Page 299 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 8 Design of Biomass Gasifiers 275

and mechanical components, such as grate, main reactor body, insulation,
cyclone, and others, that are specific to the reactor type. This section focuses
on gasifier process design.

8.6.1 Design Specification
For any design, specification of the plant is very important. The input
includes the specification of the fuel, gasification medium, and product gas.
A typical fuel specification will include proximate and ultimate analysis,
operating temperatures, and ash properties. The specification of the gasifying
medium is based on the selection of steam, oxygen, and/or air and their
proportions.
These parameters could influence the design of the gasifier as follows:

1. The desired heating value of the product gas dictates the choice of gasifi-
cation medium. Table 7.1 gives typical ranges of heating value of product
gases for different mediums. If air is the gasification medium, the lower
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heating value (LHV) of gas is in the range of 4 7 MJ/m , while in cases
of oxygen- and steam-based gasifiers, it is in the range of 10 20 MJ/m 3
(Ciferno and Marano, 2002, p. 4). It may be noted that when the feed-
stock is biomass, the heating value is lower due to its high oxygen and
moisture content.
2. Hydrogen can be maximized with steam, but if it is not a priority, oxygen
or air is a better option, as it reduces the energy used in generating steam
and the energy lost through unutilized steam.
3. If nitrogen in the product gas is not acceptable, air cannot be chosen.
4. Capital cost is lowest for air, followed by steam. A much larger invest-
ment is needed for an oxygen plant, which also consumes a large amount
of auxiliary power.
5. Equivalence ratio (ER) has a major influence on carbon conversion
efficiency.

For the product gas, the specification includes:
a. Desired gas composition
b. Desired heating value
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c. Desired production rate (N m /s or MW th produced)
d. Yield of the product gas per unit fuel consumed
e. Required power output of the gasifier, Q.
The outputs of process design include geometric and operating and per-
formance parameters. The geometric or basic size includes reactor configura-
tion, cross-sectional area, and height (hardware design). Important operating
parameters are (i) reactor temperature, (ii) preheat temperature of steam, air,
or oxygen, and (iii) amount (i.e., steam/biomass ratio) and relative proportion