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CPE406 Energy from Biomass and Waste > Biomass Gasification > Flashcards

Biomass Gasification Flashcards

1
Q

Define gasification

A

The conversion of solid or liquid feedstock into useful and conventient gaseous fuel or chemical feedstock that can be burned to release energy or used for production of value-added chemicals.

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2
Q

Describe a typical gasification process

A

It is a partial oxidation process in which organic compounds are converted to syngas (CO and H2) at 500-1400 degrees C at elevated pressures up to 33 bar in the presence of a gasification medium (air, oxygen or steam)

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3
Q

What three sequential steps are seen in a typical gasification process?

A

1 - preheating and drying to evaporate moisture (biomass 30-60% reduced to 10-20%)
2 - thermal decomposition or pyrolysis (no external agent is needed, thermal breakdown of larger hydrocarbon molecules of biomass into smaller gas molecules)
3 - char gasification

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4
Q

What influences the extent to which gasification reactions occur?

A

The gasification conditions such as T and P and the feedstock used

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5
Q

List the 8 reactions that happen in a gasification processs.

A

Drawing 23

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6
Q

Describe the chemical changes occur in the gasification step.

A

Partial combustion occurs due to the the oxidant not reaching stoichiometric requirements (usually 1/5-1/3), so the major products are CO and H2, with only some CO2. Heat produced by partial oxidation provides most of the energy for the endothermic gasification reactions. The three heterogeneous reactions (water-gas, Boudouard, methanation) can be reduced to two homogeneous reactions (water-gas-shift-reaction, steam-methane reforming reaction), which collectively play a key role in determining the final equilibrium syngas composition.

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7
Q

Give 6 advantages of gasification over combustion

A
  • Better public image
  • Variety of products (concentrated syngas, heat&power, transport fuels, feedstock chemicals)
  • Some gasifiers can produce a melted ash granulate, which is easier to dispose of than fly-ash
  • Lower CO2 emissions per MWh energy
  • Lower NOx, SOx and particulate emissions (no need for SCR units, sulphur appears as H2S and COS which can be converted to S or H2SO4 which are easy to sell, in combustion sulphur appears as SO2 which is converted to CaSO4 through scrubbing)
  • Less gas produced for a given energy throughput (smaller cleaning downstream equipment therefure lower cost)
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8
Q

Give an advantage of combustion over gasification

A

It is a mature technology that produces clean heat and power up to 85% efficiently

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9
Q

What are the 3 challenges surrounding gasification?

A
  • Greater fraction of energy as electricity instead of heat, requires tight specifications for feed material
  • Research needed to solve the tar problem (solved by 3-stage gasification incorporating high T tar cracking)
  • Has not been proven on a large scale
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10
Q

What is the product of air gasification and what are the consequences of this?

A

Partial oxidation of air produces a low heating value gas around 5 MJ/Nm3 (4-7) which is heavily diluted with nitrogen, which increases the volume of the gas requiring larger downstream equipment and therefore higher capital costs. It will also dilute the syngas and adversely affect the fuel synthesis resulting in poor performance.

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11
Q

What is the product of oxygen gasification and what are the cost implications of this?

A

Better quality, nitrogen free syngas (12-28 MJ/Nm3), but require additional costs for an air separation unit (ASU), but this is usually compensated by a better quality syngas.

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12
Q

What is the product of steam gasification and what are the cost implications?

A

High heating value syngas (10-18 Mj/Nm3) due to a higher hydrogen content in the syngas, but incurs cost as steam is not free whereas air is.

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13
Q

Draw a summary of the products and uses of each gasification process.

