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Continuous EmissionsMonitoringSystems

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Today, one of the major uncertainties facing plant operators is the nature of futu-re emissions regulations, with possible requirements for higher standards of data– perhaps presented in new ways.

At the same time, it is often recognized that better, more flexible emissionsmonitoring also brings the benefits of closer control and more economical opera-tion.

In this environment, where the advantages of better monitoring are apprecia-ted but the demands of the future are unknown, Opsis becomes an even betterchoice.

Opsis already has a well-proven capability to exceed the world’s toughest legalstandards for data quality, and the flexibility to meet the monitoring requirementsof any operator. Most important of all, it has the speed to allow emissions data tobecome a valuable tool in determining and adjusting plant efficiency.

Some Opsis benefitsFast and flexible. A single Opsis system will measure any gas specified in theoperating software. There are no dedicated sensors, and a system may be re-confi-gured to monitor additional substances at any time. The system’s speed meansthat real-time data is always available.Non-contact measurement. Although Opsis operates directly across a stack orflue, gases never come into contact with the system. Opsis is not affected by cor-rosives or other hostile substances.Non-extractive. Opsis does not involve sample extraction, with its heavy main-tenance requirements and possibilities for chemical change in the substancesmeasured.Low maintenance. Non-contact measurement and non-extractive samplingmean that Opsis has extremely low maintenance requirements.Simple calibrations. The German TÜV has approved the Opsis system for spancalibration once a year. However, the system also provides for automatic calibra-tion at any user-defined interval.Simple report generation. Opsis software allows any report or analysis to begenerated automatically, or ‘on demand’ through a few keystrokes. Statistical pro-grammes allow comprehensive trend analysis and other management routines.Data is easily portable to other systems for integration into wider studies.Comprehensive data logging. Apart from its own data, Opsis will log informa-tion from any other device producing a continuous output – such as flow andtemperature sensors. This may be integrated with Opsis data for truly comprehen-sive emissions information and reports.Logic outputs. An Opsis system can provide logic outputs for interface with wi-der control systems. This allows its real-time information to play a wider part inplant control and efficiency.On-line supervision. All aspects of an Opsis system are accessible via telemetryfrom a remote station. Routine checking of system parameters does not involvesite visits.Worldwide support. Opsis is supported internationally by a worldwide networkof representatives.

Installed in a powerplant burning fossilfuel, this single cross-stack monitor is con-tinuously measuringseven gases: NO, NO2,SO2, NH3, Hg, H2Oand CO2.

The Opsis emissions monitoring system

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Opsis – a fully computerized emissions monitoring system

Opsis measures gases by DOAS(Differential Optical AbsorptionSpectroscopy), using Beer-Lam-bert’s Law.

In an Opsis system, a beam oflight is projected to a receiver,and is then passed to the Opsisanalyser through a fibre optic ca-ble.

For emissions monitoring ap-plications, the light beam – orlight path – crosses the internaldiameter of a stack or flue. Here,each gas will absorb differentparts of the light spectrum in aunique way. This allows theanalyser’s software to detect andmeasure the gases specified bythe system user. Results are thenavailable for real-time display, orfor statistical operations and thegeneration of reports.

Two key Opsis features high-light its reliability and efficiencyfor emissions monitoring.

Firstly, measurements are tak-en from an inherently simple andreliable system – a lightpath. Theonly interactions betweenprocess gases and an Opsis sys-tem take place in the lightpath:no critical parts come into con-tact with potentially aggressivesubstances, and there are none ofthe problems associated withsample extraction.

Secondly, gas detection andmeasurement is a software task,using a high-quality spectrometerunder computer control. This notonly allows simultaneous analy-sis of several gases, withautomatic validation and loggingof data, but also allows the userto update system capabilities withno or minor hardware changes.

Once logged, Opsis databecomes available for display orfor statistical operations using anOpsis emissions computer. Thisallows any form of report to begenerated using a simple, user-friendly system.

