6 Flue Gas Analyzer Principle for Monitoring COX, NOX, SOX, hydrocarbons

Dr. Geeta

    Introduction

Atmospheric composition has been changing day by day with the addition of air pollutants which affects the biotic environment. The amount of air pollutant depends upon the source of air pollution and the ability of the atmosphere to absorb or disperses this emission. The concentration of air pollution varies spatially and temporarily, which causing the air pollution pattern to change with time and location. Presence of high amount of air pollutants adversely affects the health and prosperity of population. The major source of air pollutants include natural source and various industries such as power plants, incinerators and cement plants, domestic sources and vehicular pollution. To maintain the quality of air, policy maker at national and international level introduce certain rule and regulation. Government of India has enacted Air Act (prevention and control of pollution) in 1981 and Environment protection act, 1986.

16.1 Monitoring play an important part in controlling air pollution. It also provides information of various types of pollutant and particulate matter released out in the atmosphere. Monitoring therefore helps in the assessment, health and safety issue in the industrial plants. A good monitoring system requires equipments for sample collection, instrument calibration, data collection and their processing. There are various types of monitor are available which provide reliable and reproducible data. Monitor is a device which measures or senses the physical or chemical properties of a substance. It generates an electrical signal and data acquisition systems (DAS) records these signal and makes a correlation to the concentration of pollutants.

Monitoring can be performed by two methods. One is stack and continuous emission monitoring system (CEMS). In case of CEMS, it is apply on any instrument for measuring the constituent of exhaust gas.CEMS equipment are quite expensive. Another method is parametric monitoring in which indirect measurement of the emission. In this method process or control parameter are correlate to check the levels of pollutant emission. It is commonly used methods for small emission source. It provides more flexible and less expensive means of monitoring.

The selection of type of monitoring is depends upon various consideration like calibration or accuracy, maintenance requirements, chemical and physical properties of gas stream and pollutants, location, methods use in collection, processing and disposing of the sample, requirement based on quality control and quality assurance, safety management etc.

16.1.1 SOx: Sulphur is an abundant element in the earth crust and found in the form of gypsum. The fuel like coal, oil, wood also contains small quantity of sulphur. SO2 gas is produced by burning the material containing sulphur. First SO2 is formed then it slowly oxidized to SO3. Both SO2 and SO3 can form acid when hydrolyzed with water. There are two ways by which release of SO2 is controlled i.e. dry and wet process. During operation stage we can control the production of SO2 by using devices such as filters, wet scrubbers, condenser and by cyclone separators. Wet scrubbers are most commonly used method in controlling SOx. In this a pressure drop is measured by manometer which indirectly measure SO2. It is a less expensive method for monitoring of SO2.The molecular weight of SO2 is doubles that of elemental sulfur. So by monitoring the rate of fuel used, the emission of SO2 is calculating by assuming complete combustion of fuel containing sulfur. Totalizers are used to monitored gaseous and liquid fuels. Another method of monitoring is by established a relationship between fuel used and steam production for a fuel.

16.1.2 NOx: Oxide of nitrogen is formed by material containing bound nitrogen or during the process of combustion when air combines with oxygen at a high temperature. Oxides of nitrogen are harmful when they are combining with hydrocarbon in the presence of sunlight forming harmful photochemical compound. Optimization of combustion is one of the control methods. Periodic testing of temperature, excess air and load can be monitored and established a correlation between these parameter and NOx emission rate. An algorithm is developed to check the emission by DAS. Portable combustion analyzer is used to monitor NOx and excess air flow and temperature etc. NOx can be measured by discrete analyzer. NOx is also measured by dilution extractive, cold/dry, direct extractive sample. It is measured in ambient air monitoring system.

16.1.3 Hydrocarbon: In the atmospheric air hydrocarbons are complicated mixture of many substances like methane and non methane.

16.2 Flue gas analysis

Combustion flue gas analysis has been used by power plant to optimizing the fuel/air ratio. The amount of excess oxygen and CO in the flue gas and least amount of NOx and other green house gases is measured by analyzer. The stoichiomertic point is where all fuel is reacted with the available oxygen in the combustion air and no fuel or oxygen is left over.

16.2.1 Fuel: Fuel is those which on burning produce heat and power. Fuels used in various industry is available in three different forms i.e. solids (coal, wood, straw), liquid (diesel, crude oil) and gaseous fuels(CO, H2 and CH4).The composition of fuel used is play an important role for optimization of combustion process.

