Gaseous Emissions
13.5.4.1 Carbon Monoxide and Carbon Dioxide
Nondispersive Infrared Analyzers
When measuring the concentrations of carbon monoxide and carbon Dioxide from emission gases, the equipment frequently used is based on the absorption of infrared light. These instruments utilize the characteristic wavelength band of infrared light. The gas to be measured absorbs infrared light within the band. The wavelength band is selected by using an optical band-pass filter. This equipment is called a nondispersive infrared (NDlRl gas analyzer.
With Interference filters, two narrow wavelength bands are selected.1’’ These are the absorption (or measurement) and reference bands, within which the gas absorption is as high and as low as possible. The filters are mounted on a rotating disk, and the intensities are registered synchronously. The ratio of the intensities is used as the signal related to the gas concentration (Fig. 13.46).
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Lation techniques.19 M = mirror, D = detector. S = source. F — filter disk, MO = motor. FB = band pass filter. SD = synchronous detection, C = correlation cell. N = nitrogen filter. |
T he measurement principle, based on the ratio of the intensities, is insensitive to fluctuations in the source intensity. Changes associated with wavelength dependence can cause error in the measurement.
In the Gas correlation techniques, gas-filled cells mounted on a rotating disk cross the analyzing infrared beam in turn. One correlation cell is filled with a gas that will not absorb infrared light, such as nitrogen (N2). The other cell (or cells) are filled with a high concentration of the gas to be measured. The wavelength range is selected at the absorption band of the gas to be measured by an optical band-pass filter.
When measuring CO concentration, the reference signal is obtained when the beam is passed through the sample chamber and the CO cell. The absorption is then saturated due to the high CO concentration in the cell. Consequently, the reference signal is practically nondependent on the CO concentration in the sample gas. When the beam passes through the sample chamber and the N 2 filter, the absorption is dependent on the CO concentration in the sample chamber, as the N2 filter does not absorb energy from the infrared beam.
In the gas correlation method, the measurement and reference signals are obtained from the same wavelength band. The temperature changes in the light source and other wavelength-related changes do not disturb the measurement.19
Figure 13.46 shows the operation principles and arrangements of nondis — persive gas analyzers.
In nondispersive gas analyzers, interferences by other gases that possibly absorb at the measurement and reference bands should be taken into account. In the measurement of CO, interferences by overlapping in the measurement band can be caused by COS, N20, C02, and water vapor. Another source of uncertainty is interference in the reference band.
Other Techniques
Carbon monoxide and carbon dioxide can be measured using the FTIR techniques (Fourier transform infrared techniques; see the later section on the Fourier transform infrared analyzer). Electrochemical cells have also been used to measure CO, and miniaturized optical sensors are available for C02 monitoring.
! 3.5.4.2 Hydrocarbons and VOCs
Flame Ionization Detector
Volatile organic compounds (VOCs) include organic compounds with appreciable vapor pressure. They make up a major class of air pollutants.15 This class includes not only pure hydrocarbons but also partially oxidized hydrocarbons (organic acids, aldehydes, ketones), as well as organics containing chlorine, sulfur, nitrogen, or other atoms in the molecule.
Some of the analytical methods utilize highly selective and sensitive detection techniques for specific functional groups of atoms in compounds, whereas others respond in a more universal manner, i. e., to the number of carbon atoms present in the organic molecule.20
By using a flame ionization detector (FID), most compounds having a bond of carbon and hydrogen can be measured. This detector was originally developed for gas chromatography and employs a sensitive electrometer that measures the change in ion intensity resulting from the combustion of air
Containing organic compounds. The flame ionization detector can be considered an organic carbon analyzer.20
In a hydrocarbon analyzer using flame ionization, the sample gas is conducted along a heated sampling line to the detector, in the hydrogen flame of which the hydrocarbons are ionized into electrons and positive ions,
The operating principle of the flame ionization detector is shown in Fig. 13.47. The detector consists of a combustion chamber and a burner. Hydrogen passes through the burner nozzle and the combustion air through a hole around the nozzle into the combustion chamber. The collector electrode placed near the flame collects the ions and electrons. The combustion nozzle serves as one of the electrodes, and the current flowing between the electrodes is registered as the concentration signal.
