modeling circulating fluidized bed biomass gasifiers. results from a pseudo-rigorous 1-dimensional...

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Fuel Processing Technolog

Modeling circulating fluidized bed biomass gasifiers. Results from a

pseudo-rigorous 1-dimensional model for stationary state

Alvaro Sanz, Jose Corella *

Department of Chemical Engineering, University ‘‘Complutense’’ of Madrid, 28040 Madrid, Spain

Received 3 March 2005; accepted 2 August 2005

Abstract

Results from a 1-dimensional and semirigorous model for atmospheric and circulating fluidized bed biomass gasifiers (CFBBGs), presented in

the (previous) paper by Corella and Sanz [J. Corella, A. Sanz, Modeling circulating fluidized bed biomass gasifiers. A pseudo-rigorous model for

stationary state. Fuel Process. Technol. 86 (2005) 1021–1053], are shown here. Process variables predicted by the model are gas composition (H2,

CO, CO2, CH4, C2Hn, H2O and O2 contents), gas yield, tar content in the flue gas and char concentration in the solids. Both axial profiles in the

riser and values at the gasifier exit are calculated from the model and are shown here for some selected sets of process variables. Variables

analyzed in depth are: total air flow (used as equivalence ratio, ER), percentage of secondary air flow, height (location) of the secondary air flow,

biomass moisture and biomass flow rate, expressed as the biomass weight hourly space velocity in the gasifier. All the results from the model

agree both with known published data and with some tests made to check the model.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Biomass gasification; Fluidized beds; Tar; Biomass processing; Energy; Modeling

1. Introduction

It is very well known how thermochemical gasification

produces a valuable gas, a mixture of H2, CO, CO2, CH4,

C2Hn, etc., with some tar and other impurities, by using a

gasifying agent and a organic feedstock (biomass, coal,

residues, etc.). Atmospheric circulating fluidized bed (CFBs)

reactors are promising gasifiers because of their very high

throughputs, which in the case of biomass range between 1500

and 4000 kg biomass fed/h m2 of cross-sectional area of the

gasifier. Nevertheless, CFB gasifiers face some technical and

economical problems which difficult their use. A good model

for CFB gasifiers could improve both their design and

operation, reduce any associated problems and facilitate the

implantation of this technology.

Corella et al. at the Universities of Zaragoza and Madrid

(Spain) started to study the modeling of fluidized bed

biomass gasifiers in the mid-1980s (i.e. Ref. 2). More

recently [3], they discussed the reaction network existing in

0378-3820/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.fuproc.2005.08.003

* Corresponding author. Tel./fax: +34 91 394 4164.

E-mail address: [email protected] (J. Corella).

a CFB biomass gasifier and the problems associated with the

accuracy of the kinetic equations needed for the existing

complex reaction network. Further, Corella et al. [4],

presented a model for bubbling fluidized bed (BFB) biomass

gasifiers, gasifying with pure steam. That model identified

the four main, for modelling purposes, chemical reactions

among the reaction network existing in the gasification

process. With only four kinetic parameters, the model

predicted quite well the BFB gasifier. Finally, Corella and

Sanz [1] have presented a whole model for CFBBGs. Such

model is 1-dimensional and for steady state. The model has a

semirigorous character because of the assumptions that had

to be introduced by lack of accurate knowledge in some parts

of the modeling. A deep literature review on the field was

also made in that previous paper, reason why no much

literature will be cited in this paper now. This paper will now

show some significative results from the model developed by

Corella and Sanz [1].

Among the number of variables which can be calculated for

many different experimental conditions, the results presented

here will always point out the tar content in the produced gas

which is the key index of its quality. Even more, to use the

gasification gas in gas turbines or in gas engines the tar content

y 87 (2006) 247 – 258

www.else

C (wt.%, dry basis) 50.0

H (wt.%, dry basis) 5.8

O (wt.%, dry basis) 43.2

Ash content (wt.%, dry basis) 1.0

Inner diameter (upper part) 3.3 m

Total height 14.8 m

A. Sanz, J. Corella / Fuel Processing Technology 87 (2006) 247–258248

must be below 50 mg/N m3. This very low tar content can be

obtained by using nickel-based catalysts, rings as well as

monoliths, downstream from the gasifier. To avoid their

deactivation by coke, they require a tar content in the fuel

gas at the inlet of the catalyst reactor below 2 g/N m3 [5]. This

catalytic hot gas cleaning method require a quite clean fuel gas

which, as it will be shown in this paper, may be obtained in a

fluidized bed biomass gasifier only under very special

operating conditions. Results presented here will show these

conditions and serve to improve both design and operation of

existing and future CFBBGs.

