Hydrogen From Fossil FuelsHydrogen From Fossil Fuels

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    Hydrogen from Fossil Fuels

    Lecture 2

    8/28/2006

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    Hydrogen from Hydrocarbons

    Steam ReformingPartial Oxidation/

    Autothermal Reforming

    Shift Reaction

    CO2 Scrub

    H2

    CO, H2, CO2, H2O

    Highly Volatile Fuels:

    Natural Gas, NaphthaLow Volatile Fuels:

    Heavy Oil, Coal

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    Chemistry of Steam-Reforming and Water Gas

    Shift Reactions

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    Reaction Chemistry

    Water Gas Shift (WGS) Reaction:Exothermic, Lower Temperature

    (

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    Thermodynamics

    H2

    H0 kJ/mol

    2.2572.4650Butane

    1-41.1-41.1-WGS

    2.3371.1498Propane

    368.6206.3MethaneReforming

    H/C

    HC

    HydrocarbonReaction

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    Reaction Equilibrium

    P = 4 MPa

    Steam to Methane

    Ratio: 2

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    Steam Reforming and Water Gas

    Shift Reaction Equilibria

    Steam ReformingReaction

    Water Gas

    Shift Reaction

    400oC

    1000oC

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    Side Reactions Methane Cracking

    CH4 C + 2H2 ; H0 = 74.9 kJ/mol

    Boudouard Reaction

    2CO C + CO2 ; H0

    = -172.4 kJ/mol

    Formation of carbon deposit catalystdeactivation

    High temperatures (>820oC) suppress thesereactions

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    Energy for Steam Reforming? Methane as a fuel

    CH4 + 2O2 CO2 + 2H2O

    H0 = -802.4 kJ/mol

    Methane reforming Sum of the burning value of for methane and

    required heat input exactly equals the burning valueof 4 hydrogen molecules obtained

    Other hydrocarbons?

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    Technology and Production

    Process

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    Process Flow Diagram

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    Production Process

    Purification of Feed StreamHydrodesulfurization: conversion of organic

    sulfur to hydrogen sulfide (Co-Mo, Ni-Mo

    catalysts)

    Chloride removal: Absorption of HCl by

    sodium oxide

    Sulfur removal: Absorption by ZnO

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    Production Process

    Steam Reforming

    High Temperature (>850oC)

    Nickel-based catalysts

    Fixed bed catalyst tubes in combustion

    furnace

    Steam/methane ratio: 3 - 4

    Pressure: 1 - 4 MPa Product gas composition close to equilibrium

    Conversion >80%

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    Steam Reformer

    Cylindrical tubes packed with catalyst

    placed in rectangular prism furnace

    Top fired burners in furnace

    Downward parallel flow of combustiongases and feed gas

    Very high temperatures achieved in the

    combustion furnace

    Heat transfer by radiation

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    Production Process

    Water Gas Shift Reaction

    High Temperature Shift Reaction

    Iron oxide-chromia catalyst

    300-450oC

    CO conversion: 90-95%

    Low-Temperature Shift Reaction

    CuO-ZnO catalyst

    200-300oC

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    Production Process

    CO2/H2 Separation

    Absorption of CO2 Chemical: Monoethanolamine

    Physical: Methanol (rectisol), sulfolane(sulfinol), dimethylether of propylene glycol

    (selexol), etc.

    Adsorptive purification Zeolites (PSA)

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    Production Process

    CO2/H2 Separation

    Membrane Separations

    CMS (carbon molecular sieves): hydrogen

    permeation Nanoporous carbon membranes: carbon

    dioxide permeation

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    Production Process

    Methanator:

    Reverse of steam reforming reaction

    Convert unreacted CO to methane

    CO a poison for the ammonia synthesiscatalyst (Fe/Alumina-zirconia-K2O),

    methane not a poison

    Preferential Oxidation of CO

    CO + O2 CO2

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    Production Process Preformer

    Low temperature (500oC) catalytic reactorupstream of the main reformer

    Conversion of heavier feedstock (e.g.,

    naphtha) to methane

    Catalyst: Dispersed NiO on alumina

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    Steam-Reforming Process

    0.66 MJ of hydrogen produced per MJ of fossil fuel

    consumed

    Resource consumption: 3.8 tons/ton H2

    Major emission: CO2 (~0.5 m3/m3 of H2)

    Minor emissions: Methane, NOX, SOX, Non-methane

    Hydrocarbons, CO, etc.

