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CPE406 Energy from Biomass and Waste > Biomass Pyrolysis and Torrefaction > Flashcards

Biomass Pyrolysis and Torrefaction Flashcards

1
Q

Define pyrolysis

A

The thermochemical decomposition of large complex hydrocarbon chains (eg. cellulose) into relatively smaller and simpler molecules (eg. methane, CO2, CO, steam)

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2
Q

What temperature is pyrolysis typically carried out at?

A

300-650 deg C

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3
Q

Is pyrolysis endothermic of exothermic?

A

Endothermic because breaking bonds requires energy

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4
Q

What are the 3 products of pyrolysis?

A

Gas, liquid and char

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5
Q

Describe the principles of the biomass pyrolysis process

A

Biomass is fed into a pyrolysis chamber containing hot solids that heat the biomass to 300-650 deg C where decomposition starts. The condensable and non-condensable vapours released from the biomass leave the chamber and the char remaining in the gases is removed via a cyclone and cooled. The condensable gases in the gas stream are condensed as bio-oil (pyrolysis oil) and the non-condensable gases leave the chamber as product gas.
Some char, however will stay in the pyrolysis chamber.

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6
Q

Is pyrolysis cheaper or more expensive than gasification and incineration and why?

A

Cheaper to construct and operate due to the lower operating temperature.

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7
Q

What can the gases from pyrolysis be used for?

A

They are combustible so can be used as fuels

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8
Q

Why are gas cleaning requirements and costs lower for pyrolysis?

A

Because the gas volumes produced are considerably lower

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9
Q

What are liquid products from pyrolysis used for?

A

Stored and used when required for external processes or for supplying heat to the pyrolysis process.

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10
Q

What is the most significant challenge for the development of pyrolysis?

A

Heat transfer

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11
Q

By what mechanisms is heat transferred to the biomass particle in pyrolysis?

A

Heat is transferred to the particle’s surface via radiation and/or convection, and then transferred to the interior of the particle via conduction, and the pore convenction.

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12
Q

What is a typical thermal conductivity of a particle of biomass and why is this a problem?

A

Around 0.1 W/mK, because this is the main mechanism of transfer of heat to the interior of the particle so heating can be very slow even when the surface of the particle is receiving an increase of 10,000 deg/s

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13
Q

What 4 heat transfer methods can be used to provide the necessary heat for pyrolysis?

A
  1. Through heat transfer surfaces located in suitable positions in the reactor for example hot walls or tubes
  2. By heating the fluidisation gas (for fluidised bed reactors)
  3. By removing and re-heating the bed material in a separate unit (fluidised bed reactor)
  4. By combustion of some of the biomass/char to provide the heat (by addition of some air to the chamber)
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14
Q

What 6 possible fuels can be used to provide the heat for the pyrolysis chamber?

A
  1. Combustion of char (all or part)
  2. Combustion of pyrolysis gas
  3. Use of the bio-oil
  4. Combustion of fresh biomass (particularly when char is a high value product)
  5. Gasification of char, then combustion of the resultant gas (avoids alkali metal problems)
  6. Use of fossil fuels (when available at low cost, do not affect the process or product, and products are high value)
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15
Q

Apart from heating rate, what 3 other parameters are there for pyrolysis?

A
  1. Temperature
  2. Residence time
  3. Particle size
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16
Q

What defines slow pyrolysis, what does it produce and what temperature does it occur at?

A

The heating time being much larger than the reaction time. It is mainly used for high char production and occurs at low-moderate Ts (around 600 deg C).

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17
Q

What defines fast pyrolysis, what does it produce and what temperature does it occur at?

A

The heating time being much smaller than the reaction time, therefore heating rates are around 1,000-10,000 deg/s. There has to be rapid quenching of the product gas for primarily bio-oil production.

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18
Q

What is the typical residence time of the vapour in slow pyrolysis?

A

Minutes or longer

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19
Q

What is the typical residence time of the vapour in fast pyrolysis?

A

seconds/milliseconds

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20
Q

What kind of feedstock does fast pyrolysis need?

A

finely ground feedstock

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21
Q

What kind of bio-oil yields are expected from fast pyrolysis?

A

75% (dry basis)

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22
Q

Name the 6 types of pyrolysers.

