WASTE-TO-ENERGY / CEMENT KILN OVERVIEW
A cement kiln is the word's largest moving manufacturing machine. Typically, they are a huge cylindrical furnace 12 to 25 feet in diameter and 450 to 1,000 feet in length. They are set on a slight incline and rotate from 1 to 4 RPM. Cement kilns can process up 200 tons of raw material such as limestone, clay, and sand each hour.
Internal temperatures exceed 3,000ºF, nearly one third the temperature of the sun's surface.
The cement industry is the world's third largest consumer of energy and typically uses 12 tons of fuel each hour. Their tremendous appetite for energy and unique combustion environment makes cement kilns ideal devices to recover energy from waste.
Here's how the process works.
As the raw materials tumble through the kiln, they are gradually heated to at least 2,700ºF. As they are heated, the molecules of fuel, air and raw materials break apart. When temperatures reach 2,700ºF. the molecules recombine to form a stable new mineral compound called "clinker", the primary ingredient found in Portland Cement.
Liquid waste fuels, along with other fossil fuels, are used to elevate the main flame to temperatures in excess of 3,500ºF
Solid fuel enters through a special patented mid-kiln charging apparatus. At this point, temperatures greater than 1,800ºF. consume both the fuel and the container.
The intense heat of the combustion process separates the fuel into organic and inorganic components. The organics are consumed as fuel and the various inorganics either become part of the cement product or are collected in the air pollution control devices to ensure that emissions are in compliance with strict EPA standards.
PROTECTING HUMAN HEALTH AND THE ENVIRONMENT
Federal law requires the combustion of certain wastes because high temperature thermal destruction is the "Best Demonstrated Available Technology." High temperatures destroy organic chemicals and convert them to harmless carbon dioxide, water, and other common combustion gases. About 1.4 million tons of hazardous wastes are burned off-site each year in 15 commercial incinerators and about 25 cement plants.
Federal law favors resource recovery of wastes over disposal. Cement kilns are an example of the commercial success of this policy. Cement kilns destroy wastes as effectively as other thermal treatment facilities, while simultaneously making a beneficial product with the energy contained in the waste. Last year U.S. cement kilns supplanted the energy equivalent of one million tons of coal by adding hazardous waste to their fuel mix.
Superior technology ...
The conditions needed to make cement-high temperature, turbulence and long gas residence time are the same conditions required for destruction of hazardous waste. Cement kilns burn hotter and have longer gas residence times than other thermal treatment facilities such as incinerators.
Cement kilns meet specific health based standards for the destruction of hazardous wastes, regulations that are more comprehensive than the regulations governing hazardous waste incinerators.
Excellent worker health record ...
The low occupational illness rate among cement plant workers (less than one fourth the national average for manufacturing employees) is a good indicator that public health is protected.
A useful high-quality product ...
The quality of Portland cement manufactured in kilns that use hazardous waste in their fuel mix is indistinguishable from cement manufactured in kilns burning coal and other conventional fuels.
A SUPERIOR TECHNOLOGY FOR HAZARDOUS WASTE DESTRUCTION
Time, temperature, and turbulence are the three most important factors to ensure the destruction of hazardous organic wastes. Cement kilns burn hotter, have longer gas residence times, and are much larger than any other thermal treatment facility. These advantages, plus the degree of mixing in the kiln, make cement plants an excellent technology for recovering energy from hazardous waste.
High temperatures ...
Kilns heat raw materials (typically limestone, clay, and shale) to about 2,700°F, with flame temperatures more than 3,000°F, over 30 percent higher than the temperatures in a commercial hazardous waste incinerator. Cement Kilns Easily Meet Destruction and Removal Efficiency (DRE) Standards Cement kilns easily achieve a Destruction and Removal Efficiency (DRE) of at least 99.99 percent, as required by U.S. EPA regulations for most organic wastes. Because raw materials and fossil fuels account for most of a cement kiln's hydrocarbon and carbon monoxide emissions, trial bums in the 1990's have focused on chemicals of concern (Principal Organic Hazardous Constituents, or POHC's) that would not typically come from cement feed stocks or the kiln's conventional fuel. Tests using these POHC's have demonstrated DRE's greater than 99.99 percent.
Stable temperatures ...
Cement kilns are large (300 to 700 feet long) and have a high heat capacity, so kiln temperatures cannot change quickly due to process "upsets," ensuring that waste destruction is always complete.
High lime content ... The limestone used to make cement provides a natural "dry scrubber" inside the kiln to capture chlorine and sulfur compounds, as well as certain metals like arsenic.
LOW METALS EMISSIONS
While high temperature combustion effectively destroys organic chemicals in waste fuels, it cannot destroy metals and other inorganic materials that may be present. Extensive data regarding metals emissions from cement kilns using hazardous waste as fuel had been generated by state and federal regulatory agencies as well as the cement manufacturing industry. These data show that cement kilns achieve high system removal efficiencies for inorganic materials of concern and that inorganic air emissions are below the health-based levels set by state and federal regulatory agencies, including the EPA.
CEMENT KILNS ARE THE MOST STRINGENTLY REGULATED HAZARDOUS WASTE COMBUSTORS
The federal rules governing the use of hazardous waste in cement kilns known as the Boilers and Industrial Furnaces Rule ("BIF Rule") became effective August 21, 1991. These U.S. EPA regulations governing hazardous waste combustion are the most stringent regulations governing hazardous waste combustion. Under the BIF rules, cement kilns must meet total hydrocarbon emissions limits, specific emission standards for 12 metals of concern, as well as dioxin requirements, and, if indicated, site-specific "direct exposure" risk assessments.
Because the BIF Rule is so recent, all cement kilns burning hazardous waste are currently operating under "interim" permit status. These "interim" permit status regulations are the most comprehensive and stringent regulations EPA has ever issued under the Resource Conservation and Recovery Act (RCRA). They have the full force of Law and require a Certificate of Compliance similar to a commercial hazardous waste incinerator's "trial burn."
All cement kilns burning hazardous waste have now demonstrated at least 99.99 percent Destruction and Removal Efficiency (DRE), even though the interim status BIF rules do not require a DRE demonstration. For cement kilns, the only effective difference between "interim" and permit status under the BIF Rule is the implementation of the formal permit review and comment process.
(Information courtesy of the Cement Kiln Recycling Coalition)