# Mini-Kilns for Charcoal-Making: An Eco-Friendly Solution for Small-Scale Production of Charcoal and Wood Vinegar

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

_{2}, H

_{2}, and CH

_{4}) [7,8,9]. In the day-to-day carbonization process carried out with low-tech kilns, charcoal is usually the only product of interest and the other products are released as smoke into the surrounding atmosphere, constituting, in most cases, a severe source of pollution. Still, the yield in pyrolysis coproducts depends on wood type and operational conditions such as heating rate, final temperature, the raw material’s moisture content, and labor skills [1]. Therefore, the control of the process is entirely subjective, relying on personal perceptions about the external temperature of the kiln and the smoke color, which changes as the carbonization goes on. The control of the process also depends on the type of kiln [10].

_{4}is eliminated, which is an advantage since this component strongly contributes to greenhouse gas emissions. Methane can cause 21 times more greenhouse effects than carbon dioxide [21]. This is essential to consider if the interest is to obtain carbon credits from the carbonization process.

## 2. Materials and Methods

#### 2.1. Project Premises

- ✓
- Firewood loading: 2 h;
- ✓
- Carbonization time: 72 h (3 days);
- ✓
- Charcoal cooling: 120 h (5 days);
- ✓
- Charcoal unloading: 2 h;
- ✓
- Total: 196 h.

#### 2.2. Carbonization Kilns

#### 2.3. Condensing System for WV Recovery

#### 2.3.1. Condensing System Design

_{man}), referring to the water column and the atmospheric pressure (P

_{atm}) [23]. According to Equation (1), we have:

^{−3}), g is the acceleration of gravity (m s

^{−2}), and h is the manometric height or head (m).

^{−3}was considered, and for the acceleration of gravity, 9.81 m s

^{−2}with a head of 0.8 m was considered. Substituting the values into Equation (2), we have:

_{atm}). The working pressure inside the tubes (P

_{op_int_tubes}) was also considered atmospheric pressure, since this system is connected directly to the burner, which is open to the atmosphere. Still, the design pressure for the hull (P

_{p_hull}) was defined as recommended by ASME [28], whereas, in the case of vessels designed for internal pressure, the design pressure (P

_{p_int}) was adopted as the highest of the two values according to the following conditions: maximum operating pressure, increased by 5% when the pressure relief device (safety valve) is operated by a pilot valve and increased by 10% in other cases; 147.10 kPa of head. The second condition was chosen in the analyzed situation because it is more critical and increases the safety factor. It was necessary to add the atmospheric press value (P

_{atm}), according to Equation (3):

_{p_hull}) was defined for the internal part, it was observed that this same pressure acts on the external part of the tube set itself. Furthermore, it was higher than the interior design pressure of the pipes (P

_{p_int_tubes}), which was 147.10 kPa (1.5 kgf cm

^{−2}), according to ASME [28], also defined as the design pressure for the tubes (P

_{p_tubes}). The operating temperature at the entrance to the hull (T

_{op_ent_hull}) was the inlet temperature of the water at ambient temperature (25 °C). In comparison, the operating temperature at the hull exit (T

_{op_exit_hull}) assumed the value of 45 °C, so the heat exchanger could operate with a slight temperature difference. We avoided designing a compensation system for expansion in the equipment, which would generate additional costs [30]. The values for the operating temperatures at the inlet and outlet of the tubes (T

_{op_ent_tubes}) and (T

_{op_out_tubes}) had equal values because the system is a condenser where heat is exchanged. Therefore, the condensation of pyrolysis liquids occurs at a constant temperature, considering the maximum value of 115 °C as the average temperature to recover the WV [35].

^{−1}(1.5 L s

^{−1}). The average specific water heat at 35 °C was 4.178 (kJ kg

^{−1}K

^{−1}), according to [31]. The mass flow rate of the gases was estimated at 0.0025 kg s

^{−1}. The specific heat of pyrolysis gases was not defined precisely since it comprises various chemical components [36]. According to Pires et al. [37], there may be more than 300 chemical components in the composition of carbonization gases. This diversity makes it challenging to define the characteristic patterns of these gases [36,38,39]. With all that said, the operating and design features of the condensing system are shown in Table 1.

#### 2.3.2. Thermal Design of the Condensing System

^{−2}K), A

_{s}is the heat transfer area (m

^{2}), and ${\u2206\mathrm{T}}_{\mathrm{M}\mathrm{L}}$ is the logarithmic temperature mean difference (K).

