rotary kiln incinerator process

rotary kiln incinerator - design and manufacture

rotary kiln incinerator - design and manufacture

The rotary kiln incinerator is manufactured with a rotating combustion chamber that keeps waste moving, thereby allowing it to vaporize for easier burning. Types of waste treated in a rotary kiln incinerator

The picture"photo 1"gives a schematic overvieuw of the systemmanufactured totreat the waste in a rotating drum, we use a counter current rotary kiln. There are 2 different types of rotary kiln, co-current rotary kilns and counter current rotary kilns. Read here more about the different types of rotary kiln.

Energy recovery is always an individual design, and very attractive is electricity. But electricity is also the most complicated and less economical profitable for small installations ( the min. capacity is 3 ton/h of waste). If heat can be used in another process on site, for example in a dryer. It has to be taken into consideration that a connection between incinerator and the production process (dryer) can be the most efficient solution. The disadvantage can be if there is a production stop of the incinerator, the process (dryer) can not always stop at the same time.

For example : We produce steam as energy recovery, also for electricity production. The post combustion is strictly vertical and the boiler also has a vertical design for evacuation of dust. Our design is made for continuous operation of a steam boiler. The next drawing gives a possible set up of the installation. This is our set-up, created by people with operation experience with incinerators, and it results in this lay-out.

Depending on the amount of Chlorine, S, N or other chemicals in the waste stream there is a wet or/and dry scrubbing system available for the flue gas treatment. Flue gas treatment systems are standard systems, and normal chemical reaction. So, for correct flue gas treatment we need to take care for:

rotary kiln incinerator-dongding incinerator manufacturer

rotary kiln incinerator-dongding incinerator manufacturer

Rotary kiln incinerator incineration treatment is one of the garbage solutions, which can solve the harmfulness of waste and realize the reduction of hazardous waste. So everyone knows the incineration...

The incineration treatment can solve the dangerous waste in a huge level and harmless treatment. At this stage, about 90% of the national hazardous waste treatment and management centers have selected ...

The solution is highly valued in todays social development, and at this stage, it has already had relevant solutions to prevent and cure. That is to carry out special incineration treatment. To apply a...

rotary kiln incinerator for hazardous waste incineration

rotary kiln incinerator for hazardous waste incineration

The hazardous waste rotary kiln incinerator is rotary kiln equipment widely used in the field of international industrial waste treatment, and its market share in the field of hazardous waste incineration is as high as 85%. Hazardous waste rotary kiln incinerator is designed to deal with a variety of refractory substances or large changes in moisture waste incineration. Because the combustion furnace of the rotary kiln incinerator operates at a certain speed, the residence time of solid waste on the surface of the rotary kiln will change with different speeds.

During the combustion process of the solid waste rotary kiln combustion furnace, the solid waste is subjected to strong mechanical collision in the high-temperature air and excessive oxygen. After the solid waste is completely incinerated in the rotary kiln, slag with a very low content of flammable matter and putrefied matter can be obtained. Therefore, the solid waste rotary kiln can handle a wide range of waste, especially in the field of hazardous waste incineration.

The solid waste rotary kiln incinerator provided by AGICO can process various materials in the form of solid, liquid, gas, or sludge. Enterprises with the following material treatment needs are recommended to purchase rotary kiln incinerator for solid waste treatment. These materials usually include:

When the rotary kiln incinerator is working, the furnace body drum is rotated, and the material is brought to the upper part of the cylinder by the high-temperature resistant lining plate on the inner wall, and the solid waste falls due to its own gravity. The solid waste rolls up and down in the cylinder and is in full contact with the air. Complete combustion is carried out through three stages of ignition, combustion, and burnout.

When the rotary incinerator starts to work, the solid waste is sent into the kiln from one end of the drum, and the hot flue gas is used to dry it first. During this period, the temperature in the furnace continues to rise. When the ignition temperature is reached, the solid waste starts to burn. With the rotation of the cylinder, the solid waste is burned out in the rotary kiln, and finally, the ash is discharged at the outlet of the cylinder.

When the water content of the solid waste is too large, a grate can be added to the rotary kiln incinerator process, that is, the second combustion chamber link. The flue gas discharged from the drum of the rotary kiln enters the second combustion chamber, where it is mixed with the air of the second combustion in the second combustion chamber for full combustion. After the secondary combustion process, all the flue gas can be burned, so as to achieve the goal of safe exhaust gas emission.

rotary kiln incinerator for sludge incineration

rotary kiln incinerator for sludge incineration

Sludge is a common solid waste in daily life, which is commonly found in sewer pipe networks and sewage treatment plants. The amount of sludge waste produced in the industry is huge each year, among which electroplating sludge, leather sludge, pickling sludge, oily sludge, sewage sludge, and other types of sludge discharge are increasing day by day.

The sludge waste contains a lot of harmful substances, including heavy metals, bacteria, pathogens, etc. If it is not treated, it will inevitably have a greater impact on the environment and human health. Sludge treatment came into being under such circumstances. Sludge treatment is the reduction, stabilization, and harmless processing of various types of sludge such as concentration, conditioning, dehydration, stabilization, drying, or incineration.

There are roughly four methods for sludge treatment in the world, namely reclamation, landfill, incineration, and land use. Most landfills are open-air sites. After the rain, a large amount of sludge containing oil that is difficult to degrade organic matter, heavy metals, and pathogenic microorganisms will corrode and penetrate the land, posing safety hazards to the soil, and causing new pollution to the surrounding environment of the city. In Europe, the treatment of sludge through sludge incineration has become a very important way. Due to stricter restrictions on sludge accumulation, incineration of sludge may become the only way to treat sludge.

The sludge incineration process needs to be carried out in a sludge rotary kiln. Sludge waste rotary kiln is a kind of hazardous waste rotary kiln incinerator, which is specially used to treat sludge waste. It is a rotary kiln calcining equipment integrating drying, heating, and calcination. During the sludge incineration process, all germs and pathogens are completely killed by the high temperature in the sludge rotary kiln, and the toxic and harmful organic residues are decomposed by thermal oxidation. The sludge is incinerated through the sludge rotary kiln to achieve the greatest reduction in volume, and the final product incineration ash is obtained. Incineration ash can be used as a raw material for cement production to fix heavy metals in the concrete to prevent them from re-entering the environment.

The internal temperature in the sludge rotary kiln incinerator is generally between 1350-1650, or even higher. The materials in the kiln are in a highly turbulent fluidization state during the incineration of sludge. Therefore, the harmful organic matter in the sludge in the kiln can be fully burned, and the incineration rate can reach 99.999%. Even stable organic matter such as dioxins can be completely decomposed.

Since the feeding point of the dried sludge is in a decomposition furnace higher than 850C, the furnace has a large heat capacity and a stable temperature, which effectively inhibits the formation of dioxin precursors. The practice of disposing of toxic and hazardous wastes in cement kilns at home and abroad shows that the concentration of dioxin emissions produced by waste incineration is far below the emission limit.

The waste gas and dust discharged from calcination are collected by the bag filter at the end of the kiln as cement raw materials and then enter the kiln for calcination. No hazardous waste fly ash is generated.

The refractory bricks, raw materials, kiln skin, and clinker in the sludge waste rotary kiln are all alkaline, which can absorb SO2, thereby inhibiting its emission. In the process of cement firing, the heavy metals in the sludge and slag can be fixed in the structure of the cement clinker. So as to achieve the role of being cured.

