(318d) Optimizing Tar Cracking in Sorption-Enhanced Gasification: Decomposition of Naphthalene Using Dolomite Catalysts.

Sorption-enhanced gasification (SEG) of biomass is a promising technology that produces hydrogen-rich syngas. Using bed materials with high CO₂ sorption capacity shifts the equilibrium of water gas shift reaction towards producing more H₂. Tar production is considered the Achilles heel of biomass gasification. While most tar cracking studies focus on high temperatures and fixed bed reactors, SEG is operated with a fluidized bed and at lower temperatures due to limestone and dolomite's carbonation/calcination equilibrium. This study investigates the decomposition of naphthalene, a model tar compound, at 700^oC in a fluidized bed reactor. In an inert atmosphere, calcined dolomite showed 96% conversion of naphthalene. This conversion deteriorated to 33% after 4 hours of operation. Thermogravimetric analysis performed for the spent catalyst revealed mass loss above 600^oC in an air atmosphere, indicating carbon accumulation on the catalyst. The absence of oxidizing agents like CO₂ and steam exacerbated the catalyst deactivation. Introducing 10 volume % CO₂ into the reactor gave significant insight into dry reforming of naphthalene. Naphthalene conversion remained consistent over 8 hours of reaction time at an average of 28%. The catalyst weight increased from 30g to 43 g after the experiment because the CaO/CaCO3 equilibrium was in the carbonation zone. The CO₂ adsorbed on the surface of CaO, thus reducing the amount of catalytic sites available for the naphthalene. Temperatures of 800 and 900^oC resulted in 91% and 100% conversion, respectively, by shifting the CaO/CaCO₃ equilibrium in the calcination zone. However, achieving efficient conversions at SEG operating parameters remains challenging. To enhance catalytic performance at 700^oC, we propose a bimetallic Ni-Fe/dolomite doped in K₂O catalyst that is synthesized via a sequential co-impregnation method. The presence of nickel would improve the tar-cracking abilities. The combination of dolomite’s CO₂ sorption capacity and Fe catalyzing the WGS reaction would significantly enhance hydrogen production.