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PROCAL ANALYTICS L TD 5 MAXWELL ROAD WOODSTON PETERBOROUGH PE2 7HU (UK)
1. INTRODUCTION
In situ gas analysis methods are the simplest overall systems in terms of sampling and can achieve accuracy and availability levels that are specified bynational environmental authorities. An ultraviolet (UV) system, the Procal 5000,using diode array spectroscopy is described together with its advantages of highsensitivity to several pollutant gases and low cross sensitivity to water vapour.
Although continuous emission monitors are not normally used to look forunknown gases in stack, this has also become possible with the full spectrumtype described here.
The application of this full spectrum method to measure TRS gases (H2S,dimethyl sulphide, dimethyl disulphide and methyl mercaptan) in the presence ofNO, SO2, NO2 and H2O is discussed.The method is fully verifiable with zeroand span gases in stack on a process.
2. CONTINUOUS EMISSION SOURCES
These range from vents only 50mm diameter through to massive stacks ofpower plants up to 7 metres diameter. Common duct sizes are in the one tofive metres range. These carry waste gases from furnaces, process heaters,chemical plant, steel works etc. to the atmosphere. Temperatures range fromambient temperatures up to, occasionally, 1000oC, although the majority arebelow 400oC. Construction out of fabricated sheet metal is common,particularly on horizontal ducting leading to a vertical stack, which is a locationfor gas analysis instrumentation. Measuring points are often also on the stackitself, usually at a height where mixing has been established if more than oneduct enters the stack. Depending on design, both inside and open-air installa-tions occur.
Although there are usually filters or precipitators before the stack, dust loadingof up to 200 mg/nm3 is typical.
Typical pollutant gases measurable are with this system are:Gas Typical Concentration Ranges Pr ocess
NO 0 - 200ppm Cement kilns
NO 0 - 50ppm Power plant
NO 0 - 20ppm Power plant, gas turbine
SO2 0 - 200ppm Power, cement (coal fired)
NH3 0 - 20ppm Power plant (deNox)
H2S 0 - 20ppm Pulp and paper
TRS 0 - 30ppm Pulp and paper
UV absorbing volatile organic compounds (VOCs) such as aromatics andketones can also be measured.
There are many less common pollutants such as chlorine, ozone and sulphidegases, which can be measured by full spectrum UV analysis.Most of these emissions are covered by legislation. In the UK, this will be the EnvironmentalProtection Act 1990, the EC Large Combustion Plants, Industrial Plants andWaste Incineration Directives and the individual licence to operate the process.In the USA regulation of the process is to 40CFR Part 60 App B. Since regulations, backed by the force of law, are to be complied with, the instrumentation used, whatever its operating principle must satisfy demandingrequirements of availability, maintainability, accuracy, reliability, verification etc.These requirements and the nature of the environment near and in stacks haveprofoundly influenced design of instrumentation so that it is very different fromlaboratory instrumentation that may well incorporate similar operating principles.
The measurement of Total Reduced Sulphur, or TRS, is required because of theoffensive smell that these gases can give downwind of plants, mainly Kraft pulpprocesses, that emit it. Inside the plant itself, dangerous concentrations arepossible. Great progress has been made in the design of the process to reduceTRS emissions, resulting in lower measuring ranges being required.TRS is generally considered to consist of:
Hydrogen sulphide H2S typically over 80%Dimethyl sulphide CH3-S-CH3Dimethyl disulphide CH3-S—S-CH3Methyl mercaptan CH3-S-H
The limits set by environmental authorities for TRS are:
Emission limit Range of analyser Max calibration error
US EPA 5 ppm 0 to 30 ppm 1.5 ppm
8 ppm 0 to 30 ppm 1.5 ppm
UK Env A site specific, in mg/nm3
Aus EPA site specific, in mg/nm3
Can EPA similar to USA 0 to 30 ppm 1.5 ppm
The analysers are often required to display the emission levels under referenceconditions. An example from the Australian EPA requirements for the pulp and
paper industry is:
“Temperature 298 deg K, pressure 101.3 kPa (1 atmosphere), dry gas,7 % oxygen v/v”
The Environment Agency is the UK has a reporting requirement:
“Temperature 298 deg K, pressure 101.3 kPa (1 atmosphere), dry gas”
The US EPA, 40CFR 60 subpart BB states“Dry basis, corrected to 8 % oxygen (recovery furnace)”“Dry basis, corrected to 10 % oxygen (lime kiln)”Reporting under reference conditions is possible with the Procal 5000 analyserwhen combined with signals from an oxygen sensor. Other gases can be measured with a P200LR infrared analyser working on the same data highway.
