Characteristics of olivine as a bed material in an indirect biomass gasifier

•After a period of activation, olivine shows good characteristics as a catalyst of tar destruction.•When activated, olivine increases the yield of H2.•Enrichment of the olivine by inorganics from the fuel has noteworthy effects on the activity of olivine and the overall process.•Sulfur addition to the combustion side of the system shows a positive effect on the total tar reduction.•Addition of silica-sand to the system results in a poorer performance of the system.

The use of untreated olivine as the bed material in a biomass gasifier is investigated in this work, in which activation of the material is the main focus. The experiments were carried out in the Chalmers 2–4-MWth indirect biomass gasification unit and comprised analyses of the gas composition and bed material, as well as changes in tar yield. Starting from the raw material, the first signs of activation, in the form of a reduction in the tar yield, were observed already during the second day of the operation. The tar yield continued to decrease with time, and by the fourth day it was reduced by 30%, as compared to the yield on the first day of the operation. Analysis of the bed samples showed accumulation of inorganics within the bed material, with a share of potassium being present in leachable form. Thermodynamic calculations support the indications from the experiment that potassium can be released under gasification conditions and may play an important role in the activation of olivine. To examine the impacts of S and silica on the activity of olivine, two experiments were conducted. The addition of S to the combustion side gave a positive effect in terms of the tar levels in the raw gasification gas. The addition of silica sand revealed, as expected given the affinity of potassium for silicone, negative influences on the tar yield and gas composition that could not be attributed to mere dilution, as compared with the gas produced during operation with pure olivine.