Thermal Processes

Question 1

Up draught combustor

Answer 1

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.

Question 2

Down draught combustor

Answer 2

Requirement: Combustion or flue gas fan.
Fuel Combustion: Partial at base, completed in afterburner.
Control: Microprocessor-controlled for emissions.
Cost: Expensive

Question 3

Large scale combustion technology

Answer 3

• Fixed bed: combustion on a grate
• Fluidised bed: combustion in inert suspended particles.
• Pulverised fuel combustion.
• Fife: bubbling/circulating fluidised bed.

Question 4

Emissions in combustion boilers?
How are they reduced?

Answer 4

CO, NOx, HC and particulates (dioxins and furans in wood combustion)
Using secondary air

Question 5

What is gasification?

Answer 5

Conversion of solid carbonaceous fuels into a flammable mixture of gas.

Question 6

How is heat supplied for gasification?

Answer 6

Externally or by using a small amount of air for partial oxidation.

Question 7

Process of gasification

Answer 7

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

Question 8

Applications of biomass gasification

Answer 8

IC engines gas turbine, fuel cells

H production, chemical feedstocks

Question 9

What are the gas/solid reactions in gasification?

Answer 9

C + H2O  CO + H2
CO2 + C  2CO
Heat provided by:
C + 0.5O2  CO

Question 10

Gas phase reactions in gasification:

Answer 10

WGS:
* CO + H2O  CO2 +H2
Methanation:
* CO + 3H2 CH4 + H2O

Question 11

What other reactions occur in gasification?

Answer 11

Hydrogenation of biomass and char (exo)
pyrolysis, cracking, rearrangement of polymers and tars.

Question 12

Effect of increasing T and P on the syngas composition?

Answer 12

P - favours CH4 and CO2
T - Favours CO and H2

Question 13

Updraft gasifier

Answer 13

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.

Question 14

Downdraft gasifier

Answer 14

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

Question 15

Fluidised bed gasifier

Answer 15

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.

Question 16

Entrained flow gasifier

Answer 16

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.

Question 17

What gasifier is used commercially, what’s the problem

Answer 17

Fluidised bed - ammonia removal

Question 18

What are pyrolysis products used for?

Answer 18

Char: solid fuel or gasify
Liquid: liquid fuel, chemicals, refinery feedstock.
Gas: Fuel

Question 19

What is pyrolysis oil and problems associated

Answer 19

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.

Question 20

What is fast pyrolysis of biomass

Answer 20

• 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

Question 21

What compounds are water soluble/insoluble in pyrolysis oil? How to remove

Answer 21

• Water soluble: low MW acids
• Water insoluble: Aromatics (lignin, BTX, phenols)

Addition of equal volume water precipitates lignin

Question 22

Uses of pyrolysis bio oil

Answer 22

• 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.

Question 23

State of technology for pyrolysis

Answer 23

• 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.

Question 24

What do holoecullose degrade to in pyrolysis

Answer 24

holo - ugars,
levoglucosan, acids, furfural, lactones, hydroxyacetaldehyde

lignin - phenols, methoxyphenols and dimethoxyphenols

Question 25

compare pyrolysis oil to solid biomass.

Answer 25

Energy content is similar on mass, much higher on volume