Bioenergy - Tutorial

Question 1

Bioenergy is often described as “carbon neutral”. What does this mean in theory and in practice where do carbon emissions occur?

Answer 1

Main interest in bioenergy is ability to reduce CO2emissions and substitute fossil fuels.
Whether bioenergy is carbon neutral depends on the absorption and emissions of CO2 over the whole bioenergy system (including conversion, transport, fertilisers).
Life cycle assessment shows that while not quite carbon neutral bioenergy has lower CO2emissions per unit of electricity than fossil generation.

Question 2

What range of energy return on investment is feasible for bioenergy systems?

Answer 2

?

Question 3

Describe why only a small fraction of solar radiation is converted into available plant biomass. Using a Sankey diagram (or otherwise) provide an approximate model of the losses at each stage. What is a typical ‘leaf to biomass’ efficiency?

Answer 3

• UK annual solar energy 1000 kWh/m2
• ~12% of energy reaches plant and in correct season
• 50% of radiation at the ‘right’ wavelengths for photosynthesis
• 85% is captured by plant
• 21% converted into energy
• 60% retained after plant respiration
• ~⅔% solar energy converted to biomass

Question 4

Bioenergy fuels have higher or lower heating values (gross or net calorific values). What is the difference and what conditions are required to use them?

Answer 4

Heating value is the energy released when fuel is burned, typically measured on a mass (e.g. per kg) or molar (e.g. per mol) basis. Higher heating value
(HHV – or gross calorific value) assumes water vapour in the products, while lower heating value (LHV – or net calorific value) assumes liquid water in the products. Most analysis will use of the heating values, even in cases where the water in the products includes a
mix of liquid water and water vapour. It is, therefore, important to confirm whether efficiency estimates etc are calculated on a LHV or HHV basis.

Question 5

Review typical heat content for a range of bioenergy following fuels (MJ/kg). How do they tend to compare to fossil fuels? Why?

Answer 5

Common for bioenergy to have lower heat content. One important factor is composition and, particularly, water content.

Question 6

Explore typical yields (tonne/hectare/year, on a dry basis) for a range of bioenergy crop types. What factors will affect achievement of these values?

Answer 6

• Sugar cane 35
• Cereals 10
• Wood (temperate zones) 10
• Wood (tropical zones) 20

A broad range of factors can influence yield including planting method, species, site conditions and weather conditions.

Question 7

What are the main advantages and disadvantages of bioenergy?

Answer 7

Advantages:

• Large supply - particularly in tropical zones.
• Variety of sources.
• Variety of uses (transport, heat etc)
• Efficient use of ‘waste’ products.
• Builds off established agriculture/forestry.
• Encourages integrated farming.
• Boosts skilled agro industry.
• Environmental improvement.
• Integrated and efficient systems minimise water/air pollution.
• Rural development.
• Economic diversification and development.
• Potential greatest in developing nations.

Disadvantages:

• Risk of soil infertility/erosion.
• Overexploitation - depletion of biomass stocks.
• Competition with food production.
• Bulky material handling and transport needs.
• Inappropriate development of genetically modified organisms.
• Risk of pollution through poor process control.
• Disruption from large scale agricultural industry.
• Foreign investment vs local/national benefit.

Question 8

List the positive and negative environmental impacts of woody biomass.

Answer 8

Positive:

• Carbon reduction (Treated as carbon neutral because CO2 removed from atmosphere as the plant grew, therefore less CO2 emissions than fossil-based fuels).
• Could be combined with CCS for negative emissions.
• Likely to use short rotation composting, which tends to be good for local biodiversity.
• Tackles acid rain through NOx SOx reduction.

Negative
- Dispatchability (reducing the use of peaking plants that is good for carbon reduction.
- Land use (relatively large amounts of land required). In UK context particularly challenging.
If replacing original forest, might reduce biodiversity.

Question 9

What are the different advantages for Landfill compared to Waste to Energy?

Answer 9

Landfill:
No NOx and SOx emissions or CO2.
Less infrastructure.
Taking away focus from recycling.
Landfill methane can be captured and used to generate electricity.

WtE:
Less land use.
Methane gas is environmentally harmful from landfill. 
No chemical leakage into ground. 
Electricity and heat as byproduct.
Ash byproducts.