Research Project to Remove Siloxanes from Digester Gas

County Sanitation Districts of Los Angeles County

Industrial Silicone Polymer Lubricant Antiperspirant Skin

Care

DrainLandfills

DEFINITION OF SILOXANES AND FATE OF SILOXANE BEARING PRODUCTS

WWTP

Why Treat Digester Gas For Siloxane Removal?

When combusted, siloxanes form silicon dioxide, which can coat equipment, cause damage to equipment and cause a loss of heat transfer efficiency.

Heat Recovery Steam GeneratorsW/ Silicon Dioxide Deposits 20% Loss of Heating Value

Loss of Heat Value and Equipment Damage

$400,000/year

Clean

Treatment Costs $200,000/year

Digester Gas

Gas Turbine

Downstream Heat Recovery Equipment Becomes Coated with Silicon Dioxide Deposits

Existing Digester Gas Pretreatment

Digester Gas

Octamethylcyclotetrasiloxane (D4)

O

O O

O

Me2Si

Me2Si

Si Me2

SiMe2

Physical Properties of D4 Siloxane

Water Solubility

Low 74 micrograms/

L Vapor

Pressure High 1.0 mm Hg

at 25oC Henry’s Law

Constant High 3 to > 17

(Benzene is 2.2)

Existing JWPCP Gas Pretreatment System

• Venturi- Removes Dust• Mist Separator• Cooling Coil- 50oF• Compressor- 350 PSIG• Condenser- 40 oF. Installed a few years

after the original system to remove VOCs.

Siloxanes Concentrations at Various DigesterGas Pretreatment Locations

3.0

4.0

2.2

3.0

1.71.5

6.0

1.6 1.7

0.4

0.0

2.0

4.0

6.0

7.0

Meso. Dig.Thermo. Dig. Venturi Compressor Condenser

PPM

, (V/

V)

D4D5

Combined D4 AND D5 AT JWPCP AND OTHER TREATMENT PLANTS

2.1

1.20

0.20

1.05

0.670.83

1.03

0.530.75 0.63

0

0.5

1

1.5

2

2.5

JWPCP Condens

erVale

ncia Digest

erPalm

dale D

igeste

Hyperion Compre

ssoO.C. Plan

t 1 Com

pO.C. Plan

t 1 Dig.9

&10O.C. Plan

t 2 Com

pIEUA Plan

t 1 Comp

IEUA Plant 2

CompLas

Virgene

s

Siloxane Removal Technologies

• Liquid Absorption• Condensation• Solid Phase Adsorption

Liquid Absorption

Digester Gas In

Absorbent Liquid

Gas Flow(with Contaminants or Particulates)

Absorbent Liquid Flow

Absorbed Contaminant in Droplet

Nozzle

Digester Gas Out

Scrubber Testing

• Water Not Effective• Bench Scale Impingers Efficient

When Digester Gas is Bubbled into Solution (Polypropylene Glycol, Methanol, Ethoxy-based Detergent)

• Pilot-Plant Testing at 50 scfm showed that much more scrubbing agent needed in both Venturi and Packed Tower

Venturi-Condenser Pilot-Plant

Venturi /40oF Condenser Pilot-Plant Results

00.20.40.60.8

11.21.41.61.8

VenturiInfluent

VenturiEffluent

CondenserEffluent

D4D5

Silo

xane

Con

cent

ratio

n

Test Condenser

Percentage Siloxane Removal as Functionof Condenser Temperature

20

40

60

80

100

0-30 -20 -10 0 10 20 30 40 50

Perc

enta

ge S

iloxa

ne R

emov

al

With Coalescer

Additional Condenser Without Coalescer

Existing Plant Condenser (Alone)

Temperature (Degrees F)

Gas Adsorption with Solid MediaDirty Gas

Solid

Media

Gas Adsorbed onto Active Site of Solid Media

Solid Filter Gas Flow

Clean Gas

High Pressure ASME Rated Test Filter Vessels

Typical Siloxane Filter Breakthrough Curve

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

2 4 6 8 10 12Days of Operation

D4

Con

cent

ratio

n, p

pm (V

/V)

AAverage Influent Concentration During this Period was 1.35 ppm (V/V)