2. Outputs from the model

Using the model shown in detail in the previous paper [1],

the longitudinal profiles in the riser of a CFBBG have been

calculated for the following variables:

– Gas composition (H2, CO, CO2, CH4, C2Hn, O2, in vol.%,

dry basis, and H2O, vol.%).

– LHV (MJ/N m3, dry gas).

– Tar content (g/N m3, dry gas).

– Char concentration [g/kg (sand+dolomite)].

Once the axial profiles of these variables are known, their

values at the gasifier exit are further calculated. The model

also allows the calculation of the gas yield and a rough

estimation of the carbon (C) content in the fly exit ash.

Temperatures at the bottom and in the upper (dilute) zone of

the riser (before and after the 2nd air flow) are also estimated.

So, the following seven variables can be calculated at the

gasifier exit:

– Gas composition (H2, CO, CO2, CH4, C2Hn, O2, in vol.%,

dry basis and H2O, vol.%).

– LHV (MJ/N m3, dry gas).

– Tar content (g/N m3, dry gas).

– Char concentration [g/kg (sand+dolomite)].

– C content in fly ash (wt.%).

– Gas yield (N m3 dry gas/kg biomass daf).

– Temperature.

For ‘‘tar’’, all the tar is considered (unreacted ‘‘tar 2’’+ ‘‘tar

5’’). For ‘‘char’’, the whole char (‘‘char 2’’+ ‘‘char 3’’) is

considered. These lumps (tar2, tar5, char2, char3) were defined

in the previous paper [1].

3. Limitations or constrains of the results here presented

Trends and magnitude of the results presented here have a

general character, usefulness and application. Nevertheless,

some details concerning these results may change from one

gasifier to another one because these results were obtained for

only some specific conditions. They have to be taken into

consideration when the results presented here are applied to

another different situation. These conditions that may change

from one gasifier to another one are as follows.

3.1. Feedstock

The type of biomass used for all cases considered in this

paper is pine wood chips. Its main composition is:

This biomass has a very low content of nitrogen and of

alkali (K and Na) species. It does not generate neither high

NH3 contents in the gasification gas nor agglomeration or

sintering problems in the gasifier.

Other types of biomass may generate a different product

distribution in the pyrolysis step [8] and chars with different

reactivities and kinetic constants. For types of biomass

different to pine wood chips, the model will therefore have

to be adapted by using corrective factors for some kinetic

constants. In the case of gasifying a type of biomass very

different from the one considered here, results shown in this

paper will have only a semirigorous character. Nevertheless,

the trends shown here will still be useful.

3.2. In-gasifier material

The permanent or fluidizing material may have a noticeable

catalytic, besides thermal, activity (Refs. 6,7). The in-gasifier

material may contain some additives which have an effect on

the kinetic constants of the reaction network considered in the

model. The type of in-gasifier material has therefore an effect

on the product distribution. Results presented here correspond

to an in-gasifier material consisting of a mixture of 70–80

wt.% of silica sand (S) and 20–30 wt.% of a calcined dolomite

(D). When another material and/or additive is used, the kinetic

constants of the model will have to be modified with corrective

factors for the new situation.

3.3. Gasifier topology and location of the feeding point

The gasifier topology considered in this paper is:

The feeding point is located in the high-density zone at the

gasifier bottom, as shown in Fig. 1 of Ref. [1]. When the

feeding is above this high-density zone or from the gasifier top,

the model will have to be adapted to this situation because the

rate of the pyrolysis step is different in that case, as

demonstrated by Corella et al. [9] and by Chirone et al. [10].

3.4. Gasification agent

Air with some H2O (specifically the H2O coming from the

air and biomass moistures which are independent operation

variables of the process).

0.20 0.25 0.30 0.35 0.40 0.45700

750

800

850

900

950

30 %

10 %

40 %

20 %

0 %

T b. b

ed(o C

)

ER

Fig. 1. Effect of the total ER on the temperature at the bottom bed. Parameter:

% (defined as % of total ER) 2nd air flow.

A. Sanz, J. Corella / Fuel Processing Technology 87 (2006) 247–258 249

4. Operating conditions

Results will be presented for only some sets of process

parameters. The sets of operation conditions for each of the

cases studied here (ER, biomass moisture, etc.) are shown in

Table 1. Sets shown in Table 1 correspond to selected and

realistic situations in commercial CFBBGs.

