Heating of the lignocellulosic biomass in inert atmosphere for hours to a maximum temperature of 400-500 °C is called slow pyrolysis. The charcoal yield is 35-40% by weight. In general, the yield of liquid products would be less than the fast pyrolysis of biomass. Several types of catalysts can be employed for the pyrolysis of biomass and/or upgradation of the vapors produced from the thermal pyrolysis.

2.1.4 Fast Pyrolysis

The goal of fast pyrolysis is to produce liquid fuel from lignocellulosic biomass that can substitute for fuel oil in any application. The liquid can also be used to produce a range of specialty and commodity chemicals. The essential features of a fast pyrolysis process are very high heating and heat transfer rates, which often require a finely ground biomass feed. Carefully controlled reaction temperature of ca. 500 °C in the vapor phase and residence time of pyrolysis vapors in the reactor less than 1 s; and then quenching (rapid cooling) of the pyrolysis vapors to give the bio-oil product. The main product of fast pyrolysis is bio-oil, which is obtained in yields of up to 80 wt% of dry feed.

Fast pyrolysis is a promising process to produce transportable oil with a high volumetric energy density from bulky and inhomogeneous biomass. There are several applications foreseen for pyrolysis oil. It has been tested as a substitute for fuel oil or diesel in boilers, furnaces, engines, and turbines for heat and power generation and has been considered as a precursor for transportation fuels and chemicals. Water is the most abundant component in pyrolysis oil; typically, it is present in the range of 15-35 wt%. Probably all applications require different specifications with respect to the water content of pyrolysis oil. For fueling into a diesel engine, the water content should be below 30 wt% to decrease emissions of particles and to prevent ignition delay and phase separation. But there should also be a mini­mum amount of water present to limit NOx emissions and to ensure a uniform temperature distribution in the cylinders. For cofeeding pyrolysis oil in a mineral oil refinery, nearly all water and most organically bound oxygen must be removed. Generally, less water is beneficial for the energy density, transportation costs, stability, and acidity of pyrolysis oil. Fast pyrolysis oil possesses many undesirable properties including a high total acid number (TAN ^200), low heating value (^6560 BTU/lb), high oxygen content (^40%), chemical instability, high water content (20%), and incompatibility with petroleum fractions. Inherent low-energy density makes pyrolysis oil expensive to transport, and the high TAN makes it metallurgically incompatible with conventional transport vessels and refinery hydro­conversion equipment, both designed for feeds with TANs less than 2. In addition to these undesirable properties, pyrolysis oil is not miscible with petroleum fractions and if added into existing refinery equipment (hydrotreaters or hydrocrackers) will require a separate pyrolysis-oil feed system. Thus, pyrolysis oil needs effective pretreatment and upgradation before it is used as crude oil replacement. Depending on the reactors used, we have many kinds of fast pyrolysis processes.