Is Combustion of Plastics Desirable?

Bruce Piasecki, David Rainey, Kevin Fletcher. American Scientist. Volume 86, Issue 4. Jul/Aug 1998.

What should be done with the plastics in our garbage? This question mirrors in miniature the complex choices facing policymakers about what to do with solid wastes in general. For plastics, the issue is clear: Some amount of plastic waste, primarily resins that cannot be economically recycled, must either be buried in the ground or burned in a waste-to-energy plant. And considering that a good deal of energy may be derived from plastics, one must ask whether it might be wiser for industrial consumer societies to burn more of it and bury much less.

In 1993, plastics of various sorts accounted for approximately 9 percent (by weight) of all garbage, and the United States Environmental Protection Agency (EPA) estimates that this number will rise to over 10 percent by the year 2000. Right now, many people see recycling plastics as the best option. But empirical evidence shows that not more than 50 percent of the plastics in a town’s solid waste actually gets recycled, and average municipal recycling rates are closer to 18 percent across America. People do not always participate in recycling programs, and market forces have not yet made plastics recycling attractive enough for the practice to become more widespread than it is. The same trends are seen in Europe, Asia and the nations of the former Soviet Union. Some of the rare exceptions have been observed in towns in Germany and Japan.

Of the three waste-management options, burying is the least desirable. Plastics are now buried in landfills designed to minimize the kind of biological activity that would normally degrade wastes, so waste-not just plastics-dumped in landfills remains there, frequently for decades. Faced with the options, burning starts to look better, especially when one considers that plastics can be a relatively clean, reliable and economical energy source. Burning waste starts to seem particularly attractive as the world begins to grapple with the question of greenhouse gases generated by the combustion of fossil fuels.

A plastic bottle in a landfill just takes up space. The materials and energy used to produce it will never be recovered-even partially. If the same plastic bottle is recycled, the materials used in its manufacture will be partially recovered. But if a bottle is processed in a waste-to-energy plant, a portion of the initial energy invested in making the bottle will be recovered in the form of electricity, steam or heat. This becomes more significant in an age of climate change.

Of course, burning plastic carries with it certain controllable environmental risks. Burning plastic releases compounds such as polyvinyl chloride, acid gases, dioxins and carbon dioxide, which are potentially harmful to people and produce unwanted consequences to the earth’s atmosphere and weather. In addition to organic toxins, incinerated plastics may release heavy metals, such as lead and cadmium, into the atmosphere.

As policymakers consider the question of how to handle plastic wastes in the future, they will grapple again and again with how poorly scientists understand the significant trade-offs in solid-waste management choices. Carol Browner, administrator of the EPA, cautions that Americans are in an age of “environmental adolescence.” Still, given the potential environmental, resource-recovery and economic benefits of burning plastics, it is instructive to examine each of the leading technical concerns regarding this procedure.

Power from Plastics

Municipal solid waste has an energy content that is recoverable, making it suitable and valuable for combustion. In fact, when garbage is burned in a waste-to-energy facility, there is rarely a need to add supplemental fuels to maintain combustion. Of all types of garbage, plastics release the most energy per unit of weight. Plastic accounts for only about 8 percent of the municipal solid waste burned, but it already provides about 34 percent of the energy liberated from combustion. Surprisingly, wood and paper account for relatively little of the liberated energy.

Compared with combusting most carbonbased fuels, such as oil or coal, waste is a clean power source. A modern waste-to-energy facility generates less sulfur and nitrogen oxidesboth precursors to acid rain-than do most existing coal- and oil-fired power plants. Even when compared with natural gas, energy from waste looks good, emitting fewer nitrogen oxides and only slightly more sulfur oxides. That is why people must balance these environmental benefits into their management choices. Currently municipal-waste combustors contribute less than 1 percent to the total carbon dioxide emissions in the United States. Even if all of the country’s solid waste were burned in waste-to-energy plants, as opposed to the roughly 16 percent now burned, the conversion of waste to energy would contribute only about 4 percent of total carbon-dioxide emissions. This is a small figure compared with the volume of carbon dioxide emissions produced during the production of petroleum, steel, cement and chemicals.

