Flexible solutions for emissions monitoring

Fourier transforming overcomes the problems associated with conventional pollution monitors by enabling instruments to measure a range of pollutants in a single swipe. Keith Golding, managing director of Quantitech, explains how.

Almost all gaseous pollutants can be detected and measured using their property of absorbing infra-red radiation, and this has been an established monitoring technique for many years. Because each type of gas absorbs at specific frequencies, it is relatively simple to design an instrument to monitor a particular pollutant by measuring at the appropriate frequency for that molecule.

In complex mixtures, particularly those containing organic gases, the absorption patterns of different species can overlap, potentially causing serious measurement errors. There are also cost, maintenance and calibration issues associated with multiple installations.

The Fourier Transform Infra-Red (FTIR) technique overcomes the problems associated with single component analysers through its ability to measure at all the relevant frequencies simultaneously, thereby providing a complete analysis of all gaseous pollutants in a single measurement, and in real time. All the normal stack gases can be detected with ease - CO, CO2, SO2, NO, N2O, NO2 and CH4 - and many more like H2O, HCl, HF, NH3 and organic species (VOCs).

Scanning all frequencies
Very briefly, an FTIR analyser works by passing an infra-red beam through the air or gas sample and scanning all the infra-red frequencies required to identify and quantify components in the sample. Unlike conventional scanning instruments, however, with FTIR, all the frequencies are measured simultaneously, and the results interpreted by computer-based calculations using a mathematical procedure called a Fourier Transform.

This gives a number of practical advantages, not least of which is speed: a full spectral scan is completed ten times a second, so is effectively in real time. Indeed it was only with the advent of the fast, low-cost calculating power provided by modern processors that FTIR developed from a research tool to a routine technique that has almost completely replaced traditional scanning instruments.

The Gasmet series of analysers, marketed in the UK by Quantitech Ltd, are designed specifically for demanding industrial applications and were recently awarded TUV certification in Germany. UK MCERTS approval is also being applied for. Analyser installations can be fixed or portable and configured to measure up to 50 components simultaneously. The technique can therefore provide a solution for most emission monitoring problems, as the following application examples will illustrate.

Continuous collection
The new Elean Power Station near Ely, Cambridgeshire is the UK's first and world's largest straw burning power station, feeding 38MW into the National Grid. Strategically located in the UK's major arable farming area, the station's appetite for fuel - 50 to 60 half-tonne straw bales per hour - is fed by a dedicated fleet of trucks collecting from farms up to 50 miles away. Parameters like water content of the straw (clearly dependent on weather conditions), can vary widely and affect
its combustion behaviour.

With these factors in mind, and as a new concept for the UK, it was necessary to satisfy both the Environment Agency and local residents that the plant represents no risk to health or the environment by providing continuous data on stack emissions. In fact, the Elean power station was recently awarded the UK National Green Apple award for good environmental practice.
A Gasmet Cx-4000 analyser is permanently installed in a cabin next to the main stack and continuously monitors SO2, CO, NO, NO2, NOx and HCl. Concentrations are continuously available on-screen in the plant control room and can also be viewed remotely by the Environment Agency via modem using PC-ANYWHERE software. The system is calibrated automatically twice daily and all results are logged automatically for reference purposes.

Process control
The quarry at Lafarge Lime's Whitwell Works in Derbyshire is situated on one of the best quality dolomite - specifically magnesium limestone - deposits in the UK, and provides the raw material for a range of specialist burnt dolomite products and aggregates.

Dolomet, Dolofrit and Dolopel are respectively used as steelmaking fluxes, for kiln repairs and for the production of high quality refractory bricks and are produced in two large rotary kilns. Each product requires slightly different firing temperatures and kiln residence times, so accurate process control is essential to maintain product quality.

Changing legislation required an additional number of analytes to be measured in addition to the continuous monitoring already in place on site. The nature of the raw feed stone and mix of fuels required that the emissions monitoring system must be able to differentiate between a wide range of analytes, many at trace levels, in the presence of up to 18 per cent levels of CO2. A Gasmet Dx-3000 measures CO2, CO, SO2, HCl, HF and nitrogen oxides as well as a range of hydrocarbons as indicators of Total Organic Carbon emissions.

Sampling is automatic and data from the analyser is transmitted to the control room where it is used to control the process. A key requirement was the ability to move the system from one kiln to another: the transportable Dx-3000 can be transferred and up and running again in less than an hour and a half.

Flexible and portable
Emissions monitoring consultants, Anchem Laboratories, needed a totally flexible and portable instrument to cope with the diverse situations of their clients. Founded 18 years ago to provide a coal analysis service to the mines of South Wales, the company has diversified to reflect the huge changes in the industrial landscape of the region. It is now UKAS-accredited to provide a comprehensive service to the varied industries located there, including asbestos monitoring, chemical analysis and emissions monitoring.

Many Anchem clients operate under individual authorisations from the local authority or Environment Agency based on their specific processes, and require periodic confirmation that their flue emissions conform to license conditions.

Anchem use a portable Gasmet Dx-4000 to offer the necessary flexibility. The needs of two typical clients - one a titanium finishing plant and the other an aluminium smelter - illustrate the diversity of measurements required. In the first case, traces of HF down to 10 mg/m³ along with HNO3-, NO2, N-2-O and NO must be measured, whereas in the second it is H2SO4 and HCl from fluxes and CO, CO2 and nitrogen oxides from the furnaces.
Process development is an important aspect of the work at the AstraZeneca site in Macclesfield, Cheshire. The site is one of the Company's two production facilities in the UK and is involved in all stages of drug manufacture - scale-up from small-scale R&D processes, bulk production and
The varied nature of the work means that batch processing is the norm. This, and the development character of some of the work at Macclesfield, demanded a flexible emissions monitoring capability for a wide range of analytes and the ability to collect data in real time.

Multiple monitoring
A Gasmet Dx-4000 portable FTIR system is used to monitor up to ten different scrubber units around the site. A typical measuring period in any one location may be only 30-60 minutes, again emphasising the necessity for real-time data and portability. Unlike other FTIR systems, the Dx-4000 can be used with a laptop rather than a full PC, another important consideration when mobility is crucial.
A specially designed 'tent' is used to protect the unit from the elements, eliminating the need for permanent enclosure structures. Analytes include solvents such as toluene and acetone, HCl and nitrogen oxides. Emissions from a process are just as much an indicator of process performance as the end product itself, so in the process development context the ability to detect even unexpected emissions is particularly valuable.
The ability of FTIR to handle such a diversity of monitoring applications makes it a uniquely powerful tool in the environmental armoury. Analysers can be configured for fixed, continuous operation or as fully portable systems for mobile monitoring. Data can be logged or transmitted over data links to a control room, to the instrument supplier for on-line diagnostics or applications support, or even to a regulatory authority to demonstrate compliance.


| CO2 | coal | gas


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