Insineration is applied to the disposal of a wide range of waste, and the process itself is only one part of the overall complex system for the treatment of waste materials. Over the past few years there has been rapid technological development in the sector – solutions are being developed that reduce costs while offering better environmental characteristics.
The purpose of the incineration of waste is to reduce their volume and to capture or destroy the harmful components in their composition. Insineration also provides an opportunity to recover energy or raw materials from waste.
The waste flow is usually highly heterogeneous in nature, with organic and inorganic substances, metals and water in its composition. In the case of combustion, flue gases are released, in which, in the form of heat, most of the energy obtained by the process is contained. Organic substances in the waste burn when the required flash point and contact with oxygen is reached. The actual combustion process from which energy is emitted takes place in the gas phase and lasts a fraction of a second. With a sufficiently high calorific value of the waste and the presence of the required amount of oxygen, thermal chain reaction and autonomous combustion can result, i.e. eliminate the need to add other fuels.
Different types of incineration technologies apply to different types of waste. Solid municipal waste (SMW) can be treated in incinerators with a removable grille or a fluidized layer, or in rotary furnaces. The use of fluidized bed facilities necessitates pre-treatment and / or separate waste collection to ensure the correct particle size.
Sludge from waste water treatment plants (WWTPs) can be incinerated in rotary furnaces, multi-storey incinerators or those with a fluidized bed.. Co-incineration is also applied in grid systems, coal thermal plants and industrial processes. WWTP sludge often has a high moisture content and therefore requires drying or adding additional fuels to ensure stable and efficient combustion.
Hazardous and hospital waste is usually incinerated in rotary furnaces, but for some solid waste incinerators with a grid are also used. Some pre-treated materials are also burned in incinerators with a fluidized layer.
Incinerators with grid
These facilities are widely used for the incineration of mixed municipal waste. This technology is used in approximately 90% of SMW treatment plants in Europe. Other types of waste that are often incinerated with solid waste in lattice incinerators are non-hazardous waste from the commercial and industrial sector, WWTP sludge and some hospital waste.
Grid incinerators usually consist of a power supply mechanism, an incinerator grille, a bottom ash release device, an air system, a combustion chamber and auxiliary burners.
The waste is fed from the storage hopper through the supply shaft, then through a hydraulic ramp or other type of conveyor system pass over the grille. It moves waste materials through different areas of the combustion chamber.
The main purpose of the incinerator grille is the good distribution of combustion air in the chamber. Primary air duct inserts air into the waste layer through the small holes of the grille. Usually, to ensure complete combustion, more air is fed over the waste layer.
Often some fine particles of waste pass through the grid, and this material is subsequently released with the bottom ash or separated from it. It can be returned to the grille for re-incineration or removed for direct disposal. In the case of a recital, care should be taken of the risk of ignition of the waste in the feed hopper from combustion residues. Usually the retention time of the waste on the grid is no more than 60 minutes.
The grille is cooled in order to control the temperature of the metal and extend its service life. The cooling medium can be air or water, and it is possible to use other liquids such as oils and various heat-conductive fluids. The cooling fluid moves from colder areas of the grid to increasingly hot to maximize heat exchange. Heat absorbed by the cooling medium can be used in the process of iteration or in a process external to the system.
The bars are of several types – rocking upwards and forwards, performing incremental movement, conveyor or roller. The design of the combustion chamber is largely determined by the type of grille. In turn, combustion chambers are three types, differing in the direction of flue gas movement relative to the waste stream – straight, anti-current and centrally located flue gas hole.
Incinerators shall be designed and operated in such a way as to achieve good combustion of combustion gases by maintaining them at a minimum low temperature and oxygen content for as little retention time as possible. Typical values applied are a temperature of 850 to 1100°C at least 6 % oxygen content for at least 2 s.
During operation, burners are turned on automatically if the temperature drops below the specified value. When the equipment is stopped, auxiliary burners shall be used when there is unburnt waste in the chamber and for maintaining the desired temperature.
They are very stable facilities and are applicable to the incineration of almost all types of waste, regardless of their composition. Rotary furnaces are widely used for the incineration of hazardous waste.
Operating temperatures in rotary furnaces range from 500 to 1450°C. When used for conventional oxidative combustion, the temperature in the furnace is usually above 850°C.