A pure hydrogen flame produces a very low ion concentration. Instead, compounds brought into the flame, which contain bonds of carbon and hydrogen, produce carbon ions in the flame.21 If the gas measured contains several hydrocarbons, the resulting response depends on the carbon number and inter-atomic bonding. Thus, aliphatic, aromatic, alkenic, and acetylenic compounds all respond similarly to give relative responses of 1.00 + 0.10 for each carbon atom present in the molecule (e. g., 1 ppm hexane « 6 ppm C; 1 ppm methane « 1 ppm C; 1 ppm propane 3 ppm C). Carbon atoms bound to oxygen, nitrogen, or halogens give reduced relative responses.20
The flame ionization detector is capable of measuring only gaseous hydrocarbons, hi other words, hydrocarbons that have a low boiling point. Emission gases can, however, also contain hydrocarbons in liquid form at ambient temperature and pressure. Therefore, analyzers based on flame ionization detection are generally equipped with heating elements to keep the sampling line and the detector at about 200 °C.
Variations in oxygen concentration may affect the response of the flame ionization analyzer. The oxygen concentration can vary considerably, for instance.
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J L |
Combustion
Chamber
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Collector
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^ Sample gas |
Electrode
Burner
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Air
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When measuring hydrocarbon concentrations of gases from combustion processes. To minimize the error caused by this effect, the fuel used can be a mixture of hydrogen and helium, of which hydrogen accounts for 40% and helium 60%. The flow rate of the fuel has to be increased so that the hydrogen flow remains the same as without helium.21
A device based on flame ionization measures the total concentration of hydrocarbons. By using a catalyst, such as a heated platinum wire, hydrocarbons other than methane can be removed from the sample gas. With a platinum catalyst, these hydrocarbons are oxidized at a lower temperature than methane. Hence, the total concentration of hydrocarbons, methane, and hydrocarbons other than methane can be determined.
Gas chromatographic techniques and FTIR techniques are also used for the monitoring of VOCs.
Ultraviolet Fluorescence Method
Molecular Fluorescence When a gas containing sulfur dioxide is irradiated by ultraviolet light of appropriate wavelength, sulfur dioxide molecules are excited to a higher energy level than the original. On returning to the lower energy level, excited molecules give up their extra energy, giving fluorescent radiation (also in the ultraviolet wavelength range), the intensity of which can be detected.
Molecular fluorescence is a more complicated phenomenon than atomic fluorescence (e. g., x-ray fluorescence). In molecular fluorescence, energy changes in the vibrational and rotational motions are involved, in addition to the electronic transitions.
Reactions Producing Fluorescent Radiation In the reaction chamber of the ultraviolet fluorescent analyzer reactions producing fluorescence are
$Oi SO} (creation of an electronically excited state)
— — K; —
SO? SO2 + Hv (de-excitation of the excited state causing fluorescence)
Dissociation and quenching also occur, reducing the yield.
In the preceding formula, Ia denotes the intensity absorbed from the excitation radiation (intensity I0). This absorbed intensity Ia generates fluorescent radiation and other reactions.
/, = /„( l-e-“»w)
Where C is the concentration of sulfur dioxide, and describes the portion
Of the radiation transmitted through the sample gas according to the LambertBeer law. The intensity of the fluorescent radiation, If, is proportional to the intensity absorbed, La.
The proportion factors related to the reaction constants, together with the geometrical factors of the reaction chamber, can be included in one coefficient G, giving
Where orA is the absorption coefficient of sulfur dioxide to the excitation radiation, the wavelength of which is А; A’ denotes the fluorescent radiation, and I Is the absorption length. With low sulfur dioxide concentration and short ah sorption length this gives:
L/.y~k-c.
Where K is a proportionality factor. In practice, a linear relationship is achieved at concentrations lower than 500 ppmv (parts per million by volume).-’
Construction and Operation of Analyzer The construction principle of a sulfur dioxide analyzer based on the ultraviolet fluorescence principle is shown in Fig. 13.48. Undesired wavelengths are removed from the irradiating beam as far as possible using filters. The irradiating light (214 nm) is focused by a iens at the center of the reaction chamber.
The sulfur dioxide analyzer based on the ultraviolet principle is A Sensitive instrument. Its detection limit can be less than one ppbv (parts per billion by volume). When used in emission measurements, the sample gas is normally diluted prior to the measurement using a diluting stack sampler.
