Fast pyrolysis of biomass : primary products and reaction pathways

Sammanfattning: Pyrolysis of biomass to produce liquid fuel and chemicals has been considered as an alternativeto fossil fuel because biomass has a lower environmental impact; moreover, it is renewable andcould be sustainable. However, the process of bio-oil production involves a series of complexchemical reactions which are dependent on the biomass feedstock and composition,temperature, heating rate as well as residence time. In this thesis, pyrolysis was carried out in amicro-pyrolyser connected to a gas chromatograph with a mass spectrometer/flame ionisationdetector to separate and identify the formed volatile compounds.Firstly, the influence of temperature and residence time on the primary product yield andmechanistic pathways was investigated for the pyrolysis of cellulose, xylan and lignin attemperatures ranging between 400 – 600°C and residence times between 0.5 – 5 s. The resultshowed a general increase in the yield (count/μg sample) for most of the volatile compoundswith increasing temperature and residence time. Additionally, the interaction between theindividual biomass components was investigated. A comparison of the experimental andpredicted results showed that the product yields for some of the volatile compounds wereinhibited, especially for the cellulose-xylan-lignin blend and the native birch wood. This maybe due to the chemical interaction between the biomass and the presence of inorganic materials.The co-pyrolysis of palm kernel shell (PKS), mahogany (MAH) and iroko (IRO) sawdustshowed that the yield of the volatile compounds is dependent on the biomass composition andblend ratio. The co-pyrolysis of PKS, MAH and IRO in equal proportions showed an increasedrelative yield of the sugars compared to the other blend ratios investigated. Finally, the effectof dilute acid pretreatment on PKS, MAH and IRO sawdust prior to fast pyrolysis wasinvestigated. The removal of inorganic materials leads to increased yield, especially the sugarsand the furans. These results are important for understanding the formation mechanism of thepyrolysis products, selection of relevant operating conditions and the selection of a suitablemethodology that could enhance the pyrolysis product yield.