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Gas Chromatography Sampling Emissions

Published: 10th Sep 2011

The government’s Air Quality Strategy for England, Scotland, Wales and Northern Ireland 2007 sets national air quality standards to protect human health, and is the main policy instrument for improving air quality. Policies set out in the strategy support the achievement of national air quality objectives and EU air quality limit values.

The Air Quality Strategy identifies air quality standards for nine pollutants and a timescale for achieving these standards.

Objectives for seven of the pollutants have been set in regulations for the purposes of the Local Air Quality Management Regime. Local authorities are required to work toward achieving these objectives. The government runs the national monitoring network for air quality and publishes information on national and local air quality.

Local authorities must regularly review and assess air quality in their area. If air quality objectives are being breached or are at risk, they must declare an air quality management area and draw up an action plan to work towards achieving the objectives.

Local authorities may also apply for powers to undertake roadside emissions testing of vehicles. They are responsible for the regulation of emissions from around 500 installations under the Pollution Prevention and Control Regime, 17,000 small industrial processes under the Local Authority Pollution Prevention and Control (LAPPC) regime and many other sources under the Clean Air Act.

Air quality standards

Objectives based on the above national standards have been set for certain pollutants. Local authorities must assess whether the air quality standards will be met in their area by the specified target date. If the objectives are not met, local authorities must establish Air Quality Management Areas (AQMAs) and ensure that the standards will be met. The decision as to where exactly the air quality objective applies in any given case rests with the local authority.

Around 180 AQMAs have been designated so far by local authorities. The majority of these are in respect of the objectives for nitrogen dioxide and PM10 and are mainly as a result of emissions from road transport. Fewer than ten AQMAs have been designated as a result of emissions of sulphur dioxide from industrial sources regulated by local authorities, the Environment Agency or the Scottish Environment Protection Agency.

Sampling principles

The fundamental principle behind any sampling activity is that a small amount of collected material should be representative of all the material being monitored. The number and location of samples that are needed to make up a representative sample depends on how homogeneous the material is. If it is very homogeneous, only a few samples may be required. If the material is heterogeneous, many more samples will be required.

This fundamental principle applies as much to sampling stack gases as it does to any other type of sampling. Although the gas in a stack might be thought of as being more uniform than, for example, a stockpile of coal, gases in stacks can become non-homogeneous. This may be due to differences in chemical composition, or differences in temperature and velocity, which may lead to stratification and swirling. Where the gas is also carrying particulates along the duct, there is likely to be even less homogeneity. Here, special measures must be taken to ensure samples are representative.

For gases carrying particulates, the sampling approach has to address two effects.

Firstly, inertial effects introduced by gravity and the duct geometry lead to the particles being unevenly distributed in the duct. Samples must be obtained from multiple sample points (see below for definitions of terms) across the sampling plane to give an overall average of the particulate emission. Rules have been developed specifying where these sampling points should be located and they are provided in the Environment Agency TGN M1. In the case of a cross-duct CEM monitoring particulates, the average particulate concentration is obtained as an integrated measurement across the duct.

Secondly, for extractive methods, the sample must be collected isokinetically (at the same speed as the flow in the duct). Where the measurement is of concentrations of gaseous species alone, a sampling location where the gases are well mixed should be chosen. If gases are well mixed, it is possible to demonstrate that sampling can be carried out from a single sampling point in the sampling plane. However, if the mass emission rate is to be calculated, the gas volumetric flowrate will need to be measured; this will require velocity measurements to be made at several points across the sampling plane. Some pollutants, for example metals and dioxins, are present in both particulate and vapour phases. Other pollutants such as hydrogen chloride may be present in an aerosol phase and vapour phase. Aerosols are normally treated as particulates. In all such cases, isokinetic multi-point sampling is required.

Choice of sampling method, technique and instruments

There is a wide choice of monitoring approaches, analytical techniques, published methods and equipment that can be used to carry out stack emissions measurements. It is important that each of these is chosen to be suitable for the application in question.

The sampling approach, technique, method and equipment that are chosen can have different affects on the requirements for access, facilities and services.

Though the precise requirements can vary, the following will always be required:

• A safe means of access to the sampling position • A means of entry for sampling equipment into the stack • Adequate space for the equipment and personnel • Provision of essential services, such as electricity

The different approaches to monitoring stack emissions

Stack emissions monitoring can be classified into two types:

Periodic measurements - a measurement campaign is carried out at periodic intervals, for example, once every three months. The sample is usually - but not always - withdrawn from the stack (extractive sampling). An instrumental or automated technique may be used, where the sampling and analysis of the substance is fed to an on-line analyser. Alternatively, a technique may be used where a sample is extracted on site and analysed later in a laboratory. Samples may be obtained over several hours, or may be so-called ‘spot’ or ‘grab’ samples collected over a period of seconds to several minutes.

