Reduction in the volume and mass of solid waste is a crucial issue especially in the light of limited availability of final disposal sites in many parts of the world. Waste-to-Energy (WTE) or Energy-from-Waste (EfW) is the use of modern combustion and biochemical technologies to recover energy, usually in the form of electricity and steam, from urban wastes. These new technologies can reduce the volume of the original waste by 90%, depending upon composition and use of outputs.
Waste-to-energy technologies can address a host of environmental issues, such as land use and pollution from landfills, and increasing reliance on fossil fuels. In many countries, the availability of landfill capacity has been steadily decreasing due to regulatory, planning and environmental permitting constraints. As a result, new approaches to waste management are rapidly being written into public and institutional policies at local, regional and national levels.
The main categories of waste-to-energy technologies are physical technologies, which process waste to make it more useful as fuel; thermal technologies, which can yield heat, fuel oil, or syngas from both organic and inorganic wastes; and biological technologies, in which bacterial fermentation is used to digest organic wastes to yield fuel.
Major Components of Waste-to-Energy Processes
- Front end MSW pre-processing is used to prepare MSW for treatment and separate any recyclables
- Conversion unit (reactor)
- Gas and residue treatment plant (optional)
- 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
- Emissions cleanup system
Summary of Major Waste-to-Energy Technologies
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
Salman is a prolific environmental writer, and has authored more than 500 articles in reputed journals, magazines and websites. He is proactively engaged in creating mass awareness on renewable energy, waste management, sustainability and conservation all over the world.
Salman can be reached on salman@cleantechloops.com
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