A

Drawing 24

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14
Q

Name the three principal types of gasifier based on the gas-solid contacting mode

A
  • Fixed bed (or moving bed) - updraft or downdraft (biomass is supported on a grate and moves up or down in the gasifier as plug flow)
  • Fluidised bed (BFB and CFB) (gasification medium conveys the biomass particles through the reactor)
  • Entrained flow (gasification medium conveys the biomass particles through the reactor)
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15
Q

List the feed particle size requirements for each type of gasifier

A

Fixed/moving bed: <51 mm
Fluidised bed: <6 mm
Entrained bed: <0.15 mm

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16
Q

List the feedstock tolerance requirements for each type of gasifier

A

Fixed/moving bed: low rank coal and relatively high moisture biomass
Fluidised bed: low-rank coal and excellent for biomass
Entrained bed: any coal but not good for biomass

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17
Q

List the reaction zone temperature for each type of gasifier

A

Fixed/moving bed: 1090 degrees C
Fluidised bed: 800-1000 degrees C
Entrained bed: 1990 degrees C

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18
Q

List the gas exit temperature for each type of gasifier

A

Fixed/moving bed: 450-650 degrees C
Fluidised bed: 800-1000 degrees C
Entrained bed: >1260 degrees C

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19
Q

List the size application for each type of gasifier

A

Fixed/moving bed: small
Fluidised bed: medium
Entrained bed: large

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20
Q

List the typical thermal output for each type of gasifier

A

Fixed/moving bed: 10kW-10MW
Fluidised bed: 1-100MW
Entrained bed: 70-1000MW

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21
Q

Describe an updraft gasifier and the biomass spec it is suitable for

A

Counter-current flow gasification - oxidant and product gases flow upwards in the opposite direction to biomass, suitable for high ash (up to 25%) and high moisture (up to 60 wt%)

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22
Q

Draw an updraft gasifier

A

Drawing 25

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23
Q

Give 3 advantages of updraft gasifiers

A
  • A simple and low cost process
  • Able to handle biomass with high moisture and organic (ash) content eg MSW
  • It is a proven technology
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24
Q

Give the disadvantage of updraft gasifiers

A

The syngas contains 10-20% tar by weight, requiring extensive syngas clean-up before use in engines, turbines or synthesis applications, thus higher costs overall

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25
Q

Describe how a downdraft gasifier works and why they are the most commonly used design

A

Co-current flow of biomass and oxidant. The reaction products are mixed in the turbulent high-T region around the throat, which aids tar carcking. This means there is a low tar content therefore good for small-scale electricity generation.

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26
Q

Draw a downdraft gasifier

A

Drawing 26

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27
Q

List the 3 advantages of drowndraft gasifiers

A
  • Up to 99.9% of tars formed are consumed
  • Minerals remain within the char/ash thus reducing the need for a cyclone
  • Proven, simple, low cost process
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28
Q

List the 3 disadvantages of downdraft gasifiers

A
  • Requires the feed to be dried to a low moisture content (<20 wt%)
  • Syngas exits at a high temperature, requiring a secondary heat recovery system and extensive cooling
  • 4-7% of the carbon remains unconverted
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29
Q

Describe how a BFB gasifier works and it’s typical capacity and fluidising velocity

A

The biomass is fed into a fluidised bed of granular solids. Fluidised particles tend to break up the biomass fed into the bed and ensure a good heat transfer throughout. 10-15 dry tonnes/hr. This system is easily stopped and started. 0.5-1.0 m/s air.

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30
Q

Describe how a CFB gasifier works and it’s typical capacity

A

A fluidised bed with a higher air velocity, then the gas and solids are separated in a cyclone. Entrained particles are recycled. Fluidisation velocity 3.5-5.5 m/s but longer residence time therefore more attractive for gasification. 15 dry tonnes/hr

31
Q

Draw a CFB gasifier

A

Drawing 28

32
Q

List the 4 advantages of fluidised bed gasifiers

A
  • Yield a uniform product gas
  • Nearly uniform temperature ditribution throughout the reactor
  • High rate of heat and mass transfer between inert material, biomass and gas
  • High conversion possible with low tar and low unconverted carbon
33
Q

List the 4 disadvantages of fluidised bed gasifiers

A
  • Carbon loww with entrained ash and char
  • Higher maintenance requirements to maintain fluidisation
  • Loss of fluidisation due to bed sintering (particles coalescing)
  • High velocities may result in equipment erosion and blocking
34
Q

Give the operating conditions for the Varnamo fluidised bed gasifier plant

A
  • Uses wood as fuel at 1 kg/s, 15 wt% moisture
  • 18 bar (compressed air)
  • CFB
  • 6MW electricity
    9 MW district heat generation
35
Q

What are entrained flow gasifiers usually used for and why?

