentration meters establish a reproducible correlation between the display value and the dust concentration in mg/m3 of the exhaust gas at a standard condition (1,013mbar, 273oK). With dust concentration measurement the transmission property is converted to an extinction property. Using Lambert-Beer law extinction establishes a linear correlation between transmission and dust concentration.

A wide range of instruments available on the market, are highly capable of performing the mathematical function between the transmission signal and the required property, that being extinction. It is to this end one should realise when choosing a light based instrument for measurement of dust concentration it is important to understand the difference between opacity and dust concentration meters.

A light beam passes a measuring path, e.g., the cross section of a stack. The weakening of the beam's intensity is dependent on the dust load. Amount of weakening can vary with grain size, colour and the shape of the particles. The light source should emit modulated light (approx 1-2Khz) of a wavelength range between 400-700nm, since it is in this wavelength that dust particles can adequately influence the measured result. With a broad-spectrum, halogen lamps perfectly fulfil this requirement. Green LEDs and red LEDS can be affected with temperature fluctuations and have therefore been limited in many applications. However with the advancement in LED design manufacturers are overcoming many of the historically associated drift problems allowing the LED to replace the halogen light source. The typical life expectancy for an LED is likely to be double that of a halogen, thus lowering future maintenance costs.

There are three main influences that can affect light-based instruments:

  • distortion of the mounting surfaces can cause the light source and receiver to be offset,
  • drifts from ageing of the light source emitter and receiver,
  • stray light entering the inner stack walls.

The arrangement and selection of the measurement point affects the measuring of dust concentration and hence, is of great importance for the reliability of the measuring results. Therefore, when planning an installation it is important to consult an expert on the actual location of the instrument.

When selecting the measurement point it must be observed that the measuring path lies in the most trouble-free gas flow, away from deviations, branches, shut-off-devices, blowers etc.

The EPA CFR40 standards and the British Standard BS3405 both give indications on how to determine the required monitoring location. It is true to say that both standards appear to have conflicting advice, and therefore sometimes the installation and commissioning engineer are expected to sign off the installation independently.

In conclusion, it is advisable to any person faced with the daunting task of meeting the requirements of an IPPC permit or IPC authorisation to seek expert advice before spending money on continuous emissions monitors.



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