FEECO International, Inc. is an industry leader in the field of thermal processing. We have been supplying a durable line of thermal processing equipment for over 55 years. Our designs can address a vast array of bulk solid materials and processing requirements. FEECO supplies high temperature, refractory lined kilns for a number of applications and industries. All of our designs are developed with efficiency, durability and process flexibility in mind.

FEECO utilizes only the highest grades of industrial refractory, (either castable or fire brick) to cope with high temperature processes. Our engineering staff can incorporate a number of high tech, value added analyses to ensure satisfaction of each and every rotary kiln. We perform thorough heat transfer modeling to ensure proper characterization of fuel consumption and exhaust gas requirements. FEECO can perform finite element analysis on rotary kilns to ensure years of operation in spite of the mechanical rigors of elevated temperatures and massive rotating loads.

We supply rotary kiln systems to perform either endothermic (heat absorbing) or exothermic (heat liberating) processes. We have designed systems to operate in either parallel flow or counter current flow configuration. We have supplied rotary kiln units for mineral processes, such as calcining, catalyst recovery or treatment, and waste incineration from a number of sources.

Typical applications for Rotary Kilns include: waste lime recovery, proppent manufacture, clays, thermal desorption of organic / hazardous wastes, mineral roasting, specialty ceramics, plastic processing, gypsum calcining, tire pyrolysis, bauxite calcining, pigments, catalysts, phosphate productions, etc.

FEECO can offer cost competitive designs for units that process as little as one per hour in units 2 - 3 foot in diameter to massive units at up to 17 foot in diameter that process hundreds of tons per hour. We have successfully processed materials at temperatures in excess of 3000°F.

In addition to the kiln itself, FEECO can supply a complete system with services. These include:

What Is A Rotary Kiln?

Rotary kilns are used to heat solids to the point where a required chemical reaction(s) takes place. The rotary kiln is basically a rotating inclined cylinder. Solids retention time in the kiln is an important design factor and is set by proper selection of the diameter, length, speed, slope and internals design. There are two basic types of rotary kilns; direct fired and indirect fired.

Direct Fired Rotary Kiln

A direct fired rotary kiln has the combustion gases going through the rotary kiln / dryer. The combustion can either take place in a combustion chamber (normal for a dryer) or the flame can be directed down the length of the rotary kiln (typical for calciners).

In a drying application, the contact between the gases and the solids is the primary form of heat transfer. In a calcining application, the radiation from the flame is the primary form of heat transfer.

Rotary kilns can operate in either the co-current mode where the gases and solids move in the same direction or in the counter-current mode where they move in opposite directions. The kiln can also be operated in either the reduction or oxidation mode.

Indirect Fired Rotary Kiln

With an indirect fired rotary kiln the combustion or other form of heating takes place on the outside of the rotary kiln shell. This way, the material being processed does not come into contact with the combustion gases.

This can be important to the product quality or to keeping the product from reacting to the gases.

Another advantage is that the amount of gases coming from the kiln that need to go through an emission control system is very small. Indirect rotary kilns can also be used as coolers.

Direct VS Indirect Rotary Kilns

Rotary equipment has been widely accepted for over 100 years as a preferred means for the pyro-processing of bulk solids. These pyro-processes include, but are not limited to, drying, calcining, heating and cooling; as well as various exothermic and endothermic chemical reactions.
The most common of these processes is that of direct fired rotary kilns / dryers wherein the products of combustion (from a variety of fuel sources) is in intimate contact with the material being processed. In more recent years, however, indirect fired (or heated) processes have gained wide attention. The indirectly fired kiln / dryer is distinct from its direct fired counterpart due to the fact that all heat transferred to the processed material is conducted (and / or radiated) through the vessel shell wall. The indirect process was conceived of and used for over 100 years. Only in the last 25 years have indirect rotary kilns become a widespread commercial reality. This is due primarily to advances in materials of construction capable of withstanding higher temperatures.

Rotating vessels used in pyro-processes have distinct advantages over the many stationary varieties of furnaces commonly employed. As a cylinder rotates, tumbling, sliding, lifting or showering continuously agitates the material within. These action ensure that the material being processed is uniformly exposed to the heat source. This agitation ultimately leads to higher efficiencies and reduced processing times relative to a stationary process. It is not uncommon, as an example, for a process taking one hour in a stationary furnace, to take only a few minutes in a rotary kiln or dryer.

There is a distinction that must be made between direct-fired rotary dryers and kilns. The rotary dryer is a heat transfer device most commonly used for "low temperature" applications as opposed to what industry refers to as a kiln used to perform unit operations at much higher temperatures. The rotary kiln is most typically a carbon steel cylinder lined internally with refractory. In a direct-fired kiln the hot gases of combustion are in contact only with the refractory lining and product; thereby protecting the steel structure of the vessel from high temperature stresses. The result of this design is that products can be elevated to up to about 3000°F without causing damage to the rotating vessel.

