**5. Fast pyrolysis**

Fast pyrolysis typically involves high temperatures (450 and 550°C), high heating rates (10–1000°C/s), and short residence times (0.5–2 s) [36]. It is the most promising thermal process to produce a higher amount of liquid fuel (bio-oils) than other thermal conversion processes. Fast pyrolysis can produce up to 75 wt% bio-oil [37], which can be used directly or as an energy carrier after upgrading.

Fast pyrolysis suppresses secondary reactions (cracking and repolymerization) by having short vapor residence times (rapid removal and quenching of condensable primary volatile vapors) and maintaining high heating rates, thereby maximizing the yield of condensable vapors (bio-oil). This results from rapid quenching and condensing intermediate degradation products of hemicellulose, cellulose, and lignin to bio-oil without further reactions, such as breaking down larger molecular weight (MW) components into smaller MW gaseous products. The rapid quenching of intermediates results in bio-oil having many reactive species, contributing to its unusual characteristics. Rapid and simultaneous depolymerization and fragmentation of cellulose, hemicellulose, and lignin fractions with a rapid increase in temperature form condensable vapors. Rapid

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

removal and quenching shorten the residence time at high temperatures and trap many of these fractions inhibiting further reactions (depolymerize, decompose, degrade, crack or condense with other molecules) to form more non-condensable gases [4, 38, 39].

The main product of the fast pyrolysis process is bio-oil (65–75%), with smaller amounts of biochar (10–25%) and non-condensable gases (10–20%). The distribution of bio-oil, biochar, and gases depends on the biomass composition, rate, and duration of heating. The fast pyrolysis process has the capability to produce bio-oil with high fuel-to-feed ratios. To strike a balance between thorough devolatilization and minimal secondary cracking of vapors, the optimum pyrolysis temperature range for bio-oil production is 425–600°C, with a maximum yield of around 500°C [10]. Due to the higher cellulose and hemicellulose content in wood than in energy crops and agricultural residues, woody biomass (poplar, sawdust, forest, and wood residue) produces the highest bio-oil yield of around 75%, followed by energy crops (reed) and agricultural residues (wheat straw, flax straw, etc.). Product yield in fast pyrolysis is affected by the feed particle size. Smaller particle size increases the heat transfer rate, thus, increasing bio-oil yield. Feedstock particle size and pyrolysis temperature need to be optimized for maximum bio-oil yield [4, 36, 39]. A finely ground (usually <1 mm) biomass feed is required to achieve very high heat transfer rates, thereby very high heating rates reducing heat and mass transfer limitations. Due to the absence of secondary reactions, the overall fast pyrolysis process is highly endothermic. Fast pyrolysis favors low moisture content biomass (<10 wt.%) to minimize water content in bio-oil. Low moisture content also facilitates grinding the feed to give sufficiently small particles to ensure rapid heating and hence fast pyrolysis [4, 37].

The central part of the pyrolysis process is the reactor used, where the thermal conversion reactions occur. Many reactors are used in the pyrolysis process, such as entrained flow reactor, fluidized bed reactor, fixed bed reactor, autoclave, rotating cone reactor, and plasma reactor [40]. These reactors can be classified into subcategories according to the flow of material and phenomena, such as circulating, co-current, counter-current, and crossflow. The amount of bio-oil depends on the reactors used and the operating conditions. The continuous developments in pyrolysis technologies explore many reactor designs to optimize pyrolysis performance and produce high-quality bio-oil. Because of its moisture contents, a higher heating value (HHV) of the bio-oil produced is half the HHV of crude oil. However, each reactor type has specific characteristics, bio-oil yielding capacity, advantages, and limitations. The crucial characteristic steps of the fast pyrolysis process are: the pyrolysis reaction takes place with high heat and heat transfer rates, thus, the particle sizes of biomass materials need to be small enough to enhance such heat transfer; the pyrolysis reaction temperature ranges from 450 to 550°C in the vapor phase; short residence times for the vapor up to two seconds; rapid quenching and condensing the vapors into bio-oil. Common reactor types used for fast pyrolysis are described below [41–45].

#### **5.1 Packed bed reactor**

The packed bed pyrolysis reactor system contains a reactor with a gas cooling and cleaning system. These reactors are common types of reactors with cylindrical shapes and packed with solid packing materials, such as firebricks, steel, or concrete; they can be packed with catalysts too. The feed enters from one side and the product is obtained from the other. The relatively fine biomass solids move down and contact a counter-current upward-moving product gas stream. The catalyst pellets are packed in a given section and are unmovable where the pyrolysis reactions occur in this section. Some of the advantages of these packed bed reactors are catalyst recovery and recycling, which gives good economic impacts [41, 42].

### **5.2 Bubbling fluidized-bed reactor**

Fluidization is a phenomenon in which fine solids are transformed into a fluid-like state through contact with a gas or liquid. The particles in the fluidized bed are present in a semi-suspended state when the gas velocity maintains a critical value known as the minimum fluidization velocity. The fixed bed transforms into a fluidized bed at this stage when the fluid drag is equal to the particle weight. Bubbles are made at the openings at which the fluidizing gas enters the bed, where the packing solids above the gas entrance are pushed aside until they create a void space through which the gas can enter at the initial fluidization velocity. Uniform mixing, uniform temperature distribution, and operation in a continuous state are the main advantages of bubbling fluidized-bed reactors [43, 44].

#### **5.3 Circulating fluidized-bed reactor**

A circulating fluidized-bed reactor works on the same principle as the bubbling fluidized bed except that the bed is highly expanded and solids continuously recycle around an external loop comprising a cyclone and loop seal. In this circulating fluidized bed, the reactor does not contain any bed and does not have any separate upper surface. The most important advantages of circulating fluidized-bed reactors over other reactor configurations include internal recycling of huge bulk particles reaching the top of the vessel back to its bottom, a good void range, and no distinct upper bed surface in the column [42, 45].
