Paul Wilford, Area Business Manager for Casella Measurement outlines the use and application of laws and rules surrounding Volatile Organic Compounds (VOCs).
He explains the different technologies and techniques used to measure VOCs and also warns about the dangers, pitfalls and common mistakes that professionals in the industrial process emissions control industries make in relation to VOC detection.
Before we can understand the limits and techniques involved in detecting and measuring volatile organic compounds (VOCs) it helps to define their origin and the problems associated with emitting them to the atmosphere.
What is a Volatile Organic Compound?
A volatile organic compound is a broad category of chemical compounds that contain carbon and hydrogen and are characterised by their volatility (ability to evaporate) under standard conditions i.e. normal room temperature and pressure. Organic compounds that require heating or cooling to produce a vapour are considered stable. The majority of VOCs are man-made although VOCs are also found in all living things.
What problems do VOCs cause?
Some VOC vapours can be dangerous to human health if inhaled in large quantities over a long period of time and can also cause harm to plants, destroying their natural processes. Also, if VOCs are found in large quantities they can form an explosive mixture in air – the concentrations for when this can become a concern are described in chemical specifications (Material Safety Data Sheets) as the Lower Explosive Limit and the Upper Explosive Limit. VOC concentrations that fall between these two limits can be considered explosive and therefore dangerous.
The most important and common consequence of emitting VOCs to the atmosphere is the formation of low level ozone. In simple terms:
Oxygen (O 2 ) + VOCs + sunlight leads to the formation of Ozone (O 3 )
Ozone is the combination of three oxygen atoms to produce O3 and occurs naturally. Ozone is of great benefit to humans in the upper atmosphere as the ozone layer prevents ultraviolet light from the sun reaching the earth. However, at low levels i.e. the earth’s surface, ozone is a dangerous compound as it is the main component of smog. Smog is more than just an unsightly manifestation of pollution as a cloud hovering over the world’s cities. Smog causes respiratory and heart problems in humans, also destroying plant life from crops to forests, therefore smog has a debilitating effect on our entire environment.
In addition to VOCs there is also a family of chemical compounds called Hazardous Air Pollutants (HAPs) – these are defined as VOCs but with additional properties that are harmful to humans. HAPs can have carcinogenic effects when inhaled over large time periods but can also cause damage to the central nervous system and serious birth defects following catastrophic accidental releases to the atmosphere.
Where do VOCs come from?
VOCs are emitted to the atmosphere from a number of sources, some occurring naturally. The main contributors however are industrial processes that use solvents, (for example; printing, spray painting, coil coating, wood treatment etc.), inefficient combustion from road traffic engines and evaporation from landfill sites.
What is being done?
According to the UK Environment Agency, total VOC emissions peaked in 1989 and fell by 38% by 2000. Road emissions fell by 55% over the same period, mainly as a result of the introduction of catalytic converters for petrol cars. Fuel switching from non-catalyst cars to diesel cars has also had a small beneficial effect. Emissions from solvent use have changed little over the past 25 years.
Under international agreements, the UK has agreed to cut VOC emissions to 72% of 2000 levels by 2010. Several measures are in place to achieve this target. The Solvents Directive should reduce emissions from certain industries by about 57% between 1990 and 2007. Reductions will come mainly from the coating of textiles, pharmaceuticals, surface cleaning and vehicle finishing sectors. The Auto-Oil directives will cut vehicle emissions of VOCs and nitrogen oxides by about 70% through the promotion of cleaner fuels and reduced car use.
The legislation surrounding VOCs is detailed and varied and can quickly become an article on it’s own, a brief guide is given below.
The need for monitoring VOCs is required by law in most cases. As a general rule the various industrial sources that emit VOCs are divided into Part A, Part A2 and Part B processes. Those that use the largest amount of VOCs and therefore have the highest potential emissions are classed as Part A and Part A2 processes – these are regulated by the Environment Agency. Part B processes cover smaller operations and are regulated at a regional level by Local Authorities.
For each process there is a Process Guidance Note that gives details (amongst other things) on total VOC emissions allowed to be exhausted to atmosphere, usually quoted in mg/m3. The method by which these should be measured is also included and is usually quoted as a BS EN standard. Reference is also made as to whether continuous emissions monitoring or periodic sampling is required.
