Hazardous Waste Combustion in Cement Kilns
IV. Hazardous Waste Combustion in Cement Kilns
A. Cement kiln technology:
Portland cement is produced by heating calcium (usually limestone) silica and alumina (typically clay or shale) and iron (steel mill scale or iron ore) in cement kilns to temperatures of up to 2,700 degrees Fahrenheit. Under this intense heat, the raw materials blend and form a pebble-like substance called "clinker." After the clinker is cooled it is ground-up with a small amount of gypsum to produce cement.
Cement kilns are basically tilted, rotating cylinders lined with heat-resistant bricks. They vary in size depending on the particular type of cement-making process employed, and can reach 750 feet in length and 25 feet in diameter. The raw feed material mixture ("meal") is fed into the higher, elevated or "cool" end of the kiln. As the kiln slowly rotates, the raw meal tumbles down toward the hot lower, or "flame" end, gradually altering physically and chemically in the intense heat to form clinker.
Cement kilns operate in a counter-current configuration. Combustion gases enter the kiln at the hot lower end and flow upward, heating the raw materials flowing in the opposite direction as they pass over, and exit the kiln at the top end. The gases (which reach temperatures several hundred degrees hotter than the raw material) then pass through air pollution control devices before entering the atmosphere. These devices are typically either a fabric filter or electrostatic precipitator, both of which function to remove the particulate matter entrained in the gas stream before the gasses are emitted into the atmosphere. This particulate matter is referred to as cement kiln dust or CKD.
Five thermal zones exist in cement kilns. The raw material initially passes through the drying and pre-heating zone, where the temperature of the raw material is raised to about 1,480 degrees. Here the free and chemically bound water evaporates. Next, in the calcinating zone, the limestone is chemically converted into lime at temperatures of up to 2,192 degrees. In the last three zones, the upper transition zone, the sintering zone and the lower transition or cooling zone (sometimes referred to collectively as the burning zone) the raw material is heated to its maximum temperature of approximately 2,700 degrees and chemically alters under the withering heat to form clinker.
There are three basic types of cement manufacturing processes, wet process, semi-dry process, and dry process. In the wet process the raw material is blended with water to produce a slurry which is pumped directly into the cold end of the kiln. The slurrying process helps homogenize the material. The wet process is the oldest of the three processes, and is also the most energy intensive, because the water must be evaporated out of the slurry mixture.
The semi-dry or Lepol process involves mixing a smaller amount of water with the raw material which is then exposed to the exit gases from the kiln prior to entering the kiln chamber.
In the more energy efficient dry kiln process, the raw material enters the kiln in a dry powdered form. Three types of kilns utilize the dry process. The preheater kiln features a tower of heat-exchanging cyclones. The raw material enters the pre-heater in a dry powdered form where it is pre-heated by the hot exit gases from the kiln prior to entering the kiln chamber. The pre-calcinator kiln is identical to the pre-heater kiln except that a separate combustion gas inlet at the base of the preheater promotes further calcination of the material before entering the kiln. The third type of kiln is referred to as the long dry kiln and feeds dry raw material directly into the upper end of the kiln.
B. Lightweight Aggregate Kilns:
Lightweight aggregates are clays, shale, or slate material which have varying characteristics linked to their geological formation. These materials are combined with cement to produce concrete products. It is estimated that 80% of lightweight aggregate cement production utilizes the rotary kiln method, which is very similar to the technique used to produce Portland cement described above. Lightweight aggregate kilns that burn hazardous waste typically use hazardous waste as their sole fuel.
C. Use of hazardous waste fuel in cement kilns:
As one can readily conclude from the above description, the extraordinarily high temperatures involved in producing cement require large amounts of energy. Manufacturing one ton of cement requires an average of 4.4 million Btu - roughly equal to 400 pounds of coal. Cement kilns use coal, oil, petroleum coke, natural gas, or hazardous waste fuel. Most cement kilns burning hazardous waste use it to supplement - rather than replace - conventional fuel.
