PROBLEMS OF INCREASING THE HIGH COMBUSTIBILITY OF COAL BRIQUETTES AND ANALYSIS OF ADDITIVES

ABSTRACT

This study investigates the influence of industrial and agro-processing waste additives on the combustion properties and mechanical strength of coal briquettes. Paraffin residues, distillery stillage, and fruit kernel shells were introduced as supplementary binders in varying proportions under controlled briquetting conditions (90–100 °C, 20 min thermal mixing, 2.4 kN compressive force). The results demonstrate that increasing the additive content enhances porosity but reduces strength, with an optimal range identified at 10–30% for hard kernel shells. Spent clay and carbon adsorbents, rich in oils, fatty acids, and paraffinic compounds, were found to act both as energy-contributing components and natural binders. Experimental results closely matched theoretical predictions, with deviations not exceeding 3.2% and correlation coefficients of R² ≥ 0.99. The findings highlight the potential of waste valorisation in briquette production, offering a low-cost, energy-efficient, and environmentally sustainable approach to solid fuel development.

АННОТАЦИЯ

В данной работе исследовано влияние промышленных и агроперерабатывающих отходов на горючие свойства и механическую прочность угольных брикетов. В качестве дополнительных связующих использовались парафиновые отходы, спиртовая барда и скорлупа плодовых косточек, введённые в различных пропорциях при контролируемых условиях брикетирования (90–100 °C, 20 мин термического перемешивания, усилие прессования 2,4 кН). Результаты показали, что увеличение количества добавки повышает пористость, но снижает прочность; оптимальным диапазоном признано содержание твёрдой косточковой скорлупы на уровне 10–30 %. Отработанные глинистые и углеродные адсорбенты, содержащие масла, жирные кислоты и парафиновые соединения, проявляют двойной эффект: служат энергетически значимыми компонентами и одновременно природными связующими. Экспериментальные результаты хорошо согласуются с теоретическими расчётами (расхождение не превышает 3,2 %, коэффициент корреляции R² ≥ 0,99). Полученные выводы подтверждают перспективность утилизации отходов в технологии брикетирования, что обеспечивает выпуск низкозатратного, энергоэффективного и экологически устойчивого твёрдого топлива.

Keywords: coal briquettes; waste binders; paraffin residues; distillery stillage; fruit kernel shells; mechanical strength; porosity.

Ключевые слова: угольные брикеты; связующие из отходов; парафиновые остатки; спиртовая барда; скорлупа плодовых косточек; механическая прочность; пористость.

Introduction

At the present stage of expanding the raw material base for thermal power plants in Uzbekistan, particular attention is being given to the exploration and development of new coal deposits, as well as to the rational utilisation of fine coal fractions through briquetting with efficient binding agents. Considering that up to 60% of small coal particles may be generated during mining and transportation, the choice of an effective binder becomes both a practical necessity and a pressing issue in briquette production [4,5].

Currently, clay-based adsorbents are widely used in Uzbekistan’s vegetable oil and petroleum-refining industries for the purification of plant and mineral oils. In these sectors, repeated use of such adsorbents in adsorption purification processes—up to ten cycles—results in the formation of solid clay waste containing residual oils and fats [6]. From both economic and environmental perspectives, their regeneration is presently regarded as inefficient; hence, these wastes are transported to disposal a site, which in turn requires considerable transport costs.

Every year, more than 10,000 tonnes of oily clay waste are generated in the republic. These residues negatively affect soil conditions, hinder plant growth, and exert a detrimental impact on the ecological biosphere. Moreover, the oils and residues contained in the clay waste remain unutilised, which ultimately contributes to increased production costs within the relevant industries.

Materials and methods

Taking these factors into account, the study focused on examining the composition of spent clay-based adsorbents (waste products) generated at selected vegetable oil and petroleum-refining plants in Uzbekistan. The key properties of the used clay adsorbents were determined in accordance with the officially approved standards applied in the respective industries [7,8]. The results of this analysis are presented in Tables 1 and 2.

