Thermal Processes Flashcards
Up draught combustor
Design: Simple, natural draught.
Fuel: Batch above bed, secondary air above.
Combustion: Through most of bed.
Suitable for: Wood logs or large biomass.
Issue: Excessive smoke.
Variation: Can use natural or forced draught.
Down draught combustor
Requirement: Combustion or flue gas fan.
Fuel Combustion: Partial at base, completed in afterburner.
Control: Microprocessor-controlled for emissions.
Cost: Expensive
Large scale combustion technology
- Fixed bed: combustion on a grate
- Fluidised bed: combustion in inert suspended particles.
- Pulverised fuel combustion.
- Fife: bubbling/circulating fluidised bed.
Emissions in combustion boilers?
How are they reduced?
CO, NOx, HC and particulates (dioxins and furans in wood combustion)
Using secondary air
What is gasification?
Conversion of solid carbonaceous fuels into a flammable mixture of gas.
How is heat supplied for gasification?
Externally or by using a small amount of air for partial oxidation.
Process of gasification
1.) Pyrolysis (high T favours gases):
C6H10O5 5CO + 5H2 + C
2.) Partial oxidation (direct, less than stoichometric):
C6H10O5 5CO + 5CO2 + 5H2
3.) Reforming (gasification in the presence of steam, or steam-air:
C6H10O5 + H2O 6CO + 6H2
Applications of biomass gasification
IC engines gas turbine, fuel cells
H production, chemical feedstocks
What are the gas/solid reactions in gasification?
C + H2O CO + H2
CO2 + C 2CO
Heat provided by:
C + 0.5O2 CO
Gas phase reactions in gasification:
WGS:
* CO + H2O CO2 +H2
Methanation:
* CO + 3H2 CH4 + H2O
What other reactions occur in gasification?
Hydrogenation of biomass and char (exo)
pyrolysis, cracking, rearrangement of polymers and tars.
Effect of increasing T and P on the syngas composition?
P - favours CH4 and CO2
T - Favours CO and H2
Updraft gasifier
Operation: Fuel added from above, sub-stoichiometric air from below.
Zones: Combustion at bottom, gasification in middle, drying at top.
Tar Production: Yields large quantities of tar.
Downdraft gasifier
Direction: Fuel and gas flow in the same direction.
Design: Modern designs include air nozzles for hot char bed.
Tar Production: Yields small amount of tar, requires low ash, low moisture fuel.
Issue: Ash slagging in combustion zone can be problematic
Fluidised bed gasifier
Operation: Gas stream passes through bed of inert particles.
Mixture: Turbulent mixture, fuel is small fraction of solid.
Temperature Control: Uniform temperature and concentration, simultaneous gasification.
Advantages: Easily scalable, wide fuel variety usable.
Entrained flow gasifier
Conversion: High char conversion, low tar.
Suitability: Ideal for coal, less so for biomass without co-gasification.
Challenges: Expensive for biomass due to fine particle production, low energy density, high sintering.
What gasifier is used commercially, what’s the problem
Fluidised bed - ammonia removal
What are pyrolysis products used for?
Char: solid fuel or gasify
Liquid: liquid fuel, chemicals, refinery feedstock.
Gas: Fuel
What is pyrolysis oil and problems associated
Bio-oil is a low viscosity dark-brown fluid with a high water content, but generally does not phase separate. Highly oxygenated and highly unstable.
What is fast pyrolysis of biomass
- Short heat up and reaction times.
- Rapid removal and quenching of organic volatiles minimises carbonisation or further cracking. Short residence time.
- Reactor configuration acheives yields of liquid up to 70-80% based on dry biomass
What compounds are water soluble/insoluble in pyrolysis oil? How to remove
- Water soluble: low MW acids
- Water insoluble: Aromatics (lignin, BTX, phenols)
Addition of equal volume water precipitates lignin
Uses of pyrolysis bio oil
- Direct substitute for heating oil.
- Possible fuel for combustion turbines or modified diesel engines.
- Possible recovery of high value chemicals
- Interest in blending and upgrading of pyrolysis oil.
State of technology for pyrolysis
- Close to commercial for biomass
- Waste biomass (negative fuel costs) – Compact power Bristol.
- Problems of acidic and unstable bio-oil must be solved.
- Options include combustion, transport fuels, extraction of chemicals.
- Market required for char
- Poor economics compared to fossil fuels.
What do holoecullose degrade to in pyrolysis
holo - ugars,
levoglucosan, acids, furfural, lactones, hydroxyacetaldehyde
lignin - phenols, methoxyphenols and dimethoxyphenols
compare pyrolysis oil to solid biomass.
Energy content is similar on mass, much higher on volume
