Page 81: of Maritime Reporter Magazine (April 1998)

Variability In Measuring NOx Emissions

PROPU

The finalization of the text of the diesel engine NOx emission con- trols to be introduced as part of

Annex VI to MARPOL 73/78 will increasingly focus the attention of the many sectors of the marine industry on exactly how the vari- ous aspects of these controls are, in practice, to be implemented. One aspect of particular interest to the

CIMAC Exhaust Emissions

Controls Working Group was the question of the reproducibility of

NOx measurements: by just how much may emissions rates from a particular engine or engine design actually vary in practice?

The key factor that shaped the structure of Annex VI NOx con- trols and the associated verifica- tion procedures was the accep- tance that the NOx emission rate, under standardized conditions, will not differ between engines of the same design, adjustment and usage. Furthermore, the NOx emissions from a particular engine when in service will not tend to increase; either over time or as a result of poor maintenance.

However, it does not follow that by using the same test equipment and procedures that the exact same emission value would be achieved from the testing of a range of engines of identical design or even from the repeated testing of a single engine. Minor differences in any of the 13 mea- sured parameters which go towards determining the brake specific NOx emission rate, togeth- er with inaccuracies in the inlet air temperature and humidity correc- tion factors, will inevitably result in some variation in the values obtained.

Such variations are not a prob- lem for engines where the onboard verification procedure was solely to be the Engine Parameter Check method. In those cases the actual engine measurement of the NOx- emissions would only be required for the parent engine (or a family or group) while operating under highly controlled test bed condi- tions. Subsequently, it would sim- ply be necessary to demonstrate that the engine had been retained in its original condition and within the allowable range of adjustable settings -the Engine Parameter

Check method. However, the NOx

Technical Code was written to include the option of NOx

April, 1998

Monitoring as an alternative means of onboard verification -with the decision as to which method to be used resting solely with the ship's owner.

Although the broad outline of the

NOx Monitoring option is given within the NOx Technical Code, considerable work is yet to be required to produce a fully work- able set of guidelines. While it would be expected that these guidelines would take the equip- ment as a starting point, specifica- tions and test procedures are given within the NOx Technical Code for the test bed testing of engines -they will also need to allow for

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