Process activated ilmenite as catalyst for cleaning of biomass producer gas
Paper in proceedings, 2014
Catalytic tar cleaning has been identified as a promising technology for upgrading a raw gas originating from biomass gasification. In this work, ilmenite previously collected in the fly-ash from the Chalmers 12 MW circulating fluidized bed boiler was used as a catalyst for producer gas upgrading. The experiments were conducted in a bench scale bubbling fluidized bed reactor fed with a raw gas stream from the Chalmers 2-4 MW indirect biomass gasifier, in which silica sand was the bed material. This reactor is operated in a batch mode and after reduction on raw gas, the catalyst is regenerated using nitrogen diluted air. This study is an extension of earlier experiments using fresh ilmenite and where the concept was proven. Here, the ilmenite particles were exposed to actual process conditions and the possibility to control the operation more accurately is intended to yield kinetics on the tar decomposition but also give hint for a later process up-scaling. The effect of the ilmenite on both tar decomposition and gas composition was evaluated at 800°C and different gas-solid contact times by either varying the flowrate of raw gas or the quantity of ilmenite. Despite an expected short time-on-stream in the boiler at 900°C, the reduced ilmenite fines showed a significant effect on adjusting the gas composition with an H2/CO ratio increased from 1 in the raw gas to lightly higher than 3 in the reformed gas, a ratio potentially interesting for downstream methane synthesis. More importantly, at the contact time of 1.1s, the tar removal efficiency was as high as 52% on a tar-to-reformed gas basis, and up to 90% if benzene is excluded. Branched hydrocarbons and phenol were almost completely reformed while naphthalene, a stable molecular compound, decreased by 37% although the decomposition of branched tar compounds could contribute to the accumulation of this stable component.