A brief discussion on the properties of briquettes and binders

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In cold pressing of pulverized coal and medium-low pressure forming technologies, adhesive technology is crucial. The quality of the binder is an important guarantee for the quality of briquettes, and it has a direct impact on the combustion, operating conditions, and gas production capacity of industrial gas generators and industrial furnaces. Therefore, the selection and research of adhesives are particularly important. Briquette binders are substances that enable molecules of coal powder of different particle sizes to stick together. Such a medium not only needs to bind coal molecules together, but the briquettes formed through mechanical pressure due to this bonding must also possess certain cold strength and hot strength. Adhesives for industrial briquettes must meet the following performance requirements: 1. Cold strength. Briquettes (balls, rods) that have been bonded, shaped, and dried must possess a certain cold strength before being fed into the furnace. The cold strength of briquettes is not only necessary to ensure they remain intact during transportation, but it also has a direct impact on the load related to the thickness and weight of the coal balls once they are placed in the furnace. In the process of burning coal to produce gas in conventional industrial gas furnaces, the coal acts as a raw material and moves through the furnace in roughly three layers: the lowest layer is the combustion layer, the middle layer is the gas-generation layer, and the top layer is the coal distribution layer. During the gas production process, briquettes (balls, rods) are also subjected to the impact of compressed air and steam, in addition to the total weight of the coal inside the furnace. Therefore, briquettes bonded with adhesives must possess a certain load-bearing capacity (compressive strength), and this load-bearing capacity is ensured by a certain cold strength. The load-bearing capacity (compressive strength) of cold strength comes from the bonding index of the binder and the density within the briquette. The bonding index of the coal briquettes depends on the material of the binder, the proportion of its components, and the processing techniques used for the binder ； The density of briquettes largely depends on the mechanical equipment used and the pressing pressure. The cold strength of briquettes is insufficient; in industrial applications, the briquettes in the furnace do not undergo proper gasification, and they break apart due to their inadequate load-bearing capacity (compressive strength), leading to furnace collapse. Or, due to fragmentation, the scouring by compressed air and steam leads to an increase in the amount of material carried away, which not only wastes production materials but also easily causes the air nozzles to become clogged as a result of excessive amounts of such material, thereby affecting the normal operation of the furnace and gas production. Currently, based on the results of using briquettes made with adhesives domestically in various types of gas generators, it can be seen that for briquettes (balls or rods) produced from adhesives with an adhesive index that meets the requirements, their cold strength – namely the compressive strength per ball under static pressure after drying or air-drying – must be between 65 and 95 kg in order to be suitable for use in industrial gas generators. 2. Heat intensity. During the combustion process in industrial boilers, industrial furnaces, and gas generators, the heat intensity of briquetted coal (coal balls, coal sticks) is also a key parameter. At temperatures between 950 and 1250°C, the melting point of coal pellets has a significant impact on the proper operation of furnaces. During the combustion process in boilers and industrial furnaces, it is necessary for the coal pellets to remain intact and not disintegrate, so as to ensure complete combustion, increase the gasification rate of the coal pellets, or facilitate combustion. During the combustion process in a gas generator, it is necessary for coal lumps to form a structural framework. A too low ash melting point of briquettes leads to melting and flowing at certain temperatures, resulting in phenomena such as furnace melting and slag formation, which have a significant impact on the proper operation of the furnace. Therefore, sufficient attention must be paid to the thermal strength of briquettes (coal balls, coal sticks) made from coal powder. Generally speaking, in addition to the physicochemical properties of the coal itself, methods to improve the thermal strength of coal pellets involve adding, for those coal types with a low ash melting point, certain clays, bentonites, or kaolins with a high aluminum oxide content and good plasticity at room temperature. This helps to increase the ash melting point of the coal powder after it is formed into pellets, thereby meeting the requirements regarding the combustion performance of various furnaces. (It should be noted here that these materials are almost all non-combustible; adding too much of them will affect the fixed carbon content of the briquettes themselves.) Practice has shown that, in order to increase the ash fusion point of briquetted coal (coal balls, coal sticks), the amount of substances added to provide structural support should be 3–5%. In some cases, the mineral content and ash structure of the coal powder itself are sufficient to provide such support, so no additional substances are needed to ensure the appropriate level of fixed carbon. To ensure that these additives can combine uniformly and effectively with coal molecules to serve as a solid support. The fineness of these additives should be maintained between 80 and 120 mesh. 3. Drop strength. During the addition of coal raw materials to industrial furnaces and gas generators, there is often a certain height difference, which requires the manufactured briquettes (coal balls, coal rods) to have a certain strength under impact. This criterion requires that when coal pellets are dropped freely from a height of 1.5 meters onto a hardened surface, their degree of fragmentation should be greater than 13 mm. The dropping strength is determined by the adhesive penetrating into the pores of the coal particles, being shaped under mechanical pressure, and then solidified by drying equipment; it represents the combined effect of the adhesive’s binding force and mechanical pressure. 