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Appl. Phys. B 67, 297305 (1998) Applied Physics BLasersand Optics Springer-Verlag 1998

Gas monitoring in the process industry using diode laser spectroscopy

I. Linnerud, P. Kaspersen, T. Jger

Norsk Elektro Optikk A/S, Solheimv. 62 A, P.O. Box 384, N-1471 Skrer, Norway(Fax: +47 67 97 49 00, E-mail: [email protected]; [email protected]; [email protected])

Received: 19 March 1998/Revised version: 2 June 1998

Abstract.Gas monitors for industrial applications must havehigh reliability and require little maintenance. Monitors for

in-situ measurements using tuneable diode laser absorptionspectroscopy in the near infrared, can meet these require-ments. A brief description of monitors using high-frequencywavelength modulation spectroscopy with second-harmonicdetection is presented. Some problems related to industrialapplications are discussed, in particular line broadening ef-fects, and frequency drift of diode lasers. Typical applicationsfor monitoring ofO2, CO, NH3, HCland HFare describedtogether with measurements from several installations. Themonitors show continuous measurements with fast responseand good sensitivity, all of which is difficult to obtain withconventional techniques such as wet chemical analysis.

PACS:42.60.Fc; 42.62.Cf; 82.80.Ch

Over the last few decades there has been increasing concernabout man-made pollution of the atmosphere. The dilution ofpollutants in the atmosphere makes the concentrations verylow, often below 1 ppb, and therefore difficult to measure.There are numerous reported measurements of trace gasesin the atmosphere (see e.g. references in [1]), and measure-ments of some gases such as CO and NOx , are done rou-tinely in many cities. In addition to measuring the gases inthe atmosphere, it is important to monitor the many sourcesof emissions, where traffic, agriculture and industry are themost important types. Industrial processes are often large-

scale point sources with gas abatement systems which can beoptimised to reduce the emissions.Traditionally gas emissions have been measured by wet

chemical methods where the gas is extracted from the point ofmeasurement and preconditioned (cleaned, dried, etc.) beforeit undergoes chemical reactions from which the concentrationis deduced. For emissions subject to (inter)national regulationand legislation the chemical method is normally the methodnew techniques have to be compared with. There are sev-eral disadvantages with the chemical measurement technique.Due to low concentrations the gas has to be extracted fora long time, normally half an hour, before an amount suffi-cient for reliable measurement is obtained. There are many

sources of errors related to the extraction and precondition-ing of the gas, and measurements are laborious. For sticky

gases, i.e. gases which are easily adsorbed on surfaces, a largefraction can be lost in the extraction and preconditioning ofthe gas. Many important emission gases, such asHCl,HFandNH3, are of this type. There are several other general meas-urement techniques such as gas chromatography and massspectrometry. However, they are complicated and normallyrequire a lot of preconditioning of the gas before measure-ment. They are therefore slow and can suffer from similarproblems related to the preconditioning of the gas as thechemical methods do. Optical spectroscopic techniques canwork well without preconditioning of the gas. They thereforehave the potential of in-situ measurements with fast responseand are ideally suited for industrial applications provided theycan measure with sufficient sensitivity.

A number of spectroscopic techniques have been de-veloped for trace gas measurements. The traditional tech-nique has been non-dispersive infrared (NDIR) where thetransmission has been measured at two wavelength regions,one at absorbing and the other at non-absorbing wavelengths.This technique is suited for gases with broad absorptionbands. In recent years new techniques has emerged such asFourier transform infrared (FTIR), differential optical absorp-tion spectroscopy (DOAS), laser-induced fluorescence (LIF)and tuneable diode laser absorption spectroscopy (TDLAS).An overview of these techniques is given in [2]. There areseveral optical instruments commercially available for contin-uous emission monitoring based on these techniques. Some

are extractive, where the gas has to be preconditioned be-fore measurement in an internal cell, and others are in-situ systems. These are broad-band spectroscopic instrumentsworking in the infrared or ultraviolet. The middle infrared(MIR, 315m is a very rich spectral region where mostof the interesting trace gases absorb on their fundamentalrotational/vibrational modes. The absorption is so strong,particularly fromH2Oand CO2 molecules, that a very highspectral resolution is required to avoid interference betweenspecies. Tuneable diode lasers (TDL) have line widths of onlya few MHz or less and are therefore well suited for high-resolution spectroscopy. Lead-salt-based lasers are availablein the middle infrared but both the TDLs and the detectors


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require cooling to around liquid nitrogen temperatures. Inthe near infrared (NIR, 0.83mwe have the first and sec-ond overtones of the rotational/vibrational modes of the tracegases, and there are commercial III-V semiconductor lasersavailable up to approximately 2m that operate at roomtemperature. The absorption typically drops by an order ofmagnitude for every higher overtone, however, and a higherabsorption sensitivity is required to obtain sufficiently low de-tection limits for many of the important gases.