It has to be pointed out that the process operating parameters

in the above indicated sets are not independent of each other,

but they are related through heat, mass and hydrodynamic

balances and by the stoichiometry of the reactions existing in

the CFBBG.

Process variables studied and their intervals, given in

parenthesis, have been:

– Total ER (0.20–0.45)

– 2nd air inlet height (6–10 m)

– 2nd air fraction (0–40% of total air)

– Biomass moisture (5–25 wt.%)

– Biomass flow rate, used as weight hourly space velocity

(WHSV, 1.5–3 h�1).

5. Results

We now give details of the effects of the process variables

considered.

5.1. Effect of the total equivalence ratio (total ER)

Total ER is a very well-known and experimentally studied

parameter in biomass gasification. It is an index of the variable

that is probably the main independent variable in a biomass

gasification plant: the total air flow rate at the inlet. The effect

of ER is studied for the experimental conditions indicated in

the 1st column of data in Table 1.

Temperature at the bottom of the gasifier, calculated for

different values of ER (from 0.20 to 0.45) and percentage of

secondary air, is shown in Fig. 1.

The increase of temperature (DT) due to the 2nd air flow

was previously shown by Corella and Sanz [1]. With these

temperatures, the axial or longitudinal profiles of concentration

Table 1

Process operating conditions for the cases considered in this paper [heat losses=1%

(MJ/kg)=18.1]

Operation parameter Variable studied

ER Inlet 2nd air height

Figures 1–10 11–14

Tb.bed (-C) 825–890 825–890

Td.zone (-C) 925–990 925–990

Air/biomass (ER) 0.20–0.45 0.20–0.45

Biomass fed (kg a.r./h) 15000 15000

WHSV (h�1) 1.9 1.9

Biomass moisture (wt.%) 15 15

Preheated air temperature (-C) 250 250

% 2nd air (% of total air) 20 20

Inlet 2nd air (m) 6 6–10

In-bed dolomite Yes Yes

in the riser of the CFBBG for different values of ER have been

calculated. H2, CO and CO2 profiles are shown in Fig. 2; CH4

and C2H4 profiles in Fig. 3; H2O and O2 profiles in Fig. 4;

LHVof the gas in Fig. 5, and tar and char contents in Fig. 6. All

axial profiles shown in Figs. 2–6 have the same shape: the

effects of the fast pyrolysis near the biomass feeding point

(zone) and of the secondary air are clearly appreciated in these

figures.

All the above-mentioned profiles are needed for a deep

understanding of what is happening in the riser of the CFBBG.

Although all are relevant, the profiles for the tar content in the

flue gas, Fig. 6, are of particular interest and novelty.

The profiles in Figs. 2–6 show a shape similar to those

found in coal gasifiers (i.e. Ref. 11). One problem appeared

when calculating these profiles concerns the bottom zone,

between the air feeding and the biomass feeding points,

separated 1.5 m in these calculations. The char generated by

the devolatilization of coal has a much higher density than the

corresponding char formed from biomass. If the coal char

remains in back mixing in the bottom zone of a CFB [12–14],

the biomass char goes mainly upwards once it is generated near

the biomass feeding point. Its segregation in the upper part of

the high-density zone at the bottom of the gasifier is much

more relevant than with the coal char. This segregation and

of the total heat released; biomass particle size (mm)=1–5; biomass LHV d.a.f.

% 2nd air flow Biomass moisture WHSV

15–19 20–24 25–27

825–890 705–926 705–926

825–1090 805–1026 805–1026

0.20–0.45 0.20–0.45 0.20–0.45

15000 15000 11800–23700

1.9 1.9 1.5–3.0

15 5–25 15

250 250 250

0–40 20 20

6 6 6

Yes Yes Yes

0 2 4 6 8 10 12 14 16 18 200

1

2

3

4

5

6

7

8

9

10

0.20 0.25 0.30 0.35 0.40 0.45

C2H

4 (v

ol. %

, dry

bas

is)

z(m)

2

4

6

8

10

CH

4 (v

ol. %

, dry

bas

is)

Fig. 3. Effect of ER on the longitudinal profiles along the riser of CH4, C2H4

contents (2nd air=20% of total air).

0

5

10

15

20

25

O2

(vol

. %, d

ry b

asis

)

0

2

4

6

8

10

12

14

16

18

20

0.20 0.25 0.30 0.35 0.40 0.45

H2O

(vo

l. %

)

0 2 4 6 8 10 12 14 16 18 20

z(m)

Fig. 4. Effect of ER on the longitudinal profiles along the riser of H2O and O2

contents (2nd air=20% of total air).

