CFB reactor has been widely used for the pyrolysis of lignocellulosic biomass into high yield of liquid products (Rapid Thermal Process, RTP; UOP).The CFB reactor has many advantages, for example, the simple structures, high production capacity, favorable conditions of heat and mass transfer, and the convenience of operation, etc., the CFB was used as the main reactor in this study. To reduce the operation cost, part of the pyrolysis gas was used as the carrier gas, while the rest and the pyrolysis char were recycled as heat.

The CFB could be divided into two zones corresponding to the main chemical processes.

(i) pyrolysis zone: In this zone, feedstock was loaded into the bed and pyrolyzed very quickly. Since the feedstock particles were small and the heat exchanged rapidly, the heating rate was very high. For example, a small particle at 0.1-0.2 mm diameter could be heated at the rate of about 103 °C/s in an atmosphere at 1000 °C. In this zone, the main chemical process could be described as

Biomass! char + tar + H2O + gas (CO2, CO, CH4, CnHm, H2).

Temperature was another essential factor affecting the pyrolysis besides heating rate. Because the relatively high temperature was favorable to form more noncondensable gas and decrease the tar yield, moderate and carefully controlled temperature was needed.

(ii) Reduction and cracking zone: Before the pyrolysis vapors were quenched by the condenser, further reactions had taken place; for example, the tar cracked and the char was reduced. These processes produced more noncondensable gas such as CO and H2. Some CnHm also cracked at the same time. The main reactions could be expressed as

C + H2O! CO + H2,

C + CO2 ! 2CO,

CH4 + H2O! CO + 3H2,

CO + H2O! CO2 + H2,

Tar! CH4 + H2O + CnHm + H2.

Pyrolysis char contributed to secondary cracking by catalyzing secondary cracking in the vapor phase; rapid object of gasification is to get high-quality gas product. Thus, the high temperature of up to 900 ° C is wanted to increase the gas product and decrease the tar, while the relatively long residence time contributes to the secondary reactions including char reduction, tar cracking, shift reaction, etc. So the amount of CO2, CO, CH4, and H2 is far more, and the amount of CnHm is less in the gas product of gasification. By contrast, the objective of fast pyrolysis is to obtain more liquid product; it determines the operation conditions of moderate temperature and short residence time to increase the liquid pro­duction rate. Such operation conditions lead to the higher amount of CnHm and less amount of CO, CH4, and H2, which indicate that the degree of pyrolysis is not excessive.