Pressure drop prediction of a gasifier bed with cylindrical biomass pellets

Up to date, limited literature is found on pressure drop prediction of beds with cylindrical pellets and none was found for gasifying beds with cylindrical pellets.. In this paper, an av

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Pressure drop prediction of a gasiﬁer bed with cylindrical biomass

pellets

Royal Institute of Technology, Department of Material Science and Engineering, Division of Energy and Furnace Technology, Brinellvägen 23, 100-44 Stockholm, Sweden

h i g h l i g h t s

An equation was developed for pressure drop prediction with shrinking effect

Graphical representations of correlation constants were introduced

This would provide a guide to select pellet size and designing a grate

a r t i c l e i n f o

Article history:

Received 6 March 2013

Received in revised form 26 June 2013

Accepted 13 July 2013

Available online 8 August 2013

Keywords:

Biomass

Gasiﬁcation

Fixed bed

Pressure drop

a b s t r a c t

Bed pressure drop is an import parameter related to operation and performance of ﬁxed bed gasiﬁers Up

to date, limited literature is found on pressure drop prediction of beds with cylindrical pellets and none was found for gasifying beds with cylindrical pellets

In this paper, an available pressure drop prediction correlation for turbulent flows in a bed with cylin-drical pellets which has used equivalent tortuous passage method was extended for a gasifier bed with shrinking cylindrical pellets and for any flow condition Further, simplified graphical representations introduced based on the developed correlation can be effectively used as a guide for selecting a suitable pellet size and designing a grate so that it can be met the system requirements

Results show that the method formulated in the present study gives pressure drop approximation within 7% deviation compared to measured values with respect to performed runs Available empirical correlation with modiﬁed Ergun constants for cylindrical pellets gave pressure drop within 20% deviation after the effect of shrinkage was taken into account

1 Introduction

Biomass gasiﬁcation is a promising renewable energy

technol-ogy for supplying thermal energy and generating electric power

Nowadays, pelletized biomass is widely used in order to overcome

some problems when using conventional biomass in thermal

applications like gasiﬁcation including logistic problems due to

low bulk density, non-uniformity of fuel, low energy density, etc

Pressure drop is an important factor in ﬁxed bed gasiﬁcation of

Biomass Most common and widely used method for predicting

pressure drop in a packed bed is using Ergun equation which has

viscous and inertial terms corresponding to laminar and turbulent

ﬂow conditions One limitation of this model when applying for a

gasiﬁer bed with biomass pellets is due to the particle shape which

is essentially cylindrical shape when considering pellets Limited

literature is available in pressure drop prediction for a bed with

cylindrical pellets

One research group[1]has considered this effect and has devel-oped an equation for pressure drop in packed bed with cylindrical shaped particles by using equivalent tortuous passage method But, the equation is limited to turbulent ﬂow conditions and not valid for a bed with laminar or transition ﬂow conditions

Some investigators[2]have developed an empirical correlation for Ergun constants for a bed of cylindrical particles by referring the sphericity of particles But, this correlation does not show strong validity due to scatter of data and suitable only for a rough approximation of the pressure drop Lack of theoretical back-ground is another limitation for applying this correlation Further considering these models, none of the above models for cylindrical particles are developed for active beds of particles In a gasiﬁer, particles participate in the reaction and therefore particle size and the porosity of bed varies with time and along the height

of the bed Even if steady state condition is considered, the spacial variation of porosity has to be taken into account

Some researchers[3]have addressed this issue on a downdraft gasiﬁer but with particles in spherical shape and hence Ergun equa-tion and its’ another variaequa-tion called Macdonald correlaequa-tion have

⇑Corresponding author Tel.: +46 8 790 8402; fax: +46 8 207 681.

E-mail address: rmdsgu@kth.se (D.S Gunarathne).