A
  1. Fixed bed (or moving bed)
  2. BFB
  3. CFB
  4. Rotating-cone
  5. Ablative reactor
  6. Vacuum reactor
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23
Q

What are the main features of a fixed bed pyrolyser?

A
  • Batch reactor where char stays inside the reactor
  • Slow pyrolysis occurs so the main product of char (long residence time)
  • Heat is provided by external source or limited combustion of biomass
  • Product gas leaves reactor due to volume expansion, but sometimes a sweeping gas is used
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24
Q

What is the working principle of a BFB pyrolyser?

A

Crushed biomass (2-6 mm) is fed into a bubbling bed of hot sand which is fluidised by an inert gas (ie. not air). Intense mixing of the inert bed solids offers good and uniform temperature control. The residence time of the biomass is much longer than that of the gas.

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25
Q

Describe the main characteristics of a BFB pyrolyser.

A
  • It is a well understood technology
  • Simple in construction and operation
  • Easy to scale up
  • Good temperature control and efficient heat transfer
  • Small biomass particles (2-6 mm) are needed to achieve high heating rates
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26
Q

Describe the working principle of a CFB pyrolyser compared to a BFB pyrolyser.

A

The gas velocity is high so there is entrainment of particles in the gas stream, and the bed is highly expanded compared to a BFB. Entrained particles are separated in a cyclone and directed back to the pyrolyser to improve conversion efficiency. It has a larger capacity so is suitable for higher throughputs.

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27
Q

What is the major advantage of CFBs over BFBs?

A

Entrained char can easily be combusted in an external fluidised bed to provide the heat to the inert solids (sand) for the pyrolyser.

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28
Q

Describe the operating principle of an ablative pyrolyser.

A

The creation of high pressure between a biomass particle and a hot reactor wall (pressure created by centrifugal forces as the wall spine) allows unhibited heat transfer from the wall to the biomass, which then leaves behind a liquid film

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29
Q

What are the 2 main advantages of ablative pyrolysers?

A

Large biomass particles can be used, and liquid yields of 80% can be achieved due to high heat transfer rates and short residence times.

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30
Q

What is the operating principle of a rotating-cone pyrolyser?

A

Biomass particles and heat carrier solids are fed into the bottom of a heated rotating cone. The centrifugal forces (10 rps) drive the hot sand and biomass against the wall. The char and carrier solids fall over the edge of the cone into a fluidised bed and the char is combusted to provide heat to the cone, after which the carrier solids are then recycled. The product gas containing bio-oil vapour leaves the reactor through a tube at the top of the cone.

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31
Q

Is rotating-cone pyrolysis fast or slow?

A

Fast, high heating rate and short residence time (<1s) which produces a liquid yield of 60-70% of the dry feed.

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32
Q

What is the main advantage of rotating-cone pyrolysis?

A

No carrier gas is needed.

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33
Q

What is the main disadvantage of rotating-cone pyrolysis?

A

The geometry is complex to scale up.

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34
Q

Describe the operating principle of a vacuum pyrolyser.

A

Biomass particles are fed into a stack of many heated plates (200 deg C at the top and 400 deg C at the bottom), dropping to lower successive plates via scrappers. The biomass undergoes drying and pyrolysis while moving over the plates. At each stage, product gas is taken off, and only char is left by the time the biomass reaches the bottom of the stack.

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35
Q

Is vacuum pyrolysis slow or fast?

A

Slow, therefore liquid yields are low as the residence time of the vapour in the pyrolysis zone is short. This mean liquid yield is low - about 35% of dry feed.

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36
Q

What are the main disadvantages of vacuum pyrolysis?

A
  • Fouling potential of the pump

- Complex design

37
Q

What is the main advantage of vacuum pyrolysis?

A

There is less char in the liquid product because the vapour is removed at regular intervals

38
Q

Draw a fixed bed pyrolyser

A

Drawing 34

39
Q

Draw a BFB pyrolyser

A

Drawing 35

40
Q

Draw a CBF pyrolyer

A

Drawing 36

41
Q

Draw an ablative pyrolyser

A

Drawing 37

42
Q

Draw a rotating cone pyrolyser

A

Drawing 38

43
Q

Draw a vacuum pyrolyser

A

Drawing 39

44
Q

What are the 4 products made from pyrolysis?