Several types of spectroscopy have been used on stack, in particular infraredand UV spectroscopy. IR spectroscopy using optical filters and gas filter correla-tion (GFC) measures many gases and is a complementary in situ method to theUV system described here. For sulphide gases it is not the best choice.Other spectroscopic methods such as fluorescence spectroscopy and derivativespectroscopy have been used to a limited extent to measure SO2 and NO2.
4.1 UV Absorption SpectroscopyThe UV wavelength range that can be used in the atmosphere is from 190 nmto 400 nm.The Procal 5000 uses this range to monitor gases, such as chlorine,ozone, sulphur dioxide, nitrogen oxides, hydrogen sulphide, some of which donot have any infrared absorption. A great advantage of ultra-violet spectroscopy is that water vapour or CO2 do not cause serious cross sensitivity problems. Water vapour can still be measured. As this type ofabsorption depends on electronic transitions not all gases show UV absorptionat accessible wavelengths, in particular CO, HCl, and simple hydrocarbons arenot measurable by this method.
Although UV gas spectra have been known for many years, UV gas analysis hasonly become widely used for stack gas analysis since the availability of chemo-metric calibration methods and suitable ways of isolating regions of the UVspectrum in industrial environments. In comparison with infrared spectra, UVspectra of gases are much more intense.Therefore it is well suited to lower lev-els of pollutants that need to be measured.They are distinct in shape for eachgas but they overlap each other much more than gas phase IR spectra.
4.2 Array SpectroscopyThis is a method of obtaining a full spectrum in the UV, visible and nearinfrared. A simple, no continuously moving part spectrometer is used consistingof an entrance slit, a concave diffraction grating and an extended detector con-sisting of linear array of multiple sensors.The spectrum produced is similar tothat of laboratory instruments. The great advantage is that there is no need fora wavelength scanning mechanism and that the wavelengths are measured inparallel. This gives adequate robustness for site use and a combination of rapidresponse and a high signal to noise ratio.
Each gas has its own shape of spectrum although all absorb in the 190 to 230nm region
The principle of measurement is that an UV optical system sends light througha measuring tube that is within the stack and filled with stack gases. The spec-trum of the stack gas is measured and from this the concentration of individualgases is calculated.
Figure 4
Figure 1 Figure 2
Figure 3
Figure 5
5. THE FULL SPECTRUM ANALYSER
5.1 Optical UnitFlexing,vibration, dust and temperature changes are to be expected in an onstack environment.The in-situ principles used in previous and successful fixedwavelength system have been retained but a radically different optical unit basedon array spectrometry is used.A paraboloidal mirror collimates UV light from a long life deuterium arc lampsource and the beam is directed through a beam splitter to the end of thereflector tube assembly and is returned by a retro reflector to the beam split-ter.A lensed optical system is used in the reflector tube assembly and the beamis returned to the beam splitter assembly. A proportion is directed by the second off axis mirror to a spectrometer assembly. This consists of an entranceslit, a concave holographic grating and a diode array detector. A narrow regionof the spectrum is measured on each photodiode. The spectrum is recordedfrequently and logged as a vector of intensities.
A special device corrects for any change in lamp intensity and dark level everyfew minutes.
This system solves the problem of stack mechanical instability and also that ofthe calibration being related to the size of the stack in across stack designs.The advantages and disadvantages of in-situ systems are set out below with adiagram of this unit.
