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This page discusses the problems of burning tires to provide the energy for cement kilns. But much of the information seems apropos to the more general problem of tire burning. Burning Issues Editor]
HAZARDS OF BURNING TIRES
From Dr. Neil Carman regarding cement kiln emissions, stack tests and combustion upsets. 6/97 HAZARDS OF BURNING TIRES
A. It is highly inaccurate to state that TDF (Tire Derived Fuel)... "does not contain hazardous materials;" A. Tires are mfg from petrochemical feedstocks such as styrene and butadiene, which are both being classified as human carcinogens. Styrene is a benzene derivative and burning tires releases styrene and several benzene compounds. Butadiene is a highly carcinogenic four-carbon compound that may also be released from the styrene-butadiene (SBR rubber its called) polymer form during combustion.
Chemical composition tests on waste rubber show that it contains numerous toxic and hazardous air pollutants and once burned, these can become airborne. I have lab results somewhere on tests that I helped to perform as a state official, and we were surprised that waste rubber from SBR was not classified as a hazardous waste. M, P and O-Xylenes may also be found in TDF which are benzene derivatives and carcinogenic. Tires --older tires--used to be made from chloroprene, a chlorine-containing petrochemical used less and less today in tire mfg. Polybutadiene is another polymer used to make synthetic rubber for tires. Aromatic extender oils comprise about 25% of most tires today and are known to cause cancer in lab animals as well as being suspected human carcinogens. These are highly aromatic--multiple benzene-containing chemicals--petroleum waste materials with complex ring structures that are even more difficult to burn that benzene, which has a highly stable ring structure that makes good combustion far more difficult that burning natural gas or straight chain carbon compounds. Anything with benzene will require higher combustion temperatures, higher residence times and higher oxygen to break apart the six-carbon ring with electron clouds above and below that protect the ring from easy chemical breakdown.. The thick black oil and black smoke that one sees when tires are burning outdoors is due solely to the aromatic extender oils; they too require higher combustion temperatures, higher residence times and higher oxygen to break down fully to CO2 and water.
Do cement kilns really provide higher combustion temperatures, higher residence times and higher oxygen? These are complex process questions that can be debated by different technical experts to give very different sets of answers, and because there are generally two different kinds of cement kilns such as 1) old, energy inefficient wet process kilns and 2) newer, more energy efficient dry process cement kilns. Generally cement kilns run at higher combustion temperatures than incinerators, but I think it is highly questionable that anyone can make an absolute blanket statement supported by solid scientific proof that cement kilns provide longer residence times and adequate oxygen (i.e. as excess air) to give complete combustion. Why?
a) Cement kilns when stack tested show products of incomplete combustion (PICs) just like incinerators and other combustors demonstrating that perfect combustion is not being achieved. So something is not right with combustion.
b) Turbulence for good combustion may not be as perfect as some experts claim in cement kilns due to the extraordinarily large volumes of solid materials in the kiln being used to make clinker and then cement product, in part since a cement kiln is a giant oven used to bake rock and turn it into clinker.
c) cement kilns typically run on the lower limits of excess air for good combustion due to the huge quantities of air required to be heated from ambient temps to 3,000 degrees F, and to heat this much air to such high temps requires tremendous energy costs.
So every single pound of air heated in a cement kiln exacts a certain operating costs in fuel use and thus cement kilns try to keep the excess air (and oxygen) at the borderline of safe combustion.
But during stack tests of TDF cement kilns will do several things to make emissions and combustion look good-to-decent for such facilities:
a) run at higher excess air to improve combustion efficiency,
b) control kiln parameters more precisely,
c) prevent kiln solid ring formation and buildup that creates havoc for good combustion of any fuels,
d) burn lower TDF levels during stack tests than they may be seeking to burn operationally,
e) operate and maintain their ESPs or baghouses in top condition to keep particulate emissions to a reduced level; and
f) miscellaneous tricks.
B. Older tires may contain much higher amounts of lead when leaded gasoline was being used, according to Dr. Jerome Nriagu of the University of Michigan's School of Public Health. Burning these older tires means that some of the lead will go back into the environment as airborne contamination.
C. Metals. TDF does contain metals and these amount vary somewhat. They may be cleaner that real dirty coal or they may be worse than some cleaner types of coal. One also has to be skeptical about self-reported metal levels in coal, such as mercury, because coal users want to show less mercury emissions than is the case.
D. COMBUSTION UPSETS. This is a serious public health issue near cement kilns. Cement kilns certainly do have combustion upsets and smoke may be emitted during such events. Cement kilns are not designed or required to have major fail-safe combustion devices such as large afterburners that all state-of-the-art incinerators must have by federal law today (all medical, municipal,
What to do with used tires:
THE SOLUTION: TRUE RECYCLING and REDUCTION: Rubber from used tires can now replace 10% of the rubber in new tires. REUSE: Retreaded tires, New technologies
RECYCLE: Rubberized Asphalt Roads (Just 2,832 miles of CA roads out of the total 149,000 miles could reuse all of the tires not currently recycled.)
OR CREATE ANOTHER PROBLEM: TIRE BURNING
Increase in deadly, toxic pollutants known to cause well documented illnesses:
- Dioxin, Mercury, Small Particulates, Zinc, Nitrogen Oxides, etc.
- Cancer, Asthma, Nerve Disorders, Reproductive Disorders,
- Interference with the emerging new technologies of reducing and recycling.
THE TIRE PILE ISSUE
The need to diminish stockpiled used tires should not be solved by compromising the health of our communities. Proposals to burn tires are not effectively promoting tire reduction, reuse and recycling. Should we pay for this with our health? Hazardous waste incinerators cannot operate without their afterburner or secondary combustion chambers in normal operation. The afterburner is required because of the potential for flame outs and total combustion failure in the primary burn chamber, which is all that cement kilns possess. Cement kilns have no fail safe combustion devices which is unthinkable today in all incinerators.
Other kinds of upsets. Cement kilns are subject to a variety of problems, including a type of meltdown of the kiln when the ID fans lose power or fail to operate; without adequate air flow to control kiln temperatures at or below 3,000 degrees F, the kiln temperature may skyrocket quickly to 4,000 degrees F and the kiln is so hot that the steel shell sags toward the ground effectively destroying the kiln. Kiln meltdowns are not rare events and have happened here in Texas at several plants in the last ten years. Cement companies do not like to talk about this problem.
These are just a few of the technical issues surrounding combustion problems observed in cement kilns. The bottom line is that they are not designed, not built and not operated as state-of-the-art incineration devices, but are basically old model-T versions (especially old wet process kilns) of first generation incinerators of the 1950's-mid 60's which had no afterburners.