The application of methods and technologies that are designed to produce energy from waste, is called waste-to-energy. In waste-to-energy applications, energy may be produced from direct combustion of waste, or by conversion.
This energy, is considered to be renewable, and may occur in the form of electricity; or heat, from burnt waste, and may be derived from converted flammables like biogas. The latter is produced when biomass is converted through anaerobic digestion, and is a source of bioenergy.
In this article, the concept of waste-to-energy is discussed extensively, as outlined below;
Waste-to-Energy Meaning: 7 Different ways to Define the Concept
Waste-to-energy is a process, or a series of processes, which convert waste materials into useful energy .
The concept can be defined in terms of an overview of the ways by which energy is derived from waste, as follows;
Waste-to-energy is the process of producing energy from waste, either through processing and conversion to a fuel source, or through primary treatment.
Another factor that can be used to define waste-to-energy, is the forms of energy and power that can be derived from waste through this process;
Waste-to-energy is the conversion or primary treatment of waste to produce heat and/or electricity .
In order for the waste-to-energy process to be possible, technology is required. This consideration is used to define the concept below;
Waste-to-energy is a set of technologies and methods such as incineration, gasification, and anaerobic digestion, which are used to treat waste materials, thereby producing energy which can be used effectively in cogeneration (CHP) systems and turbine generators.
The above definition implies that waste-to-energy is a method of electricity generation . Another perspective from which the concept can be defined, is with regards to renewable energy and sustainability;
Waste-to-energy is a set of methods, processes and technologies which are used to produce energy from waste in a sustainable and renewable manner, that is similar to solar, wind and geothermal energies.
The reason why waste-to-energy can be considered renewable, is because waste materials (organic and inorganic) will always be produced on Earth, as a sustainable energy source .
This means that waste is a renewable material, and energy produced from waste is therefore renewable energy.
Another way to describe waste-to-energy, is as a method of waste management, as shown below;
Waste-to-energy is a collection of techniques and technologies that are used to treat and process municipal, agricultural and industrial waste, thereby converting this waste to heat and electricity for various uses.
Lastly, waste-to-energy can be viewed as a method of environmental remediation. This may be either in terms of preventing open biodegradation or by minimizing greenhouse emissions, as the following definition suggests;
How Waste is Converted to Energy: Waste-to-Energy Methods
Waste is converted to energy through processes that processes that release the stored biochemical energy in waste, either by breaking-down or transforming it. Such processes include gasification, anaerobic digestion, landfill gas recovery, incineration and pyrolysis.
The most common waste-to-energy method is incineration .
This is because it is a relatively-simple method whereby waste is made to undergo combustion (or burning) in the presence of oxygen, and the heat produced from this combustion is used to produce steam which can be used to drive a steam turbine and generate electricity.
Incineration is a thermal treatment method  because it relies solely on heat.
1). Gasification as a Waste-to-Energy Method
Gasification is a waste-to-energy method whereby waste is converted to gaseous substances like nitrogen, hydrogen. carbon dioxide and carbon monoxide, by subjecting the waste to high temperatures above 700°C, under controlled conditions of oxygen and water vapor supply.
Waste materials which can be used as feedstock in gasification are mainly organic materials (biomass) that is rich in carbon . Such materials can be found in industrial, agricultural and municipal waste.
Gasification is unique because it does not involve burning or full-combustion of the feedstock or waste . Combustion is prevented by carefully regulating the conditions of heat-application, such as the amount of air that is supplied.
The mixture of gases produced from gasification is collectively referred to as syngas, and is a flammable gas which contains significant amount of hydrogen and carbon dioxide, and can be used as a fuel.
Energy which is generated from the burning of syngas is considered to be renewable, because the feedstock which undergoes gasification is usually renewable biomass or municipal waste.
Gasification is better in terms of energy production, than other thermal waste-to-energy methods like incineration, because it produces a fuel which has higher energy density than the original feedstock.
2). Anaerobic Digestion as a Waste-to-Energy Method
Anaerobic digestion is a waste-to-energy method, whereby microorganisms are used to breakdown and transform organic waste in the absence of oxygen, to produce biogas; a renewable fuel and source of bioenergy.
Anaerobic digestion is a biological method , although it can involve the regulation of physicochemical conditions like temperature, to increase efficiency of the process.
It depends on microbial biodegradation, during which microbes like bacteria secrete enzymes that breakdown and convert organic matter.
This implies that anaerobic digestion is only effective for biodegradable waste materials like sewage, wastewater and livestock manure.
Anaerobic digestion involves four stages which are; hydrolysis, acidogenesis, acetogenesis, and methanogenesis .
The main byproduct of anaerobic digestion is biogas, which contains methane and other gases like carbon dioxide and nitrogen .
Like syngas, biogas is a renewable fuel and can be burnt to release energy that is usable for heating or electricity generation.
