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Comparison of Different Waste-to-Energy Processes


Major components of Waste-to-Energy Processes
  1. Front end MSW pre-processing is used to prepare MSW for treatment and separate any recyclables
  2. Conversion unit (reactor)
  3. Gas and residue treatment plant (optional)
  4. Energy recovery plant (optional): Energy / chemicals production system includes gas turbine, boiler, internal combustion engines for power production. Alternatively, ethanol or other organic chemicals can be produced
  5. Emissions clean up
Incineration
  • Combustion of raw MSW, moisture less than 50%
  • Sufficient amount of oxygen is required to fully oxidize the fuel
  • Combustion temperatures are in excess of 850oC
  • Waste is converted into CO2 and water concern about toxics (dioxin, furans)
  • Any non-combustible materials (inorganic such as metals, glass) remain as a solid, known as bottom ash (used as feedstock in cement and brick manufacturing)
  • Fly ash APC (air pollution control residue) particulates, etc
  • Needs high calorific value waste to keep combustion process going, otherwise requires high energy for maintaining high temperatures
Anaerobic Digestion
  •  Well-known technology for domestic sewage and organic wastes treatment, but not for unsorted MSW
  • Biological conversion of biodegradable organic materials in the absence of oxygen at temperatures 55 to 75oC (thermophilic digestion – most effective temperature range)
  • Residue is stabilized organic matter that can be used as soil amendment after proper dewatering
  • Digestion is used primarily to reduce quantity of sludge for disposal / reuse
  • Methane gas generated used for electricity / energy generation or flared
Gasification
  • Can be seen as between pyrolysis and combustion (incineration) as it involves partial oxidation.
  • Exothermic process (some heat is required to initialize and sustain the gasification process).
  • Oxygen is added but at low amounts not sufficient for full oxidation and full combustion.
  • Temperatures are above 650oC
  • Main product is syngas, typically has net calorific value of 4 to 10 MJ/Nm3
  • Other product is solid residue of non-combustible materials (ash) which contains low level of carbon
Pyrolysis
  • Thermal degradation of organic materials through use of indirect, external source of heat
  • Temperatures between 300 to 850oC are maintained for several seconds in the absence of oxygen.
  • Product is char, oil and syngas composed primarily of O2, CO, CO2, CH4 and complex hydrocarbons.
  • Syngas can be utilized for energy production or proportions can be condensed to produce oils and waxes
  • Syngas typically has net calorific value (NCV) of 10 to 20 MJ/Nm
Plasma Gasification
  • Use of electricity passed through graphite or carbon electrodes, with steam and/or oxygen / air injection to produce electrically conducting gas (plasma)
  • Temperatures are above 3000oC
  • Organic materials are converted to syngas composed of H2, CO
  • Inorganic materials are converted to solid slag
  • Syngas can be utilized for energy production or proportions can be condensed to produce oils and waxes

        Net Energy Generation Potential Per Ton MSW
Waste Management Method
Energy Potential*
(kWh per ton MSW)
Recycling
2,250
Landfilling
   105
WTE Incineration
   585
Gasification
   660
Pyrolysis
   660
Anaerobic Digestion
   250
 
Cost Economics of WTE Processes
Technology
Plant capacity
(tons/day)
Capital cost
(M US$)
O&M cost
(US$/ton)
Planning to commissioning
(months)
Pyrolysis
70-270
16 - 90
80 - 150
12 - 30
Gasification
900
15 - 170
80 - 150
12 – 30
Incineration
1300
30 - 180
80 - 120
54 – 96
Plasma gasification
900
50 - 80
80 - 150
12 – 30
Anaerobic digestion
300
20 - 80
60 - 100
12 - 24
In vessel composting
500
50 – 80
30 - 60
9 – 15
Sanitary landfill
500
5 - 10
10 – 20
9 – 15
Bioreactor landfill
500
10 – 15
15 - 30
12 – 18

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