(779e) Multistage Fluidized Bed Reactor for Gasification

An ideal gasification process for a solid organic feedstock entails direct conversion of the feedstock into a synthesis gas, primarily CO and H₂, without any volatile or char byproducts. These byproducts coat the internal surfaces of the gasifier, rapidly degrading the performance of the gasifier and decreasing the yield of synthesis gas.

A three stage fluidized bed reactor has been developed to produce a clean synthesis gas in high yield. High quality synthesis gas can be used as a fuel for a combustion turbine or as feedstock for a chemical refinery. The first stage is the fluidized bed reactor where both the feedstock and heat transfer particles are fluidized in order to quickly heat the feedstock to a high temperature to achieve rapid pyrolysis and gasification. The second stage has a larger cross-sectional area than the first stage. This increase in area minimizes entrainment of solids in the second stage and increases the residence time to gasify char and decompose volatiles into low molecular weight gases. Typically the second stage operates at a high temperature in order to minimize condensation. The diameter of the third stage is greater than the diameter of the second stage in order to permit more char gasification and volatiles decomposition. If most of the volatiles have decomposed before entering stage three then the gases can be partially cooled in stage three. Following stage three the product stream passes through a cyclone to remove some of the entrained solids. Then the gases pass through a condenser to recover residual volatiles. Following the condenser gas samples are captured periodically for chemical analysis.

In the initial trials using this reactor, pine powder was used as the feedstock and alumina particles were used as the heat transfer medium. The temperature profile was set at 600 ^oC for stage 1, 800 ^oC for stage 2 and 300 ^oC in stage 3. Steady state operation was not achieved due to tar deposits on the alumina particles and the reactor walls. A close approach to steady state has been achieved by switching from alumina to zirconia particles and setting each stage at 800 ^oC. CO₂ was used as the fluidizing gas. Steady state was maintained for about 100 minutes and then the reactor was shut down. At this high temperature less fouling was apparent and the corresponding carbon yield in the gas fraction was about 74 %.