# Co-combustion of anthracite coal and biomass-derived biocarbons: reaction kinetics and combustion performance indicators

Bénit Bouesso, María González Martínez, Ange Nzihou

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Bénit Bouesso, María González Martínez, Ange Nzihou. Co-combustion of anthracite coal and biomass-derived biocarbons: reaction kinetics and combustion performance indicators. Biomass and Bioenergy, 2025, 201, pp.108117. 10.1016/j.biombioe.2025.108117. hal-05144677

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# Co-combustion of anthracite coal and biomass-derived biocarbons: reaction kinetics and combustion performance indicators

Bénit BOUESSO¹, María GONZÁLEZ MARTÍNEZ¹, Ange NZIHOU¹,²*

1. Université de Toulouse, IMT Mines Albi, RAPSODEE CNRS UMR 5302, Campus Jarlard, F.81013 Albi Cedex 09, France

2. Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey 08544, United States.

## Abstract

This work investigates the feasibility of using biocarbon as an alternative fuel to substitute fossil fuels in industrial processes. Woody biocarbons (WBC1, WBC2) were used for this purpose. Anthracite was selected as the reference fossil fuel to be replaced and biocoke was also considered for comparison. Samples were characterized in terms of chemical composition, and physical and thermal properties. Fuel thermal stability and combustion performance were evaluated using thermogravimetric analysis. Ignition kinetics were modeled using differential scanning calorimetric-based model. The results showed that biocarbon with a lower volatile matter content presents a higher thermal stability while its behavior is close to that of anthracite. Biocoke and biocarbons showed lower ignition temperatures but a higher burning index compared to anthracite, which was shown as the most stable fuel. Kinetic parameters, namely activation energy (Ea), frequency factor (A), and oxidation rate (k), were compared for the samples of study. While oxidation rates of biocoke and biocarbons were comparable, different values were obtained for WBC samples with different volatile matter content. Synergistic effects were observed when biocarbon was mixed with anthracite, mainly due to AAEM present in the samples. To mitigate biocarbon reactivity, a 50/50 biocarbon/anthracite co-combustion was proposed, offering the advantages of higher ignition temperature and low reactivity.

## Highlights

Volatile matter was shown to be proportional to reactivity and explosivity index.

Biofuels whose behavior is close to that of anthracite present low volatile matter content.

Lower volatile matter content in biocarbon is correlated to higher ignition temperatures.

Biocarbon exhibited a higher burning index than anthracite.

50/50 biocarbon/anthracite showed good performances as an alternative carbon-based fuel.

**Keywords:** biocarbon, anthracite, combustion, volatile matter content, performance indicators.

<footnote>* Corresponding author: A. Nzihou; ange.nzihou@mines-albi.fr, ange.nzihou@princeton.edu</footnote>
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# 1. Introduction

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Since decades, high-rank fossil fuels have been the main energy source in many industrial processes
They are associated to the high calorific performance which depends to their degree of maturity [1].

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Hard coal, namely anthracite, has been used for this purpose at industrial scale. About 75% of global CO₂ emissions (38.8Gt, in 2022) come from the combustion of fossil fuels [2]. In this context, there is an urgent need in the short term to replace hard coal with fuel derived from renewable resources (biofuel).

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Anthracite is the highest coal rank compared to lignite and bituminous coal. It was reported as less reactive than coke and bituminous coal [3]. The variation in its thermal properties is related to its origin and composition (mineral content, i.e., clay, kaolinite, and elemental composition) [4–6]. Regarding the price of hard coal (around 140 USD/ton for anthracite) and its inevitable depletion, a renewable resource such as biomass which is an eco-friendly and available alternative to fossil fuels at a reasonable price (around 30 USD/ton for round wood) [7,8] can play a significant role to lower the carbon footprint of an industrial sector. Biomass-based materials, i.e., woody biomass [9], forestry residues [10], agricultural residues [11] and energy crops [12] are well known for energy production. However, to consider biomass and biowaste as renewable resources, they should be limited to non-edible crops and their use should be managed in a sustainable way.

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Ignition corresponds to the temperature at which the fuel begins to burn. The characteristics of the ignition mechanism are divided into two types: homogeneous combustion, in which oxidation of volatiles precedes oxidation of fixed carbon, and heterogeneous combustion, which enhances the simultaneous combustion of volatiles and carbonaceous substrate [13,14]. To the best of our knowledge, the effect of volatile matter on the combustion performance of biocarbon has not been explored in the literature up-to-date.

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The combination of performance indicators, namely burning index, reactivity, and ignition index allow to describe fuel behavior not only in terms of reactivity, but also in terms of performances during thermal processes. Thermogravimetric analysis was used to evaluate the combustion performance of the fuels [15,16]. Anthracite showed a lower performance in terms of burning index as its ignition temperature could be 20 to 50% higher than that of biocarbon ranging from 300-400°C [17,18]. The burning index of anthracite may be between 2.10-52.0 wt.%/min°C³, which is a typical range for coals, while that of biocarbon may range from 1.90 to 191 wt.%/min°C³, considering the large range of biomass (39.8-462 wt.%/min°C³) as a function of the resource (biomass type) [19–21].

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Alternative biosourced fuels able to replace fossil fuels at industrial scale must have a low burning rate and a high ignition temperature to be applied in energy intensive processes. Recent studies on biomass/coal mixtures have shown that the ignition index decreases with increasing biocarbon/coal ratio [22–24]. The biocarbon/anthracite mixture may meet the energy demand of high-energy processes, filling a research gap in bioresource valorization.

In this context, the objective of this work is to characterize biocarbon fuels as a biosourced alternative to replace fossil fuels in industrial combustion processes. To this end, the selected fuels for this study, namely anthracite, biocoke, and biocarbons, will be first characterized in terms of chemical composition and thermal properties. Their performance in combustion and co-combustion with anthracite will be evaluated. A modeling approach will be proposed too. The results obtained will be compared with the literature.

## 2. Materials and methods

### 2.1. Selected materials

Woody biocarbons (WBC1, WBC2) were produced from woody biomass pyrolysis at 600°C, followed by a confidential devolatilization process leading to samples with different volatile matter content. Biocoke was produced from a coking process using coconut shell. Anthracite was also considered as reference fossil fuel for comparison. Samples were dried for 24h at 105°C in an oven, then ground and sieved to particle size less than 250µm for the characterization and experiments. In a second step, these fuels were mixed with anthracite, namely biocarbon/anthracite and biocoke/anthracite, at three mass ratios of 25/75, 50/50, and 75/25.

Elemental analysis of the samples was performed following the EN ISO 16498 standard (Table 1) [25]. Ash content determined at 550°C (A$_{550}$) for the raw fuels was carried out in an oven (EN 15403) [26] and those of mixtures were deducted by calculation. The carbon content of biocarbons ranged from 81.8 to 90.1wt.%, higher than that of biocoke (68.4wt.%) and anthracite (51.8wt.%) These values for biocarbon were slightly higher than those reported for woody biocarbons (51.2-75.6wt.%) because of the operating conditions (resource, carbonization temperature) used for the production of biocarbon [27]. As a result, the H/C ratio decreases for biocarbons with a lower VM content (i.e., 0.44 for WBC1 and 0.13 for WBC2). Ash content from woody biocarbon (2.00-2.70wt.%) was lower than that of biocoke (10.7wt.%), while that from anthracite (31.9wt.%) was nearly three times higher than that of biocoke. High ash content from anthracite has also been reported in [28], which may be related
to the origin and age of coal. According to these results (Table 1), increasing the biocarbon/anthracite ratio in fuel mixtures will favor a decrease in H/C.

<table><caption>Table 1. Elemental composition of raw fuels and fuel mixtures</caption><thead><tr><th rowspan="2">Sample</th><th>C</th><th>H</th><th>O*</th><th>N</th><th>A<sub>550</sub></th><th rowspan="2">H/C</th></tr><tr><th colspan="5">wt.%</th></tr></thead><tbody><tr><td>WBC1</td><td>81.8</td><td>3.02</td><td>12.5</td><td>&lt;0.01</td><td>2.70</td><td>0.44</td></tr><tr><td>WBC2</td><td>90.1</td><td>1.01</td><td>6.91</td><td>&lt;0.01</td><td>2.00</td><td>0.13</td></tr><tr><td>Biocoke</td><td>68.4</td><td>1.74</td><td>18.0</td><td>1.24</td><td>10.7</td><td>0.31</td></tr><tr><td>Anthracite</td><td>51.8</td><td>1.01</td><td>14.2</td><td>1.01</td><td>31.9</td><td>0.23</td></tr></tbody></table>

Raw fuels were characterized in terms of metal content in ash content using an Horiba Jobin-Yvon Ultimate 2 atomic emissions spectrometer (ICP-AES) following the EN ISO 16967 standard (Table 2) [30]. The sum of sodium (Na), potassium (K), magnesium (Mg), and calcium (Ca) refers to the alkali and alkaline earth metal (AAEM) content. AAEMs were known to enhance combustion and co-combustion processes by playing a catalytic role for oxidation reactions [31]. Anthracite may have a catalytic effect in co-combustion with biocarbon and biocoke due to its higher AAEM content of 3.29g/kg<sub>ash</sub> [32]. Whereas, the sum of silicon (Si) and aluminum (Al) is referred to the inhibitor metals [33]. The inhibition effect would be less with biocarbons and coke (0.02-6.10g/kg<sub>ash</sub>) in contrast to anthracite (28.8g/kg<sub>ash</sub>). Recombination of Si with O to silicate (SiO<sub>2</sub>) may decrease the reactivity of the mixtures in co-combustion [34]. AAEMs, in particular K, may also enhance the formation of silicon-aluminum (Al<sub>2</sub>SiO<sub>5</sub>) responsible for fusion and sintering in ash content [35].

