Existing Waste-To-Energy Technologies: History
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Sandylove Afrane
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Jeffrey Dankwa Ampah
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Salah Kamel
Energy is a critical element in human existence, and is essential to a society’s sustainable economic growth. Globally, fossil fuels have been the primary source of electrical generation. Technologies that can efficiently recover energy from waste with little to no environmental damage have been considered as alternatives for conventional fossil fuel generation. The conversion pathways utilized in waste-to-energy (MSW) technologies may be classified into two main categories: thermochemical and biochemical. The thermochemical pathway involves the conversion of MSW feedstock to energy (electricity, heat, and value-added products) under high temperatures. It is most commonly utilized for dry waste with a larger proportion of nonbiodegradable material (a low water content). Combustion (incineration), gasification, and pyrolysis are the most common technologies under thermochemical conversion of MSW. Biochemical processes are optimal for wastes with a high proportion of biodegradable organic materials and a high moisture/water content that promotes microbial activity. Anaerobic digestion (AD) and methane recovery from a controlled environment in landfills are the most common technologies under the biochemical pathway.
  • municipal solid waste (MSW)
  • waste-to-energy (WtE)
  • zero waste

1. Combustion (Incineration)

The incineration of waste is the most common WtE technology integrated into waste management systems worldwide. It is the oxidation of combustible waste constituents, and is utilized for a diverse variety of MSW. The efficiency of the incineration process is estimated to be about 25–30%, and it is mainly influenced by the constituents of the waste stream, particularly for MSW, which may be extremely diverse [1][2]. In general, there are two types of incineration for energy recovery: mass-burn and refuse-derived fuels (RDF). In mass burn technology, MSW is burnt completely, without any pre-processing. In RDF plants, recycling and non-combustible elements are sorted, and the remaining waste is shredded, dried, and compressed to generate fuel with relatively homogenous characteristics. Pre-processing is performed in RDF to obtain a higher heating value. Even though RDF has proven to be a more efficient technology, mass-burn is a more common technology used worldwide [3]. MSW is combusted at high temperatures in a specially constructed chamber with a continuous air supply to guarantee turbulence and the full combustion of the elements to their stable and original molecular states. The end products of the combustion are mainly hot combusted gases like carbon dioxide, nitrogen, oxygen, flue gas, and non-combustible elements [4].
The incineration of MSW operating at an uncontrollably high temperature can produce a net energy of about 544 kWh/ton MSW, but this process is more environmentally damaging. Combusting MSW at uncontrolled temperatures produces chlorinated dibenzo-p-dioxins and corrosive gases that could destroy the steam pipes and cause health-related problems. Dioxins, particulate matter, sulfur dioxide, hydrochloric acid, and heavy metals are all possible contaminants in flue gases. In previous years, the environmental impact of dioxins was one of the most crucial challenges related to MSW incineration [5]. In recent years, however, most modern incinerators have used a sophisticated pollutant/emissions control system designed according to developed countries’ stringent regulations to reduce air pollutants and emit virtually no dioxins [6].

2. Gasification

Gasification is a process in which organic materials are partly oxidized at high temperatures (usually 500–1800 °C) with reduced oxygen [7]. The gasification process is influenced by the temperature, pressure, and oxygen concentrations. The gaseous product obtained from this process is known as synthetic gas (syngas), and it mainly consists of hydrogen, carbon monoxide, carbon dioxide, and methane, together with heat (used to generate power and process heat) [2]. The gasification agent (air, oxygen, or steam), operating temperature and pressure of the gasifier, and feed properties all impact the end products’ chemical composition and energy content. The syngas obtained from the gasification process is very high in calorific value, and may be transformed into other valuable products such as alternative transport fuels, natural gas replacements, and fertilizers, as opposed to just heat and electricity from the incineration process in a WtE plant [8][9].
Biological waste, sewage sludge, industrial waste, and wood waste have previously been treated using gasification methods. However, in recent years, the gasification of MSW is increasingly gaining popularity. Several gasification technologies for the cogeneration of heat and electricity from syngas have been developed during the previous two decades and are now commercially accessible [10]. Gasification facilities share environmental problems similar to those associated with mass-burn incinerators, including water pollution, air pollution, ash disposal, and other byproducts. During gasification, tars, alkaline compounds, halogens, and heavy metals are released and can cause environmental and operational problems.

