Biomass Gasification and Hot Gas Upgrading in a Decoupled Dual Loop Gasifier

A decoupled dual-loop gasifier (DDLG) has been developed in which biomass gasification and hot gas upgrading are separated into two parallel loops so that they can be optimized individually. In the gasification loop, the gasifier is designed so that the contact between volatiles and char is restrained and, therefore, the steam gasification of char is enhanced. In the upgrading loop, both a desulfurizer and a tar-reforming catalyst are used for desulfurization and tar destruction, respectively. As the in-bed desulfurizer, an iron-bearing olivine-supported ZnO (Zn/olivine) material was prepared and tested in a fixed-bed reactor. H2S sorption over ZnO, adversely affected by H2O, was accompanied by the evident reduction of ZnO and vaporization of Zn at 550 degrees C. In contrast, no obvious ZnO reduction was observed under the same conditions over Zn/olivine. The reduction resistance of Zn/olivine was investigated by means of temperature-programmed reduction and powder X-ray diffraction. In DDLG tests with pine sawdust as the feedstock and Zn/olivine + Ni/olivine as the upgrading bed material, a synergy was found between desulfurization and tar destruction. The H2O-involved reactions such as the steam gasification of char and the steam reforming of tar/hydrocarbons were intensified at elevated gasification temperatures and in the presence of Ni/olivine. As a result, the decrease of H2O favored H2S sorption by Zn/olivine, which, in turn, alleviated the sulfur poisoning of Ni/olivine. At a gasifier temperature of 850 degrees C, a steam-to-biomass mass ratio (S/B) of 0.3, and an upgrading reactor temperature of 600 degrees C, the H2O and tar contents were effectively decreased to 8.8% and 1.5 g/Nm(3), respectively. In a 2-h test, during which Zn/olivine experienced about four cycles of sulfidation/regeneration, the H2S in the product gas was decreased to 1.7 ppmv.