As shown in Table 1, spent adsorbents from vegetable oil enterprises (which are not subject to regeneration) contain more than 50% of the overall clay mass, while over 2.2% consists of free fatty acids. When these materials are employed as binders in coal briquette production, the resulting briquettes exhibit good burning characteristics during household use, releasing a significant amount of heat. In such cases, the non-fat fractions (up to 1.4%) pass into the ash composition.

Table 1.

Chemical composition of oily waste clay adsorbents from vegetable oil enterprises

Table 2.

Chemical composition of oily waste clay adsorbents from petroleum-refining plants

When examining the composition of spent clay adsorbents containing oils at petroleum-refining plants, a similar pattern was observed: mineral oils accounted for more than 47%, resins for 3.4%, and paraffin for 3.7%. All these organic compounds combust together with coal, while the inorganic components (up to 3.5% of the total clay mass) remain in the ash fraction.

In certain cases, to improve the efficiency of adsorption purification of vegetable and mineral oils, special composite materials consisting of clay and carbon-based adsorbents with differing properties are employed (see Tables 1 and 2). This is explained by the fact that carbon adsorbents demonstrate superior oil and fat absorption capacity compared with clay adsorbents, whose overall porosity is comparatively lower [9].

Table 3.

Chemical composition of waste composites derived from clay and carbon adsorbents in vegetable oil enterprises

To evaluate the feasibility of reprocessing such composites for briquette production, their composition was analysed using established methodologies [10,11]. The findings presented in Table 3 demonstrate that spent composites of clay and carbon adsorbents generated at vegetable oil enterprises possess potential for application in coal briquette manufacturing.

The residual oils, free fatty acids, and other organic constituents contained within these composites enhance the heat release during combustion, thereby improving the calorific performance of the briquettes. At the same time, the clay fraction functions effectively as a binder, facilitating the formation of briquettes with different levels of mechanical strength.

These results demonstrate dual benefits of industrial waste valorisation: on the one hand, contributing to the efficient utilisation of by-products otherwise destined for disposal, and on the other hand, supporting the sustainable production of energy-rich solid fuels. Such integration of waste management with briquette technology underscores the broader potential for resource efficiency and circular economy practices in Uzbekistan’s energy and processing industries.

Table 4.

Physicochemical characteristics of clay–carbon adsorbent composites generated in petroleum-refining plants

As shown in Table 4, the spent composites of clay and carbon adsorbents derived from petroleum-refining plants consist of 50–56% mineral oils, 4.0–5.8% resins, 4.0–6.3% paraffin, together with other inorganic admixtures. These characteristics suggest that such materials can be effectively utilised in briquette production, where they serve as binders for fine coal particles, enhancing both agglomeration and combustion properties.

In parallel, considerable amounts of combustible residues—particularly fruit kernel shells—are generated in canning industries. In Uzbekistan alone, more than 200,000 tonnes of fruit kernel shells are produced annually and are currently disposed of as waste. These biomass residues, however, represent an underutilised renewable resource with significant potential for energy recovery and sustainable fuel production. Their incorporation into briquette technology not only provides an efficient means of waste valorisation but also contributes to reducing environmental burdens while diversifying the raw material base for solid fuels.

Results

 To rationally utilise these wastes for coal briquette production, residues from fruit-processing factories were incorporated into the briquette mixtures. After grinding, 10–40% of their fraction displayed a particle size of 3–5 mm. Their moisture content ranged from 7–9%, while the sugar content varied between 0.5 and 1.2%.

Figure 1 illustrates the change in briquette strength when a certain proportion of hard fruit kernel shells (curve 1) and soft fruit kernel shells (curve 2) was added to the coal briquette composition. As can be seen from the graph, the incorporation of hard kernel shells (curve 1) and soft kernel shells (curve 2) up to 30% leads to an increase in briquette porosity, which rises exponentially with additive content. However, when the proportion of hard and soft kernel shells in the briquette composition exceeds 30%, the mechanical strength of the resulting briquettes begins to decline.