Through the treatment of the rotary kiln incinerator, the organic and inorganic components in the sludge can be fully utilized, and the resource utilization efficiency is high. The sludge contains some organic matter (above 55%) and combustible components, which will generate heat when calcined in the rotary kiln. The low calorific value of sludge is about 11MJ/kg, which is equivalent to lean coal in the sense of calorific value. Lean coal contains 55% ash and 10%-15% volatile matter and has a calorific value of 10-12.5MJ/kg.

The sludge rotary kiln incinerator has higher treatment efficiency, the incineration products can be used as cement raw materials, and the heat produced by incineration can be used for power generation, with significant economic benefits. It has become the first choice of incineration equipment for many sewage treatment plants and industrial solid waste treatment enterprises. As a high-quality rotary kiln supplier, AGICO welcomes you to send inquiries for details at any time.

rotary kiln incinerators on the rise in waste management

rotary kiln incinerators on the rise in waste management

Incineration has become an essential tool in managing industrial and municipal wastes more sustainably, as long-landfilled materials such as hazardous medical wastes, petrochemicals, ammunitions, and more, have proven detrimental to the environment.

While many types of incineration equipment exist, the rotary kiln, a diverse and flexible thermal processing machine, continues to gain market share as the preferred method of incineration for a number of reasons.

While an increasing focus on sustainability has improved waste generation and recycling, many wastes are simply not suitable for reuse or recycling. When recycling or reuse is not a viable option, incineration provides a much-needed outlet for wastes that cannot be recycled economically, or because of concerns over hazardous or toxic components.

Incineration significantly reduces the volume of waste by converting it to ash. This reduction in volume is a major environmental benefit, decreasing the amount of space the material will take up in a landfill and ultimately contributing to a declining need for landfills. A reduced volume also makes transportation more efficient (and more economical) because less hauling is required.

When disposed of, hazardous components pose environmental threats to soil, air, water, and human and animal life. Through thermal decomposition, incineration eliminates hazardous components from the waste, taking them out of the environment completely and preventing any potential associated environmental degradation.

Rotary kilns are used to carry out physical changes and chemical reactions in a variety of industrial settings. They are often chosen for their high-capacity and continuous production. When it comes to incineration, however, they are selected for one simple reason: flexibility.

The ability to simultaneously handle multiple waste streams has become essential in todays waste processing market. At one time, it was not uncommon for large manufacturers to have their own in-house incineration facility to handle the waste they produced, but as waste management became more complex, the industry moved to outsourcing incineration needs to avoid managing on-site facilities and the many complexities that accompany the task.

This developed into waste handling companies that collect waste from multiple sources and process it for a fee, which has since become the industry norm. In this type of business model, the rotary kiln provides the ideal processing solution, as waste processors need something that can handle the diverse range of waste streams they collect. If not for the rotary kiln, these processors would require multiple different types of systems to manage these waste streams.

The rotary kiln is also capable accepting feedstocks of varying moisture content and particle sizes, reducing the need for pre-treatment in many cases; from barrels to material fines, shredded waste, and everything in between, rotary kiln incinerators accept it all.

In addition to the industry as a whole changing, the make-up of the waste itself is changing as well. While advancements in technology are producing more sophisticated products than ever before, they are also yielding more complex waste streams.

The process parameters of incineration may vary depending on the types of wastes being handled. In general, however, incineration employs controlled temperature profiles in order to volatilize organic components in the waste, thus eliminating hazardous compounds.

Though indirect-fired kilns are available, almost all rotary kiln incinerator systems utilize a direct-fired rotary kiln, where the products of combustion are in direct contact with the waste being processed. Material and combustion gases are fed into the rotating drum, which is sealed at both ends to maintain the required temperature profile within the kiln.

Most incinerators are configured with a co-current air flow, meaning the material and products of combustion flow parallel to each other in the kiln (i.e., the waste material and combustion products are fed into the kiln at the same end).

Although these components are available separately, they are best purchased as a complete system from a single-source provider, as there are many integrated components and regulatory requirements that can be difficult to manage independently.

Incineration provides a valuable opportunity to manage both non-hazardous or hazardous wastes that are otherwise not economical or practical to recycle or reuse. As the incineration industry evolves to handle more complex and diverse waste streams, rotary kiln incinerators are becoming increasingly more widespread.

With more than 65 years of experience, FEECO is a world-renowned provider of custom thermal processing systems and rotary kiln incinerators. Our equipment is built to the highest quality standards for reliable long-term processing. Our Customer Service Team can also inspect, service, and repair rotary kilns of any brand. For more information on our rotary kiln incinerators or parts and service support, contact us today!

rotary kiln - an overview | sciencedirect topics

rotary kiln - an overview | sciencedirect topics

Rotary kilns are synonymous with cement making, being the workhorses of this industry. There are many types of rotary kiln arrangements for producing cement clinker with each incremental design goal aimed at improving energy efficiency, ease of operation, and product quality and minimizing environmental pollutants. Rotary cement kilns can be classified into wet-process kilns, semidry kilns, dry kilns, preheater kilns, and precalciner kilns. All of these are described in the book by Peray (1986) and many others, hence we will not dwell upon them here. Rather, we will briefly show the pertinent process chemistry and the heat requirements that drive them, so as to be consistent with the transport phenomena theme.

Rotary kilns have been used in various industrial applications (e.g., oil shale retorting, tar sands coking, incineration, cement production, etc.). The rotation of a cylinder-shaped vessel positioned longitudinally approximately 30 of the horizontal position ensures a continuous motion of catalyst between the entrance and exit of the kiln. With regard to the spent catalyst regeneration, the description of rotary kilns was given by Ellingham and Garrett [451]. There are two types of rotary kilns, i.e., direct fire and indirect fire.

The direct fire is a single shell vessel with rings added inside to slow the catalyst as it tumbles from the inlet (elevated part) towards outlet (lower part). The oxidation medium flows countercurrent to catalyst movement. The O2 concentration in the medium will decrease in the same direction because of its consumption. Therefore, the zone in the vessel located near the inlet may function as a stripper of volatile components of coke. The kiln is fired by gas burners directly against the outer shell of the vessel. The temperature inside the kiln is controlled by adjusting the burner heat, varying concentration of O2 in the oxidizing medium and its flow. The indirect fire kiln comprises a double-shell cylinder vessel. The inner shell is similar as that of the direct fire kiln. The space between the shells is heated either by combustion gas or steam. In some cases, the inner cylinder shell is ebullated allowing hot gases or steam to enter and contact the tumbled catalyst. The catalyst temperatures are controlled by monitoring the temperatures of the inlet and outlet gases. It is believed that Eurocat process evolved from a rotary kiln process by be improving the control of operating parameters such as temperature, gas flow, speed of rotation, etc.

The rotary kiln is used to process the lead-containing components resulting from the breaking and separation of waste batteries. The main components of a rotary kiln are an inclined cylindrical, refractory-lined reaction shaft equipped to rotate over rollers and a burner. Process heat is generated by burning fine coke or coal contained in the charge and by the exothermic heat of the PbO reduction by CO. This process produces molten lead and a slag with 35% Pb. A drawback of this technology is the short life of refractory liners.

The rotary kiln is a long tube that is positioned at an angle near horizontal and is rotated. The angle and the rotation allow solid reactants to work their way down the tube. Speed and angle dictate the retention time in the kiln. Gas is passed through the tube countercurrent to the solid reactant. The kiln is operated at high temperatures with three or four heating zones depending on whether a wet or dry feed is used. These zones are drying, heating, reaction, and soaking. Bed depth is controlled at any location in the tube with the use of a ring dam.

The most common reactor of this type is the lime kiln. This is a noncatalytic reaction where gas reacts with calcium carbonate moving down the kiln. Other reactions performed in the rotary kiln include calcination, oxidation, and chloridization.

