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 850^oC
• 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 75^oC (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 650^oC
• 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 850^oC 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 3000^oC
• 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