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EGA323 > Chapter 1: Hydrogen Generation > Flashcards

Chapter 1: Hydrogen Generation Flashcards

1
Q

Examples of oil spills

A
  • Exxon Valdez (Alaska, 1989)
  • BP (Gulf of Mexico, 2010)
  • Mauritius oil spill (2020)
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2
Q

Combustion engine by-products:

A
  • CO (carbon monoxide)
  • NOx (Urban Smog)
  • Unburned hydrocarbons (urban ozone)
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3
Q

1 Gallon of gas is equivalent to…

A

20 lbs CO2

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4
Q

‘Hydrogen Economy’

A

Potentially the future of energy in a system where hydrogen is used as the primary energy carrier and used for a wide range of applications.

It has the potential to significantly reduce greenhouse gas emissions.

There are many technical, infrastructural, and economic challenges to overcome.

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5
Q

Advantages of the ‘Hydrogen Economy’

A
  • Elimination of pollution caused by fossil fuel
  • Elimination of greenhouse gases
  • Elimination of economic dependence
  • Distributed and localized production
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6
Q

Where does the hydrogen come from?

A
  • Reforming of hydrocarbons
  • Reforming of biomass
  • Pyrolysis
  • Electrolysis of water

Hydrogen must be derived from renewable sources

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7
Q

Current uses of Hydrogen

A
  • NH3, HCl, CO(NH2)2, and other commodities
  • Fertilizers
  • ‘Bosch-Meiser’
    2NH3 + CO2 <=> H2N-COONH4
    H2N-COONH4 <=> (NH2)2CO + H2O
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8
Q

Ways to create electricity

A
  • Nuclear power
  • Hydroelectric dams
  • Wind turbines
  • Wave and tidal power
  • Geothermal power
  • Solar cells
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9
Q

Safety aspect of hydrogen as a fuel

A
  • Colourless and odourless
  • Low ignition energy
  • High flame temperature
  • Invisible flame in daylight conditions
  • Negative Joule Thompson Coefficient, (i.e. a leak may self ignite
  • Small molecular size (2.016 g/mol vs.vs.~107.0 g/mol for gasoline
  • Wide range of lower explosive limit to upper explosive limit (4.0 75.0 volume%)

However, hydrogen is less flammable than gasoline and other fossil fuels

Liquid hydrogen: spilling –> Burns, explosions

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10
Q

Safety factors to be considered for hydrogen as a fuel

A
  • Catastrophic rupture of the tank
  • Mixture of fuel cells reactant in the cells
  • Leaks due to punctures, faulty controls, stress cracks, etc.
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11
Q

How to prevent accidents involving hydrogen:

A
  • Testing tanks and equipment (leak prevention)
  • Installing more valves
  • Designing equipment for shocks, vibrations and wide T ranges
  • Inserting H 2 , O 2 , and leak detectors
  • Keeping fuel cells supply lines physically separated from other equipment
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12
Q

Methods for producing hydrogen:
Steam Reforming (most common)

A

(1) CH4 + H2O ==> CO + 3H2 (+ 206.4 kJ/mol)
Catalyst : Ni , highly endothermic, T = 700 - 1100

(2) CO + H2O ==> CO2 + H2 (-41, kJ/mol)
Catalyst: Fe, Cr Oxides, mildly exothermic, water gas-shift reaction

Heat needed in (1) is provided by
(3) CH4 + 2O2 ==> CO2 + 2H2O (-880 Kj/mol)

Combining (1) and (2)
(4) CH4 + 2H2O (+165 kJ/mol) ==> CO2 + H2
Efficiency: 65% - 75%

CO (traces) and CO2 are removed using adsorption processes “pressure swing adsorption”

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13
Q

Disadvantage of steam reforming to produce hydrogen

A

Costs: still 2-3 times higher than producing gasoline from
crude oil Improving

Steam (methane) reforming not to be confused with catalytic reforming of naphta (octane gasoline and H2 as a by product)

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14
Q

Methane Cracking

A

CH4 + 74.9 KJ/mol ==> C + 2H
(Excess of steam is effective in preventing coke formation)

2CO ==> C + CO2 + 172.4 KJ/mol
Carbon Contamination

Current natural gas steam reforming is not aimed at fuel cell use of
H2 produced, so CO is not eliminated

