Review of Condensed-Phase Reaction Kinetics

Drs. Stephen Niksa and Gui-su LiuNiksa Energy Associates, Belmont, CA

Prof. Robert HurtDiv. Of Engineering, Brown University

There are Four Distinct Stages of Coal There are Four Distinct Stages of Coal Conversion ChemistryConversion Chemistry

Devolatilization- Source of all gaseous fuels and soot.- Determines char yield, size, structure, and initial reactivity.

Volatiles Conversion- Conversion of tars into soot- Major heat source.- Partial combustion of primary volatiles.- Major source of CO, H2, CO2, and H2O.- Shifting/reforming chemistry throughout.

Char/Soot Oxidation- Major heat source.- Determines residual char yield for gasification.- Some flyash production.

Char/Soot Gasification- Determines overall conversion.- Flyash production, via char particle fragmentation + ash agglomeration.

Forget “Understanding” and Focus On Accuracy in Applications

Fuel Science SHOULD specify all the rate parameters used in process simulations (CFD, AspenPlus, HySys, etc.)Simulation practitioners should NOThave to comb literature or resort to default values.

All Rate Parameters Should Be Assigned From Readily Available Fuel Property Input

No connections to testing.

Operating Conditions

Detailed Mechanism

FuelProperties

ParameterEstimation

DVol: AD, ED,V∞

ChOx: AC, EC, nCChGs: {AG,i, EG,i, nG,i}

Predicted Rates and Products

ProcessSimulator

• FLASHCHAIN® was recently validated against a database of 332 independent tests involving 99 coals and broad ranges of heatingrates, temperatures, and pressures to 16.7 MPa.• Predicts the complete distribution of all volatile products plus tar and char properties.• Proximate and ultimate analyses are the only sample-specific fuel properties.• Already used to predict the devolatilization behavior of over 2000 coals.• Versions available for any coal, biomass, pet coke, black liquor, and petroleum asphaltenes.

FLASHCHAIN® for Devolatilization

Accurate Predictions for Any Coal Type

65 70 75 80 85 90 95 1000

10

20

30

40

50

60

70

AVCO Bomb, 1.3 MPa

Wei

ght L

oss,

daf

wt.

%

Carbon Content, daf wt. %

Depicts the distinctive yields of individual samples of even the same coal rank.Based on only the proximate and ultimate analyses.

One Framework Covers All P.F.

Only coal exhibits a continuous rank dependence.Petroleum derivatives & biomass determine (H/C)MAX.Black liquor & biomass determine (O/C)MAX.

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Pet Coke

Coal

BlackLiquor

Biomass

Petroleum Asphaltenes

H/C

O/C

Automatically Assign All Devolatilization Rate Parameters

500 750 1000 1250 1500 1750 20000

20

40

60

80

1,00010,000

100,000 K/s

1,000,000

Wei

ght L

oss,

daf

wt.

%

Temperature, K

0.6 0.8 1.0 1.2 1.40

1

2

3

4

1,000

10,000

100,000 C/s

1,000,000

LOG

10<K

>

1000/T

Even the SFOR can match the FC predictions for devolatilization during heatup.Assigned activation energies are constant over a broad range of heating rate.Assigned frequency factors expressed as a function of heating rate, along with ultimate yields.Rate laws can be specified for any product predicted by FC, including volatile-N.

The Carbon Burnout Kinetics Model/Extended Version (CBK/E) for Char Oxidation

CBK/E includes single-film char combustion, intraparticle reaction/diffusion, thermal annealing, and ash inhibition. Three-step intrinsic kinetics resolves the problems in the reaction order for conventional char oxidation kinetics.

1. C + O2 → 2C(O)2. C(O) + O2 → CO2 + C(O)3. C(O) → CO

No Systematic Discrepancies for Shock Tube or EFT Databases

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Measured Rate, g/cm2-s

Pre

dict

ed R

ate,

g/c

m2 -s

Parity plot for burning rate predictions for the shock tube database based on the best-fit assignment to A30 for each coal.

0

20

40

60

80

100

0 20 40 60 80 100

Measured Burnout, daf wt.%

Pred

icte

d Bu

rnou

t, da

f wt.%

Parity plot of burnout predictions for the EFR database based on the best-fit parameter assignment for each coal.

CBK/E was validated against a database of 235 independent tests that characterized 11 coals, 2 coal chars, and a graphite, heating rates approaching 106 °C/s, furnace temperatures to 1527 °C, pressures to 2.0 MPa, and O2 levels to 100 %.

A One-Point Calibration is Needed for Every Fuel Sample

Adjust one frequency factor, A70, to fit the measurements for each coal.Use default values/correlations for all other modeling parameters.Correlate A70 with rank to estimate default rate parameters.

The Rank Dependence of Char Burnout is Similar At Elevated Pressure

Same reactivity for subbituminous coals through low volatility coals.Low rank chars have diffusion-limited burning rates.Enhanced plasticity of low-rank coals at elevated pressure may lower the reactivity.