Hydrogen Sulfide and Other Reduced-Sulfur Compounds
TRS Converter To measure hydrogen sulfide and reduced-organic sulfur compounds, the technique used is thermal oxidation, in which sulfur dioxide is produced. Hydrogen sulfide and other reduced-sulfur compounds are measured by using methods applicable to the measurement of sulfur dioxide concentrations. One method is a technique based on ultraviolet fluorescence.
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1. Sample gas 2. Filter 3. Ultraviolet light source 4. Fluorescent radiation 5. Collimator 6. Photomultiplier 7. Electronics |
By this method it is possible to determine the total concentration of reduced — sulfur compounds or the concentration of TRS compounds, as in a paper puip plant. The oxidation temperature of the furnace is about 800 °C. The flue gas must contain a minimum of 1% oxygen to ensure that all TRS compounds are fully oxidized to sulfur dioxide.
If the gas to be measured contains sulfur dioxide, it has to be scrubbed from the gas before oxidation of the reduced compounds can occur. The gas is scrubbed using an S02 scrubber. This may contain citrate buffer solution (potassium citrate or sodium citrate). The collection efficiency of the sulfur dioxide may be as high as 99%.
Other Techniques Continuous methods for monitoring sulfur dioxide include electrochemical cells and infrared techniques. Sulfur trioxide can be measured by FTIR techniques. The main components of the reduced-sulfur compounds emitted, for example, from the pulp and paper industry, are hydrogen sulfide, methyl mercaptane, dimethyl sulfide and dimethyl disulfide. These can be determined separately using FTIR and gas chromatographic techniques.
The measurement of NO* concentration is based on chemiluminescence; this is light generation due to a chemical reaction. This occurs when nitrogen monoxide and ozone react with each other.
In chemiluminescence, some of the chemical reaction products developed remain in an excited state and radiate light when the excitation is discharged. This is particularly so at low pressures, when the collision frequency is low; the excitation is discharged as light radiation. The extra energy bound to the excited molecule can discharge through impact or molecular dissociation.
The chemiluminescence reaction between nitrogen monoxide and ozone is formulated as:
NO + O; — h» N02 + O, no; ^»no 2 + bv.
N02 refers to the excited nitrogen oxide molecule. These molecules can decay by emission of light of wavelengths longer than 600 nm.24
An instrument for measuring nitrogen oxides based on chemiluminescence is shown in Fig. 13.49. The ozone required for the reaction is produced in the ozone generator, which is part of the device. One of the reaction chamber walls is an optical filter through which a red-sensitive photomultiplier tube measures the chemiluminescence radiation intensity and converts it into a current signal.
The chemiluminescent method is very sensitive and is used in air quality monitoring.
The device for nitrogen oxides based on chemiluminescence measures the nitrogen monoxide concentration. The same equipment can be used to measure the concentration of nitrogen dioxide. Nitrogen dioxide is reduced to nitrogen monoxide in a converter by a molybdenum catalyst. In order to
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_P |
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FIGURE 13.49 Schematic diagram of an instrument for the measurement of nitrogen oxides based on chemilumirvescence. I. ozone generator; 2. pressure gauge; 3. oxygen control; 4. air; S. photomultiplier tube: 6. reaction chamber: 7. NO, converter; 8, sample gas, |
Get reliable measurements, the converter must have a conversion efficiency of over 95%.
Electrochemical cells are sometimes used to measure nitric oxide and nitrogen dioxide. These, together with nitrous oxide (N:0), are measured using the FTIR techniques.
13.5.4.5 Optical Multigas Analysis Techniques Differential Optical Absorption Spectrometer
The differential optical absorption spectrometer (DOAS) is based on the differential absorption of gaseous atoms or molecules.25 The Lambert-Beer law gives the concentration
C = log(I’0/J)/(eL),
Where
= light intensity without differential absorption,
I = light intensity due to gas absorption, f = differential absorption coefficient of the gas, and L = length of the absorption light parh.
