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Fast Pyrolysis

Although a wide range of technologies have been investigated and developed, only two basic processes have been successfully commercialised – fluid bed and transported or circulating fluid bed. A typical fast pyrolysis fluid bed is depicted in the figure below. EBRI has four laboratory scale continuous fast pyrolysis fluid bed reactors of up to 7 kg/h.
 
Pyrolysis
Fast pyrolysis research at EBRI

EBRI has a long history in developing fast pyrolysis technology with associated activities on feedstock pre-treatment, liquid product collection, analysis and characterisation of the products, and most recently on upgrading the vapours and liquids to refinery feedstock quality hydrocarbons.

Products of fast pyrolysis

There are three main products: bio-oil, gas and char. The main product, bio-oil, is obtained in yields of up to 75 wt.% on a dry-feed basis, together with by-product char and gas. Both can be used within the process to provide process heat for pyrolysis process and or drying the biomass feed.      

Bio-oil: Pyrolysis oil typically is a dark brown, homogenous, free-flowing liquid which approximates to biomass in elemental composition. It is composed of a very complex mixture of oxygenated hydrocarbons with an appreciable proportion of water. The liquid is formed by rapidly quenching and thus ‘freezing’ the intermediate products of flash degradation of hemicellulose, cellulose and lignin. The liquid thus contains many reactive species, which contribute to its unusual attributes. A very small proportion of solid char may also be present from incomplete char separation.  

The typical maximum yield of bio-oil from woody materials is around 75 wt.% which contains 70% of the energy on the biomass. Bio-oil has a higher heating value of about 17 MJ/kg as produced with about 25% wt. water that cannot readily be separated. While the liquid is widely referred to as “bio-oil”, it will not mix with any hydrocarbon liquids. It is composed of a complex mixture of oxygenated compounds that provide both the potential and challenge for utilisation.  
 
        

Char: The charcoal by-product contains virtually all the ash and is a very effective vapour cracking catalyst so rapid and effective separation from the pyrolysis product vapours is essential. In addition any residual char in bio-oil may separate out and may also deposit on liquid phase upgrading catalysts.  About 25% of the energy in the biomass is contained in the char.

In fluid bed processes, the char is separated and commercially, part would be burned externally to provide heat for the pyrolysis reactions. The surplus can be exported for other uses. Transported bed or circulating fluid bed reactors recycle all the char with the sand to a secondary reactor where the char is burned in air to reheat the sand which is recirculated to the pyrolyser to provide the heat for pyrolysis. There is, therefore, no production of char for export from the process in these processes.  
 
        

Gas: About 5% of the energy in the biomass is contained in the gas. This usually has a low heating value as it is usually heavily diluted with recycled gas for fluidisation. It can sometimes be recovered for energy use depending on the configuration of the reactor.

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