In this study of the rotary kiln, a methodical approach was adopted in order to organize the entire project. It covers the various stages, from the sizing and design of the kiln to the interpretation of the results.

The precise description of the device, as well as the choice of materials and measuring instruments, enable a study of thermal and energy performance. These methodological foundations are essential for understanding the behavior of the kiln during tests with or without a storage system, paving the way for future improvements for cooking operations efficient and sustainable firing.

This approach is summarized and illustrated in Figure 1.

Figure 1. Method of the study.

The materials used depend on the parameters we want to measure. The parameters measured include:

Fuel mass before and after combustionExternal furnace temperaturesInternal furnace temperatures

A set of experimental equipment was used to carry out the experimental study:

An SBS-LW-10000A precision balance with a measurement uncertainty of ±0.1 g, which was used to weigh the final masses of charcoal and ash at the end of the experiments. Figure 2 shows the SBS-LW-10000A scale. A CAMRY scale with an accuracy of ±50 g, used to measure fuels before combustion. This scale is also used to measure the mass of sand in the storage system. It has a mass range of 1 kg to 150 kg (see Figure 3).A FLUKE Ti480 PRO thermal camera with an accuracy of ±1.5˚C is used to visualize the heat distribution on the outer walls of the furnace. It is shown in Figure 4.

Class 1 K-type thermocouples with a temperature range of −40˚C to 1200˚C and an accuracy of ±1.5˚C were placed on the various components of the rotary kiln. The thermocouples used were connected to a Keithley DAQ6510 data logger with 24 connection terminals and an accuracy of ±0.08˚C. The assembly is used to measure the internal and external temperatures of the rotary furnace. Figure 5 shows the data logger and thermocouples assembly.

Figure 2. Balance model SBS-LW-10000A.

Figure 3. Balance CAMERY.

Figure 4. Thermal camera FLUKE Ti480 PRO.

Figure 5. Data Logger Keithley DAQ6510.

InTable 1 below, we have summarized the technical characteristics of the devices used:

Table 1.Summary of the characteristics of the devices used during the experiment.

The protocol for this study aims to characterize the thermal and energy performance of the rotary kiln. To this end, we used two types of fuel: charcoal and organic charcoal available on the market. The tests will be carried without a thermal storage system, with combustion according to fuel type.

In this phase, we first undertook a number of steps before moving on to combustion. These actions included:

Installing K-type thermocouples in nine (09) different locations, including five (05) placed inside the kiln, notably at the ash box, the vacuum storage system, the fuel box, the rotation bar, and the grate; Four (04) other thermocouples were placed respectively below the furnace, on the insulated left side, on the uninsulated left side (since both sides and the rear have the same configurations), and finally at the chimney. These thermocouples were placed in these different locations to measure the different temperatures over time.Weigh the fuels (charcoal and organic charcoal) of different masses, namely 1 kg, 2 kg, and 3 kg, before proceeding to combustion for each weighed fuel mass.

We then moved on to the combustion tests. We used the same test procedure for both charcoal and organic charcoal.

During combustion, the thermocouples connected to the data logger measure the temperatures at the nine (09) locations at five (05) minute intervals. Infrared images of the furnace taken with a thermal camera also allowed us to monitor the temperature distribution on the outer walls of the furnaces. The experiment was conducted over a long period of eight hours, which included combustion and cooling times. At the end of each experiment, the fuel was weighed to determine the amount of fuel burned.

Combustion is carried out without interruption for each type of fuel and according to mass. The different oven temperatures are measured over a long period of at least eight (08) hours to observe the behavior of the storage system at five (05) minute intervals. Our experimental setup is illustrated in Figure 6.

Figure 6. Experimental setup diagram.

i)Characteristics of charcoal and organic charcoal

When evaluating the results of tests conducted using the furnace, it is important to first understand the properties of the fuels used. The characteristics of the fuels used are presented in Table 2.

Table 2. Characteristics of charcoal and organic charcoal.

ii)Detailed Characteristics of Traditional Charcoal

1)Origin and Manufacture:

Produced by pyrolysis (heating at high temperatures in the absence of oxygen) of solid wood (bark, trunks, branches).The type of wood (oak, beech, acacia, etc.) directly influences its quality and burning time.