    Global Warming Potential: 12 ton CO2-equivalent/tonH2

    Spath P.L., Mann M.K. 2001, DOE Report, NREL/TP-570-27637

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    Developments

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    Developments Membrane Reactors

    Hydrogen removal from the reaction zone toshift the equilibrium of the steam reforming

    reaction

    Pd-Ag membrane

    Operation at less severe conditions

    Smaller reactors

    Very low CO contamination in product gas

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    Developments Sorption Enhanced Reforming (SER)

    Packed bed reactor containing catalyst andan adsorbent for carbon dioxide

    Steam/methane ratio ~ 6

    Temperature: 400-500oC

    Pressure: 0.1-0.5 MPa

    Product gas: 90% hydrogen Major impurity: unconverted methane

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    Developments - SER Minimization of side reactions (coking)

    Cyclic operation

    Reaction followed by desorption of carbon

    dioxide from exhausted adsorbent

    Adsorbent: K2CO3/Hydrotalcite (synthetic

    aluminum magnesium hydroxycarbonate)

    Catalyst? Hydrogen is required to remove carbon dioxide asmethane for nickel catalysts

    Alternate catalysts?

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    Developments Partial oxiation

    Mildly exothermic reactionCH4 + O2 CO + 2H2; H0 = -36 kJ/mol

    H2/CO ratio = 2, excellent for Fischer-Tropsch

    or Methanol synthesis

    Pure oxygen required

    Monolith catalytic (Pt or Rh) reactor

    Very short residence time (mS)

    Temperatures ~600oC

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    Developments Partial

    Oxidation Reaction mechanism?

    Combustion-Reforming Oxidation of methane to carbon dioxide

    Carbon dioxide reforming of methane

    Pyrolysis-Oxidation Dissociation of methane

    Oxidation of dissociated species

    Pathway dependent upon the catalyst

    Pyrolysis-oxidation dominant at high

    temperatures (>923K or 650oC)

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    Developments Partial

    Oxidation Efficiency: ~50%, lower than the

    conventional steam reforming Greater selectivity for synthesis gas

    Compactness Non-catalytic partial oxidation

    Higher Temperatures: 1100-1500oC

    Useful for higher hydrocarbons as well

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    Developments Partial

    Oxidation Higher Hydrocarbons

    C6H14 + 3O2 6CO + 7H2 O2/C ratio: 0.5-0.6, less than stoichiometric

    Thermal Partial Oxidation Units

    High Temperature: 1200-1600oC

    High Pressure: ~7MPa

    Use of low quality petroleum residue/coal/coke

    Noncatalytic

    H2/CO ratio < 2

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    Developments Dry Reforming of Methane

    Solution for fixing carbon dioxide (?)CH4 + CO2 2CO + 2H2; H

    0 = 247 kJ/mol

    H2/CO ratio = 1

    T:950oC+ (noncatalytic)

    Catalytic (NiO-MgO, noble metals): 600-

    700oC

    Catalyst deactivation due to

    Methane cracking

    CO disproportionation

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    Developments Tri-reforming of natural gas

    Utilization of waste flue gas from fossil-fuelpower plants to obtain synthesis gas

    ReactionsCH4 + CO2 2CO + 2H2CH4 + H2O CO + 3H2

    CH4 + O2 CO + 2H2CH4 + 2O2 2CO2 + 2H2O

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    Developments Tri-reforming

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    Developments Tri-reforming Advantages

    Eliminate carbon dioxide separation

    Hydrogen/CO ratio ~2

    Minimize coke formation

    Use of waste flue gas

    Tri-generation Disadvantages

    No existing industrial process

    No existing suitable catalyst

    Heat and Mass Management Inert gas handling

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    Thermal Cracking

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    Thermal Cracking of Methane Decomposition of hydrocarbons at high

    temperature CH4 C + 2H2 ; H

    0 = 74.9 kJ/mol

    Temperature: 700-980oC

    Absence of air/oxygen Production of carbon black for rubber tire

    vulcanization and printing industry

    Firebrick furnace: methane combustion to obtainhigh temperatures, followed by decompositionon bricks

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    Thermal Cracking of Methanol Catalytic decomposition:

    CH3OH

    CO + 2H2 ;

    H0

    = 91 kJ/mol Impurities: methanol, methane, ether

    Steam reforming:CH

    3

    OH + H2

    O CO2

    + 3H2

    H0 = 50 kJ/mol Excess of steam prevents the first reaction

    Temperature: 250-300oC

    Pressure: 1-2.5 MPa

    Catalyst: copper-zinc

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    Coal Conversion

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    Steam-Coal Gasification Process

