CHEMICAL ENGINEERING TRANSACTIONS 
VOL. 92, 2022 

A publication of 

The Italian Association 
of Chemical Engineering 
Online at www.cetjournal.it 

Guest Editors: Rubens Maciel Filho, Eliseo Ranzi, Leonardo Tognotti 
Copyright © 2022, AIDIC Servizi S.r.l. 
ISBN 978-88-95608-90-7; ISSN 2283-9216 

Design and Development of Bubbling Fluidized Bed Gasifier 
for Non-Woody Biomass Gasification 

Md Shahadat Hossaina,b, Deepak Kumarb, Nishat Paula, Syed Jahid Rahmana, Md 
Anisur Rahmana,c, M. Rakib Uddina,d, Domenico Pirozzie, M. Nazim Uddinf, Abu 
Yousufa,* 
aDepartment of Chemical Engineering & Polymer Science, Shahjalal University of Science and Technology, Sylhet - 3114, 
Bangladesh 
bDepartment of Chemical Engineering, State University of New York College of Environmental Science and Forestry, 
Syracuse, New York 13210, USA 
cDepartment of Chemical Engineering, University of Massachusetts Amherst, Amherst, Massachusetts 01003-9303, USA 
dDipartimento di Ingegneria, Università degli Studi di Napoli “Parthenope”, Napoli–80143, Italy 
eDepartment of Chemical, Materials and Production Engineering, Università degli Studi di Napoli Federico II, Naples-80125, 
Italy 
fDepartment of Irrigation and Water Management, Sylhet Agricultural University, Sylhet - 3100, Bangladesh. 
ayousuf-cep@sust.edu 

Bubbling fluidized bed (BFB) reactor is advantageous for synthesis gas or syngas production among other 
available gasifiers. In this study, a BFB gasifier has been designed, utilizing non-woody biomass (rice straw) 
and air as gasifying medium, to investigate the effect of equivalence ratio (ER), static bed height, and operating 
temperature on synthesis gas yield and gasification efficiency. To calculate various structural and operating 
parameters, a reaction chamber with a diameter of 10 cm and a bed material of 400-500 μm has been studied. 
Considering the minimum fluidization velocity, slugging velocity, and terminal velocity, the optimum operating 
velocity has been taken as 17.25 cm/s. The calculated optimum transport disengagement height (TDH) is 86 cm, 
and the freeboard height is approximately 116 cm. Thus, the overall height of the reactor has come up as 
202 cm. Almost 80% carbon conversion efficiency (CCE) was achieved from this BFB reactor with an optimum 
ER value of 0.35 during rice straw gasification. The same gasifier results in a synthesis gas yield of 3.6 Nm3 kg�  
with a lower heating value (LHV) of about 3.5 MJ Nm3⁄ at optimum ER value. Further scaling up this process 
based on the findings of this study for industrial-scale synthesis gas production can pave a way for bioenergy 
generation from non-woody biomass. 

1. Introduction
In developing countries, most of the non-woody biomass, such as rice straw, rice husk, wheat straw, is mostly 
used in direct combustion for heat energy generation mainly for cooking (Sharma et al., 2020, Shahsavari and 
Akbari, 2018). Due to a relatively lower air to biomass equivalence ratio (ER), carbon dioxide (CO2), carbon 
monoxide (CO), water, and other carbonaceous solids and gaseous waste are also produced in this process 
(Shahbaz et al., 2020, Xue et al., 2019). Lower ER results in heat energy at a relatively lower temperature which 
is not suitable for steam production for electricity generation (Liu et al., 2018). In contrast, higher ER (0.25 – 
6.25) is maintained in the gasification process, resulting in a sufficient supply of air for partial oxidation of 
biomass and self-sustain the gasification process without significantly affecting the synthesis gas (H2 and CO) 
yield (Zhao et al., 2021). 
Gasification is a thermochemical conversion process in which biomass undergoes partial oxidation in presence 
of an oxidizing agent to produce synthesis gas that can be used for liquid and gaseous fuel and electricity 
production (Guran, 2020, Maitlo et al., 2022). The most critical part of the gasification process is the reactor 
subsystem which is known as a gasifier. Several types of gasifiers are used for biomass gasification, for 