Use of rotary kilns for hazardous waste incineration is becoming more common for disposal of chlorinated hydrocarbons such as polychlorinated biphenyls (PCBs). Flow in these kilns is cocurrent. Major advantages include high temperature, long residence time, and flexibility to process gas, liquid, solid, or drummed wastes.

The rotary kilns used in the first half of the twentieth century were wet process kilns which were fed with raw mix in the form of a slurry. Moisture contents were typically 40% by mass and although the wet process enabled the raw mix to be homogenized easily, it carried a very heavy fuel penalty as the water present had to be driven off in the kiln.

In the second half of the twentieth century significant advances were made which have culminated in the development of the precalciner dry process kiln. In this type of kiln, the energy-consuming stage of decarbonating the limestone present in the raw mix is completed before the feed enters the rotary kiln. The precalcination of the feed brings many advantages, the most important of which is high kiln output from a relatively short and small-diameter rotary kiln. Almost all new kilns installed since 1980 have been of this type. Figure1.4 illustrates the main features of a precalciner kiln.

The raw materials are ground to a fineness, which will enable satisfactory combination to be achieved under normal operating conditions. The required fineness depends on the nature of the raw materials but is typically in the range 1030% retained on a 90 micron sieve. The homogenized raw meal is introduced into the top of the preheater tower and passes downwards through a series of cyclones to the precalciner vessel. The raw meal is suspended in the gas stream and heat exchange is rapid. In the precalciner vessel the meal is flash heated to ~900C and although the material residence time in the vessel is only a few seconds, approximately 90% of the limestone in the meal is decarbonated before entering the rotary kiln. In the rotary kiln the feed is heated to ~ 1500C and as a result of the tumbling action and the partial melting it is converted into the granular material known as clinker. Material residence time in the rotary kiln of a precalciner process is typically 30 minutes. The clinker exits the rotary kiln at ~ 1200C and is cooled to ~60C in the cooler before going to storage and then being ground with gypsum (calcium sulfate) to produce cement. The air which cools the clinker is used as preheated combustion air thus improving the thermal efficiency of the process. As will be discussed in section1.5, the calcium sulfate is added to control the initial hydration reactions of the cement and prevent rapid, or flash, setting.

If coal is the sole fuel in use then a modem kiln will consume approximately 12 tonnes of coal for every 100 tonnes of clinker produced. Approximately 60% of the fuel input will be burned in the precalciner vessel. The high fuel loading in the static precalciner vessel reduces the size of rotary kiln required for a given output and also reduces the consumption of refractories. A wider range of fuel types (for example, tyre chips) can be burnt in the precalciner vessel than is possible in the rotary kiln.

Although kilns with daily clinker outputs of ~9000tonnes are in production in Asia most modem precalciner kilns in operation in Europe have a production capability of between 3000 and 5000 tonnes per day.

A rotary kiln is a physically large process unit used in cement production where limestone is decomposed into calcium oxide which forms the basis of cement clinker particles under high temperatures. The modelling of rotary kilns are well documented in literature. Mujumdar et al. 2007 developed an iteration based rotary kiln simulator (RoCKS), which integrates models for a pre-heater, calciner, kiln and clinker cooling that agreed well with observations in industry. The model takes complexities in reactions and heat transfers with different sections into account by coupling multiple models with common boundaries regarding heat and mass communications. Other work (Ngadi and Lahlaouti, 2017) neatly demonstrates an experimentally proven kiln model being applied for screening of combustion fuel used for kilns, and how it may impact the production. This contribution coupled modelling of reactions and heat transfer in the bed region and another model for combustion and heat transfer in the freeboard region.

While modelling of these processes with varying degree of complexity has been performed, proper uncertainty and sensitivity analysis of these models have not been given due importance/consideration. As the use of computer aided process engineering tools increases, the need for robust uncertainty and sensitivity analysis frameworks becomes more important. There are several frameworks of uncertainty and sensitivity analysis applied for different problems, from good modelling practice (Sin et al., 2009) to process design and product design (Frutiger et al. 2016). These frameworks typically include the following steps (0) problem statement, (i) identification of input sources of uncertainties, (ii) sampling (iii) Monte Carlo simulations and (vi) sensitivity analysis. The purpose of this work is to perform a systematic uncertainty and sensitivity analysis of rotary kiln process design in order to address the following: (1) Given a certain base case design, what is the impact of uncertainties in the model and measurements on the key process design metrics (minimum required reactor length and degree of conversion), and, (2) given a certain source of uncertainties, what is the robust design to ensure process performance with 95 % confidence.

The rotary kiln is often used in solid/liquid waste incineration because of its versatility in processing solid, liquid, and containerized wastes. The kiln is refractory lined. The shell is mounted at a 5 degree incline from the horizontal plane to facilitate mixing the waste materials. A conveyor system or a ram usually feeds solid wastes and drummed wastes. Liquid hazardous wastes are injected through a nozzle(s). Non-combustible metal and other residues are discharged as ash at the end of the kiln. Rotary kilns are also frequently used to burn hazardous wastes.

Rotary kiln incinerators are cylindrical, refractory-lined steel shells supported by two or more steel trundles that ride on rollers, allowing the kiln to rotate on its horizontal axis. The refractory lining is resistant to corrosion from the acid gases generated during the incineration process. Rotary kiln incinerators usually have a length-to-diameter (L/D) ratio between 2 and 8. Rotational speeds range between 0.5 and 2.5 cm/s, depending on the kiln periphery. High L/D ratios and slower rotational speeds are used for wastes requiring longer residence times. The kilns range from 2 to 5 meters in diameter and 8 to 40 meters in length. Rotation rate of the kiln and residence time for solids are inversely related; as the rotation rate increases, residence time for solids decreases. Residence time for the waste feeds varied from 30 to 80 minutes, and the kiln rotation rate ranges from 30 to 120 revolutions per hour. Another factor that has an effect on residence time is the orientation of the kiln. Kilns are oriented on a slight incline, a position referred to as the rake. The rake typically is inclined 5 from the horizontal.

Hazardous or non-hazardous wastes are fed directly into the rotary kiln, either continuously or semi-continuously through arm feeders, auger screw feeders, or belt feeders to feed solid wastes. Hazardous liquid wastes can also be injected by a waste lance or mixed with solid wastes. Rotary kiln systems typically include secondary combustion chambers of afterburners to ensure complete destruction of the hazardous waste. Operating kiln temperatures range from 800C to 1,300C in the secondary combustion chamber or afterburner depending on the type of wastes. Liquid wastes are often injected into the kiln combustion chamber.

The advantages of the rotary kiln include the ability to handle a variety of wastes, high operating temperature, and continuous mixing of incoming wastes. The disadvantages are high capital and operating costs and the need for trained personnel. Maintenance costs can also be high because of the abrasive characteristics of the waste and exposure of moving parts to high incineration temperatures.

A cement kiln incinerator is an option that can be used to incinerate most hazardous and non-hazardous wastes. The rotary kiln type is the typical furnace used in all cement factories. Rotary kilns used in the cement industry are much larger in diameter and longer in length than the previously discussed incinerator.

