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8/7/2019 Biomass gasication in atmospheric and bubbling uidized
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Biomass gasication in atmospheric and bubbling uidizedbed: Eect of the type of gasifying agent on the product
distribution
Javier Gila, Jose Corellab, 1, Mara P. Aznara,*, Miguel A. Caballero a
aChemical and Environmental Engineering Department, University of Saragossa, 50009, Saragossa, SpainbChemical Engineering Department, University ``Complutense'' of Madrid, 28040, Madrid, Spain
Received 11 November 1998; received in revised form 10 March 1999; accepted 2 June 1999
Abstract
The eect of the type of gasifying agent used in biomass gasication on product distribution (gas, char and tar
yields) and gas quality (contents in H2, CO, CO2, CH4, F F F, tars) is analyzed. Gasifying agents taken into account
are: air, pure steam, and steamO2 mixtures. Process considered is biomass gasication in atmospheric and bubbling
uidized bed. Previous results got by Herguido et al. (Ind. Eng. Chem. Res. 1992; 31(2): 127482), Gil et al. (Energy
and Fuels 1997; 11(6): 110918) and Narva ez et al. (Ind. Eng. Chem. Res. 1996; 35(7): 211020) are compared.
Such authors carried their research on biomass gasication under similar conditions but varying the gasifying agent.
Three dierent gasifying agent-to-biomass ratios are needed and used to compare results. The relationships between
the H2, CO, F F F, tar contents in the ue gas and the type and amount of gasifying agent used are shown after a
carefully analysis. # 1999 Elsevier Science Ltd. All rights reserved.
Keywords: Biomass gasication; Pilot plant; Gasifying agents; Bubbling uidized bed
1. Introduction
This work concerns the gasier in biomass
gasication in atmospheric and bubbling uidized
bed. The only variable here studied is the gasify-
ing agent. Three gasifying agents are considered:
air (with some moisture), pure steam, and steam
O2 mixtures.
It is very well known how the heating value
and the H2-content, for instance, of / in the ue
gas are higher when gasication is made with
steam than when it is made with air.
Nevertheless, there are doubts or contradictory
papers about the eect of the gasifying agent on
other results like tar content in the raw or pro-
duced gas. Since there have been a research in
biomass gasication in uidized bed using similar
gasiers, gasifying with pure steam [1], with
steamO2 mixtures [2] and with air [3], it was
decided to deeply compare these results to clarify
Biomass and Bioenergy 17 (1999) 389403
0961-9534/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.
P I I : S 0 9 6 1 -9 5 3 4 (9 9 )0 0 0 5 5 -0
www.elsevier.com/locate/biombioe
1 Fax: +34-91-394-41-64
* Corresponding author. Fax: +34-976-76-21-42.
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Fig. 1. H/C and O/C atomic ratios in the feeding in biomass gasication with dierent gasifying agents.
Fig. 2. Values of S/B and ER ratios for the 3 processes here considered (in biomass gasication with dierent gasifying agents).
J. Gil et al. / Biomass and Bioenergy 17 (1999) 389403390
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the eect of the gasifying agent on product distribution and produced gas quality.
At least 20 operation parameters concerning the gasier and feedstock have an inuence on product
distribution and gas quality. To compare results got by dierent authors is very dicult thus.
Nevertheless, in the work made in Spain on biomass gasication in uidized bed in the last 16 years,
and here used for comparison purposes, several parameters were constant. It helps and allows to make
an useful or valuable comparison of product distributions. The operation parameters which have been the
same or very similar in the three studies under comparison were:
Gasier atmospheric and bubbling uidized bed
bed only silica sand (without in-bed dolomite)u0/umf 24
temperature (of the bed) 7507808C (for steam), 7808308C (for air or steam-O2 mixtures)
2nd air injection none (in all the three cases or processes)
Feedstock small chips of pine (Pinus pinaster ) wood
feedstock moisture 1020 wt%
feeding point near the bed bottom, using two screws
Gas and Tar sampling
and analysis
similar in the 3 cases (shown in Nava ez et al. [3]). They are deeply discussed
and compared with the ones used by other institutions in the recent paper
from Corella et al. [4]. According to the methods used in the analized papers,
the authors are speaking of a tar which will be called tar.
