Design of a Bubbling Fluidized Bed Gasifier for the thermochemical conversion of Oil Palm Empty Fruit Bunch Briquette

A. Johari1, B. B. Nyakuma1, A. Ahmad1, T. A. Tuan Abdullah1, M. J. Kamaruddin1, R. Mat1, A. Ali2

1Institute of Hydrogen Economy (IHE), Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor Bahru, Malaysia.

2Department of Engineering Science, Faculty of Science and Technology, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Terengganu, Malaysia.

Corresponding author email: anwar@cheme.utm.my

Keywords: Bubbling Fluidized Bed Reactor, Gasification, Combustion, Oil Palm, Empty Fruit Bunch, Briquette.

Abstract. This paper is focused on the design of a bubbling fluidized bed gasifier (BFBG) for EFB

briquette gasification. The annual production of palm oil in Malaysia generates large quantities of

lignocellulosic biomass which can be converted into clean, sustainable energy for the future. Hence,

the prospect of valorising palm waste using biomass gasifiers presents a viable option for energy

production. The fluidized bed gasifier (FBG) is considered the most suitable reactor for biomass

gasification due excellent mixing, efficient heat temperature control and tolerance for fuels.

Consequently, the proposed design of the bubbling fluidized bed gasifier for EFB briquette

gasification will consist of three main parts; feeding zone, gasification zone and the effluent gas zone

for syngas production. The results of feedstock physicochemical properties such as bulk density,

particle size, the bed hydrodynamic and fluidization parameters for gasification used in the design of

the gasifier are presented in this paper.

Introduction

The transition from fossil fuels to renewable sources of energy remains a key challenge for many

emerging economies. In Malaysia the large quantities of lignocellulosic waste resulting from crude

palm oil production provides a good source of fuel for biomass conversion technologies.

Biomass conversion can be carried out either by thermochemical or biochemical methods.

Gasification has gained widespread acceptance as an efficient thermochemical biomass conversion

method due to its potential environmental benefits, low set up costs and relative simplicity.

Gasification involves converting low value fuels into gaseous products at high temperatures using air,

steam or oxygen as gasifying medium in specially designed equipment called gasifiers [1, 2]. The

major gasification reactions are presented in Eq. 1-6. + → , , , , , , , ℎ , ℎ(1) + (1 2⁄ ) → (2) + → 2 (3) + → + (4) +2 → (5) + ( 2⁄ ) → + ( 2⁄ ) O(6) In practice the exothermic reactions between the fuel and air/O2 in the gasifier supply the thermal

energy for gasification.

There are three types of gasifiers; fixed or moving bed, fluidized bed and entrained flow [1, 2]. The

fluidized bed gasifier (FBG) is considered the most suitable gasifier for biomass gasification due to

its tolerance for different types of fuel, excellent mixing, and temperature control. In addition, the

operating FBGs require moderate air, oxygen and steam input, resulting in high cold gas efficiency,

and carbon conversion [2, 3, 4]. Two types of FBG gasifiers exist; bubbling fluidized bed gasifier

(BFBG) and circulating fluidized bed gasifier (CFBG).

Applied Mechanics and Materials Vol. 493 (2014) pp 3-8Online available since 2014/Jan/08 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.493.3

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This study will focus on the bubbling fluidized bed gasifier (FBG) as it is currently the most

widely used gasifier for palm waste gasification by researchers in Malaysia. A number of studies have

demonstrated the feasibility of converting palm waste such as empty fruit bunches (EFB) into syngas

using bubbling fluidized bed gasifiers [5, 6]. However, the alkali content and high moisture of EFB

increases its susceptibility to bed agglomeration during gasification [5] potentially resulting in loss of

fluidization, gasifier fouling and corrosion problems [7, 8]. Hence, EFB briquette has been proposed

as a fuel for gasification in this study. The high energy density and solid uniform nature of briquettes

ensure efficient heat and mass transfer during conversion. Additionally, the low moisture and shape

of the fuel ensures easy handling, storage and transportation [9].

The thermochemical fuel properties of EFB briquette have been investigated using

thermogravimetric analysis (TGA) [10, 11]. The results showed that EFB briquette decomposition

occurs in four stages; drying, heating, devolatization and char oxidation. The devolatization stage

commenced at 206 °C while the peak devolatization temperature was observed at 325 °C with 70 %

weight loss.

Biomass fuel analysis is vital for determining optimal reaction conditions and predicting the

overall behaviour of the thermochemical conversion. In addition, ultimate and proximate analyses

can be used to determine the steam to biomass ratio and equivalence ratio for gasification.

Additionally, biomass physical properties can be used to determine fluidization parameters [12].

