*Advances in Bioenergy Production Using Fast Pyrolysis and Hydrothermal Processing DOI: http://dx.doi.org/10.5772/intechopen.105185*

non-condensable gaseous products. In the absence of oxygen, combustion cannot occur; instead, pyrolysis happens. Pyrolysis processes can be classified as torrefaction, slow pyrolysis, intermediate pyrolysis, fast pyrolysis, flash pyrolysis, microwave pyrolysis, and hydrothermal processing. These pyrolysis processes differ from one another based on the operating conditions such as residence time, heating rate, and pyrolysis temperature, which in turn affect the yield of products (gas, bio-oil, and biochar) [3, 4]. Moderate temperatures and short residence times tend to produce more liquids. The operating conditions of these different thermal conversion processes, along with their product distribution and biomass feed particle size needed, are shown in **Table 1**.


#### **Table 1.**

*Operating conditions of various pyrolysis processes and their product fractions (bio-oil, biochar, and gas) [2].*

The three pathways char formation, depolymerization, and fragmentation describe the primary conversion of biomass during the pyrolysis process. Intra- and intermolecular rearrangement reactions generally favor char formation resulting in higher thermal stability of the residue. The formation of benzene rings and the combination of these rings into an aromatic polycyclic structure characterize char formation. The release of water or non-condensable gas (devolatilization) generally accompanies these rearrangement reactions. The breaking of polymer bonds characterizes depolymerization, a dominant reaction during the initial stages of pyrolysis. When the temperature is sufficiently greater than the activation energies for the bond dissociation, depolymerization occurs, increasing the concentration of free radicals. It is followed by stabilization reactions producing monomer, dimer, and trimer units. These volatile condensable molecules at ambient conditions are found in the liquid fraction. Fragmentation involves breaking polymer bonds and even monomer bonds, resulting in the formation of non-condensable gases and a range of organic vapors that are condensable under ambient conditions [4, 31–33].

The decomposition of three lignocellulose components (hemicellulose, cellulose, and lignin) releases condensable vapors and non-condensable gases. The condensable vapor includes methanol, acetic acid, acetone (mainly from hemicellulose), anhydrous monosaccharides, hydroxyacetaldehyde (mainly from cellulose), phenols, and heavier tars (from lignin decomposition) apart from water vapor. The waterinsoluble heavier tars contain larger molecules obtained from splitting ether and C-C bonds in lignin. The condensable vapors are condensed to form bio-oil (a dark brown and free-flowing organic liquid mixture). It usually contains 15–35 wt.% water resulting from the original moisture and as a pyrolysis product. Pyrolysis temperature determines the degree of devolatilization of biomass. There are significant differences between the pyrolysis behaviors of hemicellulose, cellulose, and lignin, which are responsible for most physical and chemical property modifications during the pyrolysis process. Hemicellulose and cellulose decompose over a narrow temperature range. Lignin decomposes over a wider temperature range than hemicellulose and cellulose [4, 31, 32, 34, 35].
