What Do We Mean?
Explanation of common terms used in emissions monitoring
1. The difference between
measurement and monitoring
These terms are often used interchangeably, but they have
specific and different meanings:
·
Measurement of an environmental
parameter involves assigning a numerical quantity its magnitude.
·
Monitoring involves interpretation of a
number of measurements carried out over a period of time.
It usually involves an element of comparison, either with itself or
with some external benchmark, e.g. an environmental quality standard or a
guideline value.
Some other terms and concepts requiring clarification
include:
·
Monitoring technique
– the scientific principle behind the sampling and analysis.
·
Monitoring method
– the documented procedure describing in detail how the monitoring technique
should be used for a particular application.
Official standard methods are published by the European Committee for
Standardisation (CEN), the International Standards Organisation (ISO), the
British Standards (BS) Institute, and others.
·
Monitoring equipment
– some standard monitoring methods specify in detail the exact instruments
and apparatus that must be used, but many do not.
·
Quantitative Monitoring
– measures how much of it is
present.
·
Semi-quantitative Monitoring
– whether it is present in trace, minor or major amounts.
·
Qualitative Monitoring
– measures what it is composed of.
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2. Sampling and analysis
An environmental measurement includes both a sampling
stage and an analysis stage (though it can sometimes be difficult to say
where one ends and the other begins). Sampling
must be representative both spatially and temporally.
Sampling is itself a very specialised subject.
The fundamental principle of
sampling is that a small amount of collected material should be a reasonable
estimate of the overall character of the material. For this to apply, the sample must be a representative sample.
The number and locations of samples needed depends on how homogeneous
the bulk material is, in both spatial and temporal terms.
For a very homogeneous material, only a few samples may be required; if
it is non-homogeneous, many more samples will be required.
Some other terms and concepts requiring clarification include:
·
Analyte –
the chemical species being
investigated (qualitatively or quantitatively).
·
Measurand
– often, the analyte is not measured directly, but by
measuring another parameter that is proportional to it.
In such cases the term measurand (or sometimes detrerminand) is used. For
example, in quantifying the temperature (the measurand) of gases emitted by a
chimney stack using a thermocouple, the actual quantity measured is millivolts
generated at the thermocouple junction.
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3. Different types of
emissions monitoring
There are many ways of characterising different types of
monitoring. A convenient
classification is:
·
Periodic monitoring
- measurements carried out at periodic intervals, e.g. once every three months
throughout the year.
·
Continuous monitoring
- measurements carried out continuously, with few if
any gaps in the data produced. Equipment
used for this purpose are referred to as continuous emissions monitors (CEMs)
or automatic monitoring systems (AMS).
Periodic monitoring can be carried out automatically or manually.
Automatic monitoring uses an instrument that (usually) gives a direct
readout of pollution levels in real time.
Alternatively, a manual technique may be used, where the pollutant is
sampled on site but is usually analysed later in the laboratory.
Samples may be obtained over fairly lengthy periods of several hours,
or may be so-called spot samples or grab samples collected over a period of
seconds to a few minutes.
CEMs, on the other hand, nearly always use
automatic techniques. Some
techniques measure the pollutant concentration in situ. Examples are opacity
meters where the reduction in intensity of an optical beam is measured after
it has travelled across the duct. Other
systems are extractive, where a sample of the exhaust gas is drawn
continuously from the stack to a nearby instrument. An example is a
chemiluminescence analyser for measuring NOx.
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4. Different
categories of CEMs
CEMs - especially those for
monitoring
particulate emissions - fall into three categories. It is important to distinguish between the types:
i)
Quantitative CEMs - fully quantitative instruments where a valid relationship exists
between the measured parameter (e.g. opacity) and the particulate
concentration, a calibration can be made, and the concentration is given in
units of mg/m3.
ii)
Indicative CEMs – these are semi-quantitative. The
readout/ output of the instrument increases as the particulate concentration
increases, but the relationship will not necessarily be linear and the readout
will not necessarily be in mg/m3 (it could be, for example,
percentage obscuration). These CEMs are indicative of the arrestment plant
performance and can show trends in levels of particulate emissions, etc.
iii)
Arrestment failure CEMs – these are simple go/no-go indicators, used to show arrestment plant
failure, e.g. bag-filter bursting. Such
a significant increase in particulate levels produce a jump in the output of
the instrument which then triggers an alarm.
Although this type of CEMs is itself sometimes referred to as an “alarm”,
it should be pointed out that audible and visual alarms can be linked to all
three types of CEM.
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There are two main categories of industrial emissions to
atmosphere:
§
Controlled releases
– the emissions are managed in some way, either as part of a process or as
part of a control/ abatement mechanism and the emissions are therefore
quantifiable.
§
Fugitive releases
– these are, literally, releases that cannot be captured. They are uncontrolled and often dependent on external
conditions (e.g. wind) which makes them difficult to quantify with any
reasonable degree of certainty. Another
definition of a fugitive emission is a release that is unintentional. An oil refinery may have a quarter of a million pumps, valves
and flanges that can potentially leak, making it impractical to measure the
emissions from every source.
Emissions can also be described in terms of their spatial characteristics,
usually as a point source, line source or area source.
It is important to recognise that that each of these can be either
controlled releases or fugitive releases, as shown in the table below.
It is controlled point sources (e.g. chimney stacks and vents) that are
most commonly monitored.
Table
1. Categories of emissions to
atmosphere
|
|
Point source
|
Line source
|
Area source
|
|
Controlled
release
|
Emissions from
fixed-location plant and often (but not always) released to atmosphere
via a vent, duct or chimney stack
|
Tail-pipe emissions
from vehicles driving along a road
|
Open process tanks,
storage tanks with floating roofs
|
|
Fugitive
release
|
Intermittently
leaking valve
|
Dust
re-suspended in a vehicle’s wake; wind whipping of dusty material on
an open conveyor belt.
|
Wind-whipping of a
stockpile of dusty material
|
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