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Pyro-Processing for Beginners: Direct-Fired and Indirect-Fired Rotary Kilns and Dryers

Progress advances rapidly, but we still have not created better equipment for pyro-processing solids than rotary. Stationary motionless processing machinery simple does not provide the same level of effectiveness. Rotary equipment constantly shakes the material under processing evenly exposing it the source of heat. This results in faster processing times and increased efficiency especially when compared to stationary variants.

The spectrum of processing operations varies greatly including both endothermic and exothermic chemical reactions, calcining, drying, etc. This versatility makes rotary processing applicable in various industrial procedures.

We will try to highlight advantageous and disadvantageous characteristics of both direct- and indirect-fired rotary driers and kilns.

Direct-Fired Machines

Dryers and Kilns share a plethora of characteristics. However, despite similarities these devices have different purposes. In general, rotary dryers are used as heating devices that reduce the moisture level of the material. Rotary dryers generate relatively low temperature. On the contrary, rotary kilns force chemical reactions and physically change substances under processing.

One of the similarities is that both types of devices fulfil their purposes by ensuring direct contact between the material and the gas used for processing. In rotary dryers you can easily enhance the efficiency of heat transferring. This can be done by installing various internals since dryers themselves work at low temperature and dryer shells do not necessarily have to be internal. Surely, you have heard of “lifters”, simple perpendicular plates of steel placed inside the dryer.

Lifters enable better heat scattering. Plates pick up the material and bring it up from where it “rains” down back to the bed. As the bed is usually quasi-static, the amount of cross-sectional material is less than 10% of the overall volume. To improve the area of contact between the gas and the material, lifters constantly pick up and drop down the material. Dispersing material loses moisture easier. Engineers should consider the density of dispersion to ensure that the material is heated evenly as it falls down. Dense showering curtain will not improve the process significantly. Conversely, thinner shower curtains will definitely make the drying process faster. Functioning lifters are represented in figure 1.

In rotary kiln you may often find a cylinder of carbon steel that is installed internally. In these devices, combustion produced hot gases have a very high temperature and usually such thermic pressure cause damage to steel details. However, in rotary kilns the gases contact directly only with the refractory lining offering a good protection from damage to the vessel itself.

Unlike rotary dryers, rotary kilns do not rely on convection and instead provide direct heat conduction. The procedural pattern of rotary kiln reminds the functionality of rotary dryers. However, the hot gas is not contacting with showering material. The arc of solids is being shuffled constantly and expose the underlying layer to the heat. While the cylinder rotates, the material is constantly in close contact with the gas and the heat is being transferred from hotter parts of the material to colder ones, while the bed of the material is exposed to the gas.

Usually, you can observe slight radiation especially when the temperature in the refractory reaches a certain point above 1200°F. This radiation also helps in transferring heat to the material.

Another factor to consider is direct convection that occurs over the surface of the material as the hot gases flow intimately over the bed. This is a very efficient heat transferring option as the typical bed loading rarely exceeds 10% enabling a larger surface to be in contact with the gases.

Small differences between rotary dryers and rotary kilns make the former more efficient at transferring heat per unit volume of the material.

Indirect-Fired Machines

Indirect-fired rotary devices function similarly to direct-fired devices, but the core difference is that the heat is being transferred from a source installed in a different position – behind vessel shell wall.

The gasses do not contact with the material directly rendering lifters and similar installments inefficient. There is simply no gas to transfer heat inside the vessel. The heat is instead transferred via conduction within the material itself. The radiation between the shell and solids inside is maintained due to high temperature above 1200°F. The combustion happens in a separate chamber and two heat transferring methods are needed to improve the efficiency of heating.

The idea behind indirect-fired devices is that the fuel, gas or oil burners, combust and produce loads of hot gas that heats up the shell surface. It is imperative to avoid uneven heating of the surface and provide balanced heat transferring. Sometimes, engineers prefer installing additional heat sources such as thermal transfer fluids, hot waste gas, and electric heating components. Rarely, these heating elements substitute combustion chambers. In such cases, the gases have to be filtered and used after emitting through a filtering element.
It is important to have suitable alloys that can warrant for a good heat transferring. If the quality of the alloys is high enough, the temperature of the shell will be uniform despite unevenly installed heat sources.

Even heating allows the shell to reach the optimal temperature as fast as possible. When the operating level of temperature is reached, the heat starts flowing radially inside the shell towards the back wall of the interior.

Another possible efficient method of heat transferring is best described in the illustration below. In this case, the heat is transferred directly to the part of the shell where the bed of solids resides. This is a highly effective arrangement often used in various situations.

