Computational Chemistry for Cleaner Coal-to-Energy Conversion

Transcription

1 Computational Chemistry for Cleaner Coal-to-Energy Conversion Jen Wilcox Department of Energy Resources Engineering Stanford University November 29, 2010

2 ! Motivation - Global Coal Use and Energy Demand cleaner coal, what does it mean?! Electronic Structure Theory example: trace metal capture! Grand Canonical Monte Carlo example: adsorption isotherm predictions! Nonequilibrium Molecular Dynamics example: permeability predictions

3 Global Coal Use 13 GtCO 2 /yr from stationary sources: 10.5 (power); ~3 (cement, refineries, iron/steel, etc) 500-MW plant emits ~11,000 tons CO 2 /day Trace metals: 5000 tons Hg/year worldwide Se and As quantities not well-known 52% of the electricity generated in the U.S. is from coal. China & India projected to account for 68% of the demand in world coal through ** Dependence on Coal: US ~50% India ~ 70% China and Australia ~80% South Africa 90% EIA 2005 annual statistics ( ** IEA, World Energy Outlook 2004, p. 34

4 Aiming for Cleaner Coal Old definition of clean coal includes the removal of: SO 2, SO 3, NO, NO 2 (precursors to acid rain); ~30% of power plants in the U.S. are equipped w/ SO x and NO x scrubbing technology Particulate matter (< 2.5 µm); all power plants are equipped with particulate scrubbing (ESP or fabric filters) Trace metals (Hg is big concern, As and Se are also important); 19 states currently have regulations; EPA regulations for trace metal regulations are imminent New definition of clean coal includes the removal of CO 2 : Magnitude and scale is difficult to grasp Strategies include other coal-to-electricity conversions: Current: Subcritical Coal Combustion (super- and ultra-supercritical!efficiency) Oxy-fuel Combustion Integrated Gasification Combined Cycle Chemical Looping Combustion

5 ! Motivation - Global Coal Use and Energy Demand cleaner coal, what does it mean?! Electronic Structure Theory example: trace metal capture! Grand Canonical Monte Carlo example: adsorption isotherm predictions! Nonequilibrium Molecular Dynamics example: permeability predictions

6 DFT: Hohenberg-Kohn Theorem 1. The ground-state energy from the SWE is a unique functional of the electron density, ˆ H " = E" 2. Define trial electron density; solve K-S eqns. to find single-electron w.f.; calculate electron density from K-S; compare and repeat as necessary Energy Functional (Kohn-Sham): Total energy of the system Non-interacting electron kinetic energy Attraction between nuclei and electrons Repulsion between electrons Exchangecorrelation functional

7 Algorithm - Self-Consistency Initialize Setup Outer Loop Update atomic positions Inner Loop Calculate energies NO Solve KSE Update Density Update Energy Converged? YES Calculate force/move ions

8 DFT Time Scale depending on System Sizes!"#$%&' ()' *+,#' -)' *+,'.&%' /&01)' 2% ' #$%7' (89:/((();-5-'!"#$%#&'()*+,-./#!012#!!# Relax: 2 layers!"#$!"#&'()*+,-./# 314#!!# (KPOINTS = 3x3x1) 56#$!"#&'()*+,-./# "13#!!# "0#$!"#&'()*+,-./# 015# 4# -89:/((();-5-<=>'!"#$!"#&'()*+,-./#!6717# "# Relax: Hg atom 56#$!"#&'()*+,-./# 5014# "# (KPOINTS = 9x9x1) "0#$!"#&'()*+,-./# 6217# "#?89:/((();-5-<*3'!"#$!"#&'()*+,-./# 5010# 5# Relax: Cl atom 56#$!"#&'()*+,-./# 6!1%# 5# (KPOINTS = 9x9x1) "0#$!"#&'()*+,-./# 6!10# 5# Relax: 2 layers 56#$!"#&'()*+,-./# 5731"# 5# (KPOINTS = 5x5x1) "0#$!"#&'()*+,-./#!"21%# 5# System 1 System 2 System 3 System 4 1) ENCUT = 300 ev 2) Total number of CPUs

9 Application: Trace Metal Capture Hg capture on activated carbon Hg oxidation across supported Au and Pd catalysts Hg oxidation across supported vanadia catalysts Hg adsorption and oxidation across fly ash Homogeneous Hg oxidation reactions Other volatile trace metals of interest: Se and As Dong-Hee Bihter Erdem Abby Ana Ondra Post-doc PhD PhD PhD PhD MS

