Research on Co-Combustion of High-Calorific Biomass Obtained Using Gasification and Lignite for Sustainable Utilisation of Resources

Abstract

As part of the transition to low-carbon energy and for the sustainable utilisation of resources, it is necessary to seek a replacement for solid fossil fuels, but unfortunately, it is impossible to completely abandon them for various reasons at the moment, so only partial replacement with new, high-calorific, biomass-based fuels is possible. The purpose of this work is to determine the typical parameters of the co-combustion of carbonisate, coal and their mixtures, taking into account the synergetic effects influencing the combustion intensity of the mixture. Carbonisate was obtained in the process of the gasification of pinewood through the counter-blowing method at a temperature of 800–900 °C, while air was used as an oxidant. Basically, this method of gasification is used for coal in order to obtain high-calorific coke for the metallurgical industry. Also, in this study, for the first time, carbonisate was obtained from 50% pinewood and 50% lignite. The O/C and H/C ratios were determined for carbonisate. A technical and elemental analysis of the investigated fuels was carried out. A thermal analysis in oxidising medium was applied to determining the typical combustion parameters in the process of slow heating of the fuels under study. According to the results of this thermal analysis, typical heating parameters such as the ignition temperature, burnout temperature, maximum mass loss rate, combustion index, etc., were determined. It was noted that the calorific value of carbonised wood is two times higher than that of coal. The combustion index of carbonisates is 2.5–36% lower compared to that of coal. According to the results of the analysis of the interaction of the components among themselves (in the process of their joint combustion), the presence of synergetic interactions between the components was determined, which affected the change in the combustion intensity and heat release intensity. The results of this study may be useful for retrofitting coal-fired boilers to run on a mixture containing carbonisate and lignite. If carbonisate is produced from biomass, the resulting gas could be used as an energy fuel by burning it in a coal-fired boiler.

1. Introduction

The majority of greenhouse gases are generated and emitted into the environment by solid fossil fuel energy production. In OECD (Organisation for Economic Co-operation and Development) countries, 80% of heat and electricity is generated from fossil fuels, of which 38% is coal, 28% gas and 14% oil. By 2050, the energy consumption in OECD countries is projected to increase by 15% [1,2].

The need to reduce carbon dioxide emissions worldwide requires minimising (ideally completely eliminating) the use of solid fossil fuels in the generation of heat and electricity [3,4,5,6]. The complete elimination of the combustion of solid fossil fuels is particularly difficult for a number of reasons in areas with large coal basins. For such territories, it is advisable to use a phase-out of coal combustion. The first stage of reducing coal consumption can be a transition to its partial combustion, and coal should partly be replaced by biomass waste [7,8,9]. The transition to the co-combustion of solid fossil fuels and carbon-containing industrial wastes (e.g., sawdust or wood chips) not only allows less coal to be burned in the production of thermal energy but also the formation of carbon dioxide, nitrogen oxides and sulphur oxides to be reduced, as well as emissions of fine dust based on the inorganic residue formed as a result of coal combustion to be reduced [10,11,12]. The involvement of biomass in the fuel and energy complex reduces the risk of fires during the storage of biomass waste in landfills, as well as minimising the risk of environmental damage during its long-term storage. For power plants, the transition to the combustion of biomass waste is positively reflected in a reduction in the heat losses with mechanical underburning and the heat loss to the environment and as a consequence an increase in the efficiency of these plants. From a practical point of view, biomass, compared to coal, has a number of disadvantages, such as a low heat of combustion; high humidity due to its hygroscopic properties; a fibrous structure, which reduces its grinding properties; and a high propensity towards spontaneous combustion due to its high content of volatile substances [13,14,15,16,17]. All of these disadvantages are remedied through thermal treatment of biomass before its use as energy fuel. There are different methods of thermal treatment for biomass, such as torrefaction, pyrolysis and gasification [18,19,20,21,22]. All of these methods are applied depending on the objectives pursued. In the following, the methodology of biomass gasification will be discussed in detail. Biomass gasification is the conversion of biological waste into gaseous, solid or liquid products.