0 4 8 10 12 14 16 18 200

5

10

15

20

CO

2 (v

ol. %

, dry

bas

is)

z(m)

5

10

15

20

CO

(vo

l. %

, dry

bas

is)

5

10

15

20

0.20 0.25 0.30 0.35 0.40 0.45

H2

(vol

. %, d

ry b

asis

)

2 6

Fig. 2. Effect of ER on the longitudinal profiles along the riser of H2, CO and

CO2 contents (2nd air=20% of total air).


hydrodynamics in the bottom zone depends on the topology of

that zone which sometimes is troncoconical. For these reasons,

it is still difficult to make accurate calculations for the zone, 1.5

m high in this case, between the air and biomass feeding points.

Results presented in Figs. 2–6 show how in the CFB biomass

gasifiers there are not so noticeable changes in the composition

as those shown by Chen et al. [11] for CFB coal gasifiers.

The axial profiles of char concentration shown in Fig. 6, as

well as those concerning the char concentration at the riser exit,

correspond to a ‘‘soft char’’ (from pine wood) of very small

particle size (in this paper below a few millimeters) which is

carried upwards by the rising gas. Different axial profiles (for

the char) might be obtained for some other types of biomass

having higher hardness and particle sizes. The charred biomass

and the char from such biomass could have initially a relatively

high particle size and would remain in the bottom bed. The

5

10

15

20

25

a)

H2

CO CO2

H2,

CO

, CO

2 (v

ol. %

, dry

bas

is)

0

1

2

3

4

5

6

7

8

0.20 0.25 0.30 0.35 0.40 0.45

LHV

(M

J/m

3 n, d

ry g

as)

0 2 4 6 8 10 12 14 16 18 20

z(m)

Fig. 5. Effect of ER on the longitudinal profiles along the riser of lowest heating

value of the flue gas (2nd air=20% of total air).


axial profiles would then be somewhat different of those shown

in Fig. 6. Nevertheless, since char attrition (in the bottom bed)

by the erosive silica sand is very high, the initially big particles

of char would be progressively broken to small ones which

would then be carried out of the bed by the rising gas (see Fig.

7 in the previous paper, Ref. 1). After some time-on-stream,

under stationary state, the axial profile for the final char would

be similar to that shown in Fig. 6. In other words, although

some hard chars of initially big size might have other axial

profiles (which remains to be studied in the future), under

0

20

40

60

80

Cha

r co

ncen

trat

ion

(g/k

g S+

D)

0

20

40

60

80

a)

b)

100

0.20 0.25 0.30 0.35 0.40 0.45

Tar

con

tent

(g/

Nm

3 , dry

bas

is)

0 2 4 6 8 10 12 14 16 18 20

z(m)

Fig. 6. Effect of ER on the longitudinal profiles along the riser of tar content

and char concentration (2nd air=20% of total air).

stationary state such profiles would probably not be very

different from those shown in Fig. 6.

From Figs. 2–6, the values of the above-mentioned

parameters can now be calculated at the gasifier exit. The

main results or outputs are shown in the following figures:

Gas composition (H2, CO, CO2, CH4, C2Hn, H2O and O2

contents) (Fig. 7)

LHV of the produced gas (Fig. 8)

Tar content (in the flue gas) and char concentration (with

respect to S +D) (Fig. 9)

Gas yield and C content in fly ash (Fig. 10).

0,20 0,25 0,30 0,35 0,40 0,450,0

0,1

0,2

0,3

0,4

6

8

10

12

14

H2O

, O2

(vol

. %, d

ry b

asis

)

ER

0

1

2

3

4

5

6

7

CH

4, C

2Hn

(vol

. %, d

ry b

asis

)

0

b)

c)

H2O O2

CH4

C2Hn

Fig. 7. Effect of ER on the gas composition at the gasifier exit: (a) H2, CO and

CO2 contents, (b) CH4, and C2Hn contents, (c) H2O and O2 contents.

0

1

2

3

4

Car

bon

cont

ent i

n fly

ash

(w

t %)

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

Gas

yie

ld(m

3 n, d

ry g

as/k

g da

f)

0.20 0.25 0.30 0.35 0.40 0.45ER

Fig. 10. Effect of ER on the gas yield and carbon content in the fly ash at the

gasifier exit.

0.20 0.25 0.30 0.35 0.40 0.450

1

2

3

4

5

6

7

8

9

10

11

12

ER

LHV

(M

J/m

3 n, d

ry g

as)

Fig. 8. Effect of ER on the lowest heating value in the flue gas at the gasifier

exit.


