WO2008024329A1

WO2008024329A1 - Biogas purification with siloxane removal

Info

Publication number: WO2008024329A1
Application number: PCT/US2007/018456
Authority: WO
Inventors: Donald White, Jr.; Brian Mcgill; Dustin Zastera
Original assignee: Donaldson Company, Inc.
Priority date: 2006-08-22
Filing date: 2007-08-21
Publication date: 2008-02-28

Abstract

System for removing siloxanes from biogas, the system comprises an adsorbent bed which comprises at least two of activated carbon, silica gel and molecular sieve. Method for removing siloxanes from a biogas stream using the above adsorbent bed. Method of using biogas as intake air for equipment, such as combustion engines or gas turbines. The resulting cleansed gas has very low levels of siloxane, e.g., to less than 0.05 mg/m3, 0.02 mg/m3 and even 0.01 mg/m3.

Description

BIOGAS PURIFICATION WITH SILOXANE REMOVAL

This application is being filed on 21 August 2007, as a PCT International Patent application in the name of Donaldson Company, Inc., applicant for the designation of all countries except the US, and Donald White Jr., Brian McGiIl, and Dustin Zastera, all citizens of the US, applicants for the designation of the US only, and claims priority to U.S. Provisional Patent Application Serial Number 60/823,180, filed August 22, 2006.

Background

The present invention relates to systems and methods for removing oily contaminants, including siloxanes, from a gas stream.

The make-up of contaminants in gas streams varies widely, depending on the source of the stream. Biogas is a vent gas extracted from sites such as farm manure digesters, landfill sites, and from municipal waste water treatment digesters. Biogas includes contaminants such as volatile organic compounds, often siloxane. The source of siloxane contamination in landfills and waste water systems has been traced to glossy parchment paper, industrial lubricants and personal care products such as hair sprays, shampoo products, and deodorants. Biogas purification systems, for example, for farm manure digesters in research programs for the University of Minnesota and for Catator AB, Ideon Research Park in Sweden, have been attempted. There is always room for additional improvements.

Summary

The present disclosure is directed to a gas purification system that removes impurities, including volatile organic compounds and/or siloxane, from a gas stream to provide a cleansed gas stream. The system is particularly suited for removing contaminants from biogas, which is an important source of energy that can be used in gas turbines, internal combustion engines, steam boilers, and fuel cell systems, for example. Because of its high level of purity, the cleansed gas stream from the system of this disclosure is particularly suited for use in gas turbine systems, oxygen sources for fuel cells and combustion engines, as make-up air, or for expelling into the atmosphere.

This disclosure provides for the use of biogas as intake air for combustion engines, fuel cell reactions, or gas turbine systems, wherein the biogas is sufficiently cleansed to cause no detrimental affects to the combustion engine, fuel cell, or gas turbine system. The removal of siloxanes to very low levels, e.g., to less than 0.05 mg/m³, e.g., 0.02 mg/m³, and even e.g., 0.01 mg/m³, is desired for the effective use of biogas in many applications, such as fuel engine systems. The system of the present disclosure is able to provide such desired low levels of siloxanes and volatile organic compounds. VOCs (hydrocarbon volatile organic compounds) include propane, butane, isopentane, and non-methane aromatic hydrocarbons including toluene and benzene. Other volatile organic compounds include hydrogen sulfide, hydrogen fluoride, methyl chloride, sulfur dioxide and nitrogen oxides.

The system of this invention includes an adsorbent bed that includes at least two adsorbent materials, activated carbon, molecular sieve, and/or silica gel, which may be mixed together and/or be present as discrete layers. Such an adsorbent bed, when used with the system of this disclosure, reduces the levels and siloxanes and/or VOCs from biogas to provide a usable, cleansed stream.

In one particular aspect, this disclosure is directed to a system for removing siloxanes from biogas, the system including an adsorbent bed comprising an inlet, an outlet, and at least two of activated carbon, silica gel, and molecular sieve present between the inlet and the outlet. Another particular aspect of the disclosure is directed to a method of removing siloxane from a biogas stream, wherein the system is an adsorbent bed. The method includes inputting a biogas stream into the adsorbent bed, wherein the adsorbent bed has at least two of activated carbon, silica gel, and molecular sieve; and removing a cleansed stream from the adsorbent bed, the cleansed stream having a siloxane level of less than 0.05 mg/m³.

