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Sustainable Energy Systems > L6 - Biomass and waste > Flashcards

L6 - Biomass and waste Flashcards

1
Q

What is biomass?

A
  • Energy derived from recently living matter. Does not include fossil fuels
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2
Q

What does biomass do?

A
  • Collects, stores and releases energy from the sun via photosynthesis.
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3
Q

How do you obtain energy from biomass?

A
  • Combustion

- Conversion to biofuels then combustion or further conversion

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4
Q

Why do we use biomass?

A
  1. Can reduce greenhouse gas emissions
  2. Biomass feedstock and biofuels allow energy to be stored (unlike some other renewable energies)
  3. Biomass economics may be attractive if fossil fuel prices increase
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5
Q

What are the technical challenges of using biomass?

A
  1. Low energy density (high cost of storage and transport)
  2. High water content (requires energy to move)
  3. It is biodegradeable
  4. Disperse feedstocks so is difficul to transport
  5. Conversion technologies are often less efficient and smaller scale than conventional energy conversion processes
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6
Q

What are the stakeholder challenges of biomass?

A
  • Due to tech being less efficient, it has an impact on agriculture, agro-industry, forestry, waste management, distribution networks, regulators and communities
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7
Q

Types of biomass for bioenergy

A
  1. Traditional biomass - most dominant. Consists of wood, peat (which takes centuries to form)
  2. Energy crops - Wood for charcoal, sugar cane, plants with oily seeds etc
  3. Plant residue (waste) - wood, straw, rice husks, used veg oil
  4. Animal residue - manure, poultry litter
  5. Domestic waste (especially biodegradeable waste) - MSW, landfill gas, sewage
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8
Q

Types of biofuels

A
  1. Solid fuels
    - Wood (crop and forest residue)
    - Charcoal obtained from the pyrolysis of wood
  2. Biogas
    - Obtained by gasification or anaerobic digestion of wastes
    - Can be combusted or further converted
  3. Liquid biofuels
    - Bio-ethanol obtained by fermentation
    - Volatile products of pyrolysis may be condensed to form bio-oil for combustion or further refining
    - Oil obtained from seeds and used directly or converted to biodiesel
    - Liquid fuels, such as methanol
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9
Q

Problems with food competition

A
  • Impact on availability, access, stability and utilisation of food.
  • Increased competition for land/water = higher and less stable food prices

However can create new employment for rural areas

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10
Q

Pros of bioenergy for sustainable development

A
  1. Reduce greenhouse gas emissions
  2. Sustainable bioenergy could contribute 25-33% global primary energy supply in 2050.
    - Future conversion tech offer efficient and low cost
  3. Increase social and economic development in rural communities
  4. Energy security
  5. Management of resources and wastes
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11
Q

Challenges of sustainable development for bioenergy

A
  1. Require high yield and reliable supply of feedstock
  2. Competition for land
  3. Needs to be cost-competitive
  4. Need more efficient and cleaner conversion of more diverse range of feed stocks (technological innovation)
  5. Logistics, infrastructure and stakeholder challenges
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12
Q

Conversion technologies

A

The type of biomass or waste and desirbed form of energy (heat, power, liquid fuel etc) determines which tech to apply

  1. Conversion by thermo-chemical, physiochemical or biological routes
  2. Direct combustion
  3. Co firing (combustion with other fuels eg fossil fuels)
  4. Gasification
  5. Pyrolysis
  6. Biochemical converstion
    - Anaerobic digestion
    - Fermentation
  7. Chemical conversion
    - Transesterification
    - Hydrogenation
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13
Q

Combustion

A
  • Biomass burned as fuel. Converts plant matter into CO2, h2o and residue
  • Amount of energy determined by heats of formation (making and breaking of chemical bonds)
  • CO2 released but not new source. = CO2 emissions close to 0.

Fuel + O2 -> CO2 + H2O + energy

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14
Q

Higher heating value

A

Energy released per unit of fuel combusted if steam is condensed and heat is recovered

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15
Q

Lower heating value

A

Ignores heat recovered by condensing steam

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16
Q

What is the effect of high water content in biomass

A
  • Adds mass but no energy content
  • Water needs to be heated to the combustion temp and evaporated. Energy is consumed in this process.
  • For green fuels, a significant amount of heat may be recovered for condensing the steam
17
Q

Direct combustion technologies

A
  1. Fixed bed
    - Fuel lies on grate, grate moves to allow ash removal.
    - Reliable and relatively inexpensive
    - Can only handle limited range of biomass feedstocks
  2. Fluidised bed
    - Suspension of solid biomass and hot inert granular bed material in ai rflow
    - Good mixing facilitates combustion of a wide range of biomass fuel types
    - High investment and operating costs
  3. Dust firing
    - Combustion takes place with dust (fineparticles eg sawdust) suspended in air
18
Q