An easy way to increase sensitivity is to increase the op-tical path length. This is possible for atmospheric measure-ments, but for industrial applications there is usually a prac-tical limit of a few meters. The optical path length can alsobe extended with multipass cells (White cell, Herriott cell).In recent paper, Ye et al. [3] discuss several high-sensitivitydetection techniques, and they apply an external cavity tech-nique with a cavity finesse of105 to obtain a record absorp-tion sensitivity of51013. Although this is an impressivesensitivity, such a technique is not practical for industrial ap-plications except in very special cases. Open multipass cellscannot usually be used because most applications have anopacity or dust load that prevents long path lengths and leads

to problems with unclean optical surfaces. The use of multi-pass cells therefore requires filtering and preconditioning ofthe gas and is only practical for extractive systems.

It is tempting to try to define an ideal gas monitor. Itshould (a) measure correctly without influence from othergases or dust, (b) automatically compensate for temperatureand pressure effects, (c) measure continuously with short re-sponse time. In addition, an instrument for industrial appli-cations should (a) require little maintenance with few or noconsumable parts, (b) have high reliability and availability, (c)be insensitive to vibrations and mechanical instabilities, and(d) use components operating near room temperature (i.e. noliquid nitrogen cooling).

With such requirements, open-path, in-situ near-infraredTDLAS is an attractive measurement technique. In this paperwe describe a TDLAS system currently being tested by a no-tified body (TV, Germany) for assessment of conformitywith regulations for continuous emission monitoring. It hasbeen designed for continuous operation in an industrial envi-ronment where it may be exposed to corrosive or explosiveatmosphere, large variations in ambient temperature, vibra-tions and electromagnetic disturbances. We discuss some ofthe special problems related to industrial applications, andshow some results from actual installations.

1 Measurement principle of a TDLAS gas monitor

Many detection techniques have been developed in conjunc-tion with TDLAS. An overview of some of these techniquescan be found in [1, 3, 4]. We give a brief description of oneof the techniques, which has received most attention in theliterature and which we have found to be one of the bestcandidates for an industrial monitor. For more detailed dis-cussions of the technique see [57].

All absorption spectroscopy is based on the BeerLambertlaw which states that the transmission will decay exponen-tially as exp(Sg(f)NL) where S is the absorption linestrength, g(f) is the line shape function, N is the concen-tration of absorbing molecules, and L is the optical path

length. For sufficiently low pressures (less than a few bar)many gases will have distinct absorption lines of width inthe order of 0.01 to 0.1 cm1 depending on pressure andother line broadening effects. Line broadening effects are dis-cussed later. By searching spectroscopic databases such asHITRAN [8], one can find absorption lines that are sepa-rated sufficiently far from those of other gases for so-calledinterference-free detection. The measurement might not betruly interference free, however, for reasons which are de-scribed later.

The most straightforward measurement principle is directabsorption measurement where the absorption at the line cen-tre is compared with the absorption slightly to the side of theline. In its simplest form this technique is not used much be-cause measuring a small difference in two large signals givesa rather low detection sensitivity. To improve on this, a mod-ulation of the laser frequency is applied. The instantaneouslaser frequency is then

f(t) = f0+msin(2fmt), (1)

where f0 is the laser centre frequency, is the half width

at half maximum of the absorption line, fm is the modula-tion frequency and m is the frequency modulation index. Iffm this is referred to as wavelength modulation spec-troscopy (WMS), otherwise it is called frequency modula-tion spectroscopy (FMS). By using a lock-in amplifier, har-monic components can be selected for measurement. First-and second-harmonic detection are conventionally labelled 1fand 2f detection respectively. The amplitudes of the harmoniccomponents generally decrease for increasing harmonics, in-dicating that 1f detection might be the best choice. However,since laser output power (in addition to laser frequency) ismodulated with injection current, there will be a residual am-plitude modulation (RAM) signal when there is no absorptionline. For an ideal diode laser where the output power is lin-ear with injection current the RAM signal is zero for 2f andhigher-harmonic detection [7]. This is the main reason why2f detection is preferred over 1f detection. In practice there isusually some RAM signal also for 2f detection, but it is muchsmaller than for 1f detection.

FMS usually operates with modulation frequencies oforder0.110 GHz. The advantage with high-frequency mod-ulation is reduced laser excess noise, but [6, 9] have shownthat high-frequency WMS (> 0.1 MHz) can give detectionsensitivities comparable to FMS. The lower cost of detectorsand electronics operating at high-frequency WMS makes thisideal for industrial instrumentation.

There are two major problems with the WMS and FMS

techniques. One is stabilising the laser centre frequency at theabsorption line centre, and the other is compensating for theRAM signal. In principle the RAM signal can be eliminatedwith a dual-beam subtraction technique, where the laser beamis split and one part is led outside the absorbing path to a sepa-rate detector. The extra path can also be used to frequency sta-bilise the laser by inserting a cell with the trace gas. However,it has been difficult to obtain good results with the dual pathtechnique because of strong sensitivity to alignment [10].

