In recent years, the output of rice has exceeded 200 million tons in China, and a large amount of straw has been produced along with it. Straw is a renewable resource and has low emission of sulfur dioxide when burning, which makes it an ideal renewable and sustainable energy source for producing briquette [ 1 2 ]. In most developing countries, biomass resources are not utilized effectively, and unsustainable utilization of straw leads to straw becoming one of the sources of pollution in agriculture [ 3 ]. In addition, straw briquette instead of heating coal is more easily accepted by farmers in rural areas, and it is easier to popularize than straw returning to the field [ 4 ]. Studies have indicated that biomass briquette has 20% higher combustion performance than raw materials, and straw is one of the few proven, cost-effective, and renewable energy biomasses that can reduce COemissions [ 5 ]. The research on the utilization of crop straw has long-term significance for the recycling of agricultural waste, the improvement of rural life quality, and employment of farmers [ 6 ]. The use of biomass will promote the development of rural electrification and industrialization [ 7 8 ].
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3, and its volume was reduced by 8&#;10 times [As straw was usually distributed on a large scale, logistics operation costs were greatly increased [ 9 10 ]. Straw briquetting was an effective way of comprehensive utilization of straw resources. After the unconsolidated biomass was compressed, its density was increased to about &#; kg/m, and its volume was reduced by 8&#;10 times [ 11 ]. Currently, machines for producing briquettes include screw presses, roll presses, and piston presses (mechanical or hydraulic) [ 12 ]. The commonly used vertical ring mold pressing machine had the advantages of high yield and easy maintenance. Due to the randomness of the material in the extrusion process, the torsional vibration of the briquetting machine was random, which affected the stability and reliability of the machine [ 13 ]. In the production of solid biomass briquettes, process and material significantly affected the densification process and therefore the final quality of the briquette [ 14 ]. The piston flow theory was applied to the pressure of the screw press, and it was discovered that the optimal die entry angle depends on the yield strength of the compacting material and the friction coefficient of the interface between the die and the compacting material [ 15 ]. When the briquetting machine is working, the radial load causes the radial strain of the straw, and the inner wall of the ring die hole will hinder the radial strain of the straw, so radial pressure and friction will be generated, so that the straw is continuously compressed in the ring die hole [ 16 ]. The periodic variation of the extrusion force was the main reason for the vibration of the briquetting machine, which shortens the life of the ring die and reduces its reliability and stability [ 17 ]. At present, there are many research methods for straw briquetting machine, mainly to improve the performance and life of the machine. The productivity, forming quality, and service life of different types of ring modules were different in actual production.
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It is difficult to widely use and promote biomass briquette due to its low bulk density, low energy density, low mechanical strength, and high reactivity [ 18 19 ]. The properties of the briquettes mainly depend on the properties of the raw materials, such as particle size and moisture content, and the operating conditions, such as pressure and temperature [ 20 ]. In the preparation of a bean straw compacting block, the impact resistance of the briquette was over 96% at the pressing temperature of 80, the particle size of 4 mm, and the pressure of 100 MPa. Meanwhile, the interaction of pressure and temperature significantly affected density, impact resistance, and compressive strength [ 21 ]. In the process of corn straw compaction, the maximum compression stress and the feed quality had a significant effect on the dimensional stability coefficient of relaxed density and specific energy consumption, and the interaction between moisture content and maximum compression stress had a significant effect on the relaxed density and specific energy consumption [ 22 ]. The briquette must be characterized by high mechanical durability to avoid various types of damage due to impact and friction during transport and storage [ 23 ]. In order to enhance the performance and efficiency of straw briquette, it is necessary to carry out the corresponding experimental research and analysis on the physical characteristics of straw briquette.
This paper first provides the working principle of the briquetting machine for producing briquette. Considering the relaxed density and impact resistance of rice straw briquette as the responses, the effects of straw moisture content, temperature, clearance between die and roller, and rotational speed on the performance of the briquette were studied. The mathematical prediction model of the relaxed density and impact resistance of rice straw briquette were established. The optimal process parameters for producing briquette were solved through the established mathematical model. This provides a reference for producing high-quality straw briquette.