Page 293 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory
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Chapter | 8  Design of Biomass Gasifiers                     269


                Entrained-flow gasifiers are essentially cocurrent plug-flow reactors,
             where gas and fuel travel. The hydrodynamics is similar to that of the well-
             known PC boiler, where the coal is ground in a pulverizing mill to sizes
             below 75 μm and then conveyed by part of the combustion air to a set of
             burners suitably located around the furnace. The reactor geometry of the
             entrained-flow gasifier is much different from the furnace geometry of a PC
             boiler. Additionally, an entrained-flow gasifier works in a sub-stoichiometric
             supply of oxygen, whereas a PC boiler requires excess oxygen.
                The gasification temperature of an entrained-flow gasifier generally well
             exceeds 1000 C. This allows production of a gas that is nearly tar-free and

             has very low methane content. A properly designed and operated entrained-
             flow gasifier can have a carbon conversion rate close to 100%. The product
             gas, being very hot, must be cooled in downstream heat exchangers that pro-
             duce the superheated steam required for gasification.
                Figure 8.15 describes the working principle of an entrained-flow gasifier
             by means of a simplified sketch. The high-velocity jet forms a recirculation
             zone near the entry point. Fine fuel particles are rapidly heated by radiative
             heat from the hot walls of the reactor chamber and from the hot gases down-
             stream and start burning in excess oxygen. The bulk of the fuel is consumed
             near the entrance zone through devolatilization; here, the temperature may
             rise to as high as 2500 C.

                The combustion reaction consumes nearly all of the oxygen feed, so the
             residual char undergoes gasification reactions in CO 2 and H 2 O environments
             downstream of this zone. These reactions are relatively slow compared to the
             devolatilization reaction, so the char takes much longer to complete its con-
             version to gases. For this reason, a large reactor length is required.
                Entrained-flow gasifier design may be classified into two broad groups:
             (i) the top-fed downflow (used by GE Energy and Siemens SFG), shown in


                        Fuel particle  Fuel particle path Gas particle




             Pulverized
              fuel + O 2
                                                                           (




                               Combustion                 Gasification
                                 zone                       zone
                  Fuel particle path  Gas path
             FIGURE 8.15 Simplified sketch of gas solid flow in an entrained-flow gasifier.
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