TGA testing of biomass char gasification
Antero Moilanen
Finnish-Swedish Flame Days 2013
Gasification Workshop
Paviljonki, Jyväskylä, 17th – 18th of April 2013
18.4.2013
2
Biomass fuel as feedstock
Fuel is fed to the reactor
Water (moisture) and volatiles are released (pyrolysis) rapidly
Residual carbon reacts slower
Reactions depend on the characteristics of biomass
H2O
Volatiles
Char residue
Ash
20°C
100 °C ---- 600 °C
---- 850 °C
18.4.2013
3
In TG, biomass char gasification is tested
The achievement of total carbon conversion is influenced by the char
gasification reactivity
Char structure formed during pyrolysis:
heating rate, temperature
gaseous environment (steam, CO2, product gases)
porosity
Catalyst material (ash forming constituents)
Chemical structure
Surface area of the catalytic particulate material in contact with carbon
Dispersion
TG result (mass vs. time) includes all these phenomena as a summary
Shape of the reaction rate vs. conversion graph (reactivity profile)
18.4.2013
4
TG char gasification (cont.)
Gasification studies published in literature are mainly directed to
char prepared separately
In the thermobalance, where fresh biomass is used as sample, the
char is formed in situ and it is difficult to define the exact point,
where char gasification starts (i.e char conversion = 0)
The problematic point is situated in the phase where post pyrolysis
and initial char gasification are overlapping.
18.4.2013
5
TG char gasification (cont.)
Heating rate, affects the char reactivity through char formation
Various char formation reactions have more time during the
slow heating leading to char having low reactivity
Also the structure and chemistry of the inorganic catalytically
active material is influenced by the heating rate.
Heating rate should resemble the feeding to the hot gasifier
Feedstock
Fast heating rate: small particles and particle surfaces
Slow: Cores of large particles ( pieces)
In literature, it is often used separate char samples prepared
with slow heating rate; the reactivity values obtained may not
be usable for reactor design or modelling
18.4.2013
6
TG char gasification (cont.)
Other factors affecting char and its reactivity:
Gaseous environment during pyrolysis
Pressure
Temperature
Char formation mechanisms and its influence on reactivity are not
actually well known
18.4.2013
Test procedure in VTT’s TG - 1
Reactivity measurements in TG are based on the sample weight change as a
function of time at constant T (isothermal) and in the desired gas (pure reactants
H2O, CO2, their mixture or mixed with product gas.
• Aiming at (pressurised) fluidised bed gasification
• Temperature level < 1000 ºC
• Pressure range 1 – 20 bar
Sample size of 50 - 100 mg fresh dried biomass is put into a sample holder having
a good gas - solid interaction without mass transfer effects
Fast heating rate of the sample to the reaction temperature is essential.
In TG tests in VTT, the sample is driven fast into the reactor conditioned with the
reaction environment, i.e. temperature, pressure and gas.
This input is arranged with a chain winch, which lowers the sample holder fast
(within few seconds (abt 7s) into the hot reactor.
During the input the sample heats up, pyrolyses and char is formed.
Fast heating rate can also be achieved with a special jump mode of the reactor
furnace (e.g. TA-instruments at SINTEF)
7
18.4.2013
8
Test procedure in VTT’s TG - 2
When the sample holder has reached the lowest position the weight recording
starts
Before the start of the weight monitoring, the sample has pyrolysed partly
during the input. In the tests, it has been observed that sample still has raw
unpyrolysed biomass at 15 seconds from the initiation of the weight reading. At
30 seconds no fresh biomass is seen, so it takes about 15 – 60 seconds for
the completion of the pyrolysis to the char
During the first 100s., there is overlapping char gasification and post-pyrolysis
at 850ºC causing the sample weight loss.