 DOI: 10.3303/CET2292049 

Paper Received: 16 December 2021; Revised: 4 March 2022; Accepted: 3 May 2022 
Please cite this article as: Shahadat Hossain M., Kumar D., Paul N., Rahman S.J., Anisur Rahman M., Rakib Uddin M., Pirozzi D., Nazim Uddin 
M., Yousuf A., 2022, Design and Development of Bubbling Fluidized Bed Gasifier for Non-woody Biomass Gasification, Chemical Engineering 
Transactions, 92, 289-294  DOI:10.3303/CET2292049 

289



instance, fixed-bed updraft and downdraft gasifier, fluidized bed gasifier. Fixed-bed gasifiers have a 
considerable biomass conversion rate to be employed for decentralized electricity generation at a smaller scale. 
However,  some other crucial problems like uneven temperature distribution in the catalyst bed, severe catalyst 
poisoning, and higher initial heating energy requirement make them unattractive for large-scale gasification 
processes (Volpe et al., 2017). In addition, those gasifiers show the limitations of scaling up alongside the lower 
heating value synthesis gas production and higher tar yield (Jahromi et al., 2021). Fluidized bed gasifier 
addresses several of these limitations and provides advantages of even temperature distribution, lower 
residence time, short heta-up period, and effective gas-solid mixing.  
Therefore, in this study, a laboratory-scale bubbling fluidized bed (BFB) gasifier was designed for non–woody 
biomass (rice straw) utilization. The effect of air to biomass (rice straw) ER on synthesis gas composition and 
yield was studied. Finally, variation of synthesis gas heating efficiency and biomass carbon conversion efficiency 
with the variation of ER was studied for gasification process performance evaluation.   

2. Methodology 
2.1 BFB gasifier design specifications 

A BFB gasifier was designed for non-woody biomass (rice straw) utilization. Several assumptions were 
considered in this design approach. For instance, a 10 cm of inner diameter (ID) of the gasifier was fixed initially 
and 750 – 850℃ and 101.325 kPa conditions were assumed for such gasifier operation (Rasmussen and Aryal, 
2020). Air and sand were used as a gasifying agent and fluidized bed material correspondingly. Table 1 shows 
other crucial design parameters for this study.  

Table 1: Physical properties of fluidized bed material and gasification medium alongside the operating 
conditions of the BFB gasifier 

Parameters Value 
Operating conditions  
ID of fluidized bed, D (cm) 10.00 
Temperature, T (℃) 750 - 850 
Pressure (kPa) 101.325 
Characteristics of fluidized bed material  
Diameter, dp (µm) 400 - 500 
Sphericity, φ 0.86 
Porosity, εmf 0.42 
Density, ρs( gm cm

3⁄ ) 2.60 
Characteristics of gasifying medium  
Viscosity, μ ( gm cm.s)⁄  0.00018 
Density, ρ (gm cm3)⁄  0.00120 

