Civil Engineering Reference
In-Depth Information
• Hydrogen
1
2
O
2
¼
kJ
kg Hydrogen
H
2
þ
H
2
O liquid
þ
141, 486
:
8
1
2
O
2
¼
kJ
kg Hydrogen
H
2
þ
H
2
O vapor
þ
½
141, 486
:
8
ð
9
2, 511
:
6
Þ
kJ
kg Hydrogen
¼
H
2
O vapor
þ
118, 882
:
4
(combustion of 1 kg hydrogen produces 9 kg liquid water which can be
vaporized by roughly 2,511.6 kJ/kg)
• Sulfur
kJ
kg Sulfur
S
þ
O
2
¼
SO
2
þ
10, 883
:
6
Most of the oxygen used for combustion comes from atmospheric air: in
volume, 21% of oxygen (O
2
), 78% of nitrogen, and traces of argon and
carbon dioxide can be assumed as typical values. Notice that because of
the nitrogen in the air, NO
x
emissions always occur, depending mainly on
flame temperatures.
Emission control in order to minimize the formation of NO
x
, as well as that of
CO and SO
2
, is a task of boiler-plant maintenance that must be performed with the
same care as energy saving. Quite often the two problems are closely related and
can be solved together with considerable economies if they are tackled at the
same time.
As shown in the reactions described above, water always forms through the
combination of hydrogen and oxygen. The water may be in liquid or vapor state, so
two different heating values (gross and net values, otherwise named higher and
lower heating values) are considered (see Sect.
2.2
). In industrial applications,
effluents flowing from the boiler are still at a temperature of more than 373.15 K
(100
C; 212
F), so that the water is always in vapor state.
The theoretical amount of air needed for complete combustion can be calculated
as follows:
• The mass
M
o
(kg) of oxygen which enhances the complete combustion of 1 kg of
solid or liquid combustibles is determined by their composition (mainly C%;
H
2
%; S%; O
2
%). The composition of different combustibles is shown in
Table
6.2
;
• The related oxygen volume
V
o
is equal to
M
o
multiplied by the oxygen specific
volume in standard conditions, that is:
