minimizing environmental impacts of coal-based energy … · 2015. 12. 16. · • china is...

Minimizing Environmental Impacts of Coal-based Energy

Generation

Jennifer WilcoxDepartment of Energy Resources

EngineeringStanford University

Woods Energy InstituteMay 7, 2008

On the Path to Cleaner Coal

Global coal use

Trace element (Hg, As, and Se) speciation

Trace element capture

CO2/H2 separation using inorganic membranes

• Worldwide, fossil sources account for80% of the energy demand.

• 52% of the electricity generated in theU.S. is from coal.‡

• The U.S. produces 1.5 billion tons/year of CO2 from burning coal.*

• China is constructing equivalent of two, 500 MW, coal-fired power plants/week (capacity comparable to the entire UK power grid each year).*

• China & India projected to account for 68% of the demand in world coal through 2030.**

†IEA Key World Energy Statistics (2006); ‡IEA 2005 annual statistics (www.eia.doe.gov)**IEA, World Energy Outlook 2004, p. 34; *MIT, The Future of Coal 2007.

Energy Outlook and the Role of CoalBreakdown by Energy Source†

Government and Policy• The Clean Air Act (Ammended 1990)† set up a cap and trade

system for SOx and NOx emissions; yielding a 38% reduction in SOx and a 25% reduction in NOx over the last 30 years.

• Clean Air Interstate Rule (2005)‡ mandates large reductions in SOx and NOx emissions across 28 eastern states and the District of Columbia.

• Clean Air Mercury Rule (2005)* permanently caps and reduces mercury emissions from coal-fired power plants. U.S. is the first country in the world to regulate mercury emissions from utilities. (currently 48 tons/year) – vacated February 2008

– First Phase Cap (2010): reduce emissions to 38 tons/year– Second Phase Cap (2018): reduce emissions to 15 tons/year

• April 2007: Supreme Court ruling allows CO2 emissions to fall under the Clean Air Act → state regulations are near

†EPA. Acid Rain Program Laws and Regulations, Title IV. 2002. ‡EPA. CAIR Caps. 2005.; *EPA. Regulatory Impact Analysis of the Final Clean Air Mercury Rule, U.S. EPA, Editor. 2005, EPA-452/R-05-003.

KNOWLEDGE EXCHANGE

ACADEMIA

GOVERNMENTINDUSTRY

FundamentalResearch

Marketing;Commercialization

Policy; Regulations

Global Integration for Cleaner Coal

INVENTION

GLOBAL SOLUTIONS

Applied Research

Applied Research

Internships;Collaborations

EnvironmentalImpact

U.S. Collaborations

Aerodyne, Extrel, EPRI, EPA, DOE, REI

Internships;Collaborations

SE Asia Collaborations

Chulalongkorn U (Tawatchai) – sorbent generationOkayama U (Sasoaka) – LCA and sorbent testingNat U Singapore (Saeys) – Heterogeneous Catalysis


Global coal use




Gasification vs. Combustion

• Gasification is similar to combustion, but w/ less than half of the amount of oxygen needed to burn the coal fully

• Using combined cycle design (gas + steam turbines), gasificationis more efficient, i.e., PCC ~ 38% vs. IGCC ~43%

• Gasification is a reducing environment where as combustion is anoxidizing environment which influences TE speciation

• Coal gasification: – Products: CO, H2, NH3, H2S, Ash– Trace elements: Hg, HgH, HgS, As, As2, AsS, AsH, AsH3, Se,

H2Se, HSe, SeS• Coal combustion:

– Products: H2O, CO2, N2, NOx, SOx, Ash– Trace elements: Hg, HgCl2, HgO, AsO, AsCl3, SeO, SeO2,

SeCl2

Mercury Reaction Pathways

Important Factors– Chlorine speciation and concentration– UBC in ash and coal composition/type– Temperature and quench rate– SO2 and other inhibitors

Ash & Unburned Carbon (UBC)

Hg0

HCl

Cl•

Hg2+

HgP

HClCl2

Homogeneous

Heterogeneous

Limited Pathway

Dominant Pathway

Determine from kinetic modelingagainst experiment:

Kinetic and Thermodynamic Calculations

]][[]][[ DCkBAkr rf −=

A + B ↔ C + D

⎟⎠⎞

⎜⎝⎛ −

= RTEa

nf eATk

c

fr K

kk =

Traditional rate expression

Parameters: A, n, Ea and Kc

obtain from ab initioobtain from ab initio

Computational Methodology

• Solving approximations to SWE, (QM, ab initio)– Gaussian03 (QCISD/B3LYP); Molpro (CCSD(T))

