Environmental Engineering Reference
In-Depth Information
and the equilibrium constant:
2
x
1
1
x
x
P
P
aa
a
2
x
K
KP
1
+
x
+
CO
H
K
=
=
=
P
=
x
2
(
)
Px
1
-
1
-
x
2
+
HO
2
Substituting the values of
K
and total pressure, give the following values of
fractional steam conversion:
P (atm)
800K
1000K
1500K
K
= 0.04406
K
= 2.617
K
= 608.1
1
0.2054
0.8506
0.9992
10
0.0662
0.4554
0.9919
34
0.0360
0.2673
0.9732
These results illustrate well the trade-off between temperature and pressure
effects, since increased temperature permits a higher operating pressure while attain-
ing the same conversion. It demonstrates the importance of operating the reactor
vessel at elevated pressure to decrease the size and cost. In this system the adverse
effect of pressure can be compensated for by operation at a higher temperature.
Similar considerations can be done for other hydrogen fuels, including liquid fuel
(kerosene, approximate formula C
11
H
21
) and waste fuels. However, the calculations
would require the gasification kinetics for the fuel examined. Data on some fuels
can be obtained in advanced topics on combustion.
6.10
HIGH TEMPERATURE AIR COMBUSTION USING
PURE OXYGEN
Combustion temperature can be raised using pure oxygen in place of low oxygen
concentration in air as the oxidant. Practical applications on the use of oxygen are
limited to situations where very high temperatures are required, such as melting
furnaces for glass and metal, because of the high cost for producing pure oxygen.
However, with increased efforts for improvements in the production process for
oxygen, significant cost reduction can be expected. One such attractive technology
for O
2
production is membrane technology and vacuum pressure swing absorption
(VPSA). Other membrane technologies are becoming attractive for producing oxy-
gen at relatively low cost. Use of pure oxygen is more popular in the United States
than in Japan because of the relatively low costs. The general trend in the United
States has been to develop systems using oxygen-enriched combustion air rather
than low oxygen concentration air.
Oxygen-enriched oxidant in itself has many benefits in the combustion of liquid
fuels.
16
The use of oxygen-enriched atomization air, using, for example, an air assist
atomizer, results in significant benefits with only a small quantity of oxygen. This
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