All these figures are useful in biomass gasification and they

are self-explanatory. Nevertheless, we should point out the

important quantitative effects of ER on LHVand on tar content.

These are shown in Figs. 8 and 9, respectively. From Fig. 9, it

can be deduced that, until another better in-bed additive be

found and used in the gasifier, ER values higher than 0.30 have

to be used to get tar contents below 2 g/mn3, which

simultaneously generates, according to results shown in Fig.

8, a fuel gas with a low heating value.

0.20 0.25 0.30 0.35 0.40 0.4515

20

25

30

35

ER

0

5

10

15

20

Cha

r co

ncen

trat

ion

(g/k

g S+

D)

Tar

con

tent

(g/m

3 n, d

ry g

as)

Fig. 9. Effect of ER on the tar content and char concentration at the gasifier exit.

A relatively curious, but not very important, result which

might be surprising is that, under some conditions, there is

some unreacted O2 at the exit of the gasifier, as Fig. 7 and some

other next figures show. To this respect, it can be said that:

i) The calculated O2 content at the gasifier is always below

0.5 vol.%, dry basis, a very small amount then. In these

small amounts, O2 can coexist with H2 and CH4.

ii) It is a real, experimental and proved fact (i.e. Refs.

15,16). If the produced gas is carefully analyzed, O2 is

present in that gas for ER values higher than 0.35.

iii) It has to be remembered that the model here used is based

on kinetics, not in equilibrium or thermodynamics, and

when there is not enough residence time in the gasifier,

which in the present case is of around 3 s, the conversion

of the O2 fed with the air cannot reach 100%. There is

some unreacted O2 in the exit gas then, as Fig. 7 and

corresponding next figures show.

5.2. Effect of the 2nd air inlet height (H2nd )

The effect of H2nd has been studied in the interval from 6 to

10 m. ER values considered range from 0.20 to 0.45. In this

section, the 2nd air flow was maintained at 20% of the total

ER. Other operating conditions in this study are shown in the

2nd column of Table 1.

Temperatures at the bottom of the riser are the same as those

shown in Fig. 1. DT values (because of the 2nd air flow) are the

5 7 9 10 11 120

1

2

3

4

Car

bon

cont

ent i

n fly

ash

(w

t %)

ER 0.20 0.25 0.30 0.35 0.40 0.45

H2nd(m)

02468

1012141618202224262830

a)

b)T

ar c

onte

nt(g

/m3 n,

dry

gas

)6 8

Fig. 12. Effect of 2nd air inlet height at different ER values on the tar content

and C content in fly ash.


same as the ones used in Section 5.1 of this study. From the

calculated results, it is deduced that this variable (H2nd) does

not have much influence on gas composition (main compo-

nents) and LHV and on gas yield. Nevertheless, H2nd has a

small influence on the O2 (Fig. 11a), tar content (Fig. 12a), and

C content in the fly ash (Figs. 11b and 12b). These contents

(O2, tar and C) at the gasifier exit increase somewhat when

H2nd is increased. This is due to the fact that the residence time

of the second O2 feed, which reacts in the upper part of the

riser, decreases when increasing the H2nd.

Since the tar content in gasification gas and the C content in

fly ash should be as low as possible, it is therefore concluded

that the 2nd air feeding point should be located in the riser,

preferably, ‘‘as low as possible’’. In practical terms, this means

just above the high solids density zone existing at the bottom of

the riser. At the same time, we have to remember that the

biomass feeding point was located (see Ref. 1) in such a high-

density zone as to obtain a gasification gas with a tar content

relatively low.

5.3. Effect of the percentage (defined as % of total ER) of the

2nd air flow

The percentage of the 2nd air flow has been studied in the

interval of 0% to 40% of the total ER value. For a given ER, on

increasing the percentage of 2nd air flow, the percentage of

primary air decreases accordingly. The temperature at the

0.20 0.25 0.30 0.35 0.40 0.450

1

2

3

4

Car

bon

cont

ent i

n fly

ash

(w

t %)

ER

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

a)

b)

6 m7 m8 m9 m10 m

O2

(vol

. % d

ry b

asis

)

Fig. 11. Effect of 2nd air inlet height at different ER values on (a) O2 content

and (b) carbon content in the fly ash at the gasifier exit.

bottom of the CFBBG decreases, as shown in Fig. 1, and DT

between the bottom and the upper zones increases, as was

shown in the previous paper [1].

For the experimental conditions indicated in the 3rd column

of Table 1, the gas composition, gas yield, gas quality (tar

content), etc., at the gasifier exit, for different values of the

percentage of secondary air flow, are shown in Figs. 13 and 14.

