Chapter 1: Hydrogen Generation Flashcards
Examples of oil spills
- Exxon Valdez (Alaska, 1989)
- BP (Gulf of Mexico, 2010)
- Mauritius oil spill (2020)
Combustion engine by-products:
- CO (carbon monoxide)
- NOx (Urban Smog)
- Unburned hydrocarbons (urban ozone)
1 Gallon of gas is equivalent to…
20 lbs CO2
‘Hydrogen Economy’
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.
Advantages of the ‘Hydrogen Economy’
- Elimination of pollution caused by fossil fuel
- Elimination of greenhouse gases
- Elimination of economic dependence
- Distributed and localized production
Where does the hydrogen come from?
- Reforming of hydrocarbons
- Reforming of biomass
- Pyrolysis
- Electrolysis of water
Hydrogen must be derived from renewable sources
Current uses of Hydrogen
- NH3, HCl, CO(NH2)2, and other commodities
- Fertilizers
- ‘Bosch-Meiser’
2NH3 + CO2 <=> H2N-COONH4
H2N-COONH4 <=> (NH2)2CO + H2O
Ways to create electricity
- Nuclear power
- Hydroelectric dams
- Wind turbines
- Wave and tidal power
- Geothermal power
- Solar cells
Safety aspect of hydrogen as a fuel
- 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
Safety factors to be considered for hydrogen as a fuel
- Catastrophic rupture of the tank
- Mixture of fuel cells reactant in the cells
- Leaks due to punctures, faulty controls, stress cracks, etc.
How to prevent accidents involving hydrogen:
- 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
Methods for producing hydrogen:
Steam Reforming (most common)
(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”
Disadvantage of steam reforming to produce hydrogen
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)
Methane Cracking
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
Partial Oxidation Reforming
(6) reactions
- 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)
Example of combustion: Conservation of mass
The number of moles of H, C, and O must be equal on both sides of the equation.
Stochiometry
Autothermal Reforming
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)
Autothermal Reforming: Reaction
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)
Coal Gasification : Reaction
C + aO2 + bH2O <===> cCO2 + dCO + eH2 + “other species”
Coal Gasification: Devolatilisation
From carbon to complex gaseous mixture and porous solid char residue
Coal Gasification: gaseous mixture
Combination of partial oxidation, SR, and WGS reactions
Coal Gasification: Char Particles
Gasified to CO through partial oxidation of C
Absorption force and Adsorption force:
Weakest: Hydrogen/Helium, Oxygen
Strongest: Water
CO cleanup
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
Selective Methanation
- 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)
Selective Oxidation of CO
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
Shale Gas
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
How ‘Fracking’ works
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.
Pros of Fracking
- 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 (???)
Cons of Fracking
- 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????)
Methane Clathrate
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?
Pressure swing adsorption
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
Palladium membrane separation
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)
What is Biomass
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
What is Bio energy
Energy produced from biomass including woody biomass, agricultural biomass, and other biological materials
Biomass : Plants
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”
Fossil Fuels
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
Biomass vs Fossil Fuels
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
Biomass: from which materials? ( the 5 materials)
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.
- Virgin wood: from forestry, arboricultural and from wood processing
activities - 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
Why to use biomass? (benefits)
- 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
Pre processing processes:
(Physical Pre processing)
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
Virgin wood:
-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
Energy Crops
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)
Agricultural residues:
Dry:
Straw
Corn Stover
Poultry Litter
Wet:
Grass silage
Animal slurry and
farmyard manure
Food waste :
Dry & Wet
About a third of all food grown for human consumption
in the UK is thrown away
Industrial waste & co-products
“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.
Gasification
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)
Pyrolysis
Pyrolysis is the thermal degradation of organic components in biomass in the absence of O 2 . Major products are oil, gas, and char.
ways of producing H2 from biomass
- Thermochemical
- Biological
Carnol Process
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
Hynol Process
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
Technical issues:
*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
Aerobic digestion
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
Anaerobic digestion
Anaerobic Digestion is the decomposition of biomass by
bacteria in the absence of O2. Biogas, or CH4 is the primary
product produced.
Fermentation
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.
Forms of crude oil
- Diesel
- Jet fuel
- Kerosene
- Gasoline
Diesel + Pros and Cons
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)