However, burning plastics is not emission free, and the two main foci of legitimate concern have been the release of chlorine and heavy metals into the environment. Plastics made of polyvinyl chloride, or PVC, contain 40 percent chlorine. Chlorine is a component of both hydrogen chloride, an acidic gas, and of polychlorinated dibenzo(p)dioxins and furans (PCDD and PCDF, respectively), compounds known to cause or suspected of causing cancer and other adverse health effects in laboratory animals, wildlife and people. A particularly intense aspect of this debate centers on the concern that chlorine and chlorine compounds in the environment might combine with other chemically active agents in the environment to make toxic compounds, including dioxins and furans.

In addition to chlorine, heavy metals are also released when plastics are burned. These metals are derived from lead-and cadmium-based pigments and stabilizers used in the plastics. Heavy-metal concentrations appear to be about 10 parts per million (ppm) of cadmium in PVC, about 200 ppm of lead in PVC and about 100 ppm of lead in polyethylene. These concentrations are, of course, lower than concentrations of heavy metals found in the metal components of municipal solid waste. However, metals tend to be removed from solid wastes prior to combustion, which in effect increases the relative contribution of heavy metals from plastics.

Still, when deciding on the desirability of burning plastics, it is crucial to determine the actual quantities of these substances released during burning. Recent studies reveal some answers that will come as a surprise to many.

In a 1994 study conducted for the Association of Plastics Manufacturers in Europe, Frederick E. Mark reported last year on the results of the controlled combustion of solid waste in a commercial waste-to-energy facility in Wurzburg, Germany. In his study, Mark looked at the emission profile of waste in which the plastic component ranged from a low of 10 percent of the total weight, to the medium case of 17.5 percent and, finally, to the high-composition level of about 25 percent.

The study was designed to test the effects of burning plastics within a real operating environment. For all tests, the incinerator was operated at the full thermal-load capacity of the boiler. Among other things, this means that as plastic was added above the base amount of 10 percent, the total amount of solid waste fed into the incinerator was reduced. The plastic materials tested contained a mixture of common commercial plastic polymers, including polyethylene, polystyrene, polyethylene terephthalate and polyvinyl chloride. Mark monitored the gases emitted from the incinerator for a variety of pollutants.

The level of hydrochloric acid emission was essentially identical for all three cases. The same is true for emissions of dioxins and furans. In all cases, after treating the flue gases with pollution-controlling lime and activated carbon, stack-gas concentrations of dioxins were well below the rigorous German emission limit of 0.1 nanograms per cubic meter.

Mark notes that the dioxin levels he measured in his study fall into the lower part of the legally acceptable range of the European solid-waste combustion industry. He goes on to conclude that the findings indicate that the furnace is well run and well designed. The resuits from the Mark study underscore the fact that the controlled combustion in and pollution-control equipment of a modern waste-toenergy facility easily reduce hydrochloric acid and prevent additional dioxin formation even when plastics constitute a relatively high proportion of the total waste composition.

With the exception of glass and metal, all principal components of municipal solid waste contain chlorine. The vast majority of materials, whether natural or manufactured, contain chloride or chlorine. Even vegetative matter contains chloride at levels that range from 200 to 10,000 ppm, so burning yard waste will release some chlorine into the environment. Although this is far less than the 400,000 ppm of chlorine emitted by polyvinyl chlorides, it is important to recognize the variety of sources for this ubiquitous chemical. Removing chlorinated plastics from municipal solid waste bound for burning will indeed reduce the chloride concentrations in the raw gas of an incinerator. However, such reductions will likely be insufficient to allow a waste combustor to operate without acid-gas emission controls, or to result in any material difference in dioxin formation and emission. This significant physical evidence must inform policy choices regarding the viability of plastics combustion in waste-to-energy plants.

