Biogas upgrading via CO2 removal onto tailor-made highly ultramicroporous adsorbent materials

Abstract

Biogas is a renewable energy source that can be an alternative to meet growing energy demand and reduce greenhouse gas emissions. The calorific value of biogas is diminished by the presence of CO2, the most abundant impurity presents in the production of this biofuel. In this study, ultramicroporous activated carbons (ACs) were developed from Prosopis ruscifolia sawdust (Vinal) by activation with KOH solution. The influence of temperature (873, 973, and 1073 K) and the KOH/biochar impregnation mass ratio (IR = 1, 2 or 3) on ACs main chemical, textural, and morphological characteristics were examined and related to its ability to capture CO2. The impregnation mass ratio was the most influential variable for the capture of this polluting species. The developed ACs were highly ultramicroporous and presented BET surface areas between 500 and 2430 m2 g−1. The AC developed at 973 K and IR = 2 showed the best performance in CO2 adsorption, under equilibrium conditions, at 273 K (6.2 mmol g−1). This result was attributed to its higher content of pores with diameters between 0.4 and 0.9 nm. Dynamic studies carried out in a fixed-bed column with this AC showed that it was able to capture 1.5 mmolg−1 of CO2 and ∼0.3–0.4 mmol g−1 of CH4 from single component streams, whereas when the conditions were similar to those of a biogas upgrade process (CH4 80% and CO2 20%), CO2 uptakes slightly decreased. Competition studies carried out using a CO2/CH4 mixture of equimolar concentrations pointed out a greater selectivity towards CO2. Regeneration studies showed that this AC retained between 97 and 98% of its maximum initial CO2 adsorption capacity after five adsorption-desorption cycles.