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Waste to Energy

Waste to energy is a process by which various types of waste are transformed into energy. Instead of simply disposing of waste in landfills or incinerating it without any additional benefit, waste-to-energy aims to convert these waste materials into a useful source of energy, such as electricity, heat, or biogas.

In addition to generating renewable energy, waste-to-energy offers other additional benefits, as it reduces the amount of waste going to landfills and decreases greenhouse gas emissions by avoiding the anaerobic decomposition of waste in landfills.

Waste-to-Energy should not be considered an alternative to reducing waste generation and recycling, as it is preferable not to have waste in the first place. Waste-to-Energy should be viewed as a complement to these efforts, especially for waste that is not easily recyclable or non-biodegradable.

The rise of circular economy policies has led to an increase in the implementation of waste treatment plants that allow the transformation of waste into energy. In all cases, we find a dual objective:

  • Find a more efficient way to manage waste
  • Obtain a new source of energy supply to reduce energy dependence

Waste-to-Energy Technologies

There are numerous types of waste that can be used as fuel for waste-to-energy, including:

  • Municipal Solid Waste (MSW)
  • Waste generated in industrial processes
  • Forest and agricultural biomass for electricity generation
  • Livestock manure and agro-industrial waste for biogas generation
  • Biomass for biofuels

There are different waste-to-energy technologies, which can be classified into biological processes and thermal processes.

The biological processes can be applied when the waste has a significant biodegradable fraction. On the other hand, thermal processes are viable when the heating value of the waste, measured by lower heating value (LHV), is medium or high.

The most common waste-to-energy processes include:

Biomethanization

Biomethanization is a biological process carried out in the absence of oxygen, involving a heterogeneous population of microorganisms. This process transforms the most degradable fraction of organic matter into biogas, which is a gas mixture primarily composed of methane and carbon dioxide, along with other gases in smaller proportions (water vapor, CO, N2, H2, H2S, etc.).

Biogas is a mixture of carbon dioxide, methane, and other minor gases (H2S, etc.), which, after a washing process, can be used to generate electricity through a cogeneration process. Biogas is a source of energy due to its high calorific value (5,750 kcal/m3), enabling its use in cogeneration engines, boilers, and turbines (for generating electricity, heat, or as a biofuel).

The type of material to be digested significantly influences the yield and composition of the obtained biogas. To maximize production, it is preferable to use waste rich in fats, proteins, and carbohydrates, as their degradation results in significant amounts of volatile fatty acids, precursors of methane.

The residual heat energy from the process can be recovered and partially used to concentrate the generated wastewater through a vacuum evaporation-concentration process. The result is high-quality water and highly concentrated waste.

Biomethanization is a suitable process for the treatment and valorization of agricultural, livestock, and urban waste, as well as for the stabilization of sludges from urban wastewater treatment.

Pyrolysis

Pyrolysis is a thermal process that involves transforming organic matter into other compounds that are easier to treat.

Pyrolysis is carried out at high temperatures (between 300 and 800 ÂşC) and in the absence of air, which means the decomposition occurs through heat without combustion reactions.

The fundamental characteristics of this process are:

  • The only oxygen present is the content in the waste to be treated.
  • Operating temperatures range from 300°C to 800°C.
  • As there is no oxidation reaction of the more volatile compounds, the calorific value of the synthesis gas from the pyrolysis process typically ranges from 10 to 20 MJ/Nm3.

As a result of the process, the following is obtained:

  • Synthesis gas, whose basic components are CO, CO2, H2, CH4, and volatile compounds resulting from the cracking of organic molecules, along with those already present in the waste. It is a gas with high LHV (a mixture of hydrogen, carbon monoxide, methane, ethane, ethylene, etc.), although some of the energy obtained from the gas must be invested in the pyrolysis process itself, which is endothermic.
  • Liquid residue, mainly composed of long-chain hydrocarbons such as tars, oils, phenols, or waxes, formed by condensation at room temperature.
  • Solid residue composed of all non-combustible materials that have not been transformed or that result from molecular condensation with a high content of carbon, heavy metals, and other inert components from the waste. This solid carbon is removed through an additional incineration process associated with the main pyrolysis process.

The low operating temperatures result in less carbon and other precursor contaminants being volatilized in the gas stream, such as heavy metals or dioxins. This theoretically requires less treatment of the combustion gases to meet the minimum emission limits set in the Incineration Directive. Compounds that do not volatilize will remain in the pyrolysis residues and must be properly managed.

To be able to treat waste by pyrolysis, a series of requirements must be met. However, it is difficult to define the typology of waste considered suitable or unsuitable, as it is closely related to the type of reactor used and the operating conditions. Basically, waste types more suitable include paper, cardboard, wood chips, garden waste, and selected plastics. Inadmissible waste types include bulky waste, metals, construction materials, hazardous waste, glass, and some plastics, such as PVC.

Gasification

Gasification is a thermal process in which partial combustion of matter takes place in the presence of lower-than-stoichiometric oxygen quantities. This produces a combustible gas, known as synthesis gas, whose composition varies (a mixture of hydrogen, carbon monoxide, water, and light hydrocarbons) depending on the waste and operating conditions.

The main characteristics of a waste gasification process are as follows:

  • It uses air, oxygen, or steam as a source of oxygen and sometimes as a carrier in the removal of reaction products.
  • The operating temperature is typically above 750°C.
  • The chemical reactions in this process include molecular cracking, where temperature causes the breakdown of weaker molecular bonds, resulting in smaller molecules, usually volatile hydrocarbons. Reforming of gases also occurs, which is specific to gasification processes and often involves water vapor as a reactant.

