The concept of refuse-derived fuels (RDFs) is one that has the potential for addressing two of the most troubling environmental problems in the world at the same time: solid waste disposal and a source of energy. The term refusederived fuel refers to any process or method by which waste materials are converted into a form in which they can be burned as a source of energy.
In regions of the world characterized by a throwaway ethic and swamped with essentially non-degradable materials, accumulation and disposal of solid waste continue to be a growing problem. In 1960, each American generated 2.7 lb (1.2 kg) of solid waste. This grew to 4.3 lb (1.9 kg) per person by 1990. Americans continue to generate more solid waste each day but the rate of growth has decreased. In 2002, every man, woman, and child in the United States produced an average of 4.5 lb (2.0 kg) of waste each day.
Materials are labeled wastes because they tend to have no future use once they are discarded. Yet, by their very nature, most solid wastes are potentially valuable as fuels. A typical sample of municipal waste in the United States, for instance, may consist of about 30–40% paper, 5% textiles, 5% wood, and 20–30% organic material. All of these materials are combustible. Metals, glass, plastics , sand, and other non-combustible materials constitute the remaining portion of a waste sample. Each of these non-combustible materials is potentially recyclable.
Even though 70% or more of municipal wastes appear to have potential value as sources of fuel, that potential has not, as yet, been extensively developed. In most countries, solid wastes are mainly disposed of in landfills. A small fraction is incinerated or used for other purposes. However, conversion of solid wastes to a useable fuel is still an experimental process.
The primary roadblock to the commercial development of RDFs is the economic cost of preparing such fuels. Given that most RDFs produced have only about half the energy value of a typical sample of industrial coal and given the relatively low price of coal, there is little economic incentive for municipalities to build energy systems based on refuse-derived fuels.
Many scientists, engineers, and environmentalists see a positive future for refuse-derived fuels. They argue that as fossil fuel reserves are consumed, the cost of traditional fuels such as coal, gas, and oil will inevitably increase. In addition, RDFs tend to burn more cleanly and have a significant environmental advantage over coal and other fossil fuels . Research must continue, therefore, to develop more efficient and less expensive RDFs and associated processes to obtain and use them.
One of the fundamental problems in the development of refuse-derived fuels is obtaining the raw materials in a physical condition that will allow the extraction of combustible organic matter. The solid waste material entering most landfills consists of a complex mixture of substances. Some of these are combustible while others are not. Some have other commercial values while others have none. Plastics provide a convenient example that illustrates both of these aspects.
The first step in preparing wastes for the production of refuse-derived fuels is known as size reduction. In this step, waste materials are shredded, chopped, sliced, pulverized, or otherwise treated in order to break them up into smaller pieces. Most methods for separating combustible from non-combustible materials require that waste particles be small in size. Both magnetic separation and air classification will work only with small particles.
Size reduction also serves a number of other functions. For example, it breaks open plastic and glass bottles, releasing any contents that may still be in them. It also reduces clumping and tangling that often occurs with larger particles. Finally, when used as a fuel, particles of small size burn more efficiently than larger particles.
One indication of the importance of size reduction in the production of RDFs is the number of different machines that have been developed to accomplish this step. Bag breakers are used to tear open plastic bags and cardboard boxes. Machines called chippers reduce cut wood into small pieces. Granulators have sharp knives on rotating blades. These devices cut materials against a stationary blade, somewhat similar to a pair of scissors. Flail mills are rotary machines that swing hammers around in a circle to smash materials. Ball and rod mills consist of cylindrical drums that hold balls or rods. When the drums spin, balls or rods smash against the inside of the drum, crushing solid wastes also present in them.
Once waste materials have been reduced in size, the portion that is combustible must be removed from that which is not. Several different procedures can be used. A common process, for example, is to pass the solid wastes under a magnet. The magnet will extract all metals containing iron (ferrous) from the mixture.
Another technique is rotary screening, in which wastes are spun in a drum and sorted using screens that have openings of either 50 mm or 200 mm diameter. Sand, broken glass, dust, and other non-organic materials that do not burn tend to be removed by this procedure.
A third process is known as air classification. When solid wastes are exposed to a blast of air, various components are separated from each other on the basis of their size, shape, density, and moisture content. A number of different kinds of air classifiers exist, and all are reasonably efficient in separating organic from inorganic materials.
One type of air classifier is the horizontal model, in which size-reduced wastes are dropped into a horizontal stream of air. Various types of material are separated on the basis of the horizontal distance they travel from their point of origin. Vertical classifiers separate on a similar principle except that particles are injected into a rising stream of air. The rotary classifier consists of an inclined cylindrical column into which wastes are injected and then separated by a rotating, rising column of air.
The organic matter that remains after size reduction and separation can be used as a fuel in four distinct ways. First of all, it can be converted to other combustible forms through pyrolysis. Pyrolysis is a process in which organic materials are heated to high temperatures in an oxygen-free or oxygen-deficient atmosphere . The product of this reaction is a mixture of gaseous, liquid, and solid compounds. The solid compounds can be used as fuels and are collectively called char.
Two particularly valuable fuels, methane (a gas) and ethanol (ethyl alcohol, a liquid), can also be produced from organic wastes. Scientists have long known that methane is produced when anaerobic bacteria act on organic compounds found in solid wastes. They have successfully extracted methane from landfills and used it as a fuel. However, they have achieved only limited success in generating methane from other refuse outside of landfills.
Some success has been obtained in the production of ethanol from wastes. This process separates materials containing cellulose from other wastes and then hydrolyzes them in an acidic medium. As of 2002, ethanol has not been generated in commercially useful amounts from solid waste streams. However, ethanol has been obtained from corn and some other crop materials. In some states, ethanol is added to gasoline , producing a fuel that burns more cleanly and efficiently than pure gasoline.
Some heating systems make use of the dried material produced as a result of size reduction and separation directly as a fuel. The Imperial Metal Industries (IMI) company in Birmingham, England, has operated such a plant since 1976. Solid wastes provided by the West Midlands County Council is delivered to the IMI plant where it is reduced in size, separated, dried, and then fed directly into furnaces, where it is burned. A similar plant operated by Commonwealth Edison and run on wastes from the city of Chicago was opened in 1978.
A convenient and popular method of handling RDFs is to process them as pellets or briquettes. Pellets are formed from wastes that have been size reduced, separated, and partially dried. The material is extruded through a dye or compacted into small nuggets in devices known as densifying machines. An important factor in the success of this method is keeping the wastes moist enough to stick together, but dry enough for efficient combustion .
Finally, research has been conducted on the use of RDFs in combustion systems, such as fluidized bed combustion . In this process, fuel is mixed with some material such as sand and suspended in a furnace by a stream of air. The second material makes thorough mixing of the fuel possible. The fuel is then burned in the air stream.
A considerable number of research and engineering studies have been conducted on refuse-derived fuels. As of 2002, many municipalities operate plants that generate electricity. With the help of modest tax benefits in some but not all areas, most of these facilities are commercially successful. They have reduced the volume of waste entering local landfills. In turn, the life of existing landfills has been extended and the need to open new landfills has been reduced. They generate a modest but significant proportion of electricity for customers in their municipalities.
[L. Fleming Fallon Jr., M.D., Dr.P.H. ]
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