For this reason it is difficult read the exact point of the char conversion = 0
18.4.2013
Sample
holder
Microbalance
data
acquisition
He-flushing
Winch
system
Ø16mm
Expansion
valve
PRESSURIZED
THERMOBALANCE
Pressure range 1- 90 bar
Temperature max = 1000°C
Sample
lock
Filter
Steam
condenser
N2
CO 2
H2
CO
Reactor
Thermocouple/
Pyrometer
9
data
acquisition
Steam
generator
Water pump
18.4.2013
Cylindrical Ø 16 mm reactor tube
sample
holder with
wire mesh
10
centre shaft
Local gas circulation
wire mesh
24 mm
Cup for comparison
17 mm
fuel sample
11 mm
Gas flow
View from the side
5 mm
10 mm
View from the top
(cross section)
18.4.2013
11
Fast heating of sample in TG -1
Sample holder
- Lowering of the sample to
the reactor with a winch
(abt 7 seconds)
Hot
reactor
·
18.4.2013
12
Fast heating of sample in TG -2
- Jump mode in the heating of the furnace, example TA Instruments of SINTEF
1000
4.5
900
4
800
3.5
700
Mass
3
Temperature
2.5
600
500
2
400
1.5
300
1
200
0.5
100
·
0
0
200
400
600
Time, s
800
0
1000
T,ºC
Real mass, mg
5
18.4.2013
13
Slow heating rate, usual in TG
Spruce bark
N2 + CO2, 850ºC
10
1000
9
900
8
800
7
700
100K/min
600
5
500
4
400
3
300
CO2 on
2
200
1
100
0
0
0
200
400
600
800
Time, s
1000
1200
1400
T ºC
Mass, mg
6
14
18.4.2013
TG curve, isothermal test VTT
Pine wood
1 bar H2O, 850 °C
900
100
Devolatilization, pyrolysis
- In the same gas as the char reactivity is measured
880
60
860
40
840
Real mass,mg
Spline mass, mg
T,C
T, ºC
Real mass, mg
80
Char gasification
820
20
800
0
0
100
200
300
Time, s
400
500
18.4.2013
15
WEIGHT AS A FUNCTION OF TIME, isothermal test
Weight, mg
Devolatilization
- In the same gas as the char reactivity
char gasification
Time
Reaction rate is
calculated from
the weight-time
curve
ash
18.4.2013
16
Fuel conversion
Fuel conversion, ash free = 100*
Sample weight – m2
Weight, mg
Sample weight – ash weight
m2
Ash
Time, seconds
%
18.4.2013
17
Reactivity: rate vs. conversion
Weight, mg
Char gasification rate
m1
m2
m1- m2
mg
t2 - t1
seconds
Instantaneous char gasification rate
1
m1- m2
( t2 - t1 )m2 s
t1 t2
Time, seconds
18
18.4.2013
Instantaneous reaction rate %/min
Result:Instantaneous reaction as a function of fuel conversion;
reactivity profile
500
400
char gasification
300
pyrolysis
200
100
0
0
20
40
60
Fuel conversion, %
80
100
19
18.4.2013
Char conversion zero, i.e. the starting point for char gasification
Bark pellet Mönsterås
1 bar H2O 850 °C
300.0
250.0
mass, mg
200.0
150.0
100.0
Area of the
simultaneous post
pyrolysis and char
gasification
50.0
0.0
0
10
20
30
40
50
60
70
80
90
100
time, s
110
120
130
140
150
160
170
180
190
200
18.4.2013
20
Char sample at 15 seconds at 850ºC
Fresh biomass
500 m
21
18.4.2013
Selection of the point where char conversion = 0
Instantaneous reaction rate, 1/s
0.02
0.015
Time from
the
beginning
0.01
30s
60s
100s
0.005
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Char conversion, -
0.7
0.8
0.9
1
18.4.2013
22
TG curve
Mass- time curve obtained in TG includes all the phenomena of
gasification
Development of pore structure
Char carbon properties
Catalysis
Changes in catalytic active material
Reactions, evaporation, mobility
Characteristic TG curve
Different biomass (species) but also plant parts (bark, heart
wood, stump)
18.4.2013
23
Instantaneous Reaction rate
Reaction rate vs. char conversion – reactivity profile
Reactivity profile biomassspecific
- increasing and
decreasing trends
- minimum rate
1
0
Conversion
18.4.2013
24
Reactivity profile - model
Ref.:Umeki, K., Moilanen, A., Gómez-Barea, A., Konttinen, J., A
model of biomass char gasification describing the change in
catalytic activity of ash. Chemical Engineering Journal vol.207–208
(2012) 616–624.
-works for CO2 gasification, steam needs to be developed
18.4.2013
Gasification
reactivity of
various
biomasses
and tree parts
Ref: Antero Moilanen & Muhammad Nasrullah. Fundamental studies
of synthesis-gas pro-duction based on the fluidized bed gasification of
biomass – Project UCGFunda. Gasification reactivity and ash
behaviour. Espoo 2010. VTT Publications 769.
