Producer Gas Cleaning in a Dual Fluidized Bed Reformer using Two Catalysts

The most elegant method to remove tars out of producer gas from biomass gasification is catalytic hot gas cleaning. However, most catalysts are poisoned after short operation due to carbon formation or by other contaminants on their active sites. A novel technique for catalytic gas cleaning based on two interconnected fluidized beds has been investigated. The idea is to reform the tar components into useful molecules by means of a circulating catalyst, also called oxygen carrier in the following. The producer gas is cleaned in one of the reactors, referred to as the fuel reactor (FR), while it is continuously regenerated in the air reactor (AR). The two reactors are separated by loop seals in such a way that gas leakage is prevented between the reactors while solid circulation occurs. By varying the circulation rate of material in the system, the residence time in the raw gas stream and the frequency of regeneration can be adjusted. Meanwhile, the required amount of oxygen for partial oxidation of the tars is transferred. The system described here is coupled with its FR to the Chalmers 2-4 MWth biomass gasifier while the AR is fed with nitrogen-diluted air. In preliminary tests, the effect of different catalysts on both the tar content and the gas composition was investigated. Some of the tested materials do not only reform tars, they also influence the H2/CO-ratio in a beneficial manner. In this paper, comparative results based on testing with manganese and iron based catalysts are presented. Results suggest that both show satisfying ability for regeneration from carbon deposits. Higher temperature enhances tar removal during experiment with both catalysts. Moreover, the iron-based catalyst enhances water gas shift activity, which in turns impacts the total amount of produced gas. On the other hand, the manganese-based catalyst seems to express higher propensity for tar conversion. These observations elicit an interesting flexibility of the process as a judicious set of catalyst in coordination with pertinent operating conditions can be chosen to achieve desired purposes