Contents lists available atScienceDirect

Applied Energy

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / a p e n e r g y

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been used along with considering the wall effects Another group[4]

has focused on cylindrical wood particle in a ﬂuidized bed

consider-ing shrinkconsider-ing effect but only pyrolysis conditions An interestconsider-ing

lit-erature [5] was found for a coal gasiﬁer and they have found

pressure drop variations within different zones in the gasiﬁer by

using Ergun equation for each zone separately This method has

incorporated lots of experimental data and may be successfully

used for that speciﬁc commercial gasiﬁer model but not for any type

of gasiﬁer Therefore, none of the above cases can be used for

pre-dicting pressure drop in any ﬁxed bed gasiﬁers of cylindrical pellets

One of our previous works [6] concentrated on developing a

model for prediction of pressure drop due to grate-bed resistance

of a gasiﬁer As the second step of that, with the objective of ﬁlling

the gap on pressure drop prediction of gasiﬁer beds with

cylindri-cal pellets, here we focus on the bed resistance Considering

limi-tations of previous models, an equation is developed based on

the model predicted in the literature[1] including the effect of

laminar and transition ﬂow conditions and also the effect of

shrinkage of particles during gasiﬁcation and will be veriﬁed based

on experimental data Further, it will also compare with the

empir-ical correlation available for cylindrempir-ical pellets[2]which will also

be upgraded by taking shrinking effect into account

2 Materials and methods

2.1 Materials

The biomass pellets used in the experiment (Fig 1) were

sup-plied by Boson Energy S.A

The length distribution of pellets considering 50 numbers of

pellets is given inFig 2

It can be seen that the majority of pellets are in the range of 11–

15 mm in length Physical properties of pellets (before the

experi-ment) can be summarized as inTable 1

Pellet proximate and ultimate analysis along with the heating

value is given inTable 2

2.2 Gasiﬁcation system

Gasiﬁcation experiments were carried out in updraft High

Tem-perature Agent Gasiﬁer (HTAG) unit with 0.4 m diameter (Fig 3)

This unit is incorporated with feed gas preheater, updraft gasiﬁer, fuel feeding system and producer gas post combustion unit De-tailed description of this experimental facility is available else-where[7]

Biomass pellets stored in the feed tank is transported to the gas-iﬁer via screw conveyor The frequency of feeder is correlated with the feeding rate Required frequency set point is predetermined in order to achieve speciﬁc biomass feed rate

Preheated air from the preheater is introduced to the gasifier at the side of bottom section below the grate The system has the facility to add steam to the feed stream if required The flow of hot gases and biomass is countercurrent The grate facilitates to build up a fixed bed of biomass and small particles left after con-siderable reaction can pass through the grate and collected below The producer gas which is flown upwards, leave the gasifier at the side of the top section and is burned out at the combustion chamber

2.3 Experimental procedure and data reduction The feeder was pre-calibrated with biomass pellets used in the experiment Once the air temperature was reached around 1000 °C

Nomenclature

A modiﬁed Ergun constant for viscous term (–)

B modiﬁed Ergun constant for inertial term (–)

c length of cylindrical particle at any time (m)

c0 initial length of cylindrical particle (m)

De equivalent diameter of tortuous passage (m)

d diameter of cylindrical particle at any time (m)

d0 initial diameter of cylindrical particle (m)

dp equivalent particle diameter (m)

K1 constant depend on c0,d0and x (for inertial term) (–)

K2 constant depend on c0,d0and x (for viscous term) (–)

K3 constant depend on c0,d0and x (for Rep) (–)

Kt constant depend on roughness of particle and packing

tortuosity (–)

L length (height) of the bed (m)

LHV lower heating value of biomass (MJ/kg)

l equivalent length of tortuous passage (m)

mbiomass mass of single biomass particle (kg)

mchar mass of single char particle (kg)

Rep particle Reynolds number (–)

rH hydraulic radius (m)

Sp particle surface area (m2)

Vp particle volume (m3)

v velocity of ﬂow through tortuous passage (m/s)

Greek letters

k angle of inclination of tortuous passage to the mean

ﬂow (°)

l viscosity (Ns/m2)

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by using preheater, desired feed rate was achieved by adjusting

frequency of feeder

Temperatures inside the gasiﬁer were measured with type S

thermocouples located along the reactor height and recorded by

data acquisition system connected to a PC Pressure inside the gasiﬁer below the grate and three more points above the grate were measured with digital manometers so that bed pressure drop can be calculated It was assumed that the horizontal gradient of temperature and pressure is not signiﬁcant

Syngas composition was measured with Gas chromatography (GC) Tar samples were collected from the gas outlet pipe and ana-lyzed later for quantity and composition

