Continuous emission monitoring systems (CEMS) are used as a tool to monitor the effluent gas streams resulting from combustion in industrial processes. CEMS can measure
flue gas for
oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
,
carbon monoxide
Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
and
carbon dioxide
Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
to provide information for
combustion
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...
control in industrial settings. They are also used as a means to comply with air
emission standard
Emission standards are the legal requirements governing air pollutants released into the atmosphere. Emission standards set quantitative limits on the permissible amount of specific air pollutants that may be released from specific sources ov ...
s such as the
United States Environmental Protection Agency
The Environmental Protection Agency (EPA) is an independent agency of the United States government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it began operation on De ...
's (EPA)
Acid Rain Program
The Acid Rain Program is a market-based initiative taken by the United States Environmental Protection Agency in an effort to reduce overall atmospheric levels of sulfur dioxide and nitrogen oxides, which cause acid rain. The program is an impleme ...
, other US federal emission programs, or state permitted emission standards.
CEMS typically consist of analyzers to measure gas concentrations within the stream, equipment to direct a sample of that gas stream to the analyzers if they are remote, equipment to condition the sample gas by removing water and other components that could interfere with the reading, pneumatic plumbing with valves that can be controlled by a
PLC to route the sample gas to and away from the analyzers, a calibration and maintenance system that allows for the injection of calibration gases into the sample line, and a Data Acquisition and Handling System (DAHS) that collects and stores each data point and can perform necessary calculations required to get total mass emissions. A CEMS operates at all times even if the process it measures is not on. They can continuously collect, record and report emissions data for process monitoring and/or for compliance purposes.
The standard CEM system consists of a sample probe,
filter, sample line (
umbilical), gas conditioning system,
calibration
In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of known ...
gas system, and a series of gas
analyzer
An analyser (British English) or analyzer (American English; see spelling differences) is a tool used to analyze data. For example, a gas analyzer tool is used to analyze gases. It examines the given data and tries to find patterns and relationsh ...
s which reflect the parameters being monitored. Typical monitored emissions include:
sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...
,
nitrogen oxides
In atmospheric chemistry, is shorthand for nitric oxide () and nitrogen dioxide (), the nitrogen oxides that are most relevant for air pollution.
These gases contribute to the formation of smog and acid rain, as well as affecting tr ...
,
carbon monoxide
Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
,
carbon dioxide
Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
,
hydrogen chloride
The Chemical compound, compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hyd ...
,
airborne particulate matter,
mercury,
volatile organic compounds
Volatile organic compounds (VOCs) are organic compounds that have a high vapor pressure at room temperature. They are common and exist in a variety of settings and products, not limited to house mold, upholstered furniture, arts and crafts sup ...
, and
oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
. CEM systems can also measure air flow, flue gas
opacity and
moisture
Moisture is the presence of a liquid, especially water, often in trace amounts. Moisture is defined as water in the adsorbed or absorbed phase. Small amounts of water may be found, for example, in the air (humidity), in foods, and in some comme ...
. A monitoring system that measures particulate matter is referred to as a PEMS.
In the U.S., the EPA requires a data acquisition and handling system to collect and report the data. Measurements of concentration can be converted to mass/hour by including flow rate measurements. The types of gases being measured and the calculations required are dependent upon the source type and each source type has its own subpart under 40 CFR part 60 and part 75.
emissions are measured in pounds per hour using both an
pollutant concentration monitor and a volumetric flow monitor. For
NO''x'', both a NO
''x'' pollutant concentration monitor and a diluent gas monitor are used to determine the emissions rate in weight per volume or weight per heat value (for example lbs/million Btu, lbs/ft
3, kg/kWh or kg/m
3). Opacity measurements are sometimes required, depending on the source type. measuring is sometimes a requirement, however if monitored, a or oxygen monitor plus a flow monitor should be used. The DAHS must be able to collect, record and store data, usually at 1-minute intervals. For compliance purposes, a DAHS must be in continuous operation 24/7/365 even when no process is on. For a valid measurement, the DAHS must record at least one reading every 15 minutes for 3 out of 4 quarters. The readings are then averaged hourly.
Operation
A small sample of flue gas is extracted, by means of a
pump
A pump is a device that moves fluids (liquids or gases), or sometimes Slurry, slurries, by mechanical action, typically converted from electrical energy into hydraulic or pneumatic energy.
Mechanical pumps serve in a wide range of application ...
, into the CEM system via a sample probe. Facilities that
combust fossil fuels
A fossil fuel is a flammable carbon compound- or hydrocarbon-containing material formed naturally in the Earth's crust from the buried remains of prehistoric organisms (animals, plants or microplanktons), a process that occurs within geologica ...
often use a
dilution-extractive probe to dilute the sample with clean, dry air to a
ratio
In mathematics, a ratio () shows how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8:6, which is equivalent to the ...
typically between 50:1 to 200:1, but usually 100:1. Dilution is used because pure flue gas can be hot, wet and, with some pollutants, sticky. Once diluted to the appropriate ratio, the sample is transported through a sample line (typically referred to as an umbilical) to a
manifold
In mathematics, a manifold is a topological space that locally resembles Euclidean space near each point. More precisely, an n-dimensional manifold, or ''n-manifold'' for short, is a topological space with the property that each point has a N ...
from which individual analyzers may extract a sample. Gas analyzers employ various techniques to accurately measure concentrations. Some commonly used techniques include:
infrared
Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...
and
ultraviolet
Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
adsorption
Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...
