Zeolite for Biogas Upgrading (CO₂·H₂S Removal)

Q: How does clinoptilolite separate CO₂ from biogas?

It works on the principle of selective adsorption. CO₂ has a kinetic diameter of about 3.3Å, smaller than CH₄ (about 3.8Å), and a larger quadrupole moment, so it adsorbs more strongly onto the micropores and framework cations of clinoptilolite. As a result, CO₂ binds preferentially to the adsorbent while CH₄ passes through, raising the methane concentration of the outlet gas. Adsorption is reversible: lowering the pressure desorbs the CO₂ and regenerates the adsorbent, which is the basic principle of PSA (Pressure Swing Adsorption).

Q: Can natural zeolite also remove H₂S (hydrogen sulfide)?

H₂S is partially adsorbed, but in biogas upgrading it is standard practice to first remove H₂S in a separate pretreatment stage. At high concentrations, H₂S corrodes adsorbents, piping, and compressors, and degrades PSA adsorbent performance. Clinoptilolite has been reported as an adsorptive desulfurization medium with sulfur-compound removal capability (Özkan & Özkan, 2022); however, for high-concentration H₂S, multi-stage pretreatment combined with iron oxide, activated carbon, or biological desulfurization is recommended.

Q: Why is natural zeolite considered as an adsorbent? How does it differ from synthetic zeolite?

Synthetic zeolites (13X, 5A, etc.) offer high CO₂ adsorption capacity and selectivity but are expensive. Natural clinoptilolite has lower adsorption capacity than synthetic products, but its low raw-material cost and easy bulk availability make it a candidate for small-scale/distributed biogas plants, pretreatment (coarse purification) stages, or as an adsorbent base material. CO₂ enters the clinoptilolite channels well enough that a micropore surface area on the order of 240 m²/g is observed when measured at 273K (De Gennaro et al., 2024). Actual adsorption capacity varies with the mine, pretreatment, temperature, and pressure, so pilot testing is necessary.

Q: How do temperature and pressure affect adsorption performance?

Due to the nature of physisorption, CO₂ adsorption capacity increases at low temperature and high pressure and decreases at high temperature and low pressure. Davarpanah et al. (2020) reported that CO₂ capture by natural clinoptilolite depends sensitively on temperature. PSA exploits this reversibility to cycle between adsorption (high pressure) and desorption/regeneration (depressurization). In addition, moisture competes with CO₂ for adsorption sites, so gas drying (dehumidification) pretreatment is a prerequisite for stable operation.

What Is Biogas Upgrading?

Raw biogas produced by anaerobic digestion typically contains 50–70% methane (CH₄) and 30–50% carbon dioxide (CO₂), along with trace impurities such as hydrogen sulfide (H₂S), moisture (H₂O), siloxanes, and ammonia. To raise this gas beyond a power-generation fuel to biomethane — i.e., to a quality suitable for injection into the city-gas grid or for use as vehicle fuel — an upgrading process is required that removes CO₂ and raises the methane concentration above 95%. Because CO₂ is an inert component with no calorific value, separating it raises the heating value per unit volume and the value of the gas.

Upgrading technologies include water scrubbing, amine chemical absorption, membrane separation, and PSA (Pressure Swing Adsorption). This page addresses how natural clinoptilolite is considered as an adsorbent or pretreatment medium in PSA/adsorption-based processes. The key is the adsorption selectivity that "captures CO₂ preferentially over CH₄."

Why Clinoptilolite Is Considered for CO₂/CH₄ Separation

The separation principle is selective adsorption. CO₂ has a kinetic diameter of about 3.3Å, smaller than CH₄ (about 3.8Å), and its large quadrupole moment makes it more strongly attracted to the micropores of the zeolite and the electric field of the exchangeable cations within the framework. As a result, in the same adsorption bed CO₂ is adsorbed preferentially while CH₄ relatively passes through, raising the methane concentration of the outlet gas. Importantly, this adsorption is physisorption (reversible): lowering the pressure desorbs the CO₂ and regenerates the adsorbent, and cycling this adsorption–desorption is exactly what PSA does.

De Gennaro et al. (2024, Environmental Science and Pollution Research) summarized clinoptilolite as a versatile material used across a wide range of applications including environmental catalysis and CO₂ removal, and reported that CO₂ enters and diffuses into the micropores so well that the specific surface area — about 22 m²/g when measured with N₂ — is confirmed to be about 240 m²/g when measured with CO₂ at 273K (De Gennaro, B. et al., 2024, doi:10.1007/s11356-024-33656-5). This is direct evidence that CO₂ is chemically compatible with the clinoptilolite channels.

