Waste‐to‐energy (WtE) or energy‐from‐waste (EfW) is the process of generating energy in the form of electricity and/or heat from the primary treatment of waste. WtE is a form of energy recovery. Most WtE processes produce electricity and/or heat directly through combustion, or produce a combustible fuel commodity, such as methane, methanol, ethanol, or synthetic fuels.
Gasification and pyrolysis processes have been known and used for centuries and for coal as early as the eighteenth century. Development technologies for processing (residual solid mixed waste) has only become a focus of attention in recent years stimulated by the search for more efficient energy recovery (Fichtner Consulting Engineers 2004).
Municipal solid waste (MSW) to a large extent is of biological origin (biogenic), e.g. paper, cardboard, wood, cloth, food scraps. Typically, half of the energy content in MSW is from biogenic material. Consequently, this energy is often recognized as renewable energy according to the waste input.
Methods
Incineration
Incineration, the combustion of organic material such as waste with energy recovery, is the most common WtE implementation. All new WtE plants in OECD countries incinerating waste (residual MSW, commercial, industrial, or refuse‐derived fuel [RDF]) must meet strict emission standards, including those on nitrogen oxides (NOx), sulfur dioxide (SO2), heavy metals, and dioxins. Hence, modern incineration plants are vastly different from old types, some of which neither recovered energy nor materials. Modern incinerators reduce the volume of the original waste by 95–96%, depending upon composition and degree of recovery of materials such as metals from the ash for recycling.
Incinerators may emit fine particulate, heavy metals, trace dioxin, and acid gas, even though these emissions are relatively low from modern incinerators. Other concerns include proper management of residues: toxic fly ash, which must be handled in hazardous waste disposal installation as well as incinerator bottom ash, which must be reused properly.
Critics argue that incinerators destroy valuable resources and they may reduce incentives for recycling. The question, however, is an open one, as European countries which recycle the most (up to 70%) also incinerate to avoid landfilling.
Incinerators have electric efficiencies of 14–28%. In order to avoid losing the rest of the energy, it can be used for, e.g., district heating (cogeneration). The total efficiencies of cogeneration incinerators are typically higher than 80% (based on the lower heating value of the waste).
The method of incineration to convert MSW is a relatively old method of WtE production. Incineration generally entails burning waste (residual MSW, commercial, industrial, and RDF) to boil water which powers steam generators that make electric energy and heat to be used in homes, businesses, institutions, and industries. One problem associated is the potential for pollutants to enter the atmosphere with the flue gases from the boiler. These pollutants can be acidic and in the 1980s were reported to cause environmental damage by turning rain into acid rain. Since then, the industry has removed this problem by the use of lime scrubbers and ESPs on smokestacks. By passing the smoke through the basic lime scrubbers, any acids that might be in the smoke are neutralized which prevents the acid from reaching the atmosphere and hurting the environment. Many other devices, such as FFs, reactors, and catalysts destroy or capture other regulated pollutants. According to the New York Times, modern incineration plants are so clean that “many times more dioxin is now released from home fireplaces and backyard barbecues than from incineration.” According to the German Environmental Ministry, “because of stringent regulations, waste incineration plants are no longer significant in terms of emissions of dioxins, dust, and heavy metals.”
Other Technologies
There are a number of other new and emerging technologies that are able to produce energy from waste and other fuels without direct combustion. Many of these technologies have the potential to produce more electric power from the same amount of fuel than would be possible by direct combustion. This is mainly due to the separation of corrosive components (ash) from the converted fuel, thereby allowing higher combustion temperatures in, e.g., boilers, gas turbines, internal combustion engines, and fuel cells. Some are able to efficiently convert the energy into liquid or gaseous fuels.
Thermal technologies:
- Gasification: produces combustible gas, hydrogen, synthetic fuels
- Thermal depolymerization: produces synthetic crude oil, which can be further refined
- Pyrolysis: produces combustible tar/bio‐oil and chars
- Plasma arc gasification or plasma gasification process (PGP): produces rich syngas including hydrogen and carbon monoxide usable for fuel cells or generating electricity to drive the plasma arch, usable vitrified silicate and metal ingots, salt, and sulfur
Non‐thermal technologies:
- Anaerobic digestion: biogas rich in methane
- Fermentation production: examples are ethanol, lactic acid, hydrogen
- Mechanical biological treatment (MBT)
- MBT + anaerobic digestion
- MBT to refuse derived fuel
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