Biomass is one of the renewable and potentially sustainable energy sources and has many possible applications varying from heat generation to the production of advanced secondary energy carriers. The latter option would allow mobile services like the transportation sector to reduce its dependency on the fossil fuel supply. This article reviews the state-of-the-art of the fluidization technology applied for the gasification of biomass aimed at the production of gas for subsequent synthesis of the liquid energy carriers via, e.g., the Fischer-Tropsch process. It discusses the advantages of the gasification technology over combustion, considers the size of the conversion plant in view of the local biomass availability, assesses the pros and cons of different gasifier types in view of the application of the product gas. Subsequently the article focuses on the fluidized bed technology to discuss the main process parameters and their influence on the product composition and the operability of the gasifier. Finally a synthesis process (FT) is introduced shortly to illustrate the necessary gas cleaning steps in view of the purity requirements for the FT feed gas.

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Because of the fibrous nature of woody and herbaceous biomass and the widely distributed appearance of the feedstock with respect to its size, the circulating fluidized bed (CFB) is a very suitable type of reactor for thermal conversion of this potentially renewable and sustainable energy source. Of the thermal conversion processes, combustion and gasification are the most well-known. The advantage of gasification above combustion is that it produces a secondary gaseous energy carrier (i.e., product gas or syngas) from solid biomass. This secondary energy carrier can subsequently be upgraded to, for example, hydrogen-rich fuel gas for fuel cells, converted to liquid transportation fuels [such as Fisher-Tropsch (FT) diesel or dimethyl ether (DME)], or used for synthesis of other chemicals. During operation, the CFB internally recirculates a certain amount of inert material, also referred to as "bed material". This bed material accumulates a part of the energy released during (partial) combustion of biomass and distributes it along the reactor, ensuring a nearly constant temperature throughout the bed. A variety of bulk solids can be used as bed material, with quartz sand being the mostly well-known. The drawback of quartz sand and other bed materials containing a significant amount of silica is that they are prone to agglomerate formation when herbaceous biomass with high alkali and chlorine content in the ash is being used as fuel. These agglomerates can cause a blockage inside the reactor and, therefore, unscheduled maintenance interruptions in the operation. To overcome this problem, alternative bed materials have been proposed, with one of them being magnesite. Because of its low silica content (<4% wt ), it proved to be a suitable bed material for continuous pilot-plant operation, even with high-alkaline biomass fuels. During recent experiments conducted using a 100 kW th CFB test rig at Delft University of Technology, the influence of magnesite on the composition of the gas produced during steam-oxygen blown gasification of biomass was observed and investigated. Magnesite showed activity in promoting the water-gas shift reaction, (steam) reforming of methane and C 2 hydrocarbons toward their equilibrium, and reducing the tar [here, being toluene, xylenes, polycyclic aromatic hydrocarbons (PAHs), and phenolics] concentration to ca. 6.7 g/m n,wnf 3 compared to ca. 9 g/m n,wnf 3 measured during a base-case experiment with quartz sand as the bed material. The concentration of PAHs and phenolics was reduced even to 1.9 g/m n,wnf 3 , which is below 2 g/m n 3 , being considered as an important limit for many downstream applications (

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Agglomeration in fluidized bed combustion can be a big operational problem, leading to unwanted defluidization and shutdown of the installation. Therefore, the onset of such events has to be reliably detected in an early stage and combined with counteractions to avoid further agglomeration and defluidization. The suitability of the attractor comparison method to detect agglomeration in combination with different counteraction strategies is investigated on a laboratory-scale (∼1 kWth) and a small commercial-scale (∼1 MWth) fluidized bed combustor. The agglomeration characteristics and the time until defluidization occurs can vary considerably depending on scale and process conditions, but also for similar operating conditions on the same scale. In all cases, attractor comparison has shown to detect the approach of defluidization early enough to prevent defluidization if a suitable counteraction strategy is applied. A temporary increase of fluidization velocity to promote agglomerate break-up is not a useful method to avoid agglomeration on laboratory-scale. A decrease in operating temperature below the melting points of potassium silicates can be a successful emergency strategy to ensure continued trouble-free operation. However, a subsequent temperature increase leads to further agglomeration, potentially very rapid, with alkali still present in the bed; therefore, some strategy to replace the bed content or neutralize the alkali is necessary. Semicontinuous replacement of bed material is shown to be a successful permanent solution to avoid defluidization. Yet, its application requires careful economic consideration. For larger reactor scales, agglomeration can occur more localized, which justifies several measuring positions. Especially for monitoring transition regions, for example, start-up, the application of a moving reference that has a constant negative time offset to the evaluation window is advantageous.

Supercritical Water Gasification is a promising approach to convert biogenic residues such as cattle manure, fruit/vegetable waste, and cheese whey into valuable biofuels.

Gasification of pelletized sugar cane bagasse, the waste residue from sugar cane, was carried out using an atmospheric pressure 100 kW th steam-oxygen blown circulating fluidized bed gasifier (CFBG). The purification of the syngas produced was carried out using a high temperature filter unit which contains 3 rigid ceramic candles with an outer diameter of 60 mm and a length of 1500 mm.Keeping the gasification temperature in the range of 800°-860°C and using magnesite M85 as the bed material, the effects of operational conditions (e.g. steam to biomass ratio (SBR) and oxygen to biomass stoichiometric ratio (ER)) and of catalytic and non-catalytic filtration on the composition distribution of the product gas and tar formation were investigated. Higher temperature and SBR values were favorable for increasing the mole ratio H 2 to CO and decreasing the tar compound (PAH) concentration on dry basis, measured via solid phase adsorption (SPA) sampling and subsequent GC analysis varying from approximately 2 to 6 g/m n 3 . At the same operational conditions, in the downstream filter, the conversion of tar content obtained using catalytic filter candles increased by a factor of 2.8 compared to the one with non-catalytic filter elements. The specification and distribution of major ash forming elements were investigated by SEM-EDS. Experimental results showed that bagasse is an interesting fuel for syngas production via the gasification process, both from a gas composition and from a tar production point of view.