As a result of the gasification process, the following is obtained:

  • Synthesis gas, primarily composed of CO, H2, CO2, N2 (if air is used as the gasifying agent), and CH4 in smaller proportion. Secondary products include tars, halogenated compounds, and particles.
  • Solid residue, consisting of non-combustible and inert materials present in the fed waste; it generally contains ungasified carbon. The characteristics of this residue are similar to the slag produced in incineration plant furnaces.

The quantity, composition, and calorific value of the gases resulting from gasification depend on the waste composition, temperature, and the amounts of air and steam used.

The synthesis gas obtained in the gasification process has several potential uses:

  • As a raw material for the production of organic compounds, such as direct synthesis of methanol, ammonia, or for conversion into hydrogen through steam reforming or catalytic reforming.
  • Electricity generation using internal combustion engines or microturbines. Synthesis gas can be used as fuel in electricity generation processes employing different thermal cycles than water vapor cycles, whether combined or simple.
  • It can be transformed into a liquid fuel that can be used as a substitute for diesel.
  • It can be injected into the natural gas grid after the removal of CO2 and remaining oxygen.
  • The hydrogen it contains in a fuel cell can be used for electricity generation.
  • As fuel in traditional boilers or furnaces.

The synthesis gas must be cleaned to be usable. It also generates solids, tars, and ashes that need to be incinerated.

As for the most suitable waste types, gasification also has the restriction of being able to treat only specific materials. The characteristics of the fed fuel must ensure that it contains at least the minimum amount of inert and very wet components, has a particle size between 80 and 300 mm, contains sufficient carbon for the gasification process reactions, does not contain hazardous substances, and ideally has a high calorific value.

Incineration, or combustion with excess oxygen

Incineration is a rapid thermal process in which complete combustion of the material occurs, leading to its oxidation and conversion into carbon dioxide and water. The key characteristics of waste incineration are as follows:

  • An excess of oxygen is required for combustion to ensure complete oxidation.
  • The combustion temperature typically ranges between 850°C and 1,100°C after the last injection of secondary air. The temperature varies based on the halogenated compound composition of the waste being treated.
  • For the material to react with oxygen and produce energy, it must contain carbon, hydrogen, or sulfur.

As a result of the incineration process, the following is obtained:

  • Combustion gas, primarily composed of CO2, H2O, unreacted O2, N2 from the air used in combustion, and other compounds in lower proportions from different elements that were part of the waste. The minor components present depend on the composition of the treated waste and can contain acidic gases derived from halogen reactions, sulfur, volatile metals, or unoxidized organic compounds. Finally, the combustion gases will contain particles carried by the gases.
  • Solid residue, primarily composed of inert slag, ash, and residues from the combustion gas purification system.

The overall process converts virtually all the chemical energy contained in the fuel into thermal energy, leaving a portion of unconverted chemical energy in the combustion gas and a very small amount of unconverted chemical energy in the ashes.

The heat generated by this process is utilized by generating superheated steam, with thermal efficiencies of approximately 80%, taking into account heat losses in both the furnace and the recovery boiler and the minimum temperature at the exit of the combustion gas from the recovery boiler.

Incineration processes are highly flexible in terms of the fuels that can be used, making it possible to treat municipal solid waste, industrial waste, hazardous waste, sewage sludge, or hospital waste.

Plasma Generation

Plasma is a state of matter formed from a gas subjected to high temperatures, in which virtually all atoms have been ionized. The result is a fluid consisting of a mixture of electrons, ions, and free neutral particles, collectively electrically neutral but conductive of electricity. The characteristics that define this process are as follows:

  • Plasma generation is achieved by flowing an inert gas through an existing electric field between two electrodes, forming what is known as a plasma arc.
  • Working temperatures range from 5,000°C to 15,000°C.
  • Within the gas, the following reactions occur: atom dissociation, loss of electrons from outer layers, and the formation of positively charged particles.
  • The principle of the process is as follows: if a gas is under the aforementioned conditions and is introduced into an electric field, an electric current will be generated, consisting of free electrons moving toward the positive pole of the electric field and positive particles toward the negative pole. This electric current determines a resistance and, thus, a transformation into heat dependent on the electric current’s intensity. By increasing the intensity of the electric field, electronic and cationic intensity increases, heat transformation, and the gas’s temperature also rise.
  • This process has practical limits related to the mechanical and thermal resistance of the electrodes.

Plasma, as a thermal method for waste treatment, offers three possibilities:

  • Treatment of hazardous gases, which are subjected to working temperatures, thus destroying their molecular structure. A clear example is the application for the destruction of PCBs, dioxins, furans, pesticides, etc.
  • Vitrification of hazardous waste, for both organic waste, destroying its molecular structure, and inorganic waste, by melting them into a vitrified mass. After cooling and solidifying the molten mass, the waste remains physically trapped within the vitrified mass, thus converting it into an inert solid, minimizing the possibility of leaching.
  • Plasma gasification, where the thermal energy contained in the plasma itself is used as a heat source, typically electrically, for its generation. This way, the final products obtained are: a gas, primarily composed of carbon monoxide and hydrogen, and solid residue, consisting of an inert slag usually vitrified.

Based on pilot plant tests, this technology could potentially treat a wide range of waste types, including municipal solid waste, industrial waste, biomass, healthcare waste, end-of-life vehicle waste, tires, plastics, and special waste, among others.

Landfill and Landfill Gas Utilization

In most current regulations, considering this option as a viable one is not advisable, as the amount of biodegradable waste deposited in landfills is decreasing. However, it is worthwhile to harness the energy of landfill gas, despite technical challenges (variable calorific value, presence of numerous contaminants in the gas, aggressive conditions for cogeneration engines or microturbines, etc.).