Recycling or
Disposal?
Hazardous Waste Combustion in Cement Kilns
V. Environmental Concerns Raised by Combustion of Hazardous Waste in Cement Kilns
A. Air pollution:
Air emissions from cement kilns include toxic organic chemicals such as dioxins and furans and various hydrocarbon compounds (referred to as products of incomplete combustion or PICs); acid gases generated by the combustion process; and heavy metals such as lead, mercury, cadmium and chromium. These pollutants are found in both stack and fugitive emissions (i.e., dust and gas stirred up from plant operations and emitted from locations other than stacks).
1. Particulate and Metals:
Attention for new regulatory initiatives has focused on controlling particulate matter and metals emissions from kilns. Particulate matter is a health concern because inhaling even relatively low airborne concentrations of dust can cause or aggravate lung diseases such as asthma or emphysema, and is associated with premature death. Particulate matter contains metals and regulators have generally focused on ten metals emitted by kilns. Four are known or suspected carcinogens (arsenic, beryllium, cadmium and hexavalent chromium); the six others (antimony, barium, lead, mercury, silver, and thallium) are non-carcinogenic but can cause a variety of health problems.
Since metals present in hazardous waste fuel are not combustible, they are either vaporized by the high temperatures in the kiln and emitted into the atmosphere, adsorbed into the cement kiln dust or adsorbed into the clinker, ending up in the cement product. Most metals and toxic organic compounds emitted with the exhaust gases are adsorbed to particulate matter.
It is difficult to discuss the fate of metals in hazardous waste fuel in general terms. Each of the metals of concern behave differently when subjected to high temperatures. In addition, the amount of the metal in the waste, the overall composition of the waste, and the type of kiln all appear to have significant bearing on metal emissions rates.
The results of most scientific investigations into the fate of metals in hazardous waste fuel show that the vast majority of metals are retained in the process solids (i.e., the clinker and cement kiln dust). Only about 1 per cent of most metals tested were emitted with the exhaust gases. Mercury and thallium, which are volatile at lower temperatures, appear to be exceptions to this outcome.
Nevertheless, metals emissions on a per unit of waste burned basis may be higher for kilns than for the best incinerators, because kilns have significantly stack gas discharge flow rates. If kilns emit flue gases at a faster rate than incinerators, they will emit more pollution than incinerators even if the concentration of pollutants per unit volume of discharge gas emitted is the same. Particulate emission gas concentrations rates (mass of particulate per unit volume of gas discharge) from hazardous waste burning kilns also appear to be significantly higher than for commercial hazardous waste incinerators, according to data compiled by the EPA.
2. Organic Chemicals/Acid Gases:
Since combustion does not destroy all organic chemical compounds in the fuel, some of the chemicals originally present in the waste feed will be emitted into the air. Incomplete combustion also produces organic compounds generically referred to as PICs (products of incomplete combustion.)
Although hazardous waste combustors emit a wide variety of compounds, the focus of concern has been on a handful of toxic organic compounds (dioxins and furans and principle organic hazardous constituents (POHCs)) and acid gases (hydrogen chloride, nitrogen oxides and sulphur dioxide).
The acid gases are formed as a result of the presence of either chlorine or sulfur in the fuel or coal, or the high temperature reaction of nitrogen in combustion air or in kiln feed and oxygen. Acid gas emissions are important because of the potential of pollutants like hydrogen chloride and sulfur dioxide to irritate the lung.
Dioxins and furans are chemical compounds created as a byproduct of the combustion process when chlorine in waste combines with other chemicals at temperatures typically found in the cool end of kilns and their air pollution control equipment, far downstream from the flame combustion zone. Certain isomers of dioxin/furan, such as the 2,3,7,8 tetra-chloro- dibenzo-dioxin and 2,3,7,8 tetra-chloro dibenzo-furan, have long been recognized as extremely toxic.
The EPA recently completed a draft reassessment of the toxicity of dioxin and related chemical compounds. The EPA's draft summary report concluded that dioxin is more toxic than previously believed, for both non-cancer effects and cancer effects, and that the vast majority of dioxin in the environment originates from human activities. Incineration of municipal, industrial and hazardous waste was identified as one of the four major sources of dioxin.
In addition to waste-burning related pollutants, cement kilns can sometimes also emit pollutants that result from the off-gassing of substances found in the kiln feed materials, such as from the kerogen oils sometimes found in shale feed materials. Incomplete combustion of kerogens in the cool end of the kiln can produce strong odorous emissions, excessive carbon monoxide and volatile organic compound emissions.
B. Cement Kiln Dust (CKD):
CKD contains heavy metals and toxic organic compounds; EPA's development of appropriate standards for CKD management and disposal is a critical issue in regulating waste-burning cement kilns.
Cement production operations generate large quantities of CKD. Generally, dry process kilns emit more CKD than wet process kilns. Cement kiln dust can be either disposed or some of it may recycled back into the cement making process.
Around half of the CKD collected from air pollution control equipment can be recycled back into the waste feed. However, in waste burning kilns, cycling of volatile metals, such as lead, may pose certain problems as a result of the recycling of CKD. CKD that is not recycled is generally stored or disposed on-site in piles unless it is utilized for construction, agriculture, transportation or other product purposes.
