A new technology utilizing whole rubber tires is opening the door to a vast, low-cost source for kiln energy

A proven technology is available that allows a significant portion greater than 25%-of the energy for long kilns to come from whole tires.

Using tires for fuel is recognized practice in preheater kilns, it never has been considered an option for long kilns- until the emergence of this new technology.

Essentially the process is simple: whole tires are bundled and introduced into the calcining zone of the kiln. However, critical elements such as point of entry, combustion control, and secondary fuel volume must remain in balance for it to work.

Point of Entry

A point in the calcining zone where temperatures typically are between 2,000ºF to 2,100ºF is the ideal entry point for whole tires. At this point, the kiln is hot enough to assure complete destruction of the tire, including the metal bead, while offering sufficient residence time to completely consume the tire before sintering begins. This location, which is typically 50% to 60% of the length of the kiln from the feed end, is sufficiently downhill so that all the energy is utilized, yet not so far downhill that the life of the drop tube is unacceptable.

Combustion Control

Controlled combustion is the second vital component of the technology. Showing control over the completeness of combustion also is important. The primary reason Ash Grove has been successful with regulators and the general public is our willingness to implement continuous combustion monitoring. Our continuous monitoring program made both groups aware of, and comfortable with, what we were doing. In addition, the information gained from monitoring the combustion process has helped us to improve it.

We have found that controlling combustion is made easier as whole tires, or whole tire bundles, are fed to the kiln. Because of the surface area to volume ratio, their combustion rate is moderated. Tests have shown that, once they

enter the kiln, whole tires burn for approximately 15 to 20 min. When two or three tires are added per revolution, as many as 50 to 60 tires can be in the kiln at one time in various stages of consumption. This fuel mass provides a stable and predictable burn rate even if individual tires vary in size and weight. Because they burn slowly, most of the energy from the tires is released in the heart of the calcining zone where it does the most good.

The use of large pieces of secondary fuel applies to other forms of organic waste material as well as tires. In the future, municipal waste processed into bales will most likely be another important
energy source for cement kilns.

Surface area to volume ratio is a critical factor. Whole tires perform well as a fuel supplement because of their surface area to volume ratio. Thus combustion is moderated and the fuel releases its energy downhill from the point of entry. This also has the positive effect of thermal load averaging, allowing fuel-addition once or twice per revolution.

Fuel Volume

A 40% substitution rate seems to be the technological barrier for feeding whole, unprocessed tires to the kiln. Inserting whole tires at a greater rate would cool the main frame dramatically and unbalance the process. In the average kiln, a 40% substitution rate would approximate three tires entering the kiln per revolution. Careful attention to the burner design and excess air control may allow higher substitution rates. It all works together in a delicate balance.

For an almost insignificant investment, older cement kilns now will be able to compete head-to-head with newer kilns on a national and international basis. The billions of discarded tires stockpiled throughout the country will give long cement kilns new fire power for the future. 0

Advantages Of Burning Whole Tires In A Kiln

More than 2 billion waste tires currently are stockpiled in the United States. The energy value of these tires is equivalent to 20 million tons of coal. Another 280 million tires-equal to 2.8 million tons of coal- are being discarded each year, When these whole tires are used to supplement a traditional fuel supply, producers can take advantage of some substantial benefits:

- At a 40% substitution rate, a typical kiln producing a quarter million tons a year of clinker has the potential to save $1 million in fuel costs annually;

- Tires burn cleaner than coal. Using tires as fuel reduces nitrogen oxides and does not adversely affect other kiln air emissions;

- Burning whole tires can lessen a kiln's thermal load in the sintering zone, resulting in a better coating and a longer refractory life;

- Because there is a disposal fee built into the price of every tire, tires are delivered to a kiln at no cost or better;

- Improved clinker nodularity, cooler performance, and a better quality end product can result from burning whole tires.