The main dierence in the three works or studies here compared concerns perhaps of the gasier free-board. Freeboard acts, in fact, as a 2nd reactor connected in series with the gasier bed, and in it several
reactions (like tar thermal cracking, CO-shift, etc F F F ) occur. The size and temperature (4007008C) of
the freeboard and the gas residence time in it have to be taken into account, thus. They have been not
the same in the three above said studies.
Other dierent operation variables in the three studies here compared are:
Gasier
Gasifying agent Inner diameter
(cm)
Total height
(m)
Feeding ow rate
(kg biomass/h)
For more details
see:
(pure) Steam 15 1.2 1.54.0 Herguido et al. [1]SteamO2 mixtures 15 3.2 512 Gil et al. [2]
Air 6 0.7 0.40.8 Narva ez et al. [3]
Table 1
Basic ratios used for comparison of results using dierent gasifying agents
Gasifying agent Name of the ratio used Symbol
Air Equivalence ratio ER
SteamO2 mixtures Gasifying ratio [(H2O+O2)/Biomass, (kg/h)/(kg daf/h)] GR
(pure) Steam Steam to biomass ratio [H2O/Biomass, (kg/h)/(kg daf/h)] SB
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Fig. 3. Equivalence between ER and GR (in biomass gasication with steam and oxygen mixtures).
Table 2
Operating conditions, gas composition and yields
Air3 (pure) Steam1 SteamO2 mixtures2
Operating conditions
ER 0.180.45 0 0.240.51
S/B (kg/kg daf) 0.080.66 0.531.10 0.481.11
T (8C) 780830 750780 785830
Gas composition
H2 (vol %, dry basis) 5.016.3 3856 13.831.7
CO (vol %, dry basis) 9.922.4 1732 42.552.0
CO2 (vol %, dry basis) 9.019.4 1317 14.436.3
CH4 (vol %, dry basis) 2.26.2 712 6.07.5
C2Hn (vol %, dry basis) 0.23.3 2.12.3 2.53.6
N2 (vol %, dry basis) 41.661.6 0 0
Steam (vol %, wet basis) 1134 5260 3861
Yields
Tars g/kg daf 3.761.9 6095 2.246
Char g/kg daf naa 95110 520
Gas Nm3/kg daf 1.252.45 1.31.6 0.861.14
LHV MJ/Nm3 3.78.4 12.213.8 10.313.5
a na not available.
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2. Basis of comparison
The gasifying agent-to-biomass fed ratio is
dierent depending on the gasifying agent used.One way of comparing the gasifying agent used
could be by using the (H/C) and (O/C), at-g/at-g,
ratios existing in the process. Fig. 1 shows the
typical values of these ratios for the 3 processes
here considered (gasication with air, steam, and
steamO2 mixtures). These ratios give a valuable
information but are not good enough. Forinstance, one atom of O has not the same eect
if it is introduced as H2O, as O2 (air) or even as
CO or CO2. These ratios do not help thus to
Fig. 4. Hydrogen contents in the raw gas at the gasier exit vs ER and S/B using dierent gasifying agents.
J. Gil et al. / Biomass and Bioenergy 17 (1999) 389403 393
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characterize and understand the eect of the gasi-
fying agent. The ratios selected thus for compari-
son of results are shown in Table 1.
The relative amount (and type) of gasifying
agent will be ER, GR and S/B thus. Fig. 2 showsthe values of the S/B and ER ratios for the 3
processes here considered. The value of the ER
ratio which correspond to each value of GR
using steamO2 mixtures as gasifying agent is
shown in Fig. 3 (for two dierent values of the
H2O/O2 ratio in the steamoxygen mixture).
The range or interval of operating conditions,
gas composition and yields here compared areshown in Table 2.