5

6

7

8

9

10

11

65 70 75 80 85 90 95

Carbon Content of Coal, daf wt%

log 1

0(A

30)

1 atm

High Pressure

Fundamental Difference Between Combustion and Gasification:

Chemistry in the gas phase determines the levels of the char gasification agents (CO2, H2O, CO, H2).

In a p. c. flame, all O2 enters the furnace through burners & OFA ports, so mixing limited extents of conversion determine the local O2 concentration.

In a gasifier, injected O2 only partly determines the conversion levels of volatiles/soot/char, whereas the levels of CO, H2, CO2, and H2O are variable. Gas mixtures are not stable at 1600°C, so gas phase chemistry is important throughout the entire gasifier.

Use CBK/G to Predict Gasification Rates

Combustion2C+O2 → C(O)+COC+C(O)+O2 → C(O)+CO2C(O) → CO

GasificationC+CO2 ↔ C(O)+COC(O) → COC+H2O ↔ C(O)+H2C+2H2 → CH4 (slow)

CBK/G was validated against a database of 452 independent tests that characterized 26 coals, heating rates approaching 105 °C/s, furnace temperatures to 1500 °C, pressures to 3.0 MPa, and broad ranges of CO2, H2O, CO, and H2 levels.

Separate surface oxide pools for the combustion and gasification reactions.Separate surface oxide pools for CO2 and H2O gasification.Currently neglecting CC(O) chemistry and CO chemisorption as marginal.

No Systematic Discrepancies in Predicted Extents of Conversion or Gasification Rates

0

20

40

60

80

100

0 20 40 60 80 100Measured Char Conversion, daf wt.%

Pred

icte

d C

har C

onve

rsio

n, d

af w

t.%

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00Measured Rate, 1/s

Pre

dict

ed R

ate,

1/s

CBK/G Performs Well Over A Broad Domain of Operating Conditions

0

20

40

60

80

100

120

0 50 100 150 200 250 300 350 400Time, s

Cha

r Con

vers

ion,

daf

wt.%

Gottelborn

Polish

Predicted (curves) and measured (data points) char conversion histories for ( and solid line) Polish and ( and dashed line) Gottelborn chars at 1500°C and 0.1 MPa pure CO2 in a WMR (Moors, 1998).

0.0E+00

5.0E-04

1.0E-03

1.5E-03

2.0E-03

2.5E-03

3.0E-03

3.5E-03

4.0E-03

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5PT, MPa

Rat

e, s

-1 H2O

CO2

Predicted (curves) and measured (data points) rates of Xiao Long Tan lignite char gasification at ( and solid line) 850°C in 80 % H2O, 10 % H2, and 10 % CO, and at ( and dashed line) 900°C in 90 % CO2 and 10 CO % (Sha et al., 1990).

CBK/G Performs Well Over A Broad Domain of Operating Conditions

0.E+00

2.E-04

4.E-04

6.E-04

8.E-04

1.E-03

0 10 20 30 40 50 60 70 80 90Char Conversion, daf wt.%

Rat

e, s

-1

1096oC

1040

983

928868

Predicted (curves) and measured (data points) reaction rate profiles for Jincheng anthracite under 0.1 MPa steam at ( and solid line) 1096, ( and dashed line) 1040, ( and dotted line) 983, ( and dotted-dashed line) 928, and ( and double dotted-dashed line) 868°C (Ma et al., 1992).

0.0E+00

5.0E-04

1.0E-03

1.5E-03

2.0E-03

2.5E-03

0 0.5 1 1.5 2 2.5 3PT, MPa

Rat

e, s

-1

Yallourn

Baiduri

Hongei

Taiheyo

Initial CO2 gasification rates of ( and solid line) Hongei, ( and dashed line) Baiduri, ( and dotted line) Yallourn, and ( and dotted-dashed line) Taiheyo at 850°C in a PTGA (Nozaki et al., 1992).

Plausible Correlations for the Reactivity, BUT Wide Dispersion

4

5

6

7

8

9

10

60 65 70 75 80 85 90 95 100Carbon Content, daf wt.%

log 1

0(A

70)

Rank dependence of corrected values for A70. Solid circles denote the best fit values for each coal. The solid line represents the correlation between A70 and a coal’s carbon content.

0

0.1

0.2

0.3

0.4

0.5

0.6

60 65 70 75 80 85 90 95 100Carbon Content, daf wt.%

Initi

al R

ate,

s-1

Rank dependence of the initial burning rate of a 90-µm coal in an 1200°C EFR, at 2.0 MPa and 1400°C and with 10 % CO2, 30 % H2O, 10 % CO and 10 % H2, based on the correlation in Eq. 64.