The manufacture of cement from limestone requires high kiln temperatures (1,400C) and long residence times, creating an excellent opportunity for hazardous waste destruction. Further, the lime can neutralize the hydrogen chloride generated from chlorinated wastes without adversely affecting the properties of the cement. Liquid hazardous wastes with high heat contents are an ideal supplemental fuel for cement kilns and promote the concept of recycling and recovery. As much as 40% of the fuel requirement of a well-operated cement kiln can be supplied by hazardous wastes such as solvents, paint thinners, and dry cleaning fluids. The selection of hazardous wastes to be used in cement kiln incinerators is very important not only to treat the hazardous wastes but also to reap some benefits as alternative fuel and alternative raw material without affecting both the product properties and gas emissions. However, if hazardous waste is burned in a cement kiln, attention has to be given to the compounds that may be released as air emissions because of the combustion of the hazardous waste. The savings in fuel cost due to use of hazardous waste as a fuel may offset the cost of additional air emission control systems in a cement kiln. Therefore with proper emission control systems, cement kilns may be an economical option for incineration of hazardous waste.

The rotary kiln gasifier is used in several applications, varying from industrial waste to cement production and the reactor accomplishes two objectives simultaneously: (1) moving solids into and out of a high temperature reaction zone and (2) assuring thorough mixing of the solids during reaction. The kiln is typically comprised of a steel cylindrical shell lined with abrasion-resistant refractory to prevent overheating of the metal and is usually inclined slightly toward the discharge port. The movement of the solids being processed is controlled by the speed of rotation of the kiln.

The moving grate gasifier is based on the system used for waste combustion in a waste-to-energy process. The constant-flow grate feeds the waste feedstock continuously to the incinerator furnace and provides movement of the waste bed and ash residue toward the discharge end of the grate. During the operation stoking and mixing of the burning material enhances distribution of the feedstocks and, hence, equalization of the feedstock composition in the gasifier. The thermal conversion takes place in two stages: (1) the primary chamber for gasification of the waste (typically at an equivalence ratio of 0.5) and (2) the secondary chamber for high temperature oxidation of the synthesis gas produced in the primary chamber (Grimshaw and Lago, 2010; Hankalin et al., 2011).

The rotary kiln ICM/Phoenix Bioenergy demonstration gasifier was operated at a transfer station in Newton, Kansas from 2009 to 2012 for more than 3200h, testing various types of biomass, RDF, tire-derived fuel or automobile shredded residue mixed with RDF. The 150-t-per-day facility reported to have tested more than 16 types of feedstock listed in Table 3.2 [13].

The gasification process consists of a horizontal cylinder with an internal auger which slowly rotates [15] allowing feedstock to move through the reactor, whereas air is injected at multiple points. Only small portion of the syngas was used to produce steam, whereas the rest was flared (Fig. 3.2).

Unfortunately, ICM had to take down the demonstration gasifier at the transfer station, upon completion of the project and financing grant, declaring that the facility did not prove to be a viable solution for the county. Some of the problems that ICM mention [16] were related to the availability of feedstock of only 90t per day, whereas the prototype was designed for 150t per day, but also insufficient investment from financial partners due to the lower projected returns. ICM announced that through a contract with the City of San Jose, CA they will have the ICM demonstration gasifier at the San Jos-Santa Clara Regional Wastewater Facility [17]. The facility will process 10 short tons per day of woody biomass, yard waste or construction and demolition materials mixed with biosolids from the WWT.

rotary kiln incinerator - an overview | sciencedirect topics

rotary kiln incinerator - an overview | sciencedirect topics

The rotary kiln is often used in solid/liquid waste incineration because of its versatility in processing solid, liquid, and containerized wastes. The kiln is refractory lined. The shell is mounted at a 5 degree incline from the horizontal plane to facilitate mixing the waste materials. A conveyor system or a ram usually feeds solid wastes and drummed wastes. Liquid hazardous wastes are injected through a nozzle(s). Non-combustible metal and other residues are discharged as ash at the end of the kiln. Rotary kilns are also frequently used to burn hazardous wastes.

Rotary kiln incinerators are cylindrical, refractory-lined steel shells supported by two or more steel trundles that ride on rollers, allowing the kiln to rotate on its horizontal axis. The refractory lining is resistant to corrosion from the acid gases generated during the incineration process. Rotary kiln incinerators usually have a length-to-diameter (L/D) ratio between 2 and 8. Rotational speeds range between 0.5 and 2.5 cm/s, depending on the kiln periphery. High L/D ratios and slower rotational speeds are used for wastes requiring longer residence times. The kilns range from 2 to 5 meters in diameter and 8 to 40 meters in length. Rotation rate of the kiln and residence time for solids are inversely related; as the rotation rate increases, residence time for solids decreases. Residence time for the waste feeds varied from 30 to 80 minutes, and the kiln rotation rate ranges from 30 to 120 revolutions per hour. Another factor that has an effect on residence time is the orientation of the kiln. Kilns are oriented on a slight incline, a position referred to as the rake. The rake typically is inclined 5 from the horizontal.

Hazardous or non-hazardous wastes are fed directly into the rotary kiln, either continuously or semi-continuously through arm feeders, auger screw feeders, or belt feeders to feed solid wastes. Hazardous liquid wastes can also be injected by a waste lance or mixed with solid wastes. Rotary kiln systems typically include secondary combustion chambers of afterburners to ensure complete destruction of the hazardous waste. Operating kiln temperatures range from 800C to 1,300C in the secondary combustion chamber or afterburner depending on the type of wastes. Liquid wastes are often injected into the kiln combustion chamber.

The advantages of the rotary kiln include the ability to handle a variety of wastes, high operating temperature, and continuous mixing of incoming wastes. The disadvantages are high capital and operating costs and the need for trained personnel. Maintenance costs can also be high because of the abrasive characteristics of the waste and exposure of moving parts to high incineration temperatures.

A cement kiln incinerator is an option that can be used to incinerate most hazardous and non-hazardous wastes. The rotary kiln type is the typical furnace used in all cement factories. Rotary kilns used in the cement industry are much larger in diameter and longer in length than the previously discussed incinerator.

The manufacture of cement from limestone requires high kiln temperatures (1,400C) and long residence times, creating an excellent opportunity for hazardous waste destruction. Further, the lime can neutralize the hydrogen chloride generated from chlorinated wastes without adversely affecting the properties of the cement. Liquid hazardous wastes with high heat contents are an ideal supplemental fuel for cement kilns and promote the concept of recycling and recovery. As much as 40% of the fuel requirement of a well-operated cement kiln can be supplied by hazardous wastes such as solvents, paint thinners, and dry cleaning fluids. The selection of hazardous wastes to be used in cement kiln incinerators is very important not only to treat the hazardous wastes but also to reap some benefits as alternative fuel and alternative raw material without affecting both the product properties and gas emissions. However, if hazardous waste is burned in a cement kiln, attention has to be given to the compounds that may be released as air emissions because of the combustion of the hazardous waste. The savings in fuel cost due to use of hazardous waste as a fuel may offset the cost of additional air emission control systems in a cement kiln. Therefore with proper emission control systems, cement kilns may be an economical option for incineration of hazardous waste.

The incinerator is, simply, a furnace for burning refuse and modern incinerators include pollution mitigation equipment such as flue-gas cleaning. There are various types of incinerator plant design: (1) simple incinerator, (2) fixed or moving grate incinerator, (3) rotary kiln incinerator, and (4) the fluidized bed incinerator.

The simple incinerator is an older and simpler kind of incinerator that is, essentially, a brick-lined cell with a metal grate over a lower ash pit, with one opening in the top or side for loading and another opening in the side for removing incombustible solids (clinker). Many small incinerators formerly found in apartment houses have now been replaced by waste compactors.

The fixed or moving grate incinerator is a large fixed hearth incinerator with a moving grate. The moving grate enables the movement of waste through the combustion chamber to be optimized to allow a more efficient and complete combustion. These incinerators are typically used for combustion of municipal waste and are thus referred to as MSW incinerators.