Advantages■ Units can be pre-calibrated prior to installation■ Simple installation on stack mounted flange without stringent alignment
tolerances that are needed in across stack designs.■ Can be calibrated or zeroed in situ by filling the reflector tube.■ Sample is not modified by condensation, but is filtered to exclude the dust■ Compact unit, compared with extractive gas analysis system or a cross
stack gas analyser suited to multi-component gas analysis■ Can be verified on the site by installation operator and pollution
controlling authorities
Disadvantages■ Sample is subject to stack temperature and pressure changes
(overcome by measurement of both and calculation)
5.2 Signal processing systemThis consists of an amplifier suited to the characteristics of the diode arraydetector and this amplifies the signal.Temperature and pressure signals are alsoamplified. Power supplies for the UV lamp and the rest of the instrument arealso provided.The signal from the detector and temperatures and pressure is converted to adigital data and concentration is calculated by the microprocessor.
6. DISPLAY OF CONCENTRATION
The initial concentration is transmitted by an RS485 serial link to an analyser c o n t rol unit.This control unit calculates further and outputs the gas concentration
Measurements are performed in the optical head unit of the initial concentrations, the temperatures and the pressure. From these, either of 2types of computer unit displays concentration, in a separate unit from the optical head largely for reasons of accessibility. In some cases this unit ismounted quite close to the optical head, in other cases it maybe hundreds ofmeters away in a control room. In one form, an industrialised computer runs adedicated programme, control is by just 4 push sealed buttons and output ofconcentration is on a small screen.
If large screen presentation of data, with bar graphs, trends, archives and graphictest data is required, this is available on a normal PC computer in a Windowsenvironment and an example is shown in Figure 7.
A further complication is the need to allow for the effects of pressure and temperature, which are certain to occur in an in situ stack gas analyser.
Corrections based on the gas law are generally valid for pressure changes thatare met with in stack but are seriously inadequate when correcting for theeffects of temperature changes. Further steps allow for effects of temperatureof the optical unit, automatic zero, automatic calibration, and on-site manual calibration / verification.
The analyser control unit also provides storage of concentration data and re-transmission onwards to pollution monitoring or process control computersin digital form, as well as analogue outputs.
Although some features of the individual gases can be seen it is a complexprocess to calibrate the instrument so that it then calculates the concentrationof each gas. This has proved possible with high accuracy and use is made ofchemometric methods. Temperature and pressure affect the data but calibrationmethods can eliminate their effect as well as effects of overlapping absorption inthe UV of each of the gases.
A diagram of a CEM system including the Procal 5000, other analysers and datadisplay is shown in Figure 8.
7.THE MEASUREMENT OF TRS
The absorbance spectra of the TRS constituents are shown below. Hydrogensulphide is considered to be over 80% of the TRS encountered gas, the balancebeing made up by dimethyl sulphide , dimethyl disulphide and methyl mercaptan.These are typical figures only.
Figure 9 Hydrogen sulphide spectrum Figure 10
Figure 11 Dimethyl Sulphide spectrum Figure 12 Dimethyl disulphide spectrum
These spectra differ from each other but they overlap considerably.There arestrong spectra present of sulphur dioxide and oxides of nitrogen withabsorbance levels up 0.5. In addition at short wavelengths there is the effect ofwater vapour. Calibration therefore needs to include each of these gases in thetraining data set, and the interfering gases dominate the spectra.As SO2, NOand NO2 are also pollutants their concentrations are normally measured at thesame time.
Figure 8
Figure 6
Figure 7
8. CASE STUDY:MEASUREMENT OF SO2 AND NO WITH THEARRAY SPECTROSCOPY UV SYSTEM
The specification for the analysis was:
Stack gas, coal firing, temperature 170 deg C
SO2 level: up to 1000 ppm
NO level: up to 1000 ppm
NO2 level up to 50 ppm
Water vapour: up to 20%
Dust loading: after precipators
Environment: inside of stack, dusty.