2.2 Calculation of BFB gasifier design parameters 

The design calculations including the minimum fluidization velocity, terminal velocity, slugging velocity, operating 
superficial velocity, height of the reactor, plenum design, and distributor plate design were carried out using 
various correlations from previous studies (Table 2). The minimum fluidization velocity (umf) refers to the velocity 
at which bed materials start to expand. This velocity was calculated from a relationship between the drag force 
(by upward moving gas) and the weight of the bed particles (Kunii and Levenspiel (1991); Yang (2003)). 
Terminal velocity (ut) was considered as the maximum superficial velocity and was calculated using an 
experimentally determined drag coefficient (CD) of the bed materials (sand) (Table 2). Bubble rise velocity was 
calculated to define the slugging velocity (ub,ms) of bed materials. Operating or superficial velocity (u0) was 
suggested to maintain higher than umf but lower than ut to overcome the slugging conditions in the riser. A 
relationship between expanded (H) and minimum heights (Hmf) of the bed (1.2<H/Hmf <1.4) was used to 
calculate u0 (Chatterjee et al., 1995). The total height of the gasifier (above distributor plate) (HTotal ,g) was 
calculated from the total freeboard height (Hfb) and height of the expanded bed (Hs). An additional 30 cm was 
added to transport disengagement height (TDH) to calculate Hfb, a location of gasifier gas outlet. This TDH was 
calculated from Fung and Hamdullahpur correlations using bubble diameter (db) (Fung and Hamdullahpur, 
1993). Finally, orifices were used in the distributor plate design. As illustrated in Table 2, whole distributor plate 
design was based on the pressure drop in distributor plate (ΔPdis), gas velocity at orifice (uorf), number of orifices 
per unit area of distributor (Nor), and distance between two orifices (pitch). An equidistance triangular layout was 
used for orifice distribution across the distribution plate in this study.  

290



 
Table 2: Correlations used for BFB gasifier design parameters calculation and performance evaluation 

Gasifier parameter Co-relation Reference 
Minimum fluidization 
velocity ( umf) 

1.75
εmf

3 ϕs
�

dpumfρg
μ

�
2

+
150(1-εmf)

εmf
3 ϕs

2 �
dpumfρg

μ
� =

dP
3 ρg �ρs-ρg� g

μ2
 

Kunii and Levenspiel 
(1991), Yang (2003) 

Terminal velocity(ut) 
�
4dp �ρs-ρg� g

3ρgCD
�

1/2

 
Kunii and Levenspiel 
(1991), Yang (2003) 

Slugging velocity (ub,ms) umf + 0.07(gD)
1
2 Kunii and Levenspiel 

(1991) 

Superficial velocity (u0) H
Hmf

=1+
10.978⋅(u0-umf)0.738⋅ρp

0.376⋅dp1.006

umf
0.937⋅ρg

0.126
 

(Chatterjee et al., 1995) 

Riser height HTotal ,g=Hfb+Hs , Hfb=30 +TDH, TDH=13.8×db Yang (2003) 

Distributor plate design ΔPdis=0.3×ΔPbed , ΔPbed=ρP(1-εmf)Hmfg 

uorf=Cd �
2ΔPdist
ρg,orf

�
0.5

, u0=
π
4

dor 
2 uor Nor ; Ppitch=�

2
√3

1
Nden

 

(Basu, 2006), Kunii and 
Levenspiel (1991) 

Synthesis gas yield 
(Nm3/kg) 

Vsyngas (drybasis)
Mafb (drybasis)

 
(Hervy et al., 2019) 

Lower heating value 
(MJ/Nm3) 

126.36 CO+107.98 H2 + 358.18 CH4
100

 
(Niu et al., 2013) 

Carbon conversion 
efficiency (%) 

Csyngas × Qsyngas
Cbiomass × Qbiomass

 ×100 
(Niu et al., 2013) 

Cold gas efficiency (%) LHV of syngas × Qsyngas
LHV of biomass × Qbiomass

 ×100 
(Niu et al., 2013) 

 

2.3 Gasification process performance evaluation 

A homogeneous biomass feedstock supply was used in this study. Rice straw was grounded in Willey mill and 
sieved through a 50 mesh screen. Proximate analysis (GB 28731-2012 method) and ultimate analysis (Flash 
EA 1112 CHNS-O analyzer, Thermo Fisher Scientific Inc., UK) of feedstock were conducted to determine 
moisture content, fixed carbon, volatile matter, and ash content. The performance of the gasification process 
was evaluated in terms of synthesis gas yield (Vsyngas) and mass of ash-free biomass (Mafb), lower heating value 
(LHV), carbon conversion efficiency (CCE) (calculated by carbon content (Ci), and flow rate (Qi) of synthesis 
gas and biomass), and cold gas efficiency (CGE) (Table 2). An online gas composition analyzer (Varian micro-
GC 490) was used for synthesis gas composition analysis continuously. 

