Design of Biomass Gasifiers - Biomass Gasification, Pyrolysis and Torrefaction - page 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

m 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

μ

Gas particle

Fuel particle Fuel particle path

Pulverized
fuel + O 2

(

Combustion
zone

Gasification
zone

Fuel particle path Gas path

FIGURE 8.15 Simplified sketch of gas solid flow in an entrained-flow gasifier.