10 Mercury Removal from Coal-Fired Power Mercury in flue gas Elemental (Hg 0 ), oxidized (Hg +2 ), particulate (Hg p ) Removing mercury from flue gas Hg p : air pollution control devices - ESP, FF Hg +2 : FGD wet scrubbers Elemental: Activated Carbon Injection (ACI) Co-benefits

11 Activated Carbon Model multiple studies have used a single layer of graphene with unsaturated zigzag edge atoms to simulate carbonaceous surfaces with active sites 1 halogens and Hg are known to interact with graphene zigzag edge sites 2 the edge sites of the graphene sheet are created by slicing Graphene Ribbon perpendicular to the 2D plane to create zigzag edge sites Slab Graphene computational cell 1. Padak, B.; Wilcox, J. Carbon 2009, 47, 12, ; Radovic, L.R.; Bockrath, B. J. Am. Chem. Soc. 2005, 127, ; Yang, F.H.; Yang, R.T. Carbon 2002, 40, ; 2. Olsen, E.S.; Laumb, J.D.; Benson, S.A.; Dunham, G.E.; Sharma, R.K.; Mibeck, B.A.; Miller, S.J. In Proceedings of mega symposium and air and waste management association s specialty conference. Washington, D.C., 2003

12 Computational Domain A graphene sheet with zigzag edge sites is used to model the reactive carbon surface two edges are required to resolve the surface dipole layer a 25 Å vacuum is used to isolate edge sites from their periodic images unit cell dimensions: x Å y Å z Å y x Vacuum region Å 12.50Å Å Computational domain of 5x9 graphene ribbon structure H-O-H-Cl-H

13 Hg DOS: Br-structures Hg s-, p-, and d-dos before and after adsorption on select brominated surfaces p-dos s-dos d-dos Hg-O-H-Br-H H-O-Hg-Br-H H-O-Br-Hg-H blue red gray

14 Experiments in Progress Burner: Methane is burned with excess oxygen Laminar premixed flame is generated A3:%'B4#' *6&76#0.61' &8 6# "9# : 6 8#!!1%9# 8 6 # 5179# ; 6# %!1"9# :<# 031!==># :&?#!416#==@# Reactor: Quartz packed-bed reactor (PBR) and Teflon tubing are used to minimize Hg surface reactions Bed T : 140 C Connecting lines are heated to 130 C Flow rate : 1 l/min Brominated (ACF-Br), virgin (ACF-V) Sorbent weight: 20 mg

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16 Custom-Built Instrument

17 Hg Adsorption on ACF-Br From XPS: Br might be desorbing from the surface Injection of Br to the flue gas on virgin AC is more efficient than AC-Br From EXAFS: - Experiments were carried out in 7-3 beam line in Stanford Linear Acceleration Center (SLAC). Mercury L III edge is investigated - Hg-Br bond distance is 2.55 Å and the coordination number is 2. The formation of Hg 2+ compounds rather than Hg + compounds on the carbon Plane-wave Density Functional Theory Initial Structure Final Structure Br Hg Br H H H Br Hg Br H H H d(hg-br)= 2.45, 2.91 Å d(hg-c)= 2.11 Å C C

18 ! Motivation - Global Coal Use and Energy Demand cleaner coal, what does it mean?! Electronic Structure Theory example: trace metal capture! Grand Canonical Monte Carlo example: adsorption isotherm predictions! Nonequilibrium Molecular Dynamics example: permeability predictions

19 Methodology Grand Canonical Monte Carlo Statistical mechanics: the bridge to connect micro- and macroscopic properties Grand canonical Monte Carlo (GCMC) Fixed: " Chemical potential " Pore volume " Temperature Adsorbates (e.g., CO 2, N 2, methane): " Displace " Remove " Insert " Rotate " Swap (different types of particles) " Initial Configuration Energy Calculation Decision? Displace Remove Insert New Energy Calculation No Accept? Yes New Configuration

20 Lennard-Jones Potentials In many simulations the interaction potential between a pair of particles is represented by the classical Lennard-Jones (LJ) 12-6 potential: Where " is the energy parameter of the potential (the maximum energy of attraction between a pair of molecules), or the LJ well depth, and # is the size parameter (or the distance at which the LJ potential passes through zero and the potential sharply rises to repulsive values), also called the collision diameter The 12-6 Lennard-Jones potential for particles i and j. The potential energy is in units of " and the distance between i and j is in units of #; when U ij is positive, the interactions for the pair of particles are repulsive; when U ij is negative, their interactions are attractive