The wavelength-specific differential absorption coefficient E is generally lower than the total absorption coefficient. The spectrometer and signal-processing system analyze UV and visible light intensities over a range of 200 to 1000 Nm In determining the concentrations of the absorbing species.25,26
When evaluating gas concentrations in practical applications, a reference spectrum is least squares fitted to the received absorption spectrum. This improves the system accuracy, since the spectral fingerprint over the whole scan ning range contributes to the result.25
Structure of Instrument The differential optical absorption spectrometer is composed of a light source, an optical light receiver, a fiber-optic cable, a spectrometer and a computer.26 The light source is a high-pressure xenon arc lamp. The spectrometer and software signal-processing system rapidly analyze the UV and visible light intensities to determine the absorbing gas concentrations.
The light collected in the receiver is focused onto the entrance fitting coupled to a fiber-optic cable. The light transmitted by the fiber-optic cable enters the spectrometer and is reflected and collimated by a mirror before a grating reflects it. A step motor controls the grating position. A second mirror focuses the light onto an exit slit. Light detection is accomplished using a photomultiplier tube. In front of the entrance to the photomultiplier tube is a rotating disk with radially arranged slits. The slit width is about 0.2 mm, and the rotation speed is 600 rpm. In this configuration, a spectral range of 40 nm is scanned within 10 ms, which results in a resolution of 0.2 nm and channel overlapping of 80%.26
LInknown broadband influences due to atmospheric aerosols, lamp fluctuations, and dust on the mirrors are minimized by dividing the spectrum by a fitted fifth-order polynomial. The construction of the measuring instrumentation is shown in Fig. 13.50.
Applications The differential optical absorption spectrometer has been used to monitor concentrations of gases or intermediate compounds such as SO2, N02, O j, HCHO, HNOi, CS2, N03, and OH in the atmosphere,25-2<> In atmospheric measurements with open paths of 100 to 1000 m, a detection limit of about 1 ppb can be achieved. In the emission measurements, the path length across the duct or the plume can range in meters.
Fourier Transform Infrared Analyzer
Fourier transform infrared (FFIR) analyzers can be used for industrial applications and M situ measurements in addition to conventional laboratory use. Industrial instruments are transportable, rugged and relatively simple to calibrate and operate. They are capable of analyzing many gas components and determining their concentrations, practically continuously. FTIR analyzers are based on the spectra characterization of infrared light absorbed by transitions in vibrational and rotational energy levels of heteroatomic molecules.
The basic principle of a Fourier transform spectrometer can be visualized using the conventional Michelson interferometer. There is a coherence of two light components coinciding at the beam splitter, and the resulting interference spectrum is registered. The two beams originate from the same source, but the light is divided into two parts using a semitransparent (“half-silvered”) mirror or a beam splitter. One part of the light is reflected from a fixed mirror, while the other part is reflected from a moving mirror, from which the interference is formed (Fig. 13.51). The interference spectrum (interferogram) is turned into
An energy spectrum using the Fourier transform performed by the instrument’s computer.
The position of the moving mirror (M2) determines the phase 8 between the intensities Jj and I2 as follows:28
8 = 2ir2(a — B)/A = 2ttvx
Where V — 1/A, x = 2(a-b) is the optical path difference, A is the wavelength, and V is the wave number.
FTIR techniques offer a high signal-to-noise ratio. This is due to these factors:
1. High throughput; i. e., high light energy is utilized compared with dispersive instruments. An increase in the throughput results in an increased signal-to-noise ratio.2
2. All the wavelengths are observed simultaneously.
In an industrial-design FTIR spectrometer, a modified form of the Genzel interferometer is utilized.29 A geometric displacement of the moving mirrors by one unit produces four units of optical path difference (compared with two units of optical difference for a Michelson type interferometer). The modified Genzel design reduces the time required to scan a spectrum and further reduces the noise effects associated with the longer mirror translation of most interferometers.
To make the actual analysis simple and fast, a large amount of precomputed information is utilized.30 By using library spectra of pure molecular gases together with a special mathematical multicomponent analysis, it is possible to calculate the partial pressures of the gases in a gas mixture.31
Applications Transportable FTIR analyzers have been used in monitoring applications such as continuous emissions monitoring, process gas analysis, and car exhaust and industrial air hygiene.32
For industrial applications, low-resolution FTIR spectrometers of simple, rugged design can be used.30 High signal-to-noise ratio spectra can be obtained without the use of the more traditional liquid nitrogen—cooled detectors. Due to low resolution, the spectral analysis speed is increased, and the data storage requirements are reduced. The dynamic range for quantitative analysis is greater for low — than for higher-resolution spectrometry, due to lower absorbance values and noise levels.