<table><caption>Table 2. Metallic element composition of raw fuels</caption><thead><tr><th></th><th>WBC1</th><th>WBC2</th><th>Biocoke</th><th>Anthracite</th></tr></thead><tbody><tr><td>Catalytic metals (g/kg (ash), dry basis)</td><td></td><td></td><td></td><td></td></tr><tr><td>Na+K</td><td>0.06</td><td>0.03</td><td>0.46</td><td>0.80</td></tr><tr><td>Mg+Ca</td><td>0.15</td><td>0.09</td><td>0.44</td><td>2.49</td></tr><tr><td>Sum</td><td>0.21</td><td>0.12</td><td>0.90</td><td>3.29</td></tr><tr><td>Inhibitor metals (g/kg (ash), dry basis)</td><td></td><td></td><td></td><td></td></tr><tr><td>Si+Al</td><td>0.07</td><td>0.02</td><td>6.10</td><td>28.8</td></tr></tbody></table>

Proximate analysis (ash content at 550°C (A550), volatile matter, Table 3) was also performed, as well as the lower heating value (LHV) of each fuel (deducted from higher heating value) using a bomb calorimeter following ISO 18125 [36].

<footnote>*by difference, S<0.01wt.%, dry ash-free basis; wt.%: mass percentage</footnote>
## 2.2. Methodology

### 2.2.1. Thermal behavior description

The thermogravimetric analysis combined with differential scanning calorimetry (TGA-DSC) was used for thermal profile characterization. Pyrolysis tests were performed under nitrogen ($N_2$) to compare the thermal stability of the fuels. In a second step, combustion experiments were carried out in air (21vol.%$O_2$, 79vol.%$N_2$) to evaluate fuel flammability. About 5mg of the sample was placed in a platinum crucible inside the thermobalance (SDT Q600). The sample was heated from ambient temperature to 900°C and kept at this temperature for 15min. TG profiles were plotted on dry basis, then normalized at 105°C following the EN 15403-3 standard [37]. Different heating rates were used (5, 10, and 20°C/min) with a gas flow rate of 6l/h. DSC expressed the heat flow of the fuel as a function of temperature such as Q(T). The heat released was determined by integrating the total values of Q between the peak area and baseline (Eq.1).

$$ \Phi = \int_{T_1}^{T_2} Q(T)dT \qquad (1) $$

where $T_1$ is 105 (°C), $T_2$ is 900 (°C), and Q, the heat flow (W/g).

The fuel co-interaction was investigated by overlapping experimental and calculated heat flows of the mixtures. The theoretical curves were calculated from the Eq.2.

$$ TG_i = aTG_1 + bTG_2 \qquad (2) $$

where a and b are the mass ratios of each fuel mixed.

### 2.2.2. Combustion performance evaluation

The combustion performance of the raw fuels and the fuel mixtures was evaluated by determining performance and explosivity indicators.

#### Performance indicators-based tangent method

The tangent method allowed the graphical determination of three key temperatures, namely ignition temperature ($T_i$), maximum combustion temperature ($T_p$), and burnout temperature ($T_b$, Figure 1) [38]. The associated performance indicators were as follows:

- burning index (S, Eq.3): it corresponds to the capability of the fuel to burn.
$$142 \qquad S = \frac{\left(\frac{dm}{dt}\right)_{\text{max}} \left(\frac{dm}{dt}\right)_{\text{mean}}}{(T_i^2 \cdot T_b)} \qquad (3)$$

143 - reactivity (R, Eq.4): it represents the decomposition rate as carbon of the fuel reacts with O₂ [39].

$$144 \qquad R = \frac{1}{m_0} \left(\frac{dm}{dt}\right) \qquad (4)$$

145 - ignition index (D, Eq.5): it measures the ignition quality of the fuel. The ignition corresponds to the temperature at which the fuel begins to burn.

$$147 \qquad D = \frac{\left(\frac{dm}{dt}\right)_{\text{max}}}{(t_i \cdot t_p)} \qquad (5)$$

148 where (dm/dt)max and (dm/dt)mean are maximum and average values of the mass loss rate (wt.%/min), 149 m₀ is normalized sample mass at 105°C (mg), and the set of tₚ, tₐ and tₑ correspond to times of ignition, 150 maximum combustion and burnout, respectively. The ignition time (tₑ-tᵢ) was also considered in the 151 discussion of the results.

<figure><img src="image_2.png" /><figcaption>152
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Figure 1. Example of application of the tangent method to anthracite combustion profile obtained on TGA</figcaption></figure>
## Explosivity index

The explosivity index (EI) evaluates the safety of the fuel use. It was calculated by comparing the volatile matter content (VM) of a sample to the sum of this value and the fixed carbon content (FC) of the sample. The set threshold of dust explosion hazard was 12% [40].

$$ EI = \frac{VM}{VM+FC} \qquad (6) $$

### 2.2.3. Ignition kinetics-based model-free and thermodynamic approach

The following method aimed at evaluating the decomposition rate of each fuel in order to predict the kinetic behavior of the fuel mixtures. The time required for half-fuel conversion ($t_{50}$, min) was determined by plotting the conversion degree ($\alpha$, Eq. 7) versus time.

$$ \alpha = \frac{m_0 - m_t}{m_0 - m_{ash}} \qquad (7) $$

where $m_0$ is normalized sample mass at 105°C (mg), $m_t$ is mass at time t (mg), and $m_{ash}$ is ash content (mg).

DSC-based model-free was chosen to describe the ignition kinetics in terms of heat flow. A prior knowledge of the combustion reaction model was not needed. Arrhenius's law was considered the main characteristic equation of DSC kinetics. Three heating rates (5, 10, 20°C/min) were used for kinetics determination. For fuel comparison, the oxidation rate constant (k, 1/min) was determined as a function of temperature [41] (Eq. 8).

$$ k(T) = A \exp(-\frac{Ea}{RT}) = (\frac{A}{\beta}) \exp(-\frac{Ea}{RT}) f(\alpha) \qquad (8) $$

where $\beta$ is the heating rate (°C/min), $Ea$ is activation energy (kJ/mol), $A$ is frequency factor (1/min), $f(\alpha)$ is the reaction model, $T$ is the temperature (K) and $R$ stands for ideal gas constant (8.316J/molK). The unknown kinetic triplet ($Ea$, $A$, $f(\alpha)$) was determined by the next two steps.

## DSC modelling

It was assumed that the ignition mechanism of biocarbon can be attributed to the transition between a homogeneous mechanism (oxidation of VM precedes that of FC) and a heterogeneous mechanism (simultaneously oxidation of VM and FC) [42]. This allowed the same model to be applied, considering the differences in the carbon structure of anthracite and biocarbon. The mathematical function of
asymmetric 2-sigmoidal (Asym2) was in agreement with the DSC data. The expression of Asym2 was exhibited in Eq. 9 [43].

$$DCS_{\text{model}} = \frac{H_p}{1+\exp\left(-\frac{(T-T_p+Z_1/2)}{Z_2}\right)} \left[ 1 - \frac{1}{1+\exp\left(-\frac{(T-T_p+Z_1)}{Z_3}\right)} \right] \quad (9)$$

where DSC<sub>model</sub> is the modelled DSC curve (W/g), H<sub>p</sub> is the curve amplitude (°C), T is temperature (°C), T<sub>p</sub> is DSC peak temperature (°C), Z<sub>1</sub> represents the half-width curve (°C) and Z<sub>2</sub>, Z<sub>3</sub> are symmetric and asymmetric shape parameters (°C).

## Kinetic triplet determination

The calculated Ea represents the energy barrier for fuel ignition. The integral form of Eq. 8 was given by the Flynn-Wall-Ozawa (FWO) expression in Eq. 10 [44]. After plotting log(β) versus -1/T, Ea was derived from the slope and intercept. A second integral expression of Kissinger-Akahira-Sunose (KAS) was used for Ea validation. KAS plot referred to ln(β/T<sup>2</sup>) versus 1/T [41].

$$\log(\beta) = \log\left(\frac{AEa}{Rg(\alpha)}\right) - 2.35 - 0.46\left(\frac{Ea}{RT}\right) \quad (10)$$

$$\ln\left(\frac{\beta}{T^2}\right) = \ln\left(\frac{AR}{Eag(\alpha)}\right) - \left(\frac{Ea}{RT}\right) \quad (11)$$

where g(α) results from integrating f(α).

