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 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 of agricultural, livestock and urban waste, as well as for the stabilization of sludge from urban wastewater treatment.
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.
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 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:
Synthesis gas from the gasification process potentially has several uses:
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.
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:
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 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:
As a thermal method for waste treatment, plasma has 3 possibilities:
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.