Some furnaces have a cooling casing (using air or water) that helps to prolong refractory and hence the period between maintenance stops.
At high temperatures, water cooling is applied in the furnace. The water cooling systems of rotary furnaces consist of two contours. The primary contour delivers water to the furnace and distributes it evenly to achieve a uniform cooling effect throughout its housing. The water then enters pools located under the furnace, from where it passes into the collection tank. The water is recirculated through a filter and heat exchanger by means of a circulation pump. The secondary contour takes heat away from the primary through heat exchangers and returns it for reuse.
The system delivers the cooling water through hundreds of nozzles located above the furnace housing, maintaining its temperature between 80 and 100°C. Cooling of rotary furnaces increases heat exchange through the refractory layer sufficiently to reduce the rate of chemical erosion to the maximum.
The rotary furnace consists of a cylindrical vessel with a slight slope along the horizontal axis. The container is usually laid on rollers, allowing its rotation and oscillation around the axis. The waste moves through the furnace thanks to rotational gravity. For liquid, gaseous and pasty waste, direct feed in the furnace shall be applied, especially where they pose a risk to the safety of operators at exposure.
The retention time of solid materials in the furnace is determined according to the angle of inclination of the vessel and the speed of rotation – usually between 30 and 90 minutes are sufficient to achieve a good degree of waste incineration.
In order to improve the destructive of toxic compounds, rotary furnaces are equipped with a post-combustion chamber, and in order to achieve a sufficiently high temperature, additional fuel may need to be added in the form of liquid waste.
Incinerators with a fluidized layer
This type of incinerators are widely used for finely separated waste, e.g. RDF (fuel from waste) and sludge from WWTP. The facility is a lined cylindrical combustion chamber, at the bottom of which is located a layer of inert material (sand or ash). Incineration waste shall be fed into the fluidized layer on its upper side or side by means of a pump, star feeder or auger.
Drying, evaporation, ignition and combustion processes take place in the fluidized layer. The temperature in the free space above the layer usually varies between 850 and 950°C. It is designed in such a way as to allow sufficient gas to stay in the fuel zone. The temperature in the fluidized layer itself is lower –about 650°C.
Due to good mixing in the reactor, the incinerator systems with a fluidized layer are distinguished by an even distribution of temperature and oxygen, which determines the stability of the process. The incineration of waste of a heterogeneous nature requires pre-treatment of the flow to ensure that it meets the particle size specifications.
Pre-treatment usually involves sorting and crushing larger inert particles, as well as shredding. The removal of materials of ferrous and non-ferrous metals may also be necessary. The waste particles should be of small size, often with a maximum diameter of 50 mm. Excess ash is usually removed from the bottom of the furnace. The heat generated by combustion may be caught by devices
located either in the fluidized layer itself or in the flue gas outlet.
The relatively high costs of pre-treatment processes for certain wastes are a limiting factor for the widespread use of these systems in large-scale projects. This challenge is addressed somewhat through the selective collection of certain waste and the development of quality standards for RDF. The combination of controlled quality waste and fluidized incineration can allow improvement in fuel process management and increase the potential for simpler and therefore cheaper flue gas treatment.
Depending on the speed of the gas and the design of the nozzles, the following technologies differ:
Stationary fluidized layer (under atmospheric or higher pressure) – the inert material is kwwed, but the resulting upward movement is not essential.
Rotary fluidized layer – a variant of the previous technology. The fluidized layer rotates, which contributes to a longer retention time in the combustion chamber. This type of equipment has been used to burn TBA since the 1990s.
Circulating fluidized layer – higher gas speeds in the combustion chamber lead to partial removal of waste and inert material from the fluidized layer. They return to the chamber through a recirculation pipeline.
At the start of the combustion process, the fluidized layer must be heated at least to the minimum flash point of the waste. This can be achieved by pre-heating the air with burners that remain switched on until combustion begins to self-sustain.
Usually, the bulk of the ash moves with the flue gas flow, which requires its release through treatment facilities. The actual percentage of bottom and flying ash is determined by the technology of the fluidized layer and the characteristics of the waste. Source: https://www.ecology-and-infrastructure.bg/bg/saorazheniya-za-izgaryane-na-otpadaci/2/377/