The general management point is this: The amount of dioxin emitted from a waste-to-energy incinerator is influenced by many factors. In spite of the emphasis on chlorine in current public debates of waste-to-energy proposals, chlorine in the waste stream should be one of the least important of these. The important factors parse into two major classes-combustion control and post-combustion control. The former includes designs and practices intended to optimize combustion efficiency. The latter includes air-pollution-control devices and additives intended to minimize the release of acid gases and particulates into the environment. It is this latter arena that presents the greatest opportunities for improvements.

Returning then to the evaluation of commercial waste-to-energy combustors, it is important to note that Mark’s results compare quite well with other studies. The New York State Energy and Research Development Authority reported on a series of tests performed at the Vicon incinerator in Pittsfield, Massachusetts. Dioxin formation was found to correlate roughly with temperature and oxygen levels, but it did not correlate with the amount of polyvinyl chloride added to the waste stream. Tests of commercial incinerators in France, Belgium and Italy similarly found that polyvinyl chloride concentrations had no measurable effect on emissions of dioxins and furans. In 1993, the U.S. Department of Energy reviewed the available literature and concluded, “Regardless whether or not [polyvinyl chloride] is present in [municipal solid waste], when [municipal solid waste] is incinerated, available control measures can limit dioxin emissions to levels that are below current regulatory concern. The presence or absence of [polyvinyl chloride] in the [municipal solid waste] stream will not reduce the need to employ control measures.” This official Department of Energy position restates the logic, public safety and managerial appeal of burning plastics in controlled settings.

In addition, after reviewing several studies, along with information from tests performed at the waste-to-energy combustors in Westchester County, New York, and Marion County, Oregon, the U.S. Office of Technology Assessment concluded that “plastics do not appear to play a major role in the formation of dioxins and furans within the combustion chamber.” In spite of these significant findings, the presumption that burning plastic releases toxic levels of dioxins and chlorides continues to inform many policy debates. One sees this popular misrepresentation of the issues in a host of public-interest campaigns to stop incineration as a policy option, as well as more focused, yet unscientific efforts to rid the world of chlorine.

Generating Energy

The Mark study, and others like it, are starting to demonstrate that emissions from plastics can be much less toxic than is popularly perceived. Yet, the essential ingredient in these findings is the requirement for a modern combustion facility with adequate pollution controls. Each waste incinerator is different in its specific design; however, they all share some general characteristics. Waste destined to go into a mass-burn incinerator is moved by crane from a refuse pit to a storage hopper, which feeds a charging mechanism. The charging mechanism controls the rate of flow of the hopper and drops the waste by gravity onto combustion grates on the bottom of the boiler. In the combustion zone, the solid waste is burned, which liberates heat and reduces the waste volume by an average of 85 percent.

Normal operation requires that adequate amounts of air be present in the combustion zone in order to ensure complete combustion. Combustion air typically enters through a grate and at ports above the grate and mixes with the burning waste. The combustion process generates products of combustion at high temperatures, ranging between 1,800-2,000 degrees Fahrenheit (982-1,093 degrees Celsius). Because municipal waste contains so many different components-wood, paper, cloth and plastics, to name a few-it is an extremely heterogeneous fuel and must travel along the grate for a considerable time before it is combusted completely.

One result of combustion is that the waste is now divided into two states: gaseous and solid. The gaseous products of combustion, called the flue gases, leave the lower grate section and flow upward to the furnace section. The residual solid mass on the grate drops into a container that collects what is called bottom ash.

