biomass gasification in atmospheric and bubbling fluidized bed: effect of the type of gasifying...

Biomass gasi®cation in atmospheric and bubbling ¯uidizedbed: E�ect of the type of gasifying agent on the product

distribution

Javier Gila, JoseÂ Corellab, 1, MarõÂ a P. Aznara,*, Miguel A. Caballeroa

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 e�ect of the type of gasifying agent used in biomass gasi®cation on product distribution (gas, char and taryields) and gas quality (contents in H2, CO, CO2, CH4, . . . , tars) is analyzed. Gasifying agents taken into accountare: air, pure steam, and steam±O2 mixtures. Process considered is biomass gasi®cation in atmospheric and bubbling

¯uidized bed. Previous results got by Herguido et al. (Ind. Eng. Chem. Res. 1992; 31(2): 1274±82), Gil et al. (Energyand Fuels 1997; 11(6): 1109±18) and NarvaÂ ez et al. (Ind. Eng. Chem. Res. 1996; 35(7): 2110±20) are compared.Such authors carried their research on biomass gasi®cation under similar conditions but varying the gasifying agent.

Three di�erent gasifying agent-to-biomass ratios are needed and used to compare results. The relationships betweenthe H2, CO, . . . , tar contents in the ¯ue gas and the type and amount of gasifying agent used are shown after acarefully analysis. # 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Biomass gasi®cation; Pilot plant; Gasifying agents; Bubbling ¯uidized bed

1. Introduction

This work concerns the gasi®er in biomassgasi®cation in atmospheric and bubbling ¯uidizedbed. 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 valueand the H2-content, for instance, of / in the ¯uegas are higher when gasi®cation is made withsteam than when it is made with air.Nevertheless, there are doubts or contradictorypapers about the e�ect of the gasifying agent onother results like tar content in the raw or pro-duced gas. Since there have been a research inbiomass gasi®cation in ¯uidized bed using similargasi®ers, gasifying with pure steam [1], withsteam±O2 mixtures [2] and with air [3], it wasdecided to deeply compare these results to clarify

Biomass and Bioenergy 17 (1999) 389±403

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1 Fax: +34-91-394-41-64

* Corresponding author. Fax: +34-976-76-21-42.

Fig. 1. H/C and O/C atomic ratios in the feeding in biomass gasi®cation with di�erent gasifying agents.

Fig. 2. Values of S/B and ER ratios for the 3 processes here considered (in biomass gasi®cation with di�erent gasifying agents).

J. Gil et al. / Biomass and Bioenergy 17 (1999) 389±403390

the e�ect of the gasifying agent on product distribution and produced gas quality.At least 20 operation parameters concerning the gasi®er and feedstock have an in¯uence on product

distribution and gas quality. To compare results got by di�erent authors is very di�cult thus.Nevertheless, in the work made in Spain on biomass gasi®cation in ¯uidized bed in the last 16 years,and here used for comparison purposes, several parameters were constant. It helps and allows to makean useful or valuable comparison of product distributions. The operation parameters which have been thesame or very similar in the three studies under comparison were:

Gasi®er atmospheric and bubbling ¯uidized bedbed only silica sand (without in-bed dolomite)u0/umf 2±4temperature (of the bed) 750±7808C (for steam), 780±8308C (for air or steam-O2 mixtures)2nd air injection none (in all the three cases or processes)

Feedstock small chips of pine (Pinus pinaster ) woodfeedstock moisture 10±20 wt%feeding point near the bed bottom, using two screws

Gas and Tar samplingand analysis

similar in the 3 cases (shown in NavaÂ ez et al. [3]). They are deeply discussedand compared with the ones used by other institutions in the recent paperfrom 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 di�erence in the three works or studies here compared concerns perhaps of the gasi®er free-board. Freeboard acts, in fact, as a 2nd reactor connected in series with the gasi®er bed, and in it severalreactions (like tar thermal cracking, CO-shift, etc . . .) occur. The size and temperature (400±7008C) ofthe freeboard and the gas residence time in it have to be taken into account, thus. They have been notthe same in the three above said studies.

Other di�erent operation variables in the three studies here compared are:

Gasi®er

Gasifying agent Inner diameter(cm)

Total height(m)

Feeding ¯ow rate(kg biomass/h)

For more detailssee:

(pure) Steam 15 1.2 1.5±4.0 Herguido et al. [1]Steam±O2 mixtures 15 3.2 5±12 Gil et al. [2]Air 6 0.7 0.4±0.8 NarvaÂ ez et al. [3]

Table 1

Basic ratios used for comparison of results using di�erent gasifying agents

Gasifying agent Name of the ratio used Symbol

Air Equivalence ratio ER

Steam±O2 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) 389±403 391

Fig. 3. Equivalence between ER and GR (in biomass gasi®cation with steam and oxygen mixtures).