Gasification and combustion are almost similar processes, differing only in that gasification takes place in an oxygen deficit and combustion takes place in an oxygen surplus. The products of gasification can be gas, liquid or solid residue depending on the objectives pursued. It is possible to improve the production of one or another product by changing the parameters of the gasifier’s operation. As fuel, coal is usually used to produce coke, which is used in metallurgical plants. In recent times, there has been a notable increase in the research focus on biomass gasification, primarily due to the fact that in most cases, biomass is a low-cost waste product when compared with solid fossil fuels. There is particular interest in the study of woody biomass, municipal solid waste, agricultural waste and sewage residues. The gasification of biomass to produce high-calorific fuel, which can be utilised in coal-fired boilers to enhance the heat of combustion and reduce coal consumption, results in the generation of a concomitant generator gas that can be employed as an energy fuel or to generate electricity for self-sufficiency [23,24,25].

The solid product, termed “biochar”, encompasses the organic unconverted fraction and the inert material present in the thermally treated biomass. This conversion constitutes partial oxidation of the carbon in the feedstock and is typically conducted in the presence of a gasifying carrier, such as air, oxygen, steam or carbon dioxide [26]. In [27], the results of the gasification of biomass (including woody biomass) in an air-blown circulating fluidised bed gasifier to produce generator gas and resin were presented. It was found that the composition and calorific value of the resulting gas depended largely on the air or O/C ratio and the temperature of the slurry.

The co-combustion of carbonisate with coal or biomass can improve the following characteristics: preventing a reduction in the calorific value (in some cases even increasing it) and reducing the volume of flue gas due to the low volatile content of carbonisate [28,29,30]. The co-combustion of carbonisate and thermally treated biomass was found to increase the synergistic effect with an increasing heating rate [31]. Liu Y. et al., in their work [32], investigated the co-combustion of carbonisate, woody biomass and agricultural waste and found that biomass had better combustion characteristics than those of carbonisate and the most optimal ratio of the components in the mixture was 20% biomass being added. Wang, P. et al. [33] analysed the co-combustion characteristics of carbonisate and four types of bituminous coals and found that carbonisate showed worse combustion behaviour than that of coal.

In the context of combustion, the interaction between multiple fuels can give rise to phenomena that are not immediately apparent. The principle of additivity suggests that given the combustion parameters of the individual fuel components, the co-combustion parameters of the mixture can be calculated. However, it is important to note that combustion is a complex physical and chemical process, and as such, synergetic interactions frequently occur between the components of a mixture. These interactions can have a significant impact on the combustion parameters, either enhancing or reducing their effectiveness. From a thermodynamic perspective, synergetic interactions can be attributed to the afterburning of volatile substances at elevated temperatures, which impacts the ignition of the carbon residue earlier. A synchronous thermal analysis is the most suitable method for identifying the regularities of such phenomena during the combustion of a solid-fuel mixture [34].

The purpose of this study is to investigate the joint heating of high-calorific biomass obtained through gasification at a temperature of 800–900 °C and lignite, taking into account the synergetic interactions affecting the combustion parameters. The results of this study can be applied in the design of coal-fired power plants for their further transfer to the combustion of carbonisates and mixtures based on them.

2. Materials and Methods

2.1. Fuels

The following fuels were used for the experiments:

• No. 1—Borodino lignite coal;
• No. 2—Carbonisate obtained from larch wood;
• No. 3—Carbonisate obtained from pinewood;
• No. 4—Carbonisate obtained from mixed fuel based on lignite No. 1 and common pinewood;
• No. 5—A mixture based on fuels No. 1 and 2;
• No. 6—A mixture based on fuels No. 1 and 3.

Borodino lignite is the main energy fuel for heat and electricity generation. The woody biomass waste was obtained from a wood processing plant in Krasnoyarsk, Russia. Fuel No. 4 was obtained by pre-mixing 50% No. 1 coal and common pinewood and applying further heat treatment as described below; fuel No. 4 was considered individually.

