MECHANISMS OF PYROLYSIS
Jim Jones
WHAT IS PYROLYSIS?
Pyrolysis is:
the thermal
decomposition of
carbonaceous materials
in the absence of oxygen
WHAT IS PYROLYSIS?
Pyrolysis is:
the thermal
decomposition of
carbonaceous materials
in the absence of oxygen
+
where the system
pressure is not
constrained by the
vapour pressure of water
This is called hydrothermal
decomposition
WHAT IS PYROLYSIS?
Pyrolysis is:
the thermal
decomposition of
carbonaceous materials
in the absence of oxygen
+
where the system
pressure is not
constrained by the
vapour pressure of water
This is called hydrocharring
Pyrolysis is:
the first step in combustion
WHAT IS PYROLYSIS?
Pyrolysis is:
the thermal
decomposition of
carbonaceous materials
in the absence of oxygen
+
where the system
pressure is not
constrained by the
vapour pressure of water
This is called hydrocharring
Pyrolysis is:
the first step in combustion
How?
WHAT IS PYROLYSIS?
Pyrolysis is:
the thermal
decomposition of
carbonaceous materials
in the absence of oxygen
+
where the system
pressure is not
constrained by the
vapour pressure of water
This is called hydrocharring
Pyrolysis is:
the first step in combustion
Heat in
WHAT IS PYROLYSIS?
Pyrolysis is:
the thermal
decomposition of
carbonaceous materials
in the absence of oxygen
+
where the system
pressure is not
constrained by the
vapour pressure of water
This is called hydrocharring
Pyrolysis is:
the first step in combustion
Decomposition reactions
WHAT IS PYROLYSIS?
Pyrolysis is:
the thermal
decomposition of
carbonaceous materials
in the absence of oxygen
+
where the system
pressure is not
constrained by the
vapour pressure of water
This is called hydrocharring
Pyrolysis is:
the first step in combustion
Mass transfer out
WHAT IS PYROLYSIS?
Pyrolysis is:
the thermal
decomposition of
carbonaceous materials
in the absence of oxygen
+
where the system
pressure is not
constrained by the
vapour pressure of water
This is called hydrocharring
Pyrolysis is:
the first step in combustion
Mix with O2
WHAT IS PYROLYSIS?
Pyrolysis is:
the thermal
decomposition of
carbonaceous materials
in the absence of oxygen
+
where the system
pressure is not
constrained by the
vapour pressure of water
This is called hydrocharring
Pyrolysis is:
the first step in combustion
Combust
WHEN THERE IS NO AIR
Pyrolysis forms three products:
• Charcoal
• Condensable liquids (tar)
• Non condensable gases
WHAT IS THE ISSUE?
Pyrolysis is:
A BLACK BOX
WHAT IS THE ISSUE?
Pyrolysis is:
We set inputs
We observe outputs
Feedstock type
Moisture content
Particle size
Additives
Heating rate
Soak temperature
Residence time
Yield
A BLACK BOX
Properties
WHAT IS THE ISSUE?
Pyrolysis is:
We set inputs
We observe outputs
Feedstock type
Moisture content
Particle size
Additives
Heating rate
Soak temperature
Residence time
To optimise pyrolysis and to dynamically
control the reactions, we need to
understand the mechanisms
Yield
Properties
WHAT IS THE ISSUE?
Pyrolysis is:
QUALITATIVELY
...we understand the mechanisms at play.
“The description of the pyrolysis process is particularly challenging because it evolves
a great deal of physical and chemical transformations and produces a large number of
product species. As a result, existing models aiming to predict the rates or yields of the
released pyrolytic volatiles are still supported by empirical data...”
[Figure and quote from Neve et al., 2011]
KINETIC MODEL 1
Primary Rxns
Secondary Rxns
gas
1
4
2
Biomass
volatiles
5
3
char
Each reaction has:
1. a heat of reaction
2. a rate
Mechanism proposed by Shafizadeh, 1975
1
2
3
Endothermic
4
5
Exothermic
KINETIC MODEL 1
Primary Rxns
Secondary Rxns
gas
1. Heat of Reaction
1
4
2
Biomass
volatiles
5
3
1
2
3
Endothermic
4
5
Exothermic
Reaction No.
1
2
3
4
5
H [kJ/kg]
420
420
420
-40
-40
char
2. Rate of Reaction
Each reaction has:
1. a heat of reaction
2. a rate
ki = Ai e − Ei / RT
Legend
A
=pre-exponential factor [1/s]
E
=activation energy [kJ/mol]
R
= gas constant = 8.314 [J/mol/K]
Mechanism proposed by Shafizadeh, 1975
[Data cited in Fantozzi, 2007]
KINETIC MODEL 1
Primary Rxns
Secondary Rxns
gas
1. Heat of Reaction
1
4
2
Biomass
volatiles
5
3
1
2
3
Endothermic
4
5
Exothermic
Reaction No.
1
2
3
4
5
H [kJ/kg]
420
420
420
-40
-40
char
2. Rate of Reaction
Each reaction has:
1. a heat of reaction
2. a rate
ki = Ai e − Ei / RT
Legend
A
=pre-exponential factor [1/s]
E
=activation energy [kJ/mol]
R
= gas constant = 8.314 [J/mol/K]
Reaction No.
