As the world pursues carbon neutrality, biomass gasification has emerged as a promising route for converting waste into valuable syngas while utilising CO₂. Energy crops such as Arundo donax (giant reed) are particularly attractive because they grow rapidly on marginal lands and naturally contain high levels of alkali and alkaline earth metals (AAEMs) like potassium and calcium. These AAEMs remain in the biochar after pyrolysis and can catalyse subsequent gasification. However, quantifying their catalytic contribution – separate from the effect of carbon structure – has been challenging.
Researchers prepared biochar from Arundo donax in a fixed‑bed reactor at 800 °C. They then removed AAEMs by acid washing to create a control sample (HCl‑char). Both raw char (R‑char) and HCl‑char were gasified with CO₂ at 800–950 °C, and their reactivities, carbon structures, surface chemistries and kinetics were systematically compared.
Gasification reactivity increased markedly with temperature for both samples. However, at any given temperature, the reactivity of HCl‑char was much lower than that of R‑char. For example, the reactivity indices R_0.5 and R_0.95 were significantly reduced after acid washing.
Structural characterisation using XRD and Raman spectroscopy revealed that the HCl‑char actually possessed a more disordered carbon matrix – with smaller crystallite sizes, larger interlayer spacing, and a higher defect density – than the R‑char. This disordered structure would normally be expected to enhance gasification. Yet its reactivity was lower, indicating that carbon structure is not the controlling factor.
X‑ray photoelectron spectroscopy showed that acid washing drastically altered oxygen‑containing functional groups: carboxyl groups (‑COO) decreased from 25.28 % to 8.12 %, while carbonyl groups (C=O) increased from 3.24 % to 9.75 %. This transformation is attributed to the removal of ‑COOM complexes (M = AAEMs). Elemental mapping confirmed that most Ca and K were leached out by acid washing.
Kinetic analysis using a model‑free method gave average activation energies of 164.30 kJ·mol⁻¹ for R‑char and 210.85 kJ·mol⁻¹ for HCl‑char – a clear demonstration of the catalytic role of AAEMs. Temperature‑programmed desorption showed that CO release from R‑char began above 790 °C, whereas HCl‑char required a higher temperature (>810 °C) and released much less CO, indicating fewer surface carbon‑oxygen complexes.
The proposed catalytic cycle is as follows: AAEMs adsorb CO₂, generating CO and forming active M‑O species; these react with adjacent carbon to form C‑M‑O intermediates, which decompose to produce more CO and regenerate the metal sites. This continuous cycle sustains the Boudouard reaction, explaining why AAEMs – not carbon structural order – dominate gasification reactivity. Even though acid washing created a more disordered carbon structure (which might be thought beneficial), the loss of AAEMs caused a dramatic drop in performance.
This work provides clear evidence that inherent AAEMs are the primary catalysts for CO₂ gasification of energy crop char, guiding future optimisation of biomass gasification processes.
DOI
10.1007/s11705-026-2650-x