High performance microspherical activated carbons for methane storage and landfill gas or biogas upgrade

Abstract

Microspherical activated carbons were successfully prepared via a novel synthetic route that involves hydrothermal carbonization of a renewable material, sucrose, and activation with K₂CO₃. The use of K₂CO₃ resulted in better yields (∼50%) and the retention of the spherical shape of the hydrochar, while with the less environmentally desirable and commonly used activating agent, KOH, the process occurs at the expense of the spherical morphology. The superior performance of the K₂CO₃ activated samples for methane storage and upgrade of landfill gas or biogas results from the combination of several key properties including high packing densities (∼0.9 g cm⁻³), high surface areas (up to 1400 m² g⁻¹) and micropore sizes suitable for methane storage and selective CO₂–CH₄ separation. In fact, the micropore size distributions assessed from CO₂ adsorption data through a methodology not imposing a Gaussian distribution gave meaningful values to explain both the selectivity and storage capacity of samples. Sample activated with K₂CO₃ at 800 °C presenting micropore sizes ∼0.8 nm and high packing density has high volumetric methane uptake (90 (V/V) at 1000 kPa), close to the best activated carbons reported in the literature. Sample activated with K₂CO₃ at 700 °C has narrower micropores (∼0.5 nm) and presents a remarkable selectivity (4–7) in CO₂–CH₄ mixtures for the upgrade of methane based fuels, like natural gas, landfill gas, and biogas. Although a superactivated carbon (∼2400 m² g⁻¹) was obtained with KOH activation, the low packing density and wider micropores rendered it less effective for both methane storage and upgrade.