Gases analyzed include hydrocarbons, carbon monoxide, carbon dioxide, sulfur dioxide, sulfur trioxide, nitrogen oxides (also nitrous oxide, N20), hydrogen chloride, hydrogen cyanide, ammonia, etc.
Unanticipated gases can be determined. Table 13.21 Shows the measuring ranges and detection limits of an FTIR analyzer. The detection limit depends on the optical path in the sample gas chamber; this can range from a few meters to about 10 m in industrial instruments.
Solid-State Gas Sensors
Voltage Cell Type Oxygen Sensor The operation of the zirconia oxygen sensor utilizes the conduction of oxygen ions by virtue of anion or oxygen ion vacancies in the crystalline lattice.34 The anion vacancies are created when the
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Component |
Range i |
Range 2 |
Typical detection limit, l~s measuring time |
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1,3-Butadiene |
0-500 ppm |
< 10 ppm |
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1-Butanol, C4H9OH |
0-200 ppm |
0-1000 ppm |
<4 ppm |
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1 — Propanol, C/H-OH |
0-200 ppm |
0-1000 ppm |
<4 ppm |
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Ac. euldehyde |
0-500 ppm |
< 10 ppm |
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Accne acid |
0-500 ppm |
< 1 0 ppm |
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Acetylene |
0-500 ppm |
<10 ppm |
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Ammonia, NHi |
0-100 ppm |
0-1000 ppm |
<2 ppm |
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Ben/ene |
0-500 ppm |
<2 ppm |
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Butane, C4H, n |
0-200 ppm |
0—1000 ppm |
<4 ppm |
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Carbon dioxide, C02 |
0-1 % |
0—20% |
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Carbon monoxide, CO |
0-500 ppm |
0-50 000 ppm |
<4 ppm |
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Carbonyl sulfide, COS |
0-500 ppm |
< 2 ppm |
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Ethane, C2H6 |
0-500 ppm |
0-2000 ppm |
<4 ppm |
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Ethylene, C2H,( |
0-500 ppm |
<4 ppm |
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Ethanol, C? H^OH |
0-500 ppm |
0-10 000 ppm |
<4 ppm |
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Formaldehyde |
0-500 ppm |
<4 ppm |
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Formic acid |
0-500 ppm |
<4 ppm |
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Hydrogen cyanide |
0-100 ppm |
0—1000 ppm |
<2 ppm |
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Isooctane, CXH, S |
0—500 ppm |
0-10 000 ppm |
<4 ppm |
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Methane, CHA |
0-1000 ppm |
0-10 000 ppm |
<4 ppm |
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Methanol, CH^OH |
0-200 ppm |
0—10 000 ppm |
<4 ppm |
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M. TBE, (CH^J^COCH. |
0-500 ppm |
<4 ppm |
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Nunc oxide, NO |
0-500 ppm |
0-10 000 ppm |
<10 ppm |
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Nitrogen dioxide, N02 |
0-100 ppm |
0-5000 ppm |
<4 ppm |
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Nitrous oxide, N-iO |
0—100 ppm |
0-1000 ppm |
<2 ppm |
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Propane, C, HH |
0-500 ppm |
0-5000 ppm |
<4 ppm |
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Propylene, C^H(S |
0-500 ppm |
<4 ppm |
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Sultur dioxide, SQ2 |
0-500 ppm |
0-2000 ppm |
<2 ppm |
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Toluene |
0-500 ppm |
<2 ppm |
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Water, H20 |
0-1 % |
0—20% |
Iower-valent stabilizing ion such as Y+3 substitutes for a Zr+4 ion in the crystal lattice. The electronic conductivity of stabilized zirconia is almost zero. The operation temperature of the zirconia oxygen sensor is normally greater than 500 °C, as at these temperatures the ionic conductivity is adequate for practical applications.
The basic zirconia oxygen sensor design is illustrated in Fig. 13.52, which shows the principle of the zirconia solid oxygen-ion electrolyte. The sensor consists of a closed-end tube of ceramic zirconia (Zr02). The zirconia ceramic
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O, +4e" —*-202~ Oxvgen partial pressure Po-, |
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(b)
FIGURE 13.52 (a) Solid electrolyte and (b) arrangement of the zirconia sensor.
