Incineration

Incineration of plastic waste either alone or as part of municipal waste is often pereeived by the public to be an environmentally unsound process because of the production of residual ash containing heavy metals and the possibility of dioxin formation from chlorine-containing waste. Like DDT, dioxins are persistent and accumulate in fatty tissue, but unlike DDT some of the many dioxins that could be produced by incineration are highly toxic to humans. Although experts agree that waste can be incinerated safely, at high temperature, without any dioxin emission, it is the potential risk that is the cause for concern to the general public. 

As would be expected from the chemical structure, the energy content of hydrocarbon-based polymers is similar to that of heating oil and is over twice that of paper-based waste. Hence plastic waste is a potentially valuable form of fuel. Incineration of plastics, as part of municipal waste, is especially suitable for small items such as thin films and small containers such as yoghurt cartons that are impractical to collect and sort for other forms of recycling. As with other forms of energy generation the heat from the combustion process is used to drive a steam turbine to generate electricity. 

The incineration of waste containing PVC has been a source of much discussion and comment, particularly related to the dioxin and HC1 emissions (66,106, 282, 341). PVC was also targeted in the EU incineration directive (297). 

In a recent study the PVC content of municipal solid waste for incineration was calculated at approximately 0.8%. In a modern incinerator, energy and HC1 recovery options are attractive (115,146). 

A rather negative view of MSW incineration has been detailed by the Environmental Defense Fund [74]. The arguments cited include the following  

Expense. To benefit from the economy of scale, most proposed incinerators are designed to handle refuse from a wide area. Construction costs typically are in the hundreds of millions of dollars. Local governments may have to participate in bond issues, special taxes, and tipping fees in order to build and maintain the operation. 

Solid residue (ash) disposal. If the plant handles unsorted MSW, as much as 20%-30% by weight of the charge is noncombustible material. The resulting residue must be disposed of by land Ailing. There are concerns about leaching of heavy metals from the landflll into potable water sources. 

Air emissions. Aside from CO2, about which there is concern over the greenhouse effect, there are fears that other objectionable gases may be discharged. 

Effect on conservation. A major objection by environmentalists is that mass-burn incineration removes any incentive by consumers and governments to get on with recycling, which is regarded as the most environment-friendly alternative. 

A more recent concern (which is also contended) is the possible formation of dioxins during municipal incineration of plastics waste. This comes up regularly (and is as regularly refuted) in the case of PVC, but there seems no doubt that inefficient combustion (of anything) can produce dioxins. 

Thermal treatment of hazardous wastes can be used to accomplish most of the common objectives of waste treatment— volume reduction removal of volatile, combustible, mobile organic matter and destruction of toxic and pathogenic materials. The most widely applied means of thermal treatment of hazardous wastes is incineration. Incineration utilizes high temperatures, an oxidizing atmosphere, and often turbulent combustion conditions to destroy wastes. 

FIGURE 16.14 Major components of a hazardous waste incinerator system. In the figure, POTW refers to a publicly owned treatment works, usually a municipal wastewater treatment facility. 

Depending on the mix of waste being burnt, the incinerator may or may not require auxiliary firing from fuel oil or natural gas. 

Liquid injection incinerators. This t3q)e of incinerator has a cylindrical refractory-lined combustion chamber mounted verti- 

Hearth incinerators. This type of incinerator is designed primarily to incinerate solid waste. Solids are moved through the combustion chamber mechanically using a rake. 

The policy for waste heat recovery from the flue gas varies between incinerator operators. Incinerators located on the waste producer s site tend to be fitted with waste heat recovery systems, usually steam generation, which is fed into the site steam mains. Merchant incinerator operators, who incinerate other people s waste and 

While incineration is the preferred method of disposal for wastes containing high concentrations of organics, it becomes expensive for aqueous wastes with low concentrations of organics because auxiliary fuel is required, making the treatment expensive. Weak aqueous solutions of organics are better treated by wet oxidation (see Sec. 11.5). 

This process is used for elimination of particularly harmful liquid wastes and sludges, such as black liquors from pulp production, and acid condensates from oil processing factories. It is carried out in a dry or wet mode. 

In the (incineration) reactor, used for the dry process, water evaporation is followed by pyrolysis of organic matter to form pyrolysis gas and carbonized residue. 