2)Physical and Combustion Properties:

High Energy Density: It has a very high calorific value, which means it releases a lot of heat per kilogram.Long and Stable Combustion: Ideal for long cooking times (grilling, slow barbecues). It forms stubborn embers that last a long time.Low Ash Content: High-quality pure charcoal produces little residue.Ignition: May take longer to light but provides intense, consistent heat once lit.

3)Environmental Impact:

Negative: Its production is often associated with deforestation and forest degradation if the wood does not come from sustainably managed sources (e.g., FSC label).CO₂ emissions: It emits fossil carbon dioxide (from wood) which contributes to climate change.

iii)Detailed Characteristics of organic charcoal (made from stalks andbinders)

This product is often referred to as green charcoal, densified biomass, or charcoal briquettes.

a) Origin and Manufacture:

Raw Material: Uses agricultural waste: corn stalks, cotton stalks, rice husks, cashew or peanut shells, sawdust, etc.Process:

 Carbonization: The residues are first carbonized at low temperature.

 Grinding: The resulting charcoal is ground into a fine powder.

 Mixing: The charcoal powder is mixed with a natural binder (cassava starch, corn flour, clay, molasses) and sometimes water.

 Briquetting: The mixture is compressed under high pressure in a briquetting machine to give it its shape (usually balls or paving stones).

b)Physical and Combustion Properties:

Correct Calorific Value: Although generally lower than that of pure charcoal, it offers a very good alternative. Its calorific value depends on the raw material and the binder used.Faster Combustion: Often burns faster than traditional charcoal.Ignition: May require a more powerful firelighter, especially if the briquettes are very dense.Smoke and Odor: The smoke may be different and give off an odor related to the binder or residue used (e.g., a slight sweet smell with a starch binder). It is generally less harmful than charcoal, which may contain chemicals if the wood has been treated.Ash Content: Produces more ash than traditional charcoal because binders and agricultural residues contain non-combustible minerals.

c) Environmental and Socioeconomic Impact (Major Advantages):

Waste Recovery: An environmentally friendly solution for managing agricultural waste, which is often burned in the open air, causing pollution.Carbon Neutral: The CO₂ released during combustion was previously absorbed by plants during their growth. The carbon footprint is therefore much better.Reduced Deforestation: Reduces pressure on forests by offering an alternative to charcoal made from solid wood.Local Job Creation: The collection of residues and the production of green charcoal can create local economic circuits.

The choice between the two types of charcoal depends on the use and priorities:

For pure performance and long cooking times (e.g., restaurants, large barbecues), traditional high-quality charcoal remains unbeatable.For regular, more environmentally friendly, and economical use, organic charcoal is an excellent alternative. It transforms a problem (agricultural waste) into a useful energy resource, with a positive environmental and social impact. Its quality continues to improve with innovations in manufacturing processes.

It is crucial to note that the quality of “organic charcoal” can vary greatly depending on the raw materials and the rigor of the manufacturing process. It is advisable to purchase it from reputable producers.

3.1.1. Evolution of Temperatures in the Fuel Box

Figure 7shows the changes in experimental fuel box temperatures (TCComb) during combustion as a function of time, fuel type, and fuel mass.

(a) Charcoal

(b) organic charcoal

Figure 7. Temperature evolution in the fuel box as a function of the mass of fuel used. (a) Charcoal; (b) organic charcoal.

The results obtained indicate that the evolution of temperatures over time is broadly similar for both types of fuel. A rapid rise in temperature is observed from the start of combustion, demonstrating good calorific value and reactivity for both charcoal and organic charcoal. Furthermore, Figures 7(a)-(b) show that the maximum temperatures reached vary according to the mass of fuel used. A gradual increase in temperature is recorded with increasing masses: for 1 kg, 2 kg, and 3 kg of charcoal, the maximum temperatures reach 566˚C, 615˚C, and 649˚C, respectively; for organic charcoal, they are 370˚C, 381˚C, and 415˚C. These results confirm the direct effect of fuel mass on thermal intensity, in agreement with the work of Zhang et al. (2017) [10], who showed that the higher the fuel feed rate, the higher the temperature reached during combustion.

