|Gossman Consulting, Inc.|
GCI TECH NOTES©
David Constans and David Gossman
Hazardous waste fuel has been used as an alternate or supplemental fuel in cement kilns since 1980. Since 1992, with the promulgation of the BIF regulation, this activity has become heavily regulated. Additionally, partly because of BIF and other hazardous waste regulations, the growth of hazardous waste volumes suitable for use in HWF have stopped and, in some respects, the supplies are shrinking. This is to be expected. Indeed, this was one of the stated goals of these regulations. However, the driving force behind the desirability of alternate fuels to fire cement kilns has not abated. That is, the desire to reduce total fuel costs through substitution of waste-derived fuels for the normal fossil fuels, primarily coal, used in these facilities. Consequently, there is renewed interest in the use of non-hazardous waste fuels. One of the most readily useable and highest heat content non-hazardous wastes is used tires. This Tech Notes discusses the use of tire derived fuel (TDF) in cement kilns. The various forms of TDF, how it is utilized in the different clinker production processes, the impact on product quality, and stack emissions from utilizing TDF.
Bits, Pieces or Whole Tires
TDF can be provided in a number of forms. The tires can be ground into "crumb". There are a number of advantages to utilizing this form. 1) The steel in the bead and radial bands can be removed via air classification, 2) The crumb can then be blown in with powdered coal fuel directly substituting for the powdered coal, 3) The transportation storage and management of the crumb is very similar to managing coal fines, both the good and the bad aspects of such management. There are disadvantages, or more properly unnecessary advantages, for cement kilns utilizing crumb TDF. The removal of the steel is unnecessary since cement kilns have a need for iron in its process. Producing the crumb is quite expensive, likely making the crumb as costly as the coal or coke it is replacing.
Tire "chips" of varying size are routinely utilized as cement kiln fuel. These chips range in size from 2 cm x 2 cm up to 15 cm x 15 cm squares. A variation on this is a "quartering" of the tires. In all cases, the transportation, storage and management is essentially the same. Transportation is via dump truck. Storage is generally in an open air pile similar to storage of coal or limestone. The feeding of the chips into the kiln is via a conveyor fed from a hopper. A front end loader is used to load the hopper from the storage pile.
The use of tire chips has a couple of advantages. The feed rate can be continuous and carefully regulated. Also, there is very little manual labor involved in handling chips versus whole tires. There are, however, a couple disadvantages. Like the tire "crumb", producing chips from whole tires is still expensive. Certainly not as costly as the production of crumb, but often half the cost of acquiring and delivering the TDF. The wire in the bead and radial belts do not shear smoothly when the tires are chipped. Consequently, the chips are ragged with these wires hooking onto everything they come in contact with; the front-end loader, the trucks, and other vehicles passing near the storage areas. In short, the chips migrate throughout the facility. One facility utilizing tire chips, after several months of use, plugged their raw mill with migrating chips. This necessitated a shut down and time consuming removal of the chips from the screen in the mill.
The use of whole tires as kiln fuel is common in the cement kiln industry. In this case, truck loads of whole tires, usually enclosed vans, are delivered to the end of a conveyor. Tires are manually unloaded from the truck onto the conveyor. The conveyor feeds the tires to a mechanism that inserts one tire at a time into the kiln at specified time intervals. The advantage of utilizing whole tires is that there is no processing costs in addition to the acquisition costs. Also, unlike tire chips, the whole tires do not migrate throughout the facility. Nor, like the crumb, are they subject to possible "dust fires". Transportation, storage and management of whole tires requires more logistical care and more manual labor than the management of the other TDF forms. Obviously, piles of whole tires are to be avoided. No one wants a tire pile fire. The most reasonable solution is to have the tires delivered, stacked in trucks. Multiple trucks may be on-site waiting to be unloaded onto the conveyor. Handling whole tires requires manual labor as such an activity is difficult to automate.
Input of the Tire into the Kiln
For the purpose of utilizing tires, there are two cement processes; long wet/long dry process kilns, or preheater/precalciner process kilns.