Most cement kilns burning hazardous wastes are the "wet" process type. The reasons hazardous waste fuel is used mostly by wet-process kilns are rooted in the economics of the cement industry. As noted above, the wet process is the most energy intensive of the cement making processes. Since fuel costs are such a significant part of the cost of producing cement, wet-process facilities are under great pressure to reduce fuel costs. Critics of the use of hazardous waste fuel by the cement industry argue that energy "savings" through the use of hazardous waste fuel are illusory because the waste is burned by less energy-efficient facilities to begin with, and that the practice will render the U.S. cement industry less competitive in the long run by slowing the phase-out of older, less efficient wet-process facilities.
Liquid hazardous wastes are combusted at the lower or "hot" end of the kiln. Solid hazardous wastes can enter the kiln at one of several locations, most commonly in the calcinating zone, but also directly into the pre-calcinator vessel or preheater inlet in pre-calcinator or preheater kilns, or by projection devices into the hot end of the kiln.
D. Nature of Hazardous Waste Fuel:
The term "hazardous waste" as used in this paper means a waste material which is classified and regulated as a hazardous waste under the Federal Resource Conservation and Recovery Act (RCRA). RCRA regulations define hazardous waste as a solid waste which is either listed as a hazardous waste under 40 CFR Part 261 Subpart D, or exhibits any of the four characteristics of a hazardous waste found in 40 CFR Part 261 Subpart C. These characteristics are ignitability, corrosivity, reactivity or toxicity (based on the Toxicity Characteristic Leaching Procedure (TCLP)).
Hazardous wastes used by cement kilns include spent and off-specification industrial solvents from paint and coatings, auto and truck assembly, solvent reclamation, ink and printing, cosmetics, toy, medical and electronic industry operations. Paint thinners, waste oils and other petrochemical byproducts are also burned.
Not all hazardous waste is directly suitable for use as BIF fuel. Non-combustible waste and waste with little or no energy value obviously cannot be burned as generated. The waste-burning cement industry has published a narrative identifying wastes which it is claimed the industry avoids: highly corrosive, reactive or chlorinated waste fuels which could damage their equipment or facilities, or waste fuels with high concentrations of metals which could affect the quality of the cement product.
However, it would be a mistake to conclude that non-combustible, corrosive, reactive or chlorinated wastes are not received in cement and aggregate kilns. Fuel blenders routinely mix such wastes with higher fuel value waste, and even with petroleum products to provide hazardous waste derived fuel (HWDF) to kiln operators. The practice has grown in recent years because higher-fuel value wastes are increasingly recycled or disposed of at the manufacturing facility. Consequently, it has become more profitable for kilns, through their fuel blenders, to accept poor or non-existent fuel value solid waste and blend it with clean liquid solvents or virgin oil to produce a "milkshake," that is, a pumpable mixture of liquids and solid waste which can be used as fuel.
RCRA Land Disposal Restrictions (LDRs) require destruction, removal, or immobilization of the hazardous constituent of wastes to levels achievable by the Best Demonstrated Technology (BDT) before land disposal is permitted.
The LDRs prohibit using "dilution" as a substitute for adequate treatment because dilution does not result in the destruction of the waste. EPA has recently interpreted the LDRs to prohibit combustion of certain inorganic metal bearing waste on grounds that combustion constitutes dilution. The list includes 8 heavy metals typically emitted by BIFs (arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver) and 44 metal contaminated wastes including various sludges and wastewater from electroplating operations, production of metal alloys, or smelting operations.
The interpretation does not apply to any of these wastes if they contain hazardous organic constituents exceeding treatment levels; are contained in organic, debris like material; or have a heating value greater than 5,000 Btu. The organic constituents or heating value must be contained in the waste at the point of generation. This is important because it forbids fuel blending to add organics to inorganic metal-bearing waste.