The produced gas in the three gasication pro-
cesses here compared would probably have a
Fig. 5. CO contents in the raw gas at the gasier exit vs ER and S/B with dierent gasifying agents.
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dierent end-use. Electricity production seems to
be the best end-use of the produced gas when
gasifying with air, but other end-uses (fuelcells?) could appear for the H2-rich gas gener-
ated in gasication with steam or steamO2 mix-
tures. The optimum scale of gasication (Tn
biomass/h) could be thus dierent for these three
gasication processes. So, comparison of results,
regarding gas composition, has to be made care-fully, taking into account that both, at least,
scale and end-use of the produced gas can not be
the same.
Fig. 6. CO2 contents in the raw gas at the gasier exit vs ER and S/B with dierent gasifying agents.
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3. Eect of the gasifying agent on the composition
and heating value of the produced gas
The main components in the produced raw gas
at the gasier exit for the three gasifying agentsare shown in the following gures:
H2-content Fig. 4
CO-content Fig. 5
CO2-content Fig. 6
CH4-content Fig. 7
C2 hydrocarbons Fig. 8
The gas composition being known, the low
heating value (LHV) of the produced gas is easily
Fig. 7. Methane contents in the raw gas at the gasier exit vs ER and S/B with dierent gasifying agents.
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calculated. It is shown in Fig. 9 for the three
gasication processes. As it is well known, the
LHV is very dierent depending on whether thegasication is made with air or with steam, but
Fig. 9 also shows how O2 addition to steam does
not lower very much the LHV of the gas
(decrease of only 12 MJ/m3n, depending, of
course, of the amount of O2 fed).
Although the authors consider that each gurecan be understood with the attached meaning of
symbols used, a short explanation of them is as
follows:
Fig. 8. C2 hydrocarbons contents in the raw gas at the gasier exit vs ER and S/B with dierent gasifying agents.
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Gasifying with air, the nitrogen dilutes the pro-
duced gas and softens the increase (vs ER) of
some parameters. So, although the trends are the
same, the magnitude of the variations in someparameters are dierent using air or other gasify-
ing agents. The clearest examples of this beha-
viour are shown in Figs. 6, 7 and 8 for the CO2,
CH4 and C2-hydrocarbons, respectively, and in
Fig. 9 for the LHV of the gas.
Considering all data using dierent gasifying
agents, it is clearly shown (as it was expected)how the H2 and CO2 contents in the gas at the
gasier exit increase (Figs. 4 and 6) and the CO
content decrease (Fig. 5), by the shift reaction, as
Fig. 9. Low heating value of the raw gas at the gasier exit vs ER and S/B with dierent gasifying agents.
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the S/B ratio is increased. In the same way, the
H2 and CO contents decrease and the CO2 con-
tent increase as the ER is increased (Figs. 4, 5
and 6).Methane and C2-hydrocarbons also follow the
expected trend (Figs. 7 and 8). Their contents in
the gas decrease as ER or S/B increases. This
behaviour is due to partial oxidation and steam
reforming reactions.
One question that arises is which is the ``opti-
mum'' S/B ratio: Using air as gasifying agentNarva ez et al. [3] found that an increase in the
H/C ratio in the feeding (equivalent to an
increase in the moisture content in the biomass)
Fig. 10. Gas yields at the gasier exit vs ER and S/B with dierent gasifying agents.
J. Gil et al. / Biomass and Bioenergy 17 (1999) 389403 399
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improves the gas quality. They recommend H/C
ratios around 2.2 which is equivalent to S/B
ratios around 0.28 kg/kg daf. Higher values have
not an important eect on the gas quality and
reduce the LHV of the gas (Fig. 9).
Using steam as gasifying agent, the H2-con-tent in the gas is maximum (around 55 vol%) for
S/B ratios of 0.80.9 kg/kg daf (Fig. 4). For this
S/B ratio, the steam content in the gas is
around 50 vol% (wet basis). This high steam
content in the gas could be a waste of energy
but the steam addition doubles the H2 content
in the gas respect to the pyrolisis experiments
(Fig. 4).