For each run, 20 min time interval was selected for analysis This time interval was selected based on stable temperatures and gas compositions Average values of gas compositions and temper-atures within this time interval were taken for analysis

From ultimate analysis of fuel, the average chemical formula of pellets was obtained as CH1.43O0.65and it was used to calculate the stoichiometric air to fuel ratio for calculating Equivalence Ratio (ER)

Gas ﬂow rate was calculated by applying Nitrogen balance over the gasiﬁer

The average gas properties such as density and viscosity within the gasiﬁer bed were calculated by taking volume average with gas composition data at average bed temperature

3 Results and discussion 3.1 Process performance

Table 3gives experimental data andFigs 4–6show variation of temperature along the gasiﬁer height, gas compositions and char-acteristic ratios respectively.Table 4gives the tar composition and tar characteristic ratios of each run

From temperature data, we see different bed temperature behaviours with two cases Run 1 with low ER shows gradual tem-perature drop throughout the bed height and run 2 with high ER shows high temperature adjacent to the grate with sudden drop after that Then it can be expected with run 2, CO2and H2O gener-ated by exothermic combustion reactions at high temperature zone near the grate has reduced to CO and H2by endothermic bou-douard and water gas reactions at the subsequent low temperature region As a result, high CO and H2content can be seen with run 2 They were respectively 4% and 5% increment compared to run 1

In overall, low temperature was seen throughout the gasiﬁer with low ER in run 1 and comparatively high hydrocarbon and also tar content was observed compared to run 2 due to cracking reac-tions Even with low CO and H2contents, as a result of high CH4

and CxHycontents which were around 11% and 140% higher than run 2, LHV is slightly higher with run 1 However, high CO and

H2content with run 2 resulted in high gas yield and hence consid-erably higher efﬁciency

Fig 2 Length distribution of pellets.

Table 1

Physical properties of pellets.

Particle density (g/cm 3

Diameter of equivalent spherical particle (mm) – d p 11

Table 2

Composition of pellets.

Ultimate analysis

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When characteristic ratios are considered, CO/CO2 ratio was

higher with run 2 CH4/H2and CxHy/CH4ratios were higher with

run 1 CH4/H2and CxHy/CH4ratios show the effectiveness of

hydro-carbon cracking and cracking shows more effective in run 2 due to

high temperature and longer bed There was no signiﬁcant differ-ence seen with H2/CO ratio of two cases

Signiﬁcant reduction of almost all the tar components was seen with run 2 Tar characteristic ratios were also reduced and it repre-sents that high temperature and longer bed has a positive impact

on tar decomposition reactions

Referring toTable 5, it was observed a considerable bed height achieved with each run When bed height is higher, residence time for both solid and gas phase reactions are larger and it is reﬂected

by high CO and H2content, gas yield and gasification efficiency ob-tained with run 2 Specially, significant reduction of tar content is also positive

However, the drawback of such large bed is large pressure drop

of the system which ultimately affects the system performance Therefore, prediction of pressure drop of a gasiﬁer bed is a quite interesting topic for anyone concerning the system performance 3.2 Prediction of pressure drop

3.2.1 Developing the correlation Total pressure drop through a gasiﬁer bed is mainly a sum of pressure drop through the particle bed and pressure drop through the grate However, in this study, grate resistance can be consid-ered as negligible since grate opening area is high as much as 40% and the grate thickness is low which is 6 mm

Literature[1]has derived Eq.(1)based on equivalent tortuous passage method for pressure dropDP over a bed height of L in a turbulent ﬂow using Blasius smooth pipe equation for a packed bed with cylindrical particles of diameter d and length c,

Table 3

Experimental data.

Run Dry

biomass

(kg/h)

Feed gas a

(N m 3

/h)

(MJ/

N m 3 )

Gas yield (N m 3

/kg dry biomass)

Efﬁciency (%)

a

Feed gas contains 17% O 2 , 81% N 2 and 2% CO 2

Fig 4 Temperature proﬁle along the gasiﬁer height.

Fig 5 Gas compositions.

Fig 6 Gas characteristic ratios.

Table 4 Tar composition data.

Total (g/N m 3

Characteristic ratios

Table 5 Bed heights and pressure drop.

