DESIGN OF FLUDIZED BED GASIFER
The fluidized bed gasifier was designed with innovative modifications. The design calculation
was made separately for each of the parts or sub-systems of the gasifier. The preliminary
operating conditions (such as fluidization velocity and equivalence ratio) required for the
energetic gas production on pilot scale were also determined.
The minimum fluidization velocity was calculated separately for the sand and the rice husk
using the following expression

The terminal velocity of the particles was determined for both the bed materials as per the
following expression.

Fluidization velocity during the gasification was determined by considering the relation
between the expanded bed height and the height of the bed at minimum fluidized condition as
follows.

The above equation is also observed to be valid for the bubbling fluidized bed with the following
limitation.

Initially, a value of 1.25 was selected for the above ratio. The fluidization velocity was found to
be 0.7 m/s using Eq. (3).
Overall height of the reaction chamber was calculated by the following expression.


The maximum expanded height of the bed was assumed to be 0.5 m (more than twice the internal
diameter of reactor).
D screw outlet diameter, cm
Umf Minimum fluidization velocity, cm/sec
g Acceleration due to gravity, m/sec2
φ Sphericity
µ Air viscosity (approximately 750°c and latm), kg/m-sec
ρbm Biomass density, kg/cm
ρp Particle density, kg/cm3
ρf Fluid density, kg/cm
ε Porosity
Cyclone Separator
This subsystem consists of a high efficiency cyclone intended to collect the particulate materials
which could be released during the gasification process. A cyclone separator has been designed
for which the considered parameters are listed in Table-1.
Table -1 : Parameters considered for the design of cyclone separator
Parameter Value
Gas inlet velocity (m/s) 15-30
Pressure drop (kPa) < 2.5
Collection efficiency (%) > 90
Table - 2: Design data (Dimensions) of cyclone separator
Parameter Value
Cyclone diameter (cm) 20
Cyclone gas exit diameter (cm) 9.5
Cyclone body cylindrical height (cm) 36
Cyclone total height (cm) 69
Cyclone solids exit diameter (cm) 3
Separation efficiency (%) 99.7
Pressure drop (kPa) 0.45

The density and volumetric flow rate of inlet gas to the cyclone were calculated from the mass
balance. The design data of the cyclone along with its efficiency and pressure drop are listed in
Table-2. Cyclone separator with the dimensions is shown in Fig-1.
Biomass Feeding System (Screw Feeder)
The feeding system consists of a hopper along with a gear box and a screw in the feeding
assembly for feeding the biomass. The screw is driven by a motor with a variable frequency
drive (VFD) as a speed controller. The feeding screw introduces the biomass and the bed
material to the chamber and operates at a greater speed like dosing screw to avoid feed
accumulation which causes system blockages.
Screw sizing:
The relation between the biomass and bed material flow with the bed diameter, fillet height and
revolutions of the screw has been described by the following expression. The designed screw-
feeder has been shown in Fig.-2.
mbm =60×π×n×s×ϕ×ρbm ×\D×h-h2) (6)
Where,
D screw outlet diameter in cm
n speed of screw, rpm
h Fillet height in cm
mbm Biomass mass flow rate, kg/h
ρbm Biomass density, kg/cm3
φ Sphericity
S step screw, m
Distributor Plate (Bubble Caps)
A bubble cap type distributor was selected because of its advantage in reducing the backflow of
bed materials towards the plenum. Four numbers of orifices, each of 5mm dia are made on the
Riser of each bubble cap to facilitate the flow of fluid. Six nos. of bubble caps are arranged on a
circle with one more at the centre of the circle. Air passes through these orifices of risers and is

then distributed uniformly into the reactor through these bubble caps by creating a eddy mixing
of biomass and bed material in the upward direction.
The bubble cap distributor plate was designed and results obtained are presented in Table-4. The
necessary parameters considered for the distributor plate design are listed in Table -3. The
designed distributor plate with the dimensions is shown in Fig.-3.
Table 3 Design parameters for the distributor plate.
Parameter Value
Fluidization velocity (m/s) 0.5
Minimum fluidization velocity (m/s) 0.05
Minimum fluidization height (m) 0.35
Particle density (kg/m3) 2,650
Mean particle size (µm) 385
Bed porosity 0.46
Bed zone diameter (m) 0.25
Table 4 Calculated parameters for the distribution plate.
Parameter Value
Orifice diameter (mm) 5
Number of orifices openings on pipe 4
Number of Bubble cap pipes 7
Total Height of bubble cap/ Riser (cm) 8.5
Skirt Height (cm) 5
Cold model gasifier was designed and shown in Fig.-4. Various experiments were carried out on
cold model gasifier for the optimization of system parameters. With the optimized parameters
hot model gasifier was designed where steam and heat supply were considered in addition to the
other parameters. Designed Hot model unit of gasifier has been shown in Fig.-5. A blower of
750-100 WGP, 5hp Motor with 2840 RPM and Cromption Greaves Ltd.- make, has been used

for continuous air supply. A special square type header of 45cm length has been provided for the
pressure head safety.
Experimentation :
The physical properties of the bed material and biomass handled during experimentation are
given in Table-5. The characteristics of both the feed sample and bed material determined by
proximate and ultimate analysis are listed in Table- 6 and 7. The rice husk and the saw dust were
used as feed samples. Silica sand and Dolomite were used as the bed materials for the cold model
unit. Experiments were carried out by varying different system parameters such as air flow rate,
static bed height, feed rate and pump speed.