Some variations in the calculated CO2, CH4, C2H4 and O2

contents at the gasifier exit are observed in Fig. 13 when the

percentage of the 2nd air flow is increased. But the most

important effect concerns, in the authors’ opinion, the tar

content, as Fig. 14a shows. As the percentage of the secondary

air increases, for the same ER value, the tar content at the

gasifier exit also increases. It is due to the fact that when the

percentage of 1st air flow decreases, the tar formation at the

bottom zone increases quite a lot (Fig. 6a). For a commercial

CFBBG operating at a given ER value, an important

conclusion is that the percentage of 2nd air flow must therefore

not to be high. High values (above 20%) of the percentage of

2nd air flow would decrease the quality (expressed by its tar

content) of the gas produced quite a lot as shown in Fig. 14a.

Increasing the percentage of the 2nd air flow has, on the

other hand, some simultaneous positive effects: the gas yield

increases (Fig. 14c) and the C content in fly ash decreases

(Fig. 14b). Since there are both positive and negative effects,

the final decision about the optimum percentage of 2nd air

flow remains open both for the manufacturer of the gasifier

and for the operating engineer. When balancing the positive

and negative effects, the authors conclude that the percentage

of the 2nd air flow should not exceed the 20% of the total

air flow.

0,20 0,25 0,30 0,35 0,40 0,450

5

10

15

20

25

0 % 10 % 20 % 30 % 40 %

ER

0,20 0,25 0,30 0,35 0,40 0,45ER

0,20 0,25 0,30 0,35 0,40 0,45ER

5

10

15

20

25

30

35

0

5

10

15

20

25

a)

0

1

2

3

4

5

6

7

0

1

2

3

4

5

6

7 0 % 10 % 20 % 30 % 40 %

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

O2 (

vol.

%, d

ry b

asis

)

6

8

10

12

14

16

18

0 % 10 % 20 % 30 % 40 %

b)

c)

CO

2 (v

ol. %

, dry

bas

is)

H2

(vol

. %, d

ry b

asis

)

CH

4 (v

ol. %

, dry

bas

is)

C2H

N (

vol.

%, d

ry b

asis

)H

2O (

vol.

%)

CO

(vo

l. %

, dry

bas

is)

Fig. 13. Effect of percentage (defined as % of total ER) of 2nd air flow on the gas composition at the gasifier exit, for different values of total ER. (a) H2, CO and

CO2 contents; (b) CH4 and C2Hn contents; (c) H2O and O2 contents.


0 10 20 30 40 501.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

0.20 0.25 0.30 0.35 0.40 0.45

2nd Air flow (%)

0

1

2

3

4

c)

b)

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

a)

Car

bon

cont

ent i

n fly

ash

(w

t %)

Tar

con

tent

(g/m

3 n, d

ry g

as)

Gas

yie

ld(m

3 n, d

ry g

as/k

g da

f)

Fig. 14. Effect of % (defined as % of total ER) 2nd air flow on (a) the tar content,

(b) C content in fly ash and (c) the gas yield, for different total ER values.

0 5 10 15 20 25 30600

650

700

750

800

850

900

950

ER 0.20 0.25

0.30

0.35

0.40

0.45

Tb.

bed

(o C)

Biomass moisture (%)

Fig. 15. Effect of the biomass moisture on the temperature at the bottom bed.

Parameter: ER.

0

1

2

3

4

0.20 0.25 0.30 0.35 0.40 0.45

Car

bon

cont

ent i

n fly

ash

(w

t %)

0

10

20

30

40

c)

b)

0

1

2

3

4

5

6

7

8

9

10

11

12

a)

0 5 10 15 20 25 30

Biomass moisture (%)

Tar

con

tent

(g/m

3 n, d

ry g

as)

LHV

(M

J/m

3 n, d

ry g

as)

Fig. 16. Effect of the biomass moisture on the tar content and C content at the

gasifier exit, for different values of ER.


5.4. Effect of biomass moisture

Temperatures calculated for the bottom bed for different

biomass moistures (between 5 and 25 wt.%) at different ER

values (for a 2nd air flow of 20% of total air) are shown in Fig.

15. This temperature decreases with increasing moisture

content. This is not a new and surprising fact. Fig. 15 has

been provided here because the calculated values shown there

can be useful for operators, scientists or engineers involved in

biomass gasification.

With these temperatures, and for the process variables

indicated in the 4th column of Table 1, the H2, CO2 and H2O

contents increase with biomass moisture and the CO decreases.

