Catalytic Performance of Coated and Non-Coated Bed Materials in Fluidized Bed Steam Gasification of Biomass

Abstract: Besides producing a carbon-neutral gas suitable for several applications, gasification of biomass in fluidized beds also leads to the formation of aromatic hydrocarbon compounds (tars). When condensing the gas, tars are plugging pipes and other process equipment which increases the need of high-cost gas cleaning systems.
The aim of this thesis was to improve the understanding of how tar compounds can be decomposed by using catalytically active bed materials. Earlier studies concluded a significant increase of reactivity by using bed material with outer layers rich in calcium. In this study, several materials with outer calcium-enriched layers were compared with fresh, non-coated, materials.
The materials catalytic performances were investigated in a fluidized bed lab-scale reactor by applying two reactions; water-gas shift and steam reforming of a model tar compound. The materials investigated in the water-gas shift reaction were two fresh samples and five coated samples. The fresh samples were silatherm (aluminosilicate) and k feldspar (potassium aluminum silicate), and the coated materials were three samples of quartz sand and two samples of impurities. The quartz sand samples were gathered from different fluidized bed combustion plants while the impurities samples were gathered from one fluidized bed gasification plant and further separated from olivine particles.
1H-indene was used as model tar compound in the steam reforming reaction, where four bed material samples with varying calcium-layer thickness were investigated.
From the experiments with water-gas shift reaction the catalytic enhancement was proved by using calcium-enriched materials. Similar gas compositions could be measured for two of the three quartz sand samples. The third sample showed a lower catalytic activity due to a thinner outer layer of calcium. The two samples of impurities showed the highest catalytic activity of all investigated materials. At a temperature of 850°C the maximum carbon conversion was 42% for one of the impurities samples. At the same temperature the corresponding CO conversion for k-feldspar was 0,6%. Silatherm had a slightly increased CO conversion compared to k-feldspar, but the catalytic performances of the fresh materials are significantly lower compared to the coated materials.
In the steam reforming reaction, 98,9% of 1H-indene were decomposed in the reaction when a calcium oxide was used as bed material. In contrary, 0% of 1H-indene was decomposed when feldspar was used.
From the experimental results it can be concluded that the materials coated with outer layers rich in calcium had excellent catalytic performance compared to the fresh materials. Although further investigations must be conducted, it can be confirmed that calcium is a component which significantly increases the decomposition of tars in fluidized bed gasification processes.