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CPE406 Energy from Biomass and Waste > Biofuels via Biochemical Conversion > Flashcards

Biofuels via Biochemical Conversion Flashcards

1
Q

Define biorefinery

A

The sustainable processing of biomass into a spectrum of marketable bio-based products and Bioenergy

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2
Q

What is the aim of biorefinery?

A

To maximise biomass value by producing multiple products analogous to a petroleum refinery

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3
Q

Give 3 ways in which value is maximised in a biorefinery and the direct benefit of each

A
  1. Production of one or several low-volume, high-value chemicals (enhance profitability)
  2. Production of low value, but high-volume liquid transportation fuels (meet national energy needs)
  3. Generation of electricity and process heat for its own use and perhaps enough for sale (reduce costs and GHG emissions)
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4
Q

Give two examples of biorefineries in the UK

A
  1. Ensus (feedstock wheat (1st gen), produces >400,0000 m3 bioethanol/yr, co-products: animal feed and CO2)
  2. Beta Renwables, Italy (feedstock: non-food biomass (2nd gen), first of its type, 40,000 tons bioethanol/yr)
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5
Q

When did bioethanol become popular and why?

A

In the oil crisis of the 70s, and there has been increased interest ever since and it is the most extensively used biofuel in transportation worldwide.

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6
Q

What percent of transportation fuel does bioethanol make up in Brazil and the US?

A

30% and 10% repectively

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7
Q

What is octane number and is it low or high in bioethanol?

A

A measure of gasoline quality for prevention of early ignition, which leads to cylinder knocking.
It is high in bioethanol which permits the rising of the compression ratio and results in lower emissions

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8
Q

Give 6 disadvantages of bioethanol

A
  1. Lower energy density than gasoline
  2. Corrosive
  3. Low vapour pressure (difficult cold starts)
  4. Miscibility with water
  5. Toxicity to ecosystems
  6. Vapour pressure increases (and therefore evaporative emissions) when blended with gasoline
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9
Q

What are the most common blends of bioethanol with gasoline?

A

10% by volume bioethanol, 90% gasoline (E10)

In the EU E5 is used

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10
Q

Do engines need to be modified for E5 or E10 use?

A

No

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11
Q

Up to what percentage bioethanol blend can be used with engine modifications?

A

E85

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12
Q

Which are the best and worst biomass sources to use for with respect to GHG emission savings, and how much energy do these feedstocks yield compared to that which is used to make the bioethanol?

A

Corn offers limited savings (18%) (20-30% more energy than is used to make it)
Sugarcane and cellulosic are good (almost 90% lower emissions) (9x more energy than is used to make it)

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13
Q

Name 4 sucrose-containing feedstocks and whether they are 1st or 2nd gen biomass

A

1st gen

  1. Sugarcane
  2. Sugarbeet
  3. Sweet sorghum
  4. Fruits
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14
Q

Name 5 starch feedstocks and whether they are 1st or 2nd gen biomass

A

1st gen

  1. Corn
  2. Wheat
  3. Rice
  4. Potatoes
  5. Barley
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15
Q

Name 3 lignocellulosic feedstocks and whether they are 1st or 2nd gen biomass

A

2nd gen

  1. Wood
  2. Straw (rice straw, wheat straw)
  3. Grasses (miscanthus)
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16
Q

What bioethanol feedstock is mosst commonly used in Brazil?

A

Sugarcane, as they produce about 31% of the global production (nearly 9 million hectares)

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17
Q

What bioethanol feedstock is mosst commonly used in the US?

A

Corn - production is abundant in the midwestern states

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18
Q

What bioethanol feedstock is mosst commonly used in Europe?

A

Wheat, barley and sugar beet

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19
Q

Where has lignocellulosic feedstock started being produced on a large scale?

A

Italy

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20
Q

What is the main feedstock issue with bioethanol production?

A

Availability - changes from season to season and depends on location

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21
Q

Give 15 characteristics that can vary between different feedstocks that affect the process

A
  1. Chemical composition of the biomass
  2. Cultivation practice
  3. Availability of land and land use practice
  4. Use of resources
  5. Energy balance
  6. GHG emissions
  7. Absorption of minerals to water and soil
  8. Injection of pesticides
  9. Soil erosion
  10. Contribution to biodiversity
  11. Farm-gate prince of the biomass
  12. Logistic costs
  13. Economic value from co-products
  14. Creation or maintenance of employment
  15. Water requirements and water availability
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22
Q

Why is lignocellulosic biomass the most promising feedstock for 2nd gen biofuels?

A

It’s great availability and low cost

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23
Q

What is a key factor of conversion efficiency for lignocellulosic biomass?

A

Chemical composition

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24
Q

What is cellulose?

A

A linear polymer of glucose that forms the major component of plant biomass (30-60% dry basis). The orientation of the linkages and additional hydrogen bonding make the polymer rigid and difficult to break.

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25
Q

What is hemicellulose?

A

Highly branched polymers of 5 (pentoses eg. xylose and arabinose) and 6 (hexoses eg. galactose, glucose and mannose) carbon sugars that forms 20-40% of plant biomass.