^{−1}), ${\mathrm{c}}_{\mathrm{p}\mathrm{f}}\text{}$is the specific heat of the cold fluid (kJ kg

^{−1}K

^{−1}), ${\mathrm{T}}_{\mathrm{f},\mathrm{s}\mathrm{a}\mathrm{i}}\text{}$is the cold fluid exit temperature (K), and ${\mathrm{T}}_{\mathrm{f},\mathrm{e}\mathrm{n}\mathrm{t}}\text{}$is the cold fluid inlet temperature (K).

^{−1}) and ${\mathrm{h}}_{\mathrm{f}\mathrm{g}}\text{}$is the enthalpy of vaporization (kJ kg

^{−1}).

^{−1}. Due to the composition of the carbonization gases, which makes it difficult to determine the enthalpy of vaporization at the specified temperature, Equation (5) was used to calculate the heat transfer rate. In Equation (5), a value of 4.178 kJ kg

^{−1}K

^{−1}was used, corresponding to the specific heat value for water at 35 °C (the mean temperature between 45 and 25 °C) [31]. The cold fluid’s mass flow rate was considered, corresponding to a commercial centrifugal pump with an average flow value of 1.5 L s

^{−1}with a power of 0.5 CV, a head of 3 m, suction, and a discharge diameter of 0.75 inches [40]. In addition, the values of 25 °C (298.15 K) and 45 °C (318.15 K) were also considered for the water inlet and outlet temperatures, respectively. These values are based on heat exchangers, like those cited by [31]. Substituting the values into Equation (5), we have:

^{−2}K

^{−1}, referring to the steam condenser, as it is close to the objective of the equipment, which is the condensation of wood vinegar. To calculate the logarithmic mean temperature difference, Equation (7) was used, considering a countercurrent flow:

_{int_hull}) was placed in the Pythagorean theorem to determine the distance (A), dividing the square formed into two right triangles to allow for the calculation of the longest edge (hypotenuse). Knowing that the spacing between tube centers is 38.1 mm, the values of edges B and C are equal to 4 × 38.1, namely 152.4 mm; therefore, substituting the values in the Pythagorean theorem equation, we have:

#### 2.3.3. Mechanical Design of the Condensing System

#### Hull Sizing

_{hull}is the minimum thickness needed to resist internal pressure (m), P is the design internal pressure, adding the effect of the head when necessary (Pa), R is the inner radius of the cylinder (m), S is the allowable stress of the material (Pa), E is the welding efficiency coefficient, C is the corrosion over-thickness (m), and D

_{i}is the inner diameter of the hull (mm).

#### Tube Set Sizing

_{0}) and the wall thickness (e) was calculated using Equation (14):

_{0}is 25.4 mm (according to Section 2.3.2) and arbitrating the value of 0.3 mm for wall thickness (e), the value of the relationship was 84.67 mm, a value that is compatible in the sizing stage for cylinders with a D

_{0}/e ratio ≥ 10. Next, the value of the ratio of the length of the tubular set (L) and the external diameter (D

_{0}) was determined using Equation (15):

#### Maximum Allowable Working Pressure (MAWP)

_{hull}), Equation (17) was used, according to ASME [28]:

_{hull1}and T

_{hull2}, and the values of T and S correspond to the design condition. Considering that the allowable stress of the hull material (S) at room temperature was equal to 80.6 MPa, the welding efficiency coefficient (E) was equal to 1, hull wall thickness (e) was equal to 0.00575 m (calculated), and the internal radius of the hull (R) was equal to 0.12725 m, when we substitute these values into Equation (17), we have:

#### Mirrors Sizing

_{m}) must be, at least, the highest of the two calculated values. In the heat exchanger under analysis, the design pressure is the same for both the shell and the tube bundle (248.42 × 103 Pa), as per Table 1, making it necessary to carry out the calculation to determine the mirror thickness only once. From Equations (18) and (19) below, the value of T is selected:

_{m}is the effective thickness calculated for the mirror (m), Cc is the corrosion over-thickness on the hull side (m), Ct is the corrosion over-thickness of the tubes (m), and Rc is the depth of the slot for fitting the baffles (m).

