APPLYING THE RELATIONSHIP BETWEEN THE AMOUNT OF CO 2 AND 0 2 IN FLUE GASES TO REDUCE THE NO EMISSION MEASUREMENT UNCERTAINTY

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1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St., New York, N.Y GT-235 S The Society shall not be responsible for statements or opinions advanced to papers or discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is published in an ASME Journal. Authorization to photocopy for internal or personal use is granted to libraries and other users registered with the Copyright Clearance Center (CCC) provided $3/article or $4/page is paid to CCC, 222 Rosewood Dr., Danvers, MA Requests for special permission or bulk reproduction should be addressed to the ASME Technical Publishing Department. Copyright by ASME AD Rights Reserved Printed in U.S.A. APPLYING THE RELATIONSHIP BETWEEN THE AMOUNT OF CO 2 AND 0 2 IN FLUE GASES TO REDUCE THE NO EMISSION MEASUREMENT UNCERTAINTY M.L.E. van Oost, Performance Test Engineer Thomassen International b.v., the Netherlands ABSTRACT A method has been found to reduce the high measurement uncertainty of NOx emissions of gas turbines. Calculation of the measurement uncertainty of NO emission measurements shows that the high uncertainty is not only due to the uncertainty of the NO x emission measurement itself but is also due to the correction to 15 % 0 2 in the flue gases. A second, independent method to determine the percentage oxygen in the flue gases is introduced in addition to the direct sampling method. This second method provides additional information resulting in a significant reduction of the measurement uncertainty. The solution found has the advantage that the additionally required measurement equipment is kept to a minimum. Experience with the new method in field tests are excellent. INTRODUCTION The NO x emission levels of gas turbines have to be determined by Thomassen International for various reasons: to test it with a value set by environmental considerations (imposed by governments) to test it with a contract value to compare it with the emission level of another type of gas turbine or combustor design for reasons of research (e.g. which parameters affect the NOx emission level). Since Thomassen International does not have a test stand to test the emissions of gas turbines operating at base load, such tests have to be carried out in the field. In order to compare NO x emission levels, measured at different ambient or load conditions, they are corrected to 15 % 0 2 in the flue gases: NO = NO ref (1) e.mc cm' ,m NOMENCLATURE CO 2, mmeasured percentage carbon dioxide in flue gases CO2,Sto;ch Percentage carbon dioxide in stoichiometric flue gases NOx, m Nitrogen oxide and dioxide, measured [ppmvd] NOx,mc Nitrogen oxide and dioxide, measured and corrected [ppmvd] 0 2,caic. Calculated percentage oxygen in flue gases 2, m Measured percentage oxygen in flue gases 02, fef Reference percentage oxygen in flue gases (=15 % vd) RC Slope of the combustion triangle [-] Greek symbols S Absolute measurement uncertainty T Relative measurement uncertainty In addition to this correction the NO emission can also be corrected to e.g. ISO ambient conditions. However, since the additional uncertainties in the measured ambient conditions are small in comparison to the uncertainty introduced by the correction to 15 % 0 2, it has been omitted from this discussion. In the following the NOx emission measurement principle during field tests is explained, the different factors that contribute to the total measurement uncertainty are discussed and the total measurement uncertainty is calculated. It is demonstrated that the measurement uncertainty of the corrected NO emission is high when both the amount of NO x and the percentage 0 2 in the flue gases are determined by sampling. Usually a low measurement uncertainty is preferred to a high measurement uncertainty. Especially for reasons of research a higher accuracy may lead to a better understanding of the underlying principles. Therefore the different factors are examined more closely in search for a reduction of the measurement uncertainty (uncertainties) of one or more of these factors resulting in a reduction of the Presented at the International Gas Turbine & Aeroengine Congress & Exhibition Stockholm, Sweden June 2 June 5,1998

2 total measurement uncertainty. The search proved worthwhile since the measurement uncertainty of one of the major contributing factors can be reduced. The solution found was applied during field tests. These results are also presented. THE NO EMISSION MEASUREMENT In order to understand which elements determine the measurement uncertainty, the different elements which can be distinguished in the NO emission measurement are described. point is used at a fixed position. To reduce this measurement uncertainty two solutions are available. The first solution consists of carrying out traverse measurements over the exhaust plane in order to obtain a representative sample (while it is assumed that the gas turbine is operating in steady state conditions). The second solution consists of augmenting the number of sample points by measuring with a sampling grid (ISO 9096 recommends 16 sampling points for rectangular shaped exhaust ducts larger than 1.5 m 2). The most information about the flue gas inhomogeneity is obtained by using the first solution. With the second solution, the sampling grid, the most representative sample is obtained during the period of measuring. 