Commonly in SMR: CO: 0.3-3%
CO: in the range of 10-50 ppm

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15
Q

Partial Oxidation Reforming
(6) reactions

A
  • CH4 + 1/2 O2 ==> CO = 2H2 (-35.7 kJ/mol)
    Exothermic , partial oxidation
  • CH4 + O2 ==> CO2 + 2H2 (-319.1KJ/mol)
    Exothermic, partial oxidation
  • CH4 ==> C + 2H2 (+75 KJ/mol)
    Endothermic, Thermal decomposition

CH4 + 2O2 ==> CO2 + 2H2O (liq) (-880 KJ/mol)
Exothermic, Methane Combustion

CO + 1/2 O2 ==> CO2 (-283.4 KJ/mol)
Exothermic, CO combustion

H2 + 1/2 O2 ==> H2O (liq) (-286 KJ/mol)
Exothermic, H2 combustion

Complete combustion: CO2 , H2O; no H2 , CO, O2 or fuel
Incomplete combustion: H2 , CO (in the presence of a catalyst)

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16
Q

Example of combustion: Conservation of mass

A

The number of moles of H, C, and O must be equal on both sides of the equation.

Stochiometry

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17
Q

Autothermal Reforming

A

It is a combination of:
- Steam reforming reaction
- Partial oxidation reaction
- The water gas-shift reaction

(1) in the same chemical reactor
(2) The heat required by the (endothermic) SR and WGS reaction is exactly provided by the (exothermic) POX reaction
(3) Steam (for SR) as a reactant
(4) Substoichiometric amount of O2 for (POX)

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18
Q

Autothermal Reforming: Reaction

A

CxHy + zH2O (l) + (x-0.5z)O2 <===> xCO2 + (z+0.5y)H2

z/x = steam to carbon ratio

It should be chosen such that the reaction is energy neutral
(neither exothermic nor endothermic)

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19
Q

Coal Gasification : Reaction

A

C + aO2 + bH2O <===> cCO2 + dCO + eH2 + “other species”

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20
Q

Coal Gasification: Devolatilisation

A

From carbon to complex gaseous mixture and porous solid char residue

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21
Q

Coal Gasification: gaseous mixture

A

Combination of partial oxidation, SR, and WGS reactions

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22
Q

Coal Gasification: Char Particles

A

Gasified to CO through partial oxidation of C

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23
Q

Absorption force and Adsorption force:

A

Weakest: Hydrogen/Helium, Oxygen

Strongest: Water

24
Q

CO cleanup

A

Reformative stream: CO = 0.2% (2000 ppm)
Fuel cell catalyst tolerance: CO = 1-10 ppm

Goal: Reduce CO yield to extremely low level

Physical separation: Pressure swing adsorption, Palladium membrane separation

Chemical reaction: Selective methanation of CO
Selective oxidation of CO

25
Q

Selective Methanation

A
  • Manage heat of reaction
  • Designing a catalyst that maintains its activity after prolonged exposure to high T

(1) CO + 3H2 <==> CH4 + H2O
Catalyst: Co, Fe, Ru; highly exothermic; Δ H= 206.1KJ/mol
Reduce CO and H2 yield

(2) CO2 + 4H2 <===> CH4 + 2H2O
ΔH= 165.2KJ/mol
CO is not reduced; it consumes even more H2

Need a catalyst that to promote (1) while suppressing (2)

26
Q

Selective Oxidation of CO

A

A catalyst selectively promotes a reaction that removes CO over
another that consumes H2

CO + 0.5 O2 <==> CO2
H2 + 0.5 O2 <==> H2O

The change in Δ G (Gibbs free energy) for the CO reaction is
increasingly more negative at lower T

CO reaction is favourite at lower T
(ie a higher of CO adsorbs onto the catalytic surface)

Series of consecutive selective oxidation catalyst beds that operate at increasingly lower T

27
Q

Shale Gas

A

Shale gas is methane (natural gas) which is trapped in impermeable shale rock deep underground

The gas cannot flow through the shale, so simply drilling a well, as you would for conventional natural gas, is not enough

The shale rock must be cracked to free the gas, so large quantities of water, sand, and a range of chemicals are pumped in under high pressure (fracking = hydraulic fracturing)

Tens or hundreds as many wells are needed to produce as much gas as in a conventional gas field

28
Q

How ‘Fracking’ works

A

stage 1: Drill down to shale level

stage 2: Water sand and chemicals pumped to a pressure of approx 1500 lb/inch^2, forcing the rock apart releasing the gas in the pores

stage 3: Liquid pumped out of well, sand keeps the rock separated allowing gas to seep out of the broken shale layer to be piped to the surface.