Simple LSimple L--H/NthH/Nth--Order Rate Laws Order Rate Laws Reproduce the Rates from CBK/GReproduce the Rates from CBK/G

SCOCO

nSCOCOCO

CO PKPRTEA

RCO

,

,

1)/exp( 2

222

2 +

−⋅⋅=ϑ

SHH

nSOHOHOH

OH PK

PRTEAR

OH

,

,

22

2

222

2 1

)/exp(

+

−⋅⋅=ϑ

2

2222 ,)/exp( HnSHHHH PRTEAR −⋅⋅=ϑ

55

44

33

2210 XaXaXaXaXaawhere +++++=ϑ

Must Also Apply a RateMust Also Apply a Rate--Reduction Reduction PolynomialPolynomial

conversionofExtentX =where1.0 + 1.679X - 2.4287X + 1.7825X- = RR 23

CO2

CO

0

2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 0.2 0.4 0.6 0.8 1Carbon Conversion

RC

O2/R

CO

20

Use CBK/G to Extrapolate to Actual Use CBK/G to Extrapolate to Actual GasifierGasifier ConditionsConditions

XCHAR

RGAS

Lab-Scale Gasification

Data for Calibration

CBK/GPredict RGAS, XCHAR for any

operating conditions

Complexgas mixture

compositionsCouple to

equilibrium gas mixturecompositions

Estimate gas product

compositions

Accurately estimate char and

soot conversions

Simpler gasification

rate laws for CFD

ExtrapolationsExtrapolations Based on CBK/GBased on CBK/G

0

20

40

60

80

100

0 10 20 30 40 50 60Residence Time, s

Coa

l Con

vers

ion,

daf

wt.%

0.51.02.0

P, MPa

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 10 20 30 40 50 60Residence Time, s

d p/d

p0

Faster gasification for higher pressures.Inhibition by CO & H2 stronger than the impact of doubling the CO2and H2O levels.

0

20

40

60

80

100

0 10 20 30 40 50 60Residence Time, s

Coa

l Con

vers

ion,

daf

wt.%

10% CO2, 30% H2O, 10% CO, 10% H210% CO2, 30% H2O20% CO2, 60% H2O

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 10 20 30 40 50 60Residence Time, s

d p/d

p0

The FQ Impacts are Evident in a 1D The FQ Impacts are Evident in a 1D GasifierGasifierSimulation With Detailed ChemistrySimulation With Detailed Chemistry

First-stage calculation based on full kinetics to determine Xchar and Xsoot.Steam injection into a reducing second stage.Equilibrium gas compositions shift throughout the second stage.Steam gasification with strong CO inhibition.Soot persists.

0 5 10 15 20 25 30 35Time, s

0

10

20

30

40

50

60

70

80

Maj

or P

rodu

cts,

daf

wt.%

CO

H2O

CO2

H2

Soot

0

20

40

60

80

100

120

140

160

180

0 0.02 0.04 0.06 0.08 0.1

Maj

or P

rodu

cts,

daf

wt.%

hv bituminous

O2

Char

The FQ Impacts are Evident in a 1D The FQ Impacts are Evident in a 1D GasifierGasifierSimulation With Detailed ChemistrySimulation With Detailed Chemistry

Similar trends with subbituminous coal.Comparable product gas quality.Soot persists.

0 5 10 15 20 25 30 35Time, s

0

10

20

30

40

50

60

70

80

Maj

or P

rodu

cts,

daf

wt.%

CO

H2O

CO2

H2

Soot

0

20

40

60

80

100

120

140

160

180

0 0.02 0.04 0.06 0.08 0.1

Maj

or P

rodu

cts,

daf

wt.%

subbituminous

O2

Char

The FQ Impacts are Evident in a 1D The FQ Impacts are Evident in a 1D GasifierGasifierSimulation With Detailed ChemistrySimulation With Detailed Chemistry

Similar trends with lvbituminous coal.Richer product gas quality.Soot and char persist.

0 5 10 15 20 25 30 35Time, s

0

10

20

30

40

50

60

70

80

Maj

or P

rodu

cts,

daf

wt.%

CO

H2O

CO2

H2

Soot0

20

40

60

80

100

120

140

160

180

0 0.02 0.04 0.06 0.08 0.1

Maj

or P

rodu

cts,

daf

wt.%

lv bituminous

O2

Char

Where’s the Soot ?!

Most of the Volatile-C incorporated into soot. Soot competes for available O2, scavenges radicals. Omitted in every reported gasifier simulator.No equilibrium until the soot is consumed.Kinetics determine the gas composition in, perhaps, the first half of a gasifier.Without the correct gas composition, predicted gasification rates will be incorrect.Must work with multiple gasification agents (CO, CO2, H2O, H2, CH4).

Coal Soot Yield, daf wt. %

% of Wt. Loss

Pit. #8 22.9 - 29.1 43 – 57 Ill. #6 21.0 40 PRB 9.1 19

Critical NeedsTests with >20 coals under standard conditions are needed to develop improved correlations between coal properties and the initial char gasification reactivity. Monitor loadings of alkali and alkaline earth cations. Divert lab testing away from cases with a single gasification agent to characterize (i) inhibition by CO and H2 and (ii) the complex mixtures that arise in gasifiers.Monitor gasification rates for coal-derived soot. Characterize the coupling among secondary volatiles pyrolysis, gas phase chemistry, and the conversion of char and soot throughout gasification at realistic suspension loadings.