In our case, the gas burner is acting as a primary source of heat and creates both radiation and convection. The heat transferred from the interior transfers directly to solids. This results in a controllable thermal process.

The most efficient way of designing a shell is best described by a formula below:
Q = h*A*DT
Q = Transferred amount of heat in Btu/hr
h = Heat transfer coefficient in Btu/ft2 • Hr • degrees Fahrenheit
A = ft2 measurement of the surface area of the shell interior
DT = Log mean temperature of the coolest and hottest parts of the material and shell surface

The equation helps in modeling an efficient indirect device and illustrates the heating process within the system and its parts. The best way to imagine the system is to use meticulous analysis of the process applied to a finite amount of the material within a certain time frame. This way the whole process will be viewed as a complex consisting of several process stages each happening at specific temperature level. Imagine a material consisting of multiple compounds such as moisture, organic elements, and chemical reagents that react when heated. One can observe a system where 6 distinct phases occur:

  • The operation starts with heating the material up to 212°F so the moisture would start evaporating.
  • Water evaporates completely.
  • The temperature is increased further to the point where organic elements start vaporizing..
  • Organic elements evaporate completely.
  • The temperature is increased to the point where the chemical reaction occurs.
  • Endothermic reaction starts.

Each of the processes listed above happen within a certain temperature range. Knowing the values of temperature indicating the start of each phase helps in designing a system where the log temperature difference is defined and every single step is calculated appropriately.

Direct- vs. Indirect-Fired Rotary Kilns: Choosing the Best Solution for Your Particular Case

If you can perform a process in a direct-fired rotary dryer or kiln, chances are quite high that you can carry out the same process in an indirect-fired device. However, there are pros and cons in using one or another type of rotary dryers and kilns. In general, it is more cost-efficient to use direct-fired units as they are notably cheaper. The price and economic considerations are the main reasons why direct-fired units dominate the market. Nonetheless, using indirect-fired units may be more reasonable in a plethora of applications.

Indirect-fired rotary dryers and kilns became popular fairly recently. We have been using them for only quarter of a century, while the technology is out there for more than 100 years. Slow progress of metallurgy and the lack of high quality heat-resistant materials were the obstacles in front of popularization of indirect units. There is still some room for improvement.

Indirect-heated devices are most used in industries where thermal reactions have to be controlled and the processes themselves are specialized. Let’s take a look at various cases where indirect-fired units prove to be more efficient than their direct-fired counterparts.

Finely Divided Solids

Indirect-fired units are more efficient at processing finely divided solids simply due to the fact that there is no hot gas flowing inside the vessel. In a direct-heated rotary unit, hot gas flows within the vessel and often carries away particles causing material loss and inefficient chemical reactions. The amount of loss depends on a plethora of various gas characteristics like velocity and density. Additionally, the density and shape of material particles have to be accounted for as well. This makes the whole modeling of a process to align with the permissible gas qualities instead of focusing on heat transfer requirements.

On the contrary, indirect-fired units do not need the material to intimately contact with the gases. Direct-fired rotary units use up to 100 times more gas compared to an indirect-fired units. This discrepancy is one of the reasons why indirect units are more commonly used to efficiently process finely divided solids.
Indirect units are used on a variety of industrial objects to process specific materials such as filter cakes, ground solids, carbon black, and others.

Inert Systems

Inert systems are inherently dangerous as many processes that are usually carried out in an inert environment demand free oxygen. Having free oxygen is a necessity when processing carbonaceous solids. There are quite some combustible materials that often need to be thermally processed, the list includes coal, organic solids, petroleum coke, and others. These materials can be processed in a direct-fired rotary unit, but the amount of safety protocols and installations will make this process overly complicated, expensive, and in general less efficient. Secure systems usually consist of the unit itself equipped with oxygen detection devices, explosion relief gates, fire extinguishing installments, etc. Every single addition to the system makes it more complicated and costlier. In addition, a whole new bunch of calculations have to be made to ensure that the process is designed appropriately.
Inert environments are also adopted for the processes where there is a risk of deterioration in case of unwelcome oxidation. In this case, using indirect-fired units where the presence of oxygen can be greatly reduced inside the vessel is more appropriate. Some metallic products that do not stand the presence of oxygen are best processed in indirect-fired units.

Another example of inert application is processing nitrogen filled compounds. This process is nearly impossible to perform in a direct rotary unit where eliminating nitrogen is hard. However, removing nitrogen is a fairly easy task when the process is done in an indirect unit.

