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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 Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. Quite often, the flue gas refers to the combustion exhaust gas produced a ...
for
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
,
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
and
carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
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. Combusti ...
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 over ...
s such as the
United States Environmental Protection Agency The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it be ...
'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 implem ...
, 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 Filter, filtering or filters may refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Filter (software), a computer program to process a data stream * Filter (video), a software component tha ...
, 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 know ...
gas system, and a series of gas
analyzer An analyser or analyzer 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 relationships. An analyser can be a piece of hardware or software. Autoan ...
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 toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activ ...
,
nitrogen oxides Nitrogen oxide may refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds: Charge-neutral *Nitric oxide (NO), nitrogen(II) oxide, or nitrogen monoxide *Nitrogen dioxide (), nitrogen(IV) oxide *Nitrogen trioxide (), or ni ...
,
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
,
carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
,
hydrogen chloride The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chloride ga ...
,
airborne particulate matter Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM) or suspended particulate matter (SPM) – are microscopic particles of solid or liquid matter suspended in the air. The ter ...
,
mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...
,
volatile organic compounds Volatile organic compounds (VOCs) are organic compounds that have a high vapour pressure at room temperature Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a ...
, and
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
. 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. Small amounts of water may be found, for example, in the air (humidity), in foods, and in some commercial products. Moisture also refers to the amount of water vapo ...
. 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 Measures of pollutant concentration are used to determine Risk assessment#Risk assessment in public health, risk assessment in public health. Secondary sector of the economy, Industry is continually Chemical synthesis, synthesizing new chemicals, t ...
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/ft3, kg/kWh or kg/m3). 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 slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the method they u ...
, into the CEM system via a sample probe. Facilities that
combust Combust can refer to: * Combust (astrological aspect), the obscuring of the unassisted viewing of a planet by the Sun's light * Combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the r ...
fossil fuels A fossil fuel is a hydrocarbon-containing material formed naturally in the Earth's crust from the remains of dead plants and animals that is extracted and burned as a fuel. The main fossil fuels are coal, oil, and natural gas. Fossil fuels ...
often use a
dilution Dilution may refer to: * Reducing the concentration of a chemical * Serial dilution, a common way of going about this reduction of concentration * Homeopathic dilution * Dilution (equation), an equation to calculate the rate a gas dilutes *Tradema ...
-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 those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
and
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
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 f ...
,
chemiluminescence Chemiluminescence (also chemoluminescence) is the emission of light (luminescence) as the result of a chemical reaction. There may also be limited emission of heat. Given reactants A and B, with an excited intermediate â—Š, : + -> lozenge -> ...
,
fluorescence Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...
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 during the process of beta decay. There are two forms of beta decay, β∠...
absorption Absorption may refer to: Chemistry and biology * Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which ...
. 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. Small amounts of water may be found, for example, in the air (humidity), in foods, and in some commercial products. Moisture also refers to the amount of water vapo ...
. 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. There may also be limited emission of heat. Given reactants A and B, with an excited intermediate â—Š, : + -> lozenge -> ...
. 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 the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
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 ensure 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 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 attestation or confirmation of certain characteristics of a ...
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'', an ...
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'', while ''precision'' is how close the measurements are to each ot ...
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