• Level of Theory (Method/Basis Set) Validation– Experimental kinetics often unavailable for comparison– Geometries– Vibrational frequencies– ∆Hrxn prediction of reactions– Keq prediction and comparison to experiment

• Potential Energy Surface Development– Reaction assumptions (degrees of freedom)– Surface crossing and excited states– Inclusion of ZPEs and thermal corrections

• Apply Chemical Kinetics, i.e., TST, VTST, or RRKM• Obtain a rate expression from first principles, i.e., ab initio

Ψ=Ψ EH

Potential Energy Surfaces

Hg + Br2 → HgBr + BrHgBr2 → HgBr + Br

Homogeneous Mercury Kinetics

HgCl (+M) → Hg + Cl (+M) QCISD/RECP60VDZ 16.13 4.25 (13)

HgCl + HCl → HgCl2 + H QCISD/RECP60VDZ 30.27 4.5 (13)

Hg + HCl → HgCl + H B3LYP/RECP60VDZ 82.06 2.62 (12)

Hg + Cl2 → HgCl + Cl B3LYP/RECP60VDZ 42.8 1.34 (12)

Hg + HOCl → HgCl + OH B3LYP/RECP60VDZ 36.638 3.09 (13)

HgCl2 (+M) → HgCl + Cl (+M) B3LYP/ECP60MDF 80.55 2.87 (13)

HgCl2 (+M) → Hg + Cl2 (+M) B3LYP/ECP60MDF 86.98 3.19 (11)

HgCl + Cl2 → HgCl2 + Cl B3LYP/ECP60MDF 0 1.43 (9)-2.465 (10)

HgCl + HOCl → HgCl2 + OH B3LYP/ECP60MDF 0.485 1.74 (9)-3.48 (10)

Reaction †,‡Level of Theory

ForwardAct En

kcal/mol

Preexp (A)

cm3/mol*s

†ECP60MDF - Martin, Jan M. L. Sundermann, Andreas, J. Chem. Phys. 2001, 114(8), 3408.‡RECP60VDZ - Stevens, Walter J. Krauss, Morris, Can. J. Chem. 1992, 70, 612.

Components of the CHEMKIN Model

Global mechanism ~ 376 reactions• Detailed kinetic mechanism

− Hydrocarbon chemistry− SOx chemistry− NOx chemistry

• Cl chemistry by Roesler - 29 reactions• Hg-Cl reactions by Wilcox - 9 reactions• Collaborating w/ REI in Salt Lake City, Utah on a

comparison of ab initio-based (Gaussian03) predictions with experiment

0

2

4

6

8

10

12

14

16

18

20

0 100 200 300 400 500 600

Chlorine Concentration (ppmv equivalent HCl)

% O

xida

tion

Wilcox - ab initio

Fry - experiment

Is Homogeneous Hg Oxidation Taking Place?

Quantum mechanical calculations

Simulated flue gas using flamewith methane combustion

Ab initio-generated kinetic parameters were compared against experimental speciation data using CHEMKIN software.

2500 °F

Hg0

Boiler(Lower Furnace)

Economizer

Boiler(Upper Furnace)

Temperature Quench2500 °F – 1200 °F


Isothermal 700 °FSelective Catalytic Reducer

Air Pre-HeaterTemperature Quench

700 °F – 350 °F

Isothermal 350 °F

ElectrostaticPrecipitator

Wet Flue Gas Desulfurization


Stack150 °F

Small conversionHg0 → Hg2+

Dependant on HCland Carbon in Ash

Large conversionHg0 → Hg2+

Dependant on HCl

Medium conversionHg0 → Hg2+ → HgP

Dependant on HCland Carbon in Ash

~ 95% RemovalHg2+

~ Total RemovalHgP

Application of Hg Modeling Efforts?

Co-benefits exist but further reduction is still required to meet Clean Air Mercury Rule (CAMR).