Run Bed height (m) Pressure drop across the bed (Pa)

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DP ¼ LKtqu2ð1 eÞ5=4

e3

1 2cþ

1 d

5=4

l

qu

1=4

ð1Þ

Ktis a constant combining roughness of the particles and packing

tortuosity all together and has determined experimentally and

re-lated to porosityeas follows

Kt¼ 112e3:2

Since they have considered only the inertial term of pressure

drop, it can be modified to fit to laminar or transitional flows also

by adding a viscous term

Hagen–Poiseuille equation for pressure drop in laminar ﬂow is,

DP ¼32l vl

D2e

Fixing to the deﬁnitions in[1]which are tabulated inTable 6

assuming equivalent inclined passage with an angle k to the

direc-tion of mean ﬂow, Hagen–Poiseuille equadirec-tion can be re-arranged

as,

DP ¼ 32luLð1 eÞ2

e3

1 2cþ

1 d

ð2Þ

Then, by combining Eqs.(1) and (2) and rearranging, Rangel

equation can be modiﬁed for any ﬂow condition as,

DP

L

1

qu2

¼ K1

Re1=4

p

þK2

Rep

ð3Þ

where Rep¼ð

q u

l Þ

K3

K1¼ 112e0:2ð1 eÞ 1

2cþ

1 d

K2¼ 32ð1 eÞ

e3

1

2cþ

1 d

K3¼ ð1 eÞ 1

2cþ

1 d

Graphical representations of above correlation constants for

typical biomass pellet sizes available in the market are annexed

3.2.1.1 Relation of porosity and sphericity in a bed of cylindrical

particles Some researchers[8]have formulated a relationship

be-tween porosity and sphericityUfor loose random packing of

cylin-drical particles as given in Eq.(4) This correlation shows very good

agreement with their experimental data

lne¼U5:58exp½5:89ð1 UÞ ln 0:4 ð4Þ

The sphericity of a cylindrical particle depends on its length

Very long or very short particles give low sphericity The sphericity

of cylindrical particle is given by,

U¼ð36pV2

Þ1=3

Sp

where Vpand Spare cylinder volume and area respectively

Substituting for volume and area,

U¼ 2:25 ð

cÞ2 0:5 þ c 3

" #1=3

ð5Þ

Then, porosityeof a bed with cylindrical particles is obtained as

a function of particle size c and d

3.2.1.2 Shrinking effect of particles Due to the reaction happening

in the gasiﬁer bed, the particle size is changing along the bed This results in change of sphericity and consequently the porosity of the bed Wall effect and thickness effects on porosity variation can be neglected for the cases with tube to particle diameter ratio D/dp

and bed height to particle diameter ratio L/dpare high According

to[9]the values should be D/dpP10 and L/dp> 3 in order to ne-glect those effects This assumption was applied here assuming the gasiﬁcation is done in a pilot scale unit with considerable diameter compared to particle size and achieving considerable height of bed which only necessitates pressure drop prediction Particle size of a reacting bed can be calculated by applying mass balance for one particle and mass of char particle mcharand mass of initial biomass particle mbiomasscan be related as,

mchar¼ ð1 xÞmbiomass

Practically two types of size reductions can be expected in a gasiﬁer; fragmentation and conversion Fragmentation can be ta-ken as less important when it comes to wood pellets compared

to wood chips gasiﬁcation due to high density of pellets [10] Therefore, surface conversion was assumed to dominate in this case With surface conversion, density of biomass particle can be taken as constant throughout the conversion period Then, volume

of char particle and volume of initial biomass particle can be re-lated same as above

If initial length and diameter was considered as c0 and d0 it becomes,

For a cylindrical wood pellet, assuming uniform thickness h is reduced for a certain time period from all its dimensions[11], after

a certain time new length c and diameter d of particle is given by,

c = c0 2h and d = d0 2h Avoiding unknown h,

Knowing c0, d0and x,ccan be obtained from Eqs.(6) and (7)and used in Eq.(5), in order to calculate sphericity And then, sphericity can be used in Eq.(4)for calculating porosity These values along with ﬂow properties such as velocity, density and viscosity can

be used in Eq.(3)in order to calculate pressure drop along the gas-iﬁer bed for a known conversion and bed height