,
chemiluminescence
Chemiluminescence (also chemoluminescence) is the emission of light (luminescence) as the result of a chemical reaction, i.e. a chemical reaction results in a flash or glow of light. A standard example of chemiluminescence in the laboratory se ...
,
fluorescence
Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with colore ...
and
beta ray
A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus, known as beta decay. There are two forms of beta decay, β− decay and � ...
absorption. After
analysis
Analysis (: analyses) is the process of breaking a complex topic or substance into smaller parts in order to gain a better understanding of it. The technique has been applied in the study of mathematics and logic since before Aristotle (38 ...
, the gas exits the analyzer to a common manifold to all analyzers where it is vented out of doors. A Data Acquisition and Handling System (DAHS) receives the signal output from each analyzer in order to
collect and record emissions data.
Another sample extraction method used in industrial sources and utility sources with low emission rates, is commonly referred to as a "dry extractive", "hot dry" extractive, or "direct" CEMS. The sample is not diluted, but is carried along a heated sample line at high temperature into a sample conditioning unit. The sample is filtered to remove particulate matter and dried, usually with a
chiller
A chiller is a machine that removes heat from a liquid coolant via a vapor-compression refrigeration, vapor-compression, adsorption refrigeration, or absorption refrigerator, absorption refrigeration cycles. This liquid can then be circulated th ...
, to remove
moisture
Moisture is the presence of a liquid, especially water, often in trace amounts. Moisture is defined as water in the adsorbed or absorbed phase. Small amounts of water may be found, for example, in the air (humidity), in foods, and in some comme ...
. Once conditioned, the sample enters a sampling manifold and is measured by various gas analyzers, typically and (and sometimes CO) for combustion turbines and engines running natural gas or diesel. analyzers typically work using
chemiluminescence
Chemiluminescence (also chemoluminescence) is the emission of light (luminescence) as the result of a chemical reaction, i.e. a chemical reaction results in a flash or glow of light. A standard example of chemiluminescence in the laboratory se ...
. analyzers a magnetic field which attracts to measure the concentration. The causes movement of a suspended mirror within the analyzer which then changes the amount of light being reflected by that mirror onto a photocell. The amount of current required to move the mirror back to center is proportional to the concentration. The ability to measure %
oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
in the sample is required to perform the required calculations.
Quality assurance
Accuracy of the system is demonstrated in several ways. An internal
quality assurance
Quality assurance (QA) is the term used in both manufacturing and service industries to describe the systematic efforts taken to assure that the product(s) delivered to customer(s) meet with the contractual and other agreed upon performance, design ...
check is achieved by daily introduction of a
certified
Certification is part of testing, inspection and certification and the provision by an independent body of written assurance (a certificate) that the product, service or system in question meets specific requirements. It is the formal attestatio ...
concentration
In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', '' number concentration'', ...
of gas to the sample probe. The CEMS measurement is then compared against the known concentration to arrive at a Calibration Error percentage. A zero gas reading is also taken and compared. If the calibration error % exceeds 2x the performance specification for 5 consecutive days or 4x the performance specification in 24 hours, the CEMS is considered out of control meaning the data can not be relied upon as accurate until it is brought back into control. Data substitution will be used for out of control periods. The data substitution method is generally not advantageous so it is critical to get the CEMS back into control as soon as possible.
The EPA also allows for the use of Continuous Emissions Monitoring Calibration Systems which dilute gases to generate calibration standards. The analyzer reading must be
accurate
Accuracy and precision are two measures of ''observational error''.
''Accuracy'' is how close a given set of measurements (observations or readings) are to their ''true value''.
''Precision'' is how close the measurements are to each other.
The ...
to a certain percentage. The percent accuracy can vary, but most fall between 2.5% and 5%. In power stations affected by the Acid Rain Program, annual (or bi-annual) certification of the system must be performed by an independent firm. The firm will have an independent CEM system temporarily in place to collect emissions data in parallel with the plant CEMS. This testing is referred to as a Relative Accuracy Test Audit (RATA).
In the U.S., periodic evaluations of the equipment must be reported and recorded.
[ This includes daily calibration error tests, daily interference tests for flow monitors, and semi-annual (or annual) RATA and bias tests. CEMS equipment is expensive and not always affordable for a facility. In such cases, a facility will install non-EPA compliant analysis equipment at the emissions point. Once yearly, for the equipment evaluation, a mobile CEMS company measures emissions with compliant equipment. The results are then compared to the non-compliant analyzer system.][{{Cite web, title = Flue Gas & Emissions Analyzers {{! Nova Gas, url = http://www.nova-gas.com/analyzers/flue-emissions, website = Nova Gas, access-date = 2016-02-23, language = en-US]
References
Industrial engineering