Figure 8 shows the thermal profiles obtained with 3 kg of each fuel, allowing for a clearer comparison of the thermal performance of charcoal and organic charcoal.

Figure 8. Temperature evolution in the fuel box for 3 kg of charcoal and organic charcoal used.

The results presented in Figure 8 show that, for a mass of 3 kg, charcoal reaches a maximum temperature of 649˚C, compared to 415˚C for organic charcoal, after approximately 40 minutes of combustion. This difference reflects the higher calorific value of charcoal, in line with data from Otieno et al. (2022) [11], who report a PCS of 25.7 MJ/kg for charcoal and 19.8 MJ/kg for biochar.

This difference in performance can be attributed to the composition of biochar, which is characterized by a higher content of volatile matter and ash, limiting the fraction that is actually combustible. Mierzwa-Hersztek et al. (2019) [12] demonstrated that net calorific value is strongly influenced by the chemical composition of fuels, particularly their ash content. Finally, unburned residues are observed at the end of the cycle for charcoal (Figures 9-11), with remaining masses of 179.9 g, 180.4 g, and 176.8 g for 1 kg, 2 kg, and 3 kg of initial fuel, respectively, while organic charcoal is completely consumed, as shown in Figure 12. This difference is illustrated in Figures 12-13, where the combustion of 3 kg of organic charcoal generates 1567.6 g of ash, compared to only 237.1 g for charcoal. In addition, the temperature curve for organic charcoal shows a slower decrease, reflecting a more gradual and evenly distributed combustion over time.

Figure 9. Remains of charcoal after combustion.

Figure 10. Remains of organic charcoal after combustion.

Figure 11. Remains of charcoal after combustion.

Figure 12. Quantity of ash after the consumption of 3 kg of charcoal.

Figure 13. Quantity of ash after the consumption of 3 kg of biochar.

3.1.2. Evolution of Temperatures in the Combustion Chamber

In the combustion chamber where the fuel box, the ash box and the storage system are housed, temperature differences are also observed with the same quantity of fuel. Figure 14 shows the evolution of the experimental temperatures in this chamber.

(a) Charcoal (b) organic charcoal

Figure 14. Evolution of combustion chamber temperature for 3 kg of fuel used; (a) Charcoal; (b) organic charcoal.

Figure 14highlights significant temperature differences between the fuel box and the lower compartments, particularly the empty thermal storage system (TSTV) and the ash box (TCCend). The temperatures recorded in these two areas show little variation and remain much lower than those measured in the fuel box. This difference is mainly explained by the thermal configuration of the device: located below the combustion zone, these compartments receive little heat, which naturally tends to rise by convection [8][13][14]. Furthermore, the presence of the ventilation system between the fuel box and the storage system reinforces this effect by facilitating the rise of hot air towards the upper part of the enclosure.

3.1.3. Temperature Evolution in the Cooking Chamber

In the cooking chamber, particularly at the rotating bar and the grate, temperature differences are observed despite the same amount of fuel used. Figure 15 illustrates the temperature evolution measured at these points.

(a) Charcoal

(b) organic charcoal

Figure 15. Evolution of the cooking chamber temperature for 3 kg of fuel used; (a) Charcoal; (b) organic charcoal.

Figure 15 shows the evolution of temperatures at the rotating bar (TBR) and the grate (TG) for the two fuels tested. With charcoal, the temperature increases rapidly during the first 40 minutes, reaching peaks of 381.06˚C (TBR) and 368.64˚C (TG). These valuescoincide with the maximum temperatures recorded in the fuel box, demonstrating efficient heat transmission in the cooking chamber, mainly ensured by radiation and convection [15]-[17]. Heat is therefore quickly available, but also decreases more markedly after the peak. In contrast, organic charcoal shows a slower and more gradual rise over nearly 100 minutes, with maximum temperatures of 291˚C (TBR) and 279.81˚C (TG). This gentler evolution reflects a less intense but more stable thermal propagation over time.

