1. Tezpur University
Mechanical Engineering
Down Draft Gasification Modelling of
Some Indigenous Biomass for Thermal
Applications
Partha Pratim Dutta
Assistant Professor
Department of Mechanical Engineering,
Tezpur University, Assam, India.)

2. Overview
INTRODUCTION
Biomass
Gasification
BIOMASS GASIFICATION REACTORS
BIOMASS CLASSIFICATION AND PROPERTIES
MODELLING-THERMOCHEMICAL EQUILIBRIUM APPROACH
EXPERIMENTAL DEVELOPMENT AND RESULTS
Fuel characteristics result
Modelling result
CONCLUSIONS
REFERENCES

3. INTRODUCTION
Biomass definition:
“all organic matter produced by living organisms”
Wood
=
complex biomass main compounds in wt %
arrangement of three main
organic
polymers
+
inorganic
compounds
(ash)
Biomass is
characterized by low
energy density
Pratical applications
require biomass
trasformation into gas
or liquid derived fuel
mean composition of biomass in wt %
volatiles
ash
cellulose
hemi-cellulose
lignine
C
H
O
N
S
Ash

4. GASIFICATION:
It is essentially an oxygen limited thermochemical conversion of
carbonaceous material to a useable gaseous fuel
Biomass gasification process
One of the best way to optimize the extraction of energy from biomass
and to obtain a standardized gas starting from very different materials
Air,
Steam,
CO2,
and/or O2
CO, H2, CO2, H2O, CH4, C2H4
BIOMASS
Low Calorific Value:
Medium Calorific Value:
+
unconverted tars
(all organic compound with
mass > C6H6)
4 - 6 MJ/Nm3
12 - 18 MJ/Nm3
Using air and steam/air
Using oxygen and steam

5. BIOMASS GASIFICATION REACTORS
Fixed bed technology
Updraft gasifier
Downdraft gasifier
Advantages: simple design, good maturity.
Drawbacks: low calorific value gas with a high
tar and fines content.
Fluidized bed technology
Bubbling
fluidized bed
Circulating fluidized bed
Advantages: Good gas and solid mixing, uniform temperatures and
high heating rates, greater tolerance to particle size range safer
operation due to good temperature control compared to fixed bed
gasification .
Drawbacks: segregation of low density biomass fuel.

6. Different steps in Gasification
1. Drying
2. Pyrolysis
3. Oxidation
4. Reduction
Fig.1 .A typical downdraft gasifier

7. Biomass classification and properties
Biomass : wood and non woody
Woody biomass is characterized by high bulk density, less void
age, low ash content, low moisture content, high calorific value.
Non-woody biomass is characterized by lower bulk density, higher
void age, higher ash content, higher moisture content and lower calorific
value
Properties
Bulk chemical analysis
These provide information on volatility of the feedstock, elemental composition and
heat content.
Physical properties
This provide information about shape, size, void age, thermal conductivity, heat
capacity, diffusion coefficient etc.
Biochemical analysis
Provide information about biological degradation.

8. Modelling- thermochemical equilibrium approach
The equilibrium model assumes that all the reactions are in thermodynamic
equilibrium. It is expected that the pyrolysis product burns and achieves
equilibrium in the reduction zone before leaving gasifier; hence an equilibrium
model can be used in the downdraft gasifier. The reactions are as follows:
C + 2H2
CH4
CO + H2O
CO2 + H2
(+75000 J/mol)
(+41200 J/mol)
The equilibrium constant for methane generation (K1) is
K1
P 4
CH
(3.1)
( P 2 )2
H
And equilibrium constant for the shift reaction (K2) is
K2
P 2 PH 2
CO
P PH 2O
CO
(3.2)

9. The global gasification reaction can be written as:
CH1.65 O0.71 wH 2 O mO2 3.76 mN 2
x1 H 2 x2 CO x3CO2 x4 H 2 O x5 CH 4 3.76 mN 2 (3.3)
Where w is the amount of water per kmol of wood, m is the amount of
oxygen per kmol of wood, x1 to x5 are the coefficients of constituents of
the products.
If MC is taken to be the content of moisture per mole of biomass, then
MC
mass of water
(100%)
mass of dry biomass
MC
18 w
(100%)
24 18 w
MC
mass of water
(100 %)
mass of dry biomass
MC
18 w
(100 %)
24 18 w
Then,
w
26.8 MC
18 (1 MC )