Aside biogas. anaerobic digestion also produces digestate; a nutrient-rich residue that can be used as a fertilizer.
3). Landfill Gas Recovery as a Waste-to-Energy Method
Landfill gas recovery is a waste-to-energy method whereby the gaseous products of decomposition, are collected from solid waste in landfills .
The main gas which is recovered from landfills is methane . It usually occurs as part of a mixture of gases, collectively referred to as ‘landfill gas’ (LFG).
The two main gases in LFG are methane and carbon dioxide .
A vacuum pump system is usually used to collect the gas, which is subsequently processed to remove unwanted components, so that is can be used as a fuel.
In addition to producing energy, landfill gas recovery provides a means of reducing atmospheric greenhouse emissions .
4). Incineration as a Waste-to-Energy Method
Due to its simplicity, incineration is the most common waste-to-energy method.
Basically, incineration is a waste-to-energy method which involves the burning of waste to produce heat energy that is used to generate electricity.
Incineration is particularly effective for municipal solid waste (MSW) . It can also be used for industrial waste.
It is recommendable to use this option as a means of waste management only when recycling is not possible.
The heat energy produced from burning waste, can be used to produce steam, which may in turn be used to drive a steam turbine generator. It is important to note that incineration involves the destruction of waste, rather than its conversion.
Incineration has a number of disadvantages.
These include the cost of operation and maintenance of incinerators, and the emission of greenhouse gases from burning waste. Therefore, it is not the best option with consideration of global warming and climate change.
5). Pyrolysis as a Waste-to-Energy Method
Pyrolysis is a waste-to-energy method which involves subjecting waste to high temperature in the absence of oxygen, to produce liquid and gaseous fuels .
Like gasification and incineration, pyrolysis is a thermal method, which depends mainly on temperature to be effective.
One of the biggest advantages of pyrolysis as a waste-to-energy method, is the fact that it is versatile.
Pyrolysis can be used to treat a wide range of waste types, including municipal, agricultural, industrial, and hazardous waste (like e-waste).
It is known to be effective for biomass conversion, producing biochar, bio-oil, and syngas . For other types of waste, pyrolysis can lead to the production of different chemicals with varying chemical compositions and physical properties.
In some cases, pyrolysis is performed alongside catalytic reforming , which increases the effectiveness of the waste-to-energy conversion process.
Syngas and other biofuels produced as a result of pyrolysis, can be used for heating as well as electricity generation.
Because it is a cleaner waste-to-energy method, pyrolysis is considered a good alternative to incineration . However, the process requires more operational control to ensure that it occurs in the absence of oxygen.
The temperature required for pyrolysis is also typically high, reaching up to 900°C 
Environmental Impact of Waste-to-Energy Practices
In general, waste-to-energy practices have a positive environmental impact. This is due to various reasons.
One of the reasons is the fact that waste-to-energy practices are a means of managing waste .
Rather than allow these waste materials to accumulate and pollute the environment, waste-to-energy methods can be used to convert them to useful energy. This is a very helpful function, as it reduces the overall demand on natural energy resources.
Waste-to-energy practices also reduce the total volume of greenhouse gas emissions.
One of the ways by which this is achieved, is by reducing the dependence on fossil fuels. Because most renewable fuels produced from waste-to-energy processes emit less carbon dioxide than fossil fuels , they are a good replacement in their capacity.
When waste-to-energy methods are used to generate electricity, it is estimated that greenhouse gas emissions reduce by 35-60% .
Lastly, waste-to-energy practices help to recycle nutrients in the ecosystem.
This is because the conversion products, such as biochar and digestate, are nutrient-rich and can be applied to soil as fertilizer . This prevents the loss of nutrients that would likely occur when waste is not converted or treated.
There are however some environmental problems that may be caused by waste-to-energy practices.
These include air pollution due to gaseous emissions like carbon dioxide, hydrogen sulfide and nitrous oxide; and unpleasant odors produced from waste-to-energy plants.
Waste-to-energy is good because of its advantages and benefits to the environment and economy, through waste management, pollution control, and energy production. The disadvantages can be reduced with further technological improvement.
Waste-to-Energy Plants in the World
There are estimated to be more than 2,500 active waste-to-energy plants in the world.
These plants include incinerators, pyrolyzer facilities and anaerobic digestion plants, among others. They also vary widely in their scale, complexity and efficiency.
The combined capacity of waste-to-energy plants in the world is about 420 million tons of waste per annum. This capacity is rapidly increasing, with the addition of numerous treatment facilities each year.
By the year 2030, projections suggest that the combined global capacity of waste-to-energy plants will be about 650 million tons per annum.
Asia is one of the leading continents with respect to waste-to-energy practices.