25
18.4.2013
Conditions in the fluidised bed gasification affecting
char conversion and to be tested in TG
H2
CO
CO2
H2O
Temperature 700 - 1000 °C H2
Pressure 1 – 20 bar (abs) CO
CO2
H2O
(Fragmented and attrited char)
Product gas
- H2,CO, CH4
- tar
-CO2, H2O, N2
- unreacted char
In the reactor, there is char material
(particles) at different conversion
degree having different gasification
reaction rates
Accumulated char
Fuel
O2/H2O
26
18.4.2013
Instantaneous reaction rate,
%/min
Effect of temperature
spruce bark, 1 bar, 100% H2O
200
150
900 C
100
850 C
800 C
50
0
75
80
85
90
95
Fuel conversion (Ash free), %
100
27
18.4.2013
28
Temperature dependence of
gasification (steam) rates of
various fuel chars
Ea= ca. 210 kJ/mol
29
18.4.2013
Effect of pressure
Instantaneous reaction rate,
%/min
Spruce bark
T = 850°C , 100% steam
200
150
10 bar
100
5 bar
1 bar
50
0
75
80
85
90
95
Fuel conversion (Ash free), %
100
18.4.2013
30
150
30 bar H2O
1 bar H2O
30 bar CO2
1 bar CO 2
100
50
Biomass
Kenaf
Sweet sorghum
Miscanthus
Reed canary grass
Barley straw
Wheat straw
Salix
Forest residue (pine)
Pine bark
0
Pine sawdust
r" min. (%/min)
Effect of pressure (cont.) on steam and
CO2 gasification
Under pressure the reactivity
higher in steam
18.4.2013
31
Effect of product gas:
Instantaneous reaction
rate, % /m in
Different biomasses in gas mixtures
T= 875 °C & total pressure 1 bar ( 0.3 bar H2O, 0.2 bar H2, 0.25 bar CO2, 0.15 bar
CO & 0.1 bar N2)
Bark:
200
Aspen
bark (o)
Aspen
150
Spruce
bark (o)
Spruce
100
Birch
Birch
bark (o)
50
Pine
Pine
bark (o)
0
75
80
85
90
95
Fuel conversion (Ash free), %
100
= spruce bark 1 bar steam (100%) at 875°C for comparison
Moilanen, A., Nasrullah, M. Variation in fuel Reactivity and ash characteristics of biomass feedstock for
large-scale gasification. Pres. in 17th European Biomass Conference and Exhibition. 29 June - 3 July 2009,
Hamburg, Germany
18.4.2013
Product gases also inside the char pores
char
H2O
CO
CO2
H2
H2O
CO
CO2
H2
H2O
CO
CO2
H2
32
18.4.2013
33
Formulas
Steam – hydrogen :
r 1PH2O
R
1 r 2 PH 2 O r 3 PH 2
R
r 4P CO2
1 r 5 PCO 2 r 6 PCO
Gas mixture:
R
r1 PH2O r 4PCO2
1 r 2 PH 2 O r 3 PH 2 r 5 PCO 2 r 6 PCO
Temperature dependence
ri
kie
CO2- CO:
E / RT
34
18.4.2013
Gasification reactivity of various biomasses
steam 850°C
Instantaneous Reaction Rate ,%/min
1 bar
150 Sawdust
100 (pine)
50
0
80
100
50
90
100
0 80
100
90
100
0 70
80
90
100
0
250
80
90
100
90
Fuel conversion, %
100
0
90
100
Sweet sorghum
80
400
90
100
90
100
Kenaf
200
50
80
80
100
100 Reed canary grass
Salix
0
200
Barley straw
50
50
0
200 Miscanthus
100 Wheat straw
100 Pine bark
500
30 bar
0 80
90
100
0
Fuel conversion, %, %
3 bar H2 + 27 bar H2O
80
Fuel conversion, %, %
35
18.4.2013
Classification of ash sintering/melting after a TG test
under microscope
powder
o
molten
sintered
***
*
-
**
18.4.2013
36
TGA testing of biomass char gasification
Summary
Fast heating rate
In the determination of the kinetic parameters for gasification
reactions, the suitable testing conditions shoud be sought for each
biomass
The effect of pyrolysis conditions on char reactivity
Temperature – pressure - gaseous environment
need to be studied and fixed for testing
Char gasification tests in steam, CO2, and their mixtures and
product gas components H2 and CO
18.4.2013
37
TGA testing of biomass char gasification
Acknowledgements:
Financing
1.
Nordic Energy Research Top-Level Research Initiative
(Nordsyngas)
2.
The Academy of Finland (GASIFREAC-project)
Collaboration
Prof. Kentaro Umeki LTU; Prof. Alberto Gómez-Barea Univ.
Seville; Prof. Jukka Konttinen JYU; Mr. Jasob Kramb JYU; Dr.
Liang Wang SINTEF; Prof. Rainer Backman UMU; Dr. Nikolai
DeMartini ÅA;
Mr. Jere Lehtinen VTT; Ms. Mirja Muhola VTT