In the gas monitors described in this paper the dual modu-lation (or ramp scanning) technique is applied. In addition tothe high-frequency modulation at approximately 200 kHz, thelaser centre frequency is scanned slowly over the absorption


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line at50100 Hz. This is illustrated in Fig. 1a where the de-tector signal, S, (normalised to 1) is plotted as a function oftime,t, and instantaneous laser frequency, f. The shaded sur-face represents the absorption line. When the laser frequencyis ramped slowly with a high-frequencymodulation, as shownby the curve in the tfplane, the detector signal will be rep-resented by the space curve embedded in the surface. Theprojection of the space curve onto the tSplane is the detectorsignal as a function of time and is shown again in Fig. 1b. Theenvelope of the second-harmonic component of the signal isused for the detection of the gas. The envelope is hereafterreferred to as the second-harmonic or 2f signal.

Because of the scanning of the laser centre frequency, therequirement for laser frequency stability can be somewhat re-laxed, compared with fixing the laser centre frequency to theabsorption line. We obtain adequate frequency stability bystabilising the laser temperature to within (5 mK from theideal temperature. The RAM signal can be reduced by ana-logue and digital signal processing.

For small modulation indexes (m 1)the 1f and 2f sig-nals are proportional to the first and second derivatives of theabsorption line shape function, and similar for higher har-

monics. The technique is then called derivative spectroscopy.For higher modulation indexes the envelope will deviate fromthe derivative. The peak of the second-harmonic signal will

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Fig.1a,b.A schematic drawing of the dual modulation technique. a Sis thedetector signal as a function of time, t, and instantaneous laser frequency, f.bThe envelope of the second-harmonic component of the detector signal isused for detection of the gas. See text for explanation

increase with increasing m until it reaches a maximum form =mo = 2.2 [5,11].

The detection sensitivity is limited by etalon and laserfeedback noise to 105 106 relative absorption. Etalonnoise is optical interference due to small reflections fromoptical surfaces. It will appear as oscillations superimposedon the second-harmonic signal. Small temperature varia-tions will cause the oscillations to drift slowly relative to thesecond-harmonic signal and give oscillations in the measuredgas concentration. In particular, optical surfaces separated by2to 6 cmtend to give etalon noise, which interferes in a waythat is difficult to eliminate with digital signal processing. Tominimise these effects the monitors have been designed witha minimum of optical components between laser and detec-tor. We only use a collimating lens for the laser, a focusinglens for the detector and wedged protective windows for thetransmitter and receiver units. No beam steering is used. Thealignment (after installation) relies on a sufficiently stablemounting. Since the distance between receiver and transmit-ter is limited to approximately5 m, vibrations have not givenalignment problems.

The mechanical layout of a monitor installed on a stack

is shown in Fig. 2. The laser and detector are located in thetransmitter and receiver units, respectively. Both units aremounted on the stack by using standard flanges purged withdry air to keep the optical windows clean. The detected signalis transmitted through a cable to the electronics unit, whichcontains signal-processing electronics. The measured concen-tration is displayed on an LCD display, and for data recordingand logging it is sent through an RS232 digital output anda standard420mAanalogue output.

An alternative to this design would be to place the laserand possibly detector in the electronics unit and use opti-cal fibres to the point of measurement. However, it has beendifficult to obtain similar detection sensitivities without sacri-ficing the simplicity of the above solution.

2 Line broadening effects

It is well known that the absorption strength, S, depends onlyon temperature, and the integral of the line shape function isindependent of line width such that the integral can be nor-malised to unity

g(f) df= 1.The peak amplitude of theabsorption and the second-harmonic signal therefore decrease

Fig. 2. Schematic drawing of the TDLAS gas monitors described in thispaper


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with increasing line width, . For a Lorentzian line shape theproportionality between the peak of the 2f signal andis be-tween 1/3 (formmo) and 1/(formmo). Variationsin line width are therefore of great importance for precisemeasurements of concentration.

There are three contributions to the line width of an ab-sorption line: natural line broadening, Doppler broadeningand collision broadening. In most industrial processes colli-sion broadening dominates, which results in a Lorentzian lineshape. For a more careful account of the line shape one hasto combine the effects of Doppler and collision broadening,which gives a Voigt line shape (see e.g. [12, 13]). Pressure andtemperature therefore strongly influence the line width.

In atmospheric trace gas measurements the temperature,pressure and major constituents are usually well known. Thismakes it relatively easy to calibrate or correct the measure-ment for line broadening effects. In an industrial context,however, there may be large variations in operating condi-tions both from one installation to another and within oneand the same installation. When measuring emissions froma stack, the pressure is usually close to 1 bar, but the tempera-ture may vary typically from100350 C. For some types of

gas abatement systems it is of great interest to measure ata point even closer to the process where the temperature canbe greater than 700 C. In the chemical the industry it is ofinterest to measure at pressures ranging from a small fractionof a bar to several bars. Temperature and pressure are easy tomeasure and may be continuously compensated for.