As at 2016, China had about 434 waste-to-energy plants, most of which were thermal-based. Japan also has a large waste-to-energy thermal treatment capacity, with numerous treatment plants installed.
With regards to energy production, several countries have made significant progress, these include developed countries like the United States, which is estimated to produce significant amounts of energy from waste.
Examples of some waste-to-energy plants in the world, and their energy-production capacities (in Megawatts electrical (MWe);
1). Golfo/Penoles Thermal Power Plant, San Lius Potos, Mexico: 520MWe
2). Louisiana 1 Waste Plant, Louisiana, United States: 406.3MWe
3). Pietersaari Biomass CHP Power Plant, Western Finland: 240MWe
4). Vartan CHP 8 Power Plant, Sweden: 130MWe
5). Los Angeles Refinery Wilmington Waste Plant, California, United States: 68.5MWe
Waste-to-Energy Companies in the World: 10 Major Waste-to-Energy Companies
1). Covanta Energy Corporation
Founded in 1939 as Ogden Corporation, Covanta Energy Corporation is a public company with its headquarters in Morristown, New Jersey .
It was founded as an investment and utility business, but now operates as a waste management and waste-to-energy company, since its change of name in 2001.
The company derives its revenue from waste disposal, recycling and recovery. and electricity sales.
2). Clean Association of Tokyo 23
Founded in 1999, the Clean Association of Tokyo 23 is a waste-to-energy company which is headquartered in Chiyoda, Japan.
The main method used by the company is incineration, from which heat energy is produced. Residue of the incineration process is ash, which is used as a fertilizer or disposed in landfills.
3). Abu Dhabi National Energy Company
Abu Dhabi National Energy Company, PJSC (TAQA) (ADX: TAQA) is an energy company which is controlled by the Abu Dhabi government of the United Arab Emirates.
Founded in 2005, the company’s functions include oil and gas production, and waste-to-energy technology.
The Abu Dhabi National Energy Company is operational in up to 11 countries and four continents .
4). Veolia Environement S. A.
Founded in 1853 as Compagnie Générale des Eaux, Veolia Environement S.A. is a French environmental management company with headquarters in Paris, France.
In 2014, the company was branded as Veolia, after major restructuring had been carried out.
In addition to waste-to-energy, other services provided by this company include street lighting, HVAC, waste management, and facility management.
5). China Metallurgical Group
Founded in 1982, the China Metallurgical Group Corporation is a state-owned Chinese enterprise with headquarters in Beijing.
Services provided by the company include papermaking. real estate development, natural resource exploitation, engineering procurement and construction, electrical installation, water supply and waste-to-energy plant design.
6). Ramboll Group
Ramboll Group is a Danish consulting group which was founded as Rambøll & Hannemann in 1945.
With its headquarters in Copenhagen, Denmark, the company is specialized in engineering consulting services.
Ramboll Group has handled waste-to-energy projects in various areas, including waste-to-energy-to-water, and carbon capture and storage.
7). Babcock and Wilcox Enterprises
Babcock and Wilcox Enterprises (B&W), is an environmental science and technology company, founded in 1867, with its headquarters in Ohio, United States.
Services provided by the company include engineering, construction, renewable energy, recycling, and waste-to-energy facilities management.
The company has used waste-to-heat and renewable cogeneration systems to supply over 300,000 megawatts of energy to regions across more than 90 countries of the world.
8). Suez Environment
Suez Environement is a French environmental remediation and utility company, founded in 2008.
With its headquarters in La Défense, Paris, the company provides services in water treatment, waste management and waste-to-energy.
Suez Environement is said to be the largest private water supply company in the world.
9). China Everbright International Limited
Also known as China Everbright Environment Group Limited, this company was founded in 1993, and has its headquarters in Hong Kong, China.
It is specialized in water restoration, waste-to-energy, hazardous waste treatment, wind power, solar energy, anaerobic digestion, biomass utilization, environmental remediation, and research and development.
The company operates in over 70 countries of the world, and has been involved in a number of important environmental and energy projects.
10). C&G Environmental Protection Holdings
C&G Environmental Protection Holdings Limited was founded in 2004, and is headquartered in Hong Kong, China.
The services provided by the company include design, development, construction, operation and maintenance of waste-to-energy plants.
Waste-to-energy is a group of methods and technologies that convert waste to useful energy or fuels that can be used to produce renewable energy.
Waste-to-energy methods include;
- Anaerobic Digestion
- Landfill Gas Recovery
Waste-to-energy companies in the world include;
- Covanta Energy Corporation
- Clean Association of Tokyo 23
- Abu Dhabi National Energy Company
- Veolia Environement S. A.
- China Metallurgical Group
- Ramboll Group
- Babcock and Wilcox Enterprises
- Suez Environment
- China Everbright International Limited
- C&G Environmental Protection Holdings
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