1. Performance and Economics of Currently Available Technologies for Removal of Siloxane from Biogas
Jeffrey L. Pierce, P.E.
Senior Vice President
SCS Energy
SWANA WasteCon 2010
August 15-17, 2010
Boston, Massachusetts

2. Siloxanes – What and Why?
Siloxanes are volatile organic silicon compounds (VOSCs)
Widely used in personal health and beauty products and in commercial applications
Found in the ppmv level in landfill gas and WWTP digester gas
When burned as a fuel, the silicon (Si) in siloxane oxidizes to silica (SiO2)
Silica deposits cause performance and maintenance problems
Silanes and silanols are also VOSCs

3. Impacts on Microturbines

4. Impacts on Engines

5. Impacts on Turbines

6. Impacts on Post-Combustion Catalysts

7. Impacts on Boilers

8. Impacts on Boilers

9. Common VOSC Compounds in Landfill Gas
Formal Name
AKA
Formula
Hexamethylcyclotrisiloxane D3
Si3-O3-(CH3)6
Octamethylcyclotetrasiloxane D4
Si4-O4-(CH3)8
Decamethylcyclopentasiloxane D5
Si5-O5-(CH3)10
Hexamethyldisiloxane L2
Si2-O-(CH3)6
Octamethyltrisiloxane L3
Si3-O2-(CH3)8
Trimethylsilanol
MOH
Si-(CH3)3-OH

10. Properties of Selected VOSC CompoundsMW
% Silicon
Vapor Pressure mmHg
Boiling Point (ºF)
Water Solubility (mg/l) D3
222
38.0% 10
273
1.560 D4
297
37.8%
1.3
347
0.056 D5
371
37.9%
0.4
410
0.017 L2
162
33.4% 42
214
0.930 L3
236
35.7%
3.9
306
0.034
MOH 90
31.4%
15.8
210
0.0004

11. Methods of Sampling and Analyzing for Siloxanes
Methanol impinger method (Air Toxics)
Jet Care method
OSB method
AtmAA method
AnSol method
Deutz method
Jenbacher method

12. Methanol Impinger Sample Train

13. Jet Care Sampling Kit

14. Siloxanes in Landfill Gas per SCS Database
Most common siloxanes are D4 (found 90% of the time); D5 (found 83% of time); and MOH (found 77% of the time)
Next most frequently found are L2 (45% of the time) and D3 (20% of the time)
Ten other siloxane species were seen (each no more than 7% of the time)
Siloxanes varied from 4.5 mg/m3 (0.41 ppmv) to mg/m3 (8.88 ppmv) by Air Toxics method

15. Current Siloxane Limits by Power Equipment Manufacturer
Microturbines
Ingersoll-Rand (60 ppbv)
0.25 mg Si/m3
Capstone (5 ppbv)
0.02 mg Si/m3
Reciprocating Engines
Caterpillar
10.7 mg Si/m3
Jenbacher
15.0 mg Si/m3
Deutz
5.0 mg Si/m3
Combustion Turbine
Solar – Mercury 50
2.5 mg Si/m3
Solar – Other
All limits cited at 50% LFG methane content

16. Siloxane Removal Theory
Siloxanes are a “gas in a gas”
Siloxanes cannot be filtered, since they are not in a particulate form
Must apply “advanced” physical operations
Condensation
Absorption
Adsorption

17. Siloxane Removal Technology
Condensation
Conventional chilling
Advanced chilling
Absorption
Water
Solvent
Adsorption
Activated carbon
Silica gel
Molecular sieve

18. Mountaingate Gas Plant
All three technology categories in simultaneous operation
Conventional chilling = 25% removal
Selexol absorption = 77% removal
Non-regenerative activated carbon = 95% removal
Overall removal = 99.2%
Product gas = 10 ppbv siloxane or mg Si/m3

20. Conventional Chilling
Typically 40º F
Effect may largely be due to absorption of siloxane into condensing water
Effective post-chilling water separation is critical to the performance of this technology
Generally used for moisture removal – so siloxane removal is an added benefit
Generally installed after gas pressurization and air-to-gas cooling – thus, an incremental cost
When specifically installed in siloxane removal service, chilling is usually installed to support a downstream process

21. Conventional Chilling
SCS’s experience is that conventional chilling will remove 15% to 35% of raw landfill gas siloxanes
Incremental installed capital costs range from $300/scfm to $350/scfm
Operating costs range from $25/mmscf to $30/mmscf at a power cost of $0.10/kWh
Cost ranges from the equivalent of $0.0019/kWh to $0.0022/kWh of power produced

