TQM-5134-1

The analysis of complex multi-component gas mixtures for emission monitoring

31 July 2017Test and MeasurementPrelly Marebane

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.

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Fit-for-purpose reference materials for air pollution monitoring in SA and SADC region

TQM-5134-1

TQM-5134-1

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

Pro

duction

Preparation of compressed gas mixtures by gravimetry

- Six components for stack mixtures

-five for automotive mixtures

Target mass, purity analysis M

easure

ment

Confirm or reject the hypothesis of synergistic effects resulting from differences in matrix gas of sample and calibration mixture

Impro

vem

ents

Check if matrix matching will result in better measurement of myriad complex mixtures

- Sort out other calibration methods

TQM-5134-1

Methodology and experimental

Hig

h p

ure

Sulp

hur

dio

xid

e,

nitri

c o

xid

e,

pro

pane,

carb

on m

onoxid

e,

carb

on d

ioxid

e,

oxygen a

nd n

itro

gen

𝑚𝑖 =𝑥𝑖𝑃𝑓𝑉𝑐𝑦𝑙𝑀𝑖

𝑅𝑇𝑍𝑓

Assume Z=1

1. ISO 6142-1: 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) 580-588. http://dx.doi.org/10.1016/j.snb.2013.06.006

TQM-5134-1

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

TQM-5134-1

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)

TQM-5134-1

Results –stack gas

10-1

00 p

pm

CO

, 10-1

6%

CO

2,

10-1

00

ppm

NO

, 100 p

pm

SO

2,

1-1

0 p

pm

C3H

8

in N

2

The accuracy of the measurement for matrix matching = 0.14% and at % Relative expanded uncertainty of 0.5%

TQM-5134-1

Results –stack gas

7.8

7.85

7.9

7.95

8

8.05

8.1

07

Dec

em

be

r 2

01

6

08

Dec

em

be

r 2

01

6

09

Dec

em

be

r 2

01

6

10

Dec

em

be

r 2

01

6

11

Dec

em

be

r 2

01

6

Gra

vim

etr

ic c

on

cen

trat

ion

, µm

ol/

mo

l

Analysis

C3H8 - D19 4899

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%

TQM-5134-1

Results –stack gas

The accuracy of the measurement for matrix matching = 0.09% and at % Relative expanded uncertainty of 1.5%

Source of Sulphur dioxide in stack gas-Elemental Sulphur impurities in coal, Hancox et al., (2014).-combustion of conventional fuels

Non-dispersive infrared spectroscopySynergistic effects observed when other stack mixtures where used for calibrationDefied the hypothesisSO2/N2 calibration gave better results

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)

TQM-5134-1

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 C3H8

• 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: N2 and H2

• FID and TCD: both at 175 ᵒC

• Run time: 6 min

TQM-5134-1

Results –Automotive

• 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

• 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

O2 ≤0.3 ≤0.2-0.4

CO ≤0.6 ≤0.3-0.8

CO2 ≤0.3 ≤0.3-0.5

C3H8 ≤0.2 ≤0.3-0.8

TQM-5134-1

Conclusion

3

The new method used for automotive gas mixtures is fit

for purpose

2

CO2 stack – good, both types of matrices

C3H8 – stability problems but good, only matrix matching

evaluated

NO – matrix matching results in better capability

SO2 – matrix matching doesn’t result in better measurement

CO – needs improvement

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

TQM-5134-1

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