29
Q

Pros of Fracking

A
  • Energy security
  • Job creation
  • Lower emission than liquefied natural gas
  • Natural gas is cleaner than coal and oil (clean oil is very expensive)
  • Horizontal drilling
  • Cheaper (???)
30
Q

Cons of Fracking

A
  • Environmental pollution (aquifer, soil, water, air,
    chemicals): Barnett shale: heavy metals release, Se, Sr, As .
  • Use of huge quantity of water (but compared to what?)
  • Earthquakes
  • Impact on habitat, landscape?

(Assessment and Impact studies are still very
limited, REGULATIONS????)

31
Q

Methane Clathrate

A

Methane trapped in the crystal structure of water ( (CH4) 5.75(H2O) )

Present in seabeds and permafrost

1m^3 of methan clathrate = 168 m^3 CH4

Reservoir (sediments) = 2- 10 times conventional natural gas
Estimates = 1 × 10^15 and 5 × 10^15 m³ (500 2500 Gt C)

Widely distributed, but low total volume and low
concentration at the sites, so is it economically viable?

32
Q

Pressure swing adsorption

A

Physical separation process that removes CO2, CO but also other
species except H2

H2 = 99,99%

Beds composed by zeolites, silica, and carbons

Adsorption based on molecular weight at high P

33
Q

Palladium membrane separation

A

Palladium membrane hydrogen purifiers operate via pressure
driven diffusion across palladium membranes

H2 molecules in contact with the Pd membrane surface dissociates into monatomic hydrogen and passes through the membrane

On the side of the Pd membrane, the monatomic hydrogen is recombined into molecular hydrogen

Only hydrogen can diffuse through the palladium membrane

Pd is very expensive (less expensive than Pt)

34
Q

What is Biomass

A

Biomass is a biological derived from living or recently living organisms

Biomass is composed of a mixture of organic molecules
containing hydrogen usually including atoms of oxygen,
nitrogen and also small quantities of other species, such
as alkali, alkaline earth and heavy metals

35
Q

What is Bio energy

A

Energy produced from biomass including woody biomass, agricultural biomass, and other biological materials

36
Q

Biomass : Plants

A

For plants, The carbon is absorbed from the atmosphere as CO2 by plant life, using energy from the sun

Microbial degradation: CO2, CH4

Burning: CO2

“Carbon Cycle”

37
Q

Fossil Fuels

A

Coal,oil, gas It derives too from biological materials (i e
CO2 adsorbed millions of years ago

fossil fuels => burning => CO2 + energy (and heat)

Difference of timescale from biomass

38
Q

Biomass vs Fossil Fuels

A

Biomass: While it is growing C is taken out from atmosphere and returns to it when it is burned (in relatively shorter times) - sustainable

Fossil: Oil, gas, coal: produced from C sequestered millions of years ago. CO2 generated released quickly. - not sustainable

39
Q

Biomass: from which materials? ( the 5 materials)

A

Huge resources of residues, by products and waste are potentially become available, in quantity, at relatively low, or even negative costs where there is currently a requirement to pay for disposal.

  1. Virgin wood: from forestry, arboricultural and from wood processing
    activities
  2. Energy crops high yield crops grown specifically for energy
    applications

3.Agricultural residues residues from agriculture harvesting or
processing

4.Food waste from food and drink manufacture, preparation and
processing, and post consumer waste

5.Industrial waste and by products from manufacturing and industrial
processes

40
Q

Why to use biomass? (benefits)

A
  • Renewable
  • Widely available
  • Significant reduction in net carbon emissions when compared with fossil fuels
  • Sustainable

It is a “carbon lean, so it produces a fraction of carbon
emissions of fossil fuels

Sourced locally

Local business opportunity and support rural economy

If not used they would generally rot. This will generate CO2 in
any case, and may also produce methane (CH4 ), a greenhouse
gas 21 times more potent that CO2

41
Q

Pre processing processes:
(Physical Pre processing)

A

Pre processing may be required to change the physical form or
to reduce the moisture content.

