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CO2 Separation from Biogas, Landfill Gas

Biogas, specifically landfill gas, often contains too much CO2 and too low a methane concentration to fuel a natural gas engine for electrical power generation. Typically a 50% methane concentration (minimum heating value (LHV) = 457 Btu/scf) is required for most gas engines to operate properly. PermSelect® silicone membranes can be used to effectively remove CO2 from biogas to a 50% methane concentration or higher if the balance of the biogas composition is primarily CO2. Because CO2 is roughly three times more permeable than methane in silicone, and silicone is a highly permeable membrane material, it is possible to effectively remove CO2 from biogas even at relatively low feed pressures such as 45 psi. The advantage of low pressure CO2 removal is the significant reduction in equipment and operating cost associated with compressing and handling a high pressure flammable gas. Silicone has been shown to be very effective in various methane separations including removal of heavy hydrocarbons (C3+), siloxanes, hydrogen sulfide, water, and other VOC; Indeed, silicone is well known to operate satisfactorily in the field for methane purifications in the presence of combinations of water, carbon dioxide and C3+ hydrocarbons.

How to Separate Methane from Landfill Gas? CO2 separation from biogas (landfill gas upgrading) can be performed in a single stage membrane separation, or in two separation stages as illustrated in the figure below. The single stage membrane separation plant is the simplest since there are no moving parts, except for a compressor to pressurize the biogas feed to a pressure of 45 psi. The pressurized feed gas enters the membrane module and drives the CO2 and other contaminants across the membrane (permeate) thus concentrating the methane on the feed side (retentate). The permeate with high concentration of CO2 and low concentration of CH4 can be flared or otherwise disposed properly. The retentate becomes the upgraded biogas which is fed to a gas engine.

Figure 1. Low pressure single stage (left) and two stage (right) membrane separation plant to remove CO2 from landfill biogas. Figure 1. Low pressure single stage (left) and two stage (right) membrane separation plant to remove CO2 from landfill biogas.
In the two stage membrane separation plant, the permeate is re-compressed and directed to a second membrane to further extract CO2, thus reducing overall methane loss. Note that the additional membrane area and additional compressor compared to the single stage membrane separation plant will require additional equipment and higher operating cost.

The following graphs present results from simulations of upgrading landfill gas feed concentrations ranging from 20% to 45% methane to a fuel gas concentrations of 50%, 55% and 60% methane suitable for fueling a gas engine. Figure 2 presents the membrane surface area required to accomplish the desired upgraded gas composition and flow as a function of the biogas feed composition.

Figure 2. Figure 2. Membrane area per upgraded biogas flow required for CO2 removal from biogas as a function of feed landfill gas composition.

For example, if the landfill gas composition is 35% methane (65% CO2) and it is desired to upgrade the biogas to 50% methane (50% CO2) to run a gas engine, then from 35% biogas feed (x axis) move vertically to the 50% methane line (blue), then move horizontally to the y axis to find the required membrane area, or 7.2 m2 per scfm fuel required. Thus, if the engine requires 350 scfm of fuel (50% methane) then the membrane area required is 7.2 x 350 = 2,520 m2.
Figure 3 presents the required biogas feed flow to provide needed fuel gas flow. In our example above, at 35% feed biogas, the required landfill gas feed is 2.1 times the necessary fuel gas flow or 2.1 x 350 = 735 scfm.

Figure 3.Figure 3. Feed biogas flow per upgraded biogas flow required for CO2 removal from biogas as a function of feed gas composition.

Figures 4 and 5 present a performance comparison between a single stage and two stage membrane separation system for upgrading to a fuel gas methane content of 50%. Note that, as shown in Figure 4, the two stage separation system requires more membrane area for the same level of upgrading; however there is more methane recovery with the two stage system (Figure 5). In other words more of the methane from the biogas feed ends up in the upgraded fuel gas.

Figure 4.Figure 4. Comparison of the membrane area per upgraded biogas flow required for a single stage and a two stage membrane separation plant as a function of feed biogas composition.

Figure 5.Figure 5. Comparison of the percent methane recovery for a single stage and a two stage membrane separation plant as a function of feed biogas composition.

PermSelect manufactures silicone membrane modules for small scale liquid contacting and gas separation applications. Currently, our largest commercial membrane module has 2.1 m2. However, our hollow fiber membrane configuration and module manufacturing process enable effective packaging of large amounts membrane surface area in small volumes, typically greater than 4,000 m2/m3 regardless of module size. Figure 6 illustrates the relative scale of a 3,500 m2 membrane module skid composed of individual 100 m2 modules assembled in parallel to upgrade landfill gas. Based on model simulations, this skid can upgrade landfill gas from 35% to 50% methane and deliver 500 cfm fuel to an IC engine.

