Environmental Engineering Reference
In-Depth Information
High
Temperature Air Gasification
Carbon (Hydrocarbon) + O
+ N
CO + CO
+ H + H
O + N
2
2
2
2
2
+[-
Q
]
L
Exothermic
1
High Temperature Steam Reforming
Carbon (Hydrocarbon) + H
O
CO + H
+[+
Q
]
L
Endothermic
2
2
2
, represents the usual
meaning in thermodynamics, i.e., heat evolved from the system is given with a
negative sign (exothermic reaction) and the heat supplied to the system is positive
(endothermic). It should also be recognized that the above reactions are very simple
representations. The exact mechanistic pathways for the reaction are much more
complex than that given above.
In wastes containing moisture, high temperature air promotes reactions associ-
ated with pyrolysis, gasification, and steam reforming — all occurring simulta-
neously. Higher temperatures are advantageous for both pyrolysis and gasification
and to form slag. It should be noted that the temperature of air should be higher
than the melting point of ash for slag to form. However, much higher air temperatures
can result in ash vaporization. In case the ash vaporization occurs, caution must be
exercised to capture the submicron-sized particles from the syngas evolved. The
properties of the gases evolved can be controlled somewhat with the aid of some
gasifying agent. A schematic diagram of gasifier used for wastes and low grade coals
is given schematically in
Figure 6.3
.
The coal or waste is fed from the top into the
gasifier. The waste rests on top of a grate so that the high temperature air can be
fed from the bottom chamber. The TIT group used relatively large-size (about 2 in.
diameter) ceramic balls in place of the grate and the high temperature air was
supplied from the top of the gasifier. High temperature air was obtained from a
regenerative air enthalpy intensifier manufactured by NFK. The syngas obtained had
a heating value of approximately 1000 to 1500 kcal/ . No information is given
on the removal of submicron-sized particles evolved from the gasification or pyrol-
ysis process. Particle-containing gas stream can be cleaned for particulates using a
suitably designed cyclone. The syngas produced is claimed to provide clean com-
bustion in furnaces and combustors without any additional concerns for the waste
and emission of undesirable pollutants.
where the - or + sign in the above equations, involved with ±
Q
m
3
industrial gas turbine and furnace is shown in
Figure 6.4
.
The gases evolved from
the gasification unit can also be used in micro gas turbines for independent power
generation;
A schematic diagram of the combined
5
y other designs for the gasifier can also be used in
place of the slope-shaped gasifier. The relative merits or drawbacks of each system,
or some other configuration, require further examination. The nonleachable slag
produced from the gasification process can be used for building and construction
material. In the TIT studies the low to medium gases produced from the pebble bed
gasifier have been used in a boiler to produce steam. In principle, a visionary zero
waste emission production facility, incorporating a waste to energy unit, can be
see
Figure 6.5
.
Man
3
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