In the fixed or moving grate incinerator the waste is introduced through the throat at one end of the grate, from where it moves down over the descending grate to the ash pit in the other end. Here, the ash is removed through a water lock. Part of the combustion air (primary combustion air) is supplied through the grate from below. This airflow also has the purpose of cooling the grate itself. Cooling is important for the mechanical strength of the grate, and many moving grates are also water cooled internally. Secondary combustion air is supplied into the boiler at high speed through nozzles over the grate. It facilitates complete combustion of the flue gases by introducing turbulence for better mixing and by ensuring a surplus of oxygen.

The incinerator must be designed to ensure that the flue gases reach a temperature of at least 850C (1560F) in order to ensure proper breakdown of organic toxins. This includes backup auxiliary burners (often fueled by oil), which are fired into the boiler in case the heating value of the waste becomes too low to reach this temperature alone. The flue gas is then cooled by heat transfer and heat the steam to typically 400C (750F) at a pressure of 550600psi for the electricity generation in the turbine. At this point, the flue gas has a temperature of around 200C (390F) and is passed to the flue gas cleaning system.

The rotary kiln incinerator has a primary chamber and secondary chamber. The primary chamber consists of an inclined refractory lined cylindrical tube. Movement of the cylinder on its axis facilitates the movement of waste. In the primary chamber, there is conversion of solid fraction to gases, through volatilization, destructive distillation, and partial combustion reactions. The secondary chamber is necessary to complete gas phase combustion reactions.

The clinker spills out at the end of the cylinder. A tall flue gas stack, fan, or steam jet supplies the needed draft. Ash drops through the grate, but many particles are carried along with the hot gases. The particles and any combustible gases may be combusted in an afterburner. To control air pollution the combustion product gases are further treated with acid gas scrubbers to remove sulfuric acid and emissions of nitric acid and then routed through baghouses to remove particulate matter before the gas is released into the atmosphere.

The fluidized bed incinerator uses a strong airflow through a sand bed until a point is reached where the sand particles separate to let the air through and mixing and churning occurs, thus a fluidized bed is created and fuel and waste can now be introduced. The sand with the pretreated waste and/or fuel is kept suspended on pumped air currents and takes on a fluid-like character. The bed is thereby thoroughly mixed and agitated keeping small inert particles and air in a fluid-like state. This allows all of the mass of waste, fuel, and sand to be fully circulated through the furnace.

The heat produced by an incinerator can be used to generate steam that may then be used to drive a turbine in order to produce electricity. However, the amount of energy produced is very much dependent upon the type and composition of the waste used as the feedstock. Much of the current thinking involves coincineration of a specified amount of the waste (calculated on the basis of the carbon content of the waste) with, for example, coal to produce a consistent amount of energy. However, the incineration process has a number of outputs such as (1) the mineral ash and (2) the emission to the atmosphere of flue gas. Before the flue gas cleaning, the flue gases may contain significant amounts of particulate matter, heavy metals, dioxin derivatives, furan derivatives, sulfur dioxide, hydrochloric acid, and polynuclear aromatic hydrocarbon derivatives (carcinogens).

The most publicized concerns related to the incineration of MSWs involve the fear that it produces significant amounts of emissions of dioxin and furan derivatives which are considered by many to be serious health hazards. Older generation incinerators that were not equipped with modern gas cleaning technologies were indeed significant sources of dioxin emissions. However, modern incinerators (due to advances in emission control designs and stringent regulations) must limit and even mitigate such emissions.

The quantity of pollutants in the flue gas from incineration plants is reduced by several processes. For example, taking an example from the gas processing industry (Chapter 9: Hydrocracking), particulate matter is collected by particle filtration most often electrostatic precipitators and/or baghouse filters (Mokhatab et al., 2006; Speight, 2019). Hydrogen chloride and sulfur dioxide are removed in scrubbers or as in a dry desulfurization process by injection of a limestone (CaCO3) as a slurry into the flue gas stream before the particle filtration step. Wastewater from scrubbers must subsequently pass through a wastewater treatment plant. Nitrogen oxide (NOx) emissions are either reduced by catalytic reduction with ammonia in a catalytic converted (a selective catalytic reduction process) or by a high-temperature reaction with ammonia in the furnace (a selective noncatalytic reduction process). Heavy metals are often adsorbed on injected active carbon powder (such as activate charcoal), which is collected by the particle filtration. A as is the case when coal is combusted, incineration of a biomass-based feedstock or organic waste also produces fly ash and bottom ash.

By the way of explanation, heavy metals are generally defined as metals with relatively high density, atomic weight, or atomic number. Fly ash is a combustion product that is composed of the particulates (fine particles of burned fuel) that are driven out of coal-fired boilers together with the flue gases. Fly ash is, thus, that portion of the ash that escapes up the chimney or stack. On the other hand, bottom ash is part of the noncombustible residue of combustion in a power plant, boiler, furnace, or incinerator. In an industrial context, it typically comprises traces of combustibles embedded in forming clinkers and sticking to hot side walls of a furnace during its operation. The clinker materials fall by themselves into the bottom hopper of a coal-burning furnace and are cooled. The above portion of the ash is also referred to as bottom ash.

The total amount of ash produced by MSW incineration ranges from 15% to 20% w/w of the waste and the fly ash amounts to 0%10% w/w of the total ash. The fly ash, by far, constitutes more of a potential health hazard than does the bottom ash because the fly ash often contains high concentrations of heavy metals such as lead, cadmium, copper, and zinc as well as small amounts of dioxin derivatives and furan derivatives. The bottom ash seldom contains significant levels of heavy metals.

While fly ash is always regarded as hazardous waste, bottom ash is generally considered safe for regular landfills after a certain level of testing defined by the local legislation. Ash, which is considered hazardous, may generally only be disposed of in landfills which are carefully designed to prevent pollutants in the ash from leaching into underground aquifersor after chemical treatment to reduce the leaching characteristics of the ash.

In spite of the promise shown by the use of incinerators for waste disposal and, in some cases, energy production (in the form of fuel gases), the use of incinerators for waste incineration is often controversial. However, the concerns over the health effects of the emissions of furan derivatives and dioxin derivatives have been significantly lessened by advances in emission control designs and very stringent new governmental regulations that have resulted in large reductions in the amount of dioxin derivatives and furan derivatives emissions emitted into the environment. However, there may be concerns related to the health effects of furan and dioxin emissions into the atmosphere from older generation incinerators.

Also, incinerators emit varying levels of heavy metals such as (alphabetically) arsenic, cadmium, chromium, lead manganese, mercury, nickel, and vanadium, which can be toxic at very minute levels. Other advanced alternative technologies are available such as biological treatment combined with anaerobic digestion and gasification.

Incineration units vary in size, design, and operation. They will all operate a combustion system that works mainly to reduce the volume and size of the feedstock materials (typically commingled SW) and temperature range. Incineration units can be divided into a number of categories based on the ability to destroy contaminants [7]. These categories are rotary kiln, moving grate, multiple heart, liquid injection, fluidized bed, and finally, multiple chamber.

However, before going into detail and before examining case studies of plastics used as feedstock to each type, in addition to the application of these units to waste materials in general, it should be noted that this classification is based on unit type and process specific categorization. In other words, there is another way of categorizing incineration units based on the feedstock treatment of the combustion system, which are mass burning and homogenized feedstock burning.

It should also be noted that mass burning is the most widely used and well-developed technology when it comes to incineration. Mass burning is the combustion of unsorted MSW, with the aim of converting feedstock into useable energy [8] under certain operating conditions. This technology requires little and almost no pretreatment of feedstock. The majority of treatment plants around the world operate this type of incineration using a moving grate operation.