A

Most widely used gasifier for large scale coal gasification, but may not be suitable for biomass as the residence time is very short so very fine pieces would be required, which is hard to achieve.

36
Q

Draw an entrained flow gasifier

A

Drawing 29

37
Q

Draw an entrained flow gasifier and a schematic of the reaction zones inside

A

Drawing 29

38
Q

What type of reactor can an entrained flow gasifier be compared to and what temperatures do they tend to run at?

A

A co-current plug flow reactor, >1000 degrees C (1200-1500 deg C)

39
Q

What type of reactor can an entrained flow gasifier be compared to and what temperatures and pressures do they tend to run at?

A

A co-current plug flow reactor, >1000 degrees C (1200-1500 deg C), pressurised

40
Q

List the 4 advantages of entrained flow gasifiers

A
  • Simple reactor design
  • Low tar and hydrocarbon content
  • Good scale-up potential
  • Carbon conversion almost 100%
41
Q

List the 3 disadvantages of entrained flow gasifiers

A
  • Short residence time so fine particles required which is not easy with biomass
  • Limited materials selection
  • Molten ash shortens the life of the gasifiers refactory lining as biomass has a generally high alkali metals content
42
Q

Why is syngas cleaning carried out?

A
  • Gas quality requirements are very high - for turbines and liquid fuel synthesis in particular
  • Alkali metal content is an issue
  • Tar is the biggest problem with gasification
43
Q

Which contaminant from gasification causes what problem?

A

Particulates - erosion
Alkali metals - hot corrosion
Nitrogen and chlorine compounds - NOx formation and corrosion
Tars - clogged filters, internal deposits and blockages
Sulphur compounds - corrosion, emissions

44
Q

What does the level of contamination of the syngas depend on?

A

The feedstock and the gasification process

45
Q

Draw out the BFD for hot gas cleaning

A

Drawing 30

46
Q

What is the advantage of hot gas cleaning over cold gas cleaning?

A

HGCU (hot gas cleaning unit) increases the overall energy efficiency by 10% by reducing heat loss as latent heat associated with quenching in cold gas cleaning

47
Q

What technologies are currently used in hot gas cleaning?

A

Metal oxide sorbents for sulphur removal and ceramic filters for particulate removal. Molten tin can also be used for both of these things.

48
Q

What are the environmental requirements for hot gas cleaning?

A
  • Regulations on acid rain SO2, NOx and HCl emissions
  • Regulations on GHG effects (CO2)
  • Regulations on particulate matter emissions (fly ash, trace components and metals)
49
Q

What ppm should particulates and sulphur be reduced to to protect the gas turbine?

A

Particulates: 2ppm (<10 mm)

Suplhur (H2S, COS, CS2): 20ppm

50
Q

What are the 6 options for particulate control in hot gas cleaning and what is the disadvantage to each?

A
  • Ceramic candle filters (susceptible to thermal shock and broken filter elements (low mechanical strength))
  • Metal candle filters (corrosion by oxidation)
  • Fabric filters (low filtration velocity (5cm/s) and long-term durability)
  • Granular filters (low operating temperatures)
  • Cyclones (low efficiency for smaller size particles)
  • ESPs (high energy loss and temperature limitations)
51
Q

What is tar?

A

A thick, black, highly viscous liquid that condenses in the low temperature zones of a gasifier

52
Q

How is tar formed?

A

Through depolymerisation during pyrolysis stage of gasification. When biomass is fed into the gasifier it first undergoes pyrolysis between 200-500 deg C, the same temperature range where cellulose, hemicellulose and lignin break down into primary tar. Then above 500 deg C the primary tar breaks down into lighter, non-condensable gases (CO2, CO, H2O) and heavier molecules called secondary tar. Even at higher Ts, primary tar is destroyed and tertiary products are formed.

53
Q

What is the chemical composition of tar?

A

It is a mixture of hydrovarbons; mainly benzene, toluene and napthalene

54
Q

Does tar from biomass have any commercial use and why?

A

No because it is mainly ocygenated

55
Q

List the types of gasifier by their amount of tar production from lowest to highest

A

Entrained flow, downdraft, BFB, CFB, updraft

56
Q

Where in the process can tar be removed and how?

