Agglomeration of Olivine with Potassium- or Silicon-Rich Agricultural Residues Under Conditions Relevant to Dual Fluidized Bed Gasification

Abstract

The agglomeration characteristics of olivine-and potassium (K)-rich grape marc or silicon (Si)-rich wheat straw were studied in a laboratory-scale fluidized reactor and compared under air combustion, steam gasification, or CO 2 gasification atmospheres at temperatures relevant to a dual fluidized bed gasifier. Agglomeration with grape marc is found to be induced mostly by the formation of adhesive K-rich layers onto the olivine, a process that is significantly augmented in a steam atmosphere compared to that in the air or CO 2 . The formation of an initial Krich outer layer, which is caused by the mere physical attachment of K-species onto the olivine surface, facilitated the formation of an inner layer. This inner layer exhibits chemical reactions between grape marc ash and olivine, with the ash elements involved in the chemical reactions differing between combustion and steam gasification environments. Agglomeration with a wheat straw is most significant under air combustion conditions and depends primarily on the temperature rather than on the atmosphere.

Calcite or Kaolinite Addition for Mitigating Bed Agglomeration during Combustion and Steam Gasification of Potassium-Rich Agricultural Residues with Olivine as the Bed Material

“…Interestingly, it was observed from the SEM analyses performed on the cross-sections that continuous layers are unlikely to form on olivine sand. This implies that the addition of calcite may have inhibited the K-rich layer formation process, which has been reported for olivine sand without an additive due to the interactions between K-rich grape ash and olivine. For instance, the presence of a Ca-rich additive in the bed might have interfered with the substitution reactions that would otherwise occur between K + from grape marc and Mg 2+ /Fe 2+ from olivine.…”

“…This is also supported by the EDS point analysis performed on the layer that the atomic composition of Ca is 19% compared to 6% of K. The raw data of the EDS point analysis can be found in the Supporting Information (Table S1). Both Ca and K have been reported to react with olivine sand to form layers through ion-exchange reactions, , i.e., Ca 2+ or K + to substitute Mg 2+ /Fe 2+ in olivine. It has been suggested that statically Fe 2+ is likely to be replaced first before Mg 2+ is substituted .…”

“…However, all of these previous studies on the use of additives have been based on the use of silica sand as the bed material. In contrast to the reference case for silica sand, for which agglomeration is typically induced mainly by the chemical reactions between silica and K species forming low-melting-temperature K 2 O*SiO 2 eutectics, agglomeration with the steam gasification of K-rich feedstock with olivine sand has been suggested to be induced by the formation of viscous K-rich layers on olivine through ion-exchange reactions . This implies that the influence of additives will also be different when alternative bed materials are used.…”

“…In addition, the studies identified that the positive effects of kaolin on the prevention of agglomeration of olivine were based on the case where agglomeration is induced by ash-derived melts due to the high concentration of both Si and K in the ash , or high concentration of both K and Cl . The effects of kaolin/kaolinite on agglomeration triggered by reactions between olivine and feedstock ash, as what was reported by Li et al, are still not clear and require further investigation.…”

“…Olivine sand is the most common type of bed material used in full-scale DFB gasifiers, mainly due to its catalytic properties for gasification reactions . Olivine sand is a silica-free bed material, which has also been shown to be more resistant to bed agglomeration than silica sand in a fluidized bed combustor and/or gasifier. ,, Nevertheless, the recent study by Li et al reported that significant bed agglomeration was identified during the steam gasification of K-rich fuel with olivine as the bed material. It is therefore important to identify additives with strong potential to mitigate agglomeration in olivine bed-based systems, especially for use with K-rich feedstock.…”