Brief Description of the Drawing FIG. 1 is a schematic diagram of a contaminant removal system of the present disclosure.

FIG. 2 is a graphical representation of a breakthrough curve for hexamethyldisiloxane (L₂) through a carbonaceous adsorbent. Detailed Description

The present disclosure is directed to a gas purification system that removes impurities, including volatile organic compounds and/or siloxane, from an incoming gas stream to provide a cleansed gas stream. The system is particularly suited for removing contaminants from biogas.

Biogas is an important source of energy that can be used in gas turbines, internal combustion engines, steam boilers, and fuel cell systems. The cleansed gas stream obtained from the gas purification system of this disclosure is particularly suited for use in gas turbine systems, oxygen sources for fuel cells and combustion engines, as make-up air, or for expelling into the atmosphere.

Biogas from landfill sites and municipal water treatment digesters contains siloxanes that are detrimental to gas turbines, internal combustion engines, and fuel cells. The source of siloxane contamination in landfills and waste water systems has been traced to glossy parchment paper, industrial lubricants and personal care products such as hair sprays, shampoo products, and deodorants. Siloxanes form hard, abrasive particles of silica (SiO₂), silicates (such as aluminosilicates), and siloxicon (S1₂OC₂). In some engines, for example, these abrasive particles precipitate in engine bearings and can coat surfaces, causing considerable damage. The particles erode and damage the turbine blades in gas turbines, producing unbalanced forces on the turbine rotors. The particles coat boiler tubes, thus degrading heat transfer and damaging the exterior of the tubes. The particles plug the pores of proton exchange membranes in fuel cell systems and damage compressor blades. The removal of siloxanes to very low levels, e.g., to less than 0.05 mg/m³, e.g., 0.02 mg/m³, and even e.g., 0.01 mg/m³, is desired for the effective use of biogas in many applications, such as fuel engine systems.

Referring to FIG. 1, a system 10 according to the present disclosure is generically illustrated. System 10 includes an inlet 12 for an incoming dirty gas stream, such as from a landfill; incoming dirty gas stream is contaminated with at least siloxane(s) and/or volatile organic compounds. Typically, the biogas entering the coalescing filter contains oil and water mist.

In some embodiments of the system of this disclosure, this bulk liquid (e.g., oil and water) from the biogas may be first removed by a cyclone separator, a de- mister, or other such separator (illustrated generically as separator 15) before entering a coalescing filter 20. Depending on the specific construction of the coalescing filter, it may or may not be configured to remove bulk liquid condensate from the system. In FIG. 1 , bulk liquid is removed via drain 17 and droplet laden air progresses to coalescing filter 20.

The oil and water mist droplets, which contain dissolved volatile organic compounds and siloxanes, are coalesced and drained from the system by filter 20, which may be a high efficiency filter. An example of a suitable high efficiency filter is a prefilter sold by Donaldson Company, Inc. of Minneapolis, MN under its Ultrafilter™ model FFAGO 192 designation. The partially cleaned gas then passes through an adsorber 30, where any volatile organic compounds and siloxane vapors are adsorbed.

Downstream from filter 20 is an adsorber 30. Adsorber 30 includes a bed of adsorbent material that includes at least two different adsorbent materials: activated carbon, molecular sieve, and/or silica gel. In many embodiments, all three adsorbent materials are present. The at least two adsorbent materials can be mixed uniformly thorough the bed, but preferentially they are layered within the bed. In some embodiments, two adsorbents may be mixed and then layered with the third. The embodiment of FIG. 1 illustrates adsorber 30 with three distinct layers 32, 34, 35. As arranged, layer 32 is the upstream-most layer, proximate the inlet into adsorber 30, and layer 34 is the downstream-most layer, proximate the outlet from adsorber 30. Layer 35 is the center layer. In embodiments having only two layers (e.g., only two adsorbent materials or two adsorbent materials mixed to form one layer and a third adsorbent material for the second layer), center layer 35 would not be present. If activated carbon is one of the adsorbent materials used, the activated carbon may be nonimpregnated or impregnated with a reactant. Activated carbon materials typically have large size micropores, typically 20 to 100 Angstroms, which are beneficial for the removal of the highest molecular weight hydrocarbons (for example, about 300 atomic mass units (AMU) formula weight or larger) and the highest molecular weight siloxanes (for example, about 225 AMU formula weight or larger).

