Investigation of sulfidation and regeneration of a ZnO-adsorbent used in a biomass tar removal process based on catalytic steam reforming

Abstract

In the field of allothermal biomass gasification with steam in a fluidized bed, the removal of tar is the critical step in the process chain. In the approach presented in this work, in a catalytic steam reforming process the tar species are converted to permanent gas components by means of a Ni-catalyst at 500°C. The issue with catalysts based on nickel is that these are poisoned by sulfur components in the synthesis gas. This leads to an increased catalyst deactivation and consequently to elevated operating costs of the facility. The main sulfur species in the synthesis gas of an allothermal steam gasifier is hydrogen sulfide (H2S). In order to reduce the sulfur-related deactivation of the Ni-catalyst, zinc oxide (ZnO) can be used as an upstream adsorbent to remove the main sulfur compound H2S.The aim of this work is to investigate the loading process and the regenerability of a commercial ZnO-adsorbent under realistic conditions. The sulfidation of the ZnO was done with a real synthesis gas produced from wood pellets with a steam blown laboratory gasifier. In order to examine the sulfidation process in the packed bed, the permanent gas composition and the H2S-concentration of the synthesis gas were measured at five sampling points in the reactor and at the reactor outlet. The adsorbent was operated at a constant temperature of 300°C.As a second focus, the regeneration of the used zinc oxide was examined. Regeneration cycles were carried out with mixtures of air, nitrogen and steam in different compositions at elevated temperatures between 650°C and 800°C.The sulfided and regenerated adsorbents were analyzed with a scanning electron microscope (SEM) in combination with energy-dispersive X-ray spectroscopy (EDXS).The sulfidation tests showed the development of the sulfidation process in the packed bed during the experiment. In contrast to the related literature, the ZnO-adsorbent used here turned out to be unsuitable for regeneration under the tested conditions. The used adsorbent was a commercially available product and mainly consisted of ZnO. The SEM–EDXS examination showed that the sulfur was still bonded on the adsorbent after different regeneration experiments.