Conclusions

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Unformatted text preview: monstrated
SNG
generation
from
biomass
 (Mozaffarian
et
al.
2003,
2004).
In
this
process,
indirect
gasification
is
used,
and
both
gasification
and
 methanation
are
carried
out
at
atmospheric
pressure.
The
biomass
is
gasified
in
the
riser
of
a
circulating
 fluidized
bed
(CFB)
and
the
remaining
char
is
circulated
to
the
combustor
(downcomer
of
CFB).
In
this
 process,
the
heat
required
for
gasification
is
supplied
by
char
combustion
in
the
combustor.
Steam
is
 used
for
gasification
and
air
is
used
for
char
combustion.
The
lab‐scale
gasifier,
developed
in
2004,
has
a
 biomass
capacity
of
5
kg/h
and
operates
at
temperatures
of
750°
to
900°
C
(Zwart
et
al.
2006).
Direct
 gasification
was
also
tested,
which
uses
oxygen
and
steam
for
gasification
(bubbling
fluidized
bed)
and
 operates
at
850°
C.
The
gas
treatment
in
the
integrated
bio‐SNG
system
consists
of
tar
removal
with
 organic
scrubbing
liquid
technology,
and
sulfur
and
HCl
removal
with
adsorbents.
 
 Based
on
the
experiments,
an
SNG
system
was
optimized
which
consists
of
an
indirect
gasifier,
a
tar
 removal
system
which
recycles
tar
to
the
gasifier,
a
gas
cleaning
reactor
and
shift,
and
a
methanation
 combined
reactor.
The
indirect
gasifier
working
at
850°
C
produces
nearly
nitrogen‐free
syngas
and
a
 high
amount
of
methane.
The
tars
are
recycled
to
the
gasifier
in
order
to
increase
efficiency,
whereas
 the
tar
free
syngas
is
cleaned
from
other
contaminants
(e.g.,
sulfur
and
chlorine).
The
clean
syngas
is
fed
 to
a
combined
shift
and
methanation
process,
converting
the
syngas
into
SNG.
After
methanation,
 further
upgrading
(e.g.,
CO2
and
H2O
removal)
is
required
in
order
to
comply
with
the
desired
SNG
 specifications.
The
overall
net
thermal
efficiency
is
reported
as
70%
Low
Heat
Value
(LHV)
 (approximately
64%
High
Heat
Value
[HHV]).
Forty
percent
of
the
carbon
of
the
biomass
becomes
part
 of
the
SNG
and
an
equal
amount
of
carbon
is
captured
as
CO2.
The
remaining
20%
of
the
carbon
in
 biomass
becomes
as
flue
gas
from
the
process
(Zwart,
2008).
The
cost
of
the
bio‐SNG
production
 proposed
by
the
ECN
and
its
sensitivity
to
biomass
price
are
analyzed
in
sections
3.2.4.
and
3.2.5.
 3.
Environmental
Implications
and
Economics
of
SNG
 As
described
earlier,
steam‐oxygen
gasification
is
the
only
...
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