Consequently, this paper will focus on the design of a bubbling fluidized bed gasifier (BFBG) for

EFB briquette steam gasification using established knowledge on palm waste gasification.

Theory & Methodology

Biomass Gasifier. A schematic of the proposed bubbling fluidized bed gasifier (BFBG) is

presented in Fig. 1. The proposed BFBG will comprise; feeding, gasification and effluent zones. The

feeding zone is made up of the feed for air (supplied by a compressor), steam (supplied by a steam

generator) and EFB briquette (supplied by a feeder). The gasification zone consists of a tubular

reactor made of 316 stainless steel with freeboard length 400 mm, internal diameter (ID) 150 mm and

reaction zone of length 650 mm, ID 100 mm. A perforated plate type air distributor (diameter 100

mm, thickness 3mm) will be attached to the gasifier to hold the bed materials and uniformly distribute

the fluidizing gas into the bed of solids. The effluent zone consists of a cyclone of length 1050 mm,

ID 100 mm, gas condenser, separator and gas analyser.

Fig. 1. Schematic of the proposed Bubbling Fluidized Bed Gasifier

4 Advances in Applied Mechanics and Materials

During operation the EFB briquette and sand (used as bed materials) will be heated using ceramic

band heaters to obtain the desired gasification temperatures. A number of K type thermocouples will

be used to observe and control the temperature profile during gasification. Air and steam will be used

as fluidization agent and steam will be used and gasifying medium respectively. A cyclone will be

used to collect particulate matter and ash elutriated from the gasifier. The effluent gas from the

gasifier will be cooled by a condenser for analysis in a gas chromatograph to determine the gas yield

and composition.

Design considerations. The major operating parameters in this study include; equivalent ratio

(ER), steam to biomass ratio (S/B), and gasification temperature (GT). Fluidizing parameters include;

fluidization, bed pressure drop ∆Pb, minimum fluidization velocity Umf, and terminal velocity Ut.

Equivalence ratio (ER) is a measure of the actual air fuel ratio to the stoichiometric air fuel ratio

which is always < 1.0 for gasification reactions. It can be represented mathematically as [1];

(< 1.0) = ℎ (7) Steam to Biomass ratio (S/B) is a measure of the steam feed to biomass feed ratio. It can be

expressed mathematically by the relation [1];

= ( ) ( )(8) Fluidization is the process of transforming fine solid particles into a fluid like state by passing a

gas or liquid through a bed of the particles [4, 13]. Over a given period of time, the flow of a rising

fluid (of known velocity) through a bed of particles results in a pressure drop (∆Pb) across the bed

equal to the apparent weight of the particles [4, 14]; ℎ = ℎ (9)

Or expressed mathematically by the relation;

∆ = (1 − ) . . (10) Where ∆Pb is bed pressure drop, H bed height or depth, ε, ρp density of bed particles and g is

acceleration due to gravity. The minimum fluidization velocity, Umf is the velocity at which a packed

bed becomes fluidized and is proportional to the size and density of the bed particles and can

expressed mathematically as [4];

= −150. . .1 − (11)

Where Umf is minimum fluidization velocity, dp mean particle diameter, ρp density of bed particles, ρf

fluid density, g acceleration due to gravity, εmf bed voidage at minimum fluidization, µ viscosity of

fluidizing agent and ϕs particle sphericity.

Terminal velocity, Ut; The terminal velocity Ut can be expressed by the relation [15];

= 4. −225. . (12) By adjusting the defined operating parameters the yield and composition of effluent gas can be

obtained.

The ideal design for an air distribution plate is vital for realizing a distributor pressure drop ∆Pd. In

bubbling fluidized beds, the distributor pressure drop ∆Pd is 15 to 30 % of the bed pressure drop ∆Pb

[3, 12]. The relationship between bed pressure drop ∆Pb and distributor pressure drop ∆Pd is given by

the relation [14];

Applied Mechanics and Materials Vol. 493 5

= 0.15 → 0.3(13) However the pressure drop in the bed, ∆Pb, due to the fuel feed is a function of the mass of particles in

the bed, MB as can be deduced from the expression [14];

= (1 − ) (14)

The bed pressure drop, ∆Pb can be calculated from the equation;

∆ = 1 − (15) However, the distributor pressure drop ∆Pd can be calculated from the equation [4];

∆ = 2 (16) Where Cdor is orifice coefficient, ρg, gas density, kg/m

3 and uor is the orifice gas velocity. The orifice

coefficient Cdor can be determined from the Reynolds number relation for the column;

= (17) Where dt column diameter in m; uo is superficial gas velocity through the bed material in m/s. Taking

the column diameter as 0.1 m, uor as 0.4 m/s (5 Umf ), ρg as 1.01 kg/m3 and µ as 1.85 X 10

-0.5 kg/ms, the

Re > 2000 which corresponds to 0.6.

