Emission Monitoring
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.
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. 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
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. Sample integrity is a must for accurate reporting.
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 accurate 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 total hydrocarbon, oxides of nitrogen,
oxygen and moisture.
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 separated 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.