Table 2

Operating conditions, gas composition and yields

Air3 (pure) Steam1 Steam±O2 mixtures2

Operating conditions

ER 0.18±0.45 0 0.24±0.51

S/B (kg/kg daf) 0.08±0.66 0.53±1.10 0.48±1.11

T (8C) 780±830 750±780 785±830

Gas composition

H2 (vol %, dry basis) 5.0±16.3 38±56 13.8±31.7

CO (vol %, dry basis) 9.9±22.4 17±32 42.5±52.0

CO2 (vol %, dry basis) 9.0±19.4 13±17 14.4±36.3

CH4 (vol %, dry basis) 2.2±6.2 7±12 6.0±7.5

C2Hn (vol %, dry basis) 0.2±3.3 2.1±2.3 2.5±3.6

N2 (vol %, dry basis) 41.6±61.6 0 0

Steam (vol %, wet basis) 11±34 52±60 38±61

Yields

Tars g/kg daf 3.7±61.9 60±95 2.2±46

Char g/kg daf naa 95±110 5±20

Gas Nm3/kg daf 1.25±2.45 1.3±1.6 0.86±1.14

LHV MJ/Nm3 3.7±8.4 12.2±13.8 10.3±13.5

a na Ð not available.


2. Basis of comparison

The gasifying agent-to-biomass fed ratio isdi�erent depending on the gasifying agent used.One way of comparing the gasifying agent usedcould 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 processeshere considered (gasi®cation with air, steam, andsteam±O2 mixtures). These ratios give a valuableinformation but are not good enough. Forinstance, one atom of O has not the same e�ectif it is introduced as H2O, as O2 (air) or even asCO or CO2. These ratios do not help thus to

Fig. 4. Hydrogen contents in the raw gas at the gasi®er exit vs ER and S/B using di�erent gasifying agents.


characterize and understand the e�ect 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 gasifyingagent will be ER, GR and S/B thus. Fig. 2 showsthe values of the S/B and ER ratios for the 3processes here considered. The value of the ERratio which correspond to each value of GR

using steam±O2 mixtures as gasifying agent isshown in Fig. 3 (for two di�erent values of theH2O/O2 ratio in the steam±oxygen mixture).

The range or interval of operating conditions,gas composition and yields here compared areshown in Table 2.

The produced gas in the three gasi®cation pro-cesses here compared would probably have a

Fig. 5. CO contents in the raw gas at the gasi®er exit vs ER and S/B with di�erent gasifying agents.


di�erent end-use. Electricity production seems tobe the best end-use of the produced gas whengasifying with air, but other end-uses (fuelcells?) could appear for the H2-rich gas gener-ated in gasi®cation with steam or steam±O2 mix-tures. The optimum scale of gasi®cation (Tn

biomass/h) could be thus di�erent for these threegasi®cation 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 bethe same.

Fig. 6. CO2 contents in the raw gas at the gasi®er exit vs ER and S/B with di�erent gasifying agents.


3. E�ect of the gasifying agent on the compositionand heating value of the produced gas

The main components in the produced raw gasat the gasi®er exit for the three gasifying agentsare shown in the following ®gures:

H2-content Fig. 4

CO-content Fig. 5CO2-content Fig. 6CH4-content Fig. 7C2 hydrocarbons Fig. 8

The gas composition being known, the lowheating value (LHV) of the produced gas is easily

Fig. 7. Methane contents in the raw gas at the gasi®er exit vs ER and S/B with di�erent gasifying agents.


calculated. It is shown in Fig. 9 for the threegasi®cation processes. As it is well known, theLHV is very di�erent depending on whether thegasi®cation is made with air or with steam, butFig. 9 also shows how O2 addition to steam doesnot lower very much the LHV of the gas

(decrease of only 1±2 MJ/m3n, depending, of

course, of the amount of O2 fed).Although the authors consider that each ®gure

can be understood with the attached meaning ofsymbols used, a short explanation of them is asfollows:

Fig. 8. C2 hydrocarbons contents in the raw gas at the gasi®er exit vs ER and S/B with di�erent gasifying agents.


