Environmental Engineering Reference
In-Depth Information
The steep asymptotic approach to 100% efficiency above 20 MPa is a result of
the sharp decrease of
H
vap
to zero beyond that pressure. For heat exchangers
with a finite surface area, the effect of the operating pressure is less pronounced.
In practice, a hundred percent transfer of the available heat in the reactor
effluent to the feedstock stream is impossible. In fact, efficiencies of approxi-
mately 75% are typical for liquid
Δ
liquid shell and tube heat exchangers. For
such an efficiency, the operating pressure should be about 20 MPa in case of
50 m
2
per kg
-
s
−1
throughput or 30 MPa in case of 25 m
2
per kg
s
−1
throughput
(see Figure 10.13).
3.
Additional Heat.
Mass and energy balance calculations show that in most cases
additional heat input is required for the reactor. This heat can be supplied exter-
nally and/or by
in situ
generation of oxidation heat. For the latter, oxygen has to
be added to the process.
In situ
oxidation is preferable over external heating
because of efficiency and construction reasons. However, it is then crucial to
do this selectively, i.e., without consuming any desired products. This is dis-
cussed further under point 4.
4.
Incomplete Conversion of the Biomass
. This is something that has to be taken
into account in reactor design considerations. Thermal decomposition of real
biomass will definitely result in carbon formation, either simultaneously with
the production of tars (condensable vapors) and gaseous compounds or as a
result of secondary reactions of tar. It was found that even under the high water
pressure prevailing in the SCWG process, steam gasification of char is still
extremely slow. This result excludes recycling of the carbon from the water out-
let stream or applying very long residence times, as a reactor design option.
In
situ
oxidation of the carbon indeed seems interesting, because of the corre-
sponding heat production inside the reactor. Besides, in case of catalyst addition
(discussed under point 5), regeneration is required to burn off any carbon depos-
ited on the catalyst surface. While considering
in situ
oxidation of the carbon, it
should be appreciated that oxygen introduced into the SCWG reactor will react
preferentially with gaseous products and dissolved organic molecules while
leaving the carbon largely unconverted. Reactor staging, or creating separate
combustion and gasification zones inside a single reactor, may be a solution.
Alternatively, a secondary wet-oxidation reactor can be used to clean the efflu-
ent water from the SCWG reactor.
5.
Catalysis
. This has been suggested to lower the required gasification tempera-
tures for complete conversion and to improve the selectivity to either H
2
or CH
4
in case SNG is aimed at. For reactor design, the application of a heterogeneous
catalyst has complicating consequences, e.g., regeneration is required (see fol-
lowing text). On the other hand, it also creates the opportunity for additional
removal of minerals that are deposited together with the coke on the catalyst
surface. Moreover, particle circulation can be used to prevent the heat
exchanger from coking and plugging. The formation of ash and coke occurs
in a temperature range of roughly 200
400
C and possibly mainly in a confined
-
region of the tubular heat exchanger.
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