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As a type of clean coal technology, briquette technology requires relatively less investment compared to other such technologies, making it an appropriate clean coal technology for China’s national conditions. However, in recent years, the quality of briquettes in our country has been poor and production costs high, making large-scale industrial adoption difficult. This is mainly due to the poor performance and low cost of the binders used for briquetting, with binder technology becoming one of the key factors restricting the development of briquettes. Currently, the binders used for briquetting coal fall into three categories: organic, inorganic, and organic-inorganic composite. However, briquettes made with organic binders have low thermal strength; some of these formulations cause secondary pollution during combustion, and they are also costly ； Briquettes made using inorganic binders have a higher ash content and a lower fixed carbon content, and some of these formulations exhibit flame-retardant properties ； The use of organic-inorganic composite binders complicates the production process of briquettes, increases costs, and makes it difficult to control measurements during production. In recent years, some researchers have also used biomass to produce briquette binders, which can improve energy utilization efficiency and reduce environmental pollution caused by simple direct combustion. However, issues such as incomplete combustion leading to black smoke have arisen, making this a topic of interest among scholars. In this study, corn straw and wheat straw were used as the main raw materials for briquette binders, which were modified using NaOH. The effects of the amount of modified biomass added on the mechanical strength and ignition temperature of the briquettes were investigated, as well as the impact of a composite binder composed of inorganic binders and modified biomass on the water resistance of the briquettes. 1 Experiment 1.1 Experimental Materials and Equipment (1) Corn straw and wheat straw: Obtained from rural areas near Taiyuan City, Shanxi Province; after natural drying, the corn straw and wheat straw were processed into strips with a length of 5 cm to 7 cm for use. (2) NaOH, analytical grade, Tianjin Chemical Reagents No. 3 Factory. (3) Raw material coal blending: A mixture of Yuci anthracite and Ningwu bituminous coal (with a blending ratio of 3:1), which is crushed to below 3 mm using a crusher and set aside for use. (4) Additive: MgO, analytical grade, Beijing Tongguang Fine Chemicals Company ； MgCl2, analytical grade, produced by Tianjin Chemical Reagents No. 3 Factory. 1.2 Industrial analysis of coal and straw: The raw coal and straw used in the experiments were subjected to industrial analysis using a CT5000A multi-functional calorimeter (developed by China University of Mining and Technology), a CTM300 automatic temperature controller (also developed by China University of Mining and Technology), and a WDL-9 microcomputer-based rapid sulfur determination instrument with Chinese display (developed by Hebi Kelice Control Technology Co., Ltd.). The analysis results are shown in Table 1. 1.3 Testing methods for the physical properties of briquettes: The performance of modified biomass binders is reflected and measured through the physical properties of briquettes, which mainly include compressive strength, drop strength, water immersion strength, and re-drying strength. The methods for determining these properties are as follows. 1.3.1 Compressive strength is determined using the XY-01 briquette hydraulic compressive strength tester (manufactured by Shunda Factory in Niulanshan, Shunyi, Beijing). The briquettes were placed one by one at the center of the force-applying surface of the specified testing machine, and a force was applied in one direction at a specified uniform displacement rate. The pressure displayed by the testing machine at the moment the briquette cracked was recorded. The arithmetic average of the values obtained for each briquette was used as the compressive strength of the biomass briquettes, expressed in N per unit. 1.3.2 Drop strength shall be tested in accordance with the method specified in GB/T15459. 1.3.3 Immersion strength shall be tested in accordance with the method specified in MT/T749-1997. The key points of the testing method are: a certain amount of briquettes is immersed in water at room temperature for 24 hours, then taken out, and subsequently tested using the compressive strength method. 1.3.4 Complex dry strength: A certain amount of briquettes is soaked in water at room temperature for 24 hours, then taken out, dried at (105±5)°C and cooled to room temperature to reach an air-dried state, after which it is tested using the method for compressive strength. 1.3.5 Ignition point The combustion experiments of biomass briquettes were carried out in an SGM5800 artificial intelligence box-type resistance furnace (produced by Luoyang Sigma Instrument Manufacturing Co., Ltd.). The resistance furnace used enables precise temperature control and measurement, and is equipped with a digital display that shows the current temperature in real time, facilitating the observation and recording of data. During the experiment, the temperature was increased at a rate of 20°C/min to observe the changes in the combustion behavior of the briquettes, and the ignition points of various biomass briquettes were determined based on these changes. 