16.2.2 Flue gas: It is also known as stack gas generated through combustion processes. Composition of flue gas is depends on the type of fuel and condition during the combustion process. Flue gases are composed of high concentration of water and carbon dioxide, oxides of sulphur, oxides of nitrogen, fine dust, hydrogen halides, trace element such as mercury, nickel and dioxins etc.

General composition of flue gas is given below:

Nitrogen (77%),

CO2, CO (13%)

Oxide of N2, H2O, S and other (5%)

Oxygen (5%)

All these compounds are unwanted byproduct generated from various industries. Flue gas analysis indicates the air to fuel ratio. These flue gas components are part of air pollutants so these should be minimized or eliminated by cleaning procedure before released out in the atmosphere. These flue gases are vaporized or converted chemically into harmless gases with the help of cleaning procedure.

16.2.3 Analyzer: Analyzer is a species–specific sensor. The function of analyzers is based upon various physical and physio-chemical principles like absorption, ionization, transmission and adsorption etc.

Analyzers are classified into different categories

Portable analyzers: It can use to measured different location in very short period of time.

Fixed analyzers: It is used to measure in a fixed permanent location for long period of time.

In situ analyzers: It is installed and worked in the process unit.

Extractive analyzers: It is installed outside the process stream.

Flue gas analyzer: It is an instrument which is capable of analyzing the gases and measured the quantity present in the mixture or sample. There are various types of gas analyzer depending upon the principle used such as, gas chromatography, electrochemical analyzer, photometry (IR, Visible and UV), flame photometry, ionization, chemiluminescence, conductivity and paramagnetic etc.

16.3 Photometry: It uses the absorption of infrared (1000-10000 nm), visible (400-800 nm) and ultraviolet (200-400 nm) radiation by gases. Photometry based upon principle that gases absorb energy at certain wavelength and loss of radiation intensity is measured (Fig 1). It based upon Lambert-Beer law which states a linear relationship between absorbance and concentration of an absorbing species.

Io-intensity of entering radiation, I-Intensity of emitting radiation, ɛ- extinction coefficient, C-concentration of gas ,l-optical pathlength

Fig1: Basic instrumentation of Spectrophotometer

16.4 Infrared absorption (IR) gas analyzer: Infrared radiation is absorbed by gases such as CO, CO2, SO2 or NO at specific wavelength particular to gas. The IR radiation from lamp is passed through the measuring cell filled with sample gas. Different molecules of gas absorb different frequencies of light. If concentration of gas is increased then IR absorption is increased and there is decrease in radiation intensity. A very low concentration of SO2 and NO2 is difficult to analyze because IR area is cross sensitive to H2O.It is classified into two categories:

a) Dispersive IR: It uses the radiation which is dispersed by prism. Two wavelength are used one is reference and other is that is being absorbs by sample gas. Gas concentration is measure by calculating the ratio of two wavelengths.

Fig 2: A Typical Infrared photometer

  b) Non Dispersive IR: It uses a broad band of radiation from the lamp without dispersion. When components present in sample gas absorb IR, the intensity of radiation is used at specific spectrum when reached to the detectors. Two methods are used to detect the level of absorption. One is a gas detector and other is solid state IR detectors. NDIR instruments is primarily used for measurement of CO, NO, SO2, H2O, CH4, CO2 and many hydrocarbons.

Ultraviolet absorption: Gases like NO and SO2 absorb UV radiation. The major limitation in using UV radiation is its high cost.

16.5 Fourier transform IR spectroscopy: FTIR spectroscopy is used to measured infrared active gases like SO2, CO2, CO, NO, HCl etc. It is based on the Michelson interferometer which has function of monochromator. In this radiation hits a beam splitter which reflects fifty percentage of the radiation and fifty percentages are transmitted. These two beam hits two mirrors which is perpendicular to one another and reflected back to the beam splitter. These reflected beams are recombines by beam spillter. The recombined beams are then passed through cell full of the product to be measured and focused on IR detectors. (Fig 3)

