Incineration of solid wastes serves the function of reducing the volume of these wastes from their row or collected states to more management levels, thus obtaining lower transportation costs to the ultimate disposal site, and accommodating the wastes of a greater number of people for a given acreage available for landfill.
The two main disadvantages of the technique are that both capital and operating costs are high and equipment is expensive. There is a considerable cost to the quality of the air.
Vertical compaction ram, may be mechanically or hydraulically operated, usually hand-fed, wastes compacted into corrugated box containers or paper or plastic bags, used in medium and high- rise apartments.
Image Source : dnrec.delaware.gov
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Ram mechanism used to compact wastes into paper or plastic bags on rotating platform; platform rotates as containers are filled, used to medium-and high-rise apartments.
Compactor can be chute-fed, either vertical or horizontal rams, single or continuous multistage, single bags must be replaced and continuous bags must be tied off and replaced; used in medium and high-rise apartments.
Small compactors used in individual residences and apartment units; wastes compacted into special paper bags. After wastes are dropped through a panel door into bag and door is closed, they are sprayed for odor control and button is pushed to activate compaction mechanism.
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Compactor with vertical or horizontal ram, waste co pi pressed into steel container, compressed waste are manually tied and removed; used in low medium-and high-rise apartments, commercial and industrial facilities.
Incineration has had a cyclical existence. First used in 1885, incinerators rose and fell in popularity, so that by 1952, over one-third of the cities, operating incinerators, had fazed them out. One of the main problems was that incineration furnaces were modified from the existing manufacturer’s design, rather than being specifically designed for solid wastes.
However, since 1950 there has been a resurgence of the trend towards incineration shows the growth in number and capacity of operating incinerators in the United States the principal components in the design of large municipal incinerators. Comparative air pollution control data.
Two types of incinerators are generally used for unsorted wastes. The batch-type plant is manually stoked and has a relatively small rated capacity. The operation is intermittent (hence the name “batch”) so there is a lack of uniform burning temperature.
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This promotes inadequate combustion and, thus, an attendant heavy output of particulate matter, less than optimal volume reduction and an unstable residue often containing putrescibles. These units are generally unsuitable for large urban centers, although they are used in the smaller, surrounding communities.
The continuous feed plant has larger storage bins, automatic feed hoppers, and a variety of types of moving gates and ash removal systems. The unit maintains a uniform combustion temperature range, can be fitted with pollution control devices, albeit at a higher cost, and generally yields a stable residue.
The reason incineration is experiencing rapid growth is the volume reduction that van be achieved in a well designed and operated incinerator with controlled furnace temperature in the range 1,400-1,800 F. A ton of refuse, at the generating source, occupies 13.3 cubic yards-a density of 150 lb/cubic yard.
After substantial compaction in the collection vehicle, and further compaction in a sanitary landfill, it will occupy 2.22 cubic yards and its density will be 900 lb/cubic yard. However, if the refuse is incinerated and, then, land filled, after one year, the original ton will only take up 194 cubic yards of space and have a bulk density of 2,700 lb/cubic yard.
With land growing increasingly more scarce, more expensive and available only at longer distances from the collection points, the value of incineration is that it cuts the land requirement to one-third of that required if the refuse were to be land filled. Moreover, a stable residue cuts drastically, or eliminates entirely, the need to import cover material to the landfill site.
Public acceptance of incineration is retarded by the existence of a substantial number of old and inefficient (mainly batch-type) units which frequently operate substantially below their rated capacities, are visibly dirty in particulate emission, and cannot be economically modified to meet emission standards.
A well-run, modern, large-capacity incinerator is not a major source of air pollutants. In an ordinary ton of mixed refuse, for example, sulphur constitutes as little as one-tenth of one percent.
In a study of major atmospheric contaminants in several large US cities, it was found that particulates emitted from incinerators accounted for 18.2 percent of total particulates from all sources.
However, two devices, the wet-scrubber and electrostatic precipitator, can effect removal of between 90-99 percent of incinerator pollutants, and even at 90 percent efficiency, the result of fitting these devices would be to cut total emission of particulates by 13.9 percent.
Marginal costs of efficiency of a precipitator from 90 to 99 percent can increase the cost of the unit by 50 percent. Scrubbers, although less expensive, have high water consumption, emit unsightly vapor plumes, and create unwanted pressure drop in the furnace.