The gaseous component of the waste is very hot, and this heat is extracted and used to produce energy. The gases pass by steel tubes with water inside. The heat is transferred through the metal tube to the water inside, which absorbs the heat and transfers it to a steam-producing system. It is important that the temperature be adequately controlled and that the flue gases be retained in the combustion zone for the required amount of time to ensure proper combustion and to reduce the toxins in the emissions. Generally, flue gases should be retained in the combustion zone for 2 seconds, and the combustion temperature should be at least 1,800 degrees Fahrenheit. In the upper sections of the furnace, there are water-cooled surfaces to pre-heat the boiler water and additional sections to superheat the steam. This superheated steam is generally used to turn steam-powered turbine-generators, which produce electricity.

The flue gases include toxic gaseous acids and organic molecules. Large waste-to-energy plants incorporate numerous air-pollution control devices, such as carbon filters and lime slurries to reduce the toxic load in these gases. For example, sulfur, a precursor to sulfur dioxide, as well as sulfur dioxide itself, can be removed by either wet or dry scrubbers. Wet scrubbers use an alkaline reagent, such as limestone, to remove approximately 85 percent of the sulfur dioxide. Dry scrubbers use lime to remove approximately 75 percent of the sulfur dioxide. Scrubbers can also remove hydrochloric acid, one of the waste products evolved from plastic combustion specifically. There is always the possibility that incomplete combustion will produce carbon monoxide. The key to eliminating this gas from the emissions is to ensure the proper functioning-combustion at the correct temperature and with the correct degree of oxygenation-to guarantee complete combustion of the waste products.

Ashes and Dust

The flue gases that leave the furnace section contain particulate matter, including ash and trace metals. (The larger particles, too heavy to remain airborne in the gases, generally leave the grate section as bottom ash.) Smaller, lighter particles are carried in the flue gases, as is mercury gas. These are the particles that are most likely to pose a risk to human health. Heavy metals pose a particular concern, since they have the potential to leach into groundwater. However, it is important to note that this is more likely to happen to heavy metals disposed of in a landfill. In contrast, metals that go through a waste-to-energy facility can be recovered or controlled and are less likely to become an environmental hazard. Waste-to-energy facilities often use simple technology to separate out and recover recyclable metals, such as iron.

Currently there exist two means of removing these particles. Electrostatic precipitators use electrical charges to energize the particulate matter that then can be attracted to oppositely charged plates, where the particles are collected. Using this method it is possible to collect about 99.7 percent of the particulates in the flue gases. The second method employs cylindrical bags, called baghouses, that mechanically filter out particulates from the gas.

Of particular concern in waste combustion are trace amounts of heavy metals that either enter the bottom ash or vaporize into the air emissions. Scrubbers and baghouses are both effective in reducing the metal compounds in the flue gases. The great majority of waste-to-energy facilities in operation in the United States today make use of these advanced pollution controls.

Unlike dioxins and furans, metals are neither destroyed nor formed during combustion. The release of metals into the air from an incinerator bears a more direct relationship to the metals put into the furnace, minus, of course, those metals removed between the furnace and the exiting gas. As they leave the furnace, the flue gases cool, and essentially all of the potentially toxic metals become part of the ash released by the combustor. The efficiency with which heavy metals are removed from the ash is determined by the efficacy with which particulates in general are removed from the emissions. Mass-balance studies show that approximately 99 percent of particulate-bound heavy metals can be removed through existing affordable means.

Plastics contribute primarily cadmium and lead to the total heavy-metal load of incinerator ash, but, unfortunately, there are still few reliable data on the exact percentage of heavy metals in ash derived from combusted plastics. The Wirzburg study previously discussed did test the ash derived from combustion of the medium- and high-polymer wastes. In this study, Mark reported that compared with the total concentration of regulated heavy metals in the combustor feed stream, heavy-metal contributions from polymers were insignificant. Furthermore, he found that increasing the amounts of plastics in the feed did not increase heavy-metal concentrations found in the ash. Finally, he concluded that, overall, heavy-metal concentrations matched the typical ranges common for Western European municipal waste combustor operations. Since the amount of particulate matter is not yet measured directly, measuring levels of heavy metals in the emissions is the best current indicator of how much particulate matter in general is being produced by waste-to-energy combustors.

