CEM - Abstract

Abstract Information :

In Europe there is a tendency to set emission limits at the lower BAT-Associated Emission Levels within the EU the BAT-reference documents. Subsequently the technical specifications on continuous emission measurement systems are becoming more difficult to achieve. Within our presentation we will elaborate on how to prevent overtight criteria on a CEM system, for which we use mercury measurement as an example. A daily emission limit of 1 μg/Nm3 is in the Netherlands already permitted for a new to build waste incinerator.

1) Whether a parameter requires a 40% Hg or 20% measurement uncertainty makes a large difference in the established performance of a CEM system. The type testing criteria of EN 15267 are fixed and set to values as to attain a 20% measurement uncertainty, for the CEM system. Consequently, the uncertainty in the type testing certificate easily fulfils the limit, but at a higher than necessary concentration. By recalculation of the uncertainty this issue can be overcome.

2) A BAT-AEL yearly average concentration cannot be one on one imposed as yearly emission limit value. Neither can a yearly emission limit be one on one imposed as daily emission limit value. The BREF-Large combustion plants gives a set of equations for the relation between these. At the relatively low emission concentrations of mercury the annual and daily emission limit can differ by a factor two to three.

3) At a certain low concentration, the reference method will not meet the required measurement uncertainty to calibrate a CEM system. Through optimisation of the EN 13211 reference method a 40% measurement uncertainty can be attained below 0.5 μg/Nm3.

4) At a certain low concentration of the emission limit the required uncertainty can no longer be met. This makes it problematic to fulfil the quality control criteria of EN 14181 “Quality assurance of AMS” nor the stricter EN 14884 “mercury AMS”. The implications will be shown:
a. Meeting the required uncertainty at 1 μg/Nm3 is not yet possible for any CEMS. The Netherlands uses a legal fixed uncertainty requirement below 10 μg/Nm3.
b. The QAL2 criterium for variability in EN 14884 is difficult to attain.
c. The AST criterium for drift in EN 14884 can be difficult to attain.
d. The QAL3 control limits need to be set extremely tight.

If the emissions are so much below the daily emission limit value and stable, the outcome could be that periodic measurements would be adequate also. Which is in general the case for a stable emission concentration below 50% of the annual emission limit value.