Continuous emissions monitoring systems (CEMs) - automated measurements carried out continuously, with few if any gaps in the data produced. Measurement may be carried out in situ in the stack (for example, cross-duct monitoring), or extractive sampling may be used with an instrument permanently located at or near the stack. CEMs are also referred to as Automated Monitoring Systems (AMS), particularly in mainland Europe.

The main characteristics of the two approaches are summarised in the table below. One approach is not inherently superior to the other; both have their own strengths and weaknesses depending upon the application. In general, however, CEMs provide increased confidence for both regulatory purposes and process control.

Periodic sampling of gases

For monitoring gases, the range of sampling equipment and apparatus is very wide. However, they can be grouped conveniently into automated techniques and manual techniques. For automated techniques, the sampling and quantification stages are conventionally considered to take place almost simultaneously, within an analyser. With manual techniques, the sample is taken and then the quantification and analysis takes place as a discrete, later stage.

The main steps in measuring gaseous pollutants are as follows: • Stage 1: representative sample of source gas extracted through a probe and filtered • Stage 2: gases collected in an appropriate medium • Stage 3: the sampled substance is analysed using an appropriate technique

When gases are measured using automated/instrumental techniques, such as gas analysers, Stage 2 is omitted, and the sample goes directly to the analysis stage (Stage 3). In contrast, when gases are measured using manual techniques, Stage 3 is usually carried out away from the site at an analytical laboratory.

The location requirements for measuring gas concentrations are less exacting than for particulates, as variations in velocity tends not to affect the homogeneity of the gas concentration. This means that the proximity to bends, branches, obstructions and fans is less important. However, sampling after the ingress of dilution air must be avoided.

However, sometimes it is necessary to report mass emissions rates, such as g s-1, to demonstrate environmental compliance, or for pollution inventory reporting or emissions trading purposes.

Calculation of mass emissions rate requires the measurement of gas volumetric flow rate through the duct. This requires velocity measurements to be taken at different points across the sampling plane. Measurements to determine stack gas velocity and volumetric flow rate should be made in accordance with ISO 10780:1994 or BS EN13284-1. A suitable sampling location should therefore conform to the particulate monitoring flow stability requirements.

Isokinetic sampling

Due to the wide range of particle sizes normally present in process emission streams, it is necessary to sample isokinetically to ensure that a representative sample of the particulate emission is obtained. Only very fine particles below 5 microns aerodynamic diameter behave like a gas and do not normally require isokinetic sampling. Isokinetic sampling is achieved when the gas enters the sampling nozzle at the same velocity and direction as the gas travelling in the stack or duct.

If the sampling velocity is less than the isokinetic ratio (usually expressed as a percentage), the actual volume sampled will be less than it should be. At first sight, it would appear that the emission will be underestimated. However, because the sampling rate is too low, there is a divergence in flow around the sampling inlet.

Small particles are able to follow the flow and a percentage of them will not be sampled. Larger particles, on the other hand, are not able to follow the flow because of their greater inertia, and more of these particles will enter the sampler. Thus a sub-isokinetic sampling ratio will lead to a bias in the sampled particle size distribution towards the larger particles. This could lead to an overestimate of the particulate concentration depending on the original size distribution.

Sampling at a rate in excess of the isokinetic ratio will lead to a bias in the sampled particle size distribution towards the smaller particles. This could lead to an underestimate of the emission concentration depending on the original size distribution.

In situ or extractive monitoring

As the term suggests, monitoring can be done in situ (e.g. within the stack itself) or extractive (sample withdrawn from the stack). Both periodic and continuous monitoring can be performed using in situ or extractive techniques.

Remote sensing

Remote sensing methods allow measurements to be made directly in the atmosphere without obtaining samples. The average concentration of a specifically targeted pollutant is determined over an extended measurement path, rather than at a localised point. Some methods allow the concentration to be spatially resolved. Others give the average concentration over the whole path length, which finds application in assessing the transfer of pollutants across site boundaries and along roads and runways, but the difficulty of interpreting integrated-path data should be recognised.

Differential optical absorption spectroscopy (DOAS) instruments use a double-ended system, which measures the average concentration between the instrument and a reflector up to hundreds of metres away. The system is able to measure many common pollutants including SO2, NO, NO2, H2S, O3, benzene, toluene, xylenes and formaldehyde.