As an addition, the Solvent Emissions Directive has now been implemented for the majority of processes and has been transposed into current legislation. Existing processes that operate above a given threshold for solvent consumption (e.g. coil coating processes using >25 tonnes of solvent per year, pharmaceutical manufacturers using >50 tonnes of solvent per year) have until 31st October 2007 to comply.
The Solvent Emissions Directive provides further regulation for industry, requiring the following:
- To meet an emission limit, fugitive limit or combined total emission limit
- Commission solvent reduction techniques that would lower emissions to those below the required limits
- To conform with further restrictive emission limits for more harmful substances that are classed as carcinogenic, mutagenic or toxic to reproduction
Motor vehicles and landfill sites are also covered by rafts of legislation, however, these are less relevant for the purpose of this article in terms of monitoring VOCs.
Technologies used to measure total VOCs
In general terms, the two most widely used technologies to measure total VOCs are Flame Ionisation Detection (an example of this is the Casella Eti / M&A Flame Ionisation Detector) and Photo Ionisation Detection (an example of this is the Casella CEL VocPro). Both technologies rely on the principle that when most organic vapours burn they produce positively charged carbon ions as an intermediate product of combustion. These positively charged ions are then collected on an electrode and an electrical current corresponding to the amount of carbon ions present is produced. If the instrument has been calibrated against a known source of carbon / VOCs a reading of the total carbon as parts per million (ppm) can be taken. Further calculations can then be performed to infer the actual VOC concentration assuming the proportions of VOCs being measured is known e.g. 80% xylene, 20% isopropyl alcohol and assuming these proportions haven’t changed as part of the process.
Flame Ionisation Detectors (FIDs) use a hydrogen flame to ionise the VOC and can be cumbersome to manoeuvre around the work area, this is exacerbated by the need to carry a hydrogen cylinder required to provide fuel and also a calibration gas cylinder. A heated sample line should also be used when extracting VOCs from an exhaust stack to prevent the VOC condensing in the sample line and therefore giving an inaccurate reading. This also means however that the heater needs to be carried with the FID.
Photo Ionisation Detectors (PIDs) work in the same way as FIDs except that ultra-violet light is used to ionise the VOC. This enables the PID to be simply made explosion proof. Another feature of the PID is that it does not respond to methane when measuring total VOCs – this can be of use when measuring VOCs but wishing to ignore background levels of methane in the atmosphere. The FID can be equipped to remove methane too but this requires an additional methane cutting catalyst.
The technology used to measure and monitor VOCs is often defined by the source of the VOC emission and also the data accuracy that is required. In many cases legislation will also dictate which method should be used e.g. MCerts document M2 (Monitoring of Stack Emissions to Air) prescribes that FID analysers are used for continuous emissions monitoring. Process guidance note PG6-34 for example which provides guidance to those wishing to commercially respray road vehicles specifically requires an FID to be used when measuring total organic compounds in the exhaust stack to standard method BS EN 13526.
Both FIDs and PIDs can be used for periodic extractive sampling – although there is no standard method for using a PID in this situation and can be used only where FID response is poor to certain organic vapours. PIDs are particularly useful however where portability is paramount due to their relatively small size e.g. screening for VOC hotspots in landfill sites or in the workplace. FIDs also cope very well with high moisture content in the gas streams being analysed.
As mentioned previously, FIDs and PIDs will only provide measurements of total VOCs – this is acceptable to comply with most legislation. If fully speciated measurements are required i.e. each individual component of the VOC stream requires identification and quantification, then a Gas Chromatagraph or Mass Spectrometer can be employed. The cost of these items is usually prohibitive unless speciation is absolutely necessary.
Techniques used in measuring VOCs
Sampling and measurement of VOCs can generally be done in three different ways depending on the information required and the legislation in place, these are:
- Periodic measurements
- Continuous emissions monitoring (CEMs)
Periodic measurements describe a measurement regime that is carried out at specific intervals, every six months, for example. The information is then extrapolated to reflect the general operating conditions for the periods between monitoring. The monitoring equipment is usually brought to the site of interest only when the monitoring is required.
Continuous emissions monitoring describes automatic continuous measurements taken with very few gaps in the data. The sampling equipment is therefore located at the site of interest permanently.
Screening is done using easily portable measuring equipment, traversing a large area looking for incidences of high VOCs (hotspots) that may prove a problem either in the workplace or for example, at the boundary of a landfill site.