21 CO 2 potential model One-center Lennard-Jones Model C C C C " CO 2 = 3.615Å CO 2 # CO 2 k C C C C = 242.0K HMT model: three LJ dispersive sites and discrete charges are located on the C and O atoms O C O Å Å Å TraPPE model: TraPPE model is effective in the description of vapor-liquid equilibrium (VLE) Å Å Å MOM model: involves a set of five discrete charges so that the quadrupole moment and higher order moments are accurately included Å Å O O C C O O D.D. Do, H.D. Do / Colloids and Surfaces A: Physicochem. Eng. Aspects 277 (2006)

22 Application: Carbon Capture and Storage CO 2 adsorption isotherms on carbon systems Methane adsorption isotherms on carbon systems CO 2 transport mechanisms in carbon systems CO 2 -selective membrane separation H 2 -selective membrane separation N 2 -selective membrane separation Mahnaz Yangyang Ni Ekin Keith Post-doc PhD PhD PhD MS

23 Carbon Model Geometry Porosity in coal is comprised of: 70-80% micropores (< 20 Å) 1 -with ultramicropores (< 7Å) 2 5% mesopores ( Å) % macro-pores (500 Å 0.1 $m) 1 1 5% cleats (~ 0.1 $m 2 mm) 1 The table below shows the specified types of pores, d [nm], based on IUPAC classification 3 Macro- Meso- Micro- Supermicro- Ultramicro- Submicro- > < <0.7 <0.4 1 F.Y. Wang, et. al., Chemical Engineering Science 62(2007), pp P.L. Walker, Philosophical Transactions of the Royal Society of London A 300(1981), pp B.D. Zdravkov, et. al., Central European Journal of Chemistry 5(2), (2007), pp

24 Adsorption Predictions Model Validation 1C_LJ model TraPPE model Current work Do et al. (2006) 1C_LJ vs. TraPPE (with & without charge) Low pressure adsorption (10Å T=273K) D.D. Do, H.D. Do / Colloids and Surfaces A: Physicochem. Eng. Aspects 277 (2006)

25 Local Density Predictions 1C_LJ CO 2 Model: Local CO 2 density in the different sizes of clean slit pores (@ 273K,100 bar): from single-layer to multi-layer adsorption 10 Å 20 Å

26 Inclusion of Reactivity Bridging electronic structure with GCMC Perfect graphite: the basic slit-pore surface Structural heterogeneity: the existence of vacancy sites mono vacancy, V , V5-8-5, etc. Chemical heterogeneity: the mono vacancy site in the environment of volatile components environment (e.g. water) 20! 20!

27 Inclusion of Defect Sites on Slit Pore Surface Hydrated defective graphite surface (mono-vacancy) side view Carbon Oxygen top view Hydrogen Partial charge distribution: dangling bonds represent basic site Mono-vacancy Unit: e-

28 Inclusion of Hydrated Carbon Surfaces TraPPE CO 2 Model Clean surface Hydrated surface (H) Hydrated surface (O) Cyan: Carbon Red: Oxygen White: Hydrogen Slit pore size: 3.8 Å Slit pore size: 11.1 Å

29 CO 2 Density and Orientation in the Pore TraPPE CO 2 Model: Local CO 2 density and CO 2 orientation Local Density Slit pore size 3.8 Å CO 2 Orientation Slit pore size 11.1 Å

30 Adsorption Influenced by Hydrated Surface TraPPE CO 2 Model: ultramicro pore (effective pore size = 3.8 Å) Cyan: Carbon Red: Oxygen White: Hydrogen

31 Computational Intensity CO 2 : One-atom Model C C C C CO 2 C C C C

32 Computational Intensity CO 2 : Three-atom Model vs. One-atom Model Pore wall: perfect vs. defective surface; various unit cell Three-atom CO 2 model within larger unit cell makes it more computational intensive O C O

33 ! Motivation - Global Coal Use and Energy Demand cleaner coal, what does it mean?! Electronic Structure Theory example: trace metal capture! Grand Canonical Monte Carlo example: adsorption isotherm predictions! Nonequilibrium Molecular Dynamics example: permeability predictions

34 What is Molecular Dynamics? Over the past decades, Molecular Dynamics (MD) simulations have become an important tool for investigating and predicting various static as well as dynamical properties of materials. We call molecular dynamics a computer simulation technique where the time evolution of a set of interacting atoms is followed by integrating their equations of motion. For a system with N molecules, this involves solving a set of 3N second order differential equations (Newton s equations of motion): The force on the i th particle is related to the potential energy and the fluid interactions are pairwise additive since the potential calculations are computationally costly.