## Thermodynamic analysis

Thermodynamics was used to illustrate the change of energy during fuel conversion through three typical parameters such as Gibbs free energy (ΔG, kJ/mol), reaction enthalpy (ΔH, kJ/mol), and entropy (ΔS, kJ/molK). ΔG measured the spontaneity of combustion based on its correlation with ΔH and ΔS in Eq. 13. This related to the sign of ΔG value, as ΔG<0 evolves ΔS>0 so far spontaneous combustion. ΔH and ΔS were determined using Eq. 14 and Eq. 15, respectively [41]. The energy barrier (ψ, kJ/mol) corresponding to the difference between the activation energy (Ea) and the enthalpy of the reaction (ΔH) was calculated (Eq. 16) to evaluate if complete combustion is achieved.

$$\Delta G = E_a + RT_m \ln\left(\frac{k_B T_m}{hA}\right) = \Delta H - T_m \Delta S \quad (12)$$

$$\Delta H = E_a - RT \quad (13)$$
$$205 \qquad \Delta S = \frac{\Delta H - \Delta G}{T_m} \qquad (14)$$

$$206 \qquad \psi = E_a - \Delta H \qquad (16)$$

where T_m is maximum peak temperature, k_B refers to Boltzmann constant (1.38 10⁻²³ J/K) and h, Plank constant (6.63 10⁻³⁴ J/s).

## 3. Results and discussion

### 3.1. Thermal behavior

Thermal behavior of raw fuels was assessed in pyrolysis and combustion using TGA, so as to link it with the resource properties.

#### 3.1.1. TG pyrolysis profile

The thermal stability of woody biocarbons and biocoke (Figure 2) was compared to anthracite. Lower mass loss was observed for biocarbon with a low volatile matter content (WBC2) compared to that with a higher volatile matter content (WBC1). Anthracite was in the middle of the scale and biocoke behavior was close to that of low-volatile content biocarbon. Thus, the VM content would be directly related to the fuel thermal stability. Indeed, WBC1 has the highest volatile matter content (18.5wt.%, daf), thus the highest mass loss of 18wt.%. Biocarbon with low volatile matter content (<19-4wt.%, daf) exhibited higher thermal stability than anthracite. Anthracite thermal stability was slightly higher than that previously reported (mass loss, Δm: 10wt.%) [45], which may be mainly due to differences in physicochemical properties and inorganic content (Table 2).
<figure><img src="image_3.png" /><figcaption>Figure 2. TGA mass loss as a function of temperature for woody biocarbons and biocoke, and comparison with anthracite in pyrolysis</figcaption></figure>

### 3.1.2. TG combustion profile

The combustion profile of woody biocarbons and biocoke (Figure 3) was investigated and compared to that of anthracite. Anthracite was shown as the most stable fuel in combustion compared to biocarbons and biocoke. This may be explained by its lower volatile content (3.85wt.%, daf) as anthracite has ignited at higher temperatures because of its high-grade rank and inhibition effect enhanced by Si [15]. Indeed, the inhibitor metal (Si+Al) content of anthracite was twice higher than that of other fuels (<10g/kg<sub>ash</sub>, Table 2). Lower ash content was found for biocarbons (≤1wt.%) and biocoke (7wt.%) compared to anthracite (18wt.%). This is in agreement with the fact that low ash content is typically expected for woody biomass-derived biofuels, i.e., biocarbons and biocoke [46].

The temperature gap (ΔT) between biocarbon and anthracite thermal profiles was calculated at 50wt.% of the total mass loss on the TG curve (Figure 3). The gap was approximately of 170°C between anthracite and biocarbon with a high volatile content (WBC1). Biocarbon with lower volatile matter content (WBC2) and biocoke present a reduced ΔT to 115-130°C, which was consistent with the increase in thermal stability of biocarbon with a low volatile content. The maximum decomposition rate of WBC2 was two times higher than that of anthracite (10wt.%/min). These results provide insights into ignition behavior, indicating that biocarbon with a low volatile content burns much faster.
<figure><img src="image_4.png" /><figcaption>Figure 3. TGA mass loss as a function of temperature for woody biocarbons and biocoke, and comparison with anthracite in combustion</figcaption></figure>

## 3.2. Ignition characteristics

A thorough investigation of the combustion profiles was achieved by Differential Scanning Calorimetry (DSC) with the aim of describing the ignition mechanisms. The tangent method was applied to the heat flow signal to determine characteristic temperatures i.e. Tᵢ, Tₚ, T₉, and thus calculate combustion performance indicators.

### 3.2.1. Ignition mechanism

The ignition behavior was described by an accurate analysis of the heat flow signal of the raw fuels (Figure 4). Anthracite was characterized by a symmetrical peak, which is typically an accurate pattern of heterogeneous ignition [13]. Biocoke and biocarbons presented asymmetric peaks. Similar curve shapes were found for woody biocarbons. Their peaks were straight and narrow. This particular shape could be associated with fuel reactivity [47]. Whereas, woody biocarbons showed heat flows two to three times higher than other fuels. In fuel combustion, the volatile matter combustion occurs first, so a fuel with low to medium volatile matter content (<30wt.%) can behave to a transition mechanism towards heterogeneous ignition, so-called homo-heterogeneous mechanism [14]. The heat flow curve of biocarbon with a lower VM content shifts towards higher temperatures (>450°C). Finally, biocoke has the broadest peak due to the resource type and cooking conditions. The integrated area peaks (W/g, Figure 4) confirm the high reactivity and calorific energy of woody biomass derived-biocarbons.
<figure><img src="image_5.png" /><figcaption>Figure 4. Heat flow as a function of temperature for woody biocarbons and biocoke, and comparison with anthracite in combustion</figcaption></figure>

### 3.2.2. Ignition temperature analysis

Ignition temperature ($T_i$) corresponds to the temperature at which the fuel begins to burn. It was calculated from DSC heat flow curves and was compared to fuel properties (Table 3). Woody biocarbons showed $T_i$ between 327 and 371°C, corresponding to the typical range of biocarbons produced from biomass [48]. Moreover, biocarbons were compared with those produced by carbonization at 500 and 600°C in [48], [49] and [21]. No significant effect of biomass carbonization on $T_i$ was found (327°C for WBC1 and 330°C for stone pine biocarbon [48]). In contrast, biocarbon volatile content showed an effect on $T_i$. The higher the volatile matter content, the lower the $T_i$. For instance, WB2 showed an increase in $T_i$ of 12%, presenting the highest $T_i$ (371°C), while biocoke exhibited the lowest $T_i$ (276°C).

$T_i$ can be correlated with physicochemical properties as well as LHV of the fuels (biocarbon, coals, Table 3). $T_i$ typically increases with FC, as observed in biocarbon thermal stability (Figure 2) and previously reported [48]. $T_i$ increases when VM decreases as this trend may be independent from the biomass type [50]. Hence, WBC2 was the most stable biocarbon with the lowest VM (4.70wt.%, daf) and highest FC (93.3wt.%, daf). The same trend was reported for the coal family (e.g. $T_i$ for anthracite 481°C vs. 700°C for semi-anthracite [51]), while $T_i$ of anthracites was in the range of 415 to 700°C. As opposed to the above $T_i$, $T_i$ of biocoke was 276°C, two times lower than the value reported for pet coke (580°C) [52]. These coke samples were remarkably different in terms of VM and FC (i.e., 0.60
and 98.6 for pet coke vs. 12.6 and 76.7 for biocoke). Biocoke seems to be closer to sub-bituminous coal (with Ti of 284°C) [42], which is considered a reference point as coke is mainly derived from it. Some authors reported up to 600°C the Tᵢ of a coal cooked at 1000°C [53].

The VM/FC ratio was calculated to predict fuel stability (Table 3). The value of VM/FC of anthracite (0.06) were lower than those reported in a previous study (0.11) [51]. By comparison to other coals, VM/FC decreases with the coal rank [54], so VM/FC decreases when Tᵢ increases [55].

According to the results, the LHV of the fuel typically increases when ash content decreases (Table 3). This corresponds to the coal cooking resulting in a decrease of ash content in coal by heating it at higher temperatures to reduce its volatile matter content [56]. This may imply an increase of Tᵢ with LHV, as Tᵢ increases when ash content decreases.

Finally, the explosivity index (EI) was calculated to evaluate the evaluation to explosion risks (Table 3). EI of WBC2, as well as anthracite, was less than the ceiling value (12%) in contrast to WBC1 and biocoke. This suggested that precautions must be taken for monitoring the combustion of these fuels at a higher scale.