Figure 6. Rendition of a membrane module skid composed of 35- 100m2 membrane modules, suitable for upgrading 500 cfm of LFG from 35% to 50% CH4Figure 6. Rendition of a low pressure single stage silicone membrane separation plant with 3,500 m2, composed of thirty five - 100m2 membrane modules on a skid, suitable for removing CO2 from landfill gas to upgrade the methane concentration from 35% to 50% with 500 cfm fuel output.

Removing siloxanes from Biogas (landfill and digester gas). Organo-silicon compounds, known as siloxanes, are found in household and commercial products that are discarded in landfills. Siloxanes find their way into landfill gas, although the amounts vary depending on the waste composition and age. When landfill gas is combusted, siloxanes are converted to silicon dioxide (the primary component of sand). Silicon dioxide is a white substance that collects on the inside of the internal combustion engine and components of the gas turbine, reducing the performance of the equipment and resulting in significantly higher maintenance costs. Removal of siloxane can be both costly and challenging and biogas treatment to remove siloxanes may be required to meet engine manufacturer specifications.

However, with the proposed membrane system, siloxane removal from biogas can be achieved simultaneously with CO2 removal from biogas. Other contaminants such as hydrogen sulfide, heavy hydrocarbons (C3+), and other VOC are also removed from the biogas because their permeability in silicone membrane is higher than the permeability for methane. Thus it is possible to remove siloxanes from biogas in a very simple membrane separation process, which requires low capital and operating costs, compared to adsorption and absorption methods.

We are entertaining inquiries from potential collaborators interested in working with us on acquiring the capability to produce membrane modules with 100 m2 suitable for landfill biogas upgrading. We seek established partners with significant experience in landfill biogas processing including engineering, integrating, and financing. If your organization is interested in collaborating with PermSelect in this endeavor, please contact us or call us at +1 (734) 769-1066 x21 to discuss opportunities.

Silicone has been used effectively for the purification of natural gas commercially and by many researchers. The following list of publications describes using PDMS for purifying methane:

    International Journal of Advanced Engineering Technology Vol.III/ Issue I/January-March, 2012/311-315
    Swanand Kalambe et al.

    Continuous research has been going on to find out easy and cheap technique for separation of biogas employing membrane technology. Work has been carried out to find the best suited membrane for gas separation with low operational pressure and cost. Membrane gas separation technique is very advantageous as it doesn’t require huge infrastructure for plant set up due to low pressure requirement for the process and availability of membrane at a reasonable cost. This technique has generated immense commercial interest. This paper deals with an advanced separation technique employing poly dimethylsiloxane (PDMS) hollow fiber membrane module. The results clearly show that, PDMS double membrane module in series gave the upgraded methane with 93 % purity and carbon dioxide with 96% purity.

  • Evaluation of two gas membrane modules for fermentative hydrogen separation
    International Journal of Hydrogen Energy Volume 38, Issue 32, 25 October 2013, Pages 14042–14052
    J.E. Ramírez-Morales et al.

    The ability of (dimethyl siloxane) (PDMS) and SAPO 34 membrane modules to separate a H2/CO2 gas mixture was investigated in a continuous permeation system in order to decide if they were suitable to be coupled to a biological hydrogen production process. Permeation studies were carried out at relatively low feed pressures ranging from 110 to 180 kPa. The separation ability of SAPO 34 membrane module appeared to be overestimated since the effect concentration polarization phenomena was not taken into consideration in the permeation parameter estimation. On the other hand, the PDMS membrane was the most suitable to separate the binary gas mixture. This membrane reached a maximum CO2/H2 separation selectivity of 6.1 at 120 kPa of feed pressure. The pressure dependence of CO2 and H2 permeability was not considerable and only an apparent slight decrease was observed for CO2 and H2. The mean values of permeability coefficients for CO2 and H2 were 3285 ± 160 and 569 ± 65 Barrer, respectively. The operational feed pressure found to be more adequate to operate initially the PDMS membrane module coupled to the fermentation system was 180 kPa, at 296 K. In these conditions it was possible to achieve an acceptable CO2/H2 separation selectivity of 5.8 and a sufficient recovery of the CO2 in the permeate stream.

  • Siloxane removal using silicone–rubber membranes
    Separation and Purification Technology, Volume 89, 22 March 2012, Pages 234–244
    Marc Ajhar et al.

    Landfill and digester gas purification processes usually incorporate the removal of volatile methylsiloxanes (VMS). State-of-the-art technology is adsorption on activated carbon. This paper investigates a potential alternative: membranes. The permeabilities of common VMS in a commercially available polydimethylsiloxane (PDMS) membrane are determined as a function of temperature. A synthetic biogas mixture containing silicon in landfill gas-typical concentrations is purified in 3-end and 4-end operation. The results are presented using dimensionless numbers to facilitate upscaling. In general, PDMS can be used for siloxane removal, especially in 4-end operation using ambient air as sweep gas, where energy demand is significantly lower than in 3-end. However, depending on the desired degree of purification, methane losses of approximately 7% must be accepted. Only alternative membrane materials with higher carbon dioxide–methane selectivities have the potential for lower methane losses.