Design of Biomass Gasifiers - Biomass Gasification, Pyrolysis and Torrefaction - page 311

8.12.2.3 Entrained-Flow Gasifier
The start-up procedure for an entrained-flow gasifier takes a long time
because a start-up burner must heat up the reactor vessel wall that is lined
with heavy refractory. During this time, the reactor vessel is not pressurized.
Once oil or gas flame heats up the thick refractory wall to 1100 C, the

start-up burner is withdrawn and the fuel is injected along with the oxidizer
(Weigner et al., 2002). The hot reactor wall serves as a thermal storage and
igniter for the fuel, which once ignited the fuel continues to burn in the com-
bustion zone, consuming the oxygen. For this reason, the fuel injector in an
entrained-flow reactor is also called the “burner.” The reactor is pressurized
slowly once the main fuel is ignited.
The gasifying medium is rarely premixed with the fuel. The fuel and the
medium are often injected coaxially, as in a pulverized coal (PC) burner in a
boiler or furnace. They immediately mix on entering the reactor. The opera-
tion of a gasifier's “burner” is similar to that of conventional burners, so
design methods for PC or oil burners can be used for a rough and an initial siz-
ing. The use of a separate start-up burner involves replacing it with a fuel
injector. This is especially difficult for water-cooled walls because their lower
thermal inertia cannot hold the wall temperature long enough. Integration of
the start-up burner in the existing fuel injector is the best option.

B

8.12.2.4 Tar Cracking
Several options for tar control and destruction are available. In fixed-bed
gasifiers, thermal cracking or burning has been used with success. In one
such design, as shown in Figure 8.24 , the air entering the gasifier passes
through an aspirator that entrains the tar vapor. The mixture is then burnt in
the combustion zone. The aspirator can be outside or inside the gasifier.
Fluidized-bed gasifiers can use appropriate bed materials to crack or reduce
tars. More details are discussed in Section 8.10.3.

SYMBOLS AND NOMENCLATURE

cross-sectional area of the fluidized bed (m 2 ) A b

fractional of ash in the fuel in dry basis ( ) ASH

C fractional of carbon in the fuel in dry basis ( )

volumetric specific heat of gas i (kJ/N m 3 K) C i

C 0 initial carbon in the biomass (kg)

C p specific heat of the gas (kJ/kg C)

E a activation energy (kJ/mol)

EA excess air coefficient ( )

ER equivalence ratio ( )

amount of dry fuel required to obtain 1 N m 3 of product gas (kg/N m 3 )

F

F [C] char feed rate into the gasifier (kg/s)

H fractional of hydrogen in the fuel in dry basis ( )