Bromine as an Oxidizing Agent

Reaction∆Hrxn

(kcal/mol)

Activation Energy

(kcal/mol)

Preexponential Factor (A)

(cm3/mol*s)

HgCl + Cl →Hg + Cl2 1.27 11.34 1.57E+11HgBr + Br → Hg + Br2 1.83 2.33 6.66E+13

HgCl + Cl → HgCl2 0.272 0 1.54E+08HgBr + Br → HgBr2 1.89 0 2.04E+10

HgCl + Cl2 → HgCl2 + Cl 0.402 ~0 1.43E+09HgBr + Br2 → HgBr2 + Br 1.37 ~0 4.42E+12

H + HgCl → HCl + Hg 0.766 3.41 1.07E+12H + HgBr → HBr + Hg 4.36 1.24 2.50E+15

Experimental System

CAVKIT Mercury vapor

generator

F

F

CH4Cl2

in air

FI

Rotameter

UV Detector

Purge air

Solenoid valve

Flashback arrestor

Mass Spectrometer

vent

Mass flow controllers

Furnace

air

TemperatureControllers

Heat tape

N2

Solenoid valve

Solenoid valve

Solenoid valve

Solenoid valve

DAQsystem

Experimental System

EI-QMS (Extrel)


Global coal use




Gasification vs. Combustion• Coal gasification

– Oxygen + steam + coal = fuel gas – Pre-combustion cleaning

• Sulfur/Hg removal – fuel gas is cooled to 220 ºF (expensive)– Clean syngas can be used for multiple applications,

• Combined cycle: Gas turbine + steam turbine (capture heat from process) for electricity generation

• WGS: CO + steam = CO2 + H2 can lead to effective CO2separation and H2 production

• F-T liquids• Coal combustion

– Excess air (or oxygen) + coal = flue gas– Cleaning occurs throughout the quenching process

• SCR catalysts, reduces NOx, but can lead to SO2 oxidation• ESP capture fly ash and particulate matter, including activated

carbon (compromises fly ash sales)• FGD scrubbers for SO2 capture, water-soluble TEs are also

captured and can be reemitted

Pd-HgCl2

Pd

Cl

Hg

Cl

Au-SeO2

AuO

Se

• Vienna Ab initio Simulation Package (VASP) for energy predictions using a periodic slab approach.

• Density Functional Theory w/ electron exchange and correlation functionals (gradient-corrected) of Perdew and Wang (1991).

• Lattice constants compared to experiment for model validation.

• Binding energy calculations:Ebind = Ecomplex – [Eadatom + Esurface]

Simulating Surfaces Using DFT

A Novel Material for Coal Combustion Applications

E. Sasmaz and J. Wilcox, J. Chem B. in press

Pararallel Perpendicular

SO2 -1.474 -0.288HgCl -0.879 -0.905HgCl2 -0.793 -0.326HgO -1.38 -2.757

Most stable orientation

SO2

HgCl

HgCl2

HgO

A Novel Material for Coal Gasification Applications

Hg Binding hcp site on PdAu

%Au Composition0 20 40 60 80 100

E bind

(kca

l/mol

)

-30

-25

-20

-15

-10

-5

0

0% Au Surf 25% Au Surf 50% Au Surf 75% Au Surf 100% Au Surf

• Adding small amount of gold, i.e., 12.5%, 25%, increases B.E.

• Gold improves the surface reactivity when it stays at the bottom layer instead of surface (top) layer.

0% Surface Au Comp.12.5 %

Au Comp.

-24.35 p.Pd.-hcp

(Au)

-22.75 p.Pd.-hcp

(Pd)


Global coal use




Gasification vs. CombustionElectricity generation including CCS

• Gasification– IGCC (gasification) gasifier provides a syngas of CO and H2

→ CO w/ water in a shift reactor yields CO2 and H2→ CO2 is captured→ H2 is combusted in a gas turbine

• H2 separation membrane reactor leads to CO conversion efficiency requiring lower temperatures than traditional WGS reactors; higher temperatures (faster reaction kinetics) are possible for a given efficiency.

• Total pressure remains high enough so CO2 compression is not required.

• Combustion– Sub and supercritical PC efficiencies drop nearly 10% w/ CCS.– CO2 separation (MEA) from flue gas and recovery (heat),

cooling, and compressing all require energy (i.e., additional coal burning).

Local alloy composition, NN and NNN

Solubility in Pd-based Alloys

Step 1 Simulate either PdAg or PdAu using the VASP computational software package.

Step 2 Calculate the binding energy, , as a function of NN, NNN, and the lattice constant.

Step 3 Calculate all parameters required for the solubility predictions, , using Sievert’s Law and the binding energy.