Chemical Reactions in a Controlled Atmosphere

One of the prime examples of using ceramic catalyst to improve some chemical reactions. Indirect-fired units are perfectly suited for performing processes where the solids are fully exposed to an intimate contact with various gaseous reagents.

On the contrary, in direct-fired devices, hot gases act as heat transferring agents which means that gaseous reactants are diluted with hot vapors inside the vessel. Undoubtedly, you can carry out a controlled atmosphere chemical reaction in a direct type of rotary kilns, but indirect units enable a more controllable environment where one can precisely calculate gas concentrations to ensure that the reaction occurs as intended.

High Value Volatile Products

Some solid carrier compounds have valuable components that can be retrieved only via thermal processing. One of such reactions is separating oil from shale. This reaction is best performed in an indirect-heated rotary kiln.

Direct-fired units can separate oil from shale, but the separation process needs heavy additional equipment like condensers. The condensers have to be expensive and perform exceptionally due to the tramp air. Such equipment costs a fortune and can easily be priced higher than direct unit itself. Indirect-fired devices are more efficient at separating high value volatile compounds and do not produce tramp air thus greatly reducing the price of condensing equipment.

Some industrial professionals suggest using indirect-fired units to separate oils from various wastes like soil, scrap tires and resin, cuttings left in oil drilling wastes, etc. This can be highly cost efficient and profitable.

Hazardous Wastes

Some chemical reactions rely greatly on various solid substrates that absorb undesirable components. A perfect paradigm is using activated carbon to remove waste component from gaseous and liquid materials. Activated carbon itself turns into waste, but with the assistance of an indirect-rotary kiln we can make activated carbon reusable. The hazardous components will become volatile at high temperatures and will be desorbed from carbon which can be used again. This process can definitely improve the cost-efficiency of various chemical reactions performed in any facility.

The process of desorbing hazardous waste includes a multitude of phases. When desorption concludes, hazardous waste can be immediately removed from the vessel through imposed draft channel. The compound then may be incinerated and released in a form of a vapor stream or condensed drastically to a state of purity. The problem that presents itself when the same process is being performed in a direct-fired unit is that off-gases exist. These gases have to be treated appropriately and the treatment may cost additional resources rendering the process cost-inefficient in a multitude of scenarios.

There is a list of chemical reactants that act as hazardous waste absorbents. An incomplete version of the list includes a variety of soils and tank wastes.
There are multiple factors that have to be considered when modeling a thermal-processing application. Above listed various examples of cases where indirect-fired rotary dryers and kilns are more than simply suitable. A plethora of situations exist where using indirect units is more cost-efficient and allows for better performing systems. Simultaneously, when choosing the right device, one must consider a multitude of factors such as quality requirements, operating costs, resources, necessary materials and compounds, etc. Important metrics to account for also include costs of various resources like fuel, cooling reagents, electricity, instrument air, off-spec product, etc. All these factors have to be considered and after juggling them for a while one has to make the right choice. Very often, this choice will be an indirect-fired rotary unit.

One of the most notable trends in the industry is using combinations of direct- and indirect-fired units in a system for different processing phases. There are solutions that allow both direct and indirect processes to occur within the same vessel. As a paradigm of such systems consider moving flue gas from combustion chamber of an indirect unit to a direct dryer. This system can work in the same vessel.

This illustration represents a schematic of a process where flue gas is used to dry out used carbon in the pre-drying section. Carbon has a lot of moisture that presents burning.


Stationary equipment has been a disadvantageous technology ever since rotary dryers and kilns were introduced to the world. Rotary units in pyro-processing equipment became a new foundation for a plethora of various industrial processes. For a long time, direct-fired units were dominant due to the lack of high quality materials that could make a heat-resistant shell. However, new alloys allowed us to create highly efficient and relatively cheap indirect units.

Considering the upsides and downsides of direct- and indirect-fired units is highly recommended when choosing the best option for your particular application. Contact suppliers directly to receive the fullest information about both types of technology.

You also should take into consideration other related to the product factors:

  • Quality of the product
  • Cost of fuel and power
  • Capital cost
  • Source of power
  • Instrument air
  • Cost of complementary equipment and installments
  • Licensing and permitting
  • Restrictions of air quality
  • Off-spec product cost
  • Losses of dust and yields

Strommashina is an expert supplier of rotary kilns and dryers. Our specialists are experienced professionals notorious for their expertise of various materials and industrial processes. We can help you in designing the best frameworks considering your processing goals and budget!