Validation of a fluidized bed thermogravimetric method with integrated gas analysis for CO2 capture by activated hydrochar from pistachio shells

Hydrothermal carbonization (HTC) of pistachio shells was performed in a high-pressure batch reactor at 200 °C for 2 h, yielding a carbon-enriched hydrochar. Elemental analysis shows an increase in carbon mass fraction from 44.76 % to 54.09 % and a decrease in atomic O/C and H/C ratios, confirming carbonization as visualized in a Van Krevelen diagram. The hydrochar was chemically activated by potassium hydroxide (KOH) impregnation and thermal treatment, yielding 16–28 wt.% activated hydrochar. Adsorption isotherms were determined in a thermogravimetric, fluidized-bed reactor using a stepwise CO2 concentration program limited to 50 vol.% CO2 in N2 (0–5–10–25–50–0 vol.% CO2 at 100 kPa total pressure). The setup was extended by integrating online gas analysis to provide an independent, time-resolved mass-balance cross-check. Validation was performed using Lewatit VP OC 1065 by an internal Langmuir parity check and comparison with literature-based Toth model representations; gas analysis is demonstrated using a representative low-concentration step and by comparing Langmuir models derived from gas-based versus gravimetric loadings at 50 °C. For activated hydrochar, equilibrium points were obtained up to 50 vol.% CO2 (pCO2 ≈ 50 kPa) and show decreasing loading with increasing temperature. For literature comparison and indicative saturation reporting, isotherm fits were extrapolated to pure CO2 at 100 kPa: the maximum loading derived from the raw weighing signal was 1.84 mmol/g; after buoyancy correction, the corresponding value is 1.45 mmol/g.