New Directions

Like manufacturers of batteries, electronics and other products, plastics manufacturers are increasingly seeking to reduce the amount of heavy metals in their products. They are looking for substitute pigments and heat stabilizers that do not contain heavy metals. Few companies still sell lead-based pigments, but cadmium-based pigments are more difficult to replace, since the pigments containing cadmium are more resistant to fading.

Researchers have had more success in identifying replacements for cadmium- and leadbased heat stabilizers. But lead has been especially difficult to replace. A 1992 EPA report notes that “the replacement of lead-based heat stabilizers in electrical cable insulation and jacketing … has been difficult due to the critical properties of weathering, humidity resistance and thinness of the jacket that lead imparts.”

In response to public attention and the threat of further regulation, new technologies and market advantages have driven the plastics industry to actively seek replacements for heavymetal additives. In addition to preparing for potential U.S. regulation, companies are also watching international regulatory developments. For example, Sweden’s government is beginning to restrict plastics additives. In 1993, Bo Wahlstrom and Beryl Lundqvist, in a text developed for the Swedish Environment Institute, reported that cadmium for surface treatment in plastics and in dyes is banned in Sweden and has been for many years. They anticipated similar prohibitions to be introduced broadly throughout the European Community. They noted at the time that the EPA was expected to consider a proposal to restrict plastic additives in the U.S. and to monitor the transition to leadfree pigments and drying agents in paint production. The follow-up actions by the EPA are still pending. As such, when the new regulations are introduced, U.S. firms wishing to remain competitive in the European market will be required to further reduce the amounts of heavy-metal additives in plastics. This will spur further innovation across the industry.

In addition to seeing changes in the composition of the plastic product, the public can anticipate seeing some changes in the waste-to-energy industry. The industry has learned from experience that it must demonstrate the integrity of its operations to an increasingly sophisticated public in order to remain a player in the vital arena of solid-waste management.

Continued improvement is evident in technologies to control air pollution and in ash management-the two most ardent public concerns. That the public is justified in its concerns is borne out by comparisons between older and newer waste-combustion facilities. Recent enhancements to the combustion process, for example, have enabled waste-to-energy operators to significantly reduce dioxin formation. A 1992 study conducted by Herman Vogg for the International Symposium on Chlorinated Dioxins and Related Compounds showed that the overall pollution factor for an older plant was 25 nanograms per cubic meter of raw emission gas, versus one-tenth that value, or 2.5 nanograms per cubic meter, in a newer facility. Vogg’s data also showed that modern combustion techniques and equipment reduce dioxin levels in raw gas by as much as 90 percent.

One new feature of many recently completed facilities, and of those still in the planning stages, controls mercury in emissions. The EPA emissions standards for metals, which were released in 1995, require mercury controls on facilities, since mercury emissions cannot be effectively reduced by traditional control methods.

Advances in ash management are making it more likely that beneficial uses for the by-products of solid-waste combustion will increase in the future. However, in the case of Chicago v. the Environmental Defense Fund, the U.S. Supreme Court ruled in 1994 that operators must now test ash for compliance with federal standards before disposing of it in a nonhazardous-waste landfill.

The Court ruled that ash from waste-to-energy plants that use solid waste as their fuel may be hazardous, even though the products from which it is derived are not. This is because contaminants in the ash residue are more concentrated and more likely to leach into the ground water than are the preburned components of waste. The Court ruled that the Resource Conservation and Recovery Act, which is the dominant federal legislation that defines solid waste and makes distinctions between manageable and hazardous wastes, does not exempt ash derived from municipal-solid waste from the regulation of hazardous waste. Prior to that ruling, the EPA classified ash generated from nonhazardous waste as nonhazardous. The Court’s ruling was based on an argument made by the Environmental Defense Fund that the Northwest Waste-to-Energy plant in Chicago violated the Resource Conservation and Recovery Act by dumping ash that exceeds the federal standards for lead and cadmium in landfills not designed for hazardous waste. The Supreme Court’s decision directed the EPA to develop regulations concerning the type and frequency of testing necessary to determine the toxicity of the ash.