Silica gel typically has micropores of 20 to 40 Angstroms, which is beneficial to remove the smaller molecular weight hydrocarbons and siloxanes (for example, less than about 300 AMU formula weight for hydrocarbons and less than about 225 AMU formula weight for siloxanes).

If present in adsorber 30, a preferred molecular sieve to use is 13X molecular sieve having 8 Angstrom pores, although many other types are available and would provide beneficial service in the purification of biogas according to this disclosure. In one particular embodiment of adsorber 30, the first layer or inlet layer or upstream-most layer 32 includes activated carbon, the subsequent or center layer 35 includes silica gel, and molecular sieve adsorbent is present in the third layer, outlet layer, or downstream-most layer 34, to remove the smallest molecular weight contaminants. In this order of arrangement, the three adsorbent materials are arranged to sequentially remove the contaminants in reducing molecular weight order.

The particular composition of adsorbent material present in adsorber 30 is selected based on the composition of the incoming biogas stream. In most designs, however, the activated carbon, if present, is usually no more than about 80 wt-% of the adsorbent bed; for example, about 20 wt-% to about 60 wt-%, and in some embodiments about 33.3 wt-% of the bed. The silica gel, if present, comprises usually no more than 80 wt-% of the bed, for example about 20 wt-% to about 60 wt-%, and in some embodiments, about 33.3 wt-% of the bed. The molecular sieve, if present, constitutes no more than about 80 wt-% of the bed, preferably about 20 wt-% to about 60 wt-%, and in some examples about 33.3 wt-% of the adsorbent bed.

If removal of basic or alkaline gas is desired, for example, to minimize or prevent corrosion in an engine or other use equipment, an acidic impregnated adsorbent material may be added to the adsorbent bed, in addition to the at least two adsorbent materials. If present, any optional material (e.g., acidic adsorbent material) can added anywhere in the bed, but is particularly suited at the outlet end of the adsorbent bed, for example, to remove basic gases, such as ammonia. Acidic adsorbent material can be formed, for example, by impregnating a microporous medium with an acid such as, e.g., sulfuric acid, nitric acid, or citric acid. The impregnant can be selected to best react with or otherwise remove the desired contaminant. Molecular sieve, activated carbon, activated alumina or silica gel can be used as the microporous support media. The support media is usually impregnated with about 20 wt-% to about 40 wt-% acid, often about 25 wt-% to about 35 wt-% acid, and sometimes with about 30 wt-% acid.

In some embodiments, a layer of activated carbon adsorbent material can be present at the outlet end of the absorbent bed, in addition to or alternately to the layer at the inlet end or other position in the bed. This layer will adsorb any residual hydrocarbons and/or siloxanes, and will filter adsorbent fines that might be generated in the upstream portions of the adsorbent bed. Any carbon fines that might be produced in this layer are not harmful to downstream components as they are combustible as fuel in the engine or boiler without detrimental effects. Figure 2 shows the results of one test of the purification of biogas demonstrating the removal of hexamethyldisiloxane, a linear molecular siloxane having a high vapor pressure (31 mm Hg at 25° C), which is commonly detected in high concentration at landfill sites. The adsorbent was carbonaceous adsorbent. The plot shows the amount of hexamethyldisiloxane detected on the downstream side of the carbonaceous absorbent over time of exposure of the adsorbent to hexamethyldisiloxane. Virtually no hexamethyldisiloxane was detected for about 24 hours.

Adsorber 30, with the previously described composition of the at least two specified adsorbent materials, is very beneficial in the reduction of the physical size of the biogas purification system and in the length of the service time provided by the bed of purifying adsorbent material.

From adsorber 30, a cleansed biogas stream leaves system 10 via outlet 14. This cleansed or purified biogas can then be directed for use by equipment, such as a combustion engine or boiler.

One Particular Example

One particular example of a system of this disclosure includes a coalescing prefilter having a Donaldson Ultrafilter FFAGOl 92 mold cast aluminum housing with a MF3030 borosilicate filter element and an Econometer differential pressure indicator. The housing, rated at 17.24 bar g (250 psig), had 7.6 cm (3 inch) Female NPT connections and a polyester powder coating finish to prevent corrosion. A 2.54 cm sight glass was provided in the drain reservoir.

