Dioxin and Waste Combustion:
It's Not What You Burn- It's the Way You Burn It!

Introduction

Does the composition of waste burned in waste combustors determine the dioxin and furan content of air emissions from those waste combustors?1 For example, would eliminating PVC plastics from waste reduce combustion emissions of dioxins and furans? Despite huge strides in reducing dioxin and furan air emissions from waste combustors over the past several years, some still debate whether "chloride in" equals "dioxin2 out."

Modern waste combustion is a highly engineered process designed to ensure maximum destruction of waste and minimum formation of air pollutants. Environmental regulatory and industry initiatives have resulted in drastic declines in combustor emissions of pollutants such as dioxins and furans, polycyclic aromatic hydrocarbons (PAHs), and fine particulate matter over the past few decades. According to EPA data, dioxin and furan air emissions from municipal solid waste incinerators declined 86% between 1987 and 1995.3 The agency projects another 99% decline between 1995 and 2002/4 as Maximum Achievable Control Technologies (MACTs) are implemented.4 Such remarkable improvements in the environmental performance of municipal combustors are a consequence of applying the results of scientific research to the design and operation of these units.

Chloride in the Incineration Environment

As one of the most abundant chemical entities in the crust of the Earth, chloride is found in numerous organic5 (e.g., PVC) and inorganic compounds (e.g., salts), both natural and synthetic. Many of these compounds are present in waste. (Waste may also contain small amounts of dioxins and furans that are destroyed during properly designed combustion.) In fact, ordinary waste contains more than a million times more chloride than is necessary to produce the quantity of dioxins and furans generated in a modern waste combustor. Only a small fraction of the chloride present in waste reacts to form dioxins and furans. Even air from a combustor may contain nearly 20 times more chloride than is necessary to produce typical amounts of dioxins and furans from waste combustors.6 That is why removing chloride contributed by PVC does not impact dioxin and furan formation when actual mixed waste is burned, and this has been shown in numerous scientific studies.

The Critical Parameters

For the past 25 years, scientists have studied how dioxins and furans are destroyed in the combustion environment and how they may be formed just after combustion. While there does not appear to be a direct relationship between the concentration of chloride in waste material and the production of dioxins and furans during combustion, rates of dioxin and furan formation have been shown to depend on:

1. Complete destruction of dioxins and furans and their chemical "building blocks" in waste material during combustion. This is achieved through the "3-T Rule":

  • High combustion Temperature to maximize waste destruction;
  • Adequate combustion Time (usually two seconds) to maximize waste destruction; and
  • High combustion Turbulence to distribute heat evenly and ensure complete waste destruction.

2. Prevention of conditions that favor formation of dioxins and furans immediately following combustion. This is achieved by the following design specifications:

  • Use a "fast-quench" of post-combustion gases by cooling them quickly from higher temperatures through the temperature range of approximately 400C down to 250C, to avoid prolonged exposure in the temperature range known to favor dioxin and furan formation; and
  • Minimize the presence of certain metals, such as copper, on particulate matter, that are known to facilitate dioxin and furan formation.

Scientific research has made possible improvements in the design and operation of waste combustors that have proven highly effective in destroying dioxins and furans during combustion and limiting their formation immediately following combustion. In contrast, any strategy that would focus on restricting waste composition, such as eliminating PVC plastics, would be impractical, scientifically unfounded, and would not result in emissions reductions.

Technology makes the world a new place.
- Shoshana Zuboff

1Combustors are also called incinerators.
2Here, "dioxin" implies dioxins and furans as well.
3US Environmental Protection Agency Inventory of Sources of Dioxin-Like Compounds in the United States-1987 and 1995" http://cfpub.epa.gov/ncea/cfm/dioxindb.cfm?ActType=default
4Based on EPA projections assuming full compliance with regulatory levels by this period and the closure of a copper smelter (personal communication, Dwain Winters, US EPA, 9-9-02).
5Organic compounds are a large class of chemical compounds that consist mostly of carbon and hydrogen, arranged in various branching and ring-like structures; they are called organic because organisms are largely composed of these compounds. Chlorinated organic compounds are organic compounds that include chlorine in their structures.
6Grosjean (Grosjean, D. Environ. Sci. Tech. 1990, 24, 77-81) measures up to 3.9 micrograms/m3 Cl in ambient air. At 7000 m3 air/ton waste, 27 mg Cl is introduced with the air, an 18-fold excess over what would be needed to synthesize PCDD/F.

 

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