A single Opsis analyser willmonitor a number of user-specifi-

ed gases, including mercury, phe-nol, formaldehyde and styrene.

A flexible response to a widerange of applicationsAll Opsis systems are built frommodular components. This allowsthe minimum basic system – a lightpath and a multi-gas analy-ser – to be extended to meet therequirements of complex tasksthat would very often be beyondthe capabilities of other methods.

One analyser will monitor sev-eral separate lightpaths. WhileOpsis adequately repays its costwhen used to monitor three orfour gases with a single lightpath,the standard analyser will sup-

port more than one. This allows,for example, the monitoring of anumber of points in a process.This is an extremely effectivemethod of monitoring such fac-tors as gas scrubber efficiency.

Additional lightpaths may beadded to existing systems at anytime with no redundancy of equipment.

Analysers may be accessed lo-cally or remotely. While manyapplications are contained withina single site, data logged by Opsisanalysers may also be accessedvia telemetry over any distance.Because Opsis will also log othersensor outputs such as tempera-ture and pressure, this allows

The Opsis emitter and receiver units are installed on either side of the stack. The emitter projects a beam of light to the receiver. From here, a fibre optic cable carries received light to the analyser, housed in an air conditioned enclosure.The system logs data automatically and sends them to a central computer on demand. Here, they are available for a wide range of display and statistical functions. In this instance, they also form the basis of the plant’s environmental emissions charges.

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An Opsis cross-stack monitoring system includes:■ At least one emitter with xenon lamp and power supply.■ At least one receiver which captures light from the emitter.■ A fibre optic cable which takes light from the receiver to the analyser.■ An analyser, which is the system’s central unit. Here, received light is analysed and measurements evaluated. Data is then available for display or analysis.

Opsis technology is subject to a continual process of development. This, with the modular construction of Opsis systems,means that it is always possible to expand or update an installation without redundancy of viable equipment.

Analysis of more gases, amongothers SO2, NO2, NO, NH3, phenol,formaldehyde, CS2, Hg, styrene

STAT 500 statistical processing

EmVision – calculation andpresentation in real time.

Upgrading of the analyser´scomputer system and storage capacity

Analogue and digital input-output signals. Serial communication

Additional workstations

Measurement in more flues

Logging signals from external sensors

Monitoring path (typically1–10 metres in length)

EmitterReceiver

Power supply

Analyser

Fibre optic cable

Automatic calibrations

emissions data and plant perfor-mance to be gathered, analysedand compared on a multiple sitebasis from anywhere in the world.

In this respect, the Opsis emis-sions computer becomes a com-munications tool that allows re-mote monitoring of any numberof Opsis systems, uploading dataat will.

Around the world, some typicalapplications of the Opsis emis-sions monitoring system include:■ Power plants, where Opsis isable to monitor stack emissionsfrom all commonly-used fuels.With response times of only a fewseconds, Opsis is used for processcontrol applications. Installationsat SCR and SNCR are two exam-ples. Monitoring emissions reach-ing the environment is anothercommonly used application. ■ Solid waste incinerators.A common problem is the ag-gressive environment created byHCl from vegetable and plasticwaste. Here, Opsis’ non-contactmeasurement allows continuousmonitoring of the combustiongases, including Hg and HCl, withno affect from acid attack.

■ Cement plants, where themonitoring of NOx and NH3 aretypical requirements.■ Chemical plants, includingplastics producers. Opsis multi-analytical capability allows con-tinuous monitoring of phenoland formaldehyde, together withother gases selected by particularoperators.■ Aluminium smelters. Onceagain, Opsis’ multi-analytical ca-pability has allowed customers toinclude HF monitoring alongwith a capability for SO2, NO2

and other pollutants.

Opsis is also for environmen-tal monitoring Opsis lightpaths are not confinedto monitoring stacks and flues.The same technology is widelyused to monitor environmentalair quality, using a lightpath thatmay be hundreds of metres long.

