Single-step preparation of activated carbons from pine wood, olive stones and nutshells by KOH and microwaves: Influence of ultra-microporous for high CO2 capture

Abstract

Biomass residues are crucial feedstocks for facing climate change challenges due to high-value products, such as producing activated carbons (AC) for carbon capture. Two stages of pyrolysis followed by activation at high temperatures are the most used technique for converting lignocellulosic precursors into porous activated carbons. This process has shown to offer the highest surface areas; however, a two-stage process is undesirable as is an energy-intensive processes. Product characteristics are affected by feedstock and reaction rate conditions. In the present study pine wood (PW), olive stones (OS) and pecan nutshells (NS) were evaluated as feedstocks in the production of AC for selective post-combustion CO2 capture via a single-step pyrolysis-activation using microwave heating. Direct raw biomass impregnation was completed using potassium hydroxide (KOH). The ACs were synthesised in 8 min using 300 W of microwave power with 8.8 GJ t−1 specific microwave energy input. Samples exhibited large specific surface areas (SBET), up to 1340 m2g−1, with 70 % of ultra-micropores (<0.8 nm), fundamental for high CO2 adsorption capacity. Among the tested biomasses, PW was the best performing and physicochemical characterisation and CO2 capture studies indicated that PW-based AC has 79 % carbon, amorphous structure, and possessed larger ultra-micropores that resulted in high CO2/N2 selectivity (12.5), and one of the largest CO2 uptakes for ACs (6.2 and 4.2 mmol/g at 0 and 25 °C, respectively). The CO2 performance was investigated across a range of temperatures up to 100 °C, while cyclic regenerative performance was confirmed after 15 adsorption–desorption cycles. This study highlights the development of AC from different lignocellulosic resources by a fast and low-energy single microwave-pyrolysis activation process that can produce ultra-microporous structures implemented in post-combustion CO2 capture.