2. Analysers Any measuring instrument introduces a measurement uncertainty. The analysers used to determine the amount of NO X and the percentage 0 2 in the flue gas sample are no exception to this rule. P = Pump PMC = Poring n.oo,n C = Cooler I separator CLM = ChamoWminesance CO = Cahbraton gas NDIR = Non Dispersive Intl Red Figure 1. Test set-up for emission measurements during field tests A sample is taken from the flue gases with one or more sample probes (see figure 1). To obtain sufficient gas sample a pump is used to take the flue gas sample. The cooler dries the gas sample before it is fed to the analysers. Part of the sample is fed to a NO. analyser which determines the amount of NO. in the flue gases by means of the chemoluminescence principle. Another part of the sample is fed to an oxygen analyser which determines the amount of oxygen by means of the paramagnetism principle. The excess flue gas sample is blown into the atmosphere. The analyser output signals are recorded with data loggers. To obtain correct values the analysers need calibration. Before every measurement each analyser is calibrated anew with calibration gases. THE NO EMISSION MEASUREMENT UNCERTAINTY By using the measurement method as described in the previous section measurement uncertainties are introduced by: 1. The way in which the sample is taken 2. The analysers used 3. The calibration gases This is explained in more detail in the following. 1. Sampling It is not known whether the emission level is distributed homogeneously over the exhaust plane in which is measured when this has not been investigated in advance of the actual emission tests. This implies that the measurement uncertainty is high when only one sample 3. Calibration gases The calibration gases are also subject to measurement uncertainties. THE NO x EMISSION MEASUREMENT UNCERTAINTY CALCULATION In the previous section the different measurement uncertainties were identified. In this section the measurement uncertainties are quantified and the total NO. emission measurement uncertainty is calculated. The values mentioned in this section originate from an independent laboratory that often assists Thomassen International when carrying out field emission tests. The experience this laboratory has acquired over the years has led to the measurement uncertainty values mentioned below. Furthermore, the laboratory's test equipment is calibrated on a regular basis. The relative measurement uncertainty due to the flue gas inhomogeneity is assumed constant at 3 %. Analysers used by Thomassen International to measure NO X in flue gases have a measurement uncertainty of 1 % of the Full Scale. At 37 ppmvd NO,,', the Full Scale ranges from 0 to 100 ppmvd. This implies that the measurement uncertainty equals 1 ppmvd or 2.7 % relatively. Analysers used to measure the percentage oxygen in flue gases also have a measurement uncertainty of 1 % of Full Scale (which ranges from 0 to 25 vol. % with an expected value of 15 %). The calibration gases introduce a relative measurement uncertainty of 1 %. The total measurement uncertainty is calculated in steps according to the root of the sum of squares: 37 ppmvd NO is the maximum limit allowed by Dutch legislation (=65 g/gj) when natural gas with a Lower Heating Value of 37 MJ/Nm is burned in a gas turbine.

3 z 2 TNOx,me TNOn m + To2,m (2) in which: and z TNOx,m = TNOx,inhom. + T NOx,analyser + TNOx,cal.gas TNOx,,n = = 4.2 % o2,mo TO2, m % (4) which is the relative measurement uncertainty resulting from equation ,m in equation (4) follows from: 32 = 52 +S , m 02,inhom. 02,analyser Oz,eal.gas (5) (3) A reduction in measurement uncertainty cannot be expected from the analysers for two reasons. Firstly, a certain measurement uncertainty forms part of the (chemoluminescence and paramagnetic) principles used. Secondly, the relative measurement uncertainties of the analysers are already low (1 % of Full Scale). 3. The calibration gases A reduction in the measurement uncertainty of the calibration gases is also not expected. Based on the aforementioned, it is concluded that no reduction in measurement uncertainty can be expected by reducing the measurement uncertainty of any of the instruments used or of the method of sampling. To yield a reduction in measurement uncertainty additional methods have to be introduced. According to VDINDE 2040 a weighted average value and a weighted measurement uncertainty follow from respectively: S^ m = = 0.54 vol % Substituting this result into (4) yields: m 100%=9.0% and Yi IL s yaverage 1 s2 + S2, 2 (6) and verage = 1 1 (7) TNOcorrected = = 9.9 s z Vc2 + Vc2, 2 As stated in the first section the measurement uncertainty is high. REDUCING THE MEASUREMENT UNCERTAINTY To reduce the total measurement uncertainty, the measurement uncertainty of one or more contributing elements need to be reduced. In the following it is discussed whether a reduction in measurement uncertainty can be accomplished for any of the elements. 