This application of Opsis isused by many Local Authoritiesand others concerned with envi-ronmental health. These applica-tions also have important bene-fits for industrial users.

Fugitive emissions. Where haz-ardous substances are stored or

piped within a site, a long-light-path Opsis system is one of thebest available methods of detect-ing changes in atmospheric con-centrations.

Fenceline monitoring. In thesame way, Opsis is an ideal meth-od of gauging a plant’s impact onthe surrounding environment.

Where industrial sites areoccupied by more than one plantoperator, a combination of Opsis’stack emissions data and air qual-ity data may free a user fromblame for the release of toxicsubstances.

Production area monitoring.If staff are working in large enclosed spaces where there is apossibility of toxic substancesescaping, an Opsis open-path sys-tem will provide a high level ofreal-time monitoring. Apart fromthe safety of those involved,Opsis is far more effective thansmall exposure monitors carriedby individual personnel. Thisapplication is in successful usewith an aluminium smelter,where Opsis lightpaths monitor apot room for HF and other com-pounds.

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the lightpath will typically crossthe internal diameter of a stackor flue. The transmitter lamp ispowered by a separate supply,and received light is sent to theanalyser via a fibre optic cable.

Although all lightpath equip-ment has a similar function, arange aof alternatives is availableto suit different situations. Whilethe simples systems use a singlebeam, double-beam versions may

Data output. Data stored by theanalyser may be output in anumber of ways. The simplestmethod is via data cable to anOpsis emissions computer, whereit may be presented as a real-timedisplay or used to generatereports as required.

Other standard techniques in-clude transmission via modem,and data may be sent as serial oranalogue transmissions.

HardwareThe analyser. The analyser is thecentral unit in every Opsis sys-tem. While the analyser’s basicfunctions are always the same –to detect and measure gases andto log data – its operation may bevaried to meet the particularneeds of each user.

Product overview

The most obvious variable isthe number of gases monitoredby the system. As an example,one system will monitor NO,NO2, NH3, Hg, SO2, CO2, H2O andHCl. The user has complete free-dom to specify from a wide rangeand, unlike other systems, upgra-ding to handle more gases is lar-gely a software task: it does notinvolve installing additional mo-nitoring hardware.

Apart from this, the analyser’soperating characteristics maythemselves be varied. Processcontrol calls for higher speeds,for example. This flexibility, plusthe modular nature of Opsisequipment, allows virtually anysystem to be extended after in-stallation without redundancy ofequipment.

In addition, the analyser willaccept external outputs fromprocess sensors such as tempera-ture and flow monitors. This islogged along with Opsis data,when it becomes available for awide range of displays and statis-tical calculations.

The lightpath. Lightpaths arecreated by projecting light from atransmitter to a receiver. In emis-sions monitoring applications,

System accesories. Standard andspecial system accessories offerthe facilities necessary to help anOpsis system function in a cho-sen environment. The range in-cludes air conditioned cabinetsand other physical aids for analy-ser operation, as well as electricalaccessories such as signal isola-tors and a full range of automaticand bench-operated calibrationequipment.

With hundreds of systems ope-rating around the world in a verywide range of applications, Opsishas the experience to offer itscustomers proven solutions tovirtually any monitoring require-ment.

be installed where one beam isused for monitoring and the ot-her for automatic systemcalibra-tion. This allows Opsis to meetthe conditions of such authoritiesas the US EPA.

There is also equipment forvery wide stacks, where light isreturned by a reflector.

SoftwareOpsis systems generate very highlevels of data, and a range of sof-tware packages are available toallow maximum value to be ex-tracted from this information.