Combustion: biomass co-firing

A
  1. Mixture of biomass and fossil fuel eg pulverised coal.
    - Uses infrastructure, efficiency and economy of large fossil fuel power stations
    - Up to 40% biomass, typically 3-5%
19
Q

Gasification

A
  1. Partial oxidation at high temp of carbon-rich feedstock into a gaseous fuel
    - Air, oxygen or steam used to provide oxygen
    - Oil tar and inorganic residue
  2. Gas products:
    - hydrogen, CO, methane, CO2, vapour and heavier hydrocarbons
    - May need tar removed
    - Gas reformed to produce biofuels eg methanol
    - Gas can be burned in gas turbine to provide power (high efficiency, low cost)
20
Q

Gasifier tech

A
  1. Fixed and fluidised beds

Fluidised bed are preferred as more flexible with fuel type and wetness and better at cracking tar.

21
Q

Syngas from biomass (gasification)

A
  1. Gas mainly CO and H2, can also contain CO2 and methane.
  2. Energy density is 50% of natural gas. Burnt as fuel for heat or power
  3. Syngas can produce liquid fuels in Fischer-Tropsch reaction. (biomass to liquids - BTL)
22
Q

Pyrolysis

A
  1. Thermal decomposition in the absense of oxygen.
    - Products are bio-oil, combustible gases etc
    - Proportions of S, L and G products effected by temp and residence time
  2. Fast pyrolysis and flash pyrolysis aim to increase yield of bio-oil to 75-80%
23
Q

Bio-oil advantages

A

Fuel product of pyrolysis.

  1. Good for energy storage (high energy dense) - can fuel boilers, diesel engines and gas turbines for elec or CHP generation
  2. Bioenergy generation and energy consumption become independent
24
Q

Chemical conversion

A
  1. Vegetable oils
    - Oily seeds, recovered by crushing and solvent extraction
    - Similar energy content to diesel
  2. Transesterification
    - Vegetable oils react with alcohol to form esters (biodiesel) which is used in diesel engines
  3. Veg oils also supply food and cosmetic markets
25
Q

Biochemical converstion

A
  1. Anaerobic digestion
    - Bacteria break down organic matter into sugars and acids then to gas
  2. Fermentation
    - Micro-organisms convert sugars to ethanol
    - Ethanol can be separated to provide liquid fuel
  3. Bio-photochemical routes
26
Q

Anaerobic digestion

A

Organic matter converted to sugars and acids, then to gas by bacteria.

  • Feed is typically sewage, sludge, animal wastes
  • Energy produced is 50% of energy content of feed
  • High capital cost but negative cost feedstocks make large scale operation viable
27
Q

Environmental impacts of bioenergy

A
  1. Local and global environmental impacts:
    - Particulates and gaseous emissions, solid waste (ash), demand for local water resources, noise, odour, increased levels of traffic
  2. Production chains may benefit local env
    - Reduced erosion and nutrient run-off from agricultural land, disposal of waste products, increased biodiversity.
  3. Main benefit is provision of low-carbon energy.
    - Alternative to fossil fuel-based energy. Opportunities to reduce emissions
  4. Sustainable energy source should not deplete primary resources
    - Some consumption of non-renewable resources in production, transport and pre-treatment of the feedstock
28
Q

Agricultural impact of bioenergy

A
  1. Water requirements high (higher than conventional crops)
  2. Fertiliser needed (lower than conventional crops)
  3. Perennial energy crops may help avoid erosion and nutrient run off
  4. bioenergy waste products such as ash can be used as fertiliser
29
Q

Bioenergy economics

A
  1. Energy from waste economically competitive as feedstock costs low
  2. Other bioenergy routes not yet economic compared to conventional
    - this can improve with the development of high yield biomass feedstocks
    - more efficient and economic conversion processes
  3. Low energy density of biomass feedstocks means more needed
  4. Economic incentives and policies may increase competitiveness
30
Q

Summary of bioenergy

A
  1. Benefits–reduced greenhouse gas emissions; reduced reliance on fossil fuels
  2. Challenges–dispersed, bulky, biodegradable feedstocks–complex stakeholder network•
  3. Sustainability issues–competition for land use with food
  4. Technology issues–technologies not all commercially established or highly efficient•
  5. Future prospects–broad agreement on bioenergy will be important; political and economic incentives will be needed
31
Q

Waste summary

A
  1. Landfill is traditionally used for disposal of non-hazardous waste
  2. Incineration with energy recovery now the primary treatment route for household MSW
  3. Heating value of waste can be large
  4. Many methods of transforming into useful heat and power
  5. Environmental emissions a major concern

Sustainable Energy Systems (12 decks)

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