L4 Flashcards
Pathways to convert biomass/organic wastes to syngas , hydrogen and liquid fuels
- Gasification, gas reforming/methanation, liquid fuel synthesis
- Pyrolysis, bio-oil upgrading
- Anaerobic digestion
- Fermentation
Gasification, gas reforming/methanation, liquid fuel synthesis
Gasification -> gas cleaning (syngas) -> methanation (SNG)
Gasification -> gas cleaning -> steam reforming -> fischer-tropsch (liquid fuel)
Gasification -> gas cleaning -> steam reforming -> WGS -> purification (H2)
What is biomass gasification
- Conversion of coal, petroleum or biomass inot CO and H2 based mixtures through a series of reactions of the raw material with a controlled amount of gasification agent
- 2 stages, initial devolatilization and subsequent gasification reactions
- Efficient method for extracting energy from different types of organic material
Gasification advantages
- High efficiency for energy conversion
- Flexibility for using H2 rich producer gas
- Can be operated with waste materials
- Corrosive elements in feed fuels can be removed (remain in ash)
Gasification agents
Air
- Used for partial combustion to supply heat for gasification
- Producer gas H2, CO, CO2, CH4, N2 and tars
- High content of N2, induces heat loss
- Low calorific value of producer gas
Oxygen
- Used for partial combustion to supply heat for gasification
- Similar producer gas to air, N2 much lower
- Higher calorific value for gas
- Higher cost
Steam
- H2O is used for water shift of gases from decomposed wood
- Steam and char react at high temperatures to produce H2, CO, CH4 etc
- Steam cracking occurs with shifting of tars
- High content of H2 and CO
- High calorific value of producer gas
- Need external heat input, reactions overall are endothermic
Fixed bas gasifier
- Stationary grate over which the fuel is in contact with the gasification agent
- Updraft: fuel fed on top, agent fed at bottom. Producer gas is out at the top.
- Downdraft: Fuel and agent both move downward. Producer gas out bottom
Fluidised bed gasifier
- Fuel is fluidised by the gasification gas
- Bubbling fluidised bed: gas velocity is not high enough to carry solid up to the gasifier’s top, bubbles form
- Circulating fluidised bed: High gas velocity, solid char and bed material are carried out from the gasifier’s top, the bed material returns back after gas-solid separation - circulating
Updraft (counter-current)
- Usually for heat generation not power as producer gas “dirty”
- Simple construction
- High residence time of soilds
- High carbon conversion
- Producer gas high level of tars
- 1-10 MWth
Downdraft (co-current)
- Simple and reliable
- High carbon conversion
- Tar cracking occurs around throat
- Relatively clean gas
- Low MC biomass required
- Long residence times for solids
- 100 kWth - 2 MWth
Bubbling fluidised bed gasifier
- Good temperature control
- High reaction rate
- High conversion efficiency
- Moderate tar level in producer gas
- Operated at atmospheric or pressurised
- Higher ash carry-over
- < 25 MWth
Circulating fluidised bed
- Easy to scale up
- Smaller size at same capacity than BFB
- Operation more complicated
- 10 - 100 MWth
Devolatilization
1.) devolatilization (fast pyrolysis - endothermic)
Biomass -> char, H2O, tars, H2 etc
Gas-gas reactions
H2 with O2
CO with O2
CH4 with O2
WGS
Steam-methane reforming
Char and gases reactions
C with O2 (CO2) C with O2 (CO) (C + H2O) (C + CO2) (C + H2)
Issues for gasification
Tars
- High MW compounds in producer gas, bad for downstream applications
- Condensable at reduced temperatures
Contaminant gas species
- Converted from N and S in feedstock fuels
- Poisonous for downstream catalysts
