Environmental Engineering Reference
In-Depth Information
between 10 and 20 kJ/kg (10
20 MJ/Nm
3
) (Paisley et al.
2001
). Such a gas can be
used in variety of applications, such as turbine-based power generation units, fuels,
chemicals, and hydrogen production, with relatively simple downstream conversion
technologies. A note that a high calori
-
c value gas such as natural gas has a LHV of
30 MJ/Nm
3
.
In a standard directly heated gasi
cation process (Nguyen et al.
2010
), the
pyrolysis and gasi
cation reactions occur in a single vessel. An oxidant, air, or
oxygen combusts a portion of the biomass to provide the heat required for the
endothermic reactions. Pyrolysis requires about 15 % of the heat of combustion of
the feed to raise the reaction temperature and vaporize the products. In these sys-
tems, the reactor temperature is controlled by the oxidant feed rate. If the air is used
as the oxidant, the product gas has a low heating value of 4
5 MJ/m
3
due to
nitrogen dilution as shown in Table
1
(Roesch et al.
2011
; Roesch
2011
).
The results shown in the Table
1
were obtained in our laboratory, using an
Ankur Scienti
-
er (Roesch
2011
). As the gas composition results,
shown in Table
1
, indicate that a large percentage (
c downdraft gasi
50 %) of the syngas contained
high concentrations of N
2
for all the feedstocks considered in this study. The
relatively low percentages of H
2
, CO, and CH
4
are mainly responsible for the low
lower heating values of the syngas.
By gasifying biomass with an N
2
-free agent, the product gas will not be diluted
with large concentrations of the inert gas. Using either O
2
or H
2
O as the gasifying
agent produces medium grade product gas (
*
11 MJ/Nm
3
) (Ciferno and Marano
*
2002
). In addition, catalysts in the gasi
er and downstream can aid in the con-
version of tars and char to usable gas. With these and other advanced techniques, a
gas with a LHV of up to 20 MJ/Nm
3
is possible. The drawback to this higher
quality gas is the absence of the heat produced by the burning of char. The reactions
that break up the higher carbon chains in the biomass to produce H
2
and CO are
highly endothermic. Heat must be provided to the process without polluting the
produced gas with combustion products.
The best solution found to the problem of product stream dilution has been to
burn the char produced in the reactor in a separate chamber and transfer the heat
Table 1 Syngas composition for different feedstocks
Pelletized feedstock
Component mol% dry basis
H
2
O
2
N
2
CH
4
CO
CO
2
HHC
Alfalfa
12.823
0.020
53.004
2.067
12.245
18.589
1.253
Algae
10.768
0.035
59.395
1.564
9.911
17.317
1.011
Field grass
12.454
0.023
51.621
2.860
13.630
17.795
1.617
Hemp
10.362
0.019
58.546
2.098
12.622
14.891
1.463
Miscanthus
9.147
0.039
53.152
2.685
17.509
16.194
1.274
Peanut shells
13.410
0.033
51.017
2.635
15.833
15.735
1.337
Pine
12.300
0.498
53.422
2.696
16.568
13.368
1.150
Municipal waste
11.323
0.029
59.493
1.899
9.615
16.257
1.384
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