The standard cyclone design is the cylinder-on-cone cyclone with a tangential entry [17]. The

geometry of a cyclone with a ‘slot’ type inlet is determined by the following dimensions; body

diameter, D; total height of the cyclone, H; diameter of the vortex finder, Dx; Length of the vortex

finder, S; height of the inlet, a; width of the inlet, b; height of the conical section, Hc; and diameter of

the dust exit, Dd [17].

Results and Discussion

Fuel properties The EFB briquette was pulverized to obtain a particles < 1000 µm to determine

proximate, ultimate analysis and heating value. The ultimate analysis was carried out using a LECO

CHNS analyser model 932. Proximate analysis carried out using ASTM standard techniques and

heating value using a bomb calorimeter. The results of the analyses are presented in Table 1.

Table 1. Results of proximate and ultimate analysis

Ultimate Analysis (wt %) Dry Ash Free basis Proximate Analysis (wt %) Dry basis

C H N S O MC VM AC FC HHV, MJ/kg

43.15 5.73 1.2 0.04 49.88 8.17 71.83 4.56 15.44 17.57

C,H,N,S,O – Carbon, Hydrogen, Nitrogen, Sulfur, Oxygen; MC-Moisture, VM-Volatiles, AC-Ash, FC-Fixed carbon

The molecular formula CH1.59O0.88 was obtained for EFB briquette. The volatile matter to fixed

carbon (VM/FC) ratio for both fuels was > 4.0 typical of biomass in comparison to < 1.0 for coals

[14]. For the design considerations, the physical properties of the EFB briquette were determined as

presented in Table 2. The properties of inert bed material (sand) were obtained from Ramirez et. al.

2007 [18] and the dimension of EFB briquette is shown in Table 2. The fluidizing agent of choice is

air with density, ρ 1.16 kg/m3 at 27 ° C and viscosity, µ, 1.84 x 10

-05 kg/ms.

Table 2. Physical properties of EFB briquette and sand

Property Mean Particle Size,

dp (µm)

Apparent Density, ρ

(kg/m3)

Sphericity (ϕs) † Voidage (εmf)

†

Sand 385 2600 0.66 0.42

EFB Briquette 318 805 0.85 0.80 † Deduced from Kunii and Levenspiel [4]

6 Advances in Applied Mechanics and Materials

Table 3. EFB briquette dimensions

Property Mass of EFB briquette (g) Length (cm) Diameter (cm)

EFB Briquette 2.50 5.00 0.80

From the physical properties, the nature and type of fluidization, bed hydrodynamics and

fluidization parameters can be determined [4]. Therefore, the EFB briquette and sand can be

classified as Group B particles according to the Geldart classification criteria. Solids classified in

group B particles have particle sizes in the range from 40 µm to 500 µm, undergo vigorous bubbling

fluidization, bubbles appear when gas velocity exceeds Umf, and Umb / Umf ratio equals 1 [19]. The

fluidization parameters for EFB briquette and sand were calculated as presented in Table 4.

Table 4. Hydrodynamic properties of sand & EFB briquette

Property Minimum fluidization velocity Umf (m/s) Terminal velocity Ut (m/s)

Sand 0.08 3.14

EFB Briquette 0.53 1.19

The fluidizing velocity for materials in a fluidized bed is usually in the range 0.5 to 1.7 m/sec [14].

Hence the results obtained fall within the literature values required to ensure fluidized bed

gasification.

Pressure Drop Considerations The bed pressure drop ∆Pb can be calculated from Eq. 10. By

substituting the values of the parameters into Eq. 10, the value of ∆Pb = 1,479.35 Pa was deduced. In

addition, the mass of bed particles, MB, which satisfies the bed pressure drop can then be deduced

from Eq. 15.

Conclusion

The design of a bubbling fluidized bed gasifier (BFBG) for EFB briquette gasification was

presented in this study. The physical and chemical properties of the feedstock and inert bed material

required for determining the design and operating parameters of the fluidized bed reactor have also

been presented in detail. In addition the calculated values for bed hydrodynamics and fluidization

parameters presented in the study were found to be in agreement with findings in literature.

Acknowledgement

The authors are grateful for the financial support from the Universiti Teknologi Malaysia (UTM),

Ministry of Higher Education (MOHE) and Research University Grant (RUG), VOT No. 05H04.

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Advances in Applied Mechanics and Materials 10.4028/www.scientific.net/AMM.493 Design of a Bubbling Fluidized Bed Gasifier for the Thermochemical Conversion of Oil Palm Empty

Fruit Bunch Briquette 10.4028/www.scientific.net/AMM.493.3

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http://dx.doi.org/10.1002/cjce.5450730206