Continuous
Emission Monitoring Systems (CEMS)
generally refers to a packaged system of gas analyzers, gas sampling
system, temperature, flow and opacity monitors that are integrated
with a data aquisition system to demonstrate environmental regulatory
compliance of various industrial sources of air pollutants. Technical
requirements and approved analytical techniques for continuous emission
monitoring systems are found in 40 CFR (Protection of the Environment)
part 60 and 40 CFR part 75. State implementation plans are based on
the USEPA's regulatory authority in 40 CFR, though some technical
variation exists from state to state. Links to the U.S. Environmental
Protection Agency, the 10 USEPA Regional Offices and all 50 state
regulatory agencies may be found here.
The most widely used
type of continuous emission monitor is an extractive CEMS in which
a sample of gas is continuously drawn from the process point, filtered,
transported, conditioned and presented to a gas analysis system. Gas
concentrations are measured, recorded and stored as data. The data
is used to generate reports, alarms or control some aspect of the
client's process. Extractive systems offer the advantages of choosing
the most appropriate analysis technique for the desired components
and concentration ranges and a properly designed system provides the
flexibility of easily upgrading, expanding or contracting the scope
of emissions analysis.
CEMS are useful tools
in gathering process emissions data for environmental compliance demonstration
and process control and optimization. Other types of CEMS include
in-situ and electrochemical cell type monitoring and may be appropriate
in particular applications.
A CEMS consists of the
system hardware, data aquisition and system integration.
An Extractive CEMS
hardware generally consists of the following major sub-systems:
- Sample transport
and conditioning
- Sample gas analysis
- Data acquisition,
reporting and system control
Sample Transport
and Conditioning
To accurately monitor
a source, a gas sample must be presented to the gas analyzers in a
timely fashion. The gas analyzers however, must be presented with
clean gas in order to function reliably. In addition, gas analyzers
are intolerant of contamination by condensed liquids in the source
gas. Condensable water vapor and particulate can plug passages and
flow components, obscure optical sensors and lenses and cause failure
of pumps, valves and flow-meters.
Prior to presentation
the gas analyzers, the sample gas is often filtered and conditioned
to remove particulate matter and moisture. This process however, cannot
alter the composition of the gas species of interest in the conditioned
gas sample.
Various techniques may
be employed to filter and condition sample gas depending on the levels
of particulate and/or moisture present, required system response time,
and solubility of the gas sample component species of interest.
Primary filtration and
dilution using a dilution probe with clean, dry air may be appropriate
when it is required to measure on a wet-basis and when the use of
unheated or ambient-level gas analyzers is desirable. If properly
designed and maintained, such systems are sufficiently accurate for
measuring high levels of pollutants and may allow the use of un-heated
sample transport tubing. Such systems are commonly used in situations
with long sample lines and soluble constituents as in fossil fuel-powered
utilities.
A heated sample transport
system (heated filtered probe, heated
sample lines, heated pumping and
distribution, as appropriate) and the use of heated
gas analyzers allow wet-basis gas analysis without dilution
and is the most accurate means of analyzing a wet-basis gas sample.
Such systems can be used to measure very wide variences in concentrations
total hydrocarbon, oxides of nitrogen, oxygen and moisture. For the
measurement of very low levels of nitrogen oxides (without loss of
NO2, this is the only appropriate technique currently available.
Dry-basis systems are
appropriate for the measurement of a wide variety of constituents
and the use of un-heated gas analyzers. This type of system allows
the greatest flexibility in analytical technology but requires the
most thoughtful design. Most CEMS incorporate elements of dry-extractive
systems in their design.
Dry-basis CEMS generally
consist of primary filtration and a heated sample system from the
sampling point down to the gas conditioner. The heated sample system
minimizes the exposure of soluble sample constituents to liquid condensate
and their subsequent potential removal from the sample gas. In the
sample conditioner, condensable vapor is quickly removed and seperated
from the gas sample.
To remove the water content
from the gas sample, the gas may be cooled by compressor or Peltier
(thermo-electric) refrigeration. Alternatively, water vapor may be
removed using a nafion dryer system where appropriate.