Fig 3: FTIR Spectrometer arrangement

16.6 Flame ionization detection method: It is widely used to detect organic material in gas stream. In FID sample gas containing hydrocarbons is introduced into the flame (air or Hydrogen) where hydrocarbon is ionized because organics compound are easily ionizable in hydrogen flame. The ions formed are extracted in an ionization chamber by applying an electric field using electrode, generating a current which is proportional to the concentration of hydrocarbon. The data evaluation is done by comparing the different sensitivity of different hydrocarbon compounds. In this process pure hydrogen gas (taken from pressurized gas cylinder or from electrolytic hydrogen generator unit) is flow through nozzle into the combustion chamber of flame ionization detectors. Combustion air from the atmosphere is passed through annular slit around the nozzle. Steady hydrogen flame produce by ignition produce very small ion density in the absence of organic compounds. Compared the value with sample containing organic compounds is detected. The temperature and pressure of the sample gas must be constant.FID provides a non selective total measurement of organically bound carbon (Fig 4).

Fig 4: Flame ionization detector

16.7 Chemiluminescence Method: It is used in determination of low concentration of oxides of nitrogen. It is based upon the principle that when NO is react with ozone then resulting in the generation of characteristics light radiation. The analyzers are consisting of ozone generator, a reaction chamber, an enrichment and photomultipliers detector. The oxygen present in the air is partially converted into ozone by UV radiation and electrical discharges. Then a constant flow of sample gas is passed through reaction chamber via another nozzle and both are mixed in. the chemiluminescence is optically filter and measured by photo multiplier. Reaction chamber is maintained at controlled pressure and temperature. To determine the concentration of NO2, the sample gas is firstly passed through thermo-catalytic converter which reduces NO2 to NO (Fig 5).

Fig 5: Showing working of Chemiluminescence

16.8 Gas chromatography: In gas chromatography a sample is injected into the head of chromatographic column (contain liquid stationary phase adsorbed on the surface of inert solid) which is transported by inert gaseous mobile phase. Most commonly used carrier gases are nitrogen, carbon dioxide, helium and argon depending upon the type of detectors used. The temperature of the column must be controlled and maintained depending upon the boiling point of the sample. There are various types of detectors are used in gas chromatography for example ECD(electron capture detector) used for the monitoring of halides, nitriles, nitrate, peroxides and TCD(Thermal conductivity detector) is used universally for all. Finally result are recorded and displayed in data system attached with column (Fig 6).

Fig 6: Showing working of Gas chromatography

16.9 Thermal conductivity gas analyzer: Thermal conductivity is defined as ability to conduct heat at a specific rate. Each gas has different thermal conductivity for example nitrogen have 5.680 and sulphur dioxide has 1.950 thermal conductivity. The Zero and sample gas is passed onto the heated metal filaments. The amount of heat carries away by gases changes the rate of cooling of filament wire. Change in temperature resulting in change in resistance. The change in resistance is converted into electric current and an output signal (Fig 7). Thermal conductivity analyzer helps in analyzing mixture of two gases and in production of high purify gas. Other application includes food packaging mixtures, welding shield, leak detection mixture etc.

Fig 7: Thermal conductivity Detector Bridge

16.10 Para magnetic gas analyzer: It is based upon the larger magnetic susceptibility of oxygen as compared to coexisting gas. This analyzer is best for measuring oxygen concentration in flammable gas. There is no need of using reference gas so save the cost. In this instrumental arrangement two glass spheres are filled with nitrogen gas are suspended with strong metal. These sphere are first kept in homogenous magnetic filled. Oxygen molecule in sphere having large magnetic susceptibility flow, the molecules are pulled toward strong magnetic field and sphere is moves away.

Fig 8: Paramagnetic gas analyzer

(https://www.fujielectric.com/products/instruments/products/anlz_gas/genri.html)

This deviation in the sphere is detected by light source, reflecting mirror and light receiving element. Current is then flowed through the feedback loop to control and sphere can return back to initial balanced state. This current used is proportional to the oxygen concentration (Fig 7).

16.11 Remote sensing monitoring: Remote sensing device uses the detector which measure the optical properties of the sample gas, simply by reflected and transmitted signal received after pathlength through the air. It found an enormous application in chemical processing, air quality monitoring, power generation and in transport by monitoring the gaseous emission.

Light detection and ranging (LIDAR) used for online monitoring. It is use to track the pollutant for long period of time. The technique used is long path absorption, in which beam of laser light is reflected from distant retro reflector and come back to detector.

