Thermal energy production

The hierarchy of waste management establishes the treatment type and priority that waste should receive; e.g. when reuse and recycling are not possible, recovery must be considered.

“Recovery” consists of any operation whose main result is for waste to serve a useful purpose by replacing other materials that would otherwise be used for a particular function; or when waste is prepared to fulfill that function in the plant or in the economy in general.

In energy recovery, the waste is mainly used as fuel or in some other way to produce energy.

Energy recovery processes drastically reduce the volume of waste while producing usually electrical or thermal energy. Energy is often consumed in the process itself, such that waste management finds farther savings in the purchase of electricity.

There are different waste treatments by which the energy can be recovered. The most appropriate treatment depends on the type of waste and its chemical composition. Thus, in general terms, the main processes used by Condorchem Envitech are the following:

Biomethanization

Biomethanization is a biological, multi-stage process in the absence of oxygen, which transforms the most degradable fraction of organic matter into biogas. This is performed by a heterogeneous population of microorganisms forming a mixture of gases, consisting mainly of methane and carbon dioxide, with other gases in a smaller proportion (e.g. water vapor, CO, N₂, H₂, H₂S, etc.).

Biogas is a source of energy because it is a combustible gas with a high heat capacity (5,750 kcal/m3), whose energy is used in cogeneration engines, boilers and turbines (generating electricity, heat or as a biofuel).

The type of material to be digested greatly influences the yield and composition of the biogas obtained. For maximum production, it is best to use waste rich in fats, proteins or carbohydrates, as their degradation entails the formation of significant amounts of volatile fatty acids, precursors of methane.

Biomethanization is an appropriate process for the treatment and recovery

Pyrolysis

Pyrolysis is the thermal degradation of a material in the absence of added oxygen, so decomposition occurs by heat, without combustion reactions. The basic details of this process are shown below.

• The only oxygen present is that in the waste to be treated.
• Working temperatures are between 300°C and 800°C.
• As a result of the process, the following is obtained:
  • Synthesis gas (whose basic components are CO, CO₂, H₂, CH₄) and more volatile compounds from the cracking of organic molecules, together with those already existing in the waste.
  • Liquid waste, basically composed of long chain hydrocarbons such as tars, oils, phenols or waxes, formed by condensing at room temperature.
  • Solid residue, composed of all non-combustible materials, which have either not been transformed or which form as a result of a molecular condensation, along with a high content of coal, heavy metals and other inert components already in the waste.
• As the most volatile compounds do not oxidise, the calorific value of the synthesis gas from the pyrolysis process ranges from 10-20 MJ/Nm³.

Low working temperatures cause less volatilization of carbon and other precursor pollutants in the gas stream, such as heavy metals or dioxins. Therefore, combustion gases will theoretically require less treatment to meet the minimum emission limits established in the Incineration Directive. Compounds that do not volatilize will remain in the pyrolysis residues and will need to be properly managed.

To treat waste by pyrolysis, a series of requirements must be met. However, it is difficult to establish the type of waste considered adequate or inadequate, since it is closely related to the type of reactor used and the operating conditions. Basically, the waste considered most suitable is paper, cardboard, wood chips, garden waste and selected plastics. Whereas, bulky waste, metals, construction materials, hazardous waste, glass and plastics such as PVC are not acceptable.

Gasification

Gasification is a partial oxidation process of matter in the presence of quantities of oxygen lower than those required stoichiometrically. In general terms, the details for the gasification process of a waste stream are the following:

• Air, oxygen or steam is used as a source of oxygen, and sometimes as a carrier to remove reaction products.
• Working temperatures are typically over 750°C.
• The chemical reactions produced in this process are of two types: molecular cracking – the temperature causes the weaker molecular bonds to break into smaller molecules which are usually volatile hydrocarbons – and gas reforming. These gasification reactions are process specific and the water vapor in them usually intervenes as a reagent.
• As a result of the gasification process, the following is obtained:
  • Synthesis gas, composed mainly of CO, H₂, CO₂ and N₂ if air is used as a gasifier, and CH₄ in a smaller proportion. Tars, halogenated compounds and particles as secondary products.
  • Solid residue of non-combustible and inert materials present in the waste, which usually contains part of the ungasified carbon. The properties of this residue are similar to slag from incineration plant furnaces.
  • The amount, composition and calorific value of gases from gasification depend on the waste composition and the temperature and volume of air and steam used.