35 The dimensions of the system modeled are: Length = Å [10.2 nm] Width = Å [10.3 nm] Height = Å [10.4 nm] The spacing between the adjacent graphite layers in the z- direction is 3.35 Å [0.335 nm] The number density of carbon atoms is 114 nm -3 The carbon-carbon bond length is 1.42 Å [0.142 nm] Creation of a Pore Network (1) 35

36 Creation of a Pore Network (2) 3D molecular pore network model based on the Voronoi tessellation method To generate the molecular pore network model: " First, we create a 3D simulation box of structural atoms corresponding to porous structure " Then, we tessellate the atomic structural box The pore space is created by specifying the desired porosity and then selecting a number of polyhedra in such a way that their total volume fraction equals the specified porosity; the remaining structural atoms constitute the porous solid matrix, while the pore space consists of interconnected pores of various shapes and sizes

37 Creation of a Pore Network (2) In this study the sensitivity of the porosity and pore size on the permeability are investigated, which allows us to fundamentally understand the effect of the morphology of the pore space, i.e., its pore size distribution and connectivity on the transport properties of the fluids in the organic matrix of coal and gas shale The generated pore network model can be optimized with experiments to have a similar pore size distribution and average pore size The pore network model with average pore diameter of 12 Å [1.2 nm] and 25% porosity

38 PSD vs. Pore Size Pore Size Distribution Average Pore Diameter Å The pore size distribution of graphite with 15% porosity with 12, 16, 20, and 24Å average pore diameter; pores are selected randomly

39 Pore Network Model Generation Run Time The dimensions of the system: " Length = Å [10.2 nm]; Width = Å [10.3 nm]; Height = Å [10.4 nm] " total number of graphite atoms = 124,992 Run time is an exponential function of pore size (few seconds to several days) Examples of run times on a desktop computer 1 : Total number of polyhedra with 25% porosity = 1000 Average pore diameter = 12.5Å Number of carbon atoms in porous structure = 91,657 Run time to generate the porous structure : 40 min Total number of polyhedra with 25% porosity = 3000 Average pore diameter = 8.7 Å Number of carbon atoms in porous structure = 91,003 Running time to generate the porous structure : 15 hr 1 Processor: Intel Core 2 Quad 2.3 GHz, RAM: 4.0GB, Op. System: Windows 7 64-bit

40 Modeling Molecular Transport The pore network model previously described will be used Non-equilibrium molecular dynamics (NEMD) simulations are carried out The system (pore network) is exposed to an external driving force (chemical potential or pressure gradient) in a specified direction Flux and permeability are two quantities of interest that NEMD will allow us to obtain

41 Influence of Microfractures on Transport Matrix microfracture micro/meso/macropore network The dimensions of the system modeled are: Length = Å [10.2 nm] Width = Å [10.3 nm] Height = Å [10.4 nm] Microfracture The microfracture has been created in the center of the pore network structure with 6.7 Å [0.67 nm], Å [1.005 nm], and 13.4 Å [1.34 nm] aperture sizes Image Source (right): F.Y. Wang, et al., Chemical Engineering Science, 2007, 62,

42 Permeability vs. Microfracture Size 5% Porosity Permeability of CH 4, CO 2 and N 2 will increase with increasing microfracture size When the size of the microfracture is small (6.7 Å) the permeabilities of all components fairly similar since only one molecular layer can fit through the fracture With increasing microfracture size, the relative permeability of N 2 becomes larger because of molecular Permeability of CH sieving effects 4, CO 2 and N 2 with average pore diameter of 12 Å [1.2 nm] and 5% porosity versus size of microfracture in the center of the pore network LJ sizes of CH 4, CO 2 and N 2 are 3.81, 3.794, and Å respectively

43 CO 2 Snapshots vs. Microfracture Size Average pore diameter of the pore network is 12 Å [1.2 nm] with 5% porosity; sizes of the microfracture in the center of the pore network are 0.67, 1.005, and 1.34 nm

44 Density profiles along the Pore vs. Microfracture Size Adsorption increases from N 2 < CH 4 < CO 2 Multilayer formation takes place with increasing microfracture size Average pore diameter of the pore network is 12 Å [1.2 nm] with 5% porosity; sizes of the microfracture in the center of the pore network are 0.67, 1.005, and 1.34 nm

45 Acknowledgements CEES and Teragrid for supercomputing support Funding for this work supported by: DOE-NETL (2010); NSF (2010) Chevron (2008); EPRI ( ); UPS (2009) Army Research Office Young Investigator Program (2007) NSF Career Award (2005); EPA ( , 2010); ACS PRF (2006) Shell International Exploration and Production Company Inc. and Shell Hydrogen (2005) QUESTIONS?