Collision broadening depends not only on pressure butalso on the collision cross section of the molecules. Variationsin gas composition may therefore influence the line width,and hence give a strong influence on the second-harmonicsignal. It is for this reason that the measurement might not betruly interference free even though there is no direct interfer-ence from absorption lines from other gases. We will refer tothis type of interference as line broadening interference.

In most industrial applications, line broadening effectsmust be taken into account in order to achieve the requiredmeasurement accuracy. Three types of gases that occur fre-quently and have a large influence on the line width are H2O,CO2 and hydro-carbons such as CH4. The concentration ofH2Ofrom waste incinerators and boilers may typically varyfrom 10 to 25 vol. %, but in some cases it can be as highas 40 vol. %. In Fig. 3 we show an example of the influenceof H2O concentration on the peak of the second-harmonicsignal for HCl. It is clear that such a large dependence hasto be compensated for. In principle, one can compensateby measuring curves similar to that in Fig. 3 for all rele-vant gases in the process, but since the influence will depend

on pressure, temperature and instrument-specific parameterssuch as modulation index, it is difficult to obtain good re-sults this way. In addition, the need to know concentrationsand fluctuations of other gases makes this a very unattractivesolution.

Alternatively, since the laser frequency is scanned overthe absorption line, one can measure the line width. Theprice for this, however, is a significant increase in noise inthe measured gas concentration. This can be explained bythe following model. We assume that the absorption lineis collision broadened (Lorentzian). A theoretical second-harmonic signal,Hth, can then be found [11]. Hthdepends ongas concentration, g, line width, , and frequency modula-

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lSignal,normalised

H2O conc. (%vol.)

MeasuredTheor. fit

Fig. 3.Measured second-harmonic peak signal of an HCl line as functionof H2O concentration at 270 C. The signal has been normalised to 1 forzero H2O concentration. Note that the curve is not universal but will de-pend on gas temperature and pressure and on modulation index and otherinstrument-specific parameters

tion index,m[defined in (1)];Hth(f; g, ,m)= gH(f; ,m),where H(f; ,m)is the second-harmonic signal normalised

to a fixed gas concentration. We further assume that the meas-ured second-harmonic signal has some additive white noise:Hm(f) = Hth(f)+n(f), where the noise, n(f), is a stochas-tic variable with zero mean value and constant variance for allfrequencies, f. If we know the correct line width we may findthe concentration by varyinggto obtain a best (least-squares)fit between Hth(f)and Hm(f)for all frequencies. Because ofthe noise,gis a stochastic variable with uncertainty (standarddeviation)g. If we do not know the line width we have tovary bothgand simultaneously, which leads to an increaseduncertainty,g, for the measured gas concentration. We havefound that the two uncertainties are related as

g= B(m)g, (2)

where B(m)is a dimensionless function depending only onthe frequency modulation index,m.B(m)has been found nu-merically and is shown in Fig. 4. For optimal modulation in-dex,mo = 2.2, we have B(mo) = 2.9. Hence, the uncertaintyof the measured concentration increases by nearly a factor of3 when the line width is measured and compensated for. Fromthe figure we see that the improvement that can be obtainedby selecting a different modulation index is small. Notice,however, that B(m)is independent of gas concentration andthe relative uncertainty in measured concentration will there-fore decrease as g/gfor increasing concentration.

The measurement situation is shown in Fig. 5. Without

measuring the absorption line width we can have a large sys-tematic error with a small stochastic error. Correcting thesystematic error by measuring the line width will be at theexpense of an increase in stochastic error.

The reduction in measurement precision can be under-stood from the fact that the derivative of the second-harmonicsignal with respect to line width, dH(f; , m())/ d, hasa similar shape as the second-harmonic signal itself. This hasthe effect that a small reduction in line width can be misin-terpreted (depending on noise level) as a small increase ofgas concentration, or vice versa; i.e. the two fitting param-eters are correlated. Most of the information about the linewidth is in the tails of the second-harmonic signal, which im-


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0.5 1 1.5 2 2.5 3 3.5 4

Modulation index, m

0

0.5

1

1.5

2

2.5

3

3.5

4

B(m)

Fig.4. The ratio between uncertainty in gas concentration with line widthmeasurement and without, B(m), plotted as a function of modulation index

Fig.5. Illustration of measurement errors with and without measurementof line width. The measured gas concentration without compensating forvariations in line width (lower solid line) may have a systematic error.The correct concentration (upper solid line) can be obtained by measuringand compensating for variations in line width, but has a larger stochasticuncertainty

plies that measurement of line width is sensitive to the tailsof nearby absorption lines. In practice there also seems to bean increased sensitivity to RAM and etalon noise comparedto what is predicted in the above result. This is most likelyto be because these noise sources are non-white and there-fore violate the assumptions in the calculation. Such noise cangive slow fluctuations in the measured gas concentration ona time scale of several minutes to hours. Noise in this fre-quency range is also present when the line width is not beingmeasured. It is the worst possible frequency range becausenoise of higher frequencies can be averaged out and noise

of lower frequencies (drift) can be compensated by recali-bration, zero offset measurement and similar techniques. Forthis reason industrial instruments often have lower sensitivitythan when the same technique is applied in a controlled labenvironment.