The CH4 and C2Hn contents also increase with biomass

moisture but slightly and only for ER values higher than

0.30. The LHV (dry basis) of the exit gas (Fig. 16a) decreases

somewhat when increasing biomass moisture.

Tar content in the exit gas (Fig. 16b) increases with the

biomass moisture due to the fact that the temperature at the

bottom bed decreases when increasing biomass moisture. Tar

1,0 1,5 2,0 2,5 3,0 3,5700

750

800

850

900

950 b.b.ER

0.20 0.25 0.30 0.35 0.40 0.45

WHSV (h-1)

Tb.

bed

(o C)

Fig. 18. Effect of the biomass flow rate (expressed as WHSV) on the

temperature at the bottom bed. Parameter: ER.


generation at the bottom bed increases then. Although H2O is a

reactant (whose amount in the flue gas increases with biomass

moisture) which reacts and eliminates (by steam reforming)

some tar, this tar elimination does not compensate the higher

tar generation when the biomass moisture is high, with low

temperatures at the bottom bed.

Carbon content in fly ash (shown in Fig. 16c) decreases

somewhat when increasing biomass moisture. This can be

attributed to two simultaneous facts: (1) the increase of the

rate of carbon elimination reaction with H2O (C+H2OY. . .)in the upper part of the gasifier. The same fact occurs with the

tar +H2OY. . . reaction but, in this case and as already

mentioned, the amount of tar to react increases when

increasing the biomass moisture. (2) The decrease of carbon

in fly ash might be due to the results of dilution. WHSV is

calculated with the biomass in the Fas-received_ condition.

Increasing the moisture means that less carbon is fed per inert

bed material.

Other interesting effect concerns the gas yield (dry basis)

which increases as biomass moisture increases (Fig. 17). So,

averaging effects, even taking into account the important

increase of the tar content in the fuel gas and the small decrease

in its LHV, some moisture in the biomass is beneficial. A

moisture level not more than approximately 15 wt.% can be

recommended.

5.5. Effect of biomass flow rate

The biomass flow rate has been handled as WHSV (kg

biomass a.r./h)/[kg inventory of solids (sand+calcined dolo-

mite) in the gasifier] whose units are h�1. For a given

inventory of solids (S +D) in the gasifier, WHSV directly

indicates the biomass flow rate. Big pilot and commercial

CFBBGs plants work with WHSV only between 1.5 and 3.0

h�1. Extremely high (long) gasifiers might have WHSV >3

h�1 but they are not in use yet.

The temperatures calculated for the bottom bed for different

WHSV and ER values are shown in Fig. 18. These

temperatures correspond to the set of process variables shown

1,2

1,4

1,6

1,8

2,0

2,2

2,4

2,6

2,8

3,0

3,2

(wt%)

5 10 15 20 25

Gas

yie

ld

0,20 0,25 0,30 0,35 0,40 0,45

ER

moisture

(m3 n,

dry

gas

/kg

daf)

Fig. 17. Effect of the biomass moisture on the gas yield for different values

of ER.

in the 5th column of Table 1. As it is well known, other

conditions being the same, the temperature at the bottom

increases on increasing the flow rate of biomass feed.

LHV values of the gas at the gasifier exit are shown in Fig.

19. Tar content in the exit gas and C content in fly ash are

shown in Fig. 20.

From these results, it is deduced that the tar content in the

flue gas decreases when increasing the biomass flow rate

(Fig. 20a). This is due to the temperature increase in the

bottom bed. But this trend or variation has a limit which is

not shown in Fig. 20a because of the range of WHSV used

there. When increasing WHSV above 3 h�1, if the ER value

is maintained constant (0.30 for instance), the total gas flow

and the superficial gas velocity increase. For a given gasifier

height, the gas residence time drops below the minimum

required to obtain high tar conversions in the reacting

network which leads to higher tar and carbon contents in

the flue gas and fly ash, respectively. For this reason, and

working with WHSV higher than 3 h�1, it would imply the

0

1

2

3

4

5

6

7

8

9

10

Total ER 0.20 0.25 0.30 0.35 0.40 0.45

1,0 1,5 2,0 2,5 3,0 3,5

WHSV (h-1)

LHV

(M

J/m

3 n, d

ry g

as)

Fig. 19. Effect of biomass flow rate (expressed as WHSV) and ER on the

lowest heating value of the produced gas at the gasifier exit.