This technique is also the most feasible when it comes to the economy of scale, and is considered the height of in rate of return of WtE technologies. However, just like in the case of all incineration units, there are major environmental implications associated with this technology. There has always been a risk of formation of dioxins [dioxin is the term collectively given to the structurally related polychlorinated chemicals to polychlorinated dibenzo para dioxins (PCDDs) and polychlorinated dibenzofurans], nitrogen oxides (NOx), and the deterioration of local air quality that will ultimately cause a public health concern from the pollutant exposure of its chimney stack. Mass burning using movable grates is considered to be the only technology that fulfills the criteria of proven feasibility by the World Bank [1].

Unlike mass burning technologies, pretreated incineration plants are very limited due to their complications in preparing and sorting the feedstock material. The reader should also note that energy recovery is done in a different part (or section) of the incineration process after the combustion chamber. Fig. 3.4 outlines the incineration unit plant as a whole to give the reader a better understanding of the process flow diagram of waste incineration.

In Fig. 3.4, it is key to understand that all types of combustion chambers (vessels) will operate to facilitate the material feedstock reaction with O2 to produce the heat required for energy recovery from the exothermic reaction. These materials are what we can consider as fuel to this process. In our case, plastic-rich feedstock or a plastic material. The furnace can also operate depending on the mode of the flow of gases and the feedstock material as depicted in Fig. 3.5.

The first type of mass burning incinerator unit that is considered quite common is the rotary kiln incinerator. Fig. 3.6 shows a typical rotary kiln incineration plant used in MSW incineration. The unique feature of this unit is the combustion chamber, which is a heated rotating cylinder mounted at an angle with baffles to add the required turbulence for the process. The rotary kiln itself consists of a layered burning unit where the material is transported through the furnace by the rotation of the cylinder.

Most organic materials are incinerated in this type of unit. It has been also reported to incinerate the solids and sludge. The temperature of operation in this type of process can vary between 800C and 1650C, and the kiln cylinder can be of a wide range in diameter between 1 and 5m, and 8 and 20m in length, with a throughput range between 0.1 and 20 tonnes per hour.

Moving grate technologies in incineration plants are predominant in the waste management market. The main feature of this type of unit is the transportation facility of the material, which is done through the furnace in a roller grate after an overhead carne feeds waste into a dedicated hopper (chute) to the furnace (Fig. 3.7).

Grates used in this process can vary in design and operation. The majority of which will consist of rows of bars which move with or against the waste flow (Fig. 3.8). The movement of the grate is what results in good mixing of the feedstock. Roller grates are also commonly found in many plants that deal with waste feedstock. Feedstock is first dried in the combustion chamber (100C) and then heated under pyrolytic (e.g., inert) conditions at about 250C. Oxygen (O2) gas is then supplied to have a combustion reaction occurring at 450C. Oxygen is also additionally supplied and large heat is then recovered as energy.

The design and movement of the grate is key in transporting and agitating the waste material. As it is a mass burning type of unit, there is no immediate need for pretreatment of the feedstock before incineration. In energy recovery installations (Fig. 3.7), the hot gases resulting from the combustion process are used to create steam for power generation purposes. Energy recovery will be discussed at later stages of this section. Also, it should be noted that moving grates give the highest possible treatment capacity, where throughput can be managed between 10 and 4300 tonnes per day [12].

Fluidized bed incinerators are, as their name implies, based on the principle of fluidization of the feedstock material by suspending the solids using air (fluidizing agent). Fluidization is a well-defined and reported unit operation in chemical engineering that has a very versatile application and use. Fluidized bed incinerators are also reported to incinerate waste very rapidly, and can deal with a number of feedstock from waste, including MSW and PSW.

The most common type of this unit in the pressurized fluidized bed combustion process where fuel (in this case MSW or PSW) is injected under pressure into the combustion chamber hosts inert sand particles that will transfer the heat to the fuel. Hence, fluidization has always been a good process in providing good heat and mass transfer, control of product range, and good control of the combustion reaction. Heat is also supplied throughout the combustion process as necessity persists. Air is supplied into the bottom of the particle bed that leads to their suspension during the fluidization process [6]. Table 3.2 gives a summary of the advantages and disadvantages of the main incineration unit technologies used in energy production from waste and consequently, plastic materials.

Another less common process is the multiple hearth combustion furnace used in the incineration of various contaminants (solids) and sewage sludge. The concept was developed back in the 1900s for treating and roasting iron ore (Fe2O3), where air cooled vertical cylinders are used to incinerate solids and sludge. This type of unit is not used for the treatment of MSW and certainly not plastics or polymeric waste on their own, but is considered a main type of incineration unit in waste management (Fig. 3.9).

The feedstock is slowly fed from the top through the stacked hearth. The outer shell is typically manufactured of steel, where a hollow cast iron rotating shaft runs through the center. Operating temperature of this unit can significantly vary from 800C to above 1600C.

A lesser common type of waste incineration unit is the electric infrared unit used in sludge treatment plants (Fig. 3.10). A furnace, coupled with a conveyer belt, will extend the length of the unit. Infrared heating elements will be placed on the top roof of the conveyer belt. The waste containing high moisture (such as sludge) will be dried along the length of the units conveyer. Ash will also be recovered from the process and excess air might vary significantly between 20% to 70% [13].

All the various designs of incineration units will share a number of common aspects. They will all allow the thorough mixing of feedstock material with air in temperatures high enough to achieve a combustion reaction between 750C and 1000C. They will all also allow flue gases to be scrubbed for removal of hazardous chemicals, and particulate matter (PM) will be removed by electrostatic precipitation and filters. While incineration occurs, the energy is recovered as heat from the flue gases resulting from the process.

All energy recovery schemes will require the cooling down of the flue gases by using a boiler, which then will allow the gases to exit and be part of the air purification and pollution control configuration of the plant. Three main types of boilers are available to cover the wider spectrum of energy and utility production. These are the hot water boiler, and low and high pressure (HP) steam boilers. The flue gases exiting the boiler will ultimately result in having heat, steam, power, or a combination of these utilities produced for the national grid or energy distributor, to be then used by the general public. The types of boilers are summarized in Table 3.3.

Various types of incineration units can be used with the available boilers and energy recovery configurations. In addition, the type of the waste and level of contaminants govern the process due to the units operation in handling the various types of hazardous waste. Fig. 3.11 shows how the steam boiler may be configured with the rotary kiln incineration process overall. The amount of energy recovered is governed by the previously stated parameters discussed in this chapter. As noted previously, grate type incineration units are the most common incinerators of waste plants. This technology is noted to produce large amounts of heat due to the constant oxygen supply provided. HP-steam boilers are a lucrative (and possible) route for the recovery of combined heat and power (CHP). Fig. 3.12 shows a schematic flow diagram of this process, with energy recovery and the flow gases cleanup processes as described previously in [1415].

Following development of the absorption monitoring system described in Section 4.1.2 [240], field measurements of Hg emissions were performed on a rotary kiln incinerator simulator (RKIS), where various gases (NO2, SO2, HCl etc.) were injected as potential interferants. These measurements were compared with data from the Ontario-Hydro method and a commercial CEM as references. The instrument showed sufficiently fast response to changes in gas conditions, as demonstrated by corresponding spikes in the signal. The pyrolysis tube was employed successfully to convert molecular Hg to elemental Hg prior to analysis. Total Hg concentrations up to about 20ppb (by volume) were measured and detection limits near 0.6ppb achieved. Comparison with the data from the other two analysers showed reasonable agreement, so long as the pyrolysis tube was kept at a high enough temperature (1100C), in order to atomise all the Hg that had been in molecular form. Relative accuracies are then given as 23%. Compared to most (commercial) CEM systems, this instrument is compact and requires relatively low maintenance, since no chemical solutions or traps are used. This also means that losses of Hg are minimised.