10. From the global reactions, there are six unknowns x1 to x5, and T, representing
the five unknown species of the product and the temperature of the reaction
Carbon Balance:
(3.4)
1 = x 2 + x3 + x5
Hydrogen Balance:
2w + 1.65 = 2x1 + 2x4 + 4x5
(3.5)
Oxygen Balance:
w + 0.71 + 2m = x2 + 2x3 + x4
(3.6)
The heat balance for the gasification process (assumed to be adiabatic) is:
x1 H 0 fH 2
H 0 fwood
w( H 0 fH 2O (l )
3.76mH 0 fN2
H ( vap ) ) mH 0 fO2
T '(mC p O2
3.76mC p N 2 )
x2 H 0 fCO
x4 H 0 fH 2O ( vap )
x2C pCO
x3C pCO2
x3 H 0 fCO2
x5 H 0 fCH 4
T ( x1C p H 2
x4C pH 2O ( vap )
x5C pCH 4
3.76mC pN2
Where ∆T = T2 ʹ T1 and ∆Tʹ T2 ʹ T1
=
-
T1 = temperature of the inlet
T2 = temperature of the reduction zone
T2ʹ air inlet temperature
=
(3.7)

11. From Eq. (3.4)
x5 = 1 – x2 – x3
(3.8)
From Eq. (3.5)
x4 = w + 0.82 - x1 - 2x5
(3.9)
Substituting the value of x5 from the Eq. (3.4) into Eq. (3.5)
x4 = – x1 + 2x2 + 2x3 + w –1.175
(3.10)
From Eq. (3.1)
x12 K1 = 1 – x2 –x3
(3.11)
Substituting the value of x4 from the Eq. (3.10) into Eq. (3.6)
– x1 + 3x2 + 4x3 = 2m + 1.885
(3.12)
Substituting the value of x4 from the Eq. (3.10) into Eq. (3.2)
x1x3 = K2 x2 [ – x1 + 2x2 + 2x3 + w –1.175 ]
(3.13)

12. From Eq. (3.7)
H 0 fwood w( H 0 fH 2 0(l ) ) H ( vap ) ) H 0 fO2 3.76mH 0 fN2
T2 T1
T '(mC pO2 3.76mC pN2 )
x1H 0 fH 2 x2 H 0 fCO x3 H 0 fCO2 x4 H 0 fH 2O ( vap ) x5 H 0 fCH 4
( x1C pH 2 x2C pCO x3C pCO2 x4C pH 2O (vap ) x5C pCH 4 3.76mC pN2 )
(3.14)
The general equation for lnK1 is given by
ln K1
7082.848
( 6.567) ln T
T
7.466 10 3
T
2
2.164 10 6 2
T
6
0.701 10
2T 2
5
32.541
(3.15)
The general equation for lnK2 is given by
ln K 2
5870.53
T
1.86 ln T
2.7 10 4 T
58200
T2
18.007
(3.16)

13. The set of equations (3.11) to (3.16) can be solved using the following
algorithm:
1. Specify the value of m and w.
2. Assume temperature T2; find K1 & K2 using Eq. (3.15) and Eq. (3.16).
3. Find x1, x2, & x3 using Eq. (3.11), Eq. (3.12), & Eq. (3.13) respectively.
4. Find x4 & x5 using Eq. (3.8) & Eq. (3.10) respectively.
5. Calculate the new value of T2 using Eq. (3.14).
6. Repeat the above steps until successive value of T2 becomes constant.

14. Experimental development and results
Feedstock:
Bamboo char
Bamboo
Neem

15. Gulmohar
Sishum
Mixed

16. Gasifier system
FLAME
VIBRATOR MOTOR
GASIFIER
FILTER
Cooling system
CENTRIFUGAL BLOWER
Figure 2 : Gasifier (10 kW) in operating condition

17. Producer gas burner

18. Producer gas burner
• Partially premixed
• Premixed

19. Table 1. TECHNICAL SPECIFICATION OF GASIFIER
Model
Gasifier Type
Rated Gas flow
Gasification Temperature
Fuel Storage Capacity
Ash Removal
Start-Up
Fuel type and size
Permissible Moisture content in biomass
Biomass charging
Rated hourly consumption
Rated hourly Ash discharge
Typical conversion efficiency
Typical gas composition
„ANKUR‟ WBG-10 Scrubbed Gas Mode
Downdraft
25 Nm3/hr
1050-1100 oc
85 kg
Manual, Dry Ash Discharge
Through Blower
Wood/ woody waste with maximum
dimension not exceeding diameter 25mm
Less than 20% (wet basis)
On-line Batch mode ,by topping up once
every hour
9 to 10 kg
500 gm to 1 kg
>75%
CO
- 16-22%
H2 - 16-20%
CO2 - 7-13%
CH4 - upto 3%
N2
- 50%