<table><caption>Table 3. Ignition temperatures and physicochemical properties of several fuels</caption><thead><tr><th rowspan="2">Sample</th><th>T<sub>i</sub></th><th>Ash</th><th>VM</th><th>FC</th><th>VM/FC</th><th>LHV</th><th>EI</th><th rowspan="2">Ref</th></tr><tr><th>°C</th><th colspan="3">wt.%</th><th>ratio</th><th>MJ/kg</th><th>%</th></tr></thead><tbody><tr><td>WBC1</td><td>327</td><td>2.70</td><td>18.5</td><td>78.8</td><td>0.23</td><td>32.7</td><td>19.0</td><td rowspan="6">This work</td></tr><tr><td>WBC2</td><td>371</td><td>2.00</td><td>4.70</td><td>93.3</td><td>0.05</td><td>33.4</td><td>4.80</td></tr><tr><td>Biocoke</td><td>276</td><td>10.7</td><td>12.6</td><td>76.7</td><td>0.16</td><td>26.1</td><td>14.1</td></tr><tr><td>Anthracite</td><td>481</td><td>31.9</td><td>3.85</td><td>64.3</td><td>0.06</td><td>30.8</td><td>5.65</td></tr><tr><td>Sub-bituminous coal</td><td>284</td><td>20.7</td><td>50.2</td><td>29.1</td><td>1.73</td><td>21.6</td><td>63.3</td><td>[42]</td></tr><tr><td>Oak biocarbon</td><td>325</td><td>8.50</td><td>10.2</td><td>81.3</td><td>0.13</td><td>29.9</td><td>11.2</td><td>[48]</td></tr><tr><td>Stone pine biocarbon</td><td>330</td><td>2.90</td><td>9.10</td><td>88.0</td><td>0.10</td><td>33.6</td><td>9.37</td><td>[48]</td></tr><tr><td>Silver fir biocarbon</td><td>348</td><td>1.90</td><td>7.90</td><td>90.2</td><td>0.09</td><td>34.4</td><td>8.05</td><td>[48]</td></tr><tr><td>Poplar wood biocarbon</td><td>359</td><td>2.44</td><td>23.6</td><td>71.6</td><td>0.32</td><td>29.3</td><td>24.2</td><td>[49]</td></tr><tr><td>Straw biocarbon</td><td>366</td><td>52.4</td><td>25.4</td><td>22.2</td><td>1.14</td><td>12.5</td><td>53.4</td><td>[21]</td></tr><tr><td>Bituminous coal</td><td>404</td><td>33.4</td><td>33.7</td><td>32.9</td><td>1.02</td><td>19.2</td><td>50.6</td><td>[21]</td></tr><tr><td>South Africa anthracite</td><td>415</td><td>20.8</td><td>5.80</td><td>73.4</td><td>0.08</td><td>22.9</td><td>7.32</td><td>[57]</td></tr><tr><td>Australia coal</td><td>446</td><td>18.4</td><td>29.6</td><td>51.9</td><td>0.57</td><td>24.8</td><td>36.3</td><td>[58]</td></tr><tr><td>Qiyi coal</td><td>662</td><td>16.8</td><td>8.90</td><td>74.3</td><td>0.12</td><td>29.3</td><td>10.7</td><td>[59]</td></tr><tr><td>Spain semi anthracite</td><td>700</td><td>10.7</td><td>9.20</td><td>80.1</td><td>0.11</td><td>31.1</td><td>10.3</td><td>[51]</td></tr><tr><td>Pet coke</td><td>580</td><td>0.10</td><td>0.90</td><td>99.0</td><td>0.01</td><td>32.5</td><td>0.90</td><td>[52]</td></tr></tbody></table>

dry ash-free basis
### 3.2.3. Ignition and burnout correlation

Ignition and burnout temperatures were compared for all the samples considered for this study (Figure 5). The burnout temperature ($T_b$) was associated with the temperature at the end of combustion. The results showed that $T_i$ increases along with $T_b$, as expected. $T_b$ for biocarbon ranged from 530 to 560°C while anthracite had the highest $T_b$ (693°C). This difference could be explained by the fuel properties, as a lower VM/FC implies a higher $T_b$ [55]. Indeed, high VM may enhance the oxidation rate of the fuel carbon matrix, resulting in a fast burnout (WBC1) [60]. Biocoke had an exceptional trend with high $T_b$ (593°C) and low $T_i$ (276°C).

The maximum heat temperature ($T_p$) released was also investigated (Figure 5). Hence, $\Delta T_1$ ($T_p$-$T_i$) was calculated to measure ignition quality, expecting a high-temperature range for high ignition quality. Biocarbon with a lower VM content presented reduced $\Delta T_1$, bringing $T_p$ closer to $T_b$. A lower ignition quality of the fuel would predict a high reactivity. Biocoke with the highest $\Delta T_1$ might burn more slowly than biocarbon, making its ignition quality satisfactory and slightly higher than anthracite. In addition, $\Delta T_2$ ($T_b$-$T_p$) measured the burnout quality. No significant change was observed, which implied equivalent satisfactory burnout quality for the samples considered. Nevertheless, $\Delta T_2$ may be reduced in minor cases for low VM biocarbon (WBC2), leading to faster burnout.

<figure><img src="image_6.png" /><figcaption>Figure 5. Temperatures of ignition, maximum release heat and burnout of raw fuels</figcaption></figure>
### 3.3. Combustion performance

#### 3.3.1. Performance indicator assessment

The combination of S, R, and D indicators defined in section 2.2.2 (Eq.3-5) traduces fuels performance (Table 4). An overall comparison was performed on the selected operating conditions (5mg, 3l/h air, 10°C/min, 105-800°C).

$$S = \frac{\left(\frac{dm}{dt}\right)_{\text{max}} \left(\frac{dm}{dt}\right)_{\text{mean}}}{(T_i^2 \cdot T_b)} \quad (3)$$

$$R = \frac{1}{m_0} \cdot \left(\frac{dm}{dt}\right) \quad (4)$$

$$D = \frac{\left(\frac{dm}{dt}\right)_{\text{max}}}{(t_i \cdot t_p)} \quad (5)$$

The S index of WBC1 was higher than that of Biocoke and anthracite, while low VM biocarbon presented a lower value of the S index (40.1 $10^{-8}$wt.%/min°C³). Consequently, the low $T_i$ values common for biocarbons (327-371°C) and biocoke (276°C) result in a D index three to eight times higher than that of anthracite. Similar trends were found for the D and S indexes, due to the burning rate and the nature of the ignition mechanism, finally related to VM content. The lower the VM/FC ratio, the higher the probability of heterogeneous ignition and the higher the reactivity. It can be highlighted that $R_{max}$ of anthracite and Biocoke are roughly 0.1/min. In contrast, WBC samples had the highest reactivity, which is in agreement with biomass-derived biocarbons[21].

According to the literature, coal usually has a lower S index ($6.42-8.55 \ 10^{-8}$ wt.%/min°C³)[21]. A recent study reported that pyrolysis of lignite increased $T_i$ by 29% and decreased the S index by 54% [61]. Therefore, S may be related to R because more flammable fuels may present an enhanced reactivity. Anthracite was less flammable than other fuels due to its high $T_i$ (481°C).

The high reactivity of fuels (e.g. WBC1) could be a challenge in controlling the burning at industrial scale. As a result, the ideal fuel should present low S, low D, low R, and high $T_i$ values. This compromise cannot be found with all the selected materials, especially if they present a higher VM content. A trade off was considered between $T_i$, reactivity, and S index.
### 3.3.2. Heating rate influence

The influence of the heating rate (5, 10, 20 °C/min) on the three indicators was assessed (Table 4). The heating rate may affect the combustion rate without changing the fuel thermal profile due to a temperature gradient in the bed sample [62]. 10°C/min increased by 5 to 20% the $T_i$ obtained as compared to 5°C/min. $T_i$ was stabilized between 10 and 20°C/min. As the heating rate increased, the mass loss remained constant and the D index also increased. In parallel, the S index increased with the heating rate, i.e. from 2.04wt.%/min°C³ at 5°C/min to 23.2wt.%/min°C³ at 20°C/min for anthracite. From the results, R did not show a linear increase with heating rate from 5 to 20°C/min.