A

Char, liquid, gases (Co, CO2, H2, H2O, CH4 etc.), wood cubes

45
Q

Describe the main characteristics of bio-oil

A
  • Dark brown and similar elemental compossition to biomass
  • Complex mixture of several hundred different chamicals and a relatively high water content
  • Heating value 13-18 MJ/kg
  • Can be used directly in boilers
  • Viscosity increases in time and water cannot be easily separated
  • Not as stable as fossil fuels
  • Cannot be mixed with conventional fuels
  • High oxygen content, high solid content, high viscosity and chemical instability
46
Q

Why is rapid cooling of pyrolysis gas important?

A
  • To prevent further degradation, cleavage or reaction

- To convert condensable gases (made of heavier molecules) into bio-oil

47
Q

Why is char in bio-oil bad?

A

It accelerates aging and exacerbates instability

48
Q

What is biochar and what are it’s main features?

A
  • The solid product of biomass pyrolysis
  • Primarily carbon (around 85%) and some oxygen and hydrogen
  • Large pore surface area
  • High heating value, around 32MJ/kg higher than biomass and bio-oil.
49
Q

How is biochar collected?

A

Using cyclones to remove char entrained in pyrolysis gas, or using hot vapour filtration

50
Q

What gases does pyrolysis gas mainly include and what is it’s heating value?

A

CO, CO2, H2, H2O, CH4, C2H6, C2H4, heating value 11-20 MJ/Nm3

51
Q

Define torrefaction

A

A thermochemical process in an inert or limited oxygen environment where biomass is slowly heated to within a specified temperature range and retained there for a stipulated time such that it results in near complete degradation of its hemicellulose content while maximising mass and energy yield of solid product.

52
Q

What are the two main reasons for using biomass torrefaction?

A
  • Increase energy density by increasing the carbon content and decreasing the oxygen and hydrogen content - therefore easier to transport
  • Makes biomass lose its fibrous nature so that it is easily grindable - making it more suitable for further processing
53
Q

What is a typical temperature for biomass torrefaction?

A

200-300 deg C

54
Q

What is a typical torrefaction heating rate and why?

A

50 deg/min to maximise solid yield

55
Q

Does torrefaction use air, oxygen or steam?

A

None

56
Q

Describe how biomass changes at different temperatures in torrefaction

A
  • Biomass is first dried (both surface and intrinsic moisture)
  • It is heated and the components degrade:
  • Lignin softens 80-90 deg C
  • Hemicellulose 200-260 deg C
  • Cellulose 275-300 deg C
  • Lignin degrades gradually from 250-500 deg C
57
Q

How is heat applied to a torrefaction process?

A

Externally by heating medium or heat carrier

58
Q

Describe the torrefaction of a twig

A
  • The bark of the twig is peeled off and left in an air-drying oven at 70 deg C. After an hour, its surface moisture evaporates and no longer appears wet (mass lost 21%).
  • The oven temperature is raised to 110 deg C for drying of the inherent moisture within the pores.
  • The twig is bone dry at 140 deg C for 1 hour. The wood turns reddish (mass lost 42%).
  • The twig turns brown but changes are purely physical
  • Torrefaction begins: 200 deg C for 1 hour starts chemical decomposition (mass lost 4%)
  • 250 deg C for 1 hour where major chemical reaction occurs and the twig turns black (mass lost 11%)
  • 310 deg C for 1 hour twig becomes darker and brittle (mass lost 15%)
59
Q

What is the typical CV of torrefied biomass?

A

10-20 MJ/kg

60
Q

Give 4 main features of torrefied biomass

A
  • More brittle and grindable than fibrous wood
  • Production and delivery costs are significantly lower compared to wood pellets
  • Highly competitive when co-fired with coal
  • Premium feedstock for gasification and BTL
61
Q

What are the 3 products of torrefaction?

A
  • Solid char
  • Liquid (water from drying and thermal decomposition, organics, lipids)
  • Non-condensable gases (CO, CO2 and small amounts of methane)
62
Q

How does temperature influence the torrefaction process?

A

Temperature has a great influence on torrefaction as the degree of thermal degradation of each component depends on the operating temperature.

63
Q

Draw the relationship between torrefaction temperature and mass of biomass.

A

Drawing 40

64
Q

Draw the relationship between torrefaction temperature and energy of biomass

A

Drawing 41

65
Q

Draw the relationship between time and mass/energy of biomass

A

Drawing 42

66
Q

Which has more influence on the torrefaction product, temperature or residence time?