Differential absorption lidar (DIAL): Pulses of two wavelength are directed into the air by tunable laser and a cooled, detector measured the backscattered signal form particles, gas molecules and from air molecules. Difference between the two can used to calculate absorption due to gases.

16.12 Less common measurement methods: Colorimetric, heat change, conductometry and potentiometry are measurement methods used for continuous emission monitoring of gas stream. Colorimetric method, the sample gas is brought in contact with suitable reagent and change in color is measured by photometric basis. In conductometric method, the sample gas is introduced into suitable liquid reagent and change in conductivity is measured after completion of reaction. In heat change method, the temperature increase given of during exothermic catalytic oxidation of combustible gas component is measured. In potentiometric method, the sample gas in introduced into buffered electrolyte solution and change in ion concentration is measured by ion sensitive electrode change.

16.13 Flue gas analyzer by companies

Now a day many companies are involving in manufacturing of flue gas analyzer. Some of them are given below:

The Zirconium oxide fuel cell oxygen analyzer (ZrO2) has been used for measuring combustion flue gases. First used in 1970 in power generation industry but now used for many combustion process. All automobiles are now using these sensors for controlling fuel-air ratios. The main advantage of using these technologies is that its operation at hot combustion flue gases, it can easily used because theses sensors heated at 700-750oC. The sensors can directly place into the flue gas stream on the end of probe and there is no need of sampling system. The sensors are robust and can withstand the sulfur components found in many fuels. These sensors can be calibrated in place and on line. Automated calibration is also available.

NOVA flue gas analyzer: NOVA 7200 flue gas analyzer system using various technologies for different gases. For example carbon monoxide and carbon dioxide by infrared detectors and nitrogen dioxide, CO2, O2 by electrochemical sensors. It is accurate and durable method of monitoring

Testo 340 flue gas analyzer: It is used to measure CO, NO, NO2 and SO2. It is easy to use. It has precalibrated gas sensors for quick and easy sensor change.

Wohler A 550 industrial flue gas analyzer: It can measure NOx and SOx emission with 0.1 ppm resolution. It is used in complete analysis and inspection of burners and boilers.

Summary:

• Various types of flue gas analyzers are available which help in monitoring of gases in various industries, transport and in the environment.
• Flue gas analyzer and monitoring of NOx, Sox and hydrocarbon are based on certain principle like gas chromatography, photometry (IR, visible, UV), thermal ionization, chemiluminescence, conductivity, electromagnetic fields and colorimetry etc.
• Now a day remote sensing analyzer is also available which avoid the need of storage of sample and availability of result online.

    Glossary

Calibration-Relationship comparison of the instrument performance against a known standard. Note this does not mean adjustment to bring within specification, as is commonly misunderstood.

Combustion-The act or instance of burning some type of fuel, such as gasoline, to produce energy. Combustion is typically the process that powers automobile engines and power plant generators.

Conductometry– is a volumetric analytic method in which the end of titration (equivalent point) is defined by an electric conductivity appliance.

Monitoring-. Monitoring is often done by sampling the same sites over time, and these sites may be a subset of the sites sampled for the initial inventory.

Monochromator- manually tuned, presenting one wavelength or bandpass at a time from its exit slit.

Photometry– Photometry is the science of measuring the intensity of light (luminous intensity) in relation to the sensitivity of the human eye. Photometry is analogous to radiometry, weighted by the response function of the eye. The science of photometry does not deal with the perception of color, which is the realm of colorimetric.

Potentiometry –It is a method used in electroanalytical chemistry, usually to find the concentration of a solute in solution. In potentiometric measurements, the potential between two electrodes is measured using a high impedance voltmeter.

Spectrometer- a general class of instruments that collect, spectrally disperse, and reimage an optical signal. The output signal is a series of monochromatic images corresponding to wavelengths present in the light imaged at the entrance slit.

References

• Thain W,Monitroing of toxic gases in the atmosphere for hygiene and pollution control ,Pergaman press,1980.
• Thomas S and Shahnaj Haider N (2013) A Study on Basics of a Gas Analyzer”. International Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering 2(12): 6016-6025

    Web Links:

1. http://trove.nla.gov.au/work/9639622?selectedversion=NBD1667427
2. https://www.ijareeie.com/upload/2013/december/19H_A%20Study.pdf
3. https://www.fujielectric.com/products/instruments/products/anlz_gas/genri.html