<table><caption>Table 4. Effect of heating rate on fuel performance</caption><thead><tr><th>Sample</th><th>S 10<sup>-8</sup><br>wt.%/min°C<sup>3</sup></th><th>R<sub>max</sub> 10<sup>-2</sup><br>min<sup>-1</sup></th><th>D 10<sup>-3</sup><br>wt.%/min<sup>3</sup></th><th>Δm<br>wt.%</th><th>T<sub>i</sub><br>°C</th><th>Ref</th></tr></thead><tbody><tr><td colspan="7"><strong>Heating rate 5°C/min</strong></td></tr><tr><td>Anthracite</td><td>2.04</td><td>5.62</td><td>0.84</td><td>79.0</td><td>458</td><td rowspan="4">This work</td></tr><tr><td>Biocoke</td><td>6.49</td><td>4.01</td><td>1.77</td><td>93.1</td><td>254</td></tr><tr><td>WBC1</td><td>17.2</td><td>12.0</td><td>4.43</td><td>99.3</td><td>293</td></tr><tr><td>WBC2</td><td>9.34</td><td>10.5</td><td>2.66</td><td>99.4</td><td>369</td></tr><tr><td>Saray Thrace coal</td><td>6.42</td><td>2.37</td><td>0.65</td><td>31.0</td><td>305</td><td>[19]</td></tr><tr><td colspan="7"><strong>Heating rate 10°C/min</strong></td></tr><tr><td>Anthracite</td><td>6.95</td><td>10.6</td><td>5.53</td><td>81.6</td><td>481</td><td rowspan="6">This work</td></tr><tr><td>Biocoke</td><td>20.3</td><td>7.68</td><td>11.1</td><td>93.2</td><td>277</td></tr><tr><td>WBC1</td><td>80.0</td><td>34.4</td><td>45.9</td><td>98.1</td><td>327</td></tr><tr><td>WBC2</td><td>40.1</td><td>24.2</td><td>24.5</td><td>98.9</td><td>371</td></tr><tr><td>Saray Thrace coal</td><td>8.55</td><td>3.31</td><td>3.29</td><td>25.8</td><td>325</td></tr><tr><td>Straw biochar (500°C*)</td><td>1.90</td><td>5.32</td><td>-</td><td>51.5</td><td>-</td></tr><tr><td colspan="7"><strong>Heating rate 20°C/min</strong></td></tr><tr><td>Anthracite</td><td>23.2</td><td>17.8</td><td>8.01</td><td>82.0</td><td>481</td><td rowspan="6">This work</td></tr><tr><td>Biocoke</td><td>68.2</td><td>13.2</td><td>15.4</td><td>92.1</td><td>277</td></tr><tr><td>WBC1</td><td>131</td><td>29.7</td><td>137</td><td>100</td><td>345</td></tr><tr><td>WBC2</td><td>76.1</td><td>23.8</td><td>19.6</td><td>96.6</td><td>371</td></tr><tr><td>Lignite char</td><td>60.0</td><td>21.1</td><td>-</td><td>82.0</td><td>505</td></tr><tr><td>Lignite</td><td>130</td><td>24.2</td><td>-</td><td>91.9</td><td>357</td></tr><tr><td>Eucalyptus bark</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Biocarbon (450°C*)</td><td>20</td><td>30.0</td><td>-</td><td>-</td><td>300</td><td>[64]</td></tr></tbody></table>

<footnote>*Pyrolysis temperature, Δm: mass loss</footnote>
352

## 3.4. Kinetic study

353

### 3.4.1. Fuel conversion analysis

354

Conversion of biocarbons occurred at lower temperatures than that of anthracite (Figure 6). In fact, the t<sub>50</sub> was lower for biocarbons (between 34min for WBC1 and 39min for WBC2) compared to anthracite (50min). Anthracite conversion in combustion mainly occurred between 600 and 800°C, while this temperature interval was lower for biocarbons, and biocoke (from 300 to 660°C).

355

356

357

358

The total conversion (100%) was achieved at 62, 49, and 42min for anthracite, biocoke, and biocarbons except for WBC1 (36min). This could be probably due to its reactivity, higher than WBC2. This was related to its degradation mass rate (9.80wt.%/min). Biocoke was closer to anthracite due to its structure.

359

360

361

<figure><img src="image_7.png" /><figcaption>Figure 6. Comparison of fuel conversion at 10°C/min</figcaption></figure>

362

363

364

### 3.4.2. Triplet parameter examination

365

The activation energy and oxidation rate constant of the fuels were determined at 900°C (Table 5).

366

The activation energy for fuel combustion was determined by FWO and KAS methods (Table 5). The results of Ea from FWO ranged from 101 to 119kJ/mol for biocarbons while those of anthracite and coke were 121 and 109kJ/mol, respectively. Equivalent results were obtained from KAS ranged from 94.3 to 112kJ/mol for biocarbons, 112kJ/mol for anthracite, and 103kJ/mol for biocoke. This

367

368

369
demonstrated the equivalency of both methods [65], while the slight difference observed with the data from Eq. 8.

As WBC2 exhibited a $T_i$ slightly higher (Table 4), its Ea was 12% lower than that of WBC1 confirming its higher reactivity and faster conversion. Ea range for biocarbons produced by carbonization from 400 to 800°C was reported to be in the range of 100-138kJ/mol [29,63,64,66]. Ea increases along with the decrease in VM content of biocarbons, as more energy is required for their oxidation (i.e., 119kJ/mol for WBC2). The lower Ea value of biocoke (109kJ/mol) was proportional to its $T_i$ (276°C). Its Ea was 16% lower than related values of common coal-derived cokes 130-153kJ/mol [67]. The operating conditions of biocoke may be the main reason for this difference. The Ea value of anthracite (121kJ/mol) was consistent with previous studies (116-208kJ/mol) [17]. Furthermore, no significant variations in Ea were observed along all conversion ($\alpha$=10-90%). The graphs of Ea as a function of the conversion degree ($\alpha$) can be found on supplementary material (Figure S1).

<table><caption>Table 5. Activation energies by FWO and KAS and oxidation rate constant of the fuels in combustion</caption><thead><tr><th rowspan="2">Sample</th><th>Ea FWO</th><th>Ea KAS</th><th>k<sub>900°C</sub></th></tr><tr><th colspan="2">kJ/mol</th><th>min<sup>-1</sup></th></tr></thead><tbody><tr><td>Anthracite</td><td>121</td><td>112</td><td>0.31</td></tr><tr><td>Biocoke</td><td>109</td><td>103</td><td>1.99</td></tr><tr><td>WBC1</td><td>101</td><td>94.3</td><td>2.39</td></tr><tr><td>WBC2</td><td>119</td><td>112</td><td>5.27</td></tr></tbody></table>

The oxidation constant is strictly dependent on the temperature (Figure 7). The oxidation rate constants of WBC1 and biocoke were in the same range (1.99-2.39/min) except for WBC2 (5.27/min, Figure 7). This could be probably due to its conversion ($\Delta m$ = 98.9wt.%, Table 5) which was higher than that of biocoke (93.2wt.%). Anthracite had the lowest oxidation rate (0.31/min), which was slightly higher than previously reported (0.06/min) [68].
<figure><img src="image_8.png" /><figcaption>Figure 7. Oxidation rate during conversion at 10°C/min</figcaption></figure>

### 3.4.3. Thermodynamics characteristics

The main thermodynamic parameters, namely ΔG, ΔH, and ΔS were calculated at 900°C (Table 6). The positive values of the free Gibbs energy (ΔG, between 187.7 and 231.9 kJ/mol) indicate a non-spontaneous ignition. ΔG of anthracite was 11% to 19% higher than that of biocoke and biocarbons, which may be related to its higher ignition temperature (481°C) and lower reactivity (0.11/min). Also, anthracite produced slightly more combustion energy (113 kJ/mol) compared to other fuels (95.2-113kJ/mol). The difference between the activation energy (Ea) and the enthalpy of the oxidation reaction (ΔH) can be considered as the energy barrier (Ψ, Eq.15) to be overcome for the reaction to happen. A small energy barrier was found in the operating conditions used (5 mg, 6 l/h, air, 105-900 °C), which means that few additional energy is required to achieve combustion [69]. The Ψ values of biocarbon were positive for all samples considered. Anthracite, biocoke and WBC2 showed the highest Ψ value (6.20-7.50kJ/mol), so this may suggest that WBC1 is the promising alternative fuel for a future test at industrial scale (Ψ: 5.82kJ/mol). Its Tᵢ and Tₛ were 327 and 560°C, respectively.

$$ \Psi = E_a - \Delta H \qquad (15) $$

The negative values of ΔS exhibit a lower disorder occurring in combustion, which suggests with the formation of gaseous products as a result of this conversion. These results revealed the feasibility of using biocarbons to replace anthracite at equivalent Ea, with comparable trends between the reaction enthalpy (ΔH) and the oxidation rate constant (k).
410

<table><caption>Table 6. Thermodynamic characteristics at 900°C and 10°C/min</caption><thead><tr><th rowspan="2">Sample</th><th>T<sub>i</sub></th><th>T<sub>b</sub></th><th>Ψ</th><th>ΔG</th><th>ΔH</th><th>ΔS</th></tr><tr><th colspan="2">°C</th><th colspan="3">kJ/mol</th><th>kJ/molK</th></tr></thead><tbody><tr><td>Anthracite</td><td>481</td><td>693</td><td>7.50</td><td>231.9</td><td>113.5</td><td>-0.134</td></tr><tr><td>Biocoke</td><td>277</td><td>592</td><td>6.20</td><td>203.1</td><td>102.8</td><td>-0.127</td></tr><tr><td>WBC1</td><td>327</td><td>560</td><td>5.82</td><td>187.7</td><td>95.18</td><td>-0.126</td></tr><tr><td>WBC2</td><td>371</td><td>530</td><td>6.50</td><td>205.7</td><td>112.5</td><td>-0.122</td></tr></tbody></table>