Synthesis gas from the gasification process potentially has several uses:

• As a raw material in the production of organic compounds, such as the direct synthesis of methanol, ammonia, or for its transformation into hydrogen by steam or catalytic reforming.
• As a fuel in electricity production by thermal cycles other than those of water vapor, whether combined or simple cycles, in gas turbines or internal combustion engines.
• As fuel in traditional boilers or ovens.

However, not all waste is appropriate for gasification; it can be used to treat only specific materials. The material fed must be ensured to have the following properties: the minimum content of inert or wet components; a particle size of 80-300 mm; a sufficient amount of carbon to be able to carry out the gasification process reactions, without dangerous substances; and, if possible, a high calorific value.

Incineration

During incineration, combustion takes place, which is a chemical reaction based on total thermal oxidation in excess of oxygen. The general requirements for waste incineration are the following:

• Oxygen in excess, with respect to the stoichiometric reaction, during combustion to ensure complete oxidation.
• The combustion temperature is typically between 850°C and 1,100°C after the last secondary air injection, depending on the halogenated compound composition of the residue to be treated.
• As a result of the incineration process, the following is obtained:
  • Combustion gases, composed mainly of CO₂, H₂O, unreacted O₂ and N₂ from the combustion gas air supply, as well as other compounds in smaller proportions from the different waste components. The components present in lesser quantities will depend on the composition of the waste to be treated. For example, they may contain acid gases from reactions with halogens, sulfur, volatile metals or organic compounds that have not oxidized. Finally, the combustion gases will contain particles, which are carried by the gases.
  • Solid waste, mainly composed of inert slag, ash and waste from the combustion gas purification system.

The overall process converts practically all the chemical energy contained in the fuel into thermal energy, leaving some chemical energy not converted in the combustion gas and a very small part of the chemical energy not converted in the ash. The heat from this process is used to produce superheated steam, with thermal yields of the order of 80%, due to the heat losses in both the furnace and the boiler and the minimum temperature required for the combustion gas exit from the recovery boiler.

The incineration processes are very flexible in terms of heterogeneous fuels, so they can treat urban rubbish (MSW), industrial waste, hazardous waste, sewage sludge and hospital waste.

Plasma production

• Plasma is a state of matter, consisting of a gas subjected to high temperatures in which virtually all the atoms have been ionized. The result is a fluid formed by a mixture of electrons, ions and free neutral particles, being as a whole electrically neutral, but conducting electricity.

  The description of this process is as follows:

  • Plasma is produced by passing an inert gas through an electric field between two electrodes, forming the so-called plasma arc.
  • Working temperatures vary between 5,000-15,000°C.
  • The following reactions occur within the gas: dissociation of atoms, loss of electrons from the outer layers and formation of positively charged particles.
  • The rationale for the process is as follows: if a gas in the above conditions is introduced into an electric field, an electric current will be produced, as a result of the free electrons going to the positive pole of the electric field, and the positive particles to the negative. This electrical current determines a resistivity and, therefore, a transformation into heat that depends on the electrical intensity. Thus, increasing the intensity of the electric field increases the electronic and cationic intensity, the transformation into heat and the temperature of the gas.
  • A practical limit to this process is the mechanical and thermal strength of the electrodes.

  As a thermal method for waste treatment, plasma has 3 possibilities:

  • Treating hazardous gas, whose molecular structure is destroyed by subjection to the working temperatures. Clear examples are the destruction of PCBs, dioxins, furans and pesticides.
  • Vitrification of hazardous waste, for both organic waste – whose molecular structure is destroyed – and inorganic, by fusing it into a vitreous mass. After cooling and solidifying the melt, the waste is physically captured within the vitreous mass, becoming an inert solid and minimizing the possibility of leaching.
  • Plasma gasification, in which the thermal energy contained in the plasma itself is used as a source of heat from the (normally electrical) energy consumed for its production. Thus, the final products are: a gas, consisting mainly of carbon monoxide and hydrogen, and a solid residue, consisting of an inert, generally vitrified slag.

As a result of the tests carried out in the pilot plant, this technology could treat a wide variety of waste, such as MSW, industrial waste, biomass, sanitary waste, vehicle scrapping, tires, plastics and special waste.