The choice of gasification technique was based on previous experiments with coals of different metamorphic degrees, which were subjected to gasification to obtain high-calorific coke for the metallurgical industry. Gasification at 900 °C makes it possible to volatilise all of the heavy hydrocarbons that are a source of carcinogens. Therefore, the temperature regime in the thermal treatment of the biomass was not changed, only the air flow rate. When carrying out thermal treatment of the biomass using the gasification method, we used an energy-efficient combination of parameters, such as the temperature inside the reactor and the air flow rate (the temperature regime was about 900 °C, and the air flow rate was 2.7 m³/h). The main parameters of the obtained carbonate for us were the carbon and volatile contents (

Table 1. Main characteristics of the fuels under study.

Fuels	MC	Ad	VCdaf	Cdaf	Hdaf	Ndaf	Sdaf	Odaf	LHV
	%								MJ/kg
No. 1	11.6	9.2	47.3	74.8	5.1	1.0	0.3	21.1	16.1
No. 2	1.7	4.1	4.7	97.1	1.8	0.2	0.1	0.9	32.6
No. 3	4.7	3.7	3.4	98.3	1.1	0.4	0.1	0.1	31.6
No. 4	1.8	10.8	7.5	96.1	1.6	1.1	0.5	0.6	30.1
No. 5	6.7	6.7	26.0	86.0	3.5	0.6	0.2	11.0	24.4
No. 6	8.2	6.5	25.4	86.6	3.1	0.8	0.2	10.6	23.9

). Changing the air flow rate had a negative effect on the elemental composition of the fuel, decreasing carbon and increasing oxygen. The main criterion was the repeatability of the results of the experiments, which was achieved by performing each experiment at least three times.

Thermal treatment of fuels No. 2–4 was carried out through the method of reverse gasification, a scheme of which is presented in Figure 1. The experimental unit for the thermal treatment of biomass consisted of a vertical reactor with a diameter of 0.1 m and a height of 1 m. The temperature inside the reactor reached 900 °C, which was controlled along the whole height of the reactor using thermocouples. The gasification process in this study diverges from the conventional method by incorporating counter-movement of the heat wave and the oxidant (air). Specifically, the heat wave propagates from the top to the bottom of the reactor, where it encounters the oxidant, which is introduced at the base of the reactor. Subsequently, the oxidant moves from the bottom to the top, converging with the heat wave (Figure 1).

The gasifier was loaded with about 2 kg of fuel (No. 2–4). The yield of ready products after thermal treatment of the wood biomass was 14%, while after the treatment of a mixture based on 50% Borodino coal and 50% pine sawdust, it was 25%.

2.2. Main Characteristics of the Fuels Under Study

The preparation of the fuels was as follows:

• The fuels were ground using a Retsch DM200 (Germany) to a grain size of about 1000 µm;
• To obtain a grain size of 140–250 μm (this fuel size is used in solid-fuel flaring combustion), a Retsch AS200 analytical sieving machine (Germany) equipped with sieves with appropriate cells (ISO 3310-1:2016) [35] was used;
• The moisture content (MC) of the samples was determined on an MA-150 (Germany) according to (ISO 18134-1:2022) [36];
• The inorganic residue content (A^d) in a dry state was measured using a Snol 7.2/1300 (Lithuania) muffle furnace (ISO 18122:2022) [37];
• The volatile matter (VC^daf) in a dry ash-free state was also determined using the Snol 7.2/1300 (Lithuania) furnace (ISO 18123:2023) [38];
• The low heat of combustion in working conditions (LHV) was determined on a C6000 (Germany) apparatus (ISO 18125:2017) [39];
• The carbon (C^daf), hydrogen (H^daf) and nitrogen (N^daf) contents in a dry ash-free state were determined (ISO 16948:2015) [40];
• The sulphur content (S^daf) in a dry ash-free state was determined (ISO 16994:2016) [41];
• Oxygen (O^daf) was determined through subtraction (ISO/TS 20048-1:2020) [42].