411

412

## 3.5. Co-combustion tests

413

### 3.5.1. Fuel mixture ignition

414

#### Thermal profile analysis

415

416

417

418

Combustion tests were conducted in TGA at 10°C/min on biocarbon/anthracite and biocoke/anthracite mixtures at 25/75, 50/50, and 75/25 (wt.%) mixing ratios (Figure 8). Raw materials were added to the graphs for comparison (full lines), as well as the behavior of each mixture. It was assumed represent the additive behavior of its components (dotted lines).
<figure><img src="image_9.png" /><figcaption>Figure 8. Co-combustion of (a,b) WBC biocarbons and anthracite mixture, and (c) biocoke and anthracite mixture at 10°C/min</figcaption></figure>

The results showed that as the ratio of biocarbon/anthracite decreased, the degradation profiles shifted towards at higher temperatures, approaching the anthracite curve. Two main mass losses could be distinguished from the degradation profiles of the mixtures, especially for samples with a high volatile matter content (WBC1, Figure 8a) and for mixtures with low (25/75) to intermediate (50/50) biocarbon/anthracite ratio. As the ratio increased, the shifting temperature between these two mass losses slightly decreased in the range of 500 to 600°C, approaching the behavior of the mixture to that of biocarbon/biocoke samples. This could correspond to the mechanism of degradation of anthracite, which was shown to be less reactive with a higher ignition temperature, and biocarbon, with a higher reactivity, enhanced when its volatile matter content is higher. The difference in the behavior of the mixtures and thus the final mass loss compared to anthracite and biocarbon was lower when biocarbon presented a lower VM content (WBC2), which was also linked to faster conversion rates (k:5.27/min). The slope of the WBC2/anthracite mixture (Figure 8b), corresponding to the degradation rate, was
particularly different from the others. Increasing the biocarbon/anthracite ratio to 75/25 improves the thermal stability of the mixture, shown by the decrease of the final mass loss (85% for 25/75 biocoke/anthracite, **Figure 8c**). In comparison to WBC samples, the trend of final mass loss was enhanced by the reactivity and synergistic effect. This refers to the change in oxidation rate due to a mixture of two fuels [70], which was observed by the difference in terms of mass loss and TG profile of the experimental curves and the calculated curves using the mixture ratio.

As the reaction temperature was increased up to 900°C, the final mass loss can be assimilated to the ash content of the fuels and the mixtures. The ash content of the mixture may increase because of the anthracite ratio. In contrast, WBC1/anthracite and WBC2/anthracite produce almost no ash content. This translates not only the resource effect but likely also some interactions between mixed fuels compounds. Biocoke/anthracite also showed a trend of low ash production enhanced by the synergistic effects of the metals from anthracite.

### Synergistic effect evaluation

The experimental and calculated heat flows for biocarbon/anthracite and biocoke/anthracite are represented (Figure 9).
<figure><img src="image_10.png" /><figcaption>Figure 9. Heat flow of co-combustion of (a,b) woody biocarbon/anthracite and (c) biocoke/anthracite at 10°C/min (line: experiment, dash: calculated)</figcaption></figure>

The results showed that the behavior of the mixtures is comparable to that of the single fuels (biocarbon and anthracite, **Figure 4**). The heat flows of woody biocarbon/anthracite mixtures (**Figure 9a-b**) were generally higher than that of biocoke/anthracite (**Figure 9c**). WBC2/anthracite showed the highest heat flow peak, close to that of anthracite. The higher the reactivity, the higher the difference between experimental and predicted data.

In general, the heat generated by the oxidation of biocarbon (peak around 450°C to 500°C) participated in the combustion of anthracite (peak around 600°C). As a result, this second peak decreases when increasing the biocarbon/anthracite ratio in the mixtures. The metal content in biocarbon (0.12-0.21g/kg<sub>ash</sub>, **Table 2**), especially AAEMs (i.e., K, Ca, etc.), may catalyze anthracite oxidation. Anthracite is also composed of a higher AAEMs (3.29g/kg<sub>ash</sub>) content compared to biocarbon. The increase of the biocarbon/anthracite ratio might enhance the catalytic effect in co-combustion. At a moderate
ratio (50/50), the double peaks indicated an approximative thermal compensation between biocarbon and anthracite oxidation. The synergistic effect between biocoke and anthracite may be limited by the structure of coke, which appeared as better ordered than that of biocarbon [71]. The higher Si content of anthracite may inhibit the combustion rate of biocoke, enhancing a decrease in the reactivity of the mixture [34].

## Ignition characteristics

Ignition and burnout temperatures of the fuel mixtures were assessed in co-combustion (Table 7). As a result of the mixing process, $T_i$ increases at a low mixture ratio compared to single fuels. $T_i$ of 25/75 WBC1/anthracite was 8% higher than single WBC1, respectively. This difference may be due to the improved thermal stability and H/C ratio of the mixture (Table 1). Decreasing the volatile matter content of biocarbons implies an increase in $T_i$ of the fuel mixtures in co-combustion (i.e., 431°C for 25/75 WBC2/anthracite). In such mixture conditions, $T_b$ increased for all fuel mixtures considered, leading to a range of 672 to 733°C for biocarbons. The same trend was observed for biocoke, leading at $T_i$ of 699°C for the mixture.

In addition, $T_p$ providing the temperature of half fuel conversion also increases (i.e., 574°C for 25/75 WBC2/anthracite vs. 464°C for single anthracite). This observation was consistent with findings on co-combustion of 20/80 palm kernel shell biocarbon/anthracite [17]. Nevertheless, this benefit was reduced at a moderate biocarbon/anthracite ratio of 50/50. A similar observation has been made in the literature by using a biocarbon/anthracite ratio higher than 20/80 [72]. For instance, $T_i$ decreased to 351 and 411°C for 50/50 WBC1/anthracite and 50/50 WBC2/anthracite, respectively. It was also the case for 50/50 biocoke/anthracite (303°C). Consequently, $T_p$ and $T_b$ were also affected. This may be explained by the inhibiting role of the inorganic content (Si, Al, etc.) [73].
488

<table><caption>Table 7. Ignition temperatures of the single fuels and the fuel mixtures</caption><thead><tr><th rowspan="2">Sample</th><th>T<sub>i</sub></th><th>T<sub>p</sub></th><th>T<sub>b</sub></th></tr><tr><th colspan="3">°C</th></tr></thead><tbody><tr><td>WBC1</td><td>327</td><td>464</td><td>530</td></tr><tr><td>25/75 WBC1/anthracite</td><td>355</td><td>615</td><td>711</td></tr><tr><td>50/50 WBC1/anthracite</td><td>351</td><td>491</td><td>697</td></tr><tr><td>75/25 WBC1/anthracite</td><td>330</td><td>489</td><td>669</td></tr><tr><td>WBC2</td><td>371</td><td>493</td><td>560</td></tr><tr><td>25/75 WBC2/anthracite</td><td>431</td><td>574</td><td>694</td></tr><tr><td>50/50 WBC2/anthracite</td><td>411</td><td>572</td><td>673</td></tr><tr><td>75/25 WBC2/anthracite</td><td>392</td><td>558</td><td>631</td></tr><tr><td>Biocoke</td><td>276</td><td>517</td><td>592</td></tr><tr><td>25/75 biocoke/anthracite</td><td>315</td><td>609</td><td>699</td></tr><tr><td>50/50 biocoke/anthracite</td><td>303</td><td>596</td><td>676</td></tr><tr><td>75/25 biocoke/anthracite</td><td>292</td><td>499</td><td>651</td></tr><tr><td>Anthracite</td><td>481</td><td>619</td><td>693</td></tr></tbody></table>

489

### 2.1.1. Effect on combustion performance

490

Performances of the fuel mixtures were evaluated using the three biocarbon/anthracite ratios (25/75,

491

50/50, and 75/25) and compared to those of single fuels (Figure 10).

492
<figure><img src="image_11.png" /><figcaption>Figure 10. Combustion performance of the fuel mixtures at 10°C/min, (a) S index, (b) Reactivity, and (c) D index</figcaption></figure>

In general, the burning index of the fuels decreased after the mixture with anthracite. So, the S values were compared to 6.95wt.%/min²C³ (reference for anthracite, Figure 10a). The S index of biocarbon/anthracite mixtures ranged from 12.3 to 36.2wt.%/min²C³, while the range of biocoke was from 10.1 to 17.2wt.%/min²C³.