The filter element has stainless steel inner and outer support sleeves, aluminum end caps, and O-rings that were free of silicone and other parting compounds. The differential pressure through the clean, dry element was 18 m bar (0.26 psi) at the design flow conditions of the system. An increase of approximately 65% in pressure loss was expected when the element was wet. Dirt particles were trapped in the micro-fiber mat which has a retention rating of 99.999% at 0.01 microns, and oil mist was reduced to 0.1 ppm based on a maximum inlet concentration of 3 ppm. The flow was from the inside of the filter element to the outside. Liquid aerosols and mists were agglomerated into larger droplets in the coalescing filter media and were drained into the filter sump. Fine mist aerosols were removed from the biogas stream prior to entering the adsorbent tower, because the aerosols readily pass between the granules of adsorbent and contaminate the biogas downstream. An Econometer differential pressure indicator was installed on the filter housing to indicate when the filter element required change-out.

The liquid accumulated in the sump of the filter was removed manually through a hand operated valve. Because of the low operating pressure, less than 70 m bars (<1 psig), an automatic drain valve could not easily be used. An optional loop seal was used in the condensate drain line to provide continuous drainage. At the low operating pressure of the system, a 1 meter loop seal was adequate.

Downstream of the coalescing prefilter was an adsorbent tower, designed for downflow operation. The inlet and outlet connections were 150 Ib ANSI RF flanges. The adsorbent bed in each tower was designed to provide service for at least 6 months based on normal operating conditions. Two towers can be used in series to provide a service life of one year or longer.

The tower was designed and manufactured to the ASME code, "U" stamped, and registered with the National Board of Boiler and Pressure Vessel Inspectors. It was designed to contain the adsorbent bed and to provide good flow distribution and low pressure loss with minimal attrition of the granules.

The two vessels, installed in series, were designed and manufactured to ASME Section VIII Division 1, latest edition, for a pressure of 10.34 bar g (150 psig) and a temperature range of -28.9°C to 121°C (-20⁰F to 250⁰F) with a 1.6 mm (0.0625 inch) corrosion allowance. The shells and heads were 610 mm (24 inch) diameter, 6.35 mm (0.25 inch) wall thickness, carbon steel, SA-36 Grade B or SA- 516 Grade 70. Adsorbent fill and drain ports were provided on the vessels. Removable stainless steel diffiiser screens were installed in the inlet and outlet ports to provide for even flow distribution and to retain the adsorbent material.

The vessels were factory filled with adsorbent material to the knuckle in the upper head to provide for a large flow distribution plenum in the head and minimum flow channeling. The first vessel was filled 100% with activated carbon. The second vessel was filled 50% with activated carbon, 25% with silica gel, and 25% with 13X molecular sieve. The molecular sieve was installed at the outlet end of the second vessel.

The large vessel diameter provided for a large cross-sectional flow area resulting in a low flow velocity, minimum adsorbent attrition and low pressure loss. The superficial fluidization limit was 55 meters/minute (181 ft/min) while the actual superficial velocity was 9.0 meters/ minute (29.6 ft/min) at the design conditions, or 16.4% of the limit. The pressure loss through the desiccant bed was 7.0 m bar (0.1 psi). The coalescing filter was designed to be installed directly into system piping.

Adequate piping supports were used to provide sufficient rigidity to the system. The dimensions of the coalescing filter were as follows:

Height: 1194 mm (47 inches)

Width: 178 mm (7 inches) Depth: 178 mm (7 inches)

The inlet and outlet piping connections on the coalescing filter were 7.6 cm (3 inch) Female NPT.

The approximate weight of the coalescing filter was 14 kilograms (31 pounds).

The siloxane adsorber was mounted on a base frame of rugged construction, welded A36 carbon steel. The frame was securely bolted to the building foundation after installation to prevent movement resulting from earth tremors and induced piping vibration. The dimensions of the adsorber were as follows: Height: 2692 mm (106 inches)

Width: 915 mm (36 inches)

Depth: 915 mm (36 inches) The inlet and outlet piping connections on the siloxane adsorber were 7.6 cm (3 inch) 150 Ib ANSI RF flanges.

The approximate weight of the adsorber was 394 kilograms (869 pounds).

The adsorbent tower could be modified to include a base gas adsorbent. The optional adsorbent can be used reduce the concentration of base materials (e.g., ammonia) to less than 1 ppbv based on an inlet concentration of 1 ppmv.