The main characteristics of the fuels are presented in Table 1.

Analysing Table 1, we can see that coal has a high volatile matter content and a low calorific value compared to these values in carbonisate. Thermally treated biomass has a high calorific value and a low volatile matter content. When carbonisate is added to coal, the heat of combustion increases compared to that for coal, and the volatile matter content decreases.

2.3. A Van Krevelen Diagram of the Fuels Under Study

Figure 2 shows a van Krevelen diagram, analysing which it is possible to determine the quality of the fuel before and after gasification, and the values of the most promising fuels are placed in the lower-left corner of the diagram. Therefore, to obtain a complete picture of the atomic ratio changes, the atomic ratios of pine sawdust (No. 7) and larch sawdust (No. 8) were additionally analysed [43]. A thorough examination of the diagram reveals enhancement of the fuel’s characteristics following the gasification of the woody biomass, attributable to a reduction in the oxygen/carbon and hydrogen/carbon ratios. This process is facilitated by the augmentation of the carbon component of the fuel and the heat of combustion in the carbonisates. The atomic ratios of O/C and H/C within the mixtures also diminish due to the rise in the carbon content and the heat of combustion, approaching the lower-left corner of the diagram. This observation is consistent with the findings of other researchers who have examined the thermal treatment of biomass [44,45].

2.4. The Surface Morphology of the Wood Particles Before and After Gasification

The scanning electron microscopy (SEM) method was used to make a qualitative assessment of the surface of the wood particles before and after gasification. A TM-4000 scanning electron microscope (Tokyo, Japan) was used in this study.

The morphology of the fuel particles’ surfaces (i.e., the presence of breaks, holes, cracks, etc.) has been shown to have a significant effect on increases in the specific surface area of the particle. It is well established that an increase in the area of the particle results in an increase in the rate of combustion due to the faster reaction of the carbon with the oxidant. Furthermore, it has been demonstrated that the oxidant is able to penetrate deep into the particles via the channels, where it reacts with the carbon, thus ensuring more complete combustion. As can be seen in Figure 3, the SEM images of the wood particles before and after gasification at a hundredfold magnification show this effect. The wood before gasification has a large number of pores, channels and cracks, but of a small size (Figure 3a). In the process of the heat treatment, low-molecular-weight bonds are broken, accompanied by the release of volatile substances, which in turn burst outwards from the depth of the particles, leaving pores, channels and cracks of a large size (Figure 3b), which in turn affects the increase in the specific surface area of these particles. Concomitant findings have been reported by other researchers who have employed SEM to analyse the surface morphology of thermally treated biomass [46,47].

The specific surface area of the fuel particles plays an important role in the fuel combustion process. When the oxidant interacts with a large amount of carbon, the combustion of the fuel particles is faster, especially if there are pores and channels in the particles, allowing the oxidant to penetrate deep into the particles and enter into a chemical reaction with the carbon, thus contributing to burnout of the fuel particles not only on the surface but also in their interior. However, these properties only play a partial role during slow heating, as the main role is played by the combustion of volatile substances, which intensifies the ignition and combustion of the carbon residue of the fuel particles. Nevertheless, it is worth considering the morphological structure of the fuel particles in a complete analysis of the combustion properties under different heating conditions.

2.5. Methodology of Thermal Analysis (TGA)

Thermal analysers are widely used to determine the main parameters of the combustion of solid fuels in the process of non-isotropic heating. The SDT Q600 (Thermal Analysis, New Castle, DE, USA) was used in this study. The advantages of this thermal analyser are that it is possible to simultaneously analyse the profiles of the mass loss (Weight), differential mass loss (Deriv. Weight) and differential scanning calorimetry (DSC) curves. The thermograms were generated in an oxidising environment with an air supply of ±50 mL/min and at an average heating intensity of ±21 °C/min, and the mass of the test fuel loaded onto the cuvette was ±6 mg. The thermograms were processed using Universal Analysis 2000 (Thermal Analysis, New Castle, DE, USA), which is supplied with the thermal analyser. The main criterion was the repeatability of the experimental results, achieved by performing them at least three times each.