Reporting on the Court’s decision on May 3, 1994, Linda Greenhouse wrote in the New York Times that over the shortrun, ash-disposal costs may increase from three to ten times. Engineers estimate that over the long run, however, the economic impact of the Court’s decision should be minimized if ash-stabilization and reuse technologies continue to advance as expected. At some point, the ash might be put to some practical uses, as it is now in Europe. In the Netherlands, combustor ash is an ingredient in the blocks used to construct dikes. And elsewhere in Europe, ash is used for building roadbeds. Combustion will remain an important component of environmental management in the next century, and waste feeds will increasingly include plastics.

Trade-offs

Managing waste will always entail some tradeoffs. All of the three options-recycling, landfilling and combustion-have some disadvantages. Even landfilling, which produces no emissions, fails to take advantage of the energy value inherent in plastic. Waste combustion, on the other hand, recovers the energy in plastic materials and reduces the volume of disposed solid waste by up to 90 percent of its initial preburn volumes. However, this management option generates emissions and produces an ash residue that must be managed.

As demonstrated by recent test burns, improvements in combustion and air-pollutioncontrol technology have dramatically reduced the health risks from emissions and ash. Recent studies have shown that plastics-in quantities even higher than those normally found in municipal solid waste-do not adversely affect levels of emissions or the quality of ash from waste-to-energy facilities.

In addition, waste-to-energy facilities may be a relatively economical source of fuel, and may be a more economic solution to waste management than the other available options. A waste-to-energy plant generally produces electricity that is sold to the electric utilities for approximately six cents per kilowatt-hour, a rate that is competitive with those offered by nuclear power plants and power plants that generate energy by burning fossil fuels.

Waste-to-energy facilities also have an advantage over landfills, since they generate revenue that partly offsets other costs related to running the plant. Assuming that municipal solid waste contains 5,000 Btus per pound, a waste-to-energy plant produces about 500 kilowatts per ton of municipal solid waste. Thus a plant burning 50 tons each hour produces 25,000 kilowatts per hour, generating a revenue stream of $1,500, or $30 per ton of municipal solid waste. In addition to the energy recovered, burning waste has the advantage over landfills, as it avoids a tipping fee, the price that garbage collectors pay to the landfill to deposit waste there. Tipping fees generally run between $60 and $90 per ton. Overall, burning waste generally costs between $90 and $120 per ton, which includes the capital cost, the operating cost and the cost of ash disposal.

The EPA estimates the U.S. will generate approximately 220 million tons of municipal solid waste by the year 2000. Even their optimistic estimate that 35 percent of plastics will be recycled requires that the rest be burned in waste-to-energy facilities or buried in landfills. Although landfills become more expensive to construct, the cost of building a waste-to-energy facility is offset by the revenue generated by producing a usable product.

Of course, the public should continue to ask pressing questions about the appropriateness and safety of waste-disposal options for plastics. Citizens should continue to hold waste-to-energy and landfill operators to a high standard of operating integrity. Productive relations between citizens and industry can, and do, positively affect the performance of waste-management facilities. The need for sustained public scrutiny and review is clear. Just as performance standards have been aggressively improved in the combustion industry, plastics manufacturers and others are continuing to respond to public concerns by increasing their use of nontoxic additives.

In the final analysis, communities and consumers will often need to make decisions in this age of environmentalism based on evidence that may seem incomplete-or even contradictory. But the evidence regarding plastics combustion in modern waste-to-energy plants is clear. Waste-to-energy should remain an acceptable, even desirable, option for managing plastic wastes.