Steam reforming of tar in hot syngas cleaning by different catalysts: Removal efficiency and coke layer characterization

Syngas produced by biomass and waste gasification processes must be ade-quately clean of tar compounds before being utilized in value-added applica-tions. Syngas cleaning by tar cracking at high temperatures is a promisingtechnique that can utilize different kinds of catalysts. However, their use islimited by the deposition of coke layers, which induces a masking phenome-non on the active surface, and, consequently, the rapid deactivation of the cat-alyst. This study addresses how the temperature (750 and 800 C) and thesteam concentration (0% and 7.5%) can affect the extent of water–gas andreforming reactions between steam and coke deposits. Two catalysts wereused: a market-available activated carbon and an iron-based alumina catalyst.The tests showed better performance of the Fe/γ-Al2O3catalyst. A massincrease of the bed was measured in tests with both the catalysts, which con-firms the deposition of the coke layer produced by tar dehydrogenation andcarbonization. Scanning electronic microscopy-energy-dispersive X-ray analy-sis (SEM-EDX) and Raman spectroscopy were utilized to investigate the natureof coke layers over the catalyst surface, with the aim of acquiring informationabout their reactivity towards the water gas reaction. SEM-EDX observationsindicate that the thickness of these carbon layers is less than 2μm. Ramanspectra suggest a negligible effect of the reaction temperature in the testedrange and, in particular, that the amorphous nature of coke layers deposited inthe presence of steam is relatively more graphitic than that obtained withoutsteam.