Gasifying with air, the nitrogen dilutes the pro-duced gas and softens the increase (vs ER) ofsome parameters. So, although the trends are thesame, the magnitude of the variations in someparameters are di�erent 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 inFig. 9 for the LHV of the gas.

Considering all data using di�erent gasifyingagents, it is clearly shown (as it was expected)how the H2 and CO2 contents in the gas at thegasi®er exit increase (Figs. 4 and 6) and the COcontent decrease (Fig. 5), by the shift reaction, as

Fig. 9. Low heating value of the raw gas at the gasi®er exit vs ER and S/B with di�erent gasifying agents.


the S/B ratio is increased. In the same way, theH2 and CO contents decrease and the CO2 con-tent increase as the ER is increased (Figs. 4, 5and 6).

Methane and C2-hydrocarbons also follow theexpected trend (Figs. 7 and 8). Their contents inthe gas decrease as ER or S/B increases. This

behaviour is due to partial oxidation and steamreforming reactions.

One question that arises is which is the ``opti-mum'' S/B ratio: Using air as gasifying agentNarvaÂ ez et al. [3] found that an increase in theH/C ratio in the feeding (equivalent to anincrease in the moisture content in the biomass)

Fig. 10. Gas yields at the gasi®er exit vs ER and S/B with di�erent gasifying agents.


improves the gas quality. They recommend H/Cratios around 2.2 which is equivalent to S/Bratios around 0.28 kg/kg daf. Higher values havenot an important e�ect on the gas quality andreduce the LHV of the gas (Fig. 9).

Using steam as gasifying agent, the H2-con-tent in the gas is maximum (around 55 vol%) forS/B ratios of 0.8±0.9 kg/kg daf (Fig. 4). For thisS/B ratio, the steam content in the gas is

around 50 vol% (wet basis). This high steamcontent in the gas could be a waste of energybut the steam addition doubles the H2 contentin 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 treatmentafter the gasi®er to clean up the hot gas by usingdolomites or steam reforming (Ni based) cata-

Fig. 11. Tar content in the raw gas at the gasi®er exit vs ER and S/B with di�erent gasifying agents


lysts. A relatively high steam content in the ¯uegas increases then steam reforming reactions oftars and light hydrocarbons in this catalytic stageand avoids coke deposition and catalyst deactiva-tion. So, the authors' recommendation for the``optimum'' S/B ratio in gasi®cation with airwould be 0.30, and in gasi®cation with steam of0.8±0.9.

4. E�ect of the type of gasifying agent on the gasyield and the tar content in the raw produced gas

4.1. Gas yield

Gas yields obtained with the 3 gasifying agentsare shown in Fig. 10. Notice how important it isto give the value of the gas yield as dry gas. With

Fig. 12. Tar yield at the gasi®er exit for di�erent. ER and S/B values using di�erent gasifying agents.


this dry basis, gasi®cation with air produces a gas yield (1.4±2.4 m3n dry basis/kg biomass daf) quite

higher than gasifying with steam (0.8±1.1 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 detailresults 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 valuesobtained 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 producedusing the three mentioned gasifying agents are quite di�erent between themselves. According to recentstudies [4,5], tars generated in gasi®cation with steam are more ``easy-to-destroy'' with nickel-based cata-lysts or with dolomites than tars generated in gasi®cation with air. These authors are determining howsuch ``refractoriness'' to be catalytically destroyed also depends on the values of ER, GR or S/B usedbut 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) therepresentative main results for the three gasifying agents are:

Gasifying agent

Result/parameter (Air ER=0.30 H/C=2.2 Steam±O2 GR=0.90 H2O/O2=3 Steam S/B=0.90)1

H2 (vol %, dry basis) 8±10 25±30 53±54CO (vol %, dry basis) 16±18 43±47 21±22LHV (MJ/m3

n, dry basis) 4.5±6.5 12.5±13.0 12.7±13.3Ygas (m

3n, dry basis/kg daf) 1.7±2.0 1.0±1.1 1.3±1.4

Ytar� (g/kg daf) 6±30 8±40 70Tar� content (g/m3

n) 2±20 4±30 30±80

1 These numbers explain themselves the e�ect of the gasifying agent.


Acknowledgements

This work has been carried out thanks to theCAICYT Project No. PB96-0743 and also as anaddendum to the Project No. JOR3-CT95-0053of the EU, DG-XII, JOULE III Program.

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biomass gasification in atmospheric and bubbling fluidized bed: effect of the type of gasifying...

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