Catalytic secondary methods for the removal of tar derived from biomass gasification: use of low-cost materials and study of the effect of sulfur species on the steam reforming activity of the catalysts

Biomass gasification could potentially mitigate the actual dependency on fossil fuels. The practical application of this technology still faces many challenges to be considered a sustainable and profitable energy production source. One of the drawbacks of this technology is the production of undesirable by-products such as high molecular weight hydrocarbons collectively known as “tar” and sulfur compounds. These unwanted compounds must be removed before syngas end-use applications as they can foul pipes and reduced the performance of equipment downstream the gasifier as well as poison the catalyst used for upgrading the syngas. Catalytic steam reforming stands as an appealing tar removal technology in the small and medium sized gasification plants where heat management is crucial and recovery of the energy content within the tar compounds is desirable avoiding wastewater effluents and disposal of adsorbents. Nickel based catalysts have been the preferred choice in industrial applications for the reforming reactor. However, deactivation by carbon deposition is at present an unresolved problem which must be addressed before commercial application of biomass gasification technology. Moreover, the presence of sulfur compounds even at the low concentration found in most biomass feedstocks is deleterious for the steam reforming activity of the catalyst. This thesis comprises four experimental studies, each of them deal with a specific arguments of the hot gas cleanup technology of biomass syngas. The main focus was on the steam reforming activity of nickel-based catalyst and the effect of sulfur and the potassium-sulfur interactions on the steam reforming performance of the catalyst. The main contributions from these studies are; 1) the development of a less time and energy consuming synthesis procedure for the production of a mayenite-supported nickel catalyst using low-cost precursors. This new method involves the addition of the nickel precursor during the mayenite synthesis procedure. Compared to the “wet impregnation” technique the developed method showed slightly lower toluene steam reforming activity but greater stability, which was ascribed to a higher carbon deposition tolerance. 2) Better understanding on the sulfur poisoning of catalysts under steam reforming conditions at laboratory scale. The results evidenced that for a deeper knowledge of the sulfur poisoning, the calculation of the sulfur coverage should be more accurate and new methods for its measurement are required. 3) Comprehension of the mechanism of interaction between potassium and sulfur on a sulfur passivated commercial nickel catalyst under reforming conditions using real biomass syngas. The preferential adsorption site for sulfur and potassium was determined for the applied experimental conditions and catalyst and a mechanism involving the interaction of potassium with the sulfur chemisorbed on the active sites was proposed