Cranes described in Reception transfer waste from the waste bunker into the feed system, which consists of feeding hoppers and chutes that feed the combustion chamber. The aim of the charging hopper is to feed the waste continuously and smoothly, in order to ensure steady combustion conditions.
The following three incineration systems are applicable in waste-to-energy plants, with the first one being the most prevailing and popular:
The conventional incinerator based on a moving grate consists of a burning zone on the grate, transporting material through the furnace. On the grate, the waste is dried and then burned at high temperatures with the aid of air supply. The ash (including non-combustible fractions of waste) leaves the grate as slag/bottom ash through the ash chute. Slag is quenched in a water bath and discharged by a hydraulically operated ram extractor, a device specifically designed for this purpose. The water bath also provides a gas seal to the furnace and prevents ingress of air and egress of dust and fumes.
The grate forms the bottom part of the furnace. The moving grate, if properly designed, transports and agitates the waste efficiently, as well as evenly distributing combustion air. The grate may be sectioned into individually adjustable zones, and the combustion air can usually be preheated to accommodate variations in the lower calorific value of the waste.
There are several different grate designs—including forward movement, backward movement, double movement, rocking and roller. Other alternatives may be suitable as well.
Grates are designed for the combustion of waste with a calorific value of 7,800 kJ/kg to 12,500 kJ/kg, without the need for auxiliary fuel.
A pusher-type feeder is used to feed the waste onto the grate. The waste layer breaks into smaller pieces as it falls between the feeder and the drying grates. Any moisture in the waste evaporates on the drying grate.
The drying grate is used for the evaporation of waste moisture and it is inclined for efficient transport of the waste and enhanced mixing. The movement of the alternate fixed and moving grate bars has the effect of continuously stoking and mixing the burning mass of waste. This enables a homogeneous fuel bed to be formed.
Combustion takes place on the burning grates, which are inclined for efficient transport of waste and enhanced mixing. The combustion air is uniformly supplied to the entire width of the grate so that the waste can be brought into uniform and efficient contact with the air to maximise the efficiency of combustion.
The combustion air is distributed depending upon the combustion characteristics by dividing the hoppers into several compartments to the waste flow direction. The quantity and temperature of the combustion air are controlled depending on combustion characteristics and waste property respectively, and
The unburned combustibles that remain fall between the combustion grate and the burn out grate, breaking into smaller pieces. Sufficient retention time is given to ensure that the unburned combustibles are thoroughly transformed to ash. The bottom ash (or slag) falls from the end of the grate through a chute into the bottom ash discharge system.
Individual hydraulic drives move each of the grates. This allows the thickness of the waste and ash layers to be controlled by adjusting the speed of each of the drives. The grate bar elements are designed to carry out their task reliably and with a long service life. They are made of high-grade chrome steel alloy castings and are capable of withstanding high temperatures whilst maintaining close tolerance in order to ensure a close fit between adjacent bars and proper air distribution across the grate. The air flowing through the grate also ensures the cooling of the grate bars.
The combustion chamber is made of membrane water tube walls lined with a refractory coating, completely integrated with the steam boiler to reduce heat loss and avoid air leaks. The external body of the combustion chamber consists of air-sealing steel plates and refractory materials designed to prevent air leaks. The exterior structure is heat insulated in order to prevent heat losses.
The general purpose of the design of the combustion chamber is to control combustion, reduce carbon monoxide production and ensure a minimum residence time of 2 seconds at a minimum temperature of 850˚C or 1,100 ˚C depending on chlorine content.
Incineration plants are designed, built, equipped and operated in such a way that the produced gas arises at a temperature of 850oC, in a controlled and homogenous manner. The measurements occur for two seconds, near the inner wall or at other representative points in the combustion chamber, according to the guidelines of the competent authority. If hazardous waste (content of halogenated organic substances more than 1%, expressed as chlorine) has to be incinerated, the temperature must be raised to1,100°C for at least two seconds.
The air distribution system consists of primary and secondary air fans, air preheater(s) and air dusts. The combustion air fan consists of a primary and a secondary air fan, which supply air to the grate and the secondary combustion chamber respectively. Approximately two-thirds of combustion air is supplied to the grate; the remaining third is diverted to the secondary combustion chamber. The primary air fan takes in air from the waste bunker. After heating, the air is sent to the air plenum below the grate through a duct work system with adjustable control dampers. The secondary air fan absorbs air from the boiler hall and sends it to the side walls of the secondary combustion chamber, through air preheaters and manually pre-set dampers. The Primary air temperature is adjusted to the waste calorific value. The air preheaters are supplied with low and medium pressure steam, depending on the required temperature.
Secondary air is supplied at the top of the combustion chamber to fully combust any carbon monoxide present. The geometry of the combustion chamber and location of the secondary air injection nozzles are carefully designed to ensure optimised combustion condition.
An auxiliary firing system is needed to ensure that the furnace temperature is maintained at above 850˚C (or 1,100˚C if needed) whilst there is waste on the grate, and to preheat the furnace to the same temperature at start-up, prior to waste introduction onto the grate. Oil-fired burners are more commonly used as the auxiliary firing system. During shutdown, the burners are used to decrease the temperature slowly and prevent a sharp variation in temperature which could cause the non-burning of the residual waste in the grate.
An incinerator based on a rotary kiln conducts a layered burning of the waste in a rotating cylinder. The material is transported through the furnace by the rotations of the inclined cylinder.
The rotary kiln is usually refractory lined but can also be equipped with water walls. The cylinder may be 1 to 5 meters in diameter and 8 to 20 meters long. The capacity may be as low as 2.4 t/day (0.1 t/hour) and can rise to approximately 480 t/day (20 t/hour). The excess air ratio is well above that of the moving grate incinerator, or even the fluidized bed. Consequently, the energy efficiency is slightly lower and may be up to 80 percent.
As the retention time of the flue gases is usually too short for a complete reaction in the rotary kiln itself, the cylinder is followed by, and connected to an afterburning chamber which may be incorporated in the first part of the boiler.
The rotary kiln may also be used in combination with a moveable grate, where the grate forms the ignition part and the kiln forms the burning-out section. This results in a very low level of unburned material in the slag. The slag leaves the rotary kiln through the ash chute.
Fluidized bed incineration is based on a principle whereby solid particles are mixed with the fuel and fluidized by air.
The reactor (scrubber) usually consists of a vertical refractory lined steel vessel, containing a bed of granular material such as silica sand, limestone, or a type of ceramic material. The fluidized bed technology has a number of appealing characteristics in relation to the combustion technique: reduction of dangerous substances in the fluidized bed reactor, high thermal efficiency, flexibility regarding multi-fuel input, and cost.
A main disadvantage of the fluidized bed for waste incineration is the usually demanding process of waste pre-treating before the fluidized bed so as to meet the rather stringent requirements for size, calorific value, ash content, and so forth. Because of the heterogeneous composition of MSW, it can be difficult to produce a fuel that meets the requirements at any given point.