22. Non-Regenerative Activated Carbon or Silica Gel
Gas is chilled to 40º F and reheated by 30º F to 40º F to prepare the gas for treatment
Chilling/reheat reduces moisture and siloxanes and improves performance and life of activated carbon or silica gel
Activated carbon and/or silica gel is placed in one or more vessels (in parallel or in series)
Vessels sized typically for changeout every six to nine months

23. Non-Regenerative Activated Carbon or Silica Gel (cont…)
Activated carbon nor silica gel is specifically selective for siloxane; hence, media life is affected by other contaminants
Media exhaustion is detected by siloxane breakthrough
Certain siloxane specie (e.g., L2 break through earlier than others)

24. Non-Regenerative Activated Carbon or Silica Gel (cont…)
Applicable to low pressure applications (5 psig) and high pressure applications (80 psig+)
Can also be used as a final polishing step after other processes – like at Mountaingate
Media loading rates vary from 7,000 scf/lb media to 70,000 scf/lb of media depending on many factors

25. Non-Regenerative Activated Carbon or Silica Gel (cont…)
SCS’s experience is that non-regenerative systems will remove virtually 100% of the raw landfill gas siloxanes – when media is virgin
Performance deteriorates over time
Incremental installed capital costs range from $300/scfm to $1,500/scfm (including chiller)
Operating costs range from $40/mmscf to $210/mmscf at a power cost of $0.10/kWh
Cost ranges from the equivalent of $0.003/kWh to $0.011/kWh of power produced

28. Regenerative Desiccant
Regenerative systems become more cost-effective as flow rates increase
Desiccant-based systems employ media similar to those used in air dryers (e.g., a silica gel)
Regeneration is accomplished by taking a bed offline and heating it with a backflow of hot air
The hot air desorbs the water and the siloxane that was captured by the media

29. Regenerative Desiccant (cont…)
Process is called thermal swing adsorption (TSA)
The siloxane, VOC and H2S laden off-gas is directed to a small enclosed flare for combustion
Domnick Hunter manufactures and has sold several of these systems under a trade name “GES Siloxane Removal System”

30. Regenerative Desiccant (cont…)
Domnick Hunter guarantees offers varying mg Si/m3. The guarantee appears to match the requirements specified by the power equipment suppliers, rather than the technology capability.
SCS has seen data showing performance to much better than their typical guarantees, particularly at high pressure installations
Inlet gas temperature should be limited to 80º F, which in most climates imposes a small pre-chiller requirement

31. Regenerative Desiccant (cont…)
Installed capital costs range from $380/scfm to $650/scfm
Operating costs range from $90/mmscf to $120/mmscf at a power cost of $0.10/kWh
Cost ranges from the equivalent of $0.0035/kWh to $0.0055/kWh of power produced

33. Regenerative Activated Carbon
Concept is similar to the non-regenerative dessicant concept – it is TSA
Jenbacher markets a proprietary TSA system for use in conjunction with their engines
SCS configures generic systems – normally in support of PSA high-Btu plants
A gas other than air must be used in the regeneration cycle – landfill gas or carbon dioxide can be used

34. Regenerative Activated Carbon (cont…)
Jenbacher applies heat through electric coils in the activated carbon beds. The beds are relatively small
SCS uses hot carbon dioxide gas, back flowed through the beds to head the activated carbon
Regeneration gas in both cases is sent to an enclosed flare
Gas is chilled prior to being sent to the TSA

35. Regenerative Activated Carbon (cont…)
Jenbacher’s TSA reportedly achieves low siloxane levels on virgin media, but degrades to a “roughing” removal mode, due to its low media volume, and their lack of need for “ultra-pure” gas
SCS configured systems are delivering siloxane levels in the 0.1 to 0.2 mg Si/m3 well after six months into their media cycle, largely due to their larger media volume and their more thorough regeneration cycle

36. Regenerative Activated Carbon (cont…)
Incremental installed capital costs range from $720/scfm to $820/scfm
Operating costs range from $50/mmscf to $105/mmscf at a power cost of $0.10/kWh
Cost ranges from the equivalent of $0.0045/kWh to $0.0065/kWh of power produced

39. Conclusions
There is a lack of consistency in the methods of siloxane sampling and analysis now employed
Power equipment manufacturers appear to have set overly conservative siloxane limits
Most siloxane removal applications require only “coarse” levels of siloxane removal – microturbines and high-Btu gas are notable exceptions
Commerically proven siloxane removal technologies are available
The technologies are fairly generic, although they are often presented as highly proprietary processes by equipment manufacturers