Physical pre processing consists of:
Cutting to uniform length
Chipping
Grinding or shredding
Reducing moisture content

42
Q

Virgin wood:

A

-Suitable for a range of energy applications
- It can be burned for heat and/or power at a range of scales
- Virgin wood is untreated and clean
- It may range in moisture content from oven dry to 60% or
higher as freshly harvested, green wood

43
Q

Energy Crops

A

Energy crops are grown specifically for use as fuel and offer
high output per hectare with low inputs.

Short rotation energy crops

Grasses and non woody energy crops (miscanthus, oilseed rape,……)

Agricultural energy crops (sugar, starch, and oil crops)

Aquatics (hydroponics)

44
Q

Agricultural residues:

A

Dry:
Straw
Corn Stover
Poultry Litter

Wet:
Grass silage
Animal slurry and
farmyard manure

45
Q

Food waste :

A

Dry & Wet

About a third of all food grown for human consumption
in the UK is thrown away

46
Q

Industrial waste & co-products

A

“Woody” Materials and “Non Woody” Materials

Combustion: Basic process of oxidation

Gasification: It is a partial oxidation process whereby a carbon
source such as coal, natural gas or biomass, is broken down into CO, H2 CO2 and possibly hydrocarbon such as CH4

Pyrolysis: It is the precursor to gasification, and takes place as part of both gasification and combustion. It consists of thermal decomposition in the absence of O2 . Ex.: charcoal burning.

47
Q

Gasification

A

Gasification is a thermochemical process in which biomass at
high heat is turned directly from a solid into a gaseous fuel
called syngas (a mixture of CO, H 2 and some CH4)

48
Q

Pyrolysis

A

Pyrolysis is the thermal degradation of organic components in biomass in the absence of O 2 . Major products are oil, gas, and char.

49
Q

ways of producing H2 from biomass

A
  1. Thermochemical
  2. Biological
50
Q

Carnol Process

A

This is a high T two step process involving:

(i) thermal conversion of CH4 ( CH4 ==> C + 2H2)

(ii) Methanol Synthesis ( CO2 + 3H2 ==> CH3OH + H2O )

(iii) Overall (2CO2 + 3CH4 ==> 2CH3OH + 2H2O + 3C)

Near zero CO2 emission results from removal of CO 2 from the coal burning plant and emission of equivalent amount of CO2 when methanol is burnt as a fuel

51
Q

Hynol Process

A

This process produces H2 and CH3OH from biomass with reduced CO2 emissions.

Phase 1:

(1a) C + CH2 ==> CH4
(1b) C + 2H2O ==> CO + H2
(1c) CO2 + H2 ==> CO + H2O

Phase 2:

(2a) CH4 + H2O ==> CO2 + 3H2
(2b) CO2 + H2 ==> CO + H2O

Phase 3:

(3a) CO + 2H2 ==> CH3 + OH
(3b) CO2 + 3H2 ==> CH3Oh + H2O

52
Q

Technical issues:

A

*Since H2 content in Biomass is low, the yield of H2 is
low (ca 6% vs 25% of CH4)

*Energy content of biomass is also low due to 40% O2 content

*Low energy content of biomass is an inherent
limitation of the process since over half of H2 from
biomass comes from splitting of water in steam
reforming

*The production of H2 from biomass is not presently
economically competitive with natural gas SR
without the advantage of high value co products, very
low cost biomass and potential environmental
incentives

53
Q

Aerobic digestion

A

Aerobic digestion is a process in which bacteria use O2 to
convert organic material into CO2. Products include nutrient
rich fertilizers (N, P derivatives) and composts

54
Q

Anaerobic digestion

A

Anaerobic Digestion is the decomposition of biomass by
bacteria in the absence of O2. Biogas, or CH4 is the primary
product produced.

55
Q

Fermentation

A

Fermentation is a biological process in which enzymes
produced by microorganisms cause chemical reactions to
occur. Products include EtOH, commercial levels of
therapeutic and research enzymes, antibiotics, and
specialty chemicals.

56
Q

Forms of crude oil

A
  • Diesel
  • Jet fuel
  • Kerosene
  • Gasoline
57
Q

Diesel + Pros and Cons

A

Pros: Low CO, CO2 , HC emission

Cons: NOx , sulfur, particulate matter (PM)

General:
-Heavier than gasoline
(C 14 H 30 ) vs (C 9 H 20
-Easier to refine compared
to gasoline
-Cheaper
-Higher energy density
(147K BTU vs 132K)

EGA323 (7 decks)

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