Improved Techniques for Measuring and Validating

Siloxanes Present in Biogas

Improved Techniques for Measuring and Validating

Siloxanes Present in Biogas

Barbara MarshikMKS Instruments

Barbara_Marshik@MKSinst.comAWMA June 2014

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?

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

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

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

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

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

Initial Responsivity Tests

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0

5

10

15

20

25

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

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

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

Final Product 97% CH4– Two LabsUnscaled FTIR Results

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0123456789

10

Total Siloxane

 (mg/m3)

FTIR Tenax 1&2 Tenax #1 Tenax #2 FTIR Bag #1FTIR Bag #2 Bag1 Bag2

Tube Bag

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

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