These findings suggest the existence of an optimal substitution threshold of approximately 30%, where porosity is enhanced without a significant loss in structural integrity. Beyond this threshold, excessive porosity reduces the cohesive strength of the briquettes, thereby compromising their mechanical durability. This trade-off highlights a critical balance between maximising combustion efficiency (through higher porosity and volatile release) and maintaining sufficient mechanical strength for handling, storage, and transportation.

From a practical perspective, the integration of fruit kernel residues at controlled levels demonstrates both an effective pathway for agricultural waste valorisation and a means of diversifying the raw material base for solid fuel production in Uzbekistan.

1 – hard fruit kernel shells; 2 – soft fruit kernel shells.
Figure 1. Variation in the porosity of coal briquettes depending on the proportion of additive introduced

The type of additives incorporated into the briquettes influences their calorific value. Taking this into account, it becomes necessary to investigate the impact of such additives on the energy efficiency of the resulting product. Furthermore, as briquette porosity increases, their mechanical strength tends to decrease.

Based on these considerations, we examined the effect of additive content on the strength of the produced coal briquettes, when incorporating hard fruit kernel shells (curve 1) and soft fruit kernel shells (curve 2), as illustrated in Figure 2.

1 – hard fruit kernel shells; 2 – soft fruit kernel shells.
Figure 2. Effect of additive content on the mechanical strength of coal briquettes

Discussion

The results obtained demonstrate that the incorporation of fruit kernel shell additives significantly affects both the porosity and mechanical strength of coal briquettes. An optimal substitution level of approximately 10–30% was identified, at which the briquettes retain sufficient mechanical integrity while benefiting from enhanced porosity and improved combustion efficiency. Beyond this threshold, the excessive porosity leads to a decline in strength, thereby limiting the applicability of the briquettes for storage and transportation.

The close agreement between experimental and theoretical values (within 3.2%, with correlation coefficients R² ≥ 0.99) confirms the robustness of the experimental design and the reliability of the analytical methods employed. Importantly, the study highlights the dual role of additives: organic residues such as oils, fatty acids, and fruit kernels increase calorific value and combustion performance, while clay components act as natural binders, ensuring material cohesion.

From a broader perspective, these findings underscore the potential of industrial and agro-processing wastes—paraffin residues, distillery by-products, and fruit kernel shells—as valuable secondary resources in briquette technology. Their effective utilisation not only reduces environmental burdens associated with waste disposal but also contributes to the development of low-cost, energy-efficient, and sustainable solid fuels.

Figure 3. Briquette products produced at 90 °C with 20 minutes of mixing

Figure 4. Briquette products produced at 95 °C with 20 minutes of mixing

Figure 5. Appearance of briquette samples produced at 100 °C after 20 minutes of thermal mixing

Conclusion

The experimental results confirmed that additive type, content, and processing temperature are decisive parameters in coal briquette production. Increasing the proportion of fruit kernel shells enhances porosity but significantly reduces strength, with 10–30% incorporation identified as the optimal range for maintaining mechanical durability. Soft kernel shells lead to a faster decline in strength due to their higher inherent porosity, whereas hard shells provide a more favourable balance between combustion efficiency and structural stability.

Processing temperature was shown to be equally important: briquettes produced at higher mixing temperatures (95–100 °C) exhibited greater cohesion and mechanical integrity compared with those processed at lower temperatures. The exponential relationships observed between porosity and strength, with correlation coefficients of R² ≥ 0.99, validate the reliability of the experiments and confirm the soundness of the applied methodology.

The study further demonstrated that industrial adsorbent residues from oil-refining and vegetable-oil industries, which contain mineral oils, resins, paraffins, and clays, serve a dual function: the organic fraction improves calorific performance during combustion, while the clay fraction ensures structural binding. This dual role underscores the potential of these waste streams as both functional additives and binding agents.

Overall, the integration of paraffin waste, distillery stillage, and agro-residues into briquette formulations represents a sustainable pathway for transforming industrial by-products into energy-efficient solid fuels. Such valorisation reduces environmental burdens associated with waste disposal, lowers production costs, and contributes to the development of alternative fuel sources that align with circular economy and sustainability principles.

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