A

Temperature

67
Q

How does the residence time of torrefaction compare to that of pyrolysis and why?

A

It is much longer because heating rates must be much slower

68
Q

How does biomass size affect the torrefaction process?

A

For a constant length, an increase in diameter of the biomass decrease the reduction in mass, which is a direct result of heat transfer to the biomass interior and the temperature dependent reactions within it. This change is more sensitive for large biomass sizes.

69
Q

How are pyrolyser reactors classified?

A

On the gas-solid contacting mode

70
Q

What three ways can torrefaction reactors be classified?

A

By heat transfer, by mode of heating, and by mode of gas-solid mixing

71
Q

What units would you see on a torrefaction plant?

A

Handling, preparation, dryer, torrefier and product cooler

72
Q

What are the 5 heat transfer torrefaction reactor categories?

A
  1. gas-particle convection
  2. wall-particle conduction
  3. electromagnetic heating of biomass
  4. particle-particle heat transfer
  5. liquid-particle heat transfer
73
Q

What are the 2 mode of heating torrefaction reactor categories?

A
  1. Directly heated

2. Indirectly heated

74
Q

What are the 4 gas-solid mixing torrefaction reactor categories?

A
  1. plug flow (gas flows through static solids or gas and solid both move unidirectionally)
  2. Partial back-mixed (eg. fluidised bed where was is unidirectional, but solids are back-mixed)
  3. Tumbling (solids tumble around in a drum or cylindrical tunnel)
  4. Entrained (solids are pneumatically transported by gas)
75
Q

What is the most common type of torrefaction reactor?

A

Fixed/moving bed

76
Q

Draw a moving bed torrefaction reactor

A

Drawing 43

77
Q

Describe the principle of operation of a fixed/moving bed torrefaction reactor

A

The heat carrier is a hot inert gas flows through/past the biomass particles which are either stationary (fixed bed, particles don’t move with respect to the reactor walls) or moving (moving bed, particles move with respect to the reactor wall, can be moved by gravity). No back mixing of particles ocurred.

78
Q

Describe the principle of operation of a fluidised bed torrefaction reactor

A

Hot inert gas is blown through a bed of granular heat-carrier solids/approximately sized biomass particles in such a way that they behave like a fluid. The heat carrier particles easily heaet up any freah biomass by vigorous mixing. The biomass undergoes torrefaction in a well-mixed state with uniform temperature distribution.

79
Q

What is the main limitation of fluidised bed torrefaction reactors?

A

Separation of the biomass from the heat carrier particles

80
Q

Draw a fluidised bed torrefaction reactor

A

Drawing 44

81
Q

Describe the principle of operation of a microwave torrefaction reactor

A

Biomass passes through a microwave oven with waves typically at 2.45 GHz. The particles are heated from within and every part of the biomass within the path of the wave is heated simultaneously. This creates an extremely fast rate of heating of the biomass interior (a matter of seconds) due to a very different heat transfer mechanism.

82
Q

Draw a microwave torrefaction reactor

A

Drawing 45

83
Q

Describe the principle of operation of a rotary drum torrefaction reactor

A

Biomass enters an indirectly heated rotating drum that tumbles the biomass an an inert gas medium. The biomass is heaeted by the hot drum wall or by hot internals in the drum.

84
Q

What is the primary controlling factor in a rotary drum torrefaction reactor?

A

Heat transfer from the wall of the drum to the biomass particles

85
Q

Draw a rotary drum torrefaction reactor

A

Drawing 46

86
Q

Describe the principle of operation of a screw conveyor reactor

A

Biomass enters a vertical/horizontal/inclined chamber that houses a screw conveyor. The outside walls are heated and heat is conducted to the biomass through the walls. The screw mixes and moves the bioimass to enhance heat transfer and to transfer the biomass to the exit.

87
Q

Draw a screw conveyor torrefaction reactor.

A

Drawing 47

88
Q

Give an example of a torrefaction plant and it’s purpose

A

ECN (Energy Research Centre of the Netherlands) has a 50 kg/h pilot plant built in 2008.

89
Q

What is the main disadvantage of torrefaction?

A

External energy is required to supply heat to the system as the product gas does not have enough energy.

CPE406 Energy from Biomass and Waste (8 decks)

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