Most European installations (which, as a rule, are quite clean in emissions), and most planned modifications to existing domestic units are expected to use precipitators. Other types of pollution control equipment, such as subsidence chambers, baffling systems, centrifugal collectors, and cloth filters have either low efficiencies or high space requirements that render them impractical for use on incinerators.
Development of incinerators in the immediate future will probably tend to have large units of water-cooled steel walls, rather than the refractory-lined design. The reason for this is that precipitators and other pollution control devices require cooler raw temperatures while permitting higher burning temperatures.
When higher furnace temperatures are achieved in refractory-lined units, melting glass adheres to the walls and forms a bond stronger than that holding the fire-brick together and serious damage can occur.
Another advantage of the water-cooled units that the waste heat of combustion can be utilized. Recent installations have used the recovered waste heat for steam generation that is used for space or district heating, electricity, and desalination of sea water.
While these installations are still a rarity in the United States, they are almost standard in large European units, and there exists a wealth of technology to make such installations an expected near-term reality in this country. Mixed refuse has a thermal value of between 3,500 and 5,500 and about 37 percent that of high-grade coal; and the trend is towards higher heat values for refuse waste heat recovery especially if OBWs are shredded prior to incineration.
The situation in Europe may now be quite different. Natural gas production in the Netherlands has surpassed its peak output and the general attitude is in favor of conserving it. In the remainder of Europe too, efforts to conserve energy are widespread. In Britain, a pilot plant is in operation in which refuse is shredded, mixed with coal and used as a fuel for power generation.
As for European practice, Rogus has said, “There are some indications that this t end (towards waste-heat recovery) may not continue should the natural gas resources recently opened in the Netherlands be made economically available in Central Europe.
Thus, waste heat recovery modification should be weighed against alternate energy sources and their effect on the environment. A considerable recovery of non-ferrous metals is possible from municipal incinerators but this may only be significant hi developed countries. The operation of incinerators can be integrated with sewage processing. At Avonmouth, near Bristol, all of the electrical machinery in the incinerator is powered by generators which run mainly on methane from sludge digesters.
A major problem for water-cooled installations is the increasing proportion of polyvinyl chloride (PVC) in the refuse ‘mix’. Today, PVC accounts for between 25 and 35 percent of total plastics, and about 2 percent of all unsorted refuse.
The heat value of PVC is about one- half that of other plastics (which have a heat value approximating fuel oil) and when incinerated, PVC forms hydrogen chloride gas, a toxic emitting, and one that can rapidly corrode metal walls.
The rapid growth of plastics could lead to a dangerous concentration of 5-6 percent PVC of total mixed refuse, at which time the PVC would have to be removed prior to incineration, thus increasing the already high costs of incineration.
These are two technological innovations in the art of incineration, both of which have to be evaluated beyond their initial promise, before they can be put into production. As with some projects in the past, whose expectations fell with production, a great deal of caution should be exercised in their evaluation as alleviants to the solid waste situation.
The first unit, the CPU-400, is an outgrowth of aerospace technology. It is designed as a turboelectric generator plant using solid wastes as fuel. Unsorted wastes are passed through a shredder, dried by the hot gases, discharged from the combustor, and injected into the combustor at high pressure.
Then, the wastes are “fluidized” as they pass through the chamber. The plan is that the hot gases produced will drive a gas turbine which will drive an electrostatic precipitator, and the hot gases will be expelled at atmospheric pressure. The unit is currently in the development stage.
The second unit, the Melt Zit slag-tap process, derives Iron blast furnace technology. The idea is to melt everything, as the glass component of solid wastes becomes liquid at 1,800° F, and while most of the ash becomes molten at 2,350° F.
The resulting magma can either be molded into large blocks, or run into water to produce a coarse type of sand suitable for road or concrete aggregate. In Germany, the Volkswagen Works operates a slag-tap plant, but there are no estimates on when this process will become commercially feasible here.
“Of the more than 12,000 governmentally controlled land disposal sites in the United States, 94 percent are unacceptable and do not meet even the minimum standards for sanitary landfills.” This refers to open dumps. In urban areas, other factors, such as housing and crime, can magnify the role of solid wastes in contributing to urban decay. The open dump in an abandoned quarry is quite different from its counterpart in a Maddened urban building. A short drive through a central city ghetto area will confirm this contrast.