Page 32 - Impiantistica Italiana Maggio Giugno 2014

aggressive agents). Additionally calibration and
maintenance operations could be very challenging
(both in terms of safety and in terms of costs)
because of the harsh or diffcult-to-be-reached feld
locations.
The above considerations suggest that extractive
systems are more robust and should be
preferred when the analysis has to be performed
on demanding industries like oil refneries or
petrochemical plants.
Finally parameter-based methods are possible
alternatives (or supplement) to the installation
of traditional CEM systems. In this context a
parameter is defned as a property whose value
• in-situ instrumental methods; can characterize or determine the performance of
• parameter-based methods. process or control equipment directly correlated
The extractive method involves the physical with emission levels. They are further classifed as
extraction of the sample from the stack. Sample surrogate or predictive.
gas is then fed to the analyzer through a heated Surrogates are process parameters which may
sample line. Based on the used sample-processing be used for determining directly the compliance
methodology, extractive methods can be further of a source with emission standards. In this case
broken into two techniques, direct source level the process owner must establish and justify the
and dilution. As far as the direct source concerns, parameter values that assure the compliance with
sample analysis can be carried out both on dry basis the actual regulations. This usually requires an
(condensate) and wet basis (non-condensate). extensive testing and validation procedure which is
Dilution systems achieve the same goal by diluting highly application dependent.
stack gas samples with clean dry air without any The Predictive class applies where the relationships
heating. between process conditions and emission levels
In-situ systems are basically automated are not so straightforward to be fully described
instrumented techniques employing various by a single parameter and involves the concept of
detection principles for continuous or periodic modeling. It will be more extensively described in
emission measurements. This involves making the next section.
direct measurements of pollutant concentrations
with instruments able to provide immediate and
continuous readings. Instruments CEMS applications
Based on the used which are permanently installed According to Jahnke [2] “Continuous emission
sample-processing at the plant to provide continuous monitoring involves the sum of activities associated
methodology, emission monitoring systems can be with determining and reporting pollutant emissions
extractive methods either point or cross-stack (path). from stationary sources”, this meaning that
Point in-situ systems measure the
providing a CEM goes much further than just
can be further concentration at a specifc point or providing an analyzer. A qualifed supplier must
broken into two over a relatively short path length be able to provide the whole CEMS/AMS which
techniques, direct through the stack gas. Cross-stack includes at least the following six sub-systems/
source level and systems project a light beam across services:
dilution the stack gas stream and obtain • consultancy on system design and related
certifcations (hazardous area installations,
the emission data analyzing various
spectral phenomena. They may be either single- environmental conformity certifcations /
pass or double pass, depending on whether the approvals, electrical safety and environmental
light source and the detector are on the same or management);
the opposite side. The main advantages of this • sample extraction, transport and conditioning;
approach are that it gives information with a high this subsystem has to be carefully designed
time-resolution and virtually no time delay. and engineered according to the components
Moreover many of the sampling problems to be monitored, their chemical and physical
(condensation, adsorption, occurrence of chemical properties, and the operating context (process
reactions) associated with extractive systems are constraints and peculiarities);
eliminated. However in-situ systems are affected by • sample gas analyzers;
disadvantages which could become very relevant • data acquisition and reporting;
in several industries. First of all being placed on • system integration and turnkey installation;
process conduits or equipment, they are affected • professional services including maintenance
by physical disturbances (e.g. vibration, high contracts, customized staff training and
temperatures) and/or chemical ones (corrosion, system modifcations and evolution.

28 Impiantistica Italiana - Maggio-Giugno 2014