The startup of a fluidized-bed gasifier is similar to the startup of a fluidized bed combustor. The inert bed materials are preheated either by an overbed burner or by burning gas in the bed. Once the bed reaches the ignition temperature of the fuel, the feed is started. Combustion is allowed to raise the temperature. After that, the air/oxidizer-to-fuel ratio is slowly adjusted to switch to gasifica­tion mode.

One major problem with fluidized-bed gasifiers is the entrainment (escape) of fine char with the product gas. The superficial velocity in a fluidized bed is often sufficiently high to transport small and light char particles, contributing to major carbon loss. A tall freeboard can reduce the problem, but that has a

cost penalty. Instead, most fluidized-bed gasifiers use a cyclone and a recycle system to return the entrained char particles back to the gasifier.

Entrained-Flow Gasifier

The startup procedure for an entrained-flow gasifier takes a long time because a startup burner must heat up the reactor vessel wall. During this time, the reactor vessel is not pressurized. Once oil or gas flame heats up the thick refrac­tory wall to ~1100 °C, the startup burner is withdrawn and the fuel is injected along with the oxidizer (Weigner et al., 2002). The hot reactor wall serves as an igniter for the fuel, which once ignited continues to burn in the combustion 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 operation of a gasifier “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 sizing. The use of a separate startup 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 startup burner in the existing fuel injector is the best option.

Tar Cracking

Several options for tar control and destruction are available; these were dis­cussed in Chapter 4. In fixed-bed gasifiers, thermal cracking or burning has been used with success. In one such design, as shown in Figure 6.24, 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.

Symbols and Nomenclature

Ab = cross-sectional area of the fluidized bed (m2)

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

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

Ci = volumetric specific heat of gas i (kJ/nm3.K)

Co = initial carbon in the biomass (kg)

Cp = specific heat of the gas (kJ/kg. C)

Ea = activation energy (kJ/mol)

EA = excess air coefficient (-)

ER = equivalence ratio (-)

F = amount of dry fuel required to obtain 1 Nm3 of product gas (kg/nm3)

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

Fuel FIGURE 6.24 Gasifier with an aspirator for cracking tar. Fresh air picks up the tar from the gasifier and injects it into the high-temperature combustion zone.

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

HHV = higher heating value (kJ/kg)

HHVd = higher heating value of biomass on dry basis (MJ/kg)

HHVa = higher heating value of biomass on dry ash-free basis (MJ/kg)

Hbed = height of the bed (m)

Hg = enthalpy of steam at gasification temperature (kJ/kg)

Hin = heat of the input gas (kJ)

[H2O] = concentration of steam (-) k = rate constant (A1)

k0 = pre-exponential constant in the Arrhenius equation (A1)

LHVbm = lower heating value of the biomass (MJ/kg)

LHVdaf = lower heating value of biomass on dry ash-free basis (MJ/kg)

LHVf = lower heating value of the solid fuel (MJ/Nm3)

LHVg = lower heating value of the produced gas (MJ/Nm3) m = mass-flow rate of carbon or char (kg/s)

mth = theoretical air requirement for complete combustion of a unit of biomass (kg/kg) Ma = amount of air required for gasification of unit mass of biomass (kg/kg)

M = fractional of moisture in the fuel (-)

Mdaf = moisture based on dry ash-free basis Mf = fuel flow rate (kg/s)

M)h = quantity of steam (kg/s)

Mg = gas produced (kg/s) n = order of reaction (-)

П = number of moles of species i (-)

N = fractional of nitrogen in the fuel in dry basis (-) nMai = total number of moles

O = fractional of oxygen in the fuel in dry basis (-)

Pc = amount of char produced per nm3 of product gas (kg/nm3) qc = heating value of char (kJ/kg)

Q = power output of the gasifier (MWth)

Qext = external heat addition to the system (kJ/Nm3)

Qg = Lower heating value of the product gas from gasification (MJ/Nm3)

QSasifiCaa<,n = heat supplied to gasify 1 mol of biomass (kJ/mol)

Qioss = heat loss from the gasifier (kJ/Nm3) r = steam gasification reaction rate (kg/s)

R = universal gas constant (0.008314 kJ/mol. K)

S = fractional of sulfur in the fuel in dry basis (-)

SC = steam to carbon molar ratio (-) t = time (s)

T = temperature (K)

Tf = gas temperature at the exit (°C)

Tg = gas temperature (°C)

T0 = gas temperature at the entrance (°C)

Ug = fluidizing velocity (m/s)

V = volume of the fluidized bed (m3)

Vbed = volume of the bed (m3)

Vdaf = volatile based on dry mass-free basis Vg = gas generation rate (m3/s)

Vg = volumetric flow rate of product gas (Nm3/s)

V = volumetric fraction of gas species i (-)

W = total steam needed in Eq. 6.22 (kg/s)

Win = rate of the char moving in (kg/s)

Wout = rate of the char moving out (kg/s) xchar = weight of the reacting char (kg)

X = fraction of char in the feed converted (-)

Xc = fixed carbon fraction in the fuel (kg carbon/kg dry fuel)

Xchar = char fraction in bed (-)

Xg = fraction of steam used up in gasification Є = voidage of the bed (-)

AI = Lagrangian multiplier for species i (-)

pg = density of air at the opening temperature and pressure of the gasifier (kg/m3) в = residence time of char in bed or reactor (s) pb = bed density (kg/m3) ps = density of bed solids (kg/m3)

Hgef = gasifier efficiency (-)

HCf = cold gas efficiency (-)

Hcg = cold gas efficiency of the gasifier (-)

Hhg = hot gas efficiency of the gasifier (-)

H„et = net gasification efficiency of the gasifier (-)

AHT = heat of formation at temperature T (kJ/mol)