A Procal 5000 UV system was calibrated using a large number of mixed calibra-tion gas samples and the performance of the calibration was assessed by usingthe CEM, after calibration, to recalculate the concentration of each gas in thecalibration set in turn.Drift tests were run overnight and the errors of less than 2 ppm SO2 and NOresulted, in a version with a 125 mm length measurement cell double passed.
The system was installed on stack in a power station trends related well toknown process changes and expected levels, with parallel measurement froman in-situ infrared CEM installed on the same duct.
A typical trend with a relative concentration scale is shown in Figure 13
H20 NH3 SO2 NO
9. CASE STUDY: MEASUREMENT OF TOTALREDUCED SULPHUR (TRS) WITH THE ARRAYSPECTROSCOPY UV SYSTEM
The specification for the analysis was:
Recovery boiler stack gas, temperature 170 deg C
SO2 level: up to 500 ppm
NO level: up to 400 ppm
NO2 level up to 100ppm
Water vapour: up to 30%
Dust loading: low (after precipitator)
Environment: Inside of plant, temperatures to 40 deg C
A Procal 5000 UV system was calibrated using a large number of mixed calibration gas samples and the performance of the calibration was assessed byusing the CEM, after calibration, to recalculate the concentration of each gas inthe calibration set in turn. Prediction correlation coefficients were between0.85 (H2S) and 0.99 (SO2) over the very wide ranges of concentration.
Drift tests were run overnight and the errors of less than 2 ppm TRS resulted,in a version with measurement cell length of 125mm, a quarter of the fulllength.
The system was installed on a vertical duct in a Kraft pulp mill.Although theactual levels measured are confidential, the trends related well to knownprocess changes and expected level.
A US EPA calibration error (CE) test was run which did a zero and span error
test every 24 hours and a typical log is:
29/10/[email protected] 29/10/[email protected]
Zero conc. 0.4 Zero conc. 1
Zero error% 1.9 Zero error% 4.9
Gas cyl. conc. 18.7 Gas cyl. cone. 18.7
Span conc. 19.0 Span conc. 19.0
Span error% 1.4 Span error% 1.3
Status OK Status OK
10. RECENT CALIBRATION RESULTS WITH TRS
Following production of several models for sale and field trials, the P5000 designincorporated several features which increased stability and signal to noise ratio,so a calibration has been done to assess the lowest ranges of operation possible.
H2S 0 to 20 ppm
Methyl Mercaptan 0 to 5 ppm
Dimethyl sulphide 0 to 15 pm
Dimethyl disulphide 0 to 15 ppm
SO2 0 to 200 ppm
Water Vapour 0 to 25%
The calibration set consisted of 250 gas mixtures made up by mass flowcontrollers, and data analysis was by partial least squares (PLS).The predictionplots (P5000 calculated concentration versus actual concentration) are shownbelow.Figure 14 Prediction Plots from a recent calibration
These indicate that ranges shown were of an adequate quality and in the case ofdimethyl sulphide could be reduced to 0 to 10 ppm.
11. CONCLUSIONS
An array spectroscopy system has been described that can be used for continuous emission monitoring. The measurement of sulphide gases, from the wide range of pollutant gases that can be measured by the system, was considered in detail and the results were shown to meet standards of environmental regulators.
References:1. Hutchinson RJ “Spectroscopy in smoke stacks” 2. Sights “Air monitoring by spectroscopic techniques” Wiley Interscience 19943. C B Daw “Cutting the cost of continuous emission monitoring.” Procal Analytics Ltd 19974. Hutchinson, RJ “Full spectrum insitu gas analysis in continuous emission
monitoring” IFPAC 2000
Figure 1 NO Spectra2 SO2 Spectra3 NO2 Spectra4 NH3 Spectra5 Mixed gas spectrum6 The Full Spectrum Analyser7 ACW Screen Presentation 8 The Procal P5000 System9 Hydrogen Sulphide spectrum10 Methyl mercaptan spectrum11 Dimethyl Sulphide spectrum12 Dimethyl Disulphide spectrum13 Trend Display (On site results)14 Prediction plots from TRS calibrations
Figure 13