4. Water immersion strength. The water immersion resistance here is what people commonly refer to as waterproof performance. In the past, many companies would convert pulverized coal into shaped coal, which was then sold as lump coal. It took several days, or even a month, for this shaped coal to reach the end-users where it could be burned. During transportation and storage, humidity in the air could be absorbed by the dry coal particles due to weather conditions or regional factors, resulting in a decrease in its strength; therefore, water resistance was also considered an important criterion when evaluating adhesives. However, as people came to realize that briquetted coal could replace the increasingly expensive lump coal for use in combustion, and as new research and applications in the field of binders were recognized, many enterprises that relied on lump coal decided to adopt briquetted coal. This was done both to diversify their energy sources and to save significant amounts of costs related to energy and raw materials by using briquetted coal instead of lump coal. Enterprises that used large quantities of lump coal set up their own briquetting production lines; the resulting briquettes were used directly in various furnaces or stored for only a short period of time before being used. This reduced the need for transportation, and since the briquettes were exposed to moisture for less time, their water resistance became less important. However, in coastal areas and during the rainy season, humid air still causes erosion of the produced briquettes, leading to a decrease in their strength. Due to the sensitivity of waterproofing agents to temperature and cost considerations, the author recommends adding an appropriate amount of high-purity glass cleaner to the adhesive, which still provides a certain level of waterproofing effect. 5. Thermal stability. The thermal stability of coal during combustion primarily depends on the volatiles present in its composition. For anthracite, poor thermal stability is mainly due to the presence of crystalline water within the coal. The addition of heat stabilizers to the binders can have an impact on the quality of briquettes. For coal types with poor thermal stability, the use of heat stabilizers helps prevent the briquettes from cracking or disintegrating before reaching their ash fusion point. It can be seen from the above points that: 1. Adhesives play a key role in the quality of industrial briquettes. 2. Coal types are selective regarding binders; different coal types and qualities require the use of different binders. 3. Adhesives with heat stabilizers have a certain influence on the changes in coal quality. 4. Different adhesives may require different molding pressures. There are a wide variety of binders used for industrial briquettes, but broadly they can be divided into three categories: first, organic binders. Such as: coal tar pitch, coal tar, starch, animal and plant tar residues, waste liquids, lignocellulose, and high-molecular polymers such as PUA and PAA. II. Inorganic substances. Such as: clay, bentonite, kaolin, cement, water glass, quicklime and slaked lime, calcium carbide mud, sulfates, etc. III. Composite classes. Organic and inorganic loads, such as coal tar and low-consistency pulp. Combination of organic and inorganic substances, such as the combination of bentonite with sodium polyacrylate and disodium hydrogen pyrophosphate. Currently, the humic acid binders widely used are mainly sodium humate binders produced by thermally decomposing lignite with a humic acid content of 50–70%, which has been finely ground to a particle size of 80–120 mesh, and then mixed with industrial caustic soda as well as certain proportions of other materials depending on the type of coal. Its features include minimal impact on the fixed carbon content of raw coal, good activation properties of the briquettes produced, low costs for the binder materials with small amounts required. The cold strength, hot strength, thermal stability, ash fusion point, as well as the strength after drying of the briquettes produced, all meet the requirements for industrial use, which is why it is widely accepted and utilized. However, due to different preparation processes, varying ingredient ratios, and different additional materials used, the various parameters of the briquettes produced also differ. Briquette binders are, in themselves, an experimental science that must be developed through experiments, with improvements made based on the results of those tests; otherwise, it is impossible to produce qualified briquette products that meet the requirements of industrial production and gas generation. Binder materials for briquettes are being continuously developed, with particularly bright prospects for composite binders. These composite binders utilize different materials with various bonding properties; through pyrolysis, these materials complement each other, resulting in a synergistic effect that enhances the multifunctionality of the binder. This, in turn, ensures that all the required performance standards are met in the briquette manufacturing industry.