Although the line width compensation results in a largeruncertainty of the measured concentration, the detection limitis sufficiently good for many of the most important gases andapplications. Thus automatic compensation of line broaden-ing effects should be done by measuring the line width in ap-plications with large variations in gas composition. This is thetypical situation in incinerators, whereas power plants usuallyuse only one type of fuel and one may successfully compen-

sate for line broadening effects with fixed preset values for theparameters influencing the line width.

3 Diode lasers for spectroscopic applications

The development of semiconductor diode lasers in the nearinfrared has been spurred by the development of CD players(0.78m) and fibre optic communication (1.3m,1.55m).As technology has improved, lasers have been developed fornew applications such as pumping of solid-state (0.808m)and fibre (0.98m) lasers. In addition to wavelength, otherimportant laser parameters are mode stability, in order to ob-tain single-frequency operation, current tuneability, and fre-quency drift. The FabryPerot (FP) type lasers are unreliablewith respect to mode jumping, therefore other types of laserssuch as distributed feedback (DFB), distributed Bragg reflec-tor (DBR) and vertical cavity surface emitting lasers (VC-SEL) have been developed. Low current tuneability has to becompensated by higher current modulation which generallyincrease the RAM noise. It is beyond the scope of this pa-per to describe lasers in detail and we refer the reader to the

literature (e.g. [14, 15]).Long-term frequency stability is one of the most import-ant parameters for diode lasers used in industrial gas moni-tors. Most lasers have some frequency drift and Fig. 6 showssome long-term measurements for three lasers at760 nm. Wehave observed a large spread in drift, both for lasers of thesame kind, and for different types of lasers, but it is gen-erally smaller in lasers at longer wavelengths. The drift isusually smaller after several months of operation, but a driftas shown in Fig. 6 may cause instrument failure during sucha period. The drift can be overcome by adjusting the lasertemperature such that the absorption line is always in thecentre of the frequency scan. However, some applicationsnormally have zero concentration and therefore no line totrack (e.g. measurement ofO2 for explosion safety). In suchcases one can introduce a gas cell temporarily or perma-nently in the measurement path, or in a split-off beam witha separate detector. However, such solutions will increasethe optical noise and/or mechanical/electronical complex-ity. The monitors described in this paper use lasers which

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Fig. 6.Long-term frequency drift of some lasers at 760 nm. The lasers havepassed burn-in tests prior to these measurements


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Table 1.Typical wavelengths and detection limits for some gases measuredwith the technique described in this paper. The detection limit for O2 is notlimited by electronic or optical noise, but by uncertainty due to air in thereceiver and transmitter

Gas Laser type Wavelength Detection limit/m /ppm m

O2 FP, DFB, VCSEL 0.764, 0.760 1000HF DFB 1.28, 1.30 0.03

NH3 DFB, DBR 1.51 0.2CO DFB 1.56 20H2S DFB 1.57 5HCl DFB 1.74 0.1NO DFB 1.81 5

have been selected for low drift, but the testing is a costlyand time consuming process. It is difficult to specify a max-imum acceptable drift since this is coupled to the actualmechanical/optical solution. A large, persistent frequencydrift is often accompanied by drift in other parameters such asoutput power and to some extent current tuneability. Such ef-fects may result in drift in the measured gas concentration and

short life time of the laser.Most of the diode lasers mentioned above are used inhigh-volume products. Diode lasers for gas monitors are ex-pected to be required in low volumes only [16], and un-til recently only gases with strong absorption lines near theabove-mentionedwavelengths have been possible to measure.However, the advances in the manufacturing of diode lasersin recent years have made it possible to make lasers at otherwavelengths, and several of the lasers used in the monitorsdescribed in this paper have been specially designed for thepurpose of gas monitoring.

Table 1 lists some of the types of lasers and wavelengthswe have used for some gases, and the corresponding detectionlimits obtained. Note that the detection limit forO2comes notfrom electronic or optical noise, but from uncertainty due toair in the receiver and transmitter. More extensive lists of sug-gested wavelengths for several gases can be found in [4, 17].

4 Industrial applications of TDLAS

Due to the short response time, industrial TDLAS monitorsare ideal as process control tools in processes requiring a fastresponse, and they are also well suited for continuous emis-sion monitoring of gases such as HCl and HF where themaximum permissible emission levels are in the low ppmrange. Some typical examples are presented below, followedby measurements from real installations.

4.1 Process control

O2measurement. The use of TDLAS offers for the first timea reliable and accurate method for measurement of oxygenby using spectroscopic techniques. While theO2monitor canbe used in most of the traditional applications for O2 meas-urements in combustion processes, the most challenging ap-plications are found in process control and safety systems inchemical and petrochemical plants (i.e. flare gas explosioncontrol), as well as safety systems in hazardous waste andsolvents destruction plants.