As mentioned in Section 4.1.2, commercial extractive CEMs have been developed [237239] and some are already approved by national authorities (e.g. TV in Germany) for flue gas measurements and have produced many hours of data at larger scale power plants.

The new in-situ AAS monitor for atomic Hg [242, Section 4.1.2.], using LED-absorption for background correction of SO2, has been tested in the stack gas of a commercial (230MW) coal-fired power plant. The monitor was installed just after the electrostatic precipitator, where particle loads were low. Hg- and SO2-concentrations were determined simultaneously and continuously for several hours from start-up of the plant burners. Internal quantification of the measured Hg data gave concentrations in the expected range of 130g/m3.and variations in Hg concentrations were observed to correspond to various plant operations such as start-up or switching from natural gas to coal firing. However, although SO2-concentrations could be verified by a standard measurement method, an equivalent method for Hg was not available for these first tests. Thus full evaluation of this technique awaits further tests, including comparison with an accepted standard method and measuring experience under different operational conditions.

The most commonly used and versatile technique ideal for the treatment of wastes includes a combination of solid, sludge and liquid waste streams is rotary kiln incineration. This unique feature led to its installation in commercial off-site incineration facility design. In general, rotary kiln is categorised into stationary and transportable rotary kiln. Rotary kiln and the secondary combustion chamber form an interrelated system with two-stage combustion and allow the treatment of special waste with strongly varying consistencies. The key components of the system are primary (waste is thermally treated and volatilized) and secondary chamber (combustion of volatilized compounds from primary chamber). The primary chamber is a slightly inclined cylindrical refractory lined shell that rotates at a horizontal angle of 5 with a speed of 15ft/min. The angle of inclination and rotation rate controls the transportation of feed waste to the secondary chamber and residence time of waste inside the primary chamber is 2h. The volatilized compounds generated from primary chamber enter into the secondary chamber may be aligned horizontally or vertically. The kiln incinerators are equipped with air pollution control equipment, shredder, stack and chambers lined with acid-resistant refractory brick. The foremost function of the rotary kiln is converting the waste into inorganic ashes and uncombusted organic gases through partial combustion reaction. The ash is removed in the shredder or ash bin while the uncombusted organic gases are completely destructed in the secondary chamber. In the US, hazardous wastes are treated using rotary kiln incinerator with a capacity of 60106 Btu/h (Oppelt, 1987). This type of technique is used predominantly in European countries than US for dioxins treatment. The rotary kiln incinerator at Ellesmere Port in Cheshire is the most advanced high-temperature incinerator facility to treat a wide range of industrial wastes in Europe. The high temperature, bulk feed capacity and continuous removal of ash content make the rotary kiln incinerator most suitable for the destruction of toxic compounds. Therefore, the most important factors that influence the combustion conditions are retention time, temperature and turbulence. However, the high installation cost of secondary chamber and high particulate loading are the major limitations of rotary kiln incinerators.

The most traditional, economic and universal form of remediation technology is landfill capping method. This technology buries waste materials and forms a barrier between the waste and the ecosystem, thereby; manages and limits the migration and ecological risks associated with the remediation sites. The barrier and drainage layer are considered as the most critical components of a landfill capping. The cap is specific and restricts the infiltration of surface water inside and leachate generation. Therefore, this technique has the potential to avoid the redistribution of toxic compounds like dioxins and furans from the remediation site. The complexity of landfill capping system depends on the type of waste and may be temporary or permanent. This contains only hazardous waste rather than reducing the pollutants toxicity. This method is relatively ineffective and generates volatile compounds and fluid waste. Chiarenzelli et al. (1998) reported the escape of polychlorinated biphenyls from the remediation site by volatilizing into the air and soil. Moreover, tracking the movement of leached or volatilized hazardous compounds from the containment is difficult. Thus, landfill capping containment technique is not advisable for treating persistent chemicals like dioxins and furans.

Liquid waste disposal technology buries concentrated industrial wastewater, pesticide disposal waste, volatile organic compounds and explosive wastes, far beneath the geological strata and restricts its contact with the ecosystem. In 1985, US-EPA disposed dioxins containing waste rinsates by deep well injection. Over the past decades, US treated 30 trillion gallons waste by deep well injection. There are more than 250 liquid waste disposal systems in US and most of them are in Texas. However, Food and Agriculture Organization in 1996 defined deep well injection system as an unsuitable method due to its high environmental risk and lack of control. Though, this has been the suitable method for storing hazardous waste for decades, understanding the behaviour of chemicals injected in deep wells is least known. Besides, this injection well induces earthquakes, eventually; the deleterious chemicals enter into the environment. These limitations make the deep well injection method as an inappropriate method for treating dioxins and furans. Hence, it is used exceptionally or rarely.

Since the advent of WtE plants, the objectives of MSW treatment changed rapidly with more attention being directed towards heat recovery. Additionally, regulations governing the disposal of incineration ash as well as flue gas emissions were becoming more stringent. As a result, the adoption of MSWI as an option for waste management demanded the development of robust technology capable of achieving three things: volumetric reduction of the MSW, optimal recovery of heat and materials as well as cleaning the resulting flue gas to meet prevailing emission limits [43,66].

Early incinerators were categorized into continuous feed, batch-feed, ram-feed, metal conical and waste heat recovery incinerators [49]. Continuous feed incinerators were further grouped into traveling grate incinerators, reciprocating incinerators, rotary kilns and barrel-grate incinerators. They differed from batch-feed incinerators in that the latter used a system where refuse was fed at periodic intervals allowing the previously-fed batch to burn almost completely. That way, continuous feed incinerators had the capacity to handle larger amounts of waste in comparison to batch incinerators. Ram feed and metal conical incinerators were only variations of batch feed incinerators. Of these early incinerators, only waste heat recovery incinerators were incorporating mechanisms to recover heat while the rest were designed with the primary objective of MSW volume reduction and waste inertization. Among the early waste-heat recovery incinerators were low-pressure boilers, high-pressure boilers, and water wall furnaces. Low-pressure boilers were the first to be developed and the majority of them had boilers located in the combustion chamber which lowered combustion efficiency as a result of excessive cooling of the furnace [49,58]. High-pressure boilers were later developed with refractory linings that prevented excessive cooling of the furnace and had an additional advantage of effective cooling of flue gases to the required range of 250300C (482572F). Water wall furnaces were first applied in Europe and they had higher heat recovery efficiencies than low and high-pressure boilers. The major applications of the recovered heat were in providing hot water for domestic and industrial heating, sewage sludge drying as well as seawater desalinization for the provision of potable water to households in coastal areas [58]. Heat recovery for electricity began around the middle 20th century with the first plant being built in Paris, France [29]. Towards the end of the 20th century, as incinerator designs became more complex due to the need for improved combustion efficiency, more sophisticated air emissions control systems and more efficient materials handling systems were developed. Large-scale batch feed MSWI were not developed further. Only the continuous feed incinerators survived the test of time. Today, the MSWI technologies are divided into three main groups: moving grate, rotary kiln, and fluidized bed incinerators. While these three were already in use as early as the first half of the 20th century [49,58], they have been modified over the years to suit current demands of MSWI.

