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released and char is produced. The process is dependent on the properties of the carbonaceous material and determines the structure and composition of the char, which will then undergo gasification reactions. Partialcombustion occurs as the volatile products and some of the char reacts with oxygen to form CO2 and CO, which provides heat for the subsequent gasification reactions. The most Relevant gasification reactions, and their

Reaction Enthalpy are as follows,

2

kJ/mol131h

2 HCOOC 298 (1)

2COCOCkJ/mol172h

2298 (2)

4

kJ/mol75-h

2 CH2HC 298 (3)

There is three oxidants to apply; air, steam and pure oxygen. The latter is affected with high economic and

energy costs and is not considered useable in commercial applications. Since the use of pure oxygen isexpensive, but offering considerable advantages as smaller downstream equipments, lowered compressionenergy, the use of oxygen enriched air combines the advantages in a less expensive medium [6-7].The amount of air added the biomass is very important for the composition of the producer gas. More added air as oxidant reduces the efficiency, and increases the yield of gaseous products. A ratio of one corresponds tostochiometric combustion. ER = 0 corresponds to pyrolysis, ER = 0.25 – 0.50 correspond to gasification and ER > 1 corresponds to combustion.

3. FLUIDIZATION

Fluidization is defined as the process by which solid particles are transformed into a fluid like state through

suspension in a gas or liquid. Fluidized beds have been applied widely in processes involving gasification, pyrolysis and combustion of a wide range of particulate materials including biomass. Advantages of fluidizationinclude high heat transfer, uniform and controllable temperatures, favorable gas–solid contacting and the abilityto handle a wide variation in particulate properties.

Fluidized bed is a column enclosing a collection of solid particles which rest on a perforated plate. A fluid is passed up through the supporting plate and through the bed of particles ultimately exiting from the top of thecolumn. At low flow rates the particles experience the drag force of the fluid flow through the interstices but

remain fixed by their weight. There is a flow rate at which the drag force balances the weight and the particlesare suspended in the flow. This suspension behaves in many ways like a dense liquid, leading to the termsfluidized particles and fluidization. The ability to have solid particles behave as a liquid has been exploited in a

number of technologies, our immediate concern is the use of fluid beds for Gasification [8-12].

An increase of the fluid rate flow rate above that for incipient fluidization produces expansion of the suspension.This additional expansion may occur uniformly, called homogeneous or particulate fluidization, or through theappearance of bubbles, called bubbling, aggregative or heterogeneous fluidization. The bubbles are relativelystable structures which rise through the bed, have sharp boundaries and are almost completely free of particles.Further increase in fluid flow produces larger bubbles, and if the column is sufficiently narrow the bubbles can

fill the cross-section and become slugs. Yet further increases can lead to the breakdown of bubbling and eventually to the transport of the whole bed out of the column [7]. After achieving incipient fluidizationincreasing the fluid flow velocity does not result in any significant increase in the pressure drop as the bed expands to reduce the resistance to flow. Finally at conditions of entrainment the pressure drop decreases as theentrained particles offer little resistance to flow. Fig1. below depicts the pressure drop with gas velocity.

Fig1. Pressure drop across a fluidized bed as function of fluid velocity

S.Baskara Sethupathy et al. / International Journal of Engineering Science and Technology (IJEST)

ISSN : 0975-5462 Vol. 4 No.01 January 2012 317



3.1. Hydrodynamics of Fluidized bed

All types of particles cannot be fluidized satisfactorily. The particle size is an important parameter for fluidization. The particles can be classified according to classification based on density and particle size.

Category (C) whose particle size diameter is less than 30 microns are extremely difficult to fluidize. Category(A) with the range of 20–100microns referred to as powders, whose beds expand considerably when minimum

fluidized gas velocity is exceeded, are again difficult to fluidize. Category (B) whose range of particle size is400–500microns are better candidates for fluidization [8-9].

3.1.1 Bulk density

Bulk density is the overall density of loose material including interparticle distance separation. It is defined as

overall mass of material/unit volume. It is measured simply by pouring weighed quantity of sample of particlesthrough a funnel into a graduated cylinder and volume occupied determines the bulk density. For groundnutshell the bulk density is calculated as 297.72 kg/m

3.

3.1.2 Sieve size D P:

The width of the minimum square aperture through which the particle will pass.

3.1.3 Volume diameter DV :

It is the diameter of a sphere having the same volume as that of the particle.

Dv = 1.13 D p (4)

3.1.4 Surface Volume diameter DSV :

It is the diameter of a sphere having the same external surface area /volume as the particle.

Dsv = 0.87 D p (5)

3.1.5 Sphericity ( Ψ ):

It is defined as ratio surface area of equivalent volume sphere to the surface area of the particle.