3. Results and discussion 
3.1 Structural parameters of the BFB gasifier  

Correlations described in section 2.2 are used to calculate the various structural and operating parameters (umf, 
ut, ub,ms, u0, TDH, ΔPdis, uorf, Nor, and Ppitch) of the BFB gasifier. Table 3 shows that the operating velocity of 
the designed gasifier is 17.25 cm/s for non-woody biomass gasification. The TDH of the gasification column is 
calculated 86.00 cm compared to the 116 cm height of the freeboard. Those heights are merged to design the 
total height of the gasifier riser. Design parameters of the distributor plate are also tabulated in Table 3. Those 
structural parameters were used to design BFB gasifier, and the schematic of the gasifier is shown in Figure 1. 
Biomass and air are fed into the gasifier in a counter-current manner to improve the fluidization efficiency that 
consequently reduces the tar formation as well. Other contaminants, such as dust, are entrained by adding a 
cyclone separator at the end of the fluidization region before synthesis gas composition analysis (Hossain et al., 
2019). Previous studies also reported that the counter-current flow in BFB gasifiers enhanced biomass 
conversion due to effective solid-gas mass transfer in the fluidization region. In addition, counter-current flow 
prevents the slugging phenomena effectively inside the gasifier. To prevent plugging in the fluidization region, 

291



this velocity must be higher than the minimum fluidization velocity (Khezri and Ghani, 2017). Table 3 shows that 
Ub,ms is maintained at 22.23 cm/s, higher than the Umf of 14.26 cm/s. However, counter-current flow in the gasifier 
comparatively lowers the heating efficiency even at higher ER (Cardoso et al., 2018). Since this flow pattern 
improves the operational complexity (slugging phenomena), the counter-current flow pattern is widely used in 
smaller-sized biomass gasification (Shahlan et al., 2018). Finally, a two-dimensional numerical model can be 
developed based on the above laboratory-scale gasification process. Experimental data from the laboratory 
scale process can be used to validate the developed numerical model which can be used to predict the pilot-
scale gasification process afterward. Numerical results from the pilot-scale process can be validated against the 
results available in the literature for a similar process. At the same time, experimental errors must be recorded 
to predict the uncertainty in this scale-up process. 

Table 3: Parameters for BFB gasifier design for non-woody biomass utilization  

Design parameters Value 
Minimum fluidization velocity, umf (cm/s) 14.26 
Terminal velocity, ut (cm/s) 273.11 
Slugging velocity, ub,ms (cm/s) 22.23 
Operating velocity, u0 (cm/s) 17.25 
TDH (cm) 86.00  
Pressure drop across distributor plate, ΔPdis (pa) 783.26 
Gas velocity at orifice, uorf (cm/s) 22.73 
Number of orifices per unit area of distributor, Nor 4572 
Orifice layout pitch, Ppitch (cm) 1.60 

 
 

 

Figure 1: Schematic of the laboratory scale BFB gasifier (air and biomass feeding system, gasifier riser, and 
cyclone separator are shown in the schematic).  

3.2 Effects of ER on gasifier efficiency  

The characterization of rice straw feedstock (moisture content 13.38%) is presented in Table 4. Figure 2a shows 
that both the hydrogen (H2) and methane (CH4) mole percentage in synthesis gas decreased with an increase 
of ER. Increasing ER means that amount of air flow inside the gasification region increases. With the increasing 
ER, carbon monoxide (CO) molar proportion rises as well- about 40-mole percentage at 0.30 ER. CO molar 
percentage decreases with a further increase in the ER value (0.30). In contrast, the carbon dioxide (CO2) mole 
percentage shows an opposite trend of the CO molar proportion. The mole percentage of CO2 decreases at the 
beginning (0.20 – 0.30 ER) whereas this molar proportion increases with the gradual increment of ER value. 
Synthesis gas (molar) composition varies with the increase of ER value because of varying combustion 
characteristics of biomass in the gasification region. A lower amount of air flow initially increases the CO mole 
percentage due to incomplete combustion (Niu et al., 2013). With the increasing ER value, complete combustion 
of biomass generates more CO2 but this accompanies the decrease of other gaseous components, such as CO, 
H2, and CH4, the molar percentage in the synthesis gas (Hervy et al., 2019). Varying biomass combustion 