Procedure
Bed material is taken inside the gasifier unit by the screw feeder carefully, making them
uniformly distributed. Then the biomass is fed to the gasifier using the screw feeder. Pressure
drop and other parameters are noted for both the biomass and the bed materials. Experiments
were then carried out by adjusting the air flow rate and the biomass feed rate. The pressure drop
profiles against the bed height for silica sand and dolomite have been shown in Fig.-6 and 7
respectively. The minimum fluidization and terminal velocities of bed materials for different
particle sizes of dolomite are listed in Tabie-8. The velocity profile against the bed height for
dolomite has been shown in Fig.-8.
Saw dust was gasified in the real mode i.e. in hot model gasifier using dolomite as the bed
material at the conditions as determined from the cold model experiments. Temperature profile
within the gasifier has been shown in Fig.-9. The effects of biomass feed rate and equivalence
ratio on the yield of Hydrogen have been shown in Fig.-10 and 11 respectively. Gasification
experiments were carried out with and without steam. Yield of hydrogen was found to be more
with steam gasification in comparison with the only air gasification (Fig.-12).


In general 20% of stoichiometric air is required for gasification. With the increase in
stoichiometric air, percentage of the hydrogen yield or efficiency of the gasifier is observed to
increase. Again addition of steam with air improved the Hydrogen yield by gasification process.
Several observations have been concluded through the following examples. With the cold model
unit the bed hydrodynamics of gasifier were studied.
EXAMPLE1
It is observed that the pressure drop increased when there is a shift from minimum fluidization
zone to turbulent fluidization zone irrespective of the particle size of the bed materials as shown
in Fig.-7A, 7B & 7C.
EXAMPLE 2
Bed materials (dolomite) of 1.193mm with the biomass samples were observed to have lower
minimum fluidization velocity in comparison with the bed materials (dolomite) of 2.18 mm size.
Reason may be the increased particle size increases the resistance for which the bed mass needs
more fluid for fluidization. But the with the bed materials (dolomite) of particle size 2.58mm, the
fluidization velocity is observed to be decreased. Reason may be the increased void fraction of
the bed mass (dolomite and biomass sample) with the increased particle size which reduces the
resistance thereby reducing the fluid requirement for fluidization.
EXAMPLE 3
With bed height equal to 50% or about half of the bubble cap height (4cm in the present case),
pressure drop follows a steady pattern. But the bed height below or above 50% of bubble cap
height leads to improper fluidization which results in improper mixing of the bed material with
the biomass samples in the bed thereby causing improper gasification in the hot model gasifier.
EXAMPLE 4
Effect of biomass feed rate on hydrogen yield shows that with increase of feed rate hydrogen
yield decreases [Fig. - 10] . But lower the feed rate causes the wastage of heat and dilution of
product gas. On the other hand too much feed causes the improper fluidization in turn incomplete
combustion.

EXAMPLE 5
Effect of Equivalence Ratio (ER) [Fig.-11] shows that with operating condition of temperature =
800 C, steam to biomass ratio=1.5, and feed rate=10kg/hr of rice husk, hydrogen yield increases
from 36.33 to 38.84% by volume on N2 free basis when ER is increased from 0.5 to 0.25.
Hydrogen yield then decreases with further increase in ER. When ER increased from .25 to 0.35,
hydrogen yield decreased from38.84 to 36.06% by volume where as CO2 yield was found to
increase
EXAMPLE 6
Effect of steam to biomass (S/B) ratio [Fig.-13] shows that with operating condition of
temperature = 800°C, Equivalence ratio=0.25, and feed rate=10kg/hr of rice husk, hydrogen
yield increases from 35.12 to 43.54% by volume on N2 free basis when S/B ratio is increased
from 0.5 to 2.5.
EXAMPLE 7
Effect of temperature [Fig.14] shows that with operating condition of Steam to biomass ratio= 0
(without steam), Equivalence ratio=0.25, and feed rate=10kg/hr of rice husk, hydrogen yield
increases from 21.50 to 40.58% by volume on N2 free basis when temperature is increased from
500 to 1000°C.
It is also observed that when Hydrogen yield increases, yield of CO, CO2 and CH4 decrease with
the increase of each of above three parameters (viz. temperature, S/B ratio and ER).

We Claim
1. (a) Fludized bed biomass gasifier (Cold model) with which bed hydrodynamics are studied.
(b) A fluidized bed air-blown biomass gasifier (both Cold and Hot model) with the addition of
steam and LPG as a primary firing media to produce a synthesis gas; wherein the said gasifier
houses concentrically different zones of gasification completely within the fluidized bed.
2. With the biomass gasification system as in claim 1, bed height (for bed material + biomass
sample) approximately up to 50% of bubble cap height gives proper fluidization.
3. With the biomass gasification system of claim 1, a part of heat required for heating the freshly
supplied feed and the fluidized bed materials in the combustion zone is obtained from char
particles.
4. With the biomass gasification system as in claim 1, the char produced is separated as
entrained particles at the top by using a cyclone separator whereas the liquid tar and un-
reacted, condensed steam are drained periodically from the bottom of the plenum chamber.
Un-burnt biomass and bed materials are elutriated in the freeboard zone.
5. The biomass gasification system of claim 1 is having two separate feeding systems for
biomass and bed materials. Biomass feeding system has two screw arrangements namely
dosing screw and feeding screw whereas the bed material feeding is having only one screw.
5. The biomass gasification system of claim 1 can also be used continuously whereas said
gasifier yields maximum hydrogen at a feed rate in the range of approximately 5-10 kg/hr.
6. With the biomass gasification system as in claim 1, synthesis gas is produced from biomass
feedstock with an average residence time of 0.5 seconds to 2 minutes depending upon the
type of the biomass, provided the bed is attained at 800°C.

7. The biomass gasification method of claim 1, maximum of Hydrogen (43.54% by volume on
Nitrogen free basis) is produced with the operating conditions of temperature=800°C,
S/B=2.5 and ER=0.25.