Temperature and pressure dependence are also key design variables. Davarpanah et al. (2020, Journal of Environmental Management) quantitatively reported that CO₂ capture by natural clinoptilolite depends sensitively on temperature (Davarpanah, E. et al., 2020, doi:10.1016/j.jenvman.2020.111229). Due to the nature of physisorption, adsorption capacity increases at low temperature and high pressure and decreases at high temperature and low pressure — and PSA exploits precisely this reversibility. Al-Mamoori et al. (2024, Cogent Engineering), which synthesized the use of natural zeolites in the context of biogas purification and cleaning, likewise concluded that natural zeolites can be used as adsorption/purification media in the production and purification of biogas, syngas, and hydrogen (Al-Mamoori, A. et al., 2024, doi:10.1080/23311916.2024.2398912).

That said, the practical limits must also be made clear. Synthetic zeolites such as 13X and 5A lead in CO₂ adsorption capacity and selectivity, whereas natural clinoptilolite has relatively lower adsorption capacity in exchange for advantages in raw-material cost and availability. Therefore, it is reasonable to consider natural zeolite not as the sole adsorbent of a large upgrading plant, but for small-scale/distributed plants, coarse pretreatment purification, or as an adsorbent base material.

KMIZEOLITE's natural clinoptilolite, at 97% purity, is mined and processed at the Amargosa Valley mine in Nevada, USA, with a specific surface area of 40.0 m²/g, pore diameter of 4.0–7.0 Å, stable pH range of 3.0–10.0, hardness of 4.0–5.0 Mohs, and thermal stability to 700°C — providing the physical stability to withstand gas-adsorption bed operation and regeneration cycles. Under general industrial-use labeling, it corresponds to FDA GRAS (21 CFR 182.2729).

KMIZEOLITE Key Properties

Item	Value
Clinoptilolite purity	97%
Cation exchange capacity (CEC)	1.6–2.0 meq/g
Specific surface area	40.0 m²/g
Pore diameter	4.0–7.0 Å
Stable pH range	3.0–10.0
Hardness	4.0–5.0 Mohs
Thermal stability	700°C
Specific gravity	1.89
Bulk density	45–54 lbs/ft³
Certifications	OMRI KMI-10365, FDA GRAS, TSCA, EN-71-3

H₂S (Hydrogen Sulfide) Removal Requires Separate Pretreatment

In upgrading, H₂S management is just as important as CO₂ removal. At high concentrations, H₂S corrodes adsorbents, piping, compressors, and engines, and fouls the PSA adsorbent surface, degrading CO₂ adsorption performance. Therefore, in practice the standard configuration is multi-stage, placing a desulfurization (sweetening) pretreatment that removes H₂S first, before CO₂ separation (PSA).

Clinoptilolite has been reported to have sulfur-compound removal capability as an adsorptive desulfurization medium. Özkan & Özkan (2022, Natural and Engineering Sciences) addressed adsorptive desulfurization using clinoptilolite (Özkan, V. & Özkan, A., 2022, doi:10.28978/nesciences.1222495). However, for high-concentration H₂S, pretreatment combined with iron oxide (iron sponge), activated carbon, or biological desulfurization is common, and a realistic approach is to consider natural zeolite as an auxiliary medium for downstream protection (polishing) or for simultaneously reducing moisture and trace components.

Examples of Biogas Upgrading Applications

In gas purification, it is realistic to position natural zeolite not as a stand-alone universal solution, but as an auxiliary/pretreatment medium at each process stage. Representative scenarios are as follows.

PSA CO₂ separation bed: passing dried and desulfurized biogas through an adsorption tower to preferentially adsorb CO₂, raising the methane concentration and regenerating by depressurization (small-scale/distributed or coarse purification stage)
Gas drying (dehumidification) pretreatment: because moisture competes with CO₂ adsorption sites, capturing the moisture first in a packed bed to protect the downstream adsorbent
H₂S polishing / auxiliary desulfurization: used as an auxiliary medium that captures residual sulfur downstream of primary desulfurization (iron oxide / biological desulfurization)
Adsorbent base material: used as a low-cost base material for adsorbents whose CO₂ selectivity has been enhanced through modification such as cation exchange or loading
Testing / pilot application: using small samples to verify in advance the CO₂/CH₄ selectivity and breakthrough behavior under varying gas composition, pressure, temperature, and regeneration conditions

Recommended Particle Size and Product Specifications

For PSA/packed-bed adsorption columns, Medium Granule (14×40 mesh) to Coarse Granule (8×14 mesh) is generally considered to balance pressure drop and mass transfer, while Coarse to Extra Coarse Granule is considered for gas drying and coarse pretreatment beds. Fine powder is unsuitable for gas packed beds due to excessive pressure drop. When designing a packed bed, review the superficial velocity, pressure drop, and regeneration cycle together.

Product Group	Mesh	Particle Size	Typical Use
Powder	100 mesh or finer	<150μm	Pozzolan, feed, powder adsorption
Fine Granule	30×50 mesh	0.3–0.6mm	Water treatment, filtration, soil
Medium Granule	14×40 mesh	0.4–1.4mm	Filtration beds, bedding, litter
Coarse Granule	8×14 mesh	1.4–2.4mm	Swimming pools, de-icing, large-scale filtration
Extra Coarse	4×8 mesh	2.4–4.8mm	Packed beds, air scrubbers

→ View products by mesh size · Product selection guide by use

Pilot Test and On-Site Review Points

Because biogas upgrading performance is governed by gas composition, pressure, and temperature, be sure to verify the following items together.

