48 AIR POLLUTION INSTRUMENTATION
for the above-described purpose, are not considered to be
enforcement tools. This requirement is fulfilled by the EPA
certified Visual Emissions Observer as is specified in EPA
Reference Method 9.
There are two basic types of transmissometers, single-
pass and double-pass systems. The single-pass system
incorporates a light source on one side of the stack and a
detector on the opposite side. Although this is the more
economical of the two systems, it does not meet the EPA
requirements for system zero and calibration checks with-
out complete process shutdown every 24-hours. It is better
applied in a situation where direct compliance with the EPA
criteria is not a factor, such as process control or baghouse
filter bag breakage detection.
The double-pass system houses both the light source
and detector with attendant calibration and zero-check
instrumentation on the same side of the stack with only a
reflecting mirror on the opposite side. Therefore, most of
the double-pass systems satisfy the EPA design criteria.
Refer to Table 1 for a list of vendors of either single-pass or
double-pass transmissometers. The fraction of light lost in
crossing the stack is used to calculate opacity and its value
is related to the amount of dust or smoke passing through
the light path. The cost per unit including control options
is about $20,000–40,000 (1996$). The lower figure is for
a quantity of more than 30 units; the higher figure is for a
single installation.
An acid dew point meter is a related instrument pro-
duced by Land Combustion (see address above). It is useful
in estimating SO 3 /H 2 SO 4 concentration.Gaseous Emissions MonitoringStationary sources that are required by the EPA to install
a continuous gaseous emissions monitor must match their
specific process, and source emissions to the capabilities
of the continuous monitor types available. Most instrumen-
tation will fall into two categories, extractive systems and
in-situ systems. A third category, remote monitors, utilizes
concepts such as lasers and advanced spectroscopic methods
to monitor gaseous emissions at distances from 500 to 100
meters away from the source.EXTRACTIVE MONITORSThe basic principle behind an extractive monitor is the with-
drawal of a gas sample from the main exhaust stream into
the analyzer. This withdrawal must be conducted such that a
representative sample is selected, and then appropriate inter-
ferents (particulates, water vapor, etc.) must be removed
dependent upon analytical methodology. Extractive moni-
tor types can be subdivided into three general categories:
absorption spectrometers, luminescence analyzers, and elec-
troanalytical monitors. Specialized extractive methods that
do not fit into these three categories include paramagnetism
and thermal conductivity.Absorption SpectrometersSpectroscopic analyzers utilized as continuous emissions
monitors include two basic types: non-dispersive infrared
analyzers (NDIR), and non-dispersive ultraviolet analyzers
(NDUV).
NDIR detectors can monitor SO 2 , NO x , CO, CO 2 and
hydrocarbons. As the gas travels through the instrument and
is exposed to the infrared light source, light energy absorp-
tion occurs which is subsequently detected in comparison
with a reference gas. Different gases are characterized by
differing absorption characteristics, and are thereby identi-
fied and quantified.
NDUV detectors are used primarily to monitor SO 2 and
NO 2. These instruments use light in the ultraviolet and vis-
ible portions of the spectrum. They are similar to NDIR
monitors except that they do not use a reference gas for com-
parison. Instead, they use a reference wavelength with mini-
mal absorption capabilities. NDUV analysis, also known as
differential absorption, is also utilized in in-situ and remote
sensing systems.Luminescence AnalyzersLuminescence analyzers measure the emission of light
from an excited molecule. Dependent on the mode of mol-
ecule excitement, molecules can exhibit photoluminescence
(fluorescence), chemiluminescence or flame luminescence.
Fluorescence occurs when a molecule is excited by light
energy of a given wavelength, and light energy of a second
wavelength is emitted. Fluorescence analyzers are utilized
for SO 2 analysis.
Chemiluminescence analyzers are used for NO x and NO 2
determinations, and operate on the principle of the emission
of light energy resulting from a chemical reaction. In the
case of chemiluminescence analyzers, the reaction involves
ozone (O 3 ) and nitric oxide (NO).
Flame photometric analyzers use the principle of lumi-
nescence through molecule/flame interaction. These analyzers
detect sulfur compounds, and are specific to sulfur alone.Electroanalytical MonitorsFour distinct types of electroanalytical monitors are used in
continuous source monitoring. These instruments rely on
the methods of polarography, electrocatalysis, amperometric
analysis, and conductivity.
Polarographic analyzers, also known as voltametric ana-
lyzers or electrochemical transducers, are capable of detect-
ing SO 2 , NO 2 , CO, O 2 , H 2 S and other gases dependent on
instrument setup. The analytical basis is a self-contained
electrochemical cell in which a chemical reaction takes place
involving the pollutant molecule. As a result of the chemical
reaction, a current change through a specific electrode indi-
cates pollutant concentration.
Electrocatalytic analyzers are utilized for O 2 determina-
tions. These analyzers use a solid catalytic electrolyte and
are available in both extractive and in-situ models.C001_004_r03.indd 48C001_004_r03.indd 48 11/18/2005 10:12:46 AM11/18/2005 10:12:46 AM