2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
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diameter sockets provided vertically to the opposite side
of the feeder, on the reactor wall slightly above the
feeding point and arranged in axial direction) were now
opened. Dropping a little amount of kerosene oil and
igniting the biomass layer of the bed charge initiated
combustion inside the reactor. The auger water-cooling
system and the cooling circuits of the manometer
connections were immediately turned on. The biomass
feeding was continued. The peepholes and the sand
loading port were open to atmosphere and the air was
sucked downward through the bed in the reverse
direction. The temperature of the bed was continuously
monitored. After sometime, the peepholes and the sand
loading port were closed and air was allowed to flow
upward through the bed. This initial combustion of the
biomass was continued till the bed temperature was found
to be around 600
q
C. Combustion for 15-20 minutes
ensured the heating of the bed and the gasifier assembly.
D. Steady State Operation of the Gasifier
Once the fluidized bed combustion operation was
stable and the bed temperature was found to be about
600ºC, the airflow rate and the feed flow rate were
adjusted such that a desired value of equivalence ratio
was obtained. The bed temperature was found to
fluctuate, but could be made stable within
r
20
q
C by the
careful manipulation of the airflow and the biomass feed
flow while maintaining the same equivalence ratio. This
operation was tricky and a complex one as even a very
slight variation in the feed flow rate or the airflow rate
could turn the process into combustion (indicated by a
sharp rise in temperature) or disturb the process
(indicated by sharp downfall of the bed temperature).
However, once set in, the gasification process continued
without trouble. The gas from the cyclone outlet was
flared at the exit end of the pipe. To the trained eyes, it
was easy to recognize the gas of combustible quality. The
stable bed temperature (
r
20ºC) was the indication of the
attainment of steady state of the process operation. All
the temperatures and pressures were noted under steady-
state operation. Gas sampling and tar sampling were also
carried out under steady-state conditions as detailed in
[2]. At least, two samplings were done under each steady
state condition. The data were processed and the averaged
values have been reported. The bed region temperature
and the quality of the flare were the gross indicators of
the process stability.
E. Shut Down of the Gasifier
After the sampling of the tar and the gas, and the
measurements of temperature and pressure were over, the
biomass feeder was switched off and the blower was
continued to supply air for some more time, to burn off
the residual biomass left in the bed. The dust collector
was emptied and the char-ash mixture was consolidated
and stored. The reactor cooled down slowly. The unit was
completely shut off and was allowed to cool down before
the routine cleaning was initiated.
III. R
ESULTS AND
D
ISCUSSION
Thermal efficiency is defined as the ratio of heat
output rate to the energy rate of the fuel. In other words,
it is the percentage of chemical energy in the fuels that is
chemically bound by the product gas. The efficiency
calculations were made based on higher heating value of
the fuel input and the gas output. Fig. 2 and Fig. 3 depict
the thermal efficiency for village rice husk (VRH) and
saw dust (SD) respectively.
Fig. 2. Variation of gasifier thermal efficiency with fluidization air
velocity for village rice husk.
Fig. 3. Variation of gasifier thermal efficiency with fluidization air
velocity for saw dust.
Thermal efficiency is found to decrease as the
fluidization velocity increases. It seems to pass by a
maximum in its variation with equivalence ratio (ER). It
is due to the fact that the gas yield increases and the
higher heating value decreases with increase in ER at any
fluidizing air velocity. For village rice husk, a maximum
efficiency of ~57% at an ER=0.40 and the fluidization
velocity of 0.53 m s
-1
has been found. Lowest thermal
efficiency of 37.4% is obtained at the fluidizing velocity
of 0.73 m s
-1
at an ER = 0.40. For sawdust, the highest
efficiency of 72.3% was obtained at the lowest fluidizing
velocity of 0.53 m s
-1
and an ER = 0.25. Lowest
efficiency was obtained at the highest fluidizing velocity
of 0.73 m s
-1
and the lowest ER of 0.20. In [4] a gasifier
30
35
40
45
50
55
60
0.5
0.55
0.6
0.65
0.7
0.75
Fluidization Velocity (ms
-1
)
GasifierThermalEfficiency (%)
Equivalence Ratio = 0.20
Equivalence Ratio = 0.25
Equivalence Ratio = 0.30
Equivalence Ratio = 0.35
Equivalence Ratio = 0.40
30
35
40
45
50
55
60
65
70
75
0.5
0.55
0.6
0.65
0.7
0.75
Fluidization Velocity (ms-1)
Gasifier Thermal Efficiency (%)
Equivalence Ratio = 0.20
Equivalence Ratio = 0.25
Equivalence Ratio = 0.30
Equivalence Ratio = 0.35
Equivalence Ratio = 0.40