1. The flue gas inhomogeneity By means of traverse measurements at less than one hydraulic diameter downstream of the turbine exhaust the emission pattern of a MS6001 (B) gas turbine 2 was determined. Based on empirical calculations, which are not discussed in this paper, it was found that the 3 % value is on the high side. Though an overestimation of the measurement uncertainty is made, for the remainder of this paper this value is not altered to demonstrate more clearly the gain that is achieved at other points. 2. The analysers when one parameter is determined with two different independent methods. According to the latter, the measurement uncertainty drops with 30 %, when one parameter is determined in two independent ways with the same measurement uncertainties. Which alternative measurement methods can be applied to reduce the measurement uncertainty is discussed in the next section. ALTERNATIVE MEASUREMENT METHODS Reducing the measurement uncertainty by introducing a second independent measurement method is only possible when such an alternative measurement method exists. An alternative, but difficult, method to determine the amount of NO emission is presented by Hilton et al. (1997). This method uses infrared spectroscopy. In the future alternative methods for NO X emission may become available in addition to the present method (or even replace the present method when it proves more reliable and accurate). At this time the method of Hilton et al. or other alternative methods are not available for Thomassen International when tests are carried out in the field. 2 ten combustors concentrically mounted around the compressor

4 The gain in accuracy has been found by introducing a second independent method for the detemination of the percentage oxygen in the flue gases. According to Ostwald (Brandt, 1991) a linear relationship exists between the percentage carbon dioxide and oxygen in dry flue gases which is determined by the fuel gas composition (see figure 2). In other words, the percentage oxygen is a function of the percentage CO2 and the fuel gas composition. Applying this function provides additional information thus reducing the measurement uncertainty of the percentage oxygen in the flue gases which results in a lower total NO,, emission measurement uncertainty. % CO2 Measurod % CO2 MEASUREMENT UNCERTAINTY OF THE ALTERNATIVE METHOD Before the new measurement uncertainty of the measured corrected NO. emission can be calculated, the measurement uncertainty of the second independent method has to be calculated. The measurement uncertainty of the indirect method to determine the percentage 0 2 in the flue gases is determined by: 1. the measurement uncertainty of the stoichiometric volume percentage CO2 in the flue gases 2. the measurement uncertainty of the measured percentage CO2 in the flue gases. This is depicted from figure 3. Since the composition of dry air is constant, the point of dry air in the combustion triangle does not introduce any measurement uncertainties. of dry au Measurement uneertae,y n CCk fty a. 002A7 a2 xi we yeses CakubME %6Z m 1Me gases Figure 2. Ostwald's combustion triangle (Brandt, 1991) Measured % Col in M+e gases Maasuroment uncertainty in%co2in0ue gases Ostwald's combustion triangle represents the linear relationship between the percentage 0 2 and CO2 in the flue gases. The equation of the relationship is deduced with the two extreme points of the combustion triangle (figure 2). The first point is the percentage CO 2 in the dry stoichiometric flue gases (which implies that the percentage 0 2 in the flue gases is equal to zero). The second point is the percentage CO 2 and 0 2 of dry air (which implies that no combustion has taken place). Once the combustion triangle equation is known, the percentage 02 in the flue gases is calculated by substituting the measured percentage CO. into the equation. To calculate the percentage 0 2 in the flue gases with the combustion triangle the following parameters have to be determined: 1. The percentage CO 2 in the dry, stoichiometric flue gases 2. The percentage CO 2 and 0 2 in dry air 3. The percentage CO 2 in the flue gases The percentage CO 2 in the dry stoichiometric flue gases is determined with the composition of the stoichiometric flue gases. The composition of the stoichiometric flue gases follows from the fuel gas composition and the dry air composition. It is assumed that the fuel gas is burned completely. The CO and CH Y emission levels which are less than 10 ppmvd demonstrate that this assumption is permitted. The percentages CO 2 and 0 2 in dry air are given by definition and constant. The percentage CO2 in the flue gases is measured directly by sampling. This implies that only an additional analyser needs to be connected to the test set-up shown in figure 1. Meawroment uncertainly 5, cakuhtetl % flue gases Cakuleted % 02 in flue gases e, dry SW Figure 3. Uncertainties in the calculation of the percentage 02 in the flue gases The measurement uncertainty in the stoichiometric percentage CO2 is due to the measurement uncertainty in the fuel gas composition. This uncertainty is calculated with the sum of squares of the measurement uncertainties of each component containing one or more carbon elements: 62o2.sm^ch - SC o2 + Sco + &L + SC2t+s + SCE + etc. (8) The measurement uncertainties of the components follow from table 1 (originating from the Dutch laboratory GASTEC). Table 1. Uncertainty figures of fuel gas composition analysis when carried out by the Dutch laboratory GASTEC Concentration Unit Uncertainty Unit range mol % mol % 1-10 mol % mol % mol % 0.02 relative mol % 0.01 relative The measurement uncertainty in the stoichiometric percentage CO2 is less than 0.1 % relatively. Since it is small, it is neglected in the further measurement uncertainty calculation, which implies that the slope of the line in figure 2 is constant. 2

