Unit 9 - Green Hydrogen Flashcards
What is the energy density of hydrogen?
120 MJ / kg which is triple that of diesel.
How can hydrogen be produced?
Steam reforming - reaction of CH4 with steam at high temperatures.
Electrolysis - splitting water into hydrogen and oxygen using renewable electricity.
Biomass conversion - via either thermochemical or biochemical conversion to intermediate products that can then be separated or reformed to hydrogen. Also includes fermentation techniques that produce hydrogen directly.
Solar conversion - thermolysis can be done using solar-generated heat for high temperature chemical cycle hydrogen production. By photolysis, where solar photons are used in biological or electrochemical systems to produce hydrogen directly.
How does steam reforming work?
Natural gas is used as the feedstock and as fuel for the burners. Steam is added. Heat is applied through the combustion of natural gas in the presence of a catalyst and the following reaction occurs:
CH4 + H2O ↔ CO + 3H2.
The heat is required as the reaction is very endothermic (+206 kJ mol-1). Hydrogen is obtained by adsorptive separation (zeolite) leaving very high purity of hydrogen.
What is grey hydrogen?
Where the hydrogen is made through the steam reformation of methane and the carbon dioxide is emitted into the atmosphere.
What is blue hydrogen?
Where the hydrogen through steam reformation of methane but CCS is used to prevent the carbon dioxide being emitted into the atmosphere.
What is power-to-X?
Electricity conversion, energy storage, and conversion pathways from surplus energy.
What is green hydrogen?
When renewable energy is used to power the electrolysis of water.
What is pink hydrogen?
When nuclear power is used for the electrolysis of water.
What is yellow hydrogen?
Electrolysis of water with solar power as the only source of energy.
How can solar be used to store energy?
Heliostats can focus light to melt salts as they have a high heat capacity. The molten salt can be used to vaporise water.
What are the efficiency of different methods for producing hydrogen?
Steam reforming - 80% efficient
Biogas Gasifiers - 45% efficient
Electrolysis - 60% efficient
Photolysis/Photochem - 5% efficient
What is an example of where geothermal energy has been used to produce sustainable methanol?
Iceland uses carbon dioxide captured from the atmosphere, with hydrogen derived from electrolysis. This is all powered using geothermal energy.
What is coal-to-liquid Fischer Tropsch chemistry?
High temperature (300-350 degrees) with an iron catalyst.
3C (coal) + O2 + H2O → H2 + 3CO
What is gas-to-liquid Fischer Tropsch chemistry?
Low temperature (150 degrees) with an iron/cobalt catalyst.
CH4 + H2 ↔ CO + 3H2
What percentage of worldwide carbon emissions come from the Haber Process?
3-5%