EmVision is a programme spe-cially developed for stack moni-toring applications. EmVision willtypically run on a PC interfacedwith the Opsis analyser, whichmay itself also be gathering dataform other process sensors. HereEmVision provides a wide rangeof environmental monitoring andreporting functions, as well asbeing available to provide logicoutputs for process control. Alsowithin EmVision’s considerablescope are statistical operations,including those quantifying pro-cess economies.

The full Opsis software rangeprovides a wide choise of displayand orher statistical functions, aswell as local or remote communi-cations with external computersystems.

DENOX

DESOX

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systems, associated with coal-fired boilers. Designed by a con-sortium of Lentjes, Lurgi and De-gussa, these include latest tech-nology. The builders wished toinclude an efficient and reliablemonitoring and controllingsystem, and Opsis’ low mainte-nance requirement – even in theaggressive environment of un-processed flue gases – made animportant contribution to the de-cision.

In 1992 the new DESONOXsystem was installed in Unit II ofthe power plant. Here, it bothmonitors and controls the pro-cess. Measurements are taken atthree points; prior to ammoniainjection, after the DENOX stageand again after the DESOX stage,in the purified gas.

Immunity from the effects of ag-gressive gases, for instance SO3,and low maintenance costs weretwo of the main reasons for Opsisbeing chosen to monitor andcontrol the DESONOX process inthe harbour power plant inMünster. Its approval by Germa-ny’s TÜV also had an importantbearing on the decision.

The harbour power station isoperated by Stadtwerke MünsterGmbH as a combined power ge-neration and district heatingplant. Because the distributedheat is a by-product of the powergeneration process, the plant of-fers considerable environmentalbenefits.

The power station has a capaci-ty to generate 77.5 MW ofelectricity plus 210 MW of ther-mal energy. This is derived fromburning about 85% mined coal,6% natural gas and 2% fuel oil,with an operating efficiency ofmore than 80%. Begun in 1977,it is now an important supplier ofboth electricity and domestic hea-ting to the city of Münster.

The plant includes two DESO-NOX flue-gas purification

Opsis unaffected by raw gases from coal combustion

160°C450°C

50°C

>80°C

115°C

Opsis measurement path 1(NO, NO2, SO2)

Opsis measurement path 2(NO, NO2, SO2)

Opsis measurement path 3(NO, NO2, SO2)

Fresh air

CoalFlue gas

Naturalgas

Ammonia(NH3)

Sulphuric acid

1 Steam generator2 Air pre-heater3 Electric precipitator4 Fan5 Gas pre-heater6 Channel combustion7 NH3 injection8 Catalyst tower9 Heat exchanger and H2SO4 prod.

10 Sulphuric acid scrubber11 Wet electric filter12 Flue gas reheater13 Stack

Heat distribution

The DESONOX process

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A Dutch user is Hoechst HollandB.V. at its plant in the city of We-ert where a plastic called Trespa is produced. Here, Opsismonitors phenol and formalde-hyde emissions. The system wasprimarily installed to meet theenvironmental requirements of Dutch law, but it has also proved to be a source of signifi-cant cash savings for the com-pany.

In the Weert plant the Opsissystem monitors four stacks withseparate lightpaths and collectsflowmeter data via a logger, al-lowing emissions conversions ofmg/m3 to kg/h to be carried out.The system triggers alarms if

emissions exceed preset levels,and is also linked to temperaturecontrol systems.

Speaking of his company’s cho-ice of an Opsis system, Hoechst’sMr Renette said the companyidentified two alternatives; Opsisor a gas chromatograph with hea-ted sample lines. In his ownwords: ‘After testing the two al-ternatives in parallel, we foundOpsis had an interesting mainte-nance-saving feature – no main-tenance at all! – excepting, ofcourse, expected routine replace-ment of parts such as lamps andmirrors. On the other hand, ex-tracting samples from four stacksfor gas chromatography proved

extremely cumbersome and time-consuming.’

