biomass gasi®cation 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

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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

J. Gil et al. / Biomass and Bioenergy 17 (1999) 389403 391


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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.



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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.



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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.



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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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of lignocellulosic residues in a uidized bed at small pilot

scale. Eect of the type of feedstock. Ind Eng Chem Res

1992;31(5):127482.

[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

1999; (in press).

[5] Aznar MP, Caballero MA, Gil J, Martn JA, Corella J.

Commercial steam reforming catalysts to improve biomass

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