The gas temperature is typically in the range 150 to300 C and the pressure can vary from a few millibar to35 bar. For these processes the gas matrix mainly consistsof hydrocarbons (25%80%) with the rest being H2O, COand CO2 plus a low concentration ofO2 (typically from 1%to2%).

Another process control application which has now be-come feasible is the use of the O2 monitor to improvecombustion control in high-temperature furnaces, such assteel and cement ovens. Performing high-temperature COmeasurements at the same location will improve the com-bustion control even further. The temperature is typically9001200 C and the gas matrix will in this case consist ofO2,CO,CO2,N2,NOandH2O.

NH3 measuerement. At the moment there are no envi-ronmental emission standards for ammonia emissions fromindustrial processes. Ammonia is, however, widely used inpower plants and incinerators to reduce NOx emissions. Twofrequently used techniques are selective catalytic reduction(SCR) and selective non-catalytic reduction (SNCR) in whichNH3is added to the flue gas. For such installations, monitor-ing NH3 is valuable for optimising the consumption of the

gas, as well as reducing corrosion and environmental impactfrom excessive use. TheNH3slippage level should preferablybe well below5 ppmand should be measured as close as pos-sible to the injection point. Typical conditions at this stage inthe process are gas temperatures of250 350 C, dust levelsof1020 g/m3 and a gas mix of10%20%CO2, 10%30%H2O, 3%5% O2, with the rest being N2 and ppm levels ofSOx ,NOxand NH3.

COmeasurement. Accurate and fast measurements ofCOas close as possible to the process may be used for effect-ive combustion control in high-temperature processes used insteel and cement ovens. The use of TDLAS offers for the firsttime a reliable and accurate method for this type of processcontrol, by measuring the CO level at temperatures above1000 C.

H2S measurement. H2S is a hazardous gas normallypresent in refineries and other petrochemical plants, whichhave facilities for reduction/destruction of it. The gas mixtypically includesH2,O2,N2,CH4,CO2and H2O, and mon-itoring H2S on a continuous basis is an efficient tool forprocess optimisation.

4.2 Emission monitoring

The most important emission gases believed to have an im-pact on the environment are related to combustion processes.In an industrial context, emissions usually come from boil-

ers (power plants), chemical industry, waste incinerators andfurnaces such as aluminium smelters and steel and glass fur-naces. Other significant sources of emission are cars, busesand other motor vehicles.

Continuous monitoring of gas emissions from indus-trial processes has traditionally been limited to measurementof carbon monoxide (CO) nitrogen oxides (NO and NO2)and sulphur dioxide (SO2), and the techniques for con-tinuous measurements are well established. Limitations inperformance/detection levels of the equipment available forcontinuous measurements of gases such as HF,HClandNH3has in practice prevented continuous monitoring for report-ing of emissions. Standard practice has been sampling tests


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using wet chemical analyses. Earlier in this paper we men-tioned several spectroscopic methods which can be used fora large number of gases. However, for the gases that are par-ticularly difficult to measure, TDLAS offers a new and uniquemethod. Some examples are presented below.

HCl measurement.HClis an emission depending mostlyon the content of chlorine in the fuel. Experience from domes-tic waste incinerators [18] has shown that approximately 90%of the Cl in the fuel will end up as HCl in the flue gas. Themaximum permissible emission level ofHCl from industrialand domestic waste incinerators in Europe is now specified tonot exceed10mg/m3 (daily mean values). Only a few typesof instrumentation can meet this requirement, the TDLASbeing one of these, with a detection limit of approximately0.1 ppm mfor the monitors described in this paper.

HFmeasurement. HF emissions come primarily from alu-minium plants, glass works, tile manufacturers, incineratorsand alkylation plants. For incinerators, typically 10% of thefluorine in the fuel will end up as HF in the flue gas [18].The maximum permissible level is typically1 to 10mg/m3,depending on the process and application. Until recentlyHFhas been a gas which has been considered impossible to

measure on a continuous basis at these low levels due to thelack of suitable instrumentation. The use of TDLAS has nowchanged this asHFproves to be one of the most suitable gasesfor the TDLAS technique with a detection limit of30 ppbfora1-moptical path length.

4.3 Measurement of O2, COand HClfrom a wasteincinerator

In Fig. 7 we show some simultaneous time-series measure-ments from the stack of a 27-MWcirculating fluidized bed(CFB) combined boiler and incinerator at a paper mill. Theboiler produces a maximum of40 tonsof steam per hour at210 Cand 20 bar. The steam is used in the paper mill pro-duction and may vary rapidly from 20% to 100% capacity.The boiler is designed to burn municipal waste, plastic, wood,paper, waste oil and coal. These fuels have greatly differentcalorific values and this puts high demands on both the pro-cess control and the abatement system.

