Hydrogen From Fossil FuelsHydrogen From Fossil Fuels

8/13/2019 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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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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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


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


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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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



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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


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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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)



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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)

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