At a low biocarbon/anthracite ratio, the values of S for 25/75 WBC1/anthracite (H/C: 0.26) and 25/75 WBC2/anthracite (H/C: 0.17) were five and three times lower than those of single WBC1 and WBC2, respectively. This remarkable decrease may be due to the decrease of VM in mixtures affecting Tᵢ. The lower Tᵢ, the higher the S and D indexes e.g. 25/75 WBC1/anthracite had lower Tᵢ (355°C), so higher S and D than 25/75 WBC2/anthracite. The same trend was found with 25/75 biocoke/anthracite (Tᵢ: 315°C, H/C:0.38). Biocarbon/anthracite co-combustion ensured the decrease of burning rate, as so is the reactivity (Figure 10b) [74]. The mixture of highly devolatilized biocarbons
(i.e. WBC1 and WBC2) with anthracite favored a low S index (i.e., 12.9 for 25/75 WBC2/anthracite and 17.0wt.db%/min²C³ for 25/75 WBC1/anthracite), close to that of anthracite, which suggested improved thermal stability.

At a moderate biocarbon/anthracite ratio of 50/50, WBC1 showed interesting burning performance with a more decrease of S. The mixtures with biocoke and WBC1 had similar S values, while that of mixture with WBC2 was two times higher. An interesting gain of R performance was overcame for woody biocarbons, and biocoke showed reactivity less than that of individual anthracite. Furthermore, with $T_i$ ranged from 351-411°C, biocoke may compete with woody biocarbons. Considering the ash content for the fuel mixtures (Figure 8), WBC1 remains the promising fuel with $\Delta m<1$ wt.%.

At a high biocarbon/anthracite (75/25), a synergistic effect could be prevalent. This could explain the exceptional result with BC1 as well as trends observed for the WBC2 and biocoke. D of biocarbons (Figure 10c) ranged from 16.9 $10^{-3}$ to 20.2 $10^{-3}$ wt.%/min³ against 7.19 $10^{-3}$ wt.%/min³ for biocoke.

### 2.1.2. Effect on kinetics

#### Ignition time

Ignition time represents the duration to reach a complete conversion of biocarbon/anthracite and biocoke/anthracite mixtures (Figure 11). It was deducted by the difference between the time corresponding to the burnout temperature ($t_b$) and the ignition temperature ($t_i$) of fuel mixtures at different ratios. Co-combustion of biocarbon/anthracite mainly occurs between 350 and 730°C which represents at least a gain of 70°C compared to biocarbon combustion. This is another advantage of co-combustion as it increases ignition time.

At a low biocarbon/anthracite ratio, ignition time with WBC1 and biocoke remarkably increased. This was in accordance with their higher D and lower R. Biocoke exhibited the highest ignition time of 39min (slightly higher than WBC1). In comparison, the ignition time of 25/75 WBC1/Anthracite was slightly longer than that of 25/75 WBC2/Anthracite, 36.5 versus 26.3min. The same trend was observed when the mixture ratio increased to 50/50. The highest R of WBC2 made it a discriminant alternative fuel compared to WBC1. Whereas, 50/50 WBC1/anthracite and 50/50 WBC2/anthracite showed a significant increase of their ignition time of about 27 and 34%, respectively. The decrease of ignition time at a 75/25 biocarbon/anthracite ratio, especially for WBC1, WBC2, and biocoke, may be related to the higher VM of the mixture samples.
<figure><img src="image_12.png" /><figcaption>Figure 11. Ignition time of fuel mixtures at 10°C/min</figcaption></figure>

### Kinetic parameters

The kinetic parameter such as activation energy was calculated for fuel mixtures using the FWO and KAS methods (Table 8). A change of k constant with the biocarbon/anthracite ratio was observed. This may be due to a change in VM as well as to the catalytic effect favored by AAEMs. The activation energy decrease is observed. This could be attributed to a synergistic effect between fuels.

At a low biocarbon/anthracite ratio, Ea of 25/75 biocarbon/anthracite ranged from 128 to 139kJ/mol. Ea of all mixtures increased, suggesting an increase in thermal stability. This increases the energy barrier required to ignite, so increasing Tᵢ compared to single fuels (i.e. 355°C and 128kJ/mol for WBC1/anthracite). Moreover, Ea of WBC1/anthracite and Biocoke/anthracite slightly decreased. This may be explained as a synergistic effect enhanced by a catalytic effect of AAEMs. Biocoke showed also the same trend with an increase in Ea to 137kJ/mol. Ea is higher for biocarbons with a low volatile matter content. WBC2/anthracite exhibited the highest Ea of 139kJ/mol, justifying an important decrease in S and D indexes. The highest values of Tᵢ obtained in this ratio seemed to be the result of a synergistic effect as shown by the oxidation rate constant. The values of k obtained ranged from 0.03 to 0.56/min, which is much less than the rate of single fuels. This was in agreement with the decreasing reactivity.

At a moderate ratio of 50/50, WBC1/anthracite had the lowest k value of 0.17/min. Hence the range of k for biocarbons became from 0.11 to 0.17/min and 0.04/min for biocoke. The value of Ea for
biocarbon/anthracite (129-137kJ/mol) was slightly higher than the range reported for palm kernel shell biocarbon/anthracite in [17] (90.2-94.4kJ/mol).

At a high biocarbon/anthracite ratio, the increase in the value of Ea for 75/25 biocarbon/anthracite and 75/25 biocoke/anthracite has not shown to be correlated with $T_i$. This obviously could be attributed to a synergistic effect [17]. In all cases, k of mixtures was lower than those of individual fuels except for the mixture with WBC2. This could explain its particular trend as observed for S and D at this ratio (Figure 10).

<table><caption>Table 8. Activation energies by FWO and KAS and oxidation rate constant of the fuel mixtures in co-combustion</caption><thead><tr><th rowspan="2">Sample</th><th>Ea FWO</th><th>Ea KAS</th><th>k<sub>900°C</sub></th></tr><tr><th colspan="2">kJ/mol</th><th>1/min</th></tr></thead><tbody><tr><td>WBC1</td><td>102</td><td>94</td><td>2.39</td></tr><tr><td>25/75 WBC1/anthracite</td><td>128</td><td>121</td><td>0.28</td></tr><tr><td>50/50 WBC1/anthracite</td><td>137</td><td>131</td><td>0.17</td></tr><tr><td>75/25 WBC1/anthracite</td><td>163</td><td>159</td><td>0.3</td></tr><tr><td>WBC2</td><td>129</td><td>112</td><td>5.27</td></tr><tr><td>25/75 WBC2/anthracite</td><td>139</td><td>132</td><td>0.03</td></tr><tr><td>50/50 WBC2/anthracite</td><td>129</td><td>122</td><td>0.11</td></tr><tr><td>75/25 WBC2/anthracite</td><td>132</td><td>125</td><td>2.22</td></tr><tr><td>Biocoke</td><td>109</td><td>103</td><td>1.99</td></tr><tr><td>25/75 Biocoke/anthracite</td><td>137</td><td>132</td><td>0.56</td></tr><tr><td>50/50 Biocoke/anthracite</td><td>113</td><td>107</td><td>0.04</td></tr><tr><td>75/25 Biocoke/anthracite</td><td>111</td><td>104</td><td>1.17</td></tr><tr><td>Anthracite</td><td>121</td><td>112</td><td>0.31</td></tr></tbody></table>

## 4. Conclusion

Woody biocarbons exhibited higher burning and ignition indexes compared to anthracite. The characteristics of biocarbon, mainly its carbon, inorganic, and volatile matter content, had an influence on its combustion. A comparison with the literature allowed a better understanding of the resource type on the reactivity, as on ignition temperature. Indeed, WBC samples presented a lower ignition temperature than hard coal (<481°C). A mixture of fossil fuels and biocarbon can alleviate this limitation, making it suitable for high-energy processes and partial replacement of fossil fuels in industrial combustion processes. At the same time, a 50/50 biocarbon/anthracite mixture was shown
to represent a good compromise allowing the increase of biocarbon ignition temperature, and the
decrease of S and D indexes.

Meanwhile, using biocarbons with a low volatile matter content was shown to be as beneficial fuel
burning control and enhancement of its performances. In fact, the volatile matter content in
biocarbons was shown to be proportional to the reactivity and explosivity index. Biocarbon/anthracite
exhibited also a high reactivity due to their lower T<sub>i</sub> and oxidation rate close to that of anthracite.

Regarding the performances of the fuel mixtures, WBC1 was found to be a promising alternative fuel
for energy intensive processes towards the decarbonization.

To complete this study, emissions assessment may contribute to a better selection of biocarbon and
biocarbon-fossil fuel mixture to partially replace fossil fuel in industrial combustion processes.

## Authors' contributions:

B. Bouesso: Methodology, Formal analysis, Investigation, Writing – Original Draft, Visualization.

M. González Martínez: Validation, Resources, Writing – Review & Editing, Visualization, Supervision, Project administration, Funding acquisition.

A. Nzihou: Conceptualization, Validation, Resources, Writing – Review & Editing, Supervision, Project administration, Funding acquisition.

591
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# Supplementary material

## Activation energy

Activation energies of the samples considered in this study were determined along with the conversion degree in combustion (Figure S1).