3.2.2 Calculation of pressure drop Conversion x at the top of the bed is 0 and at the bottom x is as-sumed to be 1 The average mass conversion within the bed can be calculated based on the C, H and O molar balanceTable 7 summa-rizes the molar inputs, outputs and also accumulated in char From molar rates of each species accumulated in char which is equal to difference in input and output, hourly char generation can

be calculated and it is 12.78 kg/h and 12.68 kg/h respectively in two cases Then, average mass conversion x in the bed is 0.79 and 0.75 respectively

With conversion values calculated, referring to Section3.2.1, c,

d,Uandecan be calculated and given inTable 8 The particle diameter and average length has reduced respec-tively from 8 mm and 14 mm initial values to around 4.5 mm and 10 mm at the average conversion With reduced particle sizes

Table 6

Deﬁning parameters in tortuous passage.

Parameter related to tortuous passage Deﬁnition ﬁxing to [1]

e cos k

cos k

a cos k

V p ð1 eÞ

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it can be expect that low porosity since small sized particles pack

more tightly than large size ones Proving this, the initial porosity

0.445 has reduced up to 0.42 at achieved conversion

With conversion, pellets get small and porosity is reduced

Therefore, porosity at the top of the reacting bed is highest and it

is lowest at the bottom

Density, viscosity and superﬁcial velocity of gas ﬂow inside the

bed can be approximated by bed temperature, gas ﬂow rate and

gas compositions Then, these values along with c, d andecan be used as inputs to the Eq.(3).Table 9represents all the parameters and calculated bed pressure drop for both runs

3.2.3 Incorporating shrinking effect into available empirical correlation for comparison

For cylindrical particles some researchers[2]have obtained a relationship with the sphericity and Ergun constants A and B as gi-ven in the following equation:

DP ¼ L Að1 eÞ2l

e3U2d2p u þ B

ð1 eÞq

e3Udp

u2

ð8Þ

where A ¼150

U 3=2and B ¼1:75

U 4=3

By incorporating shrinking effect this equation can be improved for a reacting bed To do this, sphericity and modiﬁed Ergun con-stants were calculated for each initial length interval and their average values are given inTable 10

The modiﬁed constants calculated for two cases are as follows For Run 1,

DP ¼ L 204ð1 eÞ2l

e3U2d2p u þ 2:3

ð1 eÞq

e3Udp

u2

For Run 2,

DP ¼ L 202ð1 eÞ2l

e3U2d2p u þ 2:28

ð1 eÞq

e3Udp

u2

Ergun indices obtained are 35% and 31% increased respectively compared to original Ergun constants which are 150 and 1.75 for viscous and inertial terms respectively When bed is composed of cylindrical particles, the pressure drop is higher compared to pack-ing spherical particles The orientation of particles, tortuosity and wetted surface are blamed regarding this increase[2]

3.2.4 Validation with experimental data Pressure drop results calculated with developed correlation and empirical correlation can be compared with experimental data as given inFig 7

The method formulated in the present study gives better approximation with only 7% maximum error with respect to per-formed two runs The available empirical equation was able to pre-dict the pressure drop within 20% interval after shrinking effect was taken into account

Table 7

C, H, O molar balance.

Run Description C (kmol/h) H (kmol/h) O (kmol/h)

1 Input (biomass & feed gas) 2.29 3.85 2.93

2 Input (biomass & feed gas) 1.92 3.21 2.64

Table 8

Pellet properties after conversion.

Initial length

range (mm)

1–5 1.09 6.09 0.613 0.558 1.23 6.23 0.637 0.534

6–10 4.76 4.76 0.874 0.404 5.04 5.04 0.874 0.404

11–15 9.33 4.33 0.823 0.416 9.65 4.65 0.828 0.415

16–20 14.14 4.14 0.759 0.443 14.46 4.46 0.767 0.439

21–25 19.03 4.03 0.706 0.476 19.36 4.36 0.717 0.469

26–30 23.96 3.96 0.664 0.509 24.29 4.29 0.676 0.499

Weighted

average

9.7 4.4 0.817 0.421 10 4.7 0.822 0.419

Table 9

Summary of parameters for bed pressure drop calculation.

4.02⁄10 5

Table 10

Summary of calculating modiﬁed Ergun constants A and B.

