Design of Biomass Gasifiers - Biomass Gasification, Pyrolysis and Torrefaction - page 291

volumetric gas production rate
hearth cross-sectional area

N m 3 s m 2

Hearth load ð = Þ 5

or

energy throughput in product gas
hearth cross-sectional area

m 2

Hearth load ð MW = Þ 5 (8.28)

The hearth load in volume flow-rate of gas per unit of cross-sectional
area is also known as superficial gas velocity or space velocity, as it has the
unit of velocity (at reference temperature and pressure).
The following section discusses type-specific design considerations.

8.8.1.1 Updraft Gasifier
Updraft gasifiers are one of the simplest and most common types of gasifier
for biomass. The maximum temperature increases when the feed of air or
oxygen increases. Thus, the amount of oxygen feed for the combustion reac-
tion is carefully controlled such that the temperature of the combustion zone
does not reach the slagging temperature of the ash, causing operational pro-
blems. The gasification temperature may be controlled by mixing steam and/
or flue gas with the gasification medium.
The hearth load of an updraft gasifier is generally limited to 2.8 MW/m 2
or 150 kg/m 2 /h for biomass (Overend, 2004). For coal, it might be higher. In
an oxygen-based coal gasifier, for example, the hearth load of a moving bed
can be greater than 10 MW/m 2 . A higher hearth load increases the space
velocity of gas through the hearth, fluidizing finer particles in the bed.
Probstein and Hicks (2006, p. 148) quoted space velocities for coal on the
order of 0.5 m/h for steam

oxygen
gasification. Excessive heat generation in such a tightly designed gasifier
may cause slagging. Based on the characteristics of some commercial updraft
coal gasifiers, Rao et al. (2004) suggested a specific grate gasification rate as
100

air gasification and 5.0 m/h for steam

200 kg fuel/m 2 h for RDF pellets, with the gas-to-fuel ratio in the range

916 kg/m 2 h with air

2.5 3.0. Carlos (2005) obtained a rate of 745 steam

and air preheat at temperatures of 350 C and 830 C, respectively.
For an updraft gasifier, the height of the moving bed is generally greater
than its diameter. Usually, the height-to-diameter ratio is more than 3:1
(Chakraverty et al., 2003). If the diameter of a moving bed is too large, there
may be a material flow problem, so it should be limited to 3 4 m in diame-

ter (Overend, 2004).

8.8.1.2 Downdraft Gasifier
As we saw in Figures 8.4 and 8.6 , the cross-sectional area of a downdraft
gasifier may be nonuniform; it is narrowest at the throat. The hearth load is,
therefore, based on the cross-sectional area of the throat for a throated