2.5.1. Determination of the Basic Parameters of Fuel Combustion and Determination of the Synergistic Effects During Heating of the Mixtures

The maximum (further peak) value of the intensity of the mass change (Deriv. Weight_max) was determined according to the profile of the Deriv. Weight curve, which characterises the intensity of the combustion of gaseous substances and carbon; high values of Deriv. Weight_max during biomass heating are characteristic in the process of the combustion of volatile substances and in coal and carbonisate during the combustion of coke residue. The ignition temperature (T_i) and burnout temperature (T_b) were determined by the curve-crossing technique, widely used by many researchers [48,49,50,51]. The combustion index (S, min⁻² °C⁻³), used by many researchers [45,52,53,54], was used to compare the combustibility of the different fuels differing in their chemical composition and thermophysical properties. The higher the value of S, the better and faster the fuel burns. The combustion index was determined using formula (1) [45,52,53,54]:

$$S={\displaystyle \frac{Deriv.Weigh{t}_{\max }\cdot Deriv.Weigh{t}_{\mathrm{mean}}}{T_i^2\cdot T_b}}\times {10}^6,$$

(1)

where Deriv. Weight_max is the value of the peak fuel mass change rate (selected from the maximum value of Deriv. Weight_max) or the peak value of the combustion intensity, %/min; Deriv. Weight_mean is the average value of the mass change rate from the ignition temperature to the burnout temperature, %/min; and T_i and T_b are the temperatures corresponding to ignition and burnout, °C.

2.5.2. Determination of the Synergistic Effects During Heating of the Mixtures

To determine the synergetic effects in the co-combustion of several fuels, the well-known method [55,56,57] was used. This method consists of comparing the profiles of the Deriv. Weight curves constructed from experimental values (Deriv. Weight_exp) and calculated values (Deriv. Weight_est). The calculated values were determined using Formula (2) [55,56,57]:

Deriv. Weight_est = x₁Deriv. Weight_coal + x₂Deriv. Weight_carbonisate,

(2)

where Deriv. Weight_coal and Deriv. Weight_carbonisate are the values of the intensity of the mass change at each moment in time for coal and carbonisate, %/min, and x₁ and x₂ refer to the content of the components in the mixture, the sums of which should be equal to one.

The coincidence of the curve profiles plotted using the experimental and calculated Deriv. Weight values shows the additive combustion of the fuel mixture. In the area of divergence of the curve profiles, there are synergetic interactions, which can both improve the intensity of combustion of the fuel mixture and worsen it by reducing the maximum value of Deriv. Weight.

3. Results and Discussion

3.1. The Combustion of Individual Fuels

The heating process for individual fuels No. 1–4 is represented in Figure 4 by the profiles of the mass change curves (Weight), the mass change intensity (Deriv. Weight) and the heat release intensity (DSC).

The heating of the fuels, for convenience in understanding the processes occurring during slow heating, is divided into three different temperature ranges. In the first temperature range of 25–110 °C, moisture removal occurs, accompanied by an endothermic effect due to heat absorption during the evaporation of moisture. All of the fuels reached an air-dry state before the experiments; moreover, the main moisture was removed from the carbonisate due to the heat treatment (Table 1). In coal, the moisture content was 11.6% (Table 1), so in Figure 4a, in the temperature range of 25–110 °C, an increase in the mass loss rate accompanied by a decrease in mass (when moisture evaporates) is noticeable, and the DSC curve profile shows a small endothermic effect. For the carbonisates, the profiles of the curves in this temperature range have an almost horizontal slope and do not show any special changes due to the low moisture content (Figure 4b–d). The main combustion parameters of fuels No. 1–6 are presented in

Table 2. Basic parameters of fuel combustion.