Separates two gas atmospheres having oxygen partial pressures PQ, and PO, -34 The inner and the outer tube surfaces are covered by catalytic porous platinum electrodes, which promote the electrochemical reaction:
02 + 4e- <=> 202_
Where <r represents an electron and 02~ represents an oxygen ion. The chemical potential of oxygen gas at each electrode, /x, and /x2, is defined by the oxygen partial pressure over each electrode.
The galvanic potential E is related to the difference in chemical potential by the relation34
E = -{1 /(ZF))^~ Tl(md Ix
Where Z is the valence of the oxygen ion (Z = 2), F is the Faraciay constant (F = 9.65 x 104 C moh1), and Јjon is the ionic transference number for oxygen ions (ratio of the ionic conductivity to the total conductivity, taken as being unity). The chemical potential is related to the oxygen partial pressure by the relation
Fi = fi0 + (RT/2) In p0,
Where R is the gas constant [(8.314 J/(K mol)] and T is the absolute temperature. Combining these two equations gives
E = -(RT/4F) | (/’i0n/PQ,)dpo,
J L> o, ■ ‘
Because the ionic transference number for zirconia material is taken as being unity, then this equation reduces to the Nernst equation34
Air is normally the reference gas used in the exhaust gas sensor. If the oxygen partial pressure in the engine exhaust gas is known as a function of the engine air/fuel ratio, the theoretical galvanic potential of the sensor is easily determined by the Nernst equation.
Application The zirconia oxygen sensor is widely used for combustion control processes and for air/fuel ratio regulation in internal combustion engines. The closed-end portion of the electrode tube is inserted into the exhaust gas stream.34 In the control of industrial combustion processes, no out-stack sampling system is required.
Other 02 Measurement Techniques The oxygen concentration in the emission gases of combustion processes is often measured based on the strong paramagnetic character of oxygen. A sampling line with appropriate sample treatments is required with this method.
Semiconductor Gas Sensors Semiconductor gas sensors are used to indicate concentrations of gases such as CO and H2S. The device responds to the change in the composition of the surrounding atmosphere with a change in conductance of a gas-sensitive semiconductor.35 Oxide semiconductors with wide band gaps (Sn02, ZnO, WO3, ln203) are typical sensing semiconductors, since the sensors operate at elevated temperatures up to around 500 °C. The chalcogenides Sn02 and CdS, utilized as gas sensor materials, are n-type semiconductors with band gaps of 3.6 eV (Sn02) and
2.5 eV (CdS). CdS is a typical II-VI compound semiconductor.36 The semiconducting behavior arises in both structures from point defects, donors being related to oxygen vacancies in Sn02 to cadmium interstitials, or to sulfur vacancies in CdS.
Theory for Operation of Sn02-Based Semiconductor Gas Sensors The negative surface charge due to the ionosorbed oxygen species generates a depletion layer and a potential energy barrier on the surfaces of ionic n-type semiconductor grains of the gas sensors. For planar geometry, the height of this barrier EVs is given by the Schottky equation:37
2 Ee0Nd
Where Nf = [O2-] + [O*] is the surface density of ionosorbed oxygen species, ee0 is the permittivity, and Nd is the volumetric density of the single electron donors. In tin dioxide material in gas sensors, the certain lack of oxygen at the surface can be expressed as Sn02_t.38
In conduction models of semiconductor gas sensors, surface barriers of in- tergranular contacts dominate the resistance. Electrons must overcome this energy barrier, EVs, in order to cross from one grain to another.3’1 For these
Models, the conductance activation energy is usually assumed to be EVs, to a first approximation in the ohmic voltage range. Then the conductivity G at a temperature T is described by39
G = G0 exp(-f VykT) ,
Where KT is the thermal energy, and G0 is a factor including the bulk inn-agranular conductivity and geometrical effects.