After the combustion in the CFB reactor the flue gaspasses through two cyclones and the boiler before it iscleaned in a multi-cyclone and an electro-scrubber. The fluegas is then let out through a 40-m-high stack, where the gasmonitors are located.

O2and COconcentrations in the flue gas are the most im-portant gases for monitoring combustion efficiency. Completeoxidation can in practice be obtained only with an exces-

sive amount of air. Too much air, however, can cool downthe combustion and increase the amount of CO in the fluegas. There exists an optimal amount of air. Figure 7a,b showsthe O2 and CO concentrations for varying efficiency of thecombustion. When the O2 concentration drops below ap-proximately 5.5 vol. %theCOconcentration peaks sharply tovalues as high as 4000 mg/Nm3, and when the O2 concen-tration is approximately6 vol. % to 7 vol. % the CO concen-tration is close to 50mg/Nm3. The fluctuations in COcon-centrations are extremely fast and large. In Fig. 7 the concen-trations have been averaged over 1 min and we see changesfrom 50 to 2300 mg/Nm3 from one sample to the next. Thefastest response of theCO monitor is 15 s, and we have seen

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.(mg/Nm3)

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Fig. 7.Concentrations in flue gas from a waste incinerator measured overa period of5 hours, where each sample is averaged over 60 seconds. TheCO concentration peaks abruptly when the O2 concentration drops belowapproximately 5.5 vol. % The HCl concentrations do not show a similarcorrelation

cases where the concentration has changed from100 to9000to100 mg/Nm3 in three successive samples at this samplingrate. It is difficult to understand how conditions can changethis fast in such a large furnace, but considering a gas flowof up to 20 m3/s, we realise that all the gas in the furnacehas been replaced in less than15 s. Such fast response meas-urements can therefore give valuable information that may beused in the process control of the furnace.

HCl was also measured in the flue gas from this wasteincinerator. The concentration of HCl depends mostly onthe content of chlorine in the fuel and is not expected toshow much correlation with process-control-related parame-ters. This is consistent with the measurements shown in Fig. 7where there is no correlation between the concentration ofHCland the other two gases. The content of chlorine in mu-

nicipal waste can vary greatly, which is also seen in thisfigure. At the time of measurement the incinerator had noabatement system for removingHCl.

HFwas also measured in the stack of the waste incinera-tor. In the analysed period the concentrations were generallyvery low and rarely above the detection limit, which was ap-proximately0.05mg/Nm3.

4.4 Measurement of HFin a primary aluminium smelter

Aluminium is produced by electrolysis of alumina (Al2O3)and cryolite (a blend ofNaF, AlF3 and CaF2). The purpose


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of adding cryolite is to lower the melting point of alumina.HFgas is produced as a byproduct in the electrolysis and ledaway with air through vent hoods at a concentration of typic-ally300 mg/m3. The flue gas is cleaned with a dry scrubberwhich uses alumina to remove theHF. According to emissionrequirements the efficiency of the dry scrubber has to be bet-ter than 99.7%. Such efficiency requires thorough control ofgas flow, temperature, humidity, etc. Figure 8 shows the HFconcentration measured after the dry scrubber. The fluctua-tions seen in the measurement are due to the process controlof smelter and dry scrubber and have a period of26 min. Suchfluctuations would be invisible by traditional half-hour aver-aging measurements.

4.5 Measurement of NH3 at a coal-fired power plant withSCR abatement system

The460-MWpower plant is designed to burn pulverised coalwith a content of2.5% sulphur. The plant is equipped witha conventional limestone flue gas desulphurization processfor SO2, and ammonia-based SCR for NOx control. NH3 is

measured at the exit of the SCR reactor where the temperatureis330360 C. Typical flue gas concentrations are 11 vol. %to 15.5 vol. % CO2, 7.5 vol. % to 10 vol. % H2O, 3.9vol. %O2,600800 ppm SO2,250 ppm NOxand05 ppm NH3(ex-pected). The dust concentration is typically around 10 g/m3,and with an optical path length of2.5 mthis gives a trans-mission ranging from a few percent up to 40% at maximum.During the period of the measurements presented in Fig. 9the plant was burning 1.5% subbituminous coal. As we cansee from Fig. 9 the concentration ofNH3 was up to 10 ppm,twice that expected. Such concentrations will contaminate thedesulphurization byproduct material (gypsum) and reduce theSCR catalyser lifetime. We further see a large correlation withpower production. This is a clear indication that the injection

of NH3 is not optimised for the varying load of the powerplant.

5 Summary and Conclusions

Instruments for industrial applications have to be easy touse, require little maintenance and be reliable. For these rea-sons, cryogenically cooled TDLAS instrumentation operating

0

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0.7

0 5 10 15 20 25 30

HFconc.

