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SOURCE TESTING ASSOCIATION I Annual Guide 2018


of equipment is required. This is most likely an opportunity to saturate the active sites and reach a steady state equilibrium where the sites no longer are a sink to the pollutant under investigation. The presence of active sites etc. will be refl ected in a slow increase in the pollutant with time and then reaching a constant or plateaued value.


b) The introduction of calibration gas in a dry state rather than as a hydrated micellar state as found in the actual stack gases disturbs this equilibrium and hence results in a false calibration. The dry gas is much more reactive and will react preferentially with any of the active sites present, even after passivation.


c) Introduction of calibration gases using a wet calibrator or as in US EPA Method 321 and US EPA Method 322 where an analytical spike is used to verify that the sample system is operating correctly requires the introduction of HCl gas into the sample probe and looking at the increase above the levels in the stack. This is a QA procedure rather than a calibration method and works best on cement kilns where the stack levels can be fairly constant. It has the advantage of introducing HCl (10% of total sample fl ow) and using the stack gases themselves to attempt to maintain the actual gas conditions in the sampling system. It also minimises perturbation of the gas concentrations and the nature of the analyte.


There have been suggestions that keeping calibration cylinders in very cold conditions can affect their concentration. I suspect there are two facets that are key here:


a) Are the components of the calibration gas handling system scrupulously dry? Have they been exposed to the general atmosphere, where they can pick up moisture? The gas calibration lines also need to be insulated when cold conditions are experienced and some heated lines are supplied or can be manufactured with calibration lines in the insulated bundle and are kept warm by the heated line itself, thereby avoiding the issue altogether.


b) HCl and Ammonia are capable of being liquifi ed by pressure and temperature. In calibration gases supplied, the withdrawal


of the gas from the cylinder may result in a potential adiabatic expansion and the gases in the cylinder will cool. If the area is already very cold then potentially localised condensation of the liquid gas could occur. This would explain some of the unusual effects. This is purely supposition and open to debate but nevertheless problems with gas cylinders in the cold have been reported by other workers using other gases, so the answer is clear. Keep them at room temperature where possible.


In reality, all gas cylinders need to be maintained ideally at a constant temperature and allowed to equilibrate to a steady- state temperature prior to use to ensure constant delivery of the gas anyway.


Hydrogen chloride like ammonia is a highly reactive species. One reaction which is well known is the reaction of hydrogen chloride with ammonia to form ammonium chloride. This has two issues; it appears as a very fi ne(sub-micron) mist and passes through fi ltration systems relatively easily, and it has the potential to sublime (change from a solid to a gas without changing to a liquid at 338ºC). Hence it should be caught on fi lters however it also decomposes readily back to ammonia and hydrogen chloride and hence back into the analysis system. The temperature this occurs at is much less well defi ned and is a known problem.


If we look at the range of issues present they keep recurring but as we move to more complex species and lower levels of pollutant the issues we once ignored are more signifi cant and have to be addressed.


It is my plan to write a similar article about iso-kinetic sampling in the next annual guide. All of the above observations have been made from real sites and real projects and form a small part of the training course I run on emission testing. As you have seen as pollutant levels drop and the interest in analysing reactive gases such as HCl, HF, and NH3


by instrumental means


become more important adherence to good practices is paramount.


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