Catalytic fast pyrolysis of agricultural residues and dedicated energy crops for the production of high energy density transportation biofuels. Part I: Chemical pathways and bio-oil upgrading

Fast pyrolysis is a controlled process of biomass thermal decomposition designed to yield high volumes of liquid bio-oil, which can subsequently be upgraded into high energy density transportation biofuels such as green gasoline, green jet fuel and green Diesel. The quality of bio-oil and the efficiency of the overall biomass - to - biofuel process can be enhanced significantly by assisting the fast pyrolysis process with an appropriate catalyst. Catalytic fast pyrolysis (CFP) intends the improvement of specific bio-oil properties like thermal stability, heating value and acidity and the selective production of valuable hydrocarbons (benzene, toluene, xylene, ethene, propene) or biofuels as end products. The present work is the first communication of a thorough two-part review on the CFP of lignocellulosic biomass emphasizing on the exploitation of agricultural residues and dedicated energy crops. It presents the main types of these feedstocks in Europe and USA, their chemical composition and their potential for biofuel production. Then, the mechanism of the fast pyrolysis of biomass is studied, the most appropriate fast pyrolysis reactors are discussed and the basic routes for physical and chemical bio-oil upgrading (dehydration, decarboxylation, decarbonylation, hydrodeoxygenation and condensation) are presented. Finally, the major upgrading technologies of liquid bio-oil through hydrotreating, catalytic cracking, gasification and steam reforming, and the production of renewable hydrocarbon fuels through Fischer-Tropsch synthesis, are reviewed. In the present, first part of the review, the attention focuses on the specific mechanisms and chemical pathways and technologies of fast pyrolysis and bio-oil-upgrading. An understanding of the CFP technology requires also an extensive review on the experimental studies of the pyrolysis of agricultural residues and energy crops and the results obtained over the large number of the different catalytic systems that have been used. Due to the large volume of existing information from the relevant catalytic studies, their review is the subject of a separate publication which forms the second part sequence of the present work. The novelty of this two-part review lies both on the specific attention on the feedstocks of agricultural residues and dedicated energy crops and on the large volume of relative updated information collected, presented, and critically discussed.