ψ = Dsv/Dv (6)

3.1.6 Voidage (€):

A mass of material has particles resting on each other due to force of gravity to form a packed bed. Dependingon the shape of particles and packing characteristics, however, a certain volume of space in between the particles remains unoccupied, such space is called voidage. It is defined as,

€ =voidsrticlesVolumeofpa

umeVoidagevol

(7)

3.1.7 Minimum fluidization velocity U mf :

The minimum velocity required to fully support the solids is called as minimum velocity of completefluidization. It is given as,

2

23

22

32

375.11150

mf

mf

svg

mf

mf

svgmf g psvgU

DU

Dg D

(8)





3.2. Equations for mixtures of sand and biomass

Various works report the studies on the fluidization of mixtures of biomass and sands. The biomass materialsused are rice husk, sawdust and groundnut shell powder, and the sands employed are of two different densities

and particle sizes. Experiments are carried out in a 5 cm ID fluidized bed column to determine the minimumfluidization velocities. The percentage of biomass materials in the mixtures studied is 2, 5, 10 and 15% by

weight. Equations are developed for predicting the U mf values of these mixtures [8-9].

21

2211

ww

wweff

(9)

1

2

1

2

2

11

ww

eff dp

dpkdpdp

(10)

1650

2

,

gdpU

geff eff

mmf

(11)

2

1

2

2

11

221

2

ww

eff

dp

dpdpk dp

(12)

And k 1 is given as,

36.020 1

1 dpk (13)

The equations are also tested for their validity against the data in current literature on U mf values of mixtures of

biomass and sands and also mixtures of particles of different sizes. Biomass materials used are rice husk,sawdust and groundnut shell powder. The other solid material used is sand of two different densities and particle

sizes. Densities of the sands are 2500 and 2700 kg/m3

and average particle sizes are (-600+355 μm) and (-355+250μm), respectively. The average dimensions of the rice husks are 2 mm wide, 1 mm thick and 10 mm

long. The average particle size of the sawdust is (-1000+800 μm) and the average particle size of the groundnutshell powder is (-1200+ 800 μm).It is found that the proposed equations quite satisfactorily predict the U mf values for mixtures of different particle densities and sizes. The following Table 1. gives the comparison of experimental and predicted minimum fluidization velocities for mixtures of sands of two different sizes. Anexperimental error analysis for the experimental and predicted values shows that the equations predict theexperimental values quite satisfactorily up to about a 10 wt% of the biomass in the mixture and for a 15 wt% of

biomass gives lower values as compared to the experimental values.

Table 1. Comparison of experimental and predicted minimum fluidization velocities for mixtures of sands of two different sizes.

Xb(%)

Umf x 10-2

m/sRelativeerror %

Coefficientof

determinationExperimental

Cheunget.al

Predicted

2 8.0 8.2 8.0 0

0.985 8.3 8.21 8.25 0.6010 8.8 8.25 8.7 1.14

15 9.4 8.3 9.2 2.13

Hence biomass to sand ratio of 10% is taken for our study and Umf values were calculated for different biomasssand mixtures. Equations (4) – (13) were used and the values are tabulated in Table 2.





Table 2. Minimum Fluidization velocities for different sand to Biomass sizes

Sand Biomass

Dpeff

(µm)

ρeff

(kg/m3)

Predicted

Umf (m/s)Diameter

(µm)Density(kg/m3)

Diameter (µm)

Density(kg/m3)

500 2500 1000 500 232.9 2300 0.0548

1000 2500 2000 500 478.4 2300 0.2095

1500 2500 3000 500 736.5 2300 0.4092

In our experiment an average sand size of 1100 µm is taken and average biomass (both coconut shell and groundnut shell) is taken as 2000 µm and the Umf is calculated as 0.2477m/s.

3.3. Theoretical study of Fluidized bed gasification

To study the characteristics of FBG with various operating parameters such as Temperature, Particle size, Gasvelocity and bubble size data’s from literatures are suitably assumed. The details of earlier work are summarized in table 3. Using Microsoft Excel scenarios the variations are tabulated and presented with suitable graphs(Fig. 2 & Fig. 3). The correlations are used as presaid in the paper. The main parameters fluidization velocity,expanded bed height and bubble rise velocity are found out as the output from scenarios. The average particlesize of sand is taken as 1100µm and for biomass say groundnut and coconut shell powder the average particlesize is taken as 2000µm. Density of sand and Biomass is taken as 2500 and 500 kg/m

3respectively and assumed

to be constant. From ultimate analysis of fuels the stiochiometric air required is calculated. From those values

the minimum fuel feed required for gasification is found out.