Long wet or long dry kilns can utilize tires in two ways. Tire crumb and smaller chipped tires may be blown in with the powdered coal or through a separate feed system. Whole tires can be injected mid-kiln through a Cadence gate attached to the wall of the kiln. The whole tire is placed in a scoop attached to the gate as it passes by a platform. As the gate rotates to the top of the kiln, the gate opens and the tire drops into the kiln. The maximum feed rate of the tires is generally limited to one tire per revolution. Although it is possible to install a gate that could insert two tires at a time, this is not recommended for reasons to be discussed later. Another possibility is a second gate on the opposite side of the kiln. Cadence Environmental Energy, Inc. has a patent on this technology and actively markets it world-wide. also, Tire Management Inc. has patented feed systems which are frequently used with a Cadence gate. Theoretically, it is possible to insert tire chips into the kiln utilizing this method. However, as noted above regarding tire chips, the hooking wires will create a feed problem both in the feeding of the chips into the scoop and the passing of the chips through the gate since neither of the locations can be "force fed" and depend totally on gravity as the motive force. The installation and especially the subsequent maintenance of the gate and its attached scoop can be quite costly and may require frequent outages. Additionally, the rush of cold air that enters the kiln with each opening of the gate negatively affects the efficiency of the kiln. This, however, is the only method of utilizing whole tires in long wet or long dry kilns.
It is easier to utilize TDF in preheater and/or precalciner kilns. Certainly crumb and small chips may be utilized as a direct substitute for powdered coal as in long wet and long dry process. The preheater/ precalciner process, however, allow the injection of whole or chip tires into the kiln quite simply through a chute down on to the kiln feed as it enters the kiln from the last stage of the preheater/precalciner. Whole tires or tire chips can be fed to a double gated chute into the duct between the kiln and the kiln feed discharge of the last stage. With the outer gate closed, the inner gate opens and drops the tire or the chips directly into the kiln feed or into a chute which drops the tire or chips into the feed. Again, such a "gravity system" may have problems with chips due to the hooking wires. However, this system is excellent for the whole tires (or for tire quarters). The timing of the opening/closing of the air lock can be set to optimize the input rate of the tires to minimize emissions impacts. (This is discussed in some detail below.)
Tire chips can be continuously fed into the preheater/precalciner kiln by feeding the chips into the duct and onto the kiln feed through a rotary valve or short screw. The screw has certain advantages over the rotary valve as the hooking wires of the tire chips will pass more easily through the screw. The feed rate of the chips can be regulated by the speed of the rotary valve or the screw.
The Effect of TDF on Product Quality and Stack Emissions
TDF is a very high quality fuel having about 13,000 to 15,000 BTU per pound, (7200 to 8300 kcal/kg), about the same as a superior quality coal. TDF typically has 0.5-2.0% sulfur, this is less than or equal to most coals and coke. The hydrocarbons that make up the rubber in the TDF are no more complex or difficult to destroy than those present in coal. The steel in the bead and the radial belts constitute about 12% by weight. The cement chemist must take this into account when he formulates his raw feed mix. TDF may have metals such as lead, cadmium and zinc. While this should not be a problem for most kilns, an experienced chemist should evaluate the concentrations of these and other metals in the raw feed, clinker and cement kiln dust to ensure the additional metals added with the TDF will not become a problem.
TDF fed at a rate that does not compromise metals input/output balances has no effect on clinker quality. Stack emissions of CO, however, may be affected due to the manner in which the tires are fed to the kiln. A uniform feed rate of crumb or chips will allow the operator to increase the kiln exit oxygen, or rather allow the operator to maintain the desired kiln exit oxygen concentration and/or the kiln exit CO concentration. However, the insertion of whole tires at one to two minute intervals will often produce a CO spike and/or an oxygen dip in the kiln exit gases. This can be compensated for by increasing the interval between tire insertion and/or by increasing the normal kiln exit oxygen by ½ to two percentage points. Some plants may also experience changes in SOX and/or NOX levels depending on where and when the TDF is burned, and changes in O2 levels.
It is estimated that there are 36 cement plants utilizing tire derived fuel each year in the United States. Both whole tires and tire chips are utilized as fuel with essentially similar results.