that as the fi lter check worked the system was fi ne. A round trip of 300km was undertaken by the engineer to return with a replacement analyser, calibration and check gases. This revealed a leaking fi lter housing on the probe due to a nick in the O-ring, a poor connection to the gas conditioner, badly maintained fi ttings and an analyser which didn’t give the correct results when calibration gas was introduced to the sample port of the unit. The problems were rectifi ed. The system was checked again with gases, the analyser was changed for the replacement one, and fi nally the engine gave results within the guarantee requirements.

A very costly situation was avoided, but the mistakes could have been avoided by careful planning and having correct and well maintained equipment on site.

Hydrogen Chloride:

A Special Case, or is it? It is rare for the issues I have discussed plus more to come into play simultaneously in emission testing, however as we move to measuring more diffi cult species at lower levels, such as mercury, ammonia, hydrogen fl uoride and hydrogen chloride these issues can and do take effect.

There has been much discussion about hydrogen chloride this year at the Source Testing Association (STA) and some of the issues can be highlighted below:

1. There needs to be suffi cient time to equilibrate hydrogen chloride analysers and sample system (with all components powered up and gases being sampled)

2. The response times of the analyser system brought in to perform tests is often very different to the fi xed system (CEM/ AMS) being operated by the plant and often under evaluation as part of a QAL2 test.

3. There are problems of introducing dry gas as a calibrant versus wet humidifi ed gas as a calibrant and the results are different.

4. The effect of ambient temperature on the analyser, conditioning system, probes, calibration cylinders and calibration lines.

5. The reactivity of hydrogen chloride with other species such as ammonia to form a salt which can both sublime and decompose back to their constituent components. What are we trying to measure?

HCl (g) NH4

+ NH3 (g) Cl(S) = NH4 Cl(s) + Heat = HCl (g) + NH3 (g)

6. Is it necessary to passivate the sample line, equipment, analyser and/or, the cleanliness of lines.

If we consider each of these in turn we can see the same issues as discussed earlier and the lessons learnt need to be applied in general.

Why do we need to give instruments and sample lines time to equilibrate at full operational temperature?

Example of Temperatures measured along a Test House Un- Heated Probe (diagram supplied by ABB Ltd)

Long Response times of the sampling system and analyser with respect to hydrogen chloride (or complex species) can be a good indicator of problems with a sampling system.

a) There can be issues with microscopic dust in heated lines, fi lters etc. However even new un-used probes and fi lters can have active sites (arising from the manufacturing process) which can react and remove or form sites for condensation and removal of the analyte. It is suggested by many that passivation


a) To stabilise the instruments in order to minimise drift and allow all the working components of the gas sample path both within the analyser and sample system to be at the necessary temperatures without any cold spots, so as to minimise any potential losses of species. Cold spots can encourage condensation of water and/or the analyte and should be avoided.

b) Probe and heated lines, can have cold spots, heated lines usually due to incorrect positioning of thermocouples or heated lines being coiled up leading to isolated hot-spots which result in incorrect temperature control of the lines. Some cold spots arise due to the use of unheated probes passing through stack walls. The gases cool rapidly as they pass through the stack wall and a cold spot resulting in condensation occurs. This is exacerbated on exposed and thick-walled stacks where the natural cooling of passing through the wall, an un-insulated probe and the cold wind have a severe cooling effect on all probe related and exposed lines at the stack port.

Example of irregular temperatures measured along a Test House Heated Probe (diagram supplied by ABB Ltd)

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