Incineration is a thermal treatment or waste combustion process used to reduce the volume of waste for final disposal. The process may or may not recover any of the heat released. Incineration of waste materials converts the waste into ash, flue gas, and heat. The ash is mostly formed by the inorganic constituents of the waste, and may take the form of solid lumps or particulates carried by the flue gas. The flue gases must be cleaned of gaseous and particulate pollutants before they are dispersed into the atmosphere.
In some cases, the heat generated by incineration can be used to generate electric power. This is known as co-incineration. Incineration with energy recovery is one of several waste-to-energy (WtE) technologies such as gasification, plasma arc pyrolysis and anaerobic digestion (See energy from waste).
Incineration may also be implemented without energy and materials recovery. Incinerators reduce the solid mass of the original waste by 80–85% and the volume (already compressed somewhat in garbage trucks) by 95-96%, depending on composition and degree of recovery of materials such as metals from the ash for recycling. This means that while incineration does not completely replace landfilling, it significantly reduces the necessary volume for disposal. Incineration has particularly strong benefits for the treatment of certain waste types in niche areas such as clinical wastes and certain hazardous waste where pathogens and toxins can be destroyed by high temperatures. The problem with this is that the process produces a small quantity of material from pollution abatement plant that is classed as a hazardous waste which still end up in tightly controlled landfill sites.
Plant Designs
There are various types of incinerator plant design: moving grate, fixed grate, rotary-kiln, and fluidized bed.
Mass Burn System
This system burns the entire waste without pre-processing and is mostly used by commercial system throughout the world. The incinerator operates 24 hours per day so sufficient waste storage capacity is made available to keep the plant in operation. It is vital to keep the waste stream uniform in order to ensure suitable calorific value. Waste with low calorific value such as wet food waste should be mixed with a load of paper rich commercial waste. This helps to keep the output from the boiler constant, but also maintains constant furnace conditions, which in turn helps to minimize pollutant formation.
Mass burn systems make use of feed grate to push the waste onto the combustion grate. Once on the grate, the waste is heated and dried through radiation heat from the furnace wall. The waste is further pyrolysed with VOC's given off and readily burnt off at the secondary combustion zone. The residual carbonaceous material moves down the grate where it is burnt further. Ash is produced and discharged into a water-filled quench tank. The efficiency of the incineration system to consume waste depends on the ability to provide and maintain the correct amount of combustion air to each section of th grate. Ther are different designs of grates use in MSW incinerators each with its own advantage and disadvantages.
Moving Grate
The typical incineration plant for municipal solid waste is a moving grate incinerator. The moving grate enables the movement of waste through the combustion chamber to be optimized to allow a more efficient and complete combustion. A single moving grate boiler can handle up to 35 metric tons of waste per hour, and can operate 8,000 hours per year with only one scheduled stop for inspection and maintenance of about one month's duration. Moving grate incinerators are sometimes referred to as Municipal Solid Waste Incinerators (MSWIs).
The waste is introduced by a waste crane 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 air flow 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. In multiple/stepped hearth incinerators, the secondary combustion air is introduced in a separate chamber downstream the primary combustion chamber.
According to the Waste Incineration Directive, incineration plants must be designed to ensure that the flue gases reach a temperature of at least 850 °C (1,560 °F) for 2 seconds in order to ensure proper breakdown of toxic organic substances. In order to comply with this at all times, it is required to install 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 gases are then cooled in the super-heaters, where the heat is transferred to steam, heating the steam to typically 400 °C (752 °F) at a pressure of 40 bars (580 psi) for the electricity generation in the turbine. At this point, the flue gas has a temperature of around 200 °C (392 °F), and is passed to the flue gas cleaning system.
Fixed Grate
The older and simpler kind of incinerator was a brick-lined cell with a fixed 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 called clinkers. Many small incinerators formerly found in apartment houses have now been replaced by waste compactors.
Rotary-Kiln
The rotary-kiln incinerator is used by municipalities and by large industrial plants. This design of the incinerator has 2 chambers: a primary chamber and secondary chamber. The primary chamber in a rotary kiln incinerator consist of an inclined refractory lined cylindrical tube. The inner refractory lining serves as sacrificial layer to protect the kiln structure. This layer needs to be replaced from time to time. Movement of the cylinder on its axis facilitates 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 clinkers spill 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 treated and/or combusted through an abatement system.
Fluidized Bed
This system is made up of a furnace containing a bed of sand. Air is blown through nozzles in the base of the furnace with a velocity sufficient to fluidize the sand. The air seeps through the sand until a point is reached where the sand particles separate. Beds operating at these air velocities are known as bubbling fluidized bed (BFB) combustors. As waste gets introduced into the system, the sand with the pre-treated waste is kept suspended on pumped air currents and takes on a fluid-like character. The bed is thereby violently mixed and agitated thereby allowing all of the mass of waste, fuel and sand to be fully circulated through the furnace. Excessive air velocity may cause the bed to become more violent which may result in lighter sand particles being blown out of the bed. A system for collecting this en-trained sand and returning them to the bed while still operating at the high velocities has been designed. This is know as Circulating Fluidized Beds (CFBs).
Unlike the grate system, BFBs can burn wastes with lower calorific values as well as operate at lower temperatures. Therefore BFBs are lower contributors of NOx due to low thermal NOx formation. In both BFBs and and CFBs, the sand bed material is heated to the operating temperature by auxilliary burners. Waste is then fed to the bed by dropping it from the top of the bed or injecting from below.