1
2
3
4
5
Mechanism proposed by Shafizadeh, 1975
A
1.44E+04
4.13E+06
7.38E+05
4.28E+06
1.00E+05
E
88.6
112.7
106.5
107.5
107.5
ref Temp [K]
673
673
673
673
673
[Data cited in Fantozzi, 2007]
KINETIC MODEL 1
Primary Rxns
Secondary Rxns
gas
1. Heat of Reaction
1
4
2
Biomass
volatiles
5
3
1
2
3
Endothermic
4
5
Exothermic
Reaction No.
1
2
3
4
5
H [kJ/kg]
420
420
420
-40
-40
char
dmB
= −mB (k1 + k2 + k3 )
dt
dmgas ,1
= mB k1
dt
dmvolatiles
= mB k2 − mvolatiles (k4 + k5 )
dt
dmchar ,1
= mB k 3
dt
dmgas , 2
= mvolatilesk4
dt
dmchar , 2
= mvolatiles k5
dt
2. Rate of Reaction
ki = Ai e − Ei / RT
Legend
A
=pre-exponential factor [1/s]
E
=activation energy [kJ/mol]
R
= gas constant = 8.314 [J/mol/K]
Reaction No.
1
2
3
4
5
A
1.44E+04
4.13E+06
7.38E+05
4.28E+06
1.00E+05
E
88.6
112.7
106.5
107.5
107.5
ref Temp [K]
673
673
673
673
673
[Data cited in Fantozzi, 2007]
KINETIC MODEL 2
1
Biomass
(volatiles + gases)1
+
2
3
(volatiles + gases)2 +char2
char 1
cf KINETIC MODEL 1
gas
Reaction order is now flexible
1
4
2
Biomass
volatiles
5
3
char
[Mechanism proposed by Koufopanos et al., 1989. Elaborated by Babu and Chaurasia, 2002]
KINETIC MODEL 3
The components of biomass each
have different decomposition kinetics
4
gas
tar
2
Cellulose
Hemicellulose
Lignin
1
intermediate
3
char + gas
cf KINETIC MODEL 1
cf KINETIC MODEL 2
gas
1
1
4
2
Biomass
volatiles
(volatiles + gases)1
3
Biomass
+
(volatiles + gases)2 +char2
5
3
2
char 1
char
Mechanism proposed by Miller and Bellan, 1997
KINETIC MODEL 3
The components of biomass each
have different decomposition kinetics
4
gas
tar
2
Cellulose
Hemicellulose
Lignin
1
intermediate
3
char + gas
cf KINETIC MODEL 1
cf KINETIC MODEL 2
gas
1
1
4
2
Biomass
volatiles
(volatiles + gases)1
3
Biomass
+
(volatiles + gases)2 +char2
5
3
2
char 1
char
Mechanism proposed by Miller and Bellan, 1997
NETWORK MODELS
...are focussed on emissions
They are...
“the most fundamental pyrolysis models ... which aim to
describe the physical and chemical phenomena taking place during a particle’s
devolatilization.”
De Jong et al. 2007 use:
FG-DVC - Function Group - devolatilization, vaporisation and crosslinking model
It uses TG-FTIR data.
[Quote from de Jong et al., 2007]
NETWORK MODELS
How they work
“Network models are based on a structural description of the parent fuel...During
devolatilization, the macromolecular fuel structure is changed as a result of
depolymerization, vaporization and cross-linking of the fuel matrix. This causes the
breakup of existing bridges connecting the aromatic rings and the formation of new
bridges at rates described by network statistics. As a result, gaseous products and tars
are formed, while the solid particle converts into carbonaceous char.”
FG-DVC models have two parts:
A functional group model (FG) - describes the gas evolution as well as the elemental
and functional group compositions.
Depolymerization, vaporization and cross-linking model (DVC) - determines the
amount and molecular weight of macromolecular fragments. The lightest of these
evolve as tar.
[Quote from de Jong et al., 2007]
OUTPUT = f{INPUTS} MODEL
Neve et al. (2011) “suggest that empirical relationships
can be developed to approximate the pyrolysis behavior of
most biomasses. Moreover, the literature data structured by the
authors is related to a wide range of operating conditions”.
We set inputs
We observe outputs
Feedstock type
Moisture content
Particle size
Additives
Yield
A BLACK BOX
Properties
Heating rate
Soak temperature
Residence time
[Model proposed by Neve et al., 2011]
OUTPUT = f{INPUTS} MODEL
Dry ash free char
Liquids
Gas
Water
[Model proposed by Neve et al., 2011]
DON’T FORGET HEAT AND MASS TRANSFER
Larger particles have heat and mass transfer limitations
It is important to hold larger
particles for longer time so
that temperatures
equilibrate
Biot number =
Heat transfer resistance
Mass transfer resistance
SUMMARY
There are many approaches to understanding pyrolysis mechanisms.
Five are presented here, from purely empirical, to kinetic models
with empirical constants to fundamental mechanistic models.
The degree of complexity has to relate to the end-use of the data.