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26
Q

Is cellulose or hemicellulose more readily hydrolysed?

A

Hemicellulose due to its branched and amorphous nature

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27
Q

What are the dominating sugars in hemicellulose in both soft woods and hard woods?

A

Mannose and xylose respectively

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28
Q

What is lignin?

A

An aromatic polymer synthesised from phenylpropane precursors that forms 15-25% of plant biomass.

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29
Q

Is lignin converted during biochemcial conversion and what does this mean for biofuels?

A

No, so cannot be used for bioethanol production. It makes lignocellulosic biomass resistant to both chemical and biological degradation

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30
Q

What are the 4 basic steps of fermentation for biofuels from lignocellulosic biomass and which is the most expensive??

A
  1. Pre-treatment (most expensive)
  2. Hydrolysis
  3. Fermentation
  4. Product separation/distillation
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31
Q

What is the aim of biomass pre-treatment for hydrolysis and fermentation?

A

To make sugar polymers (cellulose and hemicellulose) more accessible to hyrolytic enzymes for its conversion to sugar monomers by producing cellulosic fibre for enzymatic attack in the hydrolysis step.

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32
Q

Name the 3 main methods for biomass pre-treatment for hydrolysis and fermentation

A
  1. Physical (milling/grinding)
  2. Physico-chemical (steam explosion/ammonia fibre explosion)
  3. Chemical (acid)
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33
Q

What biomass size is expected after chipping and grinding and what is the expected power input required to get hardwood and corn stover to 1.6 mm?

A

10-30 mm after chipping and 0.2-2 mm after milling
Hardwood - 130 kWh/ton
Corn stover - 14 kWh/ton

34
Q

What are the 3 disadvantages of physical pre-treatment for hydrolysis and fermentation?

A

It is time consuming, energy intensive and expensive

35
Q

Describe the principle of physico-chemical steam explosion pre-treatment for hydrolysis and fermentation

A

Chipped biomass is treates with high pressure saturated steam and then pressure is reduced so that the biomass undergoes explosive de-composition

36
Q

What is the most commonly used method of pre-treatment for hydrolysis and fermentation?

A

Steam explosion

37
Q

What are the 2 advantages of steam explosion pre-treatment for hydrolysis and fermentation?

A

Lower capital investment and higher energy efficiency

38
Q

What are the 2 disadvantages of acid pre-treatment for hydrolysis and fermentation?

A
  1. Generation of a high amount of inhibitory products such as furfurals, phenolic acids and aldehydes
  2. The corrosive and toxic nature requires a suitable reactor building material
39
Q

What two operating conditions can be used (T and res. time) for acid pre-treatment for hydrolysis and fermentation?

A 
High T (>180 deg C) short duration (1-5 min)
Low T (<120 deg C) long duration (30-90 min)
40
Q

What are the 3 main hydrolysis methods?

A
  1. Dilute acid hydrolysis
  2. Concentrated acid hydrolysis
  3. Enzymatic hydrolysis
41
Q

Describe the principle of hydrolysis of biomass for fermentation

A

Sugar polymers in lignocellulosic feedstock are converted to simple sugars prior to fermentation

42
Q

Define hydrolysis

A

Chemical decomposition in which a compound is split into other compounds by reacting with water

43
Q

State the decomposition of cellulose

A

Cellulose > glucan > hexoses (eg. glucose)

44
Q

State the decomposition of hemicellulose

A

Hemicellulose > pentose > xylan > xylose

Acetyl groups > acetic acid

45
Q

Which acid is used for both dilute and concentrated hydrolysis of lignocellulosic biomass?

A

H2SO4

46
Q

What is the typical temperature, pressure, main disadvantage and sugar recovery efficiency of dilute acid hydrolysis?

A

488K, high P, acid and high T mean special reactor material required (expensive), 50% recovery

47
Q

What is the typical acid concentration, reaction time compared to dilute hydrolysis, 2 critical factors for economic viability and sugar recovery efficiency of concentrated acid hydrolysis?

A

10-30% conc., longer reaction time, up to 100% yield, critical factors are to optimise sugar recovery and cost effectively recover the acid for recycling

48
Q

What is the most common method of hydrolysis?

A

Enzymatic hydrolysis

49
Q

Is enzymatic hydrolysis a slow or fast process?

A

Slow due to the biomass molecular structure

50
Q

How are enzymes produced for enzymatic hydrolysis?

A

By several microorganisms (commonly bacteria and fungi) (expensive)

51
Q

Name 3 factors that affect the success of enzymatic hydrolysis?

A
  1. Enzyme activity
  2. Reaction conditions (T, pH etc.)
  3. Strong product inhibition
52
Q

Describe the principle of fermentation of lignocellulosic biomass

A

Hydrolysate which contains sugar monomers is fermentates using microorganisms (eg. yeast) to ethanol. The standard enzymes cannot ferment xylose (hydrolysed from hemicellulose) so metabolic engineering is required to enhance this.