Thus, charcoal is more suitable for cooking requiring high initial heat, while organic charcoal is better suited to long and moderate cooking, offering better thermal stability. These trends corroborate the observations of Guo et al. (2020) [18], according to which organic charcoal briquettes, although less powerful at start-up, ensure prolonged and controlled heat emission, ideal for gentle cooking applications.

Figure 16 shows the experimental temperatures measured on the external parts of the solar rotary kiln.

(a) Charcoal (b) Organic charcoal

Figure 16. Temperature evolution of the external walls of the kiln for 3 kg of fuel used; (a) Charcoal; (b) organic charcoal.

Figure 16 illustrates a contrasting thermal distribution across the kiln zones: the temperature measured on the uninsulated left side face (TFLN) is significantly higher than those recorded on the insulated side face (TFLI) and below the kiln (TDF). This thermal distribution is observed regardless of the type of fuel used.

The relatively low (<50˚C) and almost constant temperatures observed in the insulated zones demonstrate the effectiveness of the Compressed Earth Bricks (CEB) integrated into the combustion chamber shell. This material limits heat losses, thus promoting a high temperature in the combustion zone. These results are consistent with those of Sawadogo G. (2020) [4], who showed that the use of CEB improves thermal efficiency by reducing energy losses.

On the other hand, although the uninsulated side reaches high temperatures (up to 111.34˚C with charcoal and 95.59˚C with organic charcoal), they remain lower than those measured in the cooking chamber (374.85˚C and 285.41˚C respectively). The wall, although uninsulated, includes a layer of air that slows down thermal conduction. These observations are consistent with those of Tarpilga et al. (2023) [19], who showed that air acts as an effective insulator, helping to limit heat losses between the combustion chamber (up to 600˚C) and the outer walls (100˚C to 120˚C).

Furthermore, these trends were observed by Abdoulaye Ouédraogo et al. in 2007 [14], who found temperatures below 250˚C for the oven walls.

Infrared images of the left side and front surfaces were used to monitor the temperature distribution on the furnace’s external walls. The following figures (Figures 17-21) show the infrared images at different times during furnace operation.

(a) Left side lateral face (b) Front face

Figure 17. Start of experiment.

(a) Lateral face, left side (b) Front face

Figure 18. 30 minutes after the start of the experiment.

(a) Left side lateral face (b) Front face

Figure 19. 60 minutes after the start of the experiment.

(a) Left side lateral face (b) Front face

Figure 20. 90 minutes after the start of the experiment.

(a) Left side wall (b) Front wall

Figure 21. 100 minutes after the start of the experiment.

The thermal images obtained by the infrared camera made it possible to visualize the spatial distribution of the temperature field on the external walls of the furnace, through variations in thermal coloration. In all of Figures 17-21, the colored areas on the left side wall systematically appear less intense than those on the front wall of the furnace, indicating lower temperatures. This thermal difference can be explained by the difference in wall thickness: the side frame has a thickness of 9 cm, significantly greater than that of the front wall, which is less than 3 cm [20]. This greater thickness constitutes a more effective thermal barrier, limiting heat loss.

Furthermore, in the thermograms of the left side wall, a darker coloration is observed in the lower part, corresponding to the combustion chamber area [20]. This phenomenon is particularly visible in Figure 20. This low apparent thermal emission indicates better heat retention, attributable to the presence of compressed earth bricks (CEB) used as an insulating material in the combustion chamber [16]. These observations are consistent with the results of Sawadogo G. (2020) [4], who demonstrated the effectiveness of insulation in reducing heat losses in this critical area.

Regarding the front face of the kiln, the figures reveal a bright coloration in the upper part, corresponding to the fuel box (main heat source) and the cooking chamber. On the other hand, the lower part, composed of the thermal storage system and the ash box, presents a less intense coloration, reflecting lower temperatures. This vertical thermal gradient, clearly illustrated in Figure 20and Figure 21, suggests a predominant rise of heat, with a more marked heat transfer upwards than towards the base of the kiln.