Page 105 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 3 Biomass Characteristics 83

3.6.7 Composition of the Product Gas of Gasification
The product gas of gasification is generally a mixture of several gases
including moisture or steam. Its composition may be expressed in any of the
following ways:

Mass fraction m i
Mole fraction n i
Volume fraction V i
Partial pressure P i
It may also be expressed on a dry or a wet basis. The wet basis is the
composition gas expressed on the basis of total mass of the gas mixture
including any moisture in it. The dry basis is the composition with the mois-
ture entirely removed.
The following example illustrates the relationship between different ways
of expressing the product gas composition.

Example 3.2
The gasification of a biomass yields M kg/s product gas, with the production of
its individual constituents as follows:
Hydrogen: M H , kg/s
Carbon monoxide: M CO , kg/s
, kg/s
Carbon dioxide: M CO 2
kg/s
Methane: M CH 4
Other hydrocarbon (e.g., C 3 H 8 ): M HC , kg/s
Nitrogen: M N , kg/s
Moisture: kg/s
Find the composition of the product gas in mass fraction, mole fraction, and
other fractions.
Solution
Since the total gas production rate, M, is:
1 M H 1 M N 1 M H 2 O kg=s (i)
M 5 M H 1 M CO 1 M CO 2 1 M CH 4
the mass fraction of each species is found by dividing the individual production
rate by the total. For example, the mass fraction of hydrogen is m H 5 M H /M.
The mole of an individual species is found by dividing its mass by its molec-
ular weight:
Moles of hydrogen n H 5 mass=molecular weight of H 2 5 m H =2 (ii)
The total number of moles of all gases is found by adding the moles of i spe-
cies of gases, n 5 P (n i ) moles. So the mole fraction of hydrogen is x H 5 n H /n.
Similarly for any gas, the mole fraction is:
X i 5 n i =n (iii)
where the subscript refers to the ith species.