Design of Biomass Gasifiers - Biomass Gasification, Pyrolysis and Torrefaction - page 306

Energy Balance
The heats of combustion for different gas constituents are taken from Table C.2
(Appendix C). They are:
CO: 12.63 MJ/nm 3
Hydrogen: 12.74 MJ/nm 3
Methane: 39.82 MJ/nm 3
We note that 1 kg of feed produces 0.145 kmol of gas, the volumetric com-
position of which is:
CO: 27.5%
CO 2 : 3.5%
CH 4 : 2.5%
H 2 : 15%
N 2 : 51.5%
By multiplying the heating value of the appropriate constituents of the prod-
uct gas, we can find the total heating value of the product gas (the volume of
1 kmol of any gas is 22.4 nm 3 ):

nm 3

ð 12 : 63 3 0 : 275 1 12 : 74 3 0 : 15 1 39 : 82 3 0 : 025 Þ MJ =

4nm 3

kg feed
The total energy input is equal to the heating value of the feed, which is
28.4 MJ/kg.
From Eq. (8.40) , the cold-gas efficiency is:
ð

3 0 : 145 kmol = kg feed 3 22 : = kmol 5 20 : 6MJ =

20 : 6 = 28 : 4 Þ 3 100 72 : 5%

5

8.12.1.2 Hot-Gas Efficiency
Sometimes gas is burned in a furnace or boiler without being cooled, creat-
ing a greater utilization of the energy. Therefore, by taking the sensible heat
of the hot gas into account, the hot-gas efficiency, η hg , can be defined as:

Q g M g 1 M g C p ð T f 2 T 0 Þ

η hg 5 (8.41)

LHV f M f

where T f is the gas temperature at the gasifier exit or at the burner's entrance
and T 0 is the temperature of the fuel entering the gasifier. The hot-gas effi-
ciency assumes the heating of the unconverted char to be a loss.

Example 8.4
The gas produced by the gasifier in Example 8.3 is supplied directly to a burner
at the gasifier exit temperature, 900 C, to be burnt for cofiring in a boiler. Find
the hot-gas efficiency of the gasifier.

Solution
The product gas enters the burner at 900 C (1173K). To find the enthalpy of the
product gas, we add the enthalpies of its different components. Specific heats of