Parameters	Fuels
	No. 1	No. 2	No. 3	No. 4	No. 5	No. 6
Ti, °C	355	468	464	435	375	359
Deriv. Weightmax, %/min	26.0	20.8	23.3	20.5	14.6	14.9
TDeriv. Weight, °C	408	528	509	485	514	412
DSCmax, W/g	6.5	5.1	5.2	5.7	3.3	3.6
TDSC, °C	416	523	505	482	515	421
Tb, °C	539	545	552	574	556	570
S, min–2 °C−3	1.67	1.23	1.63	1.43	0.69	0.56

.

The second temperature range of 111–350 °C is characteristic of the destruction of low-molecular-weight bonds in the process of heating, accompanied by the release of volatile substances, which on further heating are ignited and partially burned, and the remaining part of the volatile substances burns out, together with the coke residue. Coal contains 47.3% gaseous substances in its composition, and this affects the ignition in the temperature range of 240–250 °C, with a pronounced increase in the intensity of the mass change and an increase in the intensity of the heat flow, accompanied by the onset of an exothermic effect (Figure 4a). In the carbonisates (in the second temperature range of 111–400 °C), the profiles of all of the curves do not show special changes due to the minimal content of gaseous substances (3.4–7.5%) in their structure (Table 1).

In the third temperature range, ignition and combustion of carbon residue in the fuels under study took place, starting from the value of the ignition temperature of the carbon residue (Table 2) and ending with the value of the fuel’s burnout temperature. For lignite (No. 1), the process of combustion of the carbon residue occurred in a temperature range of 355–539 °C. The maximum value of the mass change intensity during the combustion of the carbon residue reached 26%/min, at a temperature of 408 °C, and the peak value of the heat flux intensity accompanied by an exothermic effect was 6.5 W/g at a temperature of 416 °C (Figure 4a). The combustion index of the coal was 1.67 min⁻²·°C⁻³.

The combustion of carbon residue in the carbonisate obtained from larch (No. 2) proceeded in the temperature range of 468–545 °C, with a peak mass change intensity of 20.8%/min at 528 °C. During the combustion of the carbon residue in fuel No. 2, the maximum heat flux intensity accompanied by an exothermic effect reached 5.1 W/g at 523 °C (Figure 4b). The value of the combustion index for the larch carbonisate reached 1.23 min⁻²·°C⁻³.

The carbon residue in the carbonisate from common pinewood (No. 3) burned in the region of 464–552 °C, with a peak mass change intensity value of 23.3%/min (slightly lower than that of coal) at 509 °C. The peak heat release rate reached 5.2 W/g at 505 °C (Figure 4c). The value of the combustion index of the carbonisate obtained from pinewood was 1.63 min⁻²·°C⁻³.

The combustion of the carbon residue of carbonisate obtained from 50% coal and 50% common pine (No. 4) proceeded in the temperature range of 435–574 °C. The peak intensity of the mass change during combustion of the carbon residue of fuel No. 4 reached 20.5%/min at 485 °C and was expressed as an exothermic reaction at a peak heat flux intensity of 5.7 W/g (at 482 °C) (Figure 4d). The value of the combustion index of the carbonisate from the mixture of coal and pinewood was 1.43 min⁻²·°C⁻³.

Analysing the combustion parameters of the individual fuels, we can note the following:

• Coal had a lower ignition temperature compared to that of biomass carbonisate;
• The carbonisate from larch and pine had a burnout temperature approximately equal to that of coal;
• The values of the combustion index and peak mass change intensity (the combustion intensity) of pine carbonisate were only slightly lower than these values in coal, provided that the volatile content of carbonisate was 14 times lower;
• It was determined that the values of the combustion index of all of the carbonisates were higher than one, which indicated their good combustibility, even under the conditions of a low content of gaseous substances in their composition, and the peak value of the combustion intensity was 20%/min, which also confirmed their high-combustibility properties.