221

HSSHSSb EEEE −−= +

( )2

2223

242

hkTI

hmkT ππα ⎟

⎠⎞

⎜⎝⎛=

( ) ( )HhHHOHE

S ehhE

hDK νβνβα

νν

β −−−−•⎟

⎟⎠

⎞⎜⎜⎝

⎛⎥⎦

⎤⎢⎣

⎡−−+−=

112exp11

23

42exp

2

2

212HS PK=θ

3-D O-site Binding Energy Contour Plots for PdAg

15% Ag

37% Ag 50% Ag

25% Ag

O-Site BE Model Predictions Compared to Exact DFT

T-site T-site

O-siteO-site

DFT O site classical binding energy (eV)-0.5 0.0 0.5 1.0 1.5

Mod

el O

site

bin

ding

ene

rgy

(eV

)

-0.5

0.0

0.5

1.0

1.514.8% Ag25.9% Ag37.0% Ag48.2% Ag

DFT O site classical binding energy (eV)-1.0 -0.5 0.0 0.5 1.0 1.5 2.0

Mod

el O

site

bin

ding

ene

rgy

(eV

)

-1.0

-0.5

0.0

0.5

1.0

1.5

2.014.8% Au25.9% Au37.0% Au

2010

29

2807

06504321

aananaana

anannaaananaaE

NNNNNNNN

NNNNNNNNNNNNO

++++

+++++=

Predicted Solubility in PdAg and PdAu Alloys

*Knapton, A. G. Plat. Met. Rev. 1977, 21, 44.

*

The solubility predictions overestimate the experimental data, but the maxima occur at agreeing alloy compositions.

Schematic of Hydrogen Diffusion

Diffusivity in Pd-based Alloys

Step 4 Calculate activation energies for the O-T and T-O hops in the alloy lattice as a function of NN, NNN, and the lattice constant

Step 5 Use transition state theory to calculate rate constants for the O-T and T-O hops

Step 6 Use a kinetic Monte Carlo (KMC) algorithm and the First Passage Method with Einstein’s Equation to predict the H diffusion coefficients.

Step 7 Use both the solubility and diffusion coefficient predictions to calculate the H permeabilities in the Pd-based alloys

( )⎟⎟⎠

⎞⎜⎜⎝

⎛ +−−=

∏

∏

=

=− Tk

aNNNNNEaNNNNNk

B

zpzpa

iiO

iiTS

TO

*

3

1,

2

1

*, ,,

exp),,(εε

ν

ν

( ) ( ) ⎥⎦

⎤⎢⎣

⎡−= ∑

=∞→

N

iiit

RtRNt

D1

206

1lim

15% PdAg 50% PdAg14.8% 37%

Ea (eV) Predictions of PdAg Alloys: O→T Hop

• Lower activation energies lead to increased diffusion

• Diffusion occurs more readily at Ag compositions less than 37%

• NN and NNN compositions of (2.5+:0-3) yield high activation barriers

• Activation barriers are > 48.2% Ag composition

15% 26%

37% 48%

Diffusion Coefficient Predictions of Pd-based Alloys

50% PdAg

Experiments: Holleck, G. L., J. Phys. Chem, 1970, 74, 503.

*No exp data

• As temperature increases, the diffusion predictions are closer toexperimental data for PdAg.

• Due to the ideal periodic nature of the modeling calculations, it is likely that material imperfections play a role in this deviation.

• The model predictions provide insight into which material may be best suited for hydrogen separation from CO2.

15% PdAg

Permeability Predictions

50% PdAg

%Au in PdAu alloy0 10 20 30 40 50

Per

mea

bilit

y (m

ol/m

/s/P

a0.5 )

10-9

10-8

10-7

456 K590 K691 K1095 K

Details: Sonwane, S.; Wilcox, J.; Ma, Y.H. J. Chem. Phys., 125, 184714, 2006.

Pd-based Membranes for H2-Separation

• Solubility estimates for PdAg (30% Ag at 456K) are 10 times that of pure Pd. For PdAu (20% Au at 456 K) alloys, the solubility was found to be 12 times higher than that of pure Pd.

• Permeability estimates in 20%Ag PdAg alloy at 456 K is 3-4 timeshigher than pure Pd, while 12% Au PdAu alloy shows ~2 times higher permeability than pure Pd.

• Pd and Au are expensive. New materials involving Fe alloys and monolayers are being investigated.

• Pd ($384/oz); Au ($911/oz); Fe ($5.50/oz)• Bifunctional catalyst to obtain both WGS and H2 separation

Funding for this work supported by:Army Research Office Young Investigator Program (2007)

NSF Career Award (2005); EPA (2006, 2007); ACS PRF (2006)Shell International Exploration and Production Company Inc. and Shell

Hydrogen (2005)

With thanks to Ph.D. candidates, Erdem Sasmaz, Bihter Padak, andTerumi Hisano, senior research associate, Dr. Shela Aboud, andcollaborator, Dr. Andrew Fry, of Reaction Engineering International.

Acknowledgements

minimizing environmental impacts of coal-based energy … · 2015. 12. 16. · • china is...

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