L8 - Incineration, Gasification, Pyrolysis and Anaerobic conversion

Question 1

What is theoretical flame temperature?

Answer 1

• Most useful parameter to obtain for combustion or incineration.
• Highest temp the reaction can provide
  The enthalpy change from initial to final state is independent of the path taken
• Need the mean molar heat capacity (cp) value to calculate TFT

TFT varies between 1800-2200 so use est 2000 C

Want our theoretical flame temp to be as high as possible to increase efficiency by lowering excess air

Question 2

Stack loss

Answer 2

-Temperature between stack temperature and ambient temperature

Higher stack loss = lower efficiency

Question 3

Qfuel

Answer 3

The sum of the stack loss and the useful heat (Qheat) = total heat contained within the fuel

Thus, furnace efficiency = qheat/qfuel = heat to process/heat released by fuel

Question 4

Higher excess air for combustion

Answer 4

• Lowers theoretical flame temperature
• Stack temp remains the same (dew point of combustion gases still the same) = higher stack loss
• Higher stack loss = lower furnace efficiency

Question 5

Ways to increase theoretical flame temp which increases furnace efficiency

Answer 5

• Lower excess air

- Increasing air preheat (air heated before going to combustion device = less fuel energy used to heat air)

Question 6

What does an incinerator with energy recovery typically comprise of

Answer 6

1. Waste reception and handling
2. Combustion chamber
3. Energy recovery plant
4. Emissions clean up for combustion gases
5. Bottom ash handling and air pollution
6. Control residue handling

Question 7

What are fluidised bed incineration good for

Answer 7

Good for fuels with high moisture and ash

• limestone can be added to the bed for SOx subsiding
• Low combustion temp gives low NOx formation

Question 8

Types of incinceration tech

Answer 8

1. Fluidised bed
2. MSW incinerator
3. Rotary kiln

Question 9

Strengths of incineration

Answer 9

1. Handle MSW with no pre-treatment needed
2. State of the art tech in global use including pollution control tech
3. Energy recovery inluding CHP plants and opportunity for district heating programmes
4. No long term liability
5. Proven and commercially available tech
6. Reduces volume of waste by 90%

Question 10

Weaknesses of incineration

Answer 10

1. High capital costs
2. Negative public perception - stack emissions and lack of understanding of tech
3. Residue quality and disposal an issue although bottom ash can be reused
4. Debate over measurement and long term health effects of dioxin emissions
5. Minimum materials recovery, except for ferrous materials
6. Minimum or guaranteed tonnage may be required by operator to cover costs

Question 11

Steps of gasification

Answer 11

1. Drying
2. Pyrolysis - produces tar = problematic. Fuel heated to 300-500 C with no oxidising agent
3. Gasification - gas liquid and solid products of pyrolysis react with oxidising agent (air or o2). = Gives gas CO, CO2 and H2.

Question 12

Types of gasifiers

Answer 12

1. updraft gasifier
2. downdraft gasifier - simplest and least expensive
3. bubbling fluidised bed gasifier
4. circulating fluidised bed gasifier

Question 13

Partial oxidation with air gasification result

Answer 13

Products CO, CO2, H2, CH4, N2 and tar, LHV 5 MJm-3

Question 14

Partial oxidation with oxygen gasification result

Answer 14

Products Co, CO2, H2, ch4 and tar. MHV 10-12 MJm-3. Cost of air separation unit offset by better quality of producer gas.

Oxygen gives high temps, less tar and N2

Question 15

Steam gasification result

Answer 15

Products CO, CO2, H2, Ch4 and tar. MHV 15-20 MJm-3.

• 2 stage process, first makes gas and char
• 2nd = char burnt to provide heat to first reactor for reaction.

Higher methane and hydrocarbon gas content

Question 16

Strengths of gasification and pyrolysis

Answer 16

1. Not incineration
2. Qualifies for the renewables obligation for a substantial proportion of the feedstock processed
3. Efficient elec generation through combustion of gas through engines
4. Potential to recycle a large proportion of residues depending on the process
5. High temps may make the syste more flexible for other waste streams eg clinical
6. Smaller units more acceptable and part of an integrated system
7. Capable of being integrated with other processes eg output from MBT/refuse derived fuel RDF production

Question 17

Weaknesses of gasification and pyrolysis

Answer 17

1. May suffer from the same negative perception as incineration
2. Requires extensive pre-treatment to be able to handle MSW unlike incineration
3. Many processes will still have residues to be disposed of, some may be hazardous (eg from flue gas treatment)
4. Unproven on a commercial scale on MSW in uk
5. More sensitive system than moving grate incineration tech
6. More expensive in terms of gate fee than energy from waste

Question 18

What ‘fuel’ does anaerobic digestion use?

Answer 18

1. Waste
   - Fats
   - Carbs
   - Proteins

Raw materials:

• Agricultural manures
• Food waste
• Sewage

Question 19

What are the 4 processes of anaerobic digestion

Answer 19

1. Waste used
2. Waste turned into sugars and fatty acids
3. Fatty acids are into CO2, alcohols etc
4. Biogas is produced

All processes occur in one vessel
Need continuous mixing

• Mesophilic range 32-45 C is easier to control

Question 20

Outputs of incineration

Answer 20

Good:

1. Heat used to produce elec
2. Heat may be used to provide local heating needs

Bad:

1. Emissions to air (CO2, NOx)
2. Residue from flue gas treatment to hazardous waste landfill
3. Bottom ash may be recycled or disposed of to landfill
4. Metals extracted from bottom ash for recycling
5. Wastewater

Question 21

Inputs and outputs of anaerobic digestion

Answer 21

Input: Organic waste stream

Output:

1. Biogas
2. Water vapour
3. Wastewater
4. Non organics to landfill

To post treatment maturation phase:

1. Water vapour
2. Digestate to agriculture/land

Question 22

Strengths of anaerobic digestion

Answer 22

1. Relative low capital costs
2. State of the art tech in global use including pollution control tech
3. Energy recovery potential
4. Reduces organic wastes from landfill
5. Totally enclosed system = reduce environmental impact
6. Complies with animal by-products legislation
7. Eligible for renewables obligation certificates

Question 23

Weaknesses of anaerobic digestion

Answer 23

1. Uncertainties over economics and application for MSW
2. AD of MSW need to rely on comprehensive pre-processing of the waste = operational difficulties
3. Odour emissions during material handling
4. Only treats the organic fraction of MSW
5. More capital intensive than composting
6. Front end processing material handling costly
7. Contamination of final product difficult to avoid = marketing problems

Question 24

Summary

Answer 24

1. Theoretical Flame Temperature can be calculated from information about fuel and amount of air required for combustion
   • Calculation of potential heat available from the combustion/incineration process
   • Many devices available for incineration, and many design schemes for making use of the heat produced
   • Gasification and pyrolysis virtually confined to biomass, although handling of other wastes is ongoing research activity
   • Gasification and pyrolysis schemes have high capital and operating costs
   • Anaerobic digestion makes use of different biomass type wastes –higher moisture content
   • Biogas from anaerobic conversion available for power and heating –mainly using gas engines – but also can be used as basis for further processing