CO2 separation from biogas using PEI-modified crosslinked polymethacrylate resin sorbent
Graphical abstract
Introduction
Climate change mitigation, eliminating greenhouse gas emissions, and demand for renewable energy sources to match the growing population are presently the most pressing issues. Biogas is a valuable renewable energy source generated from the anaerobic digestion of biodegradable organic matter, mainly made up of carbon dioxide (CO2) and methane (CH4), two potent greenhouse gases. The utilization of methane as a renewable natural gas source and carbon dioxide capture for various end-uses, such as enhanced oil recovery or sequestration, will significantly help mitigate global warming. Biogas substrate type and the operating conditions of the anaerobic digestion of different organic matter primarily determine the presence and quantity of contaminants. Although biogas could be directly utilized in combustion engines, CO2 separation from CH4 is necessary to increase biogas' calorific value to around 9.67 kWh [1] from 5.5 kWh [2] for a cubic meter of raw biogas (with an average CH4 content of 55%), so that the biomethane can be injected into the natural gas grid or used as a vehicle fuel. This translates into removing CO2, the primary non-methane component, to the point where the stream is ~ 95% methane. Partial separation of CO2 can cause engines (used for power generation from biogas) to operate with higher efficiency [3].
Currently, the removal of CO2 from biogas is performed industrially using physically or chemically based technologies such as solvents and amine scrubbing, membrane separation, pressure swing adsorption, and cryogenic separation. These technologies involve high capital and operating costs, high energy consumption, corrosion potential, and high loss of methane, leading to a lack of economic viability compared with natural gas from fossil fuel sources. In recent years, adsorptive CO2 technology via solid porous adsorbents has become an attractive and promising technique for separating CO2 from biogas because of its low energy demand and small capital investment compared to conventional biogas upgrading methods. Amine-based solid sorbents (either physically impregnated or chemically grafted onto porous supports) have been extensively researched for CO2 separation technology [4]. Solid amine-based sorbents exhibit fast adsorption and desorption rates, are tolerant, have enhanced amine-CO2 mechanism to moisture [5], possess high CO2 uptake capacity, and are regenerated by mild temperature swings [6].
Based on their different chemical and physical properties, amine adsorbents are classified into different families [7], [8], such as physically impregnated polymeric amines in porous supports [9], covalently grafted amino-silanes, covalently tethered amino-polymers grafted via in situ polymerization, self-supported polyamine adsorbents [10], and support with a hybrid of impregnation and grafting of amines [11]. The high CO2 adsorption capacity and excellent cyclic stability of polyethyleneimine (PEI)- impregnated mesoporous silica supports [12]have attracted much attention. The CO2 adsorption performance of these sorbents depends on the morphology of the supports, such as pore volume [13], pore size [14], and pore connectivity [15]. Higher pore volume, larger pore size, and excellent pore interconnectivity characterize supports exhibiting high and improved CO2 adsorption capacities. Additionally, most PEI-functionalized porous sorbent materials are in powdered form, which would have to be pelletized to overcome material and pressure loss [16].
Hyper-cross-linked polymeric (HCP) resins are organic porous materials used in CO2 adsorption, primarily because of their properties such as low density, high specific surface area, and high mechanical stability [17]. HCPs have tunable porous structures and rigid networks that prevent the porous wall from collapsing owing to their high degree of crosslinking, making them excellent solid adsorbents [18]. Owing to their properties, hyper-cross-linked polymers such as PHAP-1 (surface area = 1137 m2/g), tetraphenyl ethylene (TPE)-based HCP (surface area = 1980 m2/g) [19], and hyper-cross-linked heterocyclic porous polymers (surface area = 1022 m2/g) [20], have been studied for CO2 capture. CO2 interacts with and is adsorbed on the polymer via physisorption, leading to low CO2 selectivity and adsorption capacity at low pressures. However, the large surface area and pore structure uniqueness of the HCPs allow amine loading and CO2 adsorption, which has led to their use as support materials for amines, such as tetraethylenepentamine (TEPA) and polyethyleneimine (PEI) [21]. In the literature, poly (methyl methacrylate) (PMMA)-, polystyrene/divinylbenzene (PDVB)-, phenolic-, melamine–formaldehyde-based resins, among others, have been used as supports for amines by impregnation for CO2 adsorption [22].
Polymethacrylate PMA-based resins possess adjustable and tunable pores, high CO2 working capacity, high thermal stability, small spherical beads (0.1–5 mm diameter), relatively inexpensive materials, and scalability for use in adsorption beds [23]. Wang et al., proposed and evaluated a low CO2 concentration separation from flue gas and air streams using a PEI-impregnated PMA-based resin, HP2MGL [24]. However, its potential to separate CO2 from biogas under high CO2 concentration conditions and the effect of methane have not been evaluated. Additionally, although interest has grown over the years in amine-modified materials for CO2 adsorption from biogas streams, the economic feasibility of resin-based sorbents has not been explored.
The objective of this study was to evaluate the effectiveness of polymethacrylate resin (HP2MGL) impregnated with polyethyleneimine (PEI) for biogas purification. A PEI-impregnated resin (HP2MGL) sorbent was synthesized to separate CO2 from biogas for its subsequent upgrade into biomethane/renewable natural gas (RNG). A relatively dense PEI (MW = 1200) was selected to balance the diffusional resistance within the adsorbent pores and the thermal stability. The influence of factors such as amine loading and water content on the adsorption of CO2 was studied. Furthermore, the sorbent regeneration, cyclic stability, and underlying mechanism of CO2 adsorption and desorption on the amine-impregnated resin were explored. The economic feasibility of CO2 adsorption technology using impregnated resin sorbent for biogas upgrading was evaluated and included sensitivity analyses.
Section snippets
Materials
Branched polyethyleneimine (MW = 1200 Da, 99%) and methanol (>99.5%) were purchased from Polysciences Inc. and Sigma-Aldrich, respectively. A commercial adsorption resin, HP2MGL (bead size = 0.44 mm diameter), was purchased from Alfa Aesar Chemicals Company. All gases (>99.99% purity) used in the experiment were obtained from Airgas.
Preparation of PEI-impregnated resins
PEI-impregnated resins were synthesized using a wet impregnation method, as reported in the literature [24]. HP2MGL was pretreated at 80 °C for 1 h to remove
Characterization of adsorbents
CO2 adsorption on a solid adsorbent occurred sequentially. First, as the fluid flows on the particle, CO2 molecules must diffuse from the bulk stream to the exterior surface of the adsorbent and then diffuse into the pores of the particles and attach to the surface. Thus, CO2 adsorption occurs primarily on the particle surface, and it is essential to understand the textural properties of the adsorbent to promote its transport of CO2 to the active sites in the adsorbent pores. The textural
Conclusions
The PEI-impregnated HP2MGL adsorbent demonstrated potential for CO2 separation from biogas. After optimizing the PEI loading of the adsorbent for maximum capacity, the adsorbent removed CO2 and regenerated from methane in the surrogate biogas with and without added water. Water vapor has a promoting effect on CO2 uptake, increasing the sorbent's adsorption capacity in humid surrogate biogas and exhibiting stable performance in the presence of potential contaminants. CO2 adsorbed on the
Declaration of Competing Interest
Two authors (BJ and JNK) hold financial interests in T2C-Energy LLC, a startup commercializing a different technology to upgrade biogas to diesel fuel.
Acknowledgements
The authors acknowledge funding from the Hinkley Center for Solid and Hazardous Waste Management.
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