3. Plasma Gasification

Plasma gasification uses an electrically driven plasma torch to volatize waste and organic materials with a regulated infusion of oxygen [11]. The organic component of the waste stream is processed into synthetic gas (syngas), while the inorganic portion is processed into an inert vitrified glass that may be utilized for a variety of construction products [12]. Plasma is the fourth state of matter, consisting of heated ionized gases generated by an electrical discharge at very high temperatures (2000–5000 °C) [13]. The energy contained in a plasma enables the utilization of low-energy fuels, such as domestic and industrial waste, which are often incapable of supporting their own gasification without extra fuel. The synthesis gas may be utilized to generate efficient power and/or heat, and second-generation liquid biofuels [10]. Typically, no ash remains after the process to be dumped in a landfill. The characteristics of the waste stream, however, may have an impact on the effectiveness of the gasification process. Waste with a high proportion of inorganic materials, such as metals and construction waste, may provide less syngas and more slag.

4. Pyrolysis

Pyrolysis is the thermal breakdown of a substance in the absence of or with a restricted supply of an oxidizing agent (i.e., partial gasification) to produce the thermal energy needed for the process. Compared to gasification, relatively modest temperatures (400–900 °C, although typically less than 700 °C) are used [14]. Syngas is produced during pyrolysis, which is the thermal breakdown of carbon-based compounds using heat, rather than direct combustion. Particle matter, mercury, sulfur, and other pollutants are removed from the syngas. Pyrolysis gas, pyrolysis liquid, and solid coke are the three products produced, the proportions of which are highly dependent on the pyrolysis technique and reactor process parameters [15].
Pyrolysis processes are classified under three main categories; slow pyrolysis, rapid (or flash) pyrolysis at high temperatures, and flash pyrolysis at low temperatures [16]. The most commonly used pyrolysis process among them in present times is flash pyrolysis. This is because the slow pyrolysis process requires a very long period of time (many hours) and yields bio-char as the primary product, while flash pyrolysis produces about 60% bio-oil in only a few seconds [16][17]. Most pyrolysis technologies are environmentally friendly ways of converting feedstock; however, if the right measures are not taken, they can cause adverse environmental and health issues. For instance, technologies that do not recycle their syngas, including small, homemade technologies or traditional systems, allow these gases to escape, which can double the carbon produced rather than being carbon neutral, partially from the use of fossil fuel and partially from avoiding the use of the produced fuel. Additionally, oil and tar contain heavy organic chemicals that are harmful to the environment.

5. Anaerobic Digestion (AD)

Anaerobic digestion is the breakdown of biodegradable material by microorganisms in the absence of oxygen. For the digesting process, special reactors are utilized, and within the reactors, certain variables—such as pH, moisture content, and temperature, among others—are regulated [18]. These parameters aim to create a suitable habitat for microorganisms, allowing them to multiply and accelerate the methane breakdown process. The bioconversion process is complicated and divided into four phases. The first step is hydrolysis, in which extracellular enzymes break down complex insoluble organic molecules like proteins, lipids, and carbohydrates into soluble organic materials such as sugars, amino acids, and fatty acids. Following this is the acidogenesis process, which involves the breakdown of hydrolysis products into acetate, hydrogen, and higher-molecular-weight volatile fatty acids. Acetogenesis is the third step, in which acidogenesis products are further processed into acetic acid, CO2, and hydrogen by acetate-forming bacteria. The last step of methanogenesis, in which methanogens may develop at low redox potentials using substrate-level or electron transport phosphorylation, generates biogas, a combination of gaseous molecules—mostly methane and carbon dioxide—through volatile fatty acid breakdown [19][20].
Biogas may be used to generate electricity, fuel (biomethane) for combustion engines, and space heating, water heating, and process heating. Over 90% of the energy available in the wastes is kept in the biogas as methane during the anaerobic conversion or fermentation of MSW, with the remainder being sludge [21]. Anaerobic digestion adds value to MSW while avoiding several negative effects connected with the natural breakdown process that happens in landfills, and allows the replacement of alternative fossil source materials. Most AD facilities around the globe are used for sewage sludge and animal manure, with MSW treatment being more complex and still under development.

6. Landfill Gas Recovery

Landfilling has been the only disposal technique capable of dealing with all of the materials in the solid waste stream, as well as the easiest and, in many cases, cheapest disposal option. The generation of landfill gas from a sanitary landfill plant is similar to anaerobic digestion, with the main difference being the operational control of the sanitary landfill [22]. Because of the rapid reaction pace combined with the temperature stability, biochemical breakdown in biogas reactors is better regulated. When organic waste is disposed of in a landfill, it emits landfill gas (LFG) as it degrades under anaerobic conditions. Instead of letting these gases enter the environment and contribute to global warming, landfill gas plants may collect them, isolate the methane, and burn them to produce electricity, heat, or both. Landfill gas typically comprises 50% methane and 50% carbon dioxide, with an energy level of 18–19 MJ/m3 [23]. In most cases, land scarcity and other threats to the environment brought on by air, water, and land pollution from effluent make landfilling, despite its low cost, unsustainable and unsuitable for treating MSW.

This entry is adapted from the peer-reviewed paper 10.3390/ijerph19148428

References

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