Examples of operating conditions include:

Inlet Biogas Design Flow Rate: 150 Nm³/hr (88 scfm) Inlet Design Pressure: 60 mbar g (0.87 psig)

Inlet Design Temperature: 35°C (95°F)

Inlet Relative Humidity: 45% rh Maximum

System Design Pressure Rating: 10.34 bar g (150 psig) Ambient Air Temperature: 3⁰C Min.; 52°C Max. (38°/125°F) Total Pressure Loss (Prefilter and 2 adsorption towers): 50 mbar (0.725 psid)

Normal inlet Biogas Quality: Composition of Biogas: Methane: 60% molar volume

Carbon dioxide: 37% molar volume Nitrogen: 2.5% molar volume

Oxygen: 0.5% molar volume

Biogas physical properties: Molecular weight: 26.77 amu's

Standard density: 1.108 kg/cubic meter (Std 1.014 bar a/ 21.1°C);.

06917 nWcubic foot (Std 14.7 psia/ 70⁰F) Viscosity at 35°C: 0.01239 centipoises (0.0300 lbm/ft-hr) Contaminant:

Siloxanes L₂ to L₅ and D₃ to D₆: 2 ppmv (Approx. 12 mg/m³) Volatile organic compounds: 2 ppmv Outlet Biogas Quality based on normal inlet conditions: Residual oil content: < 0.1 ppm (based on an inlet cone, of 3 ppm) Siloxanes: < 2 ppbv

VOCs: < 2 ppbv

Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.

Claims

WE CLAIM:

1. A system for removing siloxanes from biogas, the system including an adsorbent bed comprising an inlet, an outlet, and at least two of activated carbon, silica gel, and molecular sieve present between the inlet and the outlet.

2. The system of claim 1, wherein the activated carbon is proximate the inlet of the adsorbent bed.

3. The system of claim 2, wherein the activated carbon is present as a - layer.

4. The system of claim 2, wherein silica gel is present between the activated carbon and the outlet.

5. The system of claim 4, wherein molecular sieve is present between the silica gel and the outlet.

6. The system of claim 1, wherein the activated carbon, silica gel, and molecular sieve are homogeneously combined.

7. The system of claim 1, wherein two of the activated carbon, silica gel, and molecular sieve are homogeneously combined.

8. The system of claim 1 , wherein the adsorbent bed further comprises a layer of impregnated carbon, positioned at the outlet of the adsorbent bed.

9. The system of claim 1 , further including a coalescing filter upstream of the adsorbent bed.

10. The system of claim 9, wherein the coalescing filter comprises hydrophilic material, hydrophobic material, and/or nanofibers.

11. A method of removing siloxane from a biogas stream, the system comprising an adsorbent bed, the method comprising:

(a) inputting a biogas stream into the adsorbent bed, the adsorbent bed comprising at least two of activated carbon, silica gel, and molecular sieve; and

(b) removing a cleansed stream from the adsorbent bed, the cleansed stream having a siloxane level of less than 0.05 mg/m³.

12. The method of claim 11, wherein removing a cleansed stream comprises removing a cleansed stream having a siloxane level of less than 0.02 mg/m³.

13. The method of claim 12, wherein removing a cleansed stream comprises removing a cleansed stream having a siloxane level of less than 0.01 mg/m³.

14. The method of claim 1 1, wherein inputting the biogas stream into the adsorbent bed comprises inputting the biogas stream into an adsorbent bed comprising activated carbon, silica gel, and molecular sieve.

15. A method of using biogas as intake air for equipment, comprising:

(a) cleansing a biogas stream to provide a cleansed biogas stream; and

(b) using the cleansed biogas stream as intake air for equipment, wherein the cleansed biogas stream is sufficiently cleansed to cause no detrimental affects to operation of the equipment.

16. The method of claim 15, wherein the equipment comprises a combustion engine, fuel cell reaction, or gas turbine system.

17. The method of claim 16, wherein cleansing a biogas stream comprises cleansing a biogas stream to provide a cleansed biogas stream having a siloxane level of less than 0.05 mg/m³.

18. The method of claim 17, wherein cleansing a biogas stream comprises cleansing a biogas stream to provide a cleansed biogas stream having a siloxane level of less than 0.02 mg/m³.

19. The method of claim 18, wherein cleansing a biogas stream comprises cleansing a biogas stream to provide a cleansed biogas stream having a siloxane level of less than 0.01 mg/m³.

20. The method of claim 16, wherein cleansing a biogas stream comprises passing the biogas stream into an adsorbent bed comprising at least two of activated carbon, silica gel, and molecular sieve.