Laser interferometry detection and ranging (LIDAR) can measure aerosol particles.

Measurement by remote sensing techniques tends to be expensive because of the complexity and sophistication of the equipment and data handling facilities.

Remote methods lend themselves to mobile sampling: this may be vehicle-mounted instruments for carrying out measurements at a large number of locations, or for measuring the pollution concentration profile along a given route.

Airborne systems using in situ continuous analysers have been used for some specialist applications, such as tracking power station plumes across the North Sea. Such systems have the advantage of greater freedom of movement, three dimensional capability and higher speed of traverse, but are of course so expensive as to be only justified for specialist investigations.

Microscopic analysis

Samples of particulates obtained from smoke, dust, grit or fumes can be analysed microscopically to aid identification and characterisation, which in turn may help in identifying the source, if unknown.

Optical Light Microscopy (OLM) uses the visible, or near visible, portion of the electromagnetic spectrum, whereas Scanning Electron Microscopy (SEM) analyses the surface of solid objects, producing images of higher resolution than optical microscopy. SEM produces representations of three dimensional samples from a diverse range of materials.

Gas liquid chromatography

Gas liquid chromatography involves a sample being injected onto the head of the chromatographic column. The sample is transported through the column by the flow of inert, carrier gas. The column itself contains a liquid stationary phase, which is adsorbed onto the surface of an inert solid. The principle is that the gaseous components of the air sample injected take different lengths of time to pass through the column depending upon their chemical structure.

By using known gases as standards, it is possible to identify the components within the unknown sample based upon the time it takes them to pass through the column. Different columns are used to separate different components.

Mass spectrometry

The modern gas chromatograph is a fairly complex instrument, mostly computer controlled. The samples are mechanically injected, the analytical results are automatically calculated and the results printed out, together with the pertinent operating conditions in a standard format.

In air pollution analysis, gas chromatography can be used for odour analysis, along with mass spectroscopy.

Mass spectrometry is an analytical tool used for measuring the molecular mass of a sample. For large samples such as biomolecules, molecular masses can be measured to within an accuracy of 0.01% of the total molecular mass of the sample.

For small organic molecules, the molecular mass can be measured to within an accuracy of 5 ppm or less, which is often sufficient to confirm the molecular formula of a compound. In environmental work, mass spectrometry is often used to identify organic pollutants such as Poly Aromatic Hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs).

Mass spectrometers can be divided into three fundamental parts, namely the ionisation source, the analyser and the detector.

The sample has to be introduced into the ionisation source of the instrument. Once inside, the sample molecules are ionised and the ions extracted into the analyser region of the mass spectrometer, where they are separated according to their mass-to-charge ratios. The separated ions are detected and this signal sent to a data system where the mass-to-charge ratios are stored together with their relative abundance for presentation in the format of a spectrum.

The analyser and detector of the mass spectrometer - and often the ionisation source too - are maintained under high vacuum to give the ions a reasonable chance of travelling from one end of the instrument to the other without any hindrance from air molecules. The entire operation of the mass spectrometer - and often the sample introduction process also - is under complete data system control on modern mass spectrometers.

Atomic absorption spectrophotometry

Atomic absorption spectrophotometry is an analytical technique used to measure a wide range of elements in materials such as metals, pottery and glass.

The sample is accurately weighed and then dissolved, often using strong acids. The resulting solution is sprayed into the flame of the instrument and atomised (see schematic diagram). Light of a suitable wavelength for a particular element is shone through the flame, and some of this light is absorbed by the atoms of the sample.

The amount of light absorbed is proportional to the concentration of the element in the solution and hence in the original object.

Measurements are made separately for each element of interest in turn to achieve a complete analysis of an object and thus the technique is relatively slow to use. However, it is very sensitive and it can measure trace elements down to the part-per-million level, as well as being able to measure elements present in minor and major amounts.

Author

Andrew Taylor is currently a Chartered Safety Practitioner working with SHEilds Ltd as a tutor on the NEBOSH Diploma in Environmental Management. He has extensive experience in Health, Safety and Environmental Management, most recently in consultancy and construction.

The NEBOSH Diploma in Environmental Management is delivered via e-learning and is accessible worldwide. This benefits students who have work and family commitments as a cost effective way in which to develop in an environmental career.

More information can be found at SHEilds’ website at www.sheilds.org , or by calling +44 (0)1482 806805, where you can speak to course advisors directly.

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Published: 10th Sep 2011 in AWE International

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