L6 - Biomass and waste Flashcards
1
Q
What is biomass?
A
- Energy derived from recently living matter. Does not include fossil fuels
2
Q
What does biomass do?
A
- Collects, stores and releases energy from the sun via photosynthesis.
3
Q
How do you obtain energy from biomass?
A
- Combustion
- Conversion to biofuels then combustion or further conversion
4
Q
Why do we use biomass?
A
- Can reduce greenhouse gas emissions
- Biomass feedstock and biofuels allow energy to be stored (unlike some other renewable energies)
- Biomass economics may be attractive if fossil fuel prices increase
5
Q
What are the technical challenges of using biomass?
A
- Low energy density (high cost of storage and transport)
- High water content (requires energy to move)
- It is biodegradeable
- Disperse feedstocks so is difficul to transport
- Conversion technologies are often less efficient and smaller scale than conventional energy conversion processes
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
7
Q
Types of biomass for bioenergy
A
- Traditional biomass - most dominant. Consists of wood, peat (which takes centuries to form)
- Energy crops - Wood for charcoal, sugar cane, plants with oily seeds etc
- Plant residue (waste) - wood, straw, rice husks, used veg oil
- Animal residue - manure, poultry litter
- Domestic waste (especially biodegradeable waste) - MSW, landfill gas, sewage
8
Q
Types of biofuels
A
- Solid fuels
- Wood (crop and forest residue)
- Charcoal obtained from the pyrolysis of wood - Biogas
- Obtained by gasification or anaerobic digestion of wastes
- Can be combusted or further converted - 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
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
10
Q
Pros of bioenergy for sustainable development
A
- Reduce greenhouse gas emissions
- Sustainable bioenergy could contribute 25-33% global primary energy supply in 2050.
- Future conversion tech offer efficient and low cost - Increase social and economic development in rural communities
- Energy security
- Management of resources and wastes
11
Q
Challenges of sustainable development for bioenergy
A
- Require high yield and reliable supply of feedstock
- Competition for land
- Needs to be cost-competitive
- Need more efficient and cleaner conversion of more diverse range of feed stocks (technological innovation)
- Logistics, infrastructure and stakeholder challenges
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
- Conversion by thermo-chemical, physiochemical or biological routes
- Direct combustion
- Co firing (combustion with other fuels eg fossil fuels)
- Gasification
- Pyrolysis
- Biochemical converstion
- Anaerobic digestion
- Fermentation - Chemical conversion
- Transesterification
- Hydrogenation
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
14
Q
Higher heating value
A
Energy released per unit of fuel combusted if steam is condensed and heat is recovered
15
Q
Lower heating value
A
Ignores heat recovered by condensing steam
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
- Fixed bed
- Fuel lies on grate, grate moves to allow ash removal.
- Reliable and relatively inexpensive
- Can only handle limited range of biomass feedstocks - 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 - Dust firing
- Combustion takes place with dust (fineparticles eg sawdust) suspended in air
18
Q
Combustion: biomass co-firing
A
- 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
- 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 - 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
- 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
- Gas mainly CO and H2, can also contain CO2 and methane.
- Energy density is 50% of natural gas. Burnt as fuel for heat or power
- Syngas can produce liquid fuels in Fischer-Tropsch reaction. (biomass to liquids - BTL)
22
Q
Pyrolysis
A
- 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 - 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.
- Good for energy storage (high energy dense) - can fuel boilers, diesel engines and gas turbines for elec or CHP generation
- Bioenergy generation and energy consumption become independent
24
Q
Chemical conversion
A
- Vegetable oils
- Oily seeds, recovered by crushing and solvent extraction
- Similar energy content to diesel - Transesterification
- Vegetable oils react with alcohol to form esters (biodiesel) which is used in diesel engines - Veg oils also supply food and cosmetic markets
25
Q
Biochemical converstion
A
- Anaerobic digestion
- Bacteria break down organic matter into sugars and acids then to gas - Fermentation
- Micro-organisms convert sugars to ethanol
- Ethanol can be separated to provide liquid fuel - 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
- Local and global environmental impacts:
- Particulates and gaseous emissions, solid waste (ash), demand for local water resources, noise, odour, increased levels of traffic - Production chains may benefit local env
- Reduced erosion and nutrient run-off from agricultural land, disposal of waste products, increased biodiversity. - Main benefit is provision of low-carbon energy.
- Alternative to fossil fuel-based energy. Opportunities to reduce emissions - 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
- Water requirements high (higher than conventional crops)
- Fertiliser needed (lower than conventional crops)
- Perennial energy crops may help avoid erosion and nutrient run off
- bioenergy waste products such as ash can be used as fertiliser
29
Q
Bioenergy economics
A
- Energy from waste economically competitive as feedstock costs low
- 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 - Low energy density of biomass feedstocks means more needed
- Economic incentives and policies may increase competitiveness
30
Q
Summary of bioenergy
A
- Benefits–reduced greenhouse gas emissions; reduced reliance on fossil fuels
- Challenges–dispersed, bulky, biodegradable feedstocks–complex stakeholder network•
- Sustainability issues–competition for land use with food
- Technology issues–technologies not all commercially established or highly efficient•
- Future prospects–broad agreement on bioenergy will be important; political and economic incentives will be needed
31
Q
Waste summary
A
- Landfill is traditionally used for disposal of non-hazardous waste
- Incineration with energy recovery now the primary treatment route for household MSW
- Heating value of waste can be large
- Many methods of transforming into useful heat and power
- Environmental emissions a major concern
