Prediction of Tar and Light Gas During Pyrolysis of Black Liquor and Biomass

Harland R. PondThomas H. Fletcher

Larry L. BaxterChemical Engineering Department

Brigham Young UniversityAdvanced Combustion and Engineering Research

Center (ACERC)

Wanted: Pyrolysis ModelModel to describe pyrolysis of biomass and black liquor fuelsModel based on individual components

CelluloseHemicelluloseLignin

Model suitable for inclusion in FLUENT and PCGC–3 CFD packages

Chemical Percolation Devolatilization (CPD) Model Approach

For coal, CPD looks at a lattice of aromatic rings (Fletcher et al.)Bridges between rings thermally rupture (kinetic scheme)Lattice statistics determine the fraction of free fragmentsVapor pressure of free fragments determines tar yieldFunction of coal type, heating rate, temperature and pressure

CPD LatticeStructural Parameters required

Number of attachmentsper cluster (σ + 1)Fraction of attachments that are initially bridges (po)Molecular weight per cluster (MCL)MW per side chain (Mδ)

CPD Parameters

Structural parametersMd,, po, MCL, σ + 1, elemental analysisFraction of char links (co)

Kinetic ParametersKinetics for two competing reactions

Model GoalsEach biomass fuel consists of cellulose, lignin and hemi-cellulose (hard- and softwood)Black liquor consists of Kraft lignin and residual carboxylic acidsDevelop kinetic and structural parameters for each componentUse mass-weighted distribution of CPD predictions for individual components

CPD Application to BiomassLinear structure of cellulose is modeled with coand poBridge weights for cellulose type components changed to model ether linkages

Lignin Chemical ModelLignin is similar to low rank coal with lattice structureConiferyl, sinapyland p-coumarylacids base cluster in the CPD model

Setting ParametersStructural parameters (po, MCL, Mδ and σ+1) set with 13C NMR measurements and known structuresKinetic parameters initially taken from previous research (Sricharoenchaikul, Fletcher, Serio, Azevedo)3 parameters need to be fitted to volatile yield data (co, Ec, ρ)

13C NMR Analysis on Black Liquor

Structural parameters for black liquor composite obtained from 13C NMR analysisParent samples sent to Pugmire and Solum at the University of Utah for analysisReacted samples from flat flame burner experiments were also sent for analysis (Webster, BYU, 2002)

O

*

O

O

*

OCH3

OH

R

**

R

OH

OCH3

*

?

OCH3 CH3

OCH2

H

*

OCH CH2CH

ss

Structural ParametersParameters obtained from theory for hardwood and softwood lignin and Kraft ligninCNMR parameters used for composite Black Liquor parameters

Parameter Lignin Lignin Kraft Lignin Black LiquorStructural Parameters Hardwood Softwood Composite

MWCl 207.5 186 195 296.5Md 39 34 21.5 206.43po 0.710 0.710 0.330 0.330σ+1 3.50 3.50 3.50 3.60

Pyrolysis DataVolatile yield data required for fitting the modelNeed data with heating rates and yields at different temperaturesData obtained

Lignin: Nunn, et. al. heated grid dataBlack Liquor: BYU Furnace data

CPD Prediction of Lignin Volatile Yields1000K/s 30s hold, data from

Nunn, T.R., et. al. Ind. Eng. Chem. Process Des. Dev. , 24:844 (1985).

0

10

20

30

40

50

60

500 700 900 1100 1300

Temp /K

% Y

ield

Gas Yield Tar Yield

CPD Model in Previous ResearchKinetic parameters for biomass and black liquor identified from original and previous research (Serio)Some structural parameters in previous applications of the CPD model not set correctly – misunderstood

Sheng & Azevedo (2000) – Biomass Sricharoenchaikul (2002) – Black Liquor

Sricharoenchaikul fit model only to tar yieldsMeasured tar yields were too low (not all tar accounted and carbon balance not closed)

Optimization and Data FittingOptimization required for three parameters (Ec, ρ, co)CPD model used with optimization program to find global optimized parameter values

Values of Ec = 0 gave best model resultsValue of ρ set to 3.9

Model fit to data through fine tuning kinetic parameters and co

CPD Component ParametersParameter Lignin Lignin Kraft Lignin

Structural Parameters Hardwood SoftwoodMW1 207.5 186 195Mδ 39 34 21.5Kinetic ParametersEb, kcal/mol 54.00 54.00 55.60Ab, s-1 2.60E+15 2.60E+15 2.60E+15

σb, kcal/mol 3.972 3.972 3.972Eg, kcal/mol 66.00 66.00 66.00Ag, s-1 3.00E+15 3.00E+15 3.00E+15

σg, kcal/mol 4.776 4.776 4.776ρ 3.9 3.9 3.9Ec, kcal/mol 0 0 0Ecross, kcal/mol 55.68 55.68 55.68Across, s-1 3.00E+15 3.00E+15 3.00E+15co 0.10 0.10 0.28po 0.710 0.710 0.330σ+1 3.50 3.50 3.60fst 0.60 0.60 0.60

Results

Lignin

Yields fit heated grid data well, secondary tar reactions not modeled

CPD Prediction of Lignin Volatile Yields1000K/s 30s hold, data from

Nunn, T.R., et. al. Ind. Eng. Chem. Process Des. Dev. , 24:844 (1985).

0

10

20

30

40

50

60

500 700 900 1100 1300Temp /K

Yiel

d %

Tar Yield Light Gas Yield CPD Tar Prediction CPD Light Gas Prediction

Secondary tar reactions to form light gas

Black Liquor

Yields fit well with furnace data, sodium – carbon reactions not modeled

Black Liquor Volatile Prediction with CPD100K/s with 10 min hold

data from BYU furnace data

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

650 750 800 850 900 950 1000 1100 1200 1300

Temp /K

Yiel

d

Total Volatility Data CPD+Model

Na2CO3 + 2C 2Na + 3CO

Na2CO3 + C 2Na + CO + CO2

Future Work

More volatile data needed for black liquor, cellulose and hemi-celluloseIntegration of a program calculating component composition from elemental analysisPredict volatile yield from weighted average of biomass components

Summary & ConclusionCPD modified slightly to describe biomassPrevious attempts to use CPD model were incompleteDeveloped chemical structural and kinetic parameters for Kraft lignin and lignin based on theory, literature review and curve-fitting

Summary & Conclusion cont’dTwo cases described accurately

Lignin (heated grid)Black liquor (furnace)

More data needed to see if parameters have general applicabilityMore study needed for cellulose, hemi-cellulose and carboxylic acid parametersNeed better understanding and model of secondary reactions