53
Q

What are the three operation types of fermentation and how is the choice between them made?

A

Batch, fed-batch and continuous. This choice depends on the kinetic properties of the microorganisms and type of lignocellulosic hydrolysate in addition to process economic aspects.

54
Q

What is the typical ethanol concentration of fermentation products?

A

around 5 wt%

55
Q

Give 8 challenges/opportunities of biomass fermentation to produce bioethanol.

A
  1. Low-cost/recyclable chemicals
  2. Short residence times
  3. Allows processing in low-cost vessels
  4. Degrades little or no sugar during pre-treatment
  5. Produces solids that low enzyme loadings can convert to sugars at high yields
  6. Recycle enzymes inexpensively
  7. Reduce production cost of enzymes
  8. Significantly enhance effectiveness
56
Q

Give 3 ways hydrolysis and fermentation can be kept economical?

A
  1. Produce high yields (critical)
  2. Keep feedstock and enzyme costs low
  3. Develop microorganisms that can achieve high product yields without added enzymes
57
Q

What are the biggest capital cost contributors to the hydrolysis and fermentation process?

A

Pre-treatment and hydrolysis

58
Q

What feedstock is required for AD?

A

Anything - not even necessarily lignocellulosic

59
Q

What gases are produced in AD?

A

Methane and CO2

60
Q

How long does an AD process take?

A

A period of weeks

61
Q

What can the solid residue from AD be utilised as?

A

Fertiliser

62
Q

What are 2 disadvantages of AD?

A
  1. Long retention times (20-50 days depending on feedsotck)

2. Low overall degradation efficiency for organic matter

63
Q

Why is AD controlled in a closed reactor?

A

To collect all the gas for utilisation

64
Q

What is the product of AD and how much of it is methane?

A

Biogas, around 60 vol% methane

65
Q

What can methane from biogas be used for?

A

Fuel or chemical feedstock

66
Q

What is the main advantage of AD?

A

Processing of wet biomass and waste

67
Q

What are the 4 stages of AD?

A
  1. Hydrolysis - large, complex polymers like carbohydrates, cellulose, proteins and fats are broken down by microorganisms into simple sugars, amino acids and fatty acids
  2. Acidogenesis - simple monomers are broken down into volatile fatty acids
  3. Acetogenesis - the products of acidogenesis are broken down into acetic acid, releasing hydrogen and CO2
  4. Methanogenesis - bactera produce methane either y cleaving to acetic acid molecules to form CO2 and methane, or by reduction of carbon hydroxides with oxygen
68
Q

Draw a diagram of the AD stages

A

Drawing 53

69
Q

Give 5 uses of biogas

A
  1. Heat only
  2. Electricity only
  3. CHP
  4. Bio-methane injection to a gas grid
  5. Bio-methane for transport
70
Q

How can biogas be used for heat?

A

Can be used as heat to maintain the site (T of digester/nearby buildings), after which the excess can be transferred via hot water to a district heating system which is supported by the Renewable Heat Incentive

71
Q

Give 4 advantages to using biogas for electricity rather than heat

A
  1. More profitable
  2. Biogas requires minimal investment in cleaning and upgrading
  3. Electricity is easier to transmit than heat
  4. Supply is easily measures
72
Q

Give 1 disadvantage to using biogas for electricity rather than heat

A

Electricity storage is not simple as connecting to the electricity grid is costly

73
Q

Why is CHP the most popular option for use of biogas from AD, what percentage goes to heat/power and how efficient does it make the plant?

A

Popular as the AD process requires some heat 35-40% electricity, 40-45% heat, overall efficiencies in excess of 80% at the point of use

74
Q

What further processing does biogas need to be used in a gas grid or for tranport?

A

It need updrading into bio-methane

75
Q

What are the 2 main advantages of using bio-methane over biogas in the gas grid?

A
  1. Far more flexible fuel than biogas

2. Higher energy density

76
Q

What are the 4 main disadvantages of using bio-methane over biogas in the gas grid?

A
  1. There is no specific UK standard for bio-methane
  2. Upgrading adds substantially to the cost and energy requirement
  3. Upgrading can reduce carbon savings
  4. No incentive for grid operators to accept bio-methane
77
Q

What two options are there for injection of bio-methane into the grid?

A
  1. National high pressure

2. Local low pressure

78
Q

Why does biogas need to be upgraded to bio-methane to be used in the UK gas grid?

A

It has to be clean of impurities, dried and have a methane content of >95% so that it resembles the qualities of natural gas

79
Q

What vehicles can bio-methane be used in?

A

Those designed to run on CNG or LPG

80
Q

What are the advantages of using bio-methane in vehicles?

A

Significant GHG savings

Low emissions of local pollutants such as NOx and particulates compared to petrol vehicles

81
Q

What are the current challenges of using bio-methane in vehicles?

A

Few bio-methane vehicles in the UK since infrastructure to supply bio-methane is not common
Only around 15 refuelling sites

CPE406 Energy from Biomass and Waste (8 decks)

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