1.4 Research Content 1.4.1 Effect of NaOH modification solution concentration on the performance indicators of briquettes Different concentrations of NaOH modification solutions (0%, 0.5%, 1.0%, 1.5%, 2.0%, and 2.5%) were prepared to modify the biomass; the amount of biomass used was kept at 10%. Following the established manufacturing process, briquettes were formed from this biomass, and various physical properties were tested and compared in order to study the impact of different NaOH modification solution concentrations on the mechanical strength of the briquettes. 1.4.2 Effect of the amount of modified biomass on the performance indicators of briquettes: The NaOH modification solution determined in Experiment 1.4.1 was used to modify the biomass in order to produce biomass briquettes; the optimal amount of biomass to be used was determined, and further studies were conducted to examine the impact of different amounts of biomass on the mechanical strength of the briquettes. 1.4.3 Effect of composite binders on the performance parameters of biomass briquettes: To improve the water resistance of the briquettes prepared as mentioned above, an appropriate amount of MgO and MgCl2 was added to the briquettes, forming a composite binder together with the modified biomass, in order to enhance their water resistance. The effect of these additives on the strength of the briquettes was also investigated. 1.4.4 Effect of composite binders on the ignition temperature of briquettes: An automatic temperature controller and a smart resistance furnace were used to determine the ignition temperatures of briquettes made with inorganic binders (MgO and MgCl2) compared to those made as described in 1.4.3. By observing the combustion status of the briquettes and recording the real-time temperature displayed by the resistance furnace, the trend in the ignition temperature of the briquettes after adding corn straw and wheat straw was examined. 2 Results and Discussion 2.1 Effect of NaOH modification solution concentration on the performance indicators of briquettes According to research method 1.4.1, the experimental results shown in Figures 1 and 2 were obtained. As can be seen from Figures 1 and 2, all briquette samples exhibit high drop strength and compressive strength, meeting the requirements of the Shanxi Province standard DB14/133-2005. The strength changes differently as the NaOH concentration varies. Regarding the drop strength of briquettes, the biomass briquettes prepared from wheat straw and corn straw showed a consistent trend: when the mass fraction of the NaOH solution was between 1.0% and 2.0%, the drop strength of the briquettes was high and relatively stable ； When the mass fraction of the NaOH solution is between 1.0% and 1.5%, the compressive strength of both types of biomass briquettes is high; within this concentration range, the compressive strength of corn straw briquettes is higher than that of wheat straw briquettes. To examine the effect of NaOH modification on biomass, its structure was observed under a microscope. Figure 3 on the next page shows the surface structure of wheat straw after modification at different concentrations. As can be seen from Figure 3 on the next page, in the absence of NaOH, the surface structure of wheat straw is neat and orderly, with very small gaps between the structures (see Figure 3(a)) ； The overall structure of the straw modified with a 0.5% NaOH solution changed little, with some voids appearing (see Figure 3(b)) ； After modification with a 1.0% NaOH solution, more voids appeared in the straw structure, resulting in a more complex spatial structure (see Figure 3(c)) ； After modification with a 1.5%–2.0% NaOH solution, the pores in the straw structure become larger and more porous, and interconnection between the structural elements occurs (see Figures 3(d) and 3(e)) ； After modification with a 2.5% NaOH solution, the spatial structure becomes relatively simple, and the straw turns into filamentous shapes (see Figure 3(f)). The above phenomena indicate that when biomass is modified with NaOH at 90°C, the lignin in some fibrous substances decomposes, which is due to the separation of cellulose and hemicellulose, resulting in the formation of sugar compounds with adhesive properties. Once this mucus solidifies, it can form complex three-dimensional network structures. When the modified biomass is mixed with raw coal, the complex spatial network structure formed by its fiber structure traps a large number of coal particles, and the briquette is formed into a solid with high strength due to the compressive force applied. As the concentration of the NaOH-modifying solution increases, the lignin in the modified straw is decomposed more completely, resulting in more viscous substances that can mix more evenly with the raw coal; consequently, the strength of the briquettes is also higher. However, as the NaOH concentration increases (e.g., above 2.0%), the degree of lignin decomposition further rises, and the fiber structure of the straw is essentially completely destroyed, which in turn reduces the strength of the briquettes; therefore, it is not advisable to use NaOH solutions with too high a concentration for modifying biomass. Based on the above analysis, it is appropriate to use a NaOH solution with a mass fraction of 1.0%–1.5%. 