A fast pyrolysis system consists basically of a fluidized bed reactor, a cyclone, a condenser, and a combustion chamber, generally constructed as shown in Figure 7.2.

The fluidized bed reactor is where pyrolysis actually occurs. The remaining constituents are responsible for phase separation. The reactor operates at around 450°C. Heating is done by an immersed electrical resistor covered with inert material (silicates, in general). The func­tion of this inert material is to increase the heat transfer between the air and the fluidizing material to be pyrolyzed through abrasive action, increasing the contact surface of the solids (DalmasNeto, 2012).

Once temperature is achieved, air feeding begins. Then heating stops and the material to be pyrolyzed is fed to the reactor. At this point, an initial temperature fall is observed, caused by air and material entrance in much lower temperatures. Reactor temperature can be

immediately reestablished by combustion of the pyrolysis’ incondensable gases or by con­trolled combustion of part of the material fed to the reactor.

The combustion of incondensable gases, such as CO, H2,andCH4 (Cortez et al., 2008), is the best option, generating enough heat for autothermal operation of the reactor, but this entails the acquisition of additional equipment. On the other side, controlled combustion of part of the material fed to the reactor is easier to be handled but means loss of product (about 10% of the material needs to be burned to maintain reactor temperature, according to Mesa-Perez, 2005).

Residence time is controlled based on material feeding rate, air flow, and reactor volume. Material characteristics such as density and size are taken into account to avoid dragging out the time. After pyrolysis, the gaseous mixture is sent to a cyclone by pneumatic conveying (by the fluidizing air itself). In the cyclone, gaseous and liquid components are separated by cen­trifugal force. The gaseous products enter the condenser. The condensable fractions are then separated by gravity: In the bottom an output is used for bio-oil gathering, while the acid ex­tract is collected at the middle of the condenser. Gases and very light particles enter a centri­fuge located at the top of the condenser, where some light particles condensate, increasing the yield of the liquid phases.

The condenser effluent gases are formed by four fractions. The first one is composed of inert atmospheric gases that adhered to biomass particles when the reactor was fed; the second one consists of inert gases fed with air in fluidization (nitrogen, CO2). The third fraction involves semioxidized pyrolysis gases such as CO and CH4; the fourth is composed of those gases that are combusted to provide energy to the system. Usually this gas phase is fed back to the system, especially due to the potential of the third fraction to provide energy to the system.

The combustion chamber is responsible for burning all combustible gases generated in the process. It acts as a restorative power cell besides being a security tool (preventing release of flammable gases into the atmosphere).

The following steps and reactions summarize pyrolysis processes (adapted from Gomes et al., 2008):

1. Drying: Humid material! solid material + H2O(g)

2. Pyrolysis: Dry material! coal + volatile products

3. Combustion reactions:

a. C(s) + O2!CO2(g) + energy

b. 2H2(g) + 02(g) ! 2H2O(g) + energy

4. Heat transfer

5. Mass transfer

The smooth operation of a fast pyrolysis system depends very little on the raw material conditions but strongly depends on its composition (organic matter amount). To be pyro — lyzed, the material might be dried and milled into particles smaller than 20 mm (Bridgwater et al., 1999). Low moisture content is desired to avoid wasted energy (or higher energy de­mand) and possible influence on calorific power of the final product. (High-moisture-content materials are frequently pyrolyzed but with the drawback mentioned previously.) Particle size might be big enough to avoid excessive biomass drag by fluidizing air (the flow of which is usually high), causing loss of nonpyrolyzed material, but also small enough to allow easy heat transfer and avoid secondary polymerization and carbonization reactions (this will cause coal yield increase, according to Sanchez, 2003).

Ganesh, 1990 found that both acid and alkaline catalysts tend to increase gas production. The same study noted that desmineralization caused an increase in the superficial area of coal.

Due to the high heating rate to which material is subjected in fast pyrolysis, the residence time might be very short, usually around 1 second (Gomez, 2002). In this condition, advanced stages of undesirable reactions (such as polymerization and/or decomposition) are avoided. Figure 7.3 presents the most probable mechanisms of formation of pyrolysis products.