0

1

2

3

4

b)

a)0

5

10

15

20

25

30

35

40

45

50

55

0.20 0.25 0.30 0.35 0.40 0.45

Car

bon

cont

ent i

n fly

ash

(w

t %)

Tar

con

tent

(g/m

3 n, d

ry g

as)

1,0 1,5 2,0 2,5 3,0 3,5

WHSV (h-1)

Fig. 20. Effect of biomass flow rate (expressed as WHSV, h�1) on the tar

content and C content in fly ash, for different values of ER.


use of very high (long) gasifiers which are not yet in use

today.

6. Perspectives and checking of the results

It is well known that all models need their experimental

results checked, and also that all authors provide experimental

validation in such a way that it is easy to find in literature

contradictory models with some experimental data provided to

validate each model. So, experimental checking or validation

of the results presented here is very important for these authors.

Nevertheless, no comparison between theory and experimental

will be provided here because a full or deep comparison cannot

be presented yet. Nevertheless, three types of checking were

used for validation and improvement, when required, of the

data from the model.

The first checking was and is still being made with our own

data: periodic test-runs carried out in the CFBBG small pilot

plant located at the UCM (described in Ref. 6) were used to

check the model. Nevertheless, these tests are not suitable

enough for now to fully validate the model under extreme

conditions. Checking of the model at the UCM biomass pilot

plant will continue in the following years.

A second checking was made by using the work of Gil et al.

[17] which contains a lot of relevant results on biomass

gasification in fluidized bed at small pilot plant scale under a

broad interval of experimental conditions.

Finally, a third checking was made with the results from a

survey carried out worldwide between the owners and/or

operators of the very few existing CFBBGs at commercial and

big pilot scales. When developing the model, the existing

CFBBGs were analyzed by contacting the people in charge of

these plants, and the data obtained was then taken into

account and used in our model. Unfortunately, very often

some important and vital parameters were omitted from the

survey making it impossible to totally check the model. It can

be only said that both the absolute values and the trends

shown in the figures presented in this paper agree with the

data from the analyzed pilot and commercial CFBBGs. In

fact, when the authors encountered a discrepancy, this was

analyzed and the model was modified accordingly.

7. Conclusion

The 1-dimensional model presented in the previous paper

[1] has been used to predict the gas composition, quality (tar

content, mainly) and yield from a CFBBG. The model predicts

the axial profiles of these variables and their values at the exit

of the gasifier. Their variation by the ER value, the percentage

of secondary air flow, secondary air inlet height, biomass flow

rate (as WHSV, h�1) and biomass moisture have been shown

here for a set of selected operating conditions.

The parameters with the highest influence on gas composi-

tion and gas ‘‘quality’’ are ER, percentage of 2nd air flow and

biomass moisture. Biomass flow rate has an important influence

on gas ‘‘quality’’ but not so important for gas composition. 2nd

air inlet height does not have an important effect. The data here

presented quantify the effects of all these variables.

The model used indicates that only a number of homoge-

neous and heterogeneous reactions are important during the

formation and destruction of the reactants or species in the

CFBBG. The presence in the gasifier of catalytic materials, as

olivine or calcined dolomite, plays a significant role both in the

reduction of tar content and in the particle content (in the fuel

gas) at the gasifier exit.

Both absolute values and trends from the model agree with

all the data known from pilot and commercial CFBBGs. The

model may be therefore used as a practical tool for: (1) a quick

estimation of the properties of the produced fuel gas, (2)

quantitative predictions for the new operating parameters, and

(3) to improve the design and operation of a fluidized bed

biomass gasifier.

Notation

a.r. As received

D Calcined dolomite

ER (or total ER) Total equivalence ratio, total air fed to

the gasifier/stoichiometric air, dimensionless

b.b.ER Equivalence ratio considering only the air flow fed at

the bottom bed, dimensionless

H2nd Height of the 2nd air inlet (m) (for details, see Fig. 1

in Ref. 1)

LHV Lowest heating value of the produced gas (MJ/N m3,

dry gas)

S Silica sand

Tb.bed Temperature in the bottom bed of the gasifier (-C)Td.zone Temperature in the dilute zone of the riser/gasifier (-C)


UCM University Complutense of Madrid (Dept. of Che-

mical Engineering)

WHSV Weight hourly space velocity for the biomass, [kg

biomass (as received) fed/h]/[kg solids (S +D) in the

CFBBG]

z Height in the riser, from the gas distributor (m)

Acknowledgement

The present work was made under the EC-financed project

No. JOR3-CT99-0053. The authors thank Dr. Wennan Zhang,

at Mid-Sweden University, for his effort and encouraging work

as Project Coordinator.

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