| Straw curing briquetting technology can densify unconsolidated straws, increase the energy density, improve the combustion properties, replace coals as clean energy used in every field of production and living, and solve storage and transportation problems. It is an important technical method of integrated and large-scale use of straws. However, it still needs to be deepened on the theoretical research of briquetting process properties and briquetting frictional heat, so as to reveal briquetting adhesive mechanism of straws, and provide theory support for predicting and controlling quality of briquetting products and optimizing equipments.Through the analysis of chemical compositions, physical properties and normal temperature briquetting mechanism of straws, it can be concluded that the key factor of impacting curing briquetting qualities under normal temperature is compression force and frictional heat in the briquetting process. Based on this conclusion, changes of riquetting forces and transportation of materials in the piston one-time stamping molding process are studied according to properties of piston-type briquetting technologies. Semi-closed briquetting model that suits to the piston-type briquetting technologies is built:materials are compressed in the closed environment equivalent to be made up of mould and briquetted products which remains stationary by the action of friction. The finite element analysis method is established in the primary compression stage of straw curing briquetting using the plastoelasticity, contact mechanics, viscoelastic and finite element theory. With this method, material deformation law, stress distribution and contact stress situation in the primary compression stage are studied, which provides method guideline to mold design and briquetting process improvement.Research on frictional heat in the briquetting process is carried out combing with piston-type nomal tempreture briquetting technologies based on tribology and heat transfer theroy. By introducing physical quantity such as average pressure and equivalent velocity, the equation of the frictional heat generation in the briquetting process is derived, which converts the frictional heat problem in the briquetting process into transient heat transfer problems without inner heat source under the second and third boundary condition. The finite element analysis model of briquetting frictional heat is built, and then the temperature field distribution caused by frictional heat is researched. The rule of temperature field variation caused by frictional heat is drawn through validating the correctness of finite element simulation by experiments. Based on the frictional heat simulation and analysis of prototype under different productivities conditions, it is shown that the tempreture of the mould is increasing and remain stable at a certain tempreture over time under the action of frictional heat, and the growth rate and steady temperature go up with the increasing productivities.On the basis of research on the rule of temperature field distribution caused by frictional heat, the experimental study of straw thermophysical parameters is carried out according to lack of thermophysical parameters of biomass materials currently. The change rule of thermal conductivity and specific heat with the changes of moisture content, tempreture and density is obtained. It can not only provide data support for the research on conduction of temperature field caused by frictional heat in briquetting products, but also provide basis for research on technologies related to heat of straws. The model of friction heat transfer in the straw briquetting process is built. And the distribution of frictional heat in briquetting products under different conditions is analyzed; aslo the cohesive action of lignin is studied. It is revealed that temperature field distribution in briquetting products is determined by both mold temperature and residence time that the briquetting products stay in the mold. With regard to the prototype in this paper, when the productivity is 60kg/h, the temperature field caused by frictional heat can reach 220-230℃, and when the productivity is 50kg/h, the temperature field is 180～190℃. However, because of the impact of residence time that the briquetting products stay in the mold, when the productivity is 50kg/h, the lignin in the center of briquetting products can also be softened, and the briquetting quality is high. This research can provide theoretical basis for improving briquetting process, briquetting quality and eqipment design. |
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