The fluidized bed dimensions are arrived and corresponding values of minimum height at incipient fluidization

is found out. And generally the Hmf value is twice as that of the bed diameter for deep beds and only half of diameter in shallow bed [7-9]. Here the value of Hmf is taken as same as bed diameter 0.15 m and the readingsare calculated thereof. The Voidage is assumed to be 0.45 corresponding to the stand particle of averagesphericity 0.86. Generally the bubble size ranges 30 % in excess with the bed diameter and the gas velocityshould be twice as that of the incipient fluidization velocity. Considering the following data are used for the

theoretical study,

1. Effective mean diameter Deff : 528.54 µm

2. Effective density of particle ρ p :2300 Kg/m3

3. Air Viscosity μ at 25° C : 0.0000184 Kg/ms

4. Density of Gas ρg at 25°C : 1.185 Kg/m3

5. Bed Diameter DB : 0.15 m

6. Bed Height at incipient fluidization Hmf : 0.15m

7. Bed Voidage at U mf (£mf ) :0.45

8. Minimum fluidization velocity Umf :0.2477 m/s

9. Gas Fluidization Velocity U :0.495 m/s





Particle Dia Variation Chart

0.0

0.1

0.1

0.2

0.2

0.3

0.3

0.4

0.4

0.5

Dpeff=367 µ Dpeff=440 µ Dpeff=528.5 µ Dpeff=634 µ Dpeff =761 µ

U m f i n

m / s

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

E x p a n d e d b e d h e i g h t i n m &

B u b b l e r i s e

v e l o c

i t y i n m / s

Umf Bubble_rise_veloci ty Expanded_Bed_Height

Fig 2. Variation of Umf , Expanded bed height and Bubble rise velocity with Particle diameter

Gas Velocity Variation Chart

0.0000

0.0500

0.1000

0.1500

0.2000

0.2500

0.3000

U=0.495 U=0.594 U=0.7128 U=0.8554 U=1.026

E x p a n d e d B e d H e i g h t i n m

1.00

1.10

1.20

1.30

1.40

1.50

1.60

1.70

1.80

1.90

B u b b l e r i s e V e l o c i t y i n m / s

Expanded_Bed_Height Bubble_rise_velocity

Fig 3. Variation of Expanded bed height and Bubble rise velocity with Gas Velocity

As the particle diameter is increased by 20%, Umf value increases by 35 % but bubble rise velocity and expanded bed height shows a decreasing trend bout 7%. As the inlet air temperature increases by 5°C thevariations of Umf plot shows a decreasing trend about 0.8 %. Whereas bubble rise velocity and expanded bed height shows an increasing trend of 0.18%.

When the Gas velocity increases by 20% bubble rise velocity increases by 12% and expanded bed height proportionally increases by 10%. As the particle diameter increases the fluidization velocity also increases since

the drag force exerted by the particle is more and hence more lifting force is required. But bubble rise velocityand expanded bed height decreases since the interparticle forces become less dominant and hence it prevent

bubble forming phenomenon. When temperature of gas increases the viscosity and density varies whichcorrespondingly decreases the fluidization velocity [15]. Viscosity increases with temperature and densitydecreases with temperature. Bubble rise velocity increases and expanded bed height also shows an increasingtrend. From literatures gas bubble diameter usually sizes 30% in excess with bed diameter. As the gas velocityincreases it increases the bubble rise velocity and bed height since the air excess directly lead to bubble

formation and proportionally increases the expanded height [16-21].

4. Experimental Setup

The experimental setup consists of the fluidized bed column 104 x 1400 mm in size. Fuel is fed through screwfeeder and air is supplied through blower. In the down stream side, Cyclone separator, Tar separator (water

scrubber), Diesel bath, Dryer and burner with sampling probes are placed. The entire bed is insulated withrefractories and heater is placed at the base of the bed say 75mm height. The schematic of the experimental

setup is shown in Fig 4 .





Fig 4. Experimental fluidized bed gasification system. 1 − control panel; 2 − air blower; 3 − Variable displacement drive motor; 4 − biomasshopper; 5 − steam generator; 6 − Thermo couple; 7 − free board; 8 − Suction blower; 9 − flare; 10 − cyclone; 11 − blower motor; 12 − water

scrubber; 13 − water inlet; 14 − to gas chromatography; 15 − burner; 16 − dry filter; 17 − fluidized bed gasifier.

4.1. MaterialsBiomass materials used in the present work are groundnut shell and coconut shell. Inert material used here is

sand with bulk density of 1473.44 kg/m3

and average particle size 1100 µm. Biomass particles of differentranges 1, 2 and 3mm has taken for the experiment. The fuel samples have been tested for its ultimate analysisand chemical formula of Groundnut shell and Coconut shell are calculated as shown in table 4.