The rotary dryer, on the other hand is an unlined vessel in which a high temperature product is not required. It is not necessary to internally line the dryer shell, and as a result, a wide range of internals can be installed to improve heat transfer. The most common form of internals is a lifting flight or "lifter" which is typically plate steel extending perpendicularly inward toward the cylinder centerline. These lifters act to pick up material from the bed of material that forms as an arc on the upward running face of the cylinder's inside surface.

The cross sectional area of this material bed is typically about 10% of the overall cross sectional area. The direct fired dryer internals effectively act to remove material from this quasi-static bed of tumbling solids and lift it along the shell's inside periphery and then shower or disperse a curtain of rotation. The density of the showering curtain is of key consideration to the dryer designer and operator. The efficiency and effectiveness of this equipment is determined in part by how much intimate contact exists between the solids and the hot gases flowing within the vessel.

The operation of the higher temperature, refractory lined kiln is very similar to that of the rotary dryer. The principal distinction is that a kiln's operation is not defined by showering material in the hot gas stream, rather, the arc of material that forms may intentionally remain intact and is not dispersed. Heat transfer to the solids is not typically dominated by convection. Hot gases continuously heat the internal refractory lining of the kiln shell as the cylinder rotates; the tumbling bed of solids is constantly exposed to a hot refractory surface. Since the solids are in intimate contact with a hotter surface, heat transfer is by conduction.

Radiation effects also occur, especially if the refractory is glowing at elevated temperatures above 1200°F. The surfaces of the solids bed (toward the kiln centerline) also experience heat transfer by radiation since material is in "view" of the hot-glowing refractory. Lastly, direct convection exists between the innermost surface of solids and the hot gases flowing over the bed. As in a rotary dryer, typical bed loading in kilns is typically about 10%.

As one might expect, the direct fired rotary dryer acting as dispersion device can achieve higher heat transfer rates than refractory lined kilns. The result is that the dryer can transfer more heat per unit volume than a kiln.

Indirect fired kilns or rotary dryers closely parallel the direct-fired rotary kiln in operations. In this type of device there is not an internal gas flow acting as a heat source so components such as lifting flights would offer no perceivable advantage. Heat is transferred to the bed of solids by conduction between the solids and the wetted portion of the hot shell. In most applications the shell is heated to temperatures at or above 1200°F, so radiation between shell and solids also prevails. Since products of combustion are completely isolated from the product being processed, two heat transfer mechanisms take place.

Heat is transferred to the rotating shell's outer surface by an isolated heat source, most commonly; gas or oil fired burners orientated so as to "bathe" the shell surface in hot products of combustion. Depending on size and geometry of a particular unit, a number of burners forming an array may be employed to avoid localized overheating of the shell through excessive localized heat release. Other heat sources may be employed such as electric resistance heating elements or hot waste gas or thermal transfer fluid from isolated sources. In the event that gas or oil fired burners are used, the products of combustion are emitted through a stack as clean flue gas.

In an indirect fired rotary kiln, the shell temperature tends to be uniform along the circumference regardless of the heat source being from a distinct point. This is no doubt due to the excellent conductivity of the alloys of construction.

Once the shell attains operating temperatures, heat will flow radially inward through the shell wall to the interior face.
The second heat transfer mode is that of transferring heat from the internal shell surface to the colder bed of solids. The heat transfer process is best modeled as a heat flux passing through a boundary layer (the shell).

When to use a Direct Fired Unit or an Indirect Fired Unit?

Virtually any process that can be carried out in a directly fired rotary vessel can also be carried out in an indirectly heated unit. Both types of systems have inherent advantages and disadvantage attributed to their operation. As a general rule, the direct-fired device is more efficient in operation and also has a lower capital cost than their indirect counterparts. There are numerous applications however, in which an indirect unit offers better overall economics. The following conditions are factors wherein an indirect unit would likely be the preferred system.

Finely Divided Solids

For applications where the material to be processed consists of finely divided solids, particle entrainment becomes critical. In direct fired units, the heat source is hot gas (products of combustion and air) which flows with an inherit velocity. These gases can carry discrete particles through form drag. The degree of entrainment depends on a variety of factors such as gas velocity, gas density, particle density and shape. Due to entrainment potential, direct fired rotary dryers, and rotary kilns processing fine materials (typically under 200 mesh/75 microns) require the design to be centered around permissible gas velocities as opposed to heat transfer requirements.
Indirect fired units do not rely on processed materials being in intimate contact with hot combustion gases, and in fact, the only gases which need to be in direct contact with the material are any gases that may have evolved in the unit. It is common for direct fired rotary dryers and rotary kilns to have upward of 100 times the mass or volume of process gas flowing within the vessel as opposed to an indirect unit of the same duty.

When determining the best type of unit, direct or indirect, to carry out a pyro-process on fine solids, one must consider the overall cost of gas cleaning ancillaries such as baghouses or scrubbers in addition to the cost of the pyro-processing vessel.
Some examples of fine materials commonly processed are:

• Carbon Black
• Chemical Precipitates
• Filtercakes
• Finely Ground Solids, etc.

