(WtE) or energy-from-waste (EfW)
refers to any waste treatment that creates energy in the form of electricity or heat from a waste source. Such technologies reduce or eliminate waste that otherwise would be transferred to a greenhouse gas emitting landfill. WtE is a form of energy recovery. Most WtE processes produce electricity directly through combustion, or produce a combustible fuel commodity, such as methane, methanol, ethanol or synthetic fuels.
Incineration, the combustion of organic material such as waste, with energy recovery is the most common WtE implementation. Incineration may also be implemented without energy and materials recovery, but this is increasingly being banned in OECD (Organisation for Economic Co-operation and Development) countries. Furthermore, all new WtE plants in OECD countries must meet strict emission standards. Hence, modern incineration plants are vastly different from the 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.
Concerns regarding the operation of incinerators include fine particulate, heavy metals, trace dioxin and acid gas emissions, even though these emissions are relatively low from modern incinerators. Other concerns include toxic fly ash and incinerator bottom ash (IBA) management. Discussions regarding waste resource ethics include the opinion that incinerators destroy valuable resources and the fear that they may reduce the incentives for recycling and waste minimization activities. Incinerators have electric efficiencies on the order of 14-28%. The rest of the energy can be utilized for e.g. district heating, but is otherwise lost as waste heat.
WtE technologies other than incineration
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 a higher combustion temperatures in e.g. boilers, gas turbines, internal combustion engines, fuel cells. Some are able to efficiently convert the energy into liquid or gaseous fuels:
Gasification (produces combustible gas, hydrogen, synthetic fuels)
Pyrolysis (produces combustible tar)
Plasma arc gasification PGP or plasma gasification process (produces rich syngas including Hydrogenand Carbon Monoxide usable for fuel cells or generating electricity to drive the plasma arch, useable vitrified silicate and metal ingots, salt and sulphur)
Gasplasma - A integrated process of front end gasification of shreaded waste to syngases, plasma arc treatment of syngas and syngas cleaning and conditioning. Syngas, Plasmarok, Hydrogen, Carbon monoxide and CHP are the outputs. Residuals to landfill is less than 1%.
Anaerobic digestion (Biogas rich on methane)
Mechanical biological treatment
MBT + Anaerobic digestion or Advanced MBT AMBT
MBT to Refuse derived fuel
Measurement of the biomass fraction of waste for greenhouse gas abatement protocols
The biomass fraction of waste has a monetary value under multiple greenhouse gas protocols, such as the AB 32 program in California and the Renewable Obligation Certificate program in the United Kingdom. Biomass is considered to be carbon-neutral since the CO2 liberated from the combustion of biomass is recycled in plants. The combusted biomass fraction of waste is used by waste to energy plants to reduce their overall reported CO2 emissions.
Several methods have been developed by the European CEN 343 working group to determine the biomass fraction of waste fuels, such as Refuse Derived Fuel/Solid Recovered Fuel. The initial two methods developed (CEN/TS 15440) were the manual sorting method and the selective dissolution method. Since each method suffered from limitations in properly characterizing the biomass fraction, an alternative method was developed using the principles of radiocarbon dating. A technical review (CEN/TR 15591:2007) outlining the carbon 14 method was published in 2007. A technical standard of the carbon dating method (CEN/TS 15747:2008) will be published in 2008. In the United States, there is already an equivalent carbon 14 method under the standard method ASTM D6866.
Although carbon 14 dating can determine with excellent precision the biomass fraction of waste, it cannot determine directly the biomass calorific value. Determining the calorific value is important for green certificate programs such as the Renewable Obligation Certificate program in the United Kingdom. These programs award certificates based on the energy produced from biomass. Several research papers, including the one commissioned by the Renewable Energy Association in the UK, have been published that demonstrate how the carbon 14 result can be used to calculate the biomass calorific value.