(592a) Catalytic Fast Pyrolysis of Forest Thinnings in a Bubbling Fluidized Bed Reactor With ZSM-5 Catalysts

Catalytic pyrolysis is a technique which has been receiving increasing attention in the recent years due to its ability to produce partially deoxygenated pyrolysis oil with improved fuel properties. This change in the chemical composition of pyrolysis oil can be seen as a shift from its typical polar nature more towards conventional petroleum-derived fuels. This transformation in turn opens up possibilities for the utilization of pyrolysis oil in new applications such as co-processing in conventional oil refineries. While a multitude of biomass types have been subjected to catalytic pyrolysis, a low-cost and abundantly available feedstock is one of the main prerequisites for a full-scale industrial process. In countries with strong forest industry backgrounds, forest residues and forest thinnings are an attractive source of biomass with otherwise low economic value.

In this work, conventional and catalytic fast pyrolysis of Finnish forest thinnings was carried out in a 200 g/h bench-scale bubbling fluidized bed reactor using silica sand and two ZSM-5 catalysts as the fluidized bed material. The first catalyst was a ZSM-5 based fluid catalytic cracking additive, whereas the second catalyst was a typical ZSM-5 zeolite supplied as extrudates. The resulting pyrolysis oils, which were collected both at the middle and at the end of the catalytic two hour experiments, were characterized for their moisture content, elemental composition, heating value, viscosity and pH. Chemical composition of the pyrolysis oils was analysed using ¹³C nuclear magnetic resonance spectroscopy.

When compared to the silica sand at a reaction temperature of 475°C, the ZSM-5 based fluid catalytic cracking additive did not induce major changes in the physical properties or the chemical composition of the pyrolysis oil. The second ZSM-5 zeolite proved to be more effective in converting the pyrolysis vapours, and produced pyrolysis oil with oxygen content as low as 20 wt-%. The change in elemental composition was accompanied by an increased heating value and lower viscosity. Elimination of carbohydrate degradation products resulted in an increased proportion of aromatic compounds in the pyrolysis oil. The second catalyst was selected for further experiments where the effect of varying the reaction temperature was investigated. Decreasing the reaction temperature to 400°C caused a clear increase in the yield of organic liquids, which was accompanied by an increase in the oxygen content of the pyrolysis oil. Compared to 475°C, the pyrolysis oil produced at 400°C exhibited lower viscosity, which cannot therefore be explained by elimination of oxygenated molecules. On the other hand, increasing the reaction temperature to 550°C caused an increase in the viscosity of the pyrolysis oil. Other physical properties were not affected to such a large extent by the temperature increase.