The analysis of complex multicomponent

Transcription

1 The analysis of complex multicomponent gas mixtures for emission monitoring 31 July 2017 Test and Measurement Prelly Marebane

2 Fit-for-purpose reference materials for air pollution monitoring in SA and SADC region Have you Ever wondered who is responsible for traceability of measurements in South Africa? Measurements affect our daily lives, from the food we eat, the quality of drinking water, the monitoring of air pollution, health care and even for law enforcement. NMISA provides the confidence that measurements in South Africa are accurate, comparable and internationally accepted. In air pollution monitoring, the provision of reference materials is important to ensure accuracy of results reported for compliance to national air quality ambient air standards as set out by the air quality act of 2004, health and safety limits and global climate change programmes. This accuracy often depends on; o Continuous development of reference methods o Reduction of uncertainty o Matrix reference materials o Cutting edge technology This presentation focuses on the effects of differences in matrix gases in the analysis of stack gas by the NMISA and the implications to industry. It also reports on the findings of another important emission industry; automotive.

3 Production Measurement Improvements Significance and methodology The significance of this study is to better the measurement capability of the laboratory for emission reference gases in an effort to ultimately provide suitable reference gas mixtures representative of emission samples. For consideration; o Matrix effect (78% calibrated MFCs etc. ) o Reactivity of gases o Gravimetric preparation and purity analysis o Cross interferences non dispersive infrared spectroscopy o Simultaneous determination Preparation of compressed gas mixtures by gravimetry - Six components for stack mixtures -five for automotive mixtures Confirm or reject the hypothesis of synergistic effects resulting from differences in matrix gas of sample and calibration mixture Check if matrix matching will result in better measurement of myriad complex mixtures - Sort out other calibration methods Target mass, purity analysis

4 High pure Sulphur dioxide, nitric oxide, propane, carbon monoxide, carbon dioxide, oxygen and nitrogen Methodology and experimental m i = x ip f V cyl M i RTZ f Assume Z=1 1. ISO : Gas Analysis - Preparation of calibration gas mixtures Gravimetric method, (2015) 2. Hodgkinson J., Smith R., Ho W.O., Safell J.R., Tatam R.P., Non-dispersive infrared (NDIR) measurement of carbon dioxide at 4.2 μm in a compact and optically efficient sensor. Sensors and Actuators B (2013)

5 Methodology and experimental Analytical parameters for the analysis of propane in stack gas mixtures using gas chromatography coupled with flame ionisation detector (FID) Same conditions may be used for carbon dioxide analysis by gas chromatography with thermal conductivity detector at 175 deg. Celsius 20 µl sample volume 7 injections Matrix match only

6 Methodology and experimental continued. Calibration techniques offer the ability to compare methods as well in efforts to measure emission gases better. Calibration curve γ = mx + c Single point calibration Matrix match and same mole fraction of gas component Monitor drift Time efficient Residual gas analyser (simultaneous analysis by multi-dimensional chromatography)

7 Results stack gas ppm CO, 10-16% CO 2, ppm NO, 100 ppm SO 2, 1-10 ppm C 3 H 8 in N 2 The accuracy of the measurement for matrix matching = 0.14% and at % Relative expanded uncertainty of 0.5%

8 07 December December December December December 2016 Gravimetric concentration, µmol/mol Results stack gas C 3 H 8 - D Analysis The accuracy of the measurement for matrix matching = 0.4% and at % Relative expanded uncertainty of 0.3% The accuracy of the measurement for matrix matching = 0.4% and at % Relative expanded uncertainty of 1.8%

9 Results stack gas Source of Sulphur dioxide in stack gas -Elemental Sulphur impurities in coal, Hancox et al., (2014). -combustion of conventional fuels Non-dispersive infrared spectroscopy Synergistic effects observed when other stack mixtures where used for calibration Defied the hypothesis SO 2 /N 2 calibration gave better results The accuracy of the measurement for matrix matching = 0.09% and at % Relative expanded uncertainty of 1.5% Therefore, the results show that when calibrating a measuring instrument for stack gas, traditional methods calibration methods may be used. However, this has not been investigated on the use of chromatography (Sulphur chemiluminescent detector)

10 Results automotive Method for analysis Column: Molecular sieve 13X for oxygen, nitrogen and carbon monoxide separation Shin Carbon St for carbon dioxide PLOT column for C 3 H 8 Oven; 30 ᵒC Temp program: initial 30 ᵒC for 2 min, and 85 ᵒC for 3 min 30 ᵒC/min Sample flow: 35 ml/min Injections: 7 Carrier gases: N 2 and H 2 FID and TCD: both at 175 ᵒC Run time: 6 min

11 Results Automotive Traditionally binary reference gas mixtures (one component and nitrogen balance) and multipoint calibration technique have been used to determine unknown mole fractions of samples. However, this method has been found to be associated with synergistic effects. New system exploits the linearity of gas chromatographs to provide a simultaneous and quicker analysis Matrix matching exploits the ability to measure without interferences from matrix effect by matching not only the matrix but mole fraction as well, Component %difference (max) %REU O CO CO C 3 H The capability of the method is illustrated in the table above Target: Emphasis on not only accuracy but the reduction of uncertainty Goal: 0.5% Relative expanded uncertainty, long-term

12 Conclusion 1 -Need for representative matrix reference materials -sort out and develop new methods to control challenges posed by complex mixtures -Ongoing development of stack and automotive reference materials for emission monitoring 2 CO 2 stack good, both types of matrices C 3 H 8 stability problems but good, only matrix matching evaluated NO matrix matching results in better capability SO 2 matrix matching doesn t result in better measurement CO needs improvement 3 The new method used for automotive gas mixtures is fit for purpose

13 Acknowledgements The NMISA Dr. J Tshilongo Gas Analysis section head (NMISA) Prof H Tutu Supervisor (University of the Witwatersrand) Gas team: David Mogale, Napo Ntsasa, Portia Seemane, Tshepiso Mphamo, Kennedy Ramahala, Lincon Maluleke, Mudalo Jozela, Nompumelelo Leshabane, Silindile Lushozi, Gumani Mphaphuli and Goitsemang Lekoto. National Research Foundation