On the other hand, for several end-uses of thegas it is necessary a secondary step or treatment
after the gasier to clean up the hot gas by using
dolomites or steam reforming (Ni based) cata-
Fig. 11. Tar content in the raw gas at the gasier exit vs ER and S/B with dierent gasifying agents
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lysts. A relatively high steam content in the ue
gas increases then steam reforming reactions of
tars and light hydrocarbons in this catalytic stage
and avoids coke deposition and catalyst deactiva-
tion. So, the authors' recommendation for the``optimum'' S/B ratio in gasication with air
would be 0.30, and in gasication with steam of
0.80.9.
4. Eect of the type of gasifying agent on the gas
yield and the tar content in the raw produced gas
4.1. Gas yield
Gas yields obtained with the 3 gasifying agents
are shown in Fig. 10. Notice how important it is
to give the value of the gas yield as dry gas. With
Fig. 12. Tar yield at the gasier exit for dierent. ER and S/B values using dierent gasifying agents.
J. Gil et al. / Biomass and Bioenergy 17 (1999) 389403 401
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this dry basis, gasication with air produces a gas yield (1.42.4 m3n dry basis/kg biomass daf) quite
higher than gasifying with steam (0.81.1 m3n, dry basis/kg biomass daf).
4.2. Tar content
Tar content in the produced gas is shown in Fig. 11 for the 3 gasifying agents. Analysing in detail
results in this gure it is seen how tar contents follow this order:
4.3. Tar yield
Another way of indicating the tar generated is as tar yield (g tars/kg biomass daf). The values
obtained for the 3 gasifying agents are shown in Fig. 12. The same just mentioned conclusions for tarcontent apply to tar yield.
4.4. Tar composition
Tar yield or tar content in the ue gas is not enough to fully describe the tar problem. Tars produced
using the three mentioned gasifying agents are quite dierent between themselves. According to recent
studies [4,5], tars generated in gasication with steam are more ``easy-to-destroy'' with nickel-based cata-
lysts or with dolomites than tars generated in gasication with air. These authors are determining how
such ``refractoriness'' to be catalytically destroyed also depends on the values of ER, GR or S/B used
but such study is not ready for publication yet (it will be published elsewhere).
5. Conclusions
Under the best and/or selected (indicated below) conditions (and without in-bed use of dolomite) the
representative main results for the three gasifying agents are:
Gasifying agent
Result/parameter (Air ER=0.30 H/C=2.2 SteamO2 GR=0.90 H2O/O2=3 Steam S/B=0.90)1
H2 (vol %, dry basis) 810 2530 5354
CO (vol %, dry basis) 1618 4347 2122
LHV (MJ/m3n, dry basis) 4.56.5 12.513.0 12.713.3
Ygas (m
3
n, dry basis/kg daf) 1.72.0 1.01.1 1.31.4Ytar (g/kg daf) 630 840 70
Tar content (g/m3n) 220 430 3080
1 These numbers explain themselves the eect of the gasifying agent.
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Acknowledgements
This work has been carried out thanks to the
CAICYT Project No. PB96-0743 and also as an
addendum to the Project No. JOR3-CT95-0053
of the EU, DG-XII, JOULE III Program.
References
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[2] Gil J, Aznar MP, Cabllero MA, France s E, Corella J.
Biomass gasication in uidized bed at pilot scale with
steamoxygen mixtures. Product distribution for very
dierent operating conditions. Energy and Fuels
1997;11(6):110918.
[3] Narva ez I, Orio A, Corella J, Aznar MP. Biomass gasi-
cation with air in a bubbling uidized bed. Eect of six
operational variables on the quality of the produced raw
gas. Ind Eng Chem Res 1996;35(7):211020.
[4] Corella J, Orio A, Toledo JM, Biomass gasication withair in a uidized bed: Exhaustive tar elimination with
commercial steam reforming catalysts, Energy and Fuels
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[5] Aznar MP, Caballero MA, Gil J, Martn JA, Corella J.
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J. Gil et al. / Biomass and Bioenergy 17 (1999) 389403 403