F2 Synthesis Gas Flashcards
Syngas
Mixture of H2 and CO (Colourless, odourless and highly flammamble)
Products from syngas
Methane
Transportation fuels (from Fischer Tropsch)
Ammonia
H2 (hydrogen gas)
Methanol
Electricity (from H2?)
What raw material can be used for producing syngas?
Natural gas
naphta (nafta är en del av råbensinen - mörk och lättflytande)
coal
waste, etc.
(anything that contains CH - hydrocarbons)
Vad beror valet av råmaterial av?
Cost
Availability (good to use waste)
Application (depending on the rawmaterial used you will have different outcomes/structure in the gas)
What are the three main processes for producing syngas?
* Steam reforming (natural gas CH4, light hydrocarbons)
–> most common.
* Partial oxidation of heavy hydrocarbons ex diesel (expensive because you need pure oxygen)
* Partial oxidation of coal = coal gasification
–> mostly for power generation
General flow scheme of Steam reforming, partial oxidation of heavy hydrocarbons and coal gasification.
Methods for removing sulphur from feedstock
* Adsorption - through activated carbon
* Reaction - most common to react with ex ZnO
* Scrubbing - the sulfur reacts with limestone (CaCO3) and water
* Hydrotreatment - if stable sulphur compounds –> let the gas react with H –> the hydrogen will react with the impurities
Steam Reforming reaction
CH4 + H2O → CO + 3H2 (endothermic)
Water-gas-shift reaction
CO + H2O → CO2 + H2 (exothermic)
Steam reforming process
- Temperature > 1000K in order to get good syngas
- Methane is very stable → catalyst is needed (Ni is good)
- Sulphur removal necessary
- Two sections:
- convection section (where you preheat the gas)
- radiation section (the reforming itself takes place)
Effects of externally shaped pellets vs internally shaped pellets.
Externally shaped pellets:
- Lower pressure drop
- better dispersion (spridning) of “the reaction”
Internally shaped pellets:
- higher CH4 conversion
- higher effectiveness factor
Problems with the commercial catalysts
The heat transfer is a limiting step:
Ceramic has low conductivity → the temperature in the middle of the tube will be lower → the reaction will be slower
Endothermic reaction → the wall needs to be capable to handle high temperatures → need stable materials inside as well
Why must carbon formation be avoided in Steam Reforming? And how do you prevent this?
Must be avoided for two reasons:
→ Deactivation of catalyst (hinders the methane from the active sites)
→ Blockage of reformer tubes (can increase pressure drop and create hot spots → can cause melting)
Solution: add excess steam! Favour the reaction that doesn’t produce C
The affects of high pressure and high temperature?
High pressure:
(-) is thermodynamically unfavourable (SR → more gas molecules)
(-) lower methane conversion
(+) lower syngas compression costs
(+) smaller reformer size
High temperature:
(-) material constraints! (can make the material degrade for ex)
(+) need hight temperatures for the steam reforming - for good quality syngas
The predicament of high temperature?
High temperature is needed for a high methane conversion, but will lead to degradation of the tube material. Need more water (steam) for higher methane conversion at lower temperatures.
Improvements for the stem reforming process?
- Cost reduction!
- Better materials in the tubes (that can handle heat)
- Better catalysts (sulphur tolerance and less carbon deposition)
- Want to use higher hydrocarbons in the feedstock (forms more carbon) → Use a prereformer (converts higher CHx to CH4)
- If you want CO rich syngas → add CO2 or use autothermal reforming (O2 addition)
What is the coal gasification process?
The coal is heated → the organic matter in the coal melts while gases evolve.
Further heating leads to pyrolysis (char is formed)
There are many reactions in the gas phase
(Gasification can also be used for biomass)
Applications for coal gas
Reactions in coal gasification
What are the three main reactors for coal gasification?
Moving bed gasifier
Fluidized bed gasifier
Entrained flow gasifier
Moving bed gasifier
Answer:
What type of coal?
Temperature
Quality of the product gas
Pros and cons for moving bed gasifier
(+) Counter-current operation gives high efficiency (all coal is converted but the gas is not of high quality)
(-) Need large excess of steam to keep temp low (lowers efficiency)
(-) Large amount of byproducts
(-) Not for all types of coal
Fluidized bed gasifier
Answer:
What type of coal?
Temperature
Quality of the product gas
Pros and cons for fluidized bed gasifier
(+) High temp, less impurities
(-) Highly reactive coal is needed
(-) A lot of unreacted carbon → follows with the products gas, lowers the conversion
Entraines flow gasifier
Answer:
What type of coal?
Temperature
Quality of the product gas
Pros and cons for Entrained flow gasifier
(+) High temp → high conversion
(+) High temp → only CO and H2
(+) Can handle all types of coal
(-) Short residence time requires very high temp.
Usage for product gas from Moving bed vs Fluidized bed vs Entrained flow
Moving bed gasifier: hardly any syngas but good for power generation (CH4) carries more energy
Fluidized bed gasifier: still better for energy usage
Entrained flow gasifier: the BEST for syngas production
Cleaning and conditioning of syngas
Conditioning of syngas for ammonia synthesis
WGS: CO + H2O → CO2 + H2
Methanation: 3H2 + CO → CH4 + H2O
Biogas to hydrogen
