Catalyst deactivation in the steam reforming of streams derived from plastics and biomass

Resumen

This Doctoral Thesis investigates the deactivation of the heterogeneous catalysts employed in H2 production from plastic wastes and biomass. These processes involve one or more thermal steps and a steam reforming step, using the mentioned catalyst, which suffers from a severe deactivation. In particular, the processes are: (1) pyrolysis-steam reforming of high density polyethylene (HDPE); (2) pyrolysis-steam reforming of biomass; and (3) steam reforming of bio-oil (obtained from biomass flash pyrolysis). The first two processes (1 and 2) have been performed in an in-line two-step continuous process of comprising (i) pyrolysis of the feed and (ii) steam reforming of volatiles from the previous pyrolysis step, whereas the process (3) involves (i) the thermal degradation of certain bio-oil species and (ii) the in-line steam reforming step. We have studied economically viable and kinetically interesting catalytic systems consisting of Ni- and Rh-based catalysts together with other functionalities.Results have shown a progressive catalyst deactivation with time on stream, as conversion of the feedstock and H2 yield are decreased. This progressive deactivation has been correlated with (i) an extensive reaction medium analysis in order to identify the deactivation precursors; (ii) catalyst morphological characterization and (iii) the analysis of deactivating species in terms of location, morphology and composition. Thus, we have been able to correlate the phenomena occurring in the process and their relevance on deactivation, such as metal sintering, support degradation and, more notoriously, coke deposition. Correlations have been established among these phenomena, as well as with other process variables such as temperature, steam-to-carbon ratio or time on stream. Furthermore, the development of scarcely-studied and challenging techniques for coked catalyst characterization has allowed attaining valuable insights on coke, whose versatility may lead to a potential interest for further study of other coking processes. All in all, valuable insights into the origins, dynamics or mechanisms and effects of deactivation causes were attained in the studied reforming processes, in order to enhance their economic interest for their scaling-up into waste and bio-refinery.