Page 301 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 8 Design of Biomass Gasifiers 277

basis. By using the definition of these, one can relate the HHV on moisture-
ash-free basis to that on only dry-basis value as:

1 2 M
HHV daf 5 HHV d (8.8)
1 2 ASH 2 M
where the subscripts d and daf refer to dry and moisture-ash-free basis,
respectively; M is the moisture fraction; and ASH is the ash fraction in fuel
on a raw-fuel basis.
On a dry basis, HHV d is typically in the range 18 21 MJ/kg (Van Loo and
Koppejan, 2003, p. 48). It may be calculated from the ultimate analysis for the
biomass using the following equation (Van Loo and Koppejan, 2003, p. 29):
HHV d 5 0:3491C 1 1:1783H 1 0:1005S 2 0:0151N
(8.9)
2 0:1034O 2 0:0211ASH
where C, H, S, N, O, and ASH are the mass fraction of carbon, hydrogen,
sulfur, nitrogen, oxygen, and ash in the fuel on a dry basis.

8.6.2.3 Flow-Rate of Gasifying Medium
The amount of gasification medium has a major influence on yield and com-
position of the product gas. This section discusses methods for choosing
that amount.

Air
The theoretical air requirement for complete combustion of a unit mass of a
fuel, m th , is an important parameter. It is known as the stoichiometric air
requirement. Its calculation is shown in Eq. (3.32). For an air-blown gasifier
operating, the amount of air required, M a , for gasification of unit mass of bio-
mass is found by multiplying it by another parameter equivalence ratio (ER):

M a 5 m th ER (8.10)
For a fuel feed rate of M f , the air requirement of the gasifier, M fa , is:
(8.11)
M fa 5 m th ER 3 M f
For a biomass gasifier, 0.25 may be taken as a first-guess value for ER.
A more detailed discussion of this is presented next.

8.6.2.4 Equivalence Ratio
Equivalence ratio (ER) is an important design parameter for a gasifier. It is
the ratio of the actual air fuel ratio to the stoichiometric air fuel ratio. This
definition is the same as that of excess air (EA) used for a combustion sys-
tem, except that it is used only for air-deficient situations, such as those
found in a gasifier.