Initial length

range (mm)

Average sphericity

sphericity

Weighted average 0.817 204 2.30 0.822 202 2.28

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Fig A1 Variation of K 1 with conversion for pellets of 8 mm diameter.

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4 Conclusions

A correlation for pressure drop prediction in a gasiﬁer bed with

cylindrical particles was proposed, compared with available

empir-ical correlation for cylindrempir-ical pellets and veriﬁed with

experimen-tal data

Based on the developed correlation, simpliﬁed graphical

repre-sentations were introduced for commonly available pellet sizes in

order to reduce the calculation effort The plots developed can be

effectively used as a guide for selecting suitable pellet size and

designing a grate so that it can be met the system requirements

Acknowledgements

Authors like to acknowledge KIC-innoenergy project which

pro-vided the ﬁnancial support and Boson Energy S.A which propro-vided

the biomass samples for experimental work

One of authors, Duleeka Sandamali Gunarathne would like to

acknowledge the ﬁnancial supporting from the European

Commis-sion This publication reﬂects the views only of the author, and the

Commission cannot be held responsible for any use which may be

made of the information contained therein

Appendix A Graphical representation of correlation constants

It was reported that pellet size has the more impact on the

shrinking behavior, not the composition of pellet[12,13]

Commer-cially available pellets are commonly found with 6 mm and 8 mm

in diameter with maximum length to diameter ratio being 5[14]

Then, for those pellets, following ﬁgures can be used to ﬁnd the

K values to be used in the Eq.(3)at any conversion if the initial

par-ticle size distribution is known

According toFigs A1–A6, very rapid increase of K values and

hence the pressure drop can be seen at the end of the conversion

period which is happening in the bottom of the bed By having a

grate opening area large enough to maintain conversion below

0.9 may be beneﬁcial in this case depending on the ability of the

system to overcome the pressure drop Therefore, someone can

use these ﬁgures as a guide for designing a suitable grate for the system On the other hand, smaller the pellet size, larger the pres-sure drop in the system and it is also clearly seen in these ﬁgures With lower length to diameter ratio and small diameter, all the K values and hence the pressure drop will be high Therefore, this can be another guide for selecting a suitable pellet size for the sys-tem requirements

References

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[2] Nemec D, Levec J Flow through packed bed reactors: 1 Single-phase ﬂow Chem Eng Sci 2005;60:6947–57

[3] Sharma AK Modeling fluid and heat transport in the reactive, porous bed of downdraft (biomass) gasifier Int J Heat Fluid Flow 2007;28:1518–30 [4] Sreekanth M, Kolar AK Progress of conversion in a shrinking wet cylindrical wood particle pyrolysing in a hot fluidized bed J Anal Appl Pyrolysis 2009;84:53–67

[5] Luckos A, Bunt JR Pressure-drop predictions in a ﬁxed-bed coal gasiﬁer Fuel 2011;90:917–21

[6] Donaj P, Izadpanah MR, Yang W, Blasiak W Effect of pressure drop due to grate and bed resistance on the performance of a downdraft gasiﬁer Energy Fuels 2011;25(11):5366–77

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[9] Foumeny EA, Roshani S Mean voidage of packed beds of cylindrical particles Chem Eng Sci 1991;46(9):2363–4

[10] Teixeira G, Van de Steene L, Martin E, Gelix F, Salvador S Gasiﬁcation of char from wood pellets and from wood chips: textural properties and thermochemical conversion along a continuous ﬁxed bed Fuel 2012;102:514–24

[11] Nfﬁez C, Cruells M, Garcia-Soto L A general shrinkage particle model for the chemical dissolution of all types of cylinders and discs Hydrometallurgy 1994;36:285–94

[12] Erlich C, Björnbom E, Bolado D, Giner M, Fransson TH Pyrolysis and gasiﬁcation of pellets from sugar cane bagasse and wood Fuel 2006;85:1535–40

[13] Erlich C, Fransson TH Downdraft gasiﬁcation of pellets made of wood, palm-oil residues respective bagasse: experimental study Appl Energy 2011;88:899–908

[14] Cocchi M, Nikolaisen L, Junginger M, Goh CS, Heinimö J, Bradley D, et al Global wood pellet industry market and trade study, IEA Bioenergy Task 40: Sustainable international bioenergy trade; 2011.

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