Hydrogen Economy and Production Flashcards
What are the main features of the Hydrogen Economy? Describe a feature of each.
Hydrogen Production - SMR is currently the most popular
Storage and Transport - As compressed gas, liquid H2 or chemical storage
Applications - Using H2 as a net 0 carbon fuel in transport, industry, residential
Environmental Impact - currently, H2 production by SMR is very carbon intense
What is the equation for combustion of H2 in air
(Overall reaction of a Fuel Cell)
H2 + 0.5 O2 = H2O
What is the equation for the combustion of CH4 in air
CH4 + 2O2 = CO2 + 2H2O
What are issues with the hydrogen economy
Technology - exists and we understand the challenges but not ready for nation wide application, expensive/experimental and small scale
Financial - H2 production is expensive, infrastructure for commercial storage and transport doesn’t exist
Why is H thought to be a better fuel than conventional fuels
The carnot efficiency of H is much greater than that of conventional fuels such as diesel/petrol engines or combined cycle plants.
Using H does not produce any greenhouse gases (ignoring steam). So thought to be a green replacement.
What is the cathodic reaction of a hydrogen fuel cell, and what is the potential
Reduction of Oxygen into water
O2 + 4e- + 4H+ = 2H2O
Ec = +1.23 V
What is the anodic reaction of a hydrogen fuel cell, and what is the potential
Oxidisation of Hydrogen
H2 = 2H+ + 2e-
Ea = 0 V
What is the total potential of a hydrogen fuel cell.
Using G = - n * F * Ecell find the efficiency
H(H) = -285 kJ/mol
F = 96485
Ecell = Ec - Ea = 1.23 V
G = - 2 * 96485 * 1.23 = -237 kJ/mol
Eff_elec = desired/input
= G/H = -237/-285 = 0.83
What is Steam Methane Reforming, and what is the fundamental reaction and byproduct. How can the byproduct be removed/converted.
Production of Hydrogen using the reforming of CH4
Steam Reforming
CH4 + H2O = CO + 3H2
CO is produced as a byproduct and can be converted using the following reaction
Water Shift Reaction
CO + H2O = CO2 + H2
Write the equation for steam reforming, what conditions does this reaction take place at?
Steam Reforming
CH4 + H2O = CO + 3H2
At high temperatures and pressures
Uses Ni/Al2O3 catalysts
Write the equation for water shift reaction, what conditions does this reaction take place at?
Water Shift Reaction
CO + H2O = CO2 + H2
High Shift at 350 deg
Low Shift at 250 deg
High Shift using Fe-Cr catalyst
Low Shift using Cu-Zn/Al2O3 catalyst
What are main considerations of SMR
Produces lots of CO2, for every 1kg of H we produce around 9-12 kg of CO2
CH4 is a valuable resources and is non-renewable
Output (80% H2) needs purification
Economical Viable and well-established
Explain Coal Gasification, whats the feedstock and what are the 2 main reactions (and their sub reactions).
Feedstock = Coal
Coal is superheated in fuel rich conditions and turned into syngas that is rich in H2, this can be used to produce electricity through gas turbines or H2 through further processing.
Oxidisation (exothermic)
C + 0.5O2 -> CO
CO + 0.5O2 -> CO2
H2 + 0.5O2 -> H2O
Gasification (endothermic)
C + H2O -> CO + H
C + CO -> 2CO2
C + 2H2 -> CH4
What dictates if combustion or gasification occurs.
The amount of O2 in the reaction.
Lots of O2 (fuel lean) results in combustion where CO2 and H2O are produced.
Little O2 (fuel rich) results in gasification where CO and H2 are produced.
What is Methane Pyrolysis
The decomposition of methane into carbon (coal) and hydrogen.
CH4 -> C + 2H2
What is the feedstock of methane pyrolysis and what conditions are needed
feedstock = methane (natural gas)
high temperatures = 1100/1200 deg
no catalysts needed
no air needed (pyrolysis is burning with no oxygen)
What occurs in the reactor of methane pyrolysis
natural gas is feed into the reactor
nat gas is bubbled through a column of molten salt/metal
nat gas decomposes into carbon (coal) and hydrogen
hydrogen escapes through the top of the reactor
coal remain in the molten column and has to be separated once the column becomes too carbon rich
Pros and Cons of Methane Pyrolysis
+ves
Carbon is produced as coal so no carbon capture is required
High hydrogen purity
-ves
Can have proplems seperating the solid
carbon from the molten salf/metal
Creates a stockpile of coal
Pros and Cons of Coal Gasification
+ves
Cleaner alternative to standard coal combustion
-ves
Carbon still produced as a byproduct so need carbon capture
difficult to control reaction
Overall reaction of water electrolysis and potential
H2O -> H2 + 0.5O2
Ecell = -1.23 V
Half reactions of water electrolysis in acidic electrolyte
Opposite of fuel cell
Cathode (Ec = 0 V)
H -> 2H+ + 2E-
Anode (Ea = 1.23 V)
2H2O = O2 + 4e- + 4H+
Half reactions of water electrolysis in alkaline electrolyte
Cathode (Ec = -0.828 V)
2H2O + 2e- = H2 + 2HO-
Anode (Ea = 0.401 V)
2OH- = 0.5O2 + H2O + 2e-
Consideration of alkaline and acidic water electrolysis
acidic
fast dynamic response
suitable for variable renewable energy support
alkaline
slower dynamic response
less suited for variable renewable energy support
has some applications that could be scaled up
Nernst Equation
Ecell = Ec - Ea - RT/nF * ln(
What are the 2 types of Hydrogen storage, give examples of each
Physical Storage
Compressed gas
Liquid Hydrogen and cryogenic storage
Material Based Storage
‘Chemical based storage’ in hydrides
Liquid organic
Give more detail on the examples of material based storage
Hydrides
Hydrogen stored in interstitial states
can be released at high tempertures
ie when burnt
M + (x/2)H2 = MHx
Absorbtion
Bonding of hydrogen onto a surface via weak Wan-der-val forces
required cryogenic tempertures
How is hydrogen delivered
Trucks ?
Pipelines -> repurpose CH4 lines for LH
Shipping -> most common
