An assessment of pinecone gasification in subcritical, near-critical and supercritical water

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Pinecone is a lignocellulosic forest residue with value-added industrial importance in terms of energy and materials production. Although promising, pinecone has received inadequate attention as a biofuel feedstock for thermochemical conversion. Water above its critical temperature (TC≥374°C) and critical pressure (PC≥22.1MPa), termed as supercritical water, has high kinetic energy and densities similar to that of gases and liquids, respectively. When employed in gasification, supercritical water has an ability to completely dissolve organics and gases. Therefore, this study identifies the candidacy of pinecone as a precursor for conversion to hydrogen through hydrothermal gasification. Pinecone was gasified in subcritical water (300 and 350°C; 21MPa), near-critical water (370°C; 22MPa) and supercritical water (450 and 550°C; 23MPa) to investigate the impacts of temperature (300–550°C), feed concentration (10–25wt%) and residence time (15–60min). The impacts of alkali catalysts (e.g., Na2CO3, NaOH and KOH) at a loading of 30wt% were examined to maximize hydrogen yields. The hydrochar generated from gasification in subcritical water, near-critical water and supercritical water were physico-chemically characterized through proximate and ultimate analysis (carbon-hydrogen-nitrogen-sulfur-oxygen), thermogravimetric analysis, X-ray diffraction, Fourier transform infra-red spectroscopy, Raman spectroscopy, Scanning electron microscopy and Nuclear magnetic resonance spectroscopy. In the non-catalytic gasification of pinecone, highest hydrogen (1.42mmol/g) and total gas yields (6.6mmol/g) with lower heating value (488kJ/Nm3) of the gas products were obtained in supercritical water at 550°C with 10wt% feed concentration for 60min. Moreover, at 30wt% catalyst loading, highest hydrogen yield was obtained from KOH (3.26mmol/g) followed by NaOH (2.71mmol/g) and Na2CO3 (1.96mmol/g). Hydrochars generated in supercritical water at 550°C had a greater content of aromatic carbon and were thermally stable. The findings reveal, for the first time, the potential of pinecone for hydrogen production through subcritical, near-critical and supercritical water gasification, as well as the prospective of its hydrochar for environmental and material applications.