20. Fuel characteristics result
Table 1: Ultimate analysis of different biomass
C% by
weight
48.39
44.43
45.10
44.85
45.85
Feedstock
Bamboo
Gulmohar
Neem
Dimaru
Sisham
H% by
weight
5.86
6.16
6.00
5.98
5.80
Carbon
Hydrogen
Nitrogen
N% by
weight
2.04
1.65
1.70
1.65
1.60
O% by
weight
39.21
41.90
41.50
41.84
40.25
Oxygen
50
Percentage of Elements
45
40
35
30
25
20
15
10
5
0
Bamboo
Gulmohar
Neem
Biomass type
Dimaru
Sisham
Figure 3: Elemental composition of different biomass

21. Table 2: Proximate analysis of different biomass
Feedstock
Bamboo
Gulmohar
Neem
Dimaru
Sisham
Volatiles %
db
80.30
81.25
81.75
82.00
80.00
Fixed Carbon
% db
15.20
13.25
12.65
12.20
15.40
Ash % db
Volatiles
4.50
5.50
5.60
5.80
4.60
Ash
Moisture %
wb
15.00
15.00
15.00
15.00
15.00
Fixed Carbon
90.00
Percentage of Elements
80.00
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
Bamboo
Gulmohar
Neem
Dimaru
Sisham
Biomass type
Figure 4: Compositional variation of different biomass

22. Table 3: Calorific value of different biomass
Biomass
LHV(MJ/kg)
Bamboo
18.4
Neem
16.6
Shisham
Gulmohar
Dimaru
17.15
16.2
15.95
The following empirical relation may be used to compute theoretical GCV of a typical biomass.
GCV = 0.3491XC + 1.1783XH + 0.1005XS – 0.0151XN – 0.1034XO -0.0211Xash[MJ/kg]
where Xi is the contents of Carbon, Hydrogen, Sulphur, Nitrogen, Oxygen and ashes
in wt % (wb) and it is clear from the formula that C, H and S contribute positively for
heating value and N, O and ash contents affect negatively to the heating value.
The net calorific value may be calculated from the following correlation.
NCV = GCV [(1 -
𝑤
𝑤
ℎ
𝑤
)] – 2.44100 – 2.444100 8.936(1- 100 ) MJ/kg wb
100
Where w is moisture content of fuel in wt % (wb) and h is concentration of
hydrogen in wt % (db).

23. Calorific values of biomasses
Poly. (Calorific values of biomasses)
19
y = -1.629x2 + 53.50x - 420.8
R² = 0.998
Experimental (NCV)
18.5
18
17.5
17
16.5
16
15.5
15
15.5
16
16.5
17
Theoritical (NCV)
Figure 5: Comparative studies of theoretical and experimental (NCV)
Figure 12. represents variation of theoretical net heating value and experimental values for the
all samples. The relationship may be best described with the curve fitting expression:
[NCV] exp. = -1.692(NCV)2theor + 53.50(NCV)theor) -420.8

24. Series1
Linear (Series1)
16.6
y = 0.935x - 1.849
R² = 0.998
Net Calorific Value
16.4
16.2
16
15.8
15.6
15.4
15.2
15
18.00
18.50
19.00
19.50
20.00
Gross Calorific Value
Figure 6. variation of theoretical net and theoretical gross calorific value
Figure 13 shows variation of gross calorific value with net calorific value. It is clear from
curve fitting that the relationship between the two variable follow liner relationships given
by the equation. The relationship between net calorific value and gross calorific value is
linear with slope equals to 0.935.
NCV = 0.935GCV -1.849

25. Modeling result
40
Hydrogen
Carbon monoxide
Carbon dioxide
Methane
Neem (CH1.137O0.141)
35
Gas (% v/v)
30
25
20
15
10
5
0
0
5
10
15
20
25
30
35
Moisture Content (% wt basis)
Gas Composition (%v/v)
Figure 7: Effect of moisture content in Neem on gas composition at 850 °C
30
Neem (CH1.137O0.141)
25
20
15
10
5
0
Hydrogen
Carbon
monoxide
Carbon dioxide
Methane
Syngas Component
Figure 8: experimental results for Neem biomass at a gasification
temperature of 850 °C.