Mr Renette also spoke of Opsis’money-saving features for hiscompany: ‘We have reduced la-bour costs, because we avoidtime-consuming sampling andbecause the maintenance costsfor Opsis are more or less negli-gible. And, by controlling ourprocess with Opsis, we have alsoreduced our costs for electricity,gas and water by significantamounts. Finally, because all Op-sis data is automatically stored,we can follow emissions in detail;we can go back through the yearsto evaluate progress wheneverwe need to.’

Chosen for low maintenance costs. Opsis also cuts electricity, gas and water consumption

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Under Swedish law, significantcharges are made for the releaseof NOx into the atmosphere. The-se led SYSAV, the operator of awaste incinerator in Malmö, toinstall an SNCR process.

The SNCR process involves areaction between urea andnitrous oxides, which convertsthe substances into nitrogen, car-bon dioxide and water. Beneficialthough this is, there is also a pos-sible penalty, because incompletereactions can lead to the unwan-ted formation of ammonia.

This factor led to the installa-tion of an Opsis system to moni-

tor the process. Solid waste is aheterogeneous fuel, liable tocause sudden changes in the over-all emission content. Because it isnot possible to predict the con-tent of the fuel, it is not possibleto predict the composition andthe levels of emissions it will pro-duce. However, in Malmö, theclose control over urea dosage,made possible by Opsis, has hel-ped to reduce emissions so effici-ently that the plant’s environ-mental charges have been cut byhalf. While it operates in an ag-gressive area with much dust inthe atmosphere, it consistently

operates to a high standard withnegligible down-time. The reduc-tion of emissions is achieved au-tomatically through direct datalinks between the Opsis systemand the computer controlling theSNCR process. The Opsis systemwill also trigger alarms in theplant control room.

SYSAV also operates anotherOpsis system, and data from thisactually sets the emissions char-ges paid by the operator. Thissystem monitors emissions intothe air of NO, NO2, SO2, NH3,HCl, H2O and Hg. Again, accuratemeasurement is crucial, becausedata gathered here is the basis forreports to the authorities withregard to the maximum permit-ted emissions.

In the Malmö plant, Opsis’EmVision software validates andpresents data in real time. As anexample, it shows NOx emissionsin kg/h. The programme will alsocalculate emissions in mg/MJ, or work within other parametersused to help optimize process ef-ficiency.

EmVision is also a valuable aidfor compiling databases used forinternal or external report gene-ration, on a monthly or yearlycycle.

Opsis cuts environmental charges by half for incinerator operator

System ANO, NO2, NH3

Boiler house

System BNO, NO2, NH3, SO2, Hg, HCl, H2O

Cyclone precipitator Economizer Fan StackLime reactor Trap filter

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The basis of the principle used byOpsis to identify and measureconcentrations of different gasesis scientifically well established:Differential Optical AbsorptionSpectroscopy (DOAS), which isbased on Beer-Lambert’s absorp-tion law. It states the relationshipbetween the quantity of light ab-sorbed and the number of mo-lecules in the lightpath.

Because every type of mol-ecule, every gas, has its own uni-que absorption spectrum proper-ties, or ‘fingerprint’, it is possibleto identify and determine theconcentrations of several differ-ent gases in the lightpath at thesame time.

DOAS is based on transferringa beam of light from a specialsource – a high-pressure xenonlamp – over a chosen path andthen using advanced computercalculations to evaluate and analyse the light losses from

molecular absorption along thepath. The light from the xenonlamp is very intense, and includesboth the visible spectrum andultraviolet and infrared wave-lengths.

The light is captured by a re-ceiver and conducted through anoptical fibre to the analyser. Thefibre allows the analyser to beinstalled away from potentiallyaggressive environments.

The analyser includes a highquality spectrometer, a computerand associated control circuits.The spectrometer splits the lightinto narrow wavelength bandsusing an optical grating.

This can be adjusted so that anoptimum range of wavelengths isdetected.

