Design of Biomass Gasifiers - Biomass Gasification, Pyrolysis and Torrefaction - page 289

8.8 GASIFIER SIZING
The process design described in the previous section determines such operat-
ing parameters as gasification temperature, feed rates of fuel, and gasifica-
tion medium. Now we can move to the next step, which involves the choice
of gasifier configuration and type. Section 8.1.1 discusses the choice of gas-
ifier. Table 8.5 compares the choices by their strength and weaknesses.
Table 8.6 gives a range of product gas composition and heating values
obtained in several commercial or large demonstration plants of generic
types. The data of a fixed bed may not be typical as it is the value from one
manufacturer gasifying MSW. Fixed-bed gasifiers for biomass are known for
high tars unless it uses a downdraft system. Another table of typical compo-
sition is given in Table 2.6. By carefully examining these along with the
type of plant to be designed, we can make a rational choice of gasifier type.
Once the gasifier type has been chosen, the designer can then proceed
with the geometric design, where the basic sizes (the geometric dimensions
of critical components) of the reactor are determined. At this stage, the
designer decides on the geometric configuration of the reactor and its prelim-
inary size. Both configuration and size depend on the reactor technology
used.

8.8.1 Moving-Bed Gasifiers
A moving-bed gasifier may be designed on the basis of characteristic design
parameters such as specific grate gasification rate, hearth load, and space
velocity.
Specific grate gasification rate is the mass of fuel gasified per unit of
cross-sectional area in unit time. The hearth load of a gasifier may be

TABLE 8.5 Comparison of Strength and Weaknesses of Different

Types of Gasifiers

Power
Production Class Types Strength/Weakness

Fixed bed Downdraft Low heating value, moderate dust, low tar Small to

medium
scale

Updraft Higher heating value, moderate dust, high tar

Crossdraft Low heating value, moderate dust

Fluidized
bed

Bubbling Higher than fixed-bed throughput, improved
mass and heat transfer from fuel, higher heating
value, higher efficiency

Medium
scale