Biomass is defined as the organic material of recent biological origin. Plant-based biomass, which is considered as a suitable feedstock for pyrolysis, is mainly composed of three organic polymers – hemicellulose 20-40%, cellulose 40-60% and lignin 10-25% by weight. During pyrolysis, these organic components are thermally decomposed in the absence of oxygen. This process is irreversible and generally produces numerous chemical species in the form of pyrolysis vapors, aerosols and solid residue. The condensation of pyrolysis vapors and aerosols yields a dark brown colored liquid with a distinct smoky odor called as bio-oil. Non-condensable fraction of pyrolysis vapors usually consists of gaseous species such as carbon monoxide, carbon dioxide, methane and hydrogen. Solid residue obtained from pyrolysis is called as charcoal or char. It consists of dehydration, condensation and re-polymerization products of the non-volatile fragments of hemicellulose, cellulose and lignin that are produced during pyrolysis. The pyrolysis product classification into the above mentioned three product categories based on their physical state of existence: char (solid), bio-oil (liquid) and non-condensable gases (gas) has become a standard over the years. The relative proportions of these three product fractions significantly vary depending upon the process conditions.
The purpose of fast pyrolysis is to obtain maximum yield of bio-oil from biomass or other organic feedstocks. High heat transfer rates and temperature around 500°C is necessary to obtain maximum bio-oil yield. Along with the reaction temperature, the residence time of the pyrolysis vapors also plays an important role in the process. It can have following implications on the fate of pyrolysis vapors: 1) secondary cracking in the gas phase – where pyrolysis vapors undergo further degradation producing more low molecular weight compounds and gaseous species. The residence time of the pyrolysis vapors could be minimized by using rapid condensation or by quenching them, however it is difficult to completely eliminate the secondary decomposition reactions. The char particles, which are often entrained in the pyrolysis vapors emerging from the reactor, need to be separated prior to the condensation. This operation needs pyrolysis vapors to go through a char separation unit, which adds to the vapor residence time at high temperatures. 2) Secondary decomposition on the char surface – the vapor-char interactions cause pyrolysis vapors to condense and decompose on the char surface. This decomposition is probably catalyzed by the char. These interactions can take place inside the biomass particles of sufficiently large size or outside the biomass particles before char is separated from the products. These intraparticle vapor-solid interactions are particularly important in case of large size (> 0.5 cm) particles. When such particles are exposed to pyrolysis temperatures, the thermal wave produces a thin pyrolysis front, which travels slowly inwards depending on the rate of internal heat transfer, leaving behind the layer of char. As pyrolysis front moves inwards, the devolatalized vapors have to diffuse out through the layer of char. The char provides good site for pyrolysis vapors to condense and decompose to produce secondary char (or coke) at the expense of bio-oil. Biomass particle size < 2 mm has been recommended for the maximum bio-oil yield. The secondary char formation is also possible on the surface of char entrained in the gas phase, provided enough residence time is available.