Hydrogen-Rich Gas Stream from Steam Gasification of Biomass: Eggshell as a CO2 Sorbent

The present study investigates the steam gasification of biomass with an in-process CO2 capture. The work is aimed at achieving hydrogen enrichment while reducing the carbon dioxide (CO2) concentration in the gas stream. A perceived waste resource, eggshell, was utilized as the source of the CO2 sorbent, while sawdust was used as the feedstock. The eggshell was calcined at 900 degrees C to activate it for the carbonation process. The gasification tests were conducted in a bubbling fluidized bed reactor with the calcined eggshell (CES) as the bed material in addition to being a CO2 sorbent. Thermogravimetric analysis conducted on the eggshell showed that 900 degrees C is sufficient to fully convert the calcium carbonate (CaCO3) in the eggshell to calcium oxide (CaO). The complete conversion was also evident in X-ray diffraction peaks. The effects of key process parameters, steam to biomass ratio (SBR) and calcined eggshell to biomass ratio (CEBR), were examined. Increasing the CEBR provided more CaO to the process, promoted the CO2 uptake via the carbonation reaction, and accordingly enhanced hydrogen enrichment. An increase in SBR in the CES-based tests improved the hydrogen concentration in the gas stream. A minimum CO2 volumetric concentration of 3.3 +/- 0.4% and a maximum hydrogen concentration of 78 +/- 3.6% were obtained in this study at a temperature of 650 degrees C, an SBR of 1.2, and a CEBR of 1.0. Additionally, results of the CES-based experiments showed that the water gas shift reaction is more important for the enhancement of hydrogen production than the other gasification reactions.