Table 4. Ultimate analysis of Fuel samples

Composition Coconut Shell Groundnut Shell

Carbon 53.73 51.43Hydrogen 6.15 6.06 Nitrogen 0.86 0.58Oxygen 38.45 38.82Sulphur 0.2 0.22Ash 0.61 2.89

ChemicalFormula

(a.f basis)

C1.603H2.4567O C1.388H2.368O

4.2. Preliminary experimental procedure

The stiochiometric air fuel ratio required for combustion of Coconut and groundnut shell is calculated from (14)

S O H C ma 8/8

3

8

23

100.

(14)

For an equivalence ratio of 0.3 the fuel feed rate is calculated by mass balances and found to be 18.55 and 19.4

kg/h respectively. Biomass feed rate was determined over a range of screw speeds prior to testing. Pressurereadings are measured using a manometer and temperature is measured using K- type thermocouples as positioned. The outlet producer gas composition is to be measured using Calomat6, Oxymat61 and Ultramat23gas analyzers. A complete experimentation phase to be carried out considering the theoretical aspects studied so

far, and earlier experimentation works done in this area.

Conclusion

Fluidized bed gasification is global research area with complex hydrodynamics. Hence more research is needed for better computation and prediction of gasifier performance. Present work details theoretical aspects of

biomass gasification and a procedural approach for determining the fluidization characteristics. Previous worksinsists biomass to sand ratio less than 10% (mass basis) gives better fluidization. A comparative study of





biomass fed gasification in a fluidized bed done so far is made and it reveals that operating performance of FBGmainly depends on bed temperature, particle size, superficial gas velocity, equivalence Ratio and fuel qualityetc. major hindrances faced are agglomeration, excessive tar formation, poor fluidization and bed corrosion etc.

References

[1] P.M.Lv, Z.H.Xiong and J.Chang (2004), ‘An experimental study on biomass air–steam gasification in a fluidized bed ’, Bioresource

Technology, Vol. 95, pp. 95-101.

[2] S.Rapagana, N.Jand and A.Kiennemann (2000), ‘Steam-gasification of biomass in a fluidized-bed of olivine particles’, Biomass &

Bioenergy, Vol. 19, pp. 187-197.

[3] M.M.Hoque, S.C.Bhattacharya (2001), ‘Fuel characteristics of gasified coconut shell in a fluidized and a spouted reactor’, Energy,Vol. 26, pp. 101-110

[4] L.E.Fryda, K.D.Panapoulos and E.Karkaras (2008), ‘Agglomeration in fluidized bed gasification of biomass’, Powder Technology,Vol. 181, pp. 307-320.

[5] David Ross. (2007), ‘Axial gas profiles in a bubbling fluidized bed biomass gasifier’, Fuel, 86, pp. 1417-1429.

[6] Geldart, D.,(1986) ‘Gas fluidization Technology’, University of Bradford, UK.[7] Heiping Cui. (2007), ‘Fluidization of biomass particles: A review of experimental multiphase flow aspects’, Chemical engineering

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[8] Howard, J.R.(1989), Fluidized Bed Technology, New York.

[9] Kunii D, Levenspiel O. (1969, Fluidization engineering. New York: Wiley.

[10] Natarajan, E., A. Nordin and A. Rao (1998), ‘Overview of Combustion and Gasification of Rice Husk in Fluidized Bed Reactors’, Biomass & Bioenergy, Vol. 14, pp. 533-546.

[11] K.G.Mansaray, A.E.Ghaly (1999), ‘Air gasification of rice husk in a dual distributor type fluidized bed gasifier’, Biomass &

Bioenergy, Vol. 17, pp. 315-332.[12] Rao T.R. (2001), ‘Minimum fluidization velocities of mixtures of biomass and sands’, Energy, Vol. 26, pp. 633-644.

[13] Demirbas A. (2005), Potential applications of renewable energy sources, biomass combustion problems in boiler power systems and

combustion related environmental issues. Progress in Energy and Combustion Science 31:pp.171–92.[14] Stanislav V, Vassilev, David BLKA, Christina G, Vassileva (2010), An overview of the chemical composition of biomass. Fuel 89:

pp.913–933.

[15] Kucuk MM, Demirbas A, (1997). Biomass conversion processes. Energy Conversion Management ; 38(2) : 151-65.[16] Jenkins BM, Baxter LL, Miles Jr. TR, Miles TR (1998) Combustion properties of biomass. Fuel Processing Technology, 54: 17–46.

[17] Furimsky E 1998,.Gasification of oil sand coke: Review. Fuel Processing Technology , 56:263–90.

[18] Demirbas A,2004,Combustion characteristics of different biomass fuels. Progress in Energy and Combustion Science , 30: 219– 30.

[19] Beenackers AACM,1999, Biomass gasification in moving beds, a review of European technologies. Renewable Energy , 16:1180–86.[20] Babu SP.1995, Thermal gasification of biomass technology development: end of task report for 1992 to 1994. Biomass and Bioenergy

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