It is imperative that the sample taken when performing the periodic and continuous monitoring techniques is representative of the overall VOC Detection process emissions. The usual location for a representative sampling port is just prior to the VOCs exiting the process / factory boundary, in most cases via an exhaust stack, to enable measurement of emissions to atmosphere. There are several techniques that can be applied to ensure representative samples are taken, these are generally much less onerous when dealing with VOCs compared with monitoring particulates.
Prior to performing a periodic sample a sweep of the duct should be carried out. This is simply taking VOC measurements from a point diametrically opposite the sample point located in the exhaust/duct and drawing the measuring equipment back across to the opening/ sample port, noting the concentrations at several points. The variation in concentrations should not exceed 15% – if this is the case then the gas being sampled can be considered homogeneous and therefore any samples taken should be representative. Once sampling begins in earnest it is recommended that the tip of the measuring probe be placed between one-third and half of the diameter into the stack. If the gas is found to not be homogeneous then a sweep across the duct should be performed at several predetermined intervals and the samples combined to provide an average value.
To best ensure that a homogeneous gas mix can be achieved there are several considerations that need to be taken account of when positioning the sample port in the duct / exhaust, these are more important when installing a CEM system.
As a general rule, when installing a CEM system the manufacturers advice should always be adhered to, however there are a few simple rules that are outlined in Mcerts Technical Guidance Document (Monitoring) M1 (see www.mcerts.co.uk). This states that the monitoring location should be:
- Representative of the total volume flow in the duct
- Positioned in a straight section that is at least ten hydraulic diameters long
- At least five hydraulic diameters upstream and downstream from any flow disturbance
NB. The hydraulic diameter is defined as 4 x area of the sampling plane / length of the sample plane perimeter.
The flow through the duct at the sample location should:
- Deviate ? ±10° from the duct axis
- Have no local negative gas flow
- Have variations in velocity ?10% at any sample point
If mass emissions are to be calculated from the VOC measurements taken (often required for regulatory compliance) then the sampling location must also conform to the following whether periodic or continuous measurements are to be taken:
- Deviate ? ±15° from the duct axis
- Have no local negative gas flow
- Have a differential pressure >5 Pa (Pascal) (where Pitot tubes are used to measure the velocity pressure) which is approximately 2-3m/s
- Ratio of highest to lowest local gas velocities less than 3:1
For many CEMs calibration will be performed in conjunction with a periodic type VOC analyser. This can also be carried out using calibration gas fed directly into the analyser automatically.
VOC monitoring can also be performed for reasons other than regulatory compliance. It can be used to monitor the working environment, ensuring process operators are working in a safe environment, the performance of abatement equipment e.g. thermal oxidisers or gas scrubbers, or as a design tool for specifying the aforementioned abatement equipment prior to purchase. In this instance, it is imperative that long term representative sampling is carried out to best understand the high and low concentrations of the VOCs emitted to atmosphere by the process. In many cases the abatement equipment will be designed to cope with a short-term maximum VOC output as opposed to an overall average.
Safety is always paramount when working with VOC monitoring equipment. Gas sampling is often done several metres high when working on exhaust stacks and care should be taken – permanent platforms (built to the correct standard) are ideal to work from, however, scaffold structures are more than adequate assuming they have also been constructed to the required standard.
VOCs are clearly important environmental contaminants because of their mobile, persistent and toxic nature. It is therefore important to monitor and measure, particularly within an industrial context, the complex way in which VOCs interrelate between land, water and air and the effects these have on humans and the environment. Any quality supplier of VOC detection equipment will be able to support and offer expert advice to any industry implementing and meeting legislative demands to ensure a healthy future for us all.
Paul Wilford is an Area Business Manager for Casella Measurement and since graduating from Nottingham Trent University, he has been involved in environmentally related industries. Paul deals regularly with both Local Authorities and Industry, providing support in the field of air quality monitoring and measurement.
Casella Measurement is the leading UK manufacturer and supplier of occupational and environmental monitoring equipment and meets all the requirements of the EU Directives and the relevant instrument standards. For further information contact: Casella Measurement on tel: 01234 844100, or by email: [email protected] or visit: www.casellameasurement.com
Published: 01st Jun 2006 in AWE International