    Two chemical reactions Pyrolysis: Rapid reaction expelling volatile constituents

    CHxOy (1-y)C + yCO + 0.5xH2CHxOy (1-y-x/8)C + yCO + 0.25xH2 + x/8CH4

    Gasification of residual carbon: Slow reactionC + H2O CO + H2

    T: 600-1000oC, P: 70 bar

    Endothermic reaction: 163 kJ/mol

    C + 2H2O CO2 + 2H2

    Decreases with an increase in temperature

    CO:H2 ratio ~ 1

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    Equilibrium Compositions in Coal

    Gasification

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    Gasification Reactors

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    Reactor Comparison

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    Coal Cracking HYDROCARB Thermal Cracking of Coal

    CH0.8O0.016N0.016 + CaCO3 C + 0.32H2 +0.08H2O + 0.008N2 + 0.016CaS

    C: Carbon Black as clean fuel Hydrogen: Byproduct Fuel

    Carbon Dioxide-free fossil fuel basedhydrogen

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    Steam-Iron Process Reduction of Iron Oxides

    Fe3O4 + H2 3FeO + H2O Fe3O4 + CO 3FeO + CO2

    FeO + H2 Fe + H2O

    FeO + CO Fe + CO2

    Oxidation of Iron

    Fe + H2O FeO + H2 3FeO + H2O Fe3O4 + H2

    Process cyclic with respect to iron-iron

    oxides

    Coal Gasification Research and

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    Coal Gasification Research and

    Developments

    Advanced reactor configurations:

    circulating fluidized bed reactor

    Use of pure oxygen/oxygen enriched air:membrane separations for oxygen

    Membrane separations for product gas In-Situ coal gasification

    Coal Gasification Research and

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    Coal Gasification Research and

    Developments

    CO2 separation and capture during

    reaction (e.g., capture using CaO)

    Resource recovery from byproducts (sulfurto sulfuric acid, slag for road construction)

    Mixed feedstock (biomass, municipal

    waste)

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    Conversion of Oil

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    Partial Oxidation of Oil Hydrogen from heavy oil (remainder of

    crude oil distillation) Noncatalytic process

    Temperature : 1150-1315oC

    Pressure: 1-10 MPa

    Reactions

    CnHm + nH2O nCO + (n+m/2)H2 CnHm + n/2 O2 nCO + m/2 H2 Water gas shift reaction

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    Partial Oxidation of Oil Heavy Oil:

    Residual Oil: n = 1 m = 1.3 CO/hydrogen ratio ~1

    Lighter Hydrocarbons

    Catalytic Process

    Lower Temperature

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    Hydrocracking

    Conversion of heavy oil into lighter oil andgasoline fractions

    Require input of hydrogen and heat

    High Pressure: 10 MPa

    Temperature: 320-400oC

    Catalytic process Net hydrogen yield: 3.7 Nm3/kg heavy oil

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    Fossil fuel-based hydrogen shouldnot have any greenhouse gas

    emissions associated with it forenjoying the full environmental

    benefits of the hydrogen economy.

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    Appendix

    Reaction Mechanisms

    Mechanism of Steam Reforming

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    Mechanism of Steam Reforming

    Reaction

    Dissociative Adsorption of Steam andMethaneH2O(g) H2O (s)H2O (s) O(s) + H2(g)

    CH4(g)

    CH2(s) + 2H(s)

    Reaction Between Adsorbed Species

    CH2(s) + O(s) CHO(s) + H(s)

    CHO(s) CO(s) + H(s)CO(s) + O(s) CO2(s)CHO(s) + O(s) CO2(s) + H(s)

    Mechanism of Steam Reforming

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    Mechanism of Steam Reforming

    Reaction Desorption of Products

    CO2(s) CO2(g)

    2H(s) H2(g)

    CO(s) CO(g)

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    Mechanism of WGS Reaction

    Adsorptive (Formate) Mechanism

    H2O(g) H2O (s)

    CO(g) CO(s)H2O(s) + CO(s) Intermediate(s)

    Intermediate(s) CO2(s) + H2(s)

    CO2(s) CO2(g)H2(s) H2(g)

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    Mechanism of WGS Reaction

    Regenerative Mechanism

    H2O(g) H2O (s)

    H2

    O (s) OH(s) + H(s)

    OH(s) O(s) + H(s)

    2H(s) H2(g)

    CO(g)

    CO(s)CO(s) + O(s) CO2(s)

    CO2(s) CO2(g)