2.3 waste to energy technologies

Question 1

Discuss the advantages and disadvantages of recovering energy from waste by incineration. Your answer should make specific reference to the following issues associated with energy recovery from waste incineration;

 Environmental Issues.
 Economic issues.
 Security of supply issues. {9}

The quality of written communication will be assessed in this question.

Answer 1

Answer;
Indicative content;

Environmental Issues;

 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.

 Stack emissions (flue gases) can be treated to by ‘scrubbing’ or filtering or electrostatic precipitation.

Economic issues;

 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.

Security of Supply Issues.

 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.

All relevant, valid responses will be given credit

Question 2

Explain the following key terms with anaerobic digestion. {5}

1 – Low solids;

2 – High solids;

3 – Residence time;

4 – Single-stage;

5 – Multi-stage;

Answer 2

Answer;

Low Solids - Low solid digestion is where the feedstock is around 15% solid material. {1}

High solids - High solid digestion is where the feedstock is around 55% solid material. {1}

Residence time - The time taken for the full degradation of the material in an aerobic digestion system.{1}

Single stage - The biological reactions occur in one holding tank and the biogas comes from this tank. ` {1}

Multistage - Hydrolysis, acetogenesis and acidogenesis occurs in one tank whilst the methanogenesis which produces the biogas occurs in a separate tank. {1}

Question 3

Question; ‘The diversion of biodegradable waste to anaerobic digestion can reduce greenhouse gas emissions from landfill.

For e.g. capturing the biogas from one tonne of food waste will save between 0.5 and 1.0 tonne of CO2 equivalent.

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

Answer 3

Answer;

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 4

Outline how anaerobic digestion can deliver both heat and power.

Answer 4

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 5

Outline how anaerobic digestion can be used to provide both heat and power (CHP). {2}

Answer 5

Answer;

Biogas from the AD plant can be burnt in a CHP plant to generate power (electricity) and heat {1}; Instead of losing heat, as in a traditional power plant, it is diverted into local heating systems {1}.

Question 6

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

Answer 6

Answer;

Any three from; 
 Lawn clippings. 
 Shredded stalks. 
 Vegetable peelings. 
 Hedge clippings. 
 Cut flowers. 
 Tea bags. 
 Leaves. 
 Eggshells.

Question 7

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

Answer 7

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

Question 8

Define Embodied energy

Answer 8

This is the name given to the collective sum of all the required energy needed to produce any material, product or service.

Question 9

What substances cannot be put into a Composting Bin/Heap:

Answer 9

Diseased plants
Plastics 
Glass 
Cooked food 
Weeds 
Coloured glossy paper 
Pet droppings

Question 10

What are the three types of organisms involved in the composting process:

Answer 10

Fungi
Bacteria
Actinomycetes (Bacteria that branch)