Inert Systems

Many pyro-processes are best performed in inert atmospheres. As an example, most carbonaceous solids will combust when at elevated temperatures in the presence of free oxygen. High temperature processing of combustible products such as coal, petroleum coke, sludge and numerous organic solids can be carried out in direct fired units, but careful provisions must be made to ensure safety. Fuel rich or near precise stoichiometric flames must be developed to reduce the presence of free oxygen. Such systems are complicated in their control and may involve oxygen detection, explosion relief doors and automated extinguishing systems. Such extensive provisions involve added costs and careful licensing and insurance detail.

An indirect fired unit can have an internal environment relatively free of oxygen through well-designed sealing mechanisms. Such systems can readily process combustible solids at elevated temperatures with a reduced risk of combustion or explosion.

Other applications exist in which oxidation of product does not pose an explosion or combustion risk, but the quality of the product may be deleteriously affected by oxidation. Applications where an undesirable oxide compound forms in the presence of oxygen at high temperatures are often best performed by indirectly heated means. A common example would be a metallic product.

Other isolated cases may exist where a solid may form an undesirable compound with nitrogen at high temperatures. In such a case, it is nearly impossible to eliminate nitrogen composition from processed solids in a direct fired unit, but relatively easy to do in an indirect unit.

Controlled Atmosphere Chemical Reactions

Indirect fired units have been used to perform an array of gas to solids reactions. The indirect unit can prove ideal to expose solids to a gaseous chemical reactant under high temperatures where reactions occur at accelerated rates. The "doping" of ceramic catalysts, could serve as a practical example.

Such chemical reactants can be introduced into a direct-fired unit, but the reactant becomes diluted by the hot gases, which are required, as a heat transfer medium. In indirect units any reactant utilized can be piped in and metered to achieve precise concentrations.

High Value Volatile Products

Certain pyro-processes exist in which the goal is to recover a high value volatile component from a somewhat valueless solid carrier. As an example, consider the thermal separation of oil from oil shale. Such an application is likely best suited in an indirectly heated vessel. As in the case of combustible materials, a direct fired process can be carefully designed to perform the separation, but the load on separation equipment, such as a condenser, becomes greatly increased by undesired tramp air. The cost associated with these ancillary separation devises can far exceed the cost of the heat transfer vessel.

By processing such a material in an indirect unit, the desired volatile product is highly concentrated and does not burden the recovery equipment with needless tramp air.

Another highly promising application is in using an indirect fired rotary kiln to effectively distill valuable oils from low value wastes such as:

• Scrap Tires
• Oil Saturated Soils
• Oil Drilling Wastes (cuttings)

Hazardous Waste Applications

There is an array of applications in which a hazardous component may be absorbed on an inert solid substrate. Examples of this scenario are spent activated carbons used to absorb an undesirable waste component from a liquid or gaseous stream. In such cases, the impregnated carbon can be held at high temperatures and the absorbed component will volatize and effectively be desorbed from the carbon. The volatized waste is then evacuated from the system via an imposed draft. The off-gas  from the indirect rotary kiln would be laden with a hazardous waste component that could then be condensed in high purity or incinerated as a concentrated vapor stream. This operation can be performed in a direct-fired unit, but as in previous examples, the off-gas burden would be significantly increased. Depending on the regulations placed on the volatized compounds, the treatment of this off-gas can be quite costly. Such systems are often employed where the effluent gas is treated in an isolated vessel such as a secondary combustion chamber. Regulations placed on most hazardous waste compounds stipulate that the off-gas be burned in an oxygen rich environment for two seconds of residence. The sizing of a secondary combustion chamber is based on mass of off-gas, regardless of the concentration level of the hazardous component.

Chemical laden soils; absorbents, tank wastes and other inert substrates follow the above conditions very closely.

As the above five examples indicate, there are countless applications wherein a process flowsheet that involves a more costly indirect fired kiln or dryer may result in a more cost effective overall system. The developer of a thermal process must take into account the cost of all required ancillaries, operating cost, product quality requirements and several other variables as opposed to merely the cost of the primary heat transfer device.

When evaluating an overall system approach, the designer must also take into account operating cost factors such as the cost of fuel, cooling water, power, instrument air, the air quality restrictions, and the cost of off-spec product. The overall analysis is never as simple as considering the relative attributes of primary heat transfer vessel. For quick evaluations, the rule of thumb would be that direct heat transfer devices are the most desirable and cost effective unless offset by the above five conditions.

Recent developments have been made to develop equipment flowsheets that combine direct and indirect processing steps. Some hybrid equipment has been developed with direct and indirect phases occurring in a single vessel. As an example of this, consider the ducting of clean flue gas from the furnace segment of an indirect unit to a direct rotary dryer. This scenario can be achieved in a single vessel.

In summary, there are numerous applications in which an indirectly heated rotary vessel may be the unit of choice for a given application. When selecting the proper technology, one must carefully evaluate all aspects of the operation including:

• Product Quality
• Fuel and Electricity Costs
• Capitol Cost
• Cost of Ancillary equipment such as dust control and secondary combustion of exhaust gases
• Environmental permitting and continual compliance
• Yields and dust losses