Monitoring of VOC Emissions keeps industry on the right side of the law.

The snag, however, is that VOCs are invisible and may, when partially treated, be undetectable by any other means than gas analysis. So, if a thermal oxidiser should develop a fault and work inefficiently, it is quite possible for a facility to be emitting VOCs and breaking the law without the company operating it knowing anything about the failure. And that can be both expensive and a source of trouble.

The best way to be absolutely sure that a business will not fall foul of the law by emitting VOCs from areas where solvents are used or processed is by fitting an appropriate gas analyser system to monitor the output of the thermal oxidiser.

Minimal hydrocarbon content

The thermal oxidiser is a combustion chamber which burns unwanted VOCs so that the exhaust air contains no more than a minimal level (ideally zero) of hydrocarbons. If combustion within the thermal oxidiser is incomplete for any reason, the output from the thermal oxidiser will contain carbon monoxide (CO).

To be sure that the thermal oxidiser is doing its job, it is therefore necessary to monitor the level of CO in the output airflow. Provided that the CO present remains below a set level, the VOCs have been fully oxidised. If the CO level rises, the thermal oxidiser is faulty and urgent action is needed.

Logically therefore, the way to check whether the thermal oxidiser is doing its job is to use a gas analysis system which is set up to monitor carbon monoxide.

Major advances

Dr Simon Bruce of Servomex International, emphasises the international nature of this specialised market for gas analysers to monitor the efficiency of thermal analysers.

“Major manufacturers want more than simply a source of precision gas analysis instruments of very high reliability” he explains. “They require the analyser manufacturer both to have a knowledge of international environmental standards and of the emissions legislation in all the major industrial countries. They seek a common standard of analysis appropriate to all the countries in which they manufacture.”

For obvious reasons, this is likely to require performance to the most stringent standard available worldwide. Maintaining up-to-date information on comparative standards imposes considerable responsibilities for research and analysis of complex government data and regulations emerging in many languages from bureaucracies great and small across the globe. Only a truly international company can cope. Ideally, analysers also need to be approved to these standards. These facts more than any others have concentrated the sales of gas analyser systems for thermal oxidiser plants into the leading analyser manufacturers.

Analyser components

Servomex System Designer Jeremy Dixon explains that an analyser system for an industrial thermal oxidiser consists essentially of three pieces of equipment mounted in a standard 19 inch rack. These are a Servomex Xentra 4900 continuous emissions analyser, a Servomex 4995 modular integrated sample conditioning unit and a chart recorder. The Xentra 4900 is already T†V approved for CO and oxygen measurements and other Approvals are in the pipeline.

The sample conditioning unit extracts gas samples from the extractor fans and cleans and dries them before they are delivered to the gas analyser. Any process involving combustion, which includes thermal oxidisers, inevitably produces carbon deposits, however well combustion is controlled. The sample from the gas stream at the extractor fans will therefore, in simple terms, include some soot and some water vapour. If the analyser is to continue to work with the precision for which it was designed, that soot and water vapour has to be removed before the sample is delivered to the gas analyser.

The Xentra 4900 analyser is capable of being configured to measure concentrations of several gases at once, but for the thermal oxidiser monitoring application is installed with a gas filter correlation infrared transducer designed to monitor carbon monoxide. This has two transparent gas-filled filters, one filled with nitrogen, the other with carbon monoxide, which are rotated so that an infra-red energy source passes alternately through the nitrogen-filled filter, then through the carbon monoxide-filled filter, after which it passes through the cleaned and dried gas sample from the sample conditioning unit.

When the nitrogen-filled filter intercepts the infra-red beam, all the energy passes through the measuring cell, after which some is absorbed by the sample.

When the carbon monoxide-filled filter intercepts the beam, infrared energy at the wavelengths associated with carbon monoxide is removed. The change of signal detected as the two filters are alternately placed in the infra-red beam represents only the infrared wavelengths associated with carbon monoxide, so spurious readings caused by other gases which may be present in the gas stream from the thermal oxidiser are removed. The result is a precise analysis of CO content, and therefore an accurate check on the function of the thermal oxidiser.

The results of the analysis are continuously plotted by the chart recorder to provide a quickly accessible visible record of the gas analyser’s results.

However, in most cases, the Xentra 4900, which has a serial RS232 port, will also be supplying data continuously to the production control computer so that, in the event of an emergency condition, the computer can shut the offending area of the plant down before environmental legislation infringements occur.

Wide range of companies

Simon Bruce makes the point that the potential impact of lost production and penalties for infringement of environmental legislation are problems for a wide range of industries, including plastics producers, any process industry manufacturing or using solvents and, of course, any company which is spraying finishes or other materials containing solvents.

“Every manufacturer has to comply with the law and is equally subject to the penalties which apply if accidental infringements occur” he says. “Companies of quite modest size are now recognising that monitoring VOC output with a gas analyser system makes sound financial sense”.