3.2. Combustion of the Fuel Mixtures

The heating process for fuel mixtures No. 5 and 6 is represented in Figure 5 according to the profiles of the curves of the weight change (Weight), the intensity of the weight change (Deriv. Weight) and the intensity of heat release (DSC).

It is evident from Figure 5 that when the mixtures are subjected to heating at temperatures ranging from 25 to 110 °C, there is a marginal increase in the rate of mass loss. This phenomenon is accompanied by an endothermic effect, which can be attributed to the removal of a modest amount of the moisture content, as outlined in Table 1. Further heating of the mixtures leads to the destruction of low-molecular-weight bonds, accompanied by the release of insignificant amounts of volatile substances (Table 1), which ignite and burn in the temperature range of 250–350 °C, accompanied by an exothermic effect.

On heating mixed fuel No. 5, the process of combustion of the carbon residue takes place in the temperature range of 375–556 °C, expressed by two maxima in the Deriv. Weight and DSC curves, due to the fact that the coke residue in each component burns within an individual temperature range; for coal, it is 375–450 °C, and for carbonisate from larch, it is 451–556 °C, and the highest peak is characteristic of the combustion of carbon residue carbonisate. The peak value of the mass change intensity for the combustion of the coal carbon residue was 13.6%/min at 408 °C, and for the combustion of the carbonised carbonisate residue (No. 2), it was 14.6%/min at 514 °C. The peak heat release intensity during the combustion of the coal carbon was 3.1 W/g at 410 °C, while that for the carbonisate was 3.3 W/g, at 515 °C (Figure 5a). The combustion index of mixture No. 5 was 0.69 min⁻²·°C⁻³.

During the heating of mixed fuel No. 6, as well as during the heating of mixture No. 5, two clearly visible peaks in the Deriv. Weight and DSC curves can be seen (Figure 5b). The first Deriv. Weight and DSC peak corresponds to the combustion of the carbon residue of coal in the temperature range of 359–450 °C, with a peak combustion intensity equal to 14.9%/min at 412 °C and a peak heat release of 3.6 W/g, at 421 °C, accompanied by an exothermic effect. The second Deriv. Weight and DSC peak corresponds to the combustion of the carbon residue carbonisate obtained from pinewood (No. 3) in the temperature range of 451–470 °C, with a peak mass loss rate of 11.6%/min at 470 °C and a peak heat release rate of 3.1 W/g, at a temperature of 474 °C, accompanied by an exothermic effect. The combustion index of mixture No. 6 was 0.56 min⁻²·°C⁻³.

The following observations were noted during the combustion of the carbon residue from the solid-fuel mixtures:

• The ignition temperature of mixtures No. 5 and 6 decreased in comparison with that of carbonisates No. 2–4.
• There was a decrease in the flammability of mixtures No. 5 and 6 compared to that of the individual components in these mixtures due to a decrease in the peak combustion intensity (%/min).
• The high ignition temperature of the carbonisate is due to the low content of volatile substances, which in turn can cause ignition in a lower temperature range due to the intense increase in the temperature of the carbon residue caused by the intense heat transfer from the burning gaseous substances to the carbon residue. The addition of coal with a high content of volatile substances shifts the ignition process to lower temperatures due to an increase in the volatile content of the mixture compared to the carbonisate (Table 1).

3.3. Synergetic Effects in the Combustion of the Fuel Mixtures

Figure 6 and Figure 7 compare the profiles of the Deriv. Weight and DSC curves obtained experimentally and through calculations for mixtures No. 5 and No. 6.

With the removal of moisture from both mixtures (Figure 6 and Figure 7), in the temperature range of 25–110 °C, synergetic effects are observed; in the calculated values, the maximum value of the mass loss rate is significantly higher than that in the experimental values, and at the same time, a synergetic interaction when comparing the experimental and calculated profiles of the DSC curves is observed only in mixture No. 5, while in mixture No. 6, in this temperature range, the process of moisture removal proceeds additively (the profiles of the DSC_exp and DSC_est curves coincide).

