Experimental Study And Theoretical Analysis Of Stalk Drying Process

Bio-mass energy is the only storable and transportable renewable energy, and for the time being, is the most hopeful substituting energy to realize the utilization at a large scale. The application of the bio-mass energy is very significant to improve the energy structure, guarantee the energy safety and reduce the environmental pollution. In recent ten years, the research on the applicable technologies of bio-mass has been developed vigorously. However, no matter which kind of utilizing method (gasification, liquefaction or solidification etc.) is used, the bio-mass energy requires drying and it can only become useful after the moisture content is reduced to 15% or lower. Therefore, the fast and highly-efficient drying of the crop stalks, which occupy a leading position in the bio-mass energy, was studied as an important part of "Research on Pre-processing Technology during Gasification Process of Fluidized Bed for Agricultural Waste", the sub-subject of "Gasified Bio-bass Power Generation Optimized System and Its Model Project", which is the program project of the tenth five-year plan and "863" program.In the paper various tests were conducted regarding the physical and chemical characteristics, thermogravimetric characteristics, isothermal drying characteristics and microstructure characteristics of stalks and the influence of such characteristics on the drying process is pointed out. In the test of isothermal drying characteristics of stalks, the best drying temperatures of four kinds of stalks under different working conditions were concluded. This paper raises the view that the influence of proper increase of moisture content of the stalk on the drying time can be neglected. By the stalk microstructure test, the distribution rule of the stalk drying framework and void as well as the moisture transferring path are put forward for the first time.Taking the volume averaging theory as the guide and taking the heat/mass balance as the starting point, the theoretical analysis on inward-moving gasification interface during the stalk drying process was set up. By taking the moving gasification interface as the dynamic effects of the manifestative drying process, the boundary of the dry and wet zones of the manifestative unit was redefined. After fully considering the influence of wall temperature zone T_w at the gasification interface on the diffusion coefficient D_T, the boundary conditions with wall temperature T_w as the variable were established. In the drying phase, the resistance of moisture diffusion becomes significant, and thus the inward-moving coefficient isintroduced to indicate such a resistance to avoid the increase of calculations brought by the use of complex formulas.From the comparison of the tested values and computational values, the manifestative free moisture conductivity resistance coefficient H, diffusion resistance coefficient F and inward-moving coefficient A are obtained, and the calculated values are corrected with the above-mentioned coefficients. The final corrected values are consistent with the tested values, thus it has testified the accuracy of the theoretical analysis;finally, it produces the expression of the drying speed during the constant-speed drying phase under different drying temperatures and the calculation formula of drying speed during the speed-reducing drying phase at a drying temperature of 130℃ for the com stalk with an initial moisture content of 70%.A set of crate type stalk drying tester was designed in an optimized way based on the basic data obtained in the stalk drying characteristics test and the returns of the theoretical analysis. The "jet flow heating plate" is suggested dexterously to supply different heating schemes and different energy for the dry medium on different heating plates, which has realized the energy supply depending on the demand in different drying phases. The experimental study was also conducted on the designed stalk drying tester, which has testified that the division of three phases of stalk drying, raised in the isothermal drying test, and the energy demand in different phases are measured to prove the accuracy of the theoretical analysis. In the further experiment, the various technical indexes of the tester is tested, and the results are as follows: output of 208.6kg/h, thermal energy utilization rate of 71.2%, and the dried stalk moisture content unevenness of 2%, which has met the design requirements and is significant for guiding the optimized design of the industrialized production of stalk drying equipment.