Investigation of Tar Conversion Over Biomass Char
Creators: name: Korus, Agnieszka
Publisher: name: University of Lincoln
Year: 2023

description: Tar release is one of the main challenges for a further development of gasification technology. While heavy aromatic compounds condensation causes installation clogging, lighter molecules remain in a gaseous phase, contaminating producer gas. Therefore, catalytic reforming of tar has been extensively studied. To simplify a conversion mechanism, a single tar compound is often used in a fundamental research. In this dissertation, toluene was selected as a model tar molecule. Biochar was used as a catalyst, as its availability, as a gasification by-product, makes it an economically beneficial material. Toluene conversion over biochars prepared under uniform conditions, from three differentiated tree species, was examined. The comparison of various woods revealed, that toluene conversion over a char prepared from coniferous pine was lower, compared to deciduous-trees chars. It was attributed to less abundant active sites and lower surface area of a pine char. Moreover, a higher microporosity resulted in a quicker deactivation of this char, as suggested by a conversion decrease with time. Since one of the main distinctions between deciduous and coniferous trees is a high resin content in the letter, an acetone extraction of pinewood prior to char preparation was performed. This pre-treatment influenced pyrolysis process, yielding char with slightly improved parameters and an increased short-term performance. Another objective of this work focused on plausible toluene conversion pathways. To this end, along with toluene conversion, a yield of its by-products was also measured. The experiment was carried out under pyrolytic conditions to enable separation of a heterogeneous, catalytic toluene conversion from the effects of an oxidising agent. Then, an H2O/N2 atmosphere was studied to assess the role of steam and a possibility of its interactions with toluene during reforming. It was concluded that a majority of toluene decomposed heterogeneously over char bed, yielding coke and releasing H2. Demethylation was a competitive, less favoured pathway leading to benzene and CH4 formation, with a selectivity up to ?15 %. Some secondary substitution and dehydrogenation reactions were also observed, leading to xylenes, ethylbenzene and styrene. The steam introduced in a reforming mode did not influence toluene conversion pathways. Its role was limited to char and coke gasification, and thus a prolongation of catalytic activity of the bed. Although the two processes were not directly interacting, a plausible competition for char surface between steam and toluene was suspected. An additional set of tests was designed to further clarify some of toluene conversion aspects, i.e. the roles of an oxidising agent, char surface chemistry and tar molecule’s substituents. An intermittent toluene feeding experiment suggested that no homogeneous toluene conversion was initiated by any char-released radicals. Comparison of benzene, toluene and p-xylene conversion suggested an importance of methyl group presence to the compound’s reactivity and its conversion pathways. An addition of an extra CH3– group did not however improve the conversion any further, although it enhanced secondary, rearrangement reactions. Comparison of O2 and steam as an oxidising agent suggested that while steam favoured char/coke oxidation and did not react directly with toluene, O2 was primarily consumed by toluene in ring-opening, homogeneous reactions thus it did not regenerate catalyst bed. Char pre-treatment, applied to differentiate alkali and alkaline earth metals content, resulted in drastic changes in char’s reactivity towards oxidation, yet it did not affect its performance as a catalyst in pyrolytic toluene conversion.