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Improved Techniques for Measuring and Validating
Siloxanes Present in Biogas
Improved Techniques for Measuring and Validating
Siloxanes Present in Biogas
Barbara MarshikMKS Instruments
[email protected] June 2014
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Do We Need On-line Siloxanes Monitoring in Biogas?
Siloxanes damage engines– Deposits silica on blades, inside pistons and other surfaces– Costs for repair and oil changes are higher than anticipated– Engine MFGs put responsibility on plants for “no siloxanes”
Engine emission governed by EPA RICE MACT JJJJ– Siloxanes / SiO2 plug catalyst
Feed Forward Siloxane breakthrough monitoring– Activated Carbon, silica gels, as well as PSA– Anticipation of when to change media or switch beds
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What is Wrong with Laboratory / Off-Line Siloxanes Monitoring?
Current Labs Analysis– Considered to be the “golden standard”– ~ $500 - $2000 per sample
BUT we have found that:– Biogas grab sampling not accurate enough
Sampling (repeatability) issues – Can’t detect real-time excursions
Lab results can take up to 2 weeks– Inter-Lab results are not comparable
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Current Engine MFG Siloxanes Limits
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Engine MFG Siloxane (mg/m3) Maximum Limit
Caterpillar 28Jenbacher 10Waukesha 25
Deutz 5Solar Turbines 0.1
IR Microturbines 0.06Capstone Microturbines 0.03
State of California 0.1 Si with 0.01 Si DLs
• How were these maximum limits derived?• Are they even achievable?
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Can We Even Achieve These Low Limits?
Reported Method detection limits in the Laboratory– One Lab - Tubes
~0.050 mg/m3 as Siloxane or TMS ~0.020 mg/m3 as Si or TMS
– Second Lab - Tedlar Bags <0.005 mg/m3 as Siloxane or TMS <0.001 mg/m3 as Si or TMS
What about Accuracy and Repeatability?– No NIST traceable standards– To be accurate you must be repeatable– Need to take into consideration propagation of the real Error
Analysis + Sampling errors together
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Accuracy & Repeatability are of Utmost Important
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NO Repeatability = NO Accuracy
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FTIR Before & After ScrubberTotal Si(mg/m3) Method Example
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0
5
10
15
20
25
0 50 100 150 200 250 300 350 400 450 500 550 600 650 700
Total Si (mg/m3)
Time (Arbitrary minutes)
TMS as Si(mg/m3) Siloxanes as Si (mg/m3)
Pre‐scrubber
Pre‐scrubber
Post‐scrubber
Site 1
Site 2
Post‐scrubber
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0.0
0.5
1.0
1.5
2.0
2.5
Total Siloxane
s (m
g Si / m
g3)
Outlet Siloxanes as SiBag Lab
Bag#1 Lab2
Bag#2 Lab2
Tenax#1 Lab3
Tenax#2 Lab3
FTIR #1
FTIR #2
024681012141618
Total Siloxane
s (m
g Si / m
g3)
Inlet Siloxanes as SiBag Lab1
Bag#1 Lab2
Bag#2 Lab2
Tenax#1 Lab3
Tenax#2 Lab3
FTIR #1
FTIR #2
Simultaneous Landfill Grab SamplesMultiple Lab Analyses
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FTIR
FTIR
BAG TUBE
BAG TUBE
FTIR ResultsNot Scaled
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Method Validation Issues
Which sampling method is best?– Jet Care so far off - not considered for Volatile Siloxanes
Which Laboratory method is correct?– Duplicate samples are inconsistent– All claim <0.001 mg Si / m3 MDLs Precision in the Lab
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Inlet WET / RAW (Si mg/m3)Tedlar Lab1 Tedlar Lab2 TENNAX Lab3 FTIR
4.16 12.5 13.1 17.73.41 16.3 17.9
Inlet DRY (Post Chiller) (Si mg/m3)Tedlar Lab1 Tedlar Lab2 TENNAX Lab3 FTIR
3.87 10.6 9.98 17.59.43 15.2 17.6Outlet (Si mg/m3)
Tedlar Lab1 Tedlar Lab2 TENNAX Lab3 FTIR0.332 0.647 1.56 1.90
0.563 1.68 2.17
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FTIR Field Validation Using Standard Addition
Standard Addition – Adding known quantities of analyte to matrix and measure– Used when the matrix interferes with the analyte analysis
Common practice in Process Control analyzers
How do you do it?– Measure Native Siloxane on FTIR– Measure each Siloxane “addition / Spike” added to the Native– Plot the “known” Siloxane (mg/m3) Addition Values versus the
FTIR Response– Extrapolate to y=0
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• G. Gruce and P. Gill. "Estimates of Precision in a Standard Addition Analysis." Journal of Chemical Education, Volume 76, June 1999 • J.C. Miller and J.N. Miller. Statistics for Analytical Chemistry. 2nd Edition, 1988, Ellis Horwood Limited. Pages 117-120• DC Harris. Qualitative Chemical Analysis: 7th Addition Page 87
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Initial Responsivity Tests
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30
0 100 200 300 400 500 600 700
Si (m
g/m3)
Time (Arbitrary minutes)
TMS Total Siloxanes Total Siloxanes+TMS
Post‐Chiller
Post‐Scrubber
Siloxanes
Pre‐Chiller
TMS
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Total Siloxane Standard Addition
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y = 1.1004x + 30.449
y = 1.372x + 2.4576
0
10
20
30
40
50
60
70
80
90
100
‐50 ‐30 ‐10 10 30 50 70
FTIR Total Siloxane
mg/m
3
Siloxane Addition Concentration mg/m3
Siloxane Inlet Siloxane Outlet
Inlet 27.67
Outlet 1.79
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Inlet Biogas –Two LabsUnscaled FTIR Results
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0
20
40
60
80
100
120
Total Siloxane
(mg/m3)
FTIR Tenax 1&2 Tenax #1 Tenax #2FTIR Bag 1&2 Bag1 Bag2
Tube Bag
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Final Product 97% CH4– Two LabsUnscaled FTIR Results
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0123456789
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Total Siloxane
(mg/m3)
FTIR Tenax 1&2 Tenax #1 Tenax #2 FTIR Bag #1FTIR Bag #2 Bag1 Bag2
Tube Bag
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150123456789
10
Total Siloxane
(mg/m3)
FTIR Tenax 1&2 Tenax #1 Tenax #2FTIR Bag #1 FTIR Bag #2 Bag1
Tube Bag
Final Product 97% CH4– Two LabsScaled FTIR Results
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Summary Laboratory Analysis for total Siloxane (or Si)
– Provides very low MDLs – High Precision– However sampling issues cause very low accuracy
Process FTIR for total Siloxane (or Si) – Very high repeatability– Accuracy in field by Standard Addition
or 3-5 days by Laboratory Results– Real time Monitoring
Feed forward process control of Si excursions On Line analysis of H2O, CO, CO2 and CH4
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Conclusion Siloxane detection limit requirements MUST:
– Take into account sample collection errors Increase the number of duplicates ≥ 3
– Include repeatability and inter-laboratory method errors ASTM D03.05 Special Constituents of Gaseous
Fuels – Laboratory Method– Round Robin testing on sampling techniques
Bags, absorbent tubes, Summa Canisters and MeOH impingers– Round Robin testing on laboratory test methods– Final method will include only those techniques that are
repeatable FTIR is a repeatable and accurate instrument for
on line process control17