Biogas upgrading to methane: Application of a regenerable polyethyleneimine-impregnated polymeric resin (NKA-9) via CO2 sorption

Abstract

Biogas upgrading technology is important for increasing the heating value of biogas via separating CH4 and CO2, before biogas can be utilized as a vehicle fuel or for grid injection. In this work, using a porous polymeric resin (NKA-9), we prepared a new sorbent, namely polyethyleneimine (PEI)-impregnated NKA-9, for use in biogas upgrading to methane via CO2 sorption firstly. The adsorption performance, regeneration behavior, cyclic stability and CH4/CO2 separation performance were then evaluated in detail. The sorbent exhibited high adsorption capacity of up to 151.1 mg/g with negligible CH4 adsorbed, demonstrating its high selectivity towards CO2 over CH4. In column breakthrough experiments, binary mixtures were separated effectively under each set of reaction condition, where high CH4 purities of >95% could be obtained as the final product in the outlet gas. As tested, excessive PEI loading, high temperature and low CO2 concentration in biogas would result in unfavorable effects on biogas upgrading. In addition, we tested the regenerability of the sorbent using the two most common methods-N2 and CO2 purging, where the sorbent could be regenerated completely at temperatures above 85 °C and 155 °C, respectively. During the long-term operation, PEI-impregnated NKA-9 exhibited excellent cyclic stability, with 1.4% and 5.2% decreases in adsorption capacity after 30 adsorption-regeneration cycles. Detailed characterization revealed that the sorbent was chemically stable at 155 °C, in contrast to other conventional amine-modified sorbents. Therefore, the regenerable PEI-impregnated NKA-9 sorbent could be a promising choice for biogas upgrading to methane via CO2 sorption.