Pretreatment sequence: First design a multi-stage configuration that removes H₂S, moisture, and siloxanes before CO₂ separation (PSA). In particular, pre-removal of H₂S is a prerequisite to protect the adsorbent and equipment.
Gas drying (dehumidification): Because moisture competes with CO₂ adsorption sites, sufficient drying is key to stable CO₂ selectivity and adsorbent lifetime.
Temperature/pressure operating point: CO₂ physisorption favors low temperature and high pressure. Set the pressure-swing span between adsorption (high pressure) and regeneration (depressurization), and the cycle time.
CO₂/CH₄ selectivity and methane loss: Design the breakthrough timing and recovery process to minimize methane slip (loss), in which CH₄ escapes together from the adsorption bed.
Regeneration and lifetime: Evaluate the efficiency of depressurization/purge regeneration, the decline in adsorption capacity over repeated cycles, and deactivation due to sulfur and moisture accumulation.
Adsorbent selection: The higher the required methane purity and the larger the scale, the more a comparison with synthetic zeolites (13X, etc.) is needed. Natural zeolite offers strong economics in small-scale, pretreatment, and base-material uses.

→ View TDS (Technical Data Sheet) · View MSDS (Safety Data Sheet)

Biogas Upgrading FAQ

How does clinoptilolite separate CO₂ from biogas?

It works on the principle of selective adsorption. CO₂ has a kinetic diameter of about 3.3Å, smaller than CH₄ (about 3.8Å), and a larger quadrupole moment, so it adsorbs more strongly onto the micropores and framework cations of clinoptilolite. As a result, CO₂ binds preferentially to the adsorbent while CH₄ passes through, raising the methane concentration of the outlet gas. Adsorption is reversible: lowering the pressure desorbs the CO₂ and regenerates the adsorbent, which is the basic principle of PSA (Pressure Swing Adsorption).

Can natural zeolite also remove H₂S (hydrogen sulfide)?

H₂S is partially adsorbed, but in biogas upgrading it is standard practice to first remove H₂S in a separate pretreatment stage. At high concentrations, H₂S corrodes adsorbents, piping, and compressors, and degrades PSA adsorbent performance. Clinoptilolite has been reported as an adsorptive desulfurization medium with sulfur-compound removal capability (Özkan & Özkan, 2022); however, for high-concentration H₂S, multi-stage pretreatment combined with iron oxide, activated carbon, or biological desulfurization is recommended.

Why is natural zeolite considered as an adsorbent? How does it differ from synthetic zeolite?

Synthetic zeolites (13X, 5A, etc.) offer high CO₂ adsorption capacity and selectivity but are expensive. Natural clinoptilolite has lower adsorption capacity than synthetic products, but its low raw-material cost and easy bulk availability make it a candidate for small-scale/distributed biogas plants, pretreatment (coarse purification) stages, or as an adsorbent base material. CO₂ enters the clinoptilolite channels well enough that a micropore surface area on the order of 240 m²/g is observed when measured at 273K (De Gennaro et al., 2024). Actual adsorption capacity varies with the mine, pretreatment, temperature, and pressure, so pilot testing is necessary.

How do temperature and pressure affect adsorption performance?

Due to the nature of physisorption, CO₂ adsorption capacity increases at low temperature and high pressure and decreases at high temperature and low pressure. Davarpanah et al. (2020) reported that CO₂ capture by natural clinoptilolite depends sensitively on temperature. PSA uses this reversibility to cycle between adsorption (high pressure) and desorption/regeneration (depressurization). In addition, moisture competes with CO₂ for adsorption sites, so gas drying (dehumidification) pretreatment is a prerequisite for stable operation.

Which particle size (mesh) is suitable, and can I get a sample?

For PSA/packed-bed adsorption columns, Medium Granule (14×40 mesh) to Coarse Granule (8×14 mesh) is generally considered to balance pressure drop and mass transfer, while Coarse to Extra Coarse is considered for gas drying and coarse pretreatment beds. Fine powder is unsuitable for packed beds due to high pressure drop. KMIZEOLITE supports sample provision for application review, so please leave your application purpose (PSA adsorption, H₂S pretreatment, gas drying, etc.) and desired particle size on the sample request page.

Inquiries and Sample Requests

If you are considering applying zeolite to the field of biogas upgrading (CO₂·H₂S removal), please contact us through the channels below.

Notice

Applicability may vary depending on site conditions, regulations, and test results. Before actual application, testing and review tailored to site conditions must be conducted first. Zeolite is best understood not as a universal solution for the field, but as a material that supports existing processes.