26. 40
Bamboo (CH1.65O0.71)
Gas (% v/v)
35
30
Hydrogen
Carbon
monoxide
25
20
15
10
5
0
0
5
10
15
20
25
30
35
Moisture Content (% wt basis)
Figure 9: Effect of moisture content in Bamboo on gas composition at 850 °C
40
Sisham (CH1.132O0.18)
35
Gas (% v/v)
30
Hydrogen
Carbon
monoxide
25
20
15
10
5
0
0
5
10
15
20
25
Moisture Content (% wt basis)
30
35
Figure 10: Effect of moisture content in Shisham on gas composition at 850 °C

27. 40
Gulmohar (CH1.68O0.140)
35
Gas (% v/v)
30
Hydrogen
Carbon monoxide
Carbon dioxide
Methane
25
20
15
10
5
0
0
5
10
15
20
25
Moisture Content (% wt basis)
30
35
Figure 11: Effect of moisture content in Gulmohar on gas composition at 850 °C
40
Dimaru (CH1.136O0.23)
35
Gas (% v/v)
30
Hydrogen
Carbon monoxide
Carbon dioxide
Methane
25
20
15
10
5
0
0
5
10
15
20
25
Moisture Content (% wt basis)
30
35
Figure 12: Effect of moisture content in Dimaru on gas composition at 850 °C

28. Conclusion
 It was observed that bamboo samples had highest calorific values (18.4 MJ/kg)
and Dimaru had minimum (15.95 MJ/kg) for same moisture. Out four main types
of woody biomass Shisham gave maximum calorific value (15.15 MJ/kg).
 From modelling a gasification temperature of 850 C is reached in case of
Neem which gives corresponding syngas composition as H2-16.98%, CO27.35%, CO2-8.14%, CH4-1.8%.
 For all biomass sample predicted result shows that when moisture
content increases H2 and CO2 composition increases and a decreasing trend
is observed for CO and N2. CH4 composition is almost fixed.
 In case of bamboo the increase of CO with moisture content is found to be
more as compared to Neem and almost similar trend is observed for
H2, CO, CO2, CH4 and N2 with moisture in case of all biomass.

29. References
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
Brown, A,. 2010 survey of energy resources. Technical report, World Energy Council,
2010.
Beurskens J,. Peter AF., Ingvar F., Fridleifsson Erik Lysen-David Mills Jose
Roberto Moreira Lars J. Nilsson Anton Schaap Wim C. Sinke Jose Goldemberg Amulya
K.N. Reddy Kirk R. Smith Wim C. Turkenburg, John Baker and Robert H. Williams.
World energy assessment. Technical report, United Nations Development
Program(UNDP), United Nations Department of Economic and Social
Affairs(UNDESA), and the World Energy Council(WEC), 2001.
Handbook of Biomass Downdraft Gasifier Engine Systems.
Kendall's Advanced Theory of Statistics Vol. 2: Interference and Relationship. Gri_n,
1972.
Biomass Gasi_cation: Principles and Technology. Noyes Data Corporation, 1981.
Annual energy outlook 2011. Technical report, Energy Information Administration,
2010.
Biomass feedstock fuel cost comparison. Technical report, White Technology, 2010.
Antal, M.J.; Varhegyi, G. (1995). “Cellulose pyrolysis kinetics: the current state of
knowledge,”
Ind. Eng. Chem. Res. 34, 703-717.
Aznar, M. P.; Corella, J.; Gil, J.; Martin, J. A.; Caballero, M. A.; Olivares, A.; Pérez, P.;
Francés, E. (1997). “Biomass gasification with steam and oxygen mixtures at pilot scale
and
with catalytic gas upgrading. Part I: performance of the gasifier.” from
Developments in
Thermochemical Biomass Conversion, Vol. 2, Eds: Bridgwater,
A. V.; Boocock, D. G. B.,
Blackie, London, UK, 1194-1208.
Beenackers, A.A.C.M.; Maniatis, K. (1996). “Gasification Technologies for Heat and
Power
from Biomass.”
Beenackers, A.A.C.M.; Van Swaaij, W.P.M. (1984). “Gasification of Biomass, a State of
the
Art Review,” in Thermochemical Processing of Biomass, Bridgwater, A.V.,
Ed., London,
UK: Butterworths, pp. 91-136.

30. …………………………..THANK YOU