292



characteristics also affect the synthesis gas yields in Figure 2a. Synthesis gas yield increases from 2.5 Nm3/kg 
to 4.2 Nm3/kg between 0.20 – 0.50 ER value, but nitrogen exclusion from the synthesis gas shows that only 2.1 
to 3.0 Nm3/kg yield increase in the same ER range. Nitrogen dilution improves the synthesis gas yield but lowers 
the LHV of the synthesis gas (Hervy et al., 2019). Figure 2b represents the continuous reduction of LHV due to 
increased airflow (ER) in this study. Similar nitrogen dilution also lowers the CGE after the ER value of 0.30 in 
Figure 2b. In this ER value (0.3), biomass CCE also reaches the maximum value of about 80%, beyond that 
CCE remains at the plateau value.    

Table 4: Proximate and ultimate analysis for rice straw characterization 

Proximate analysis (dry basis) (wt%) Ultimate analysis (dry basis) (wt%) 
Ash  Volatile matter  Fixed carbon  C H O N S 
15.07  
± 0.08 

73.38  
± 0.31 

11.55  
± 0.00 

42.85 ± 
0.82 

5.73  
± 0.35 

36.49  
± 0.41 

0.46  
± 0.11 

0.14  
± 0.04 

 
 

 

 

 

                                                                        (a) 

 

 

 

 

                                                                        (b) 

Figure 2: Effects of ER value on (a) synthesis gas molar composition and yield and (b) synthesis heating 
characteristics and carbon conversion efficiency. 

4. Conclusions 
Designed BFB gasifier can be used for the non-woody biomass (rice straw) gasification for synthesis gas 
production. Minimum fluidization velocity is calculated 14.26 cm/s for this gasification process. Maintaining the 
operating velocity (17.25 cm/s) higher than minimum fluidization velocity prevents the slugging conditions in the 
gasification zone. Orifice meters were used in the distributor plate for airflow to create fluidization conditions 
inside the BFB gasifier and a total of 4572 orifice meters were used in a 1.60 cm pitch distance. In addition, 
optimum ER is calculated at 0.30 in the designed BFB gasifier, using rice husk at 300 µm particle size. At 
optimum ER, both the CCE and CGE reached the maximum values, about 80% and 58% correspondingly. 
Beyond this ER value of 0.30, the gasification process starts generating more CO2 (>20%) than CO (< 38%). 
Moreover, LHV also decreases significantly after ER of 0.30, and therefore, this is considered as the optimum 
ER value for rice straw gasification in the designed BFB gasifier.   

1.0

2.0

3.0

4.0

5.0

0.0

10.0

20.0

30.0

40.0

50.0

0.10 0.20 0.30 0.40 0.50 0.60

S
yn

th
es

is
 g

as
 y

ie
ld

 
(N

m
3 /

kg
)

G
as

 C
om

po
si

tio
n 

(m
ol

%
)

ER (-)

CO H2
CH4 CO2
Syngas Syngas - N2 free

0.00

1.00

2.00

3.00

4.00

5.00

6.00

0.0

20.0

40.0

60.0

80.0

100.0

0.10 0.20 0.30 0.40 0.50 0.60

LH
V

 o
f s

yn
th

es
is

 g
as

 
(M

J/
N

m
3 )

E
ffi

ci
en

cy
 

(%
)

ER (-)

CCE CGE LHV

293



Acknowledgments 

The authors acknowledge the financial support provided by SUST Research Center, Shahjalal University of 
Science and Technology, Sylhet - 3114, Bangladesh under the project AS/2020/1/22.  

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	Design and Development of Bubbling Fluidized Bed Gasifier for Non-Woody Biomass Gasification