3 Waste To Energy Technologies

Question 1

Describe the concept of embodied energy. {2}

Answer 1

Embodied energy is the name given to the collective sum {1} of all energy required to produce any material, product or service {1}.

Question 2

Discuss the advantages and disadvantages of recovering energy from waste by incineration.
Reference env economic and security of supply {9}

Answer 2

• Reduce volume of waste sent to landfill significantly (by approx. 90%).
• There is a negative public perception about the emissions and the
technologies involved (NIMBY) – Not In My Back Yard.
• Residue from flue gas treatment process or from bottom ash may go to landfill as hazardous waste.

• Incineration plants can generate lots of electricity and can also provide local district heating (especially of close to urban areas).
• Energy from waste reduces the amount of landfill tax that has to be paid by business.
• Bottom ash residue may be recycled for building materials.
• Incineration plants are costly and so need high utilisation to be cost
effective.

• To be effective energy from waste plants need a secure supply.
• Minimum or guaranteed tonnage may be needed by operator to cover
costs.
• Risks to supply due to increased exports of waste to countries with lower gate fees.
• Risk to supply due to illegal activity regarding gate fees.

Question 3

Low solids

Answer 3

Low solid digestion is where the feedstock is around 15% solid material

Question 4

High solids

Answer 4

High solid digestion is where the feedstock is around 55% solid material

Question 5

Residence time

Answer 5

The time taken for the full degradation of the material in an aerobic digestion system.

Question 6

Single stage

Answer 6

The biological reactions occur in one holding tank and the biogas comes from this tank.

Question 7

Multistage anaerobic digester

Answer 7

Hydrolysis, acetogenesis and acidogenesis occurs in one tank whilst the methanogenesis which produces the biogas occurs in a separate tank.

Question 8

Explain how anaerobic digestion results in the production of biogas. {4}

Hydrolysis
Acidogenesis
Acetogenesis
Methanogenesis

Answer 8

Hydrolysis – insoluble polymers broken down to soluble organic compounds. {1}
Acidogenesis – soluble organic compounds fermented into volatile fatty acids and alcohols. {1}
Acetogenesis – alcohols and fatty acids converted into acetic acid and hydrogen. {1}
Methanogenesis – bacteria convert acetic acid and hydrogen into methane and carbon dioxide / biogas. {1}

Question 9

Mechanical pretreatment

Answer 9

Filtering and separation to size of material in feedstock where it is processed into thick liquified form

Question 10

Outline how anaerobic digestion can deliver both heat and power. {2}

Answer 10

Biogas from an AD plant can be burnt in a combined CHP plant, this generates power (electricity) and heat. {1}
Instead of losing the heat, as in traditional power plants, it is diverted into a local heating system. {1} This is co-generation.

Question 11

Home composting is encouraged by many local councils. Name three types of waste product which can be composted in a domestic situation. {3}

Answer 11

• Lawn clippings.
• Shredded stalks.
• Vegetable peelings.
• Hedge clippings.
• Cut flowers.
• Tea bags.
• Leaves.
• Eggshells.

Question 12

Explain briefly why composting is unsuitable for treating any form of catering waste. {1}

Answer 12

Cooked food must never be used in composting because it will attract vermin.