5 The percentage 0 2 in the flue gases is calculated with the following equation: The average 0 2 measurement uncertainty follows from substituting the measurement uncertainties of the measured and calculated percentages 0 2 into equation (7): CO 2.noich - CO2.m 02,ca1c. = RC (9) In which CO2, m is the measured percentage CO 2 in the flue gases and RC is the slope of the line in figure 2. The measurement uncertainty of the calculated percentage 0 2 is obtained by differentiating equation 9 partially to CO2,m, CO2,$to;ch and RC. Since CO2,sto;cr, and RC are constant, their partial differentials are zero and the measurement uncertainty of the indirect method for the determination of the percentage 0 2 in the flue gases is equal to: z _ RC/ S z 0 z.'ae. co,, sz or C sco2.m Soz.wic. = RC (10) (10) The measurement uncertainty of the percentage CO 2 in the flue gases is determined by the same uncertainties as the other emission measurements, i.e.: Z 2 S coz.m 6c02.inhom. + 6c02,analyser + ScOp,paz (11) In which: the uncertainty due to the flue gas inhomogeneity is estimated at 3 % relatively the uncertainty due to the analyser equals 1 % of Full Scale (which is 0 to 20 vol. %) the uncertainty due to the calibration gases equals 1 % relatively This results in: 6 co,, m = ) ( )2 = 0.23 vol % (12) The percentage CO 2 in the flue gases used for the determination of the measurement uncertainty is 3.7 %, which is a common value for the percentage CO 2 in the flue gases of a gas turbine. By substituting the values of 8c02, m and RC (which is 0.57 for natural gas) into equation 10 the measurement uncertainty for the indirect 0 2 measurement method yields 0.40 vol. % average = 1 11 = 0.32 vol % (7) Substituting this result into equation (4) leads to: T0.32 = O2 average.100%= 5.5 (13) Together with the relative measurement uncertainty of the NO,, emission, the relative measurement uncertainty of the corrected NO,, emission results in: 2 TNOx,mc = = 6.9 % (14) EXPERIENCE OF FIELD TESTS During the contract measurements at Thomassen's projects Schwarze Pumpe and Shell Per+, the percentage 02 and CO 2 were measured. Fuel samples were taken and analysed by GASTEC. The measured and calculated percentage 0 2 in the flue gases are listed in table 2 and 3. These results show that little discrepancy existed between the measured and calculated percentage 0 2. The first table also demonstrates that Ostwald's combustion triangle is not limited to natural gas fuels, because at Schwarze Pumpe the gas turbine is operated on a synthetic fuel gas; i.e. fuel gas with a high hydrogen and carbon monoxide content. The figures listed in table 3 originate from the performance tests on natural gas of the two gas turbines at Shell Per The performance tests on synthetic fuel gas are still outstanding. Table 2. Percentage oxygen in flue gases determined with two independent methods during the contract performance tests at Schwarze Pumpe, Germany (fuel gas with high percentage hydrogen and carbon monoxide) Test %02 %02 %02 number measured calculated average G1 (base G2 (base G3 (base G5 (80 % of base load + steam injection) G6 (base load steam injection) G7 (50 % of base load with steam injection) G8 (50 % of base G9 (80 % of base

6 Table 3. Percentage oxygen in flue gases determined with two independent methods during the contract performance tests at Shell Per + (2 gas turbines tested on natural gas fuel) Test number GT4700 G1 (base load + steam injection) %O measured % 02 calculated % 02 average G4 (base G8 (60 % of base GT4800 G1 (base load steam injection) G4 (base G8 (60 % of base CONCLUSIONS In addition to the direct sampling method to determine the percentage 02 in flue gases, a second independent method can be used. The combination of the two methods yields a reduction in measurement uncertainty of the percentage 02 in the flue gases. This reduction in measurement uncertainty results in a decrease in measurement uncertainty of the corrected NO,, emission measurement from 9.9 to 6.9 %. Field tests demonstrate that the percentages 0 2 determined with both methods lie close to each other. REFERENCES Brandt, F., "Brennstoffe and Verbrennungsrechnung", FDBR- Fachbuchreihe, Vulkan-Verlag. Essen, Hilton, M., Lettington, A.H., Wilson, C.W., "Gas turbine exhaust emissions monitoring using non-intrusive infrared spectroscopy", ASME IGTI Congress paper 97-GT-1 80, Orlando, 1997 ISO9096:1992(E), "Stationary source emissions - Determination of concentration and mass flow rate of particulate material in gas - carrying ducts - Manual gravimetric method", International Standard, June 1992 VDINDE 2040 Part 3, "Calculation principles for measurement of fluid flow using orifice plates, nozzles and venturi tubes. Examples of calculations", VDINDE Richtlinien, May