CKD is currently excluded from regulation as a hazardous waste under the "Bevill exemption" (RCRA Sec. 3001(b)(3)). CKD from hazardous waste burning kilns is subject to specific testing requirements under the BIF rules, however, and must be disposed of in compliance with RCRA standards if the toxic compounds are determined to be significantly higher than CKD from operations with conventional fuel or are present at levels that could pose significant risks to human health.
The EPA investigated and evaluated risks to human health and the environment both from existing on-site management practices and a variety of off-site uses, and finally released a Draft Report to Congress on Cement Kiln Dust (RTC) in December, 1993.
The RTC documents numerous instances of groundwater and surface water contamination from cement kiln dust disposal practices, many of them at hazardous waste burning kilns. Common management practices mentioned in the report, such as storage in exposed, unlined piles, disposal in water-filled abandoned quarries and landfills, caused contamination of nearby air, surface water, and groundwater. Since CKD collected in air pollution control devices typically has a small particle size, poorly managed cement kiln dust handling, transport and disposal has been shown to cause severe fugitive dust and air pollution problems.
The most prevalent metals found in CKD were cadmium, chromium, arsenic, lead, and selenium. Although the Report did not contain a comparative analysis of CKD from waste burning kilns and from fossil fuel burning kilns, it did conclude that "(o)verall, certain metals appear to be present at a consistently higher mean concentration in CKD generated by kilns burning hazardous waste than in CKD generated by kilns not using this type of alternative fuel. Lead, cadmium, and chromium are the most prominent examples." Data from the Report indicate that CKD from kilns burning hazardous waste contains on average 9 times more lead, 5 times more cadmium, and 7 times more chromium than kilns which do not burn hazardous waste, and that CKD from both waste burning kilns and fossil fuel burning kilns may not comply with existing RCRA standards for land disposal of hazardous waste. , The Report concluded that the risks associated with CKD are "generally low, however, there is a potential under certain circumstances for CKD to pose a danger to human health and environment, and it may do so in the future."
On February 7, 1995, U.S. EPA published in the Federal Register its final decision concerning how it intended to regulate cement kiln dust pursuant to the Report to Congress. EPA will use the Subtitle C hazardous waste authority under the Resource Conservation and Recovery Act to regulate CKD, but CKD sites will not be subject to all features of hazardous waste regulations. EPA will "tailor" certain hazardous waste authority to CKD piles, particularly on the matter of groundwater protection. Only after these rules are developed, which could take two years or more for final promulgation, will the current Bevill exclusion from hazardous waste regulation be removed.
EPA announced it intends to use Clean Air Act authority to regulate fugitive dust emissions from CKD management and disposal operations. The agency determined that the existing National Pollution Discharge Elimination System (NPDES) for water discharges would be sufficient to regulate surface water pollution from CKD management and disposal. EPA must yet actually list CKD as a specific hazardous waste and assign it a waste code. Finally, EPA announced that its regulation decision on cement kiln dust would apply to all CKD and not just CKD derived from the burning of hazardous waste in cement kilns.
EPA's final CKD decision has been criticized by some groups as constituting "Subtitle C Lite" regulation of CKD, and for failure to fully regulate the material as a full-fledged hazardous waste which should not escape RCRA regulation under the "derived from" rule that presently ensures the hazardous waste incinerator ash must be treated as a hazardous waste. The critics contend that EPA's decision doesn't go far enough.
C. Cement Products From Waste Burning Kilns:
Cement and clinker produced by waste burning kilns contain the same types of metals and organic compounds that are found in air emissions and CKD. As is the case with air emissions and CKD, the raw materials used, the system design, and the operation of the kiln - as well as the type of fuel - contribute to the amount of metals found in cement product.
There is concern whether cement and clinker produced by hazardous waste burning kilns contain unacceptably high percentages of metals, and the potential for metals to leach from finished cement into the environment. The presence of metals in cement in sufficient quantities could also threaten the structural integrity of roads, buildings and sidewalks. Unfortunately, there is very little data available to shed light on this topic.
The EPA Cement Kiln Dust Report found that some clinker samples from hazardous waste burning kilns exceeded Land Disposal Restrictions for hazardous waste. Technically, this clinker should be considered hazardous waste. On the other hand, a review of limited data by a cement industry consultant indicated that average metal concentrations did not differ significantly for cement and clinker produced in hazardous waste fired kilns and fossil fuel burning kilns. It should be understood, however, that the wide variability of the metal content of hazardous waste blends that can be brought to a facility will have a dramatic impact on the results of environmental and cement product testing where there is no effort to standardize or normalize the metals content of the waste being fed during any testing being conducted.
The use of metal bearing wastes as fuel should eventually lead to higher metal content in the clinker and cement product, particularly if CKD is recycled at high rates. Kilns that recycle less CKD show less metal content in their CKD, because metals do not accumulate in the CKD through repeated passes through the kiln. As regulations governing the disposal of CKD tighten, and as air pollution control devices become more efficient, greater percentages of metal contaminated CKD may be captured and recycled to the kiln. This may have an impact on the metals content of cement product. CKD containing 1300 ppm lead when recycled to the kiln under typical operating conditions yields a cement product containing 540 ppm lead.
Notwithstanding the problem of toxic contaminants being present in cement, EPA's February 7, 1995 decision anticipated that more stringent regulation of cement kiln dust would cause more of this material to be recycled back into kilns. In their decision, EPA announced that cement klinker would be excluded by rule from any inclusion as a hazardous waste under the revised regulatory program.
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