^{2}), F and G are numerical coefficients used for mirrors that have sealing joints on both sides: firstly, fixed and floating mirrors, for straight tubes, where F is one and G is the average diameter of the mirror sealing joint (m); and, secondly, fixed mirrors for U-shaped tubes, where F is 1.25 and G is the average diameter of the mirror sealing joint (m). Additionally, n is the values indicated, according to Equation (22) for square arrangements [27]:

_{0}is the external diameter of the tube set (m).

_{0}was 0.0254 m (as determined in Section 2.3.2), substituting these values into Equation (22) leads to:

^{2}. The value of C was 0.1524 × 4, namely 0.6096 m. Therefore, substituting the values into Equation (21), we have:

_{m}) was determined using Equations (17) and (18). For Equation (18), the value of C

_{c}was 3 mm and the value of C

_{t}was zero. Therefore, we have:

_{c}was zero, as baffles were not being foreseen. Substituting the values, we have:

^{−2}.

#### Caps Sizing

_{cap}) flanged and bolted was determined by Equation (23) [19,20]:

_{cap}= the allowable tension of the cap material (Pa). Considering that d totaled 0.2545 m, P totaled 248.42 kPa (Table 1), and S

_{cap}totaled 108 MPa, and substituting these values, we have:

#### Hydrostatic Pressure Test (PTH)

#### 2.4. Smoke Burner

#### Smoke Burner Sizing

#### Average Specific Mass of Smoke Gases

_{sgases}is the average specific mass of smoke gases (kg m

^{−3}), P

_{atm}is the atmospheric pressure (kN m

^{−2}), R

_{sgases}is the universal constant of smoke gases (kNm kg

^{−1}K

^{−1}), and T

_{sgases}is the= average temperature of the smoke (K).

_{2})—67.8%, oxygen (O

_{2})—15.69%, carbon dioxide (CO

_{2})—11.82%, and carbon monoxide (CO)—3.84%. For such a mixture, the values of the universal gas constant are close to that of air (0.287 kJ kg

^{−1}K

^{−1}), according to [31]. Therefore, the universal gas constant was used in the calculations, having air as a reference. For temperature, the gas entry temperature into the burner (T

_{ent}) was considered equal to 115 °C (see Table 1), and the gas exit temperature from the burner (T

_{exit}) was equal to 850 °C, according to Cardoso [49]. Furthermore, we calculated that P

_{atm}equaled 101.325 atm. Therefore, we have:

#### Mass Flow Rate of Smoke Gases

_{sgases}) was determined using Equation (25), according to Melo et al. [24]:

_{sgases}is the mass flow rate of smoke gases (kg h

^{−1}) and V

_{sgases}is the volumetric flow of smoke gases (m

^{3}h

^{−1}).

^{3}h

^{−1}, according to average data presented by Cardoso [49], we have:

#### Amount of Heat Required to Burn Smoke Gases

^{−1}), ${\mathrm{c}}_{\mathrm{p}}$ is the average specific heat of smoke gases (kJ kg

^{−1}K

^{−1}), and $\u2206\mathrm{T}$ is the temperature variation (K).

^{−1}K

^{−1}), according to Cengel [31]. Therefore, this value was used as a reference. After substituting it into Equation (26), the result is as follows:

#### Rate of Fuel to Be Consumed by the Smoke Burner

^{−1}), $\mathsf{\eta}$ is the thermal efficiency of the smoke burner, and $\mathrm{L}\mathrm{H}\mathrm{V}$ is the lower heating value of the fuel (kJ kg

^{−1}).

^{−1}), $\mathrm{H}\mathrm{H}\mathrm{V}$ is the higher heating value of the fuel (kJ/kg), and ${\mathrm{P}}_{\mathrm{H}2}$ is the fraction of hydrogen in the fuel (kg of hydrogen per kg of fuel), which is considered to have a value of 0.038, according to Silva et al. [48].

^{−1}higher than the calorific value of several solid fuels of vegetable origin, highlighting the possibility of its use as an energy product. Therefore, substituting the values into Equation (28), we have:

#### Combustion Chamber Volume

_{cc}), Equation (29) was employed, according to Melo et al. [24]:

_{cc}is the combustion chamber volume (m

^{3}), $\dot{{\mathrm{m}}_{\mathrm{c}}}$ is the fuel rate (kg h

^{−1}), and K is the energy release rate (kJ m

^{−1}h

^{−1}). The average value adopted [51] was 734,400 kJ m

^{−1}h

^{−1}. Substituting the values, we have:

#### Grate Surface Area

_{c}) for design purposes, which, in this case, gave the formula (Equation (30)):

_{g}is the grate surface area (m

^{2}) and N

_{c}is the combustion rate (kg m

^{−2}h

^{−1}), the value of which was set at 150 kg m

^{−2}h

^{−1}(based on Melo et al. [24]). Substituting the values, the result is:

#### Fans

_{2}and H

_{2}O) out of the smoke burner, optimizing the process.