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)

Time (hours)

Fig.8. HF concentration in flue gas from an aluminium smelter measuredafter the dry scrubber. The detection limit is 0.015 mg/m3 with a measure-ment length of2 m

Fig.9a,b.Measurements ofNH3 in flue gas from a coal fired power plantwith SCR abatement system. The NH3 concentration ashows a clearcorre-lation with the power production b of the plant

in the middle infrared with lead-salt lasers have not foundwidespread use in industry. Although the absorption lines areweaker in the near infrared than in the middle infrared, thesimplicity that follows from operating laser and detectors atroom temperature makes such instruments more attractive forindustrial applications. Gas monitors based on TDLAS withhigh-frequency WMS and second-harmonic detection havebeen used in several applications with large variations in op-erating conditions and requirements to detection sensitivity.The monitors have to be flexible with respect to gas tempera-ture, pressure, gas composition and optical transmission. Forseveral applications, in particular flue gas measurements, themonitors have to work in a high-dust atmosphere giving anoptical transmission down to 1% or lower. High temperatureapplications ( 7001200 C) are also of great interest formany gases, such as O2,COandNH3.

The large variations in operating conditions impel themonitors to be flexible and versatile. One important factor inthis respect is to compensate for line broadening interferencein applications with large variations in the concentration ofsome gases such as H2O, CO2 and hydrocarbons. We havefound that measuring the line width by fitting a theoreticalsecond-harmonic signal will increase the stochastic noise inthe measured concentration by a factor of3 or more. In spite

of this, a sufficiently high measurement precision has beenobtained for many of the most important gases and appli-cations. Detection sensitivities can be further improved withmultipass cells, but this requires an extractive system, whichhas a slower response and potential problems related to filter-ing and preconditioning of the gas.

To keep high reliability with low production costs itis essential to use highly reliable laser diodes with lowdrift in frequency. Lasers with high current tuneability gen-erally have lower RAM-induced noise than lasers withlow tuneability. As production technology for diode lasersimproves, problems related to such matters are expectedto be reduced, and development of lasers for new wave-


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lengths will make it possible to measure new gases in newapplications.

The great variety in the needs for gas monitoring,and acceptance of the TDLAS measurement technique inthe process industry, is generating an increasing demandfor TDLAS-based gas monitors. In this article we havedemonstrated that in-situ, open path measurements in thenear infrared using TDLAS can be done with the accu-racy and reliability required for industry in a number ofdifferent applications.

References

1. H.I. Schiff, G.I. Mackay, J. Bechara: The use of tunable diode laserabsorption spectroscopy for atmospheric measurements, In ChemicalAnalysis Series, Vol. 127 (Wiley, New York 1994)

2. H.I. Schiff (Ed.): Measurement of Atmospheric Gases, SPIE 1433(1991)

3. J. Ye, L.-S. Ma, J.L. Hall: J. Opt. Soc. Am.B 15, 6 (1998)4. M. Feher, P.A. Martin: Spectrochim. Acta, Part A 51, 1579 (1995)5. J. Reid, D. Labrie: Appl. Phys. B 26, 203 (1981)6. J.A. Silver: Appl. Opt.31, 707 (1992)

7. X. Zhu, D.T. Cassidy: J. Opt. Soc. Am. B 14, 1945 (1997)8. L.S. Rothman, R.R. Gamache, R.H. Tipping, C.P. Rinsland, M.A.H.

Smith, D. Chris, V. Benner, M. Devi, J.-M. Flaud, C. Camy-Peyret,A. Perrin, A. Goldman, S.T. Massie, L.R. Brown, R.A. Toth: J. Quant.Spectrosc. Radiat. Transfer 48, 469 (1992)

9. D.S. Bomse, A.C. Stanton, J.A. Silver: Appl. Opt.31, 718 (1992)10. G.W. Sachse, G.F. Hill, J.M. Hoell: Proc. Soc. Photo-Opt. Instrum.

Eng. 438, 99 (1983)11. R. Arndt: J. Appl. Phys. 36, 2522 (1965)12. B.H. Armstrong, R.W. Nicholls: Emission, Absorption and Transfer of

Radiation in Heated Atmospheres(Pergamon Press, Oxford 1972)13. F.G. Smith: Atmospheric Propagation of Radiation The Infrared andElectro-Optical Systems Handbook, Vol. 2 (SPIE, Bellingham, DC1993)

14. O. Svelto:Principles of Lasers2nd ed. (Plenum, New York 1989)15. M. Ohtsu: Highly Coherent Semiconductor Lasers (Artech House,

London 1992)16. M. Zchbauer: Tuneable Diode Laser Gas Analysis: Thoughts on

a possible market, VDI Berichte 1366, Freiburg (1998) pp. 12913417. P. Werle: High sensitivity Gas Analysis by mid and near Infrared

Diode Lasers, VDI Berichte 1366, Freiburg (1998) pp. 116; Spec-trochim. Acta A 54, 197 (1998)

18. Forbrenningsanlegg veiledning for saksbehandlere (Combustionplants guidelines for environmental inspectors), State Pollution Con-trol Authority, Norway, (1995) (in Norwegian) and refs. therein

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