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Chemical Mechanisms of Tar Chemical Mechanisms of Tar Formation and Destruction Formation and Destruction
during Gasification of during Gasification of BiomassBiomass--Derived FuelsDerived Fuels
Lusi HindiyartiLusi HindiyartiJennica KjällstrandJennica Kjällstrand
Jim FrederickJim Frederick
ObjectivesTo understand the mechanisms of tar formation and destruction during biomass gasificationTo simplify what might be otherwise an overwhelming body of knowledge by finding the most probable pathways for tar transformationsTo provide possible routes for kinetic modelling of tar formation and destruction, based on organic reaction pathways
Definition of Tar: organic decomposition products that have a molecular weight larger than the molecular weight of benzene (78).
Secondary Tar Reactions
Tertiary TarSecondary Tar
Primary Tar
OHSecondary tar: phenolic compounds and olefins
Classification of Tar Compounds O
O
OH
OH
Primary tar: oxygenated compounds
Tertiary tar: polycyclic aromatic compounds
Tar FormationTar Formation
Structural Features of LigninStructural Features of Lignin((MartonMarton, 1979), 1979)
H2C
O
Lignin
CH2
HOH2C
C
CH
OHOCHCH
OCH3
OCH2
OCH3
OH
OH
OCH3
CH
CH
CH2
OH
OH3CO
CH2
C
OH
O
H3CO
CH
CHSLignin
OH
CH2
OH
OH
O
C
CH
OH
OCH3
C
CH
CHC
H3CO
OH
CH
CH
OH
OCH3
OH
OH
CH2
O
C
O
HO
H3CO
OLignin
O
O OH
OH
H3CO CHOH
CH
CH2
OHOH
OCH3
Lignin
AromaticClusters
Ether Links
LabileBridges
Side Chains
Predicting Primary Tar YieldsPredicting Primary Tar YieldsAdaption of the Fletcher et al. Adaption of the Fletcher et al. Chemical Percolation Chemical Percolation
DevolatilizationDevolatilization ModelModel6
0
1
2
3
4
5
600 700 800 900 1000 1100
Model
Measured
6
0
1
2
3
4
5
600 700 800 900 1000 1100
Model
Measured
Maximum Tar Yield(wt% kraft lignin)
Final Reactor Temperature, oCSricharoenchaikul, 2001
Primary Tar Compounds Found in Rapid Pyrolysis Experiments
(Alen et. al, 1996)
VolatilesCatecholsVanillinsPropyl guaiacolsOther guaiacolsAromatic HC’sOther phenolsOther
Conditions
Relativecomposition,
%
75
50
25
04000.24
10000.13
8000.15
6000.19
Pyrolysis of lignin
Temperature, oC:Residence time, s:
Tar TransformationsTar Transformations
VaporPhase
SolidPhase
LiquidPhase
Biomass
CO, CO2,H2O
Primaryvapors
(oxygenates)
Primary Processes Tertiary Processes
Primaryliquids
PNA’s,CO, CO2,H2, H2O
CO, CO2,H2, H2O
Soot
Light HC’s, aromatics,oxygenates
Olefins, aromatics,oxygenates
CO, CO2,H2, H2O
Secondary Processes
Condensed oils(phenolics, aromatics)
Charcoal Coke
Increasing Severity of Pyrolysis
Source: Milne et al.,
1998
Naphthalene Acenaphthene
Phenalene Pyrene
Benzene
Tertiary TarsTertiary Tars
TolueneXylene
OH
Phenol
Secondary TarsSecondary Tars
OH
O
O
Dimethoxy Phenol O
O
OH
OH
Hydroxy MethoxyBenzoic acid
TrimethoxybenzeneO O
O
Primary TarsPrimary Tars
Transformation of Tar SpeciesTransformation of Tar Speciesfrom Black Liquorfrom Black Liquor
0.0
0.5
1.0
1.5
2.0
2.5
0.0 0.5 1.0 1.5Time, s
% ofCarbonInput
900oCBenzene + Naphthalene
C10-C20(Toluene, Xylenes,Methylstyrene)R
R
(Methanol, Formaldehyde,Acetaldehyde, Acetone)C-H-O
BlackLiquor
Oxygenates?