2.2 Effect of the amount of modified biomass added on the performance indicators of briquettes: The experimental results, obtained according to experimental method 1.4.2, are shown in Figures 4 and 5. As can be seen from Figures 4 and 5, within the biomass addition range of 2% to 20%, all briquette samples exhibited high drop strength and compressive strength, meeting the requirements of the Shanxi Province standard DB14/133-2005. As the amount of biomass added increases, the drop strength and compressive strength of the briquettes increase significantly. This indicates that the more biomass is used, the more network structures are formed to bind together the coal particles, making it easier to hold them together; as a result, the briquettes obtained after applying molding pressure have higher strength. In the tests on immersion strength and re-drying strength, all briquettes broke apart after being soaked for 24 hours, with no strength detectable. It can be seen that when biomass is added to briquettes, these briquettes exhibit higher drop strength and compressive strength; however, their water resistance is poor. This is mainly because substances such as sugars and sodium silicate, which act as binders on the surface of coal particles, are water-soluble and dissolve when exposed to water. Moreover, undecomposed straw also swells when wet, causing the briquettes to disintegrate under the influence of water, resulting in extremely low water resistance. Therefore, further measures need to be taken to improve the water resistance of briquettes. 2.3 Effect of composite binders on the performance indicators of biomass briquettes: In accordance with experimental method 1.4.3, while adding a fixed amount of MgO and MgCl2, the amount of biomass used was varied; the experimental results are shown in Figures 6 to 9 on the following page. As can be seen from Figures 6 to 9 on the next page, after adding a specified amount of MgO and MgCl2, the physical properties of the biomass briquettes were superior to those of the briquettes without inorganic binders (see Figures 4 and 5). Moreover, these briquettes exhibited high water absorption strength, meeting the requirements of the DBl4/133-2005 standard, as well as high re-drying strength. As the amount of modified straw added increases, both the water absorption strength and the re-drying strength decrease. This is likely because, when the briquettes are exposed to water, the undecomposed straw absorbs water and swells, thereby reducing the water absorption strength and re-drying strength of the briquettes. Therefore, to improve the water resistance of briquettes, an appropriate amount of MgO and MgCl2 can be used. Form a composite binder with modified biomass straws. 2.4 Effect of composite binders on the ignition temperature of briquettes: Based on study item 1.4.4, the ignition temperatures of briquettes made from inorganic binders (MgO and MgCl2) and those made according to study item 1.4.3 are shown in Table 2. As can be seen from Table 2, as the amount of modified straw added increases, the ignition temperature of the biomass briquette gradually decreases. This is because as the amount of straw added increases, the proportion of combustible volatiles in the briquette rises, thereby gradually lowering the ignition temperature of the biomass briquette. The degree to which the ignition temperature of briquettes decreases varies depending on the type of modified biomass used. When modified corn straw is added, the reduction in the ignition temperature of briquettes is greater compared to when modified wheat straw is used; this is because the combustible volatile content in the corn straw used in this study is higher than that in wheat straw. Compared with briquettes made from inorganic binders (MgO and MgCl2), the ignition temperature of biomass briquettes is significantly lower. This is because biomass has a high volatile content, which makes it easy to ignite; once biomass catches fire, it quickly ignites the coal surrounding it, thereby reducing the overall ignition temperature of the briquettes. 3 Conclusions Through the above research, the following conclusions can be drawn: 3.1 The mixture obtained by modifying corn and wheat straws with NaOH solutions at concentrations of 1.0% to 2.5% can be used as a binder for biomass briquettes; the proportion of biomass used can exceed 20%. As the amount of modified biomass increases, both the mechanical strength (falling strength and compressive strength) improve, but the water resistance of these biomass briquettes is poor. 3.2 The use of a modified biomass combined with composite binders made of MgO and MgCl2 enables briquettes to possess high mechanical strength and good water resistance, making it a binder with promising practical applications. 3.3 Compared with briquettes made from inorganic binders (MgO and MgCl2), the ignition temperature of biomass briquettes is significantly lower; when the biomass content is 20%, the ignition point of the briquettes can be reduced to below 510°C. In summary, by using modified biological straws as binders for biomass briquettes, along with inorganic binders (MgO and MgCl2), high-performance biomass briquettes can be produced. The production process is simple, and by replacing mineral coal with renewable biomass, costs are saved and environmental pollution is reduced. This is a production method that aligns with industrial policies and is worth promoting.