<figure><img src="image_13.png" /><figcaption>Figure S1. Activation energies as a function of conversion degree for woody biocarbons and biocoke, and compared to anthracite using (a) FWO and (b) KAS methods</figcaption></figure>

## Kinetic modelling approach validation

DSC-modeled curves were compared to the experimental data for the singular fuels and the fuel mixtures (Figure S2).

For the singular fuels (Figure S2a), a good match with high regression coefficients ($R^2$: 0.980–0.997) was found between the model and the experimental data. The higher the volatile content, the lower the regression coefficient of the model ($R^2$ = 0.952 for WBC1).
<figure><img src="image_14.png" /><figcaption>Figure S2. Heat flow co-combustion modelling at 10°C/min (line: experiment, dash: model)</figcaption></figure>

For the fuel mixtures, the DSC curves of the biocarbon/anthracite and biocoke/anthracite mixtures were represented for the experimental and modeled data (Figure S2b-d). Some differences were observed by comparing the experimental and modeled DSC curves of the mixtures with those of the singular fuels. Co-combustion showed an effect on the curve shapes and the temperature range of the release heat flow of the mixtures.

At a low biocarbon/anthracite ratio (25/75, Figure S2b), the shape of biocarbon curves was characteristic of a fast conversion as they are less inclined and shifted towards lower temperatures. This may be related to the synergistic effect resulting in a decrease of Ea and an increase of k. Hence, $T_p$ and $T_b$ were slightly higher for biocarbons with low volatile matter content (WBC2). Only WBC1 remained in the range of 300 and 600°C. In contrast, biocoke has the broadest peak with a distinguished peak at higher temperatures associated with anthracite.
At a moderate mixture ratio (50/50, **Figure S2c**), the curve of WBC1 shifts towards at higher temperatures, becoming close to WBC2 with a similar intensity. This clearly shows two distinguished peaks (first for biocarbon and second for anthracite). The biocoke curve shows an overlapping peak while the curve of WBC1 is well divided with a significant gap of T~p~ for both peaks. WBC2 had a more reactive shape curve with a small peak of anthracite.

Finally, at a high biocarbon/anthracite ratio (75/25 **Figure S2d**), WBC1 shows two distinguished peaks in contrast to WBC2. The higher peak of WBC2 (**Figure S2c**) is reduced and a small peak appears at higher temperatures. Biocoke shows a particular asymmetric peak tending to higher temperatures. In all three cases, the modeling results had high R² coefficients (0.938-0.997).

These results validated the Asym2 modeling approach to predict the fuel combustion behavior and the fuel co-combustion behavior (biocarbon/anthracite and biocoke/anthracite mixtures).

**Effect of co-combustion tests on thermodynamics characteristics**

Thermodynamic parameters were evaluated in co-combustion (Table 9).

The free Gibbs energy of the fuel mixtures increased with the biocarbon/anthracite ratio. Kinetics showed the lowest range of k values using a moderate biocarbon/anthracite ratio of 50/50. ΔG of biocarbon/anthracite ranged from 197 to 230kJ/mol while biocoke/anthracite exhibited the highest ΔG of 226kJ/mol. Moreover, T~i~ of biocoke was 303°C and the enthalpy derived from its ignition was lower (106kJ/mol) than that low-volatile content biocarbon such as WBC2 (122kJ/mol). Meanwhile, WBC1 had the highest ΔH (131kJ/mol) with the lowest energy barrier (Ψ) of 6.40kJ/mol to ensure complete combustion. At least, 7kJ/mol may be required for all mixture samples. The burnout temperature (T~b~) of woody biocarbon/anthracite (491-572°C) associated with the reactivity and lower oxidation rate, would favor the feasibility of using biocarbon as a fuel alternative. In addition, the values of ΔS were negative, which may be indicative of a low disorder during the co-combustion process.
<table><caption>Table 9. Thermodynamic parameters of fuel mixtures</caption><thead><tr><th rowspan="2">Sample</th><th>T<sub>i</sub></th><th>T<sub>b</sub></th><th>ψ</th><th>ΔG</th><th>ΔH</th><th>ΔS</th></tr><tr><th colspan="2">°C</th><th>min</th><th colspan="2">kJ/mol</th><th>kJ/molK</th></tr></thead><tbody><tr><td>WBC1</td><td>327</td><td>464</td><td>6.82</td><td>187.7</td><td>95.18</td><td>-0.126</td></tr><tr><td>25/75 WBC1/anthracite</td><td>355</td><td>615</td><td>7.40</td><td>217.1</td><td>120.6</td><td>-0.109</td></tr><tr><td>50/50 WBC1/anthracite</td><td>351</td><td>491</td><td>6.40</td><td>209.7</td><td>130.6</td><td>-0.104</td></tr><tr><td>75/25 WBC1/anthracite</td><td>330</td><td>489</td><td>6.10</td><td>214.1</td><td>156.9</td><td>-0.075</td></tr><tr><td>WBC2</td><td>371</td><td>493</td><td>16.5</td><td>112.5</td><td>112.5</td><td>-0.122</td></tr><tr><td>25/75 WBC2/anthracite</td><td>431</td><td>574</td><td>7.10</td><td>229.8</td><td>131.9</td><td>-0.116</td></tr><tr><td>50/50 WBC2/anthracite</td><td>411</td><td>572</td><td>6.80</td><td>220.1</td><td>122.2</td><td>-0.116</td></tr><tr><td>75/25 WBC2/anthracite</td><td>392</td><td>558</td><td>7.10</td><td>196.9</td><td>124.9</td><td>-0.088</td></tr><tr><td>Biocoke</td><td>276</td><td>517</td><td>6.20</td><td>203.1</td><td>102.8</td><td>-0.127</td></tr><tr><td>25/75 biocoke/anthracite</td><td>315</td><td>609</td><td>7.10</td><td>213.3</td><td>129.9</td><td>-0.095</td></tr><tr><td>50/50 biocoke/anthracite</td><td>303</td><td>596</td><td>6.80</td><td>226.3</td><td>106.2</td><td>-0.139</td></tr><tr><td>75/25 biocoke/anthracite</td><td>292</td><td>499</td><td>6.20</td><td>191.2</td><td>104.8</td><td>-0.112</td></tr><tr><td>Anthracite</td><td>481</td><td>619</td><td>7.50</td><td>231.9</td><td>113.5</td><td>-0.134</td></tr></tbody></table>
List of acronyms

(dm/dt)<sub>max</sub>: maximum value of the mass loss rate (wt.%/min)

(dm/dt)<sub>mean</sub>: average value of the mass loss rate (wt.%/min)

A: frequency factor (1/min)

Asym2: mathematical function of asymmetric 2-sigmoidal

C: carbon content (wt.%)

daf: dry ash-free

db: dry basis

DSC<sub>model</sub>: heat flow model (W/g)

Ea: activation energy (kJ/mol)

EI: explosivity index (%)

f(α): reaction model

FC: fixed carbon content (wt.%)

FWO: Flynn-Wall-Ozawa

H: hydrogen content (wt.%)

h: Plank constant (6.63.10<sup>-34</sup>J/s)

H<sub>p</sub>: DSC curve amplitude (°C)

k: oxidation rate constant (1/min)

KAS: Kissinger-Akahira-Sunose

k<sub>B</sub>: Boltzmann constant (1.38.10<sup>-23</sup>J/K)

N: nitrogen content (wt.%)

O: oxygen content (wt.%)

Q: heat flow (W/g) at a given T

R: ideal gas constant (8.316J/molK)

S: burning index (wt.%/min°C<sup>3</sup>)

T: temperature (°C)

t: tonne (t)

t<sub>50</sub>: time required for half-fuel conversion (min)

T<sub>b</sub>: burnout temperature (°C)

t<sub>b</sub>: burnout time (min)

TGA-DSC: thermogravimetric analysis combined with differential scanning calorimetry
TG: calculated TGA curve (wt.%)

T<sub>i</sub>: ignition temperature (°C)

t<sub>i</sub>: ignition time (min)

T<sub>m</sub>: maximum combustion temperature (°C)

T<sub>p</sub>: DSC peak temperature (°C)

t<sub>p</sub>: maximum combustion time (min)

VM: volatile matter content (wt.%)

m<sub>0</sub>: normalized sample mass at 105°C (mg)

m<sub>ash</sub>: ash content (mg)

m<sub>t</sub>: mass at time t (mg)

Z<sub>1</sub>: half-width curve (°C)

Z<sub>2</sub>: symmetric shape parameter (°C)

Z<sub>3</sub>: asymmetric shape parameter (°C)

α: conversion degree (%)

β: heating rate (°C/min)

ΔG: Gibbs free energy (kJ/mol)

ΔH: reaction enthalpy (kJ/mol)

ΔS: entropy (kJ/molK)

Φ: heat released by integration from T<sub>1</sub> to T<sub>2</sub> (W/g)

Ψ: energy barrier (kJ/mol)