0 4,800 9600 14,400SCF/lb Graphite

Rapid Rise in Effluent Concentration at 7 days

Types of Solid Adsorbents• Polymeric Synthetic Proprietary Resin• Zeolite (Crystalline Aluminosilicates) • Silica Gel• Activated Carbon Graphite (with

Various Adsorption Capacity Ratings)• Activated Coconut Shell Based Carbon

Proprietary Resin

•Copolymer of Styrene/ Divinylbenzene

•Hydrophobic

•Low Affinity for Methane

•Micropores 100-200 Angstroms

Crystalline Aluminosilicates Clinoptilolite Zeolite

•Inexpensive

•High Sorptive Capacity

•Pore Size 11 Angstroms

•Adsorbs Large Molecules

•Typical Use of Zeolites is Adsorption of water in presence of non-polar solvents

Graphite Based Activated Carbon

•Carbon composed of polymorphous graphite

•Pore size 10-100 Angstroms

•Different Adsorbant Grades

Coconut Shell Based Carbon

•Inexpensive

•Good Experience with Coconut Shell Activated Carbon for Air Pollution Control

•Average Pore Size 20 Angstroms

Polymeric Resin Results

•Polymeric resin had high adsorptive capacity (20,000 scf gas/lb resin) on initial run but could not be regenerated with high efficiency.

•Along with siloxanes, other high molecular weight organics were adsorbed onto active sites in the resin.

•Overall rejection of the polymeric resin for this application. Resin good for low molecular weight contaminant such as methyl ethyl ketone (MEK)

Clinoptiolite Zeolite

0

1000

2000

3000

4000

5000

6000

7000

8000Sc

f Dig

este

r Gas

/lbZe

olite

Silica Gel Had Same Performance of Zeolite

Graphite Carbon Results

0

5000

10000

15000

20000

50/60/70 90CTC Number

SCF

Dig

este

r G

asFl

owPe

r L

b G

raph

ite

VirginMicrowaveKiln

80

Activated Carbon Results

1150012000

02000400060008000

100001200014000

Virgin

Regenerated

s

cf D

iges

ter

Gas

/lb.A

ctiv

ated

Car

n

02000

6000

10000

14000

18000

50/6

0/70

Coc

onut

Shel

l G.A

.C.

80 90

VirginMicrowaveKiln

Coconut Shell Solid Media PerformanceSC

F D

iges

ter G

as F

low

/Lb

Car

bon

Sil.G

el /Z

eol

Siloxanes at JWPCP 2000 and 2005

0.86

3.0

4.3

3.0

1.31.7

0.5

1.5

0.18

1.7

3.2

0.4

0.58

0

1

2

3

4

5

6

2000 2005 2000 2005 2000 2005

Mesophilic Digester Condenser Influent Condenser Effluent

Silo

xane

s C

once

ntra

tion

ppm

(v/v

)

D5D4D3

4.5

5.3

4.7 4.5

2.1

1.1

Potential Full Scale Design

•3- Filters; 2 on-line & 1 Stand By

•Each 30,000 lbs Graphite or Coconut Shell Carbon

•Each 10 ft. Diam. X 14 ft. Ht.

Estimated Capital & Operating Costs of 9,000 scfm Adsorption Filter System

System Graphite-Based

Coconut Shell NewAdditional

Condenser*CapitalCosts

$360,000 $360,000 $750,000

AnnualOperations/Mainten-

ance Costs

$160,000 $160,000 $85,000

*JWPCP already chills digester gas from 80oF to 40oF at capital cost of $500,000 and annual O/M of $64,000

Final Conclusions and Future Studies• Removal of siloxanes can be done

with adsorption and/or condensation

•Condensation/Adsorption costs $160,000 per year over 10 year period. Not treating costs $400,000/year

2006 Survey of Treatment Used

• LACSD: Carbon at Lancaster WRF Fuel Cell;Carbon and Silica Gel at Calabasas Landfill; Chiller at Palmdale Microturbine

• City of Los Angeles: Activated Carbon at Scattergood Electrical Station (Hyperion WWTP)

• Activated Carbon, Graphite and Chillers Used Elsewhere—”Second Generation” Low Temperature Chillers