## 3. Results

## 4. Discussion

^{2}; it was also calculated that the nominal diameter of the shell would be 10 in (from = 273 mm) with an internal diameter of 254.5 mm and a wall thickness of 9.27 mm, corresponding to “Series 40” or “Std”, and with an approximate weight of 60.23 kg/m.

## 5. Conclusions

## Author Contributions

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 3.**Views of the mini-kiln (dimensions in millimeters). (

**A**) Side; (

**B**) front; (

**C**) top; and (

**D**) rear view.

**Figure 5.**Arrangement of the set tubes of the condensing system (dimensions in mm) (

**A**). Representation of the right-angled triangle formed by dividing the square (

**B**).

**Figure 6.**Condensing system in isometric perspective and orthographic views of the tube set (dimensions in mm).

**Figure 10.**Front and rear views of the kilns, condensing device, and smoke burner from different views.

**Figure 11.**Three-dimensional views of the carbonization set (kilns, condensing device, and smoke burner) from different perspectives.

Fluid | Hull | Tubes |
---|---|---|

Water | Carbonization Gases | |

Internal operating pressure (${\mathrm{P}}_{\mathrm{o}\mathrm{p}\_\mathrm{i}\mathrm{n}\mathrm{t}}$) | 109.17 kPa | 101.32 kPa |

External operating pressure (${\mathrm{P}}_{\mathrm{o}\mathrm{p}\_\mathrm{e}\mathrm{x}\mathrm{t}}$) | 101.32 kPa | 109.11 kPa |

Design pressure (${\mathrm{P}}_{\mathrm{p}}$) | 248.42 kPa | 248.42 kPa |

Inlet operating temperature (${\mathrm{T}}_{\mathrm{o}\mathrm{p}\_\mathrm{i}\mathrm{n}\mathrm{l}\mathrm{e}\mathrm{t}}$) | 25 °C | 115 °C |

Outlet operating temperature (${\mathrm{T}}_{\mathrm{o}\mathrm{p}\_\mathrm{o}\mathrm{u}\mathrm{t}}$) | 45 °C | 115 °C |

Design temperature (${\mathrm{T}}_{\mathrm{p}}$) | 65 °C | 150 °C |

Mass flow ($\dot{\mathrm{m}}$) | 1.5 kg s^{−1} | 0.0025 kg s^{−1} |

Specific heat (${\mathrm{c}}_{\mathrm{p}}$) | 4.178 kJ kg^{−1} K^{−1} | - |

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## Share and Cite

**MDPI and ACS Style**

de Albuquerque, F.B.; de Melo, R.R.; Pimenta, A.S.; de Oliveira Paula, E.A.; Scatolino, M.V.; Rusch, F.
Mini-Kilns for Charcoal-Making: An Eco-Friendly Solution for Small-Scale Production of Charcoal and Wood Vinegar. *Inventions* **2023**, *8*, 146.
https://doi.org/10.3390/inventions8060146

**AMA Style**

de Albuquerque FB, de Melo RR, Pimenta AS, de Oliveira Paula EA, Scatolino MV, Rusch F.
Mini-Kilns for Charcoal-Making: An Eco-Friendly Solution for Small-Scale Production of Charcoal and Wood Vinegar. *Inventions*. 2023; 8(6):146.
https://doi.org/10.3390/inventions8060146

**Chicago/Turabian Style**

de Albuquerque, Felipe Bento, Rafael Rodolfo de Melo, Alexandre Santos Pimenta, Edgley Alves de Oliveira Paula, Mário Vanoli Scatolino, and Fernando Rusch.
2023. "Mini-Kilns for Charcoal-Making: An Eco-Friendly Solution for Small-Scale Production of Charcoal and Wood Vinegar" *Inventions* 8, no. 6: 146.
https://doi.org/10.3390/inventions8060146