A
  • In-situ (primary): by changing the operating conditions of the gasifier (higher T) and residence time so tar formation is reduced, using catalytic cracking and reforming (addition of a catalyst to the fluidised bed) or modification of the gasifier design
  • Postgasification (secondary): physical tar removal from the product gas, or thermal cracking
57
Q

How is tar removal by catalytic cracking and reforming carried out?

A

At 800-900 deg C using dolomite (MgCO3 and CaCO3), nickel-based and other catalysts, and conversion rates of over 99% have been achieved

58
Q

What are the disadvantages of tar removal by catalytic cracking and reforming?

A

Catalyst deactivation and contamination (especially metal catalysts), and much more R&D is needed

59
Q

How is tar removal by physical removal carried out?

A

Tar is condensed before separation by cyclones, barrier filters, wet ESPs, wet scrubbers or alkali salts, with an efficiency of anywhere between 20-97%

60
Q

How is tar removal by thermal cracking carried out?

A

High Ts (around 1200 deg C) are used without a catalyst to break down the tar

61
Q

What is syngas conditioning and why is it carried out?

A

It is the adjustment of the composition of the syngas to provide the right ratio of H2 and CO for the intended use. It is usually required for fuels/chemicals production (eg. ammonia/Fischer-Tropsch fuels)

62
Q

What are the three methods of syngas conditioning?

A

Reforming, water-gas-shift (WGS) reaction and CO2 removal

63
Q

Give the methane forming reaction

A

Drawing 31

64
Q

What temperature is syngas reforming carried out at?

A

800 deg C

65
Q

How does reforming conditioning of syngas work?

A

The light hydrocarbons and methane in the gasification product gas (considered inert for the synthesis process) can be converted into H2 and CO in a reformer, otherwise the chemical energy in these compounds is lost for fuel synthesis.

66
Q

Give the water gas shift reaction

A

Drawing 32

67
Q

How does WGS conditioning of syngas work?

A

The H2/CO ratio of the syngas is increased from 0.7-1.8 to closer to the stoichiometric ratio for most fuel synthesis processes (eg. Fisher-Tropsch is H2/CO=2) by the addition of water and heat

68
Q

What temperature is WGS conditioning of syngas carried out at?

A

200-300 deg C

69
Q

How does CO2 capture conditioning of syngas work?

A

Removal of CO2 increases the yield of liquid fuels in the same way as the removal of nitrogen, as it is considered inert. WGS increases the CO2 content so then it needs to be removed to improve yield.

70
Q

Give the 3 main methods of CO2 removal from syngas

A
  • Chemical absorption (amines, MEA)
  • Physical absorption (methanol, eg. Rectisol)
  • Adsorption (activated carbon)
71
Q

What are the 4 main drivers for the co-gasification of biomass and coal?

A
  • Coal is the world’s most plentiful fossil fuel (but has serious impact on the environment)
  • Can get a compromise between increasing energy content of the fuel and the reducing the net CO2 emissions
  • Availability of a continuous biomass supply can be a problem so coal can fill in the gaps and also allow biomass to benefit from the economies of larger scale processing
  • Prevents the formation of corrosive alkali chloride compounds typical of biomass gasification
72
Q

List the 7 issues with co-gasification

A
  • Elemental and proximate analysis of coal is much different from that of biomass (higher hydrogen and oxygen content in biomass)
  • Biomass is highly variable and heterogeneous
  • Biomass generally has a higher moisture
  • Biomass is less brittle and more fibrous than coal which results in significantly different millling/grinding characteristics and requirements
  • Ash in biomass is higher K, Ca, Si so increased risk of fouling and corrosion
  • Heat output could be reduced when biomass replaces coal
  • Fire and explosion risks from combustible dust from coal
73
Q

Draw the 3 set-up options for co-gasification of biomass and coal.

A

Drawing 33

74
Q

What are the 3 set-up options for co-gasification of biomass and coal?

A

Indirect co-firing (biomass is gasified prior to joining milled coal in the boiler)
Direct co-firing (biomass joins coal pre-milling)
Parallel co-firing (biomass is fired in a separate boiler and the steam produced joins the steam from the coal boiler, with any excess purged)

CPE406 Energy from Biomass and Waste (8 decks)

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