Synergetic interactions are observed in the temperature range of 375–450 °C during the afterburning of gaseous substances and the combustion of carbon residue from coal. During the experiment, the peak value of the mass change intensity was 13.7%/min, and this was 14%/min in the calculated values (Figure 6a). Concurrently, within this temperature range, synergistic effects exerted a detrimental influence on the combustion intensity and the maximum value of the heat effect intensity, exhibiting a reduction of 19% (Figure 7a). When the mixture was heated further, positive synergistic interactions were observed in the temperature range of 451–556 °C, corresponding to the combustion of carbon residue carbonisate from larch wood, and the peak value of the combustion intensity increased by 35% (Figure 6a), and the maximum value of the heat flux intensity increased by 22% (Figure 7a).

In the process of heating mixture No. 6, positive and negative synergetic effects were observed in the temperature range of 359–450 °C, shown as an increase in the value of the combustion intensity of the gaseous substances and the carbon residue from coal by 28% (Figure 6b) and a decrease in the peak value of the heat release intensity by 3% (Figure 7b). On further heating of the mixture in the temperature range of 451–470 °C, combustion of carbon residue of carbonisate from pinewood occurred, which, as a result of the synergetic interactions of the components of the mixture among themselves, was expressed as a shift in the peak value of the intensity of the mass change during the experiment in the lower temperature range by 42 °C, and the intensity of combustion decreased by 4% (Figure 6b), while a positive synergetic effect was expressed as an increase by 14% in the peak value of the heat flux intensity compared to the calculated data (Figure 6b).

The resulting synergistic interactions are attributable to the combustion of volatile substances across both the low-temperature range and the temperature range of the combustion of carbon residue, as evidenced in the case of fuel mixture No. 6 (Figure 6b), where a shift in the maximum value of the mass loss rate to the lower temperature range was observed. The thermal conductivity of the components of the fuel mixtures themselves, their location on the cuvette in the device, the density of the filling and the presence of dispersed particles of coal and carbonisate also influence the occurrence of synergetic interactions.

When analysing the above, it can be noted that from a practical point of view, it is not possible to calculate the joint combustion of mixtures based on carbonisate and lignite due to the interaction of the components with each other, so the combustion process of the mixtures does not obey the principle of additivity. These interactions are expressed as the maximum value of the combustion rate and the maximum value of the heat release intensity (Figure 6 and Figure 7).

4. Conclusions

A thermal analysis of coal, carbonisate (obtained from wood) and their mixtures in an air environment with a heating intensity of ±20 °C/min allowed us to establish the following:

• Carbonisate produced from coal and pinewood has the following advantages: Its ignition temperature is lower by 7% compared to that of carbonisate produced from wood; the combustion index is higher than that of carbonisate produced from larch wood by 16%; and the lower calorific value is higher than that of coal by 86%.
• The combustion index of carbonisate made of pinewood is lower than that of coal by only 2.5%, that of carbonisate made of larch wood by 36%, that of carbonisate made of coal and that of coal by 17%.
• The ignition temperature of coal is lower than that of carbonisate by more than 100 °C, so the addition of 50% of highly reactive coal to carbonisate reduces the ignition temperature by 25%, and the LHV of the mixture is higher than the LHV of coal by 50%.
• The analysis of the interaction of the fuels between themselves in the process of their joint combustion showed the presence of synergetic interactions, which led both to a decrease in the intensity of combustion and heat release and to increases.
• During the combustion of the carbon residue carbonisate obtained from a mixture of coal and pinewood (No. 4), a shift in the peak values of the combustion intensity and the heat release intensity to lower temperatures was observed.
• The results of this study may be useful for retrofitting coal-fired boilers to run on a mixture containing carbonisate and lignite. If carbonisate is produced from biomass, the resulting gas could be used as an energy fuel by burning it in a coal-fired boiler.