Gas analysers play a crucial role

Paul Boughton

The Daldowie fuel plant at Daldowienear Glasgow in Scotland is operated by SMW Limiteda wholly owned subsidiary of Scottish Power Generation Limited.

The plant includes one of Europe's largest sludge drying plantwhich processes the sewage sludge from greater Glasgow’s population of approximately one million.

The plant represents the very latest in sophisticated controlthermal drying and effluent treatment practiceand produces up to 65000t/y of low odour granule. It consists of six dryers whose emissions feed three regenerative thermal oxidisers (RTOs) which each have their own emission stack.

Strict regulations

In order to comply with strict emissions regulations set by the UK’s Environment Agencyand also to ensure that the RTOs were operating correctlyit became necessary to install a continuous emission monitoring (CEM) system on each exhaust gas stack.

Three such systems were installedincorporating Servomex4900 series gas analysersServomex2700 series gas analysersServomex4995 series sample conditioning systems and ancillary equipment.

Monthly reports

Emissions regulations in Scotland are stricter than in many other European countries and oblige plants such as Daldowie to deliver monthly reports to demonstrate that emissions limits have not been exceeded.

Due to the unusual nature of the Daldowie plantit was required to meet newly introduced and strictly enforced regulations for the emission of gases and odoursas laid down by the Scottish Environment Protection Agency (SEPA).

The main area of concern was exhaust gases from the three RTOs. These pieces of equipment heat the gases released during the drying process so that the volatile organic compounds (VOCs) are broken down and burnt.

This was a particularly challenging application in terms of gas analysisbecause of the moist nature of the gasesthe continually varying mix of gases and emissionsthe elevated temperature (around 120°C)and the fact that several gases had to be monitored.

Accurate readings

The new emissions regulations imposed on the plant required the measurement not only of VOCs but also carbon monoxide (CO) and oxygen (O2). Furthermoreaccuracy was essential because if the emissions limits are exceededthe plant must be shut down; any false alarms due to inaccurate readings could therefore be extremely costly in terms of downtime and lost production.

It was already known that similar sludge treatment plants had suffered through unreliable CEM systems that had failed after being used to monitor production runs for as little as 10 hours. SMW wanted to avoid this problem with the Daldowie plantso the company wrote a tight specification.

Compliance with MCERTS (the monitoring certification scheme) requirements was also necessaryyet the equipment had to be reasonably priced.

Servomex was able to meet the specification with a bespoke walk-in cabinet housing three CEM systemsone per stack.

Each system comprises a Servomex4900 series gas analyserServomex2700 series gas analyserTOC (total organic carbon) analyser and a Servomex4995 series sample conditioning system.

Because of the moisture content in the flue gasesthe sensor probes and sample lines are heated to prevent the formation of condensation.

The 4900 gas analyser is designed specifically for CEM applications and is capable of measuring O2COCO2NO and SO2though in the Daldowie plant only CO and O2 are being monitored.

Benefits provided by the analyser include a low cost of ownershiplow maintenance requirement and easy integration with other systems.

While oxygen is measured using a paramagnetic sensor cellinfrared technology is used for carbon monoxide.

A second O2 reading is obtained using a Servomex2700 series analyser. The benefits provided by the 2700 include a continuous measurement of O2 and/or combustibleshigh temperature operation up to 1750°Clow operating/maintenance costs and is fully field maintainable.

The reason for taking two O2 readings is that the 4900 measures O2 on a dry basis and the 2700 measures O2 on a wet basis using a zirconium oxide sensor.

The O2 measurements are used in order to derive the moisture content of the gas which is used to correct the TOC measurement from a wet to dry basisand also to convert the measured gas concentrations to reference conditions of 11percent O2 as required by SEPA.

Bryan Englanda consultant to SMWcomments: “Considering the complexity of the analysis being undertakenthe Servomex equipment has performed very well: as the sampled gas is both hot and wetit is important to have a system that can handle it in such a form.

The system provided by Servomex has proved to be reliable and we have a high degree of confidence in the results it produces.

Analysis problems

Without the Servomex CEM system SMW would find it much more difficult to comply with the stringent emissions regulations here in Scotland.”

He continues: “We enjoyed excellent service from Servomexright from the initial enquiry through installation and commissioning. The engineers are extremely knowledgeable and are clearly capable of addressing even the most out-of-the-ordinary gas analysis problemsdeveloping complete CEM systems around the company's standard gas analysers plus equipment from third-party suppliers.”

More information about the Servomex 49004995 and 2700 series gas analysersas well as other Servomex gas analysersall of which are available and fully supported worldwidecan be obtained from Servomex Group by calling
+44 (0)1892 652181faxing +44 (0)1892 662253
emailing or visiting the website at


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