They incorporate varying degrees of MSW pre-processing which was non-existent in early incinerators. Over the years, technology for pre-processing has been developed to remove bulky and hazardous materials as well as non-combustibles, thereby giving MSW better combustibility and improved emission control [19,51]. Screening through the use of trommel screens, air classifiers, magnetic separators, and eddy current separators can be done to reduce the heterogeneity of MSW prior to incineration. Trommel screens utilize the interaction of MSW particle size, trommel aperture, declination angle, drum length and rotation speed to separate MSW components while air classifiers take advantage of density differences to separate light fractions from the bulk of the MSW [6,19]. More advanced systems available today make use of optical sorting devices to separate waste materials based on their optical properties [3]. Magnetic separators and eddy current separators are specialized for removing ferrous and non-ferrous but electrically conductive metals respectively [19]. Needless to be compared against one another, all these separation techniques are normally used in combinations to achieve the desired degree of separation prior to MSWI.

Moving grate incinerators employ a mass-feed approach which requires minimal pre-processing in the form of screening and pit fluffing [19]. On the other hand, fluidized bed incinerators can only handle waste that has undergone shredding and size reduction apart from basic separation [3,29]. Shredding and size reduction equipment exists ranging from high-speed low torque (HSLT) and low-speed high torque (LSHT) hammer-mill shredders. HSLT shredders have a larger capacity of around 300t/h while LSHT can only handle up to 150t/h for large-scale plants [19]

Moving grate incinerators have proved to be superior to either rotary kiln or fluidized bed incinerators primarily because of their ability to handle large volumes of MSW without prior sorting or shredding save for the removal of bulky materials such as white goods and hazardous or explosive materials that may damage the MSWI equipment [3,64]. Additionally, they are capable of accommodating large variations in waste composition and calorific value with great operational stability [29,32]. Even though rotary kiln and fluidized bed incinerators have equally been in use since the middle of the 20th century, only moving grates have been fully developed and tested thoroughly to meet the demands for large-scale technical performance. They can be built in very large units capable of burning up to 50t of waste per hour [19]. A comparison made by Lu et al. [33] showed that in the EU (2012), Germany and the US, the proportion of MSWI plants making use of moving grate technology is 88%, 94%, and 76% respectively while the rest are either fluidized bed or rotary kiln incinerators (Fig. 2). The major disadvantage of moving grates is that they require a relatively higher investment and maintenance capital outlay as compared to either fluidized bed or rotary kiln incinerators [3]. Fluidised bed incinerators require investment and operational capital which is roughly 70% that of moveable grates [19]. Nevertheless, their strict requirements in terms of feedstock homogeneity and their high sensitivity to changes in the calorific value of waste feedstock make their operation difficult [22]. This has undoubtedly lowered their competitive edge against moving grates throughout the world. China currently has the largest capacity of MSWI utilizing fluidized bed technology but the ratio is on the decrease [30,33]. Early combustion chambers were fitted with auxiliary oil burners but now the requirement is that incinerators should operate without supplementary fuel except in deliberate co-incineration configurations [62,65].

The development of MSWI has been slowest in Africa, Latin America and Australia [33,44]. In Africa, the only waste-to-energy treatment facility is still under construction in Addis Ababa and is expected to have a capacity of 1400 MT/d [42]. Brazil represents Latin America with an MSWI capacity of around 600 MT/d while Australia has a capacity of only 390MT/d [33]. Even though China has the largest capacity of MSWI using fluidized bed incinerators, most of the fluidized bed technology is still imported from Europe [30,33]. Even though new MSWI plants are being built each year, a survey carried by Lombardi et al. [32] which classified plants according to their first day of construction showed that the most rapid increase in the number of plants globally occurred betwen1990 and 2010 (Fig. 3). It can also be observed from Fig. 3 that after 2010 the number of new plants that were being constructed declined. Reviewed literature shows that the largest decline occurred in the US as compared to either Europe or Asia [7,16]. One of the factors contributing to this decline in construction of new MSWI plants was fierce public opposition arising from MSWI pollution concerns [46]. EPA [16] affirmed that the availability of land for constructing new landfills also provided a cheaper alternative for waste disposal in the US thereby making MSWI decline possible.

The overall thermal efficiency of a WtE plant depends on the end use of the recovered energy [3,41]. Many energy recovery circuits around the world today are based on the conventional Rankine cycle with combined heat and power (CHP) or combined steam and power (CSP) configurations with overall energy efficiencies up to 60% [3,13,24,32].

A MSW incineration power system generates electricity by driving turbines with high temperature steam produced by the incineration of MSW, as shown in Fig. 4 [47]. After transporting by closed trucks, MSWs were poured into a storage pool to ferment for approximately three days. The characteristics of MSW in China are unsorted coupled with low calorific values and high moisture rates, as shown in Table 2 [48]. This ferment procedure could reduce the materials humidity and increase their heat values. MSWs were then burned in incineration boilers to heat water to generate steam, which is the driving force of turbine generators. The flue gases and solid residues generated during the MSW incineration process should be treated accordingly to avoid secondary environmental pollution, especially the flue gases which contain significant amounts of dioxins, particulate matters, heavy metals, sulphur dioxide, and hydrochloric acid. The flue gases are first sent into a flue gas scrubber to remove acidic material, after which bag filters are used to remove dust particles so that the gas can meet the final emission standards. Fly ash, one of the flue gas residues, is a hazardous substance and should be dealt with in accordance with hazardous material waste laws.

The incinerator, which accounts for approximately 50% of the MSW incineration power plant costs, is the core of MSW incineration process. The technologies of its craft and design have a direct influence on MSW disposal effects and economic benefits, as well as a direct impact on the subsequent treatment of flue gases. There are various incinerators such as stoke grate incinerators, fluidised bed incinerators, rotary kiln furnaces and pyrolysis gasification furnaces. Table 3 shows the comparison of different MSW incinerators in China [32,49]. As shown in Table 3, stoke grate incinerators and fluidised bed incinerators predominate, while pyrolysis furnaces and rotary kiln furnaces are only adopted on a small scale. At present, most incineration facilities adopting mechanical stoke grate technologies are located in the more economically developed cities of eastern coastal areas, especially in the provincial capital and the sub-provincial cities. By contrast, incineration facilities utilising fluidised bed technologies are predominantly located in small and medium cities, as well as the large cities in the middle and western regions of China that are economically less developed. It is for the reason that the cost of investment and operation of fluidised beds are comparatively low, and coal, as the auxiliary fuel for fluidised beds, is abundant in central and western China. The facility costs for stoke grate incinerators vary between US$ 98 million and US$ 164 million thousand tonnes daily of treatment capacity. In comparison, the costs for fluidised beds are merely half of that. The operation and maintenance (personnel training, fuel, parts repair and replacement) of stoke grate incinerators are also costly, varying between US$ 16 and US$ 32 per tonne MSW treated, while it is approximate US$ 10US$ 20 for fluidised beds [50]. It must also be pointed out that the market share of stoke grate becomes increasingly higher than that of the fluidised bed. The increasing rates of the number, total incineration capacity and total power generation capacity of stoke grate incinerator are 26.2%, 31.5% and 32.6% respectively from 2011 to 2012, while counterparts of fluidised bed incinerator are 11.3%, 11.1% and 14.0% respectively. The reason for this tendency is that the technology of fluidised bed is not as mature and stable as stoke grate; therefore, all advantages, such as complete combustion of native MSW through mixed unsorted collection, less dioxin emission, and no additional investment in sewage treatment etc., are not as good as expected. Due to this reason, the Chinese government is prone to support promotion of stoke grate at present, which is best illustrated by the first selection of 3A Selection of MSW Incineration plants in China lasting a whole year from March, 2012 to March, 2013. All of the MSW incineration plants adopting fluidised bed technology were unconditionally excluded for the selection, which represented the governments attitude and hindered the development of fluidised bed technology to a certain extent [51].

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