In the case of wet incineration at higher temperatures (up to 350°C) and a high pressure (5-15 MPa), organic pollutants are degraded into carbon dioxide and water, or lower fatty acids (Fig. 3.61). The disadvantage of this process lies in the high investment and operating costs. 

Campbell, M.D. and Lehr, J.H. Water Wall Technology. McGraw-Hill, New York 

Tebbutt, T.H.Y. Principles of Water Quality Control. Pergamon Press, Oxford 1977. 

Molecular oxygen is the principal oxidant in the ordinary combustion of fuels. Burning is essentially a high-temperature analog of autooxidation, in which the spin barrier to the initiation step (Equation 4.67) is overcome by use of high temperatures. 

Oxygen atoms and hydroxyl and hydroperoxyl radicals are also important oxidants in flames (Gardiner, 1984 Miller and Fisk, 1987). 

A great variety of organic combustion products have been identified (Schumacher et al., 1977 Junk and Ford, 1980 Hawthorne et al., 1988). The mechanisms of the reaction of oxygen with combustible species are, however, very poorly understood. Even such a simple fuel molecule as methane has very complex behavior during combustion. The methyl free radical formed during the initiation step has a number of possible fates including recombination to form ethane (Equation 4.68), common in fuel-rich flames  

Further oxidation of CH3 proceeds by intermediate steps that are not entirely certain, involving formaldehyde, -CHO, and CO, which is finally converted to the ultimate combustion product, CO2, by the reaction 

This reaction is very exothermic and is responsible for much of the heat generation during the combustion of fuels (Miller et al., 1990). The hydrogen atoms formed in the reaction are very important chain-carriers because they produce highly reactive oxidizing free radicals when they combine with molecular oxygen  

It has been argued that the burning of plastics to recover the calorific value is a form of recycling in which the CO2 produced can then be fixed by plants in the photosynthetic pathway to create new fuels. Unfortunately, the natural recycle time for carbon is of the order of 300-400 years, but this process does recover the energy value of wastes. Although plastics account for only about 7% of the mass disposed of to landfill, they contribute 30% of the calorific value of MSW, so if plastics are removed from the post-consumer waste streams that are currently incinerated, oil or gas would have to be used to allow the material to bum. 

Energy recovery can take place by two principal means incineration of the waste, or gasification. 

The cost of incineration must rise in the future to reflect the investment that operators are having to make to scrub the flue gases and monitor the incineration processes more carefully. The NIMBY lobby is also extremely active in preventing the construction of new incinerators. Well maintained and closely controlled incinerators can dispose of waste effectively by reducing its volume by 90% and its weight by 70% while at the same time recovering the calorific value of the waste stream. 

Snowman suggested a combination of these protective devices by installing an incinerator between the product and the pump while fitting an absolute filter to the pump exhaust to prevent environmental pollution. 

Alternative physical methods for decontamination including electrostatic precipitation, ultraviolet irradiation, heat, etc., are reviewed below. 

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Treat the effluent using incineration, biplogical digestion, etc. to a form suitable for discharge to the environment, called end-of -pipe treatment. 

Combustion in an incinerator is the only practical way to deal with many waste streams.This is particularly true of solid and concentrated wastes and toxic wastes such as those containing halogenated hydrocarbons, pesticides, herbicides, etc. Many of the toxic substances encountered resist biological degradation and persist in the natural environment for a long period of time. Unless they are in dilute aqueous solution, the most effective treatment is usually incineration. 

Incineration. Incinerators were discussed in Sec. 11.1. When incinerators are used to treat gaseous pollutants in relatively low concentration, auxiliary firing from fuel or other waste material normally will be necessary. The capital and operating costs may be high. In addition, long duct lines are often necessary. 

The major products of combustion are CO2, water, SO, and NO. The products of combustion are clearly beshminimized by making the process efficient in its use of energy through improved heat recovery and avoiding unnecessary incineration through minimization of process waste. 

If air is used as stripping agent, further treatment of the stripped material will be necessary. The gas might be fed to an incinerator or some attempt made to recover material by use of adsorption. 

A greater amount of steam would be generated if the noncondensible vent was treated using catalytic incineration rather than absorption. The... 

Tail gas containing traces of SO2, H2S, COS and CS2 are usually sent to a finishing processing before being incinerated. 

Differential pulse polarography and stripping voltammetry have been applied to the analysis of trace metals in airborne particulates, incinerator fly ash, rocks. 

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