It is assumed that all the donors are ionized and that the voltage dependence of the current is ohmic. The temperature dependence of G0 may be considered constant. A schematic diagram of a porous gas sensor sample together with its band structure is shown in Fig. 13.53.40
Response The conductance response of semiconductor gas sensors has been modeled by describing the barrier energy in terms of physisorption of oxygen molecules in the surface, the ionosorption of 02 to Os according to Fermi-Dirac statistics, and the catalytic reactions of reducing gases with physisorbed 02 molecules.41
The model gives the nature of measured power law of the conductance response to oxygen:
G « Po,
With /3 = KT/E0. Ј0 is an energy constant characteristic of the sensor.42 The values of /3 vary between 0.25 and 0.5. When a reducing gas G, removes physisorbed oxygen from the surface via a catalytic reaction,
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The gas sensor material.40 The negative surface charge due to ionosorbed oxygen species generates a depletion layer. Јf is the Fermi energy. |
■ UjG, + ()2sl, rt———— > (jprod 5
The conductance changes according to
G «I Pif ( 1 + Kjpitt^ ,
Where Po2 and /?, are the partial pressures of oxygen and reducing gas G,, n; is a reaction coefficient, and K: is the sensitivity coefficient for gas Gr The response time of the sensor is determined by the kinetics of electron transitions between the conduction band and the surface states. For a commercial sensor, the sensitivity coefficient factors (K,-) have been measured:41 KCH = 5x 10-‘1 ppm 1 , XHi() = 5 x 10”’ ppm”1 , KH> -~ 0.1 ppm-2 , and Kco *lx 10”" ppm2 . This sensor is very sensitive to the presence of hydrogen.
Construction In tin dioxide semiconductor sensors, the sensing material is small sintered particles. For the sensor current flow, particle boundaries form potential energy barriers, which act as a random barrier network. Different types of semiconductor gas sensors are shown in Fig. 13.54.
The gas-sensitive material in Thick-film gas sensors has a sintered layer area of a few square millimeters, and about 30 |im in thickness, on a ceramic substrate (Fig. 13.54b). In other types of gas sensors, it is on the outer surface of a thin tube or as a sintered button (Fig. 13.54<? and C). The sensor is heated to the operating temperature of 300-500 °C by means of a resistor.
Applicability of Semiconductor Gas Sensors Research into the applications of this type of sensor has mainly been concerned with measuring carbon monoxide concentration in flue gases. Tests show that sensors follow the concentration of carbon monoxide in the flue gas. Improvement in sensor performance has resulted with the introduction of a catalytic additive (palladium or platinum).43’44
In combustion processes tests, it has been noticed that Pd-catalyzed SnO, sensors follow the variations in the concentration of CO in the combustion gases, even in the case of solid fuels.44
Optical Gas Sensors
Electrically Tunable Micromachined Fabry-Perot Interferometer The
Fabry-Perot interferometer is an optical resonator consisting of two parallel mirrors. Fabry-Perot interferometers can be made by silicon bulk micro machining.4546 Silicon surface micromachining is also a suitable technique for making interferometers for infrared wavelengths.
The principle of a gas concentration measurement system operating on an electrically tunable micromachined Fabry-Perot interferometer is NDIR (non dispersive infrared) single-beam dual-wavelength measurement.46 The interferometer is tuned so that the pass band coincides with the absorption band of the measured gas. A detector records the signal strength after the measurement chamber. The pass band of the Fabry-Perot interferometer then shifts to either side of the absorption band, and the signal now detected constitutes the reference signal (Fig. 13.55).
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Sensing |
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Contact wire |
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Hlectrode |
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1. Platinumbwircs |
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For heating resistor |
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Klee tr odes |
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(ias-sensitivc Inver |
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MiG« substrate |
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Light-Emitting Diode Sensors Some gas detection sensors utilize light — emitting diodes (LEDs) or laser emitters together with photodetectors operating at appropriate wavelengths. The radiation is emitted at the wavelength of an absorption maximum of the gas to be measured. An example of an infrared C()2 sensor is shown in Fig. 13.56.
The production of emitting sources requires further development in material technology, as the emitting wavelength is controlled by the compositions of alloy semiconductors to form heterostructures such as InGaAs/InAs48’49 or III-V alloy systems.-50
The composition must be controlled to give the required emission wavelength. Techniques utilized include molecular-beam epitaxy (MBE) and liquidphase epitaxy (LPE).
Several LED chips have been mounted together on the same thermoelectric cooler shown in Fig. 13.56b.51 This nondispersive IR analyzer utilizes sample and reference PbSe photoresistors as detecting elements. The LED emission (4.3 |лт for C02 and 4.7 |лт for CO detection) is focused onto a sample photodetector by a concave mirror.
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