The light is transformed intoelectrical signals. A narrow slitsweeps past the detector at highspeed, and a large number of in-stantaneous values are built up to

form a picture of the spectrum inthe relevant wavelength range.This scan is repeated a hundredtimes a second, and the registeredspectra are accumulated in thecomputer’s memory while awai-ting evaluation.

Evaluation is carried out forone wavelength range at a time.It works by comparing absorptioncurves.

The absorption spectrum justregistered from the light path iscompared with one calculated bythe computer. The calculatedspectrum consists of a well-bal-anced summation of the refer-ence spectra for the analysis con-cerned.

The computer proceeds by va-rying the size factors for each re-ference spectrum until it reaches the best possible match. From this the different gas con-centrations can be calculatedwith high accuracy.

How the Opsis technique works

Monitor

Rapid ScanningDevice

Spectrometer(Czerny-Turner)

Grating set

Receiver

Monitoring Path

Light Source

Opto-Fibre Grating

Detector

Trigger

A/D Converter

Computer Data Storage

Z

X1

X2

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to give the best match for Z. Thesystem achieves this by very ra-pidly creating a new curve out ofthe sum of the two referencespectra, varying values until thebest correspondence is achieved.

The equation the computeruses can be expressed as C1X1 +C2X2 = Z, where C1 and C2 arethe proportions of each gas.From C1 and C2 it is then possibleto calculate the current concent-rations.

give rise to the gradient on theabsorption curve, result from alarge number of known and un-known factors. Their influencecan be eliminated completely bymathematically matching a curvewhich does not follow the rapidvariations in the spectrum.

1. Once the data has been collect-ed, the raw spectrum is stored inthe computer’s memory.

What happens in the computer

2. First the raw spectrum is com-pared with a zero-gas spectrum.This has previously been register-ed with no absorption gases pre-sent and is used as a system refer-ence.

3. After division by the zero-gasspectrum, the total light absorp-tion between the transmitter andthe receiver is obtained. This re-sult is caused not just by the ga-ses that are present but also bye.g. dust in the flue or dirty op-tics. The task now is to separatethe light absorption of the gasesfrom other influence.

4. To do this, the system takesadvantage of the fact that onlygas molecules will cause rapidvariations in the absorption spec-trum. The slow variations, which

5. After a new division, all thatremains are the rapid variations.For the remaining calculations,the logarithm of the curve is ta-ken, which turns the curve upsi-de down. A differential absorp-tion spectrum has now been ob-tained. This spectrum is a combi-nation of the various gases pre-sent between the transmitter andthe receiver at the moment ofdetection. In the example this iscalled Z.

6–7. The gases that absorb light inthis wavelength range are alreadyknown, and a pre-recorded refe-rence spectrum for each gas isstored in the computer’s memory.In this example there are onlytwo gases, called X1 and X2. Thetask is to determine the propor-tions of X1 and X2 that combine

8. Finally, the result is checked bydetermining the difference be-tween the measured and the cal-culated curves (the shaded area).This means that every measure-ment result can be stated with astandard deviation.

The more reference curves sto-red in the computer’s memory,the more accurate the result ofthe calculation will be. However,even if there should be someunknown interference, i.e. whenthe measurements are affected bya gas whose reference spectrum isnot stored in the computer’s me-mory, the computer neverthelessevaluates the gases it is program-med for.

The influence of the unknowngas is presented as an increase inthe standard deviation in themeasurement result.

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Opsis AB, Box 244, SE-244 02 Furulund, SwedenTelephone Int. +46 46 72 25 00Telefax Int. +46 46 72 25 01E-mail info@opsis.se, URL http://www.opsis.se

The Opsis benefits

Multi-component, multi-path system

Cross-stack measurements

No sample system required

Fast response time

High accuracy

High sensitivity

Easily calibrated

Remote control capabilities

Low maintenance costs

Meets TÜV and EPA specifications

Application-tailored software packages

Several hundred systems installed worldwide