Pollution
Incineration has a number of outputs such as the ash and the emission to the atmosphere of flue gas. Before the flue gas cleaning system, if installed, the flue gases may contain significant amounts of particulate matter, heavy metals, dioxins, furans, sulfur-dioxide, methane, and hydrochloric acid. If plants have inadequate controls, these outputs may add a significant pollution component to stack emissions.
Gas Emissions
Dioxin and Furans
Dioxins are a class of chemical contaminants that are formed during combustion processes such as waste incineration, forest fires, and backyard trash burning, as well as during some industrial processes such as paper pulp bleaching and herbicide manufacturing. The most toxic chemical in the class is 2,3,7,8-tetrachlorodibenzo-para-dioxin (TCDD). The highest environmental concentrations of dioxin are usually found in soil and sediment, with much lower levels found in air and water. Humans are primarily exposed to dioxins by eating food contaminated by these chemicals (National Institute of Environmental Health Sciences, 2011).
The most publicised concerns from environmentalists about the incineration of municipal solid wastes (MSW) involve the fear that it produces significant amounts of dioxin and furan emissions. Dioxins and furans are considered by many to be serious health hazards. The US EPA announced in 2012 that the safe limit for human oral consumption is 0.7 picograms Toxic Equivalence (TEQ) per kilogram bodyweight per day, which works out to 17 billionths of a gram for a 150 lb person per year.
Other Gas emissions
Other gaseous emissions in the flue gas from incinerator furnaces include nitrogen oxides, sulfur dioxide, hydrochloric acid, heavy metals, and fine particles. Of the heavy metals, mercury is a major concern due to its toxicity and high volatility, as essentially all mercury in the municipal waste stream may exit in emissions if not removed by emission controls. The steam content in the flue may produce visible fume from the stack, which can be perceived as a visual pollution. It may be avoided by decreasing the steam content by flue-gas condensation and reheating, or by increasing the flue gas exit temperature well above its dew point. Flue-gas condensation allows the latent heat of vaporisation of the water to be recovered, subsequently increasing the thermal efficiency of the plant.
Flue-gas cleanings
The quantity of pollutants in the flue gas from incineration plants may or may not be reduced by several control measures, depending on the plant. Particulate is collected by particle filtration, most often electrostatic precipitators (ESP) and/or baghouse filters. The latter are generally very efficient for collecting fine particles. Acid gas scrubbers are used to remove hydrochloric acid, nitric acid, hydrofluoric acid, mercury, lead and other heavy metals.
Basic scrubbers remove sulfur dioxide, forming gypsum by reaction with lime. Sulfur dioxide may also be removed by dry desulfurisation by injecting limestone slurry into the flue gas before the particle filtration. NOx is either reduced by catalytic reduction with ammonia in a catalytic converter (selective catalytic reduction, SCR) or by a high temperature reaction with ammonia in the furnace (selective non-catalytic reduction, SNCR). Urea may be substituted for ammonia as the reducing reagent but must be supplied earlier in the process so that it can hydrolyze into ammonia. Substitution of urea can reduce costs and potential hazards associated with storage of anhydrous ammonia. Heavy metal are often adsorbed on injected activated carbon powder, which is collected by the particle filter.
The efficiency of removal depends on the specific equipment, the chemical composition of the waste, the design of the plant, the chemistry of the reagents, and the ability of the engineers to optimize these conditions, which may conflict for different pollutants.
Solid Outputs
Fly-Ash and Bottom-Ash
The main technical problem associated with incineration is the waste resulting from it, in the form of ash and other air pollution control residues (APC). The final residues from pollution abatement plants (about 20-50 kg per tonne of waste burned) and are classed as a hazardous waste, which must be disposed of in hazardous waste landfill sites.
Incineration produces fly ash and bottom ash just as is the case when coal is combusted. The total amount of ash produced by municipal solid waste incineration ranges from 4 to 10% by volume and 15-20% by weight of the original quantity of waste, and the fly ash amounts to about 10-20% of the total ash. The fly ash which are by-products of pollution abatement plant, are classed as hazardous waste under the European Waste Catalogue as they often contain high concentrations of heavy metals (lead, cadmium, copper and zinc), unreacted lime, soluble chlorides and sulphates, dioxins and furans. The bottom ash seldom contain significant levels of heavy metals. In testing over the past decade, no ash from an incineration plant in the USA has ever been determined to be a hazardous waste. After leaving the quench tank, bottom ash can be treated by an electromagnetic and eddy current to remove ferrous and non ferrous metals respectively. This is stored for a number of months to enable stabilization i.e, to allow alkalinity to reduce, metal oxidation to complete and the material to become more physically and chemically stable. This can be recycled either alone or with other suitable material as a substitute for natural aggregates in road building and other construction projects.
Other pollution issues
Odor pollution can be a problem with old-style incinerators, but odors and dust are extremely well controlled in newer incineration plants. They receive and store the waste in an enclosed area with a negative pressure with the airflow being routed through the boiler which prevents unpleasant odors from escaping into the atmosphere. However, not all plants are implemented this way, resulting in inconveniences in the locality.
An issue that affects community relationships is the increased road traffic of waste collection vehicles to transport municipal waste to the incinerator. Due to this reason, most incinerators are located in industrial areas. This problem can be avoided to an extent through the transport of waste by rail from transfer stations.
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