Mechanisms of Tar Mechanisms of Tar TransformationsTransformations
Mechanisms of Tar TransformationsUnimolecular and Bimolecular Decomposition– Tools: Bond Disociation Energy
Bond Dissociation Energy calculation method– Group additivity methods: THERGAS software– Density Function Theory (B3LYP/cc-pVDZ)
Calculation of Bond Disociation Energy
OH
CHO
OMe O
OH
CHO
CH3.+
BDE = Hf (R·)+ Hf(CH3·) - Hf (vanillin)
OHOMe
OHOH
OH OMe
Tar Model Compounds
OH
CHO
OMe
AnisoleAnisolePhenolPhenolCatecholCatecholGuaiacolGuaiacolVanillinVanillin
Bond Dissociation Energiesfor the Model Compounds
AnisoleAnisolePhenolPhenolCatecholCatecholGuaiacol Guaiacol VanillinVanillin
H
O
O
H
CH2
H
CH
O 86
91102 93
63112
87
112
OO CH2
H H83
8563
148
O
OH
H52
90
9052
OH
87
100 OCH2
H
102
8764
63
63 52
5287
64
Unimolecular ReactionsExample: Unimolecular reactions of vanillin
OH
CHO
OMe O
OH
CHO
CH3.+
O
OH
CHO
.
CHO
CO
OH
+
O
O H
CH O
O H
CH O
O H
RH- R
+
CHO
OH
RH- R .
CHO
OH
+
Bimolecular ReactionsExample: Bimolecular decomposition of vanillin
OH
CHO
OMeOH
CHO
OCH2.RH-R +
OH
CHO
CHO
OH
CHO
OCH2.
.H+
OH
CHO
CHOOH
CO.
CHORH-R +
OHCO.
CHO
- CO
OH
CHO
RH- R
OH
.
CHO
+
O
CHO
OMe
.
CHO
OMe
- CO
PAH TransformationsPAH Transformations
. .+
. . H H+ + +
. . H H+ + +
PAH FormationPossible mechanisms- Further reaction of pool radicals or smaller hydrocarbons via Diels Alder- Further reaction of cyclopentadienyl radical
.HH
. +HH H
H.
+
H. .
.
.
H
H+.
The mechanism can be described as:
. .
H
.
.
PAHsPAHsNaphthalene, once formed, is fairly stable at 900oC and below, but decomposes rapidly at 1000oCPAHs are formed more slowly at the lower temperatures, but re not stable at 900oC
Residence time, s
% o
f C
Inpu
t
0
0.5
1.0
10001000ooCC
700oC
800oC
900oC
0.0 0.5 1.0 1.5
C10-C20 tars0
0.5
1.010001000ooCC
700oC
800oC
900oC
Naphthalene
Conclusions
1. Advanced chemical structure models provide useful information about tar species transformations
2. Tar species transformations follow these pathways:
Homolysis of the weakest bond of the compound.
Hydrogen abstraction from the second weak bond by a radical.
Formation of cyclopentadienyl (a stable radical), leading to formation of naphthalene and PAHs via the Diels Alder reaction.
2. Another possible formation of aromatic is from the pool radicals of (ene) which are formed when the cyclopentadienyl radical breaks into smaller compounds that react further via the Diels Alder reaction to form aromatic and larger rings.
3. In pyrolysis, PAH are formed by pyrosynthesis, in which radicals undergo a series of bimolecular reactions with alkenes, alkynes, and aromatics to form larger ring structures. PAH yields change with temperature and gas residence times.
Conclusions
AcknowledgementsAcknowledgements
KnutKnut Lundquist, ChalmersLundquist, Chalmers
HelgeHelge EgsgaardEgsgaard, RIS, RISØØ National National Laboratory, DenmarkLaboratory, Denmark
Valerie Valerie ConraudConraud, National , National PolythecnicPolythecnicde Lorraine, Francede Lorraine, France
