Graduation Year


Document Type




Degree Name

MS in Chemical Engineering (M.S.C.H.)

Degree Granting Department

Chemical Engineering

Major Professor

John N. Kuhn, Ph.D.

Co-Major Professor

Babu Joseph, Ph.D.

Committee Member

Scott Campbell, Ph.D.


Biofuel, Biomethane, CO2 adsorption, LFG upgrading, Renewable natural gas


Biogas is a valuable renewable energy generated from anaerobic digestion of biodegradable organic matter. It is applicable as fuel in vehicles, for the generation of electricity, industrial heating, or as raw material to produce chemicals, liquid fuels, syngas, and compressed natural gas (CNG). Carbon dioxide (CO2) and methane (CH4) are the major components in biogas, with a trace amount of contaminants, including hydrogen sulfide (H2S), water vapor (H2O), nitrogen (N2), ammonia (NH3), oxygen (O2), carbon monoxide (CO), halides, volatile organic compounds (VOCs), siloxanes, and hydrocarbons.

The source of biogas, which is anaerobic digestion of different organic matter or landfill decomposition, determine the presence and quantities of contaminants. Separation of CO2 from CH4 is necessary for increasing the heating value of biogas prior to use as a vehicle fuel or for natural gas grid injection. Adsorptive CO2 technology via solid porous adsorbents is regarded as a promising technique for separating CO2 from biogas because of low energy demand and small capital investment in comparison to conventional biogas upgrading methods such as ammonia, water, or amine solvent absorption. Porous materials such as activated carbon (AC), zeolite, metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and mesoporous silica has been extensively researched in for application in CO2 separation technology.

Recently, amine-functionalized silica has been proposed as a sorbent for CO2. The objective of this thesis is to evaluate its potential for use with biogas upgrading. We synthesized PEI-impregnated HP2MGL adsorbent for the separation of carbon dioxide from biogas for upgrading to biomethane. The effects of loadings, adsorption, and regeneration were studied. The sorbent exhibited the highest adsorption capacity of 2.73 mmolCO2/gads at 30% amine mass loading, with negligible CH4 adsorbed in simulated biogas experiments, proving a high affinity towards CO2 over CH4. The saturation capacity of the sorbent increased to 2.92 mmolCO2/gads in the presence of moisture. The sorbent was regenerated completely at 100 °C. In the presence of water, the sorbent remains stable over at least five adsorption-desorption cycles. Adsorption and desorption mechanism study under the in-situ CO2 DRIFTS study proves that CO2 adsorption on PEI-impregnated sorbent is consistent with the zwitterion reaction mechanism. Desorption of adsorbed CO2 species from amine occurs by removal of weakly adsorbed species by reduction of CO2 partial pressure and by removal of the ammonium-carbamate ions via temperature increase to 100 °C for desorption of strongly bonded CO2 molecules from amine surface.

Techno-economic sensitivity analyses show that the amine-functionalized sorbent does not only provide the technical capacity to satisfy the requirement on gas quality, but it also provides a reduction in energy consumption in addition to cost minimization. The PEI-HP2MGL sorbent used for the process achieved economic viability with natural gas at adsorption capacity of 2.7 mmolco2/gads and 2000 regeneration cycles. PEI-modified polymeric resin is an attractive choice for biogas upgrading to biomethane through CO2-adsorptive technology from the experimental and economic feasibility study.