Gas Sample Analysis
To measure the concentration
of sample gas constituents, gas analyzers measure some physical characteristic
of the species of interest. Ideally, the characteristic is unique
to the species. Such techniques commonly include
infrared and ultraviolet adsorption, photo or flame
ionization, catalytic or chemiluminscence
photon emission from chemical reaction or excitation.
In cases where gas sample
constituents create cross-interference, techniques are used to reduce
or measure and cancel these interferences. Selection of appropriate
technology for gas conditioning and analysis is important in the production
of reliable and accurate emissions data.
Data Acquisition,
Reporting and Control
The data acquisition
system acts as a system controller as well as a means to collect and
record data. It should be capable of initiating daily calibrations,
monitor important system parameters, generate alarms and reports and
communicate over the client’s network. A means of accounting for maintenance,
validating data and future system upgrade should be included in the
package.
Often a DAS includes
some level of redundancy. All or part of the DAS may be redundant,
depending on the application requirements. An uninterruptable power
supply or battery-powered data logger is almost a requirement. Often
local storage is supplemented with a multi-pen recorder or regular
back up over the client’s network.
System Integration:
Putting it Together
Integration of a CEMS
is the packaging, arrangement and connection of all of the various
components so that they operate as one coherent and highly reliable
system. The selected components and technologies should compliment
each other and be appropriate for the application.
Successful integration
of a CEMS starts with good basic design practice. The designer(s)
must seriously consider not only source characteristics and how the
equipment will be used, but also the way the client (including outside
testing firms and regulatory officials) will be working with it. Reliable
long-term function and complete data capture are the result of careful
planning, taking into account source constituents, materials compatibility,
system siting, minimal maintenance, redundancy and ease of use.
In particular, source
characteristics and constituents in the sample gas play a very important
role in determining the best analytical technologies as well as selection
of sample transport and conditioning components. A "pre-engineered
CEMS" is more a product of a vendor’s marketing priorities than
consideration of the client’s monitoring requirements. While allowing
a vendor to streamline inventory, documentation and the production
of sales literature, the resulting monitoring system inevitably incorporates
less than optimal design features. A surprising number of large, well-known
companies that manufacture analytical products, fail to provide well-designed
custom CEMS. Mistakes with CEM system design or vendor selection are
often costly and contentious to resolve.
Often overlooked by CEMS
vendors, is the importance of periodic RATA testing. While the selection
of a reputable testing firm is most important, the CEMS must use appropriate
gas sampling rates and analytical technique(s) to produce source data
that tracks well with a reference CEMS. In addition, the data acquisition
system must be flexible and accessible to the testing firm; capable
of archiving and exporting appropriate data sets for periodic testing.
A DAS that produces graphical and scalable trending data can assist
tremendously in the set-up and monitoring of periodic testing progress.
Often, problems that would have resulted in re-testing (at considerable
expense) have been avoided by a cursory visual comparison of the two
systems’ data trends.
System redundancy and
maintainability are very important in any CEMS. They are absolutely
essential during periodic testing. The basic CEMS design must be as
simple and robust as possible. Sampling system parameters that are
important to measure performance and condition of probes, filters,
sample lines and pumps should be accessible, but not overwhelm the
operator with irrelevant data.
Corrective action for
probe-blinding or component failure should take minutes, at the maximum,
to correct. Probes should be self-cleaning and sub-assemblies within
the equipment enclosure should be either redundant or easily removed
and replaced.
Finally, when selecting
a vendor to supply a CEMS, get references and take a look at the work
they have done previously.Be sure that the displays and controls are
placed optimally for operator convenience. Check that the equipment,
wiring, tubing, valves and services within the enclosures are intelligently
and professionally arranged. If you are favorably impressed, check
to make sure that the designer whose work you admired is still on
staff. Ask questions about maintenance, technical support, consumables
usage and anything you can think of relating to how the equipment
will be used in your application. Keep in mind that this equipment
will be used for at least several years hence so operational quirks
should be excluded from its design.
If you would like to
see some recent examples of our work in progress, click on the CEMS
link or the second page of CEMs
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