Study On Moving Evaporate-interfaces Dry Model Of Straw

With the high-speed development of global economy, the energy has already become a great factor of restricting economic development, forcing the scientists to transfer the focal point studied to the new energy development to replace fossil energy, and biomass energy which having pollution-free and getting abundant advantages attracts researcher's interest . In the past ten years scientists at home and abroad have got a lot of progression in this subject. The straw stalk obtained the large-scale use,the main use way is transforms the straw stalk gas, solid, liquid fuel, as the substitute of fossil energy. At present these transformed technologies already extremely were mature, but in the switching process of the application technology to the project direction, firmly exposed some key questions and the technical difficult problem, these new questions directly will cause under the biomass energy application project economic efficiency, will display the competitive ability in the market economy to be weak, will survive with difficulty, and how highly effective dry crops straw stalk, choose the appropriate dry operating mode and others dry questions will be a thorny research aspect. For this reason, the subject of development plan of national high-tech research (863 Program ): "The biomass gasification electricity generation optimization system and its the demonstration project" takes the present paper its sub- topic "the agricultural reject fluidized gasification process pretreatment engineering research" an important content development research.The mathematic model of straw constant temperature drying was firstly established in this article. It's supposed that the straw to dry was pellet one, and according to the humidity content in the drying process three physical models which were wet zone, evaporating zone and dry zone were separately established. And flow equation, heat transfer equation and mass transfer equation were established for every zone. For the wet zone, the humidity content was stable. Mainly it's the capillary flow and mass transfer of the free moisture content. For evaporating zone, on one side it's the drying of combined water, on the other side it's the flow and mass transfer of the evaporating of free water and partial combined water. For the dry zone, the humidity content was also stable with the steam flow as the main mass transfer.Constant temperature experimental studies were done with four different crop stalks, corn straw, cotton straw and wheat straw. The effect to dry process of different dry conditions for example original humidity content, drying temperature, variety and drying time was analyzed. And optimal dry process parameters were obtained. Regression analyses were done on the three sections of the drying process and regression equations were established separately. The main conclusions are in the following. It needs longer time to dry if the original humidity content is high, no matter how high the temperature is when to reach the same drying effect. The slope of the dry curve is relatively high when the drying temperature is 100℃, and the time needed is relatively short, ultimate humidity content is low, so the drying effect is also good, no matter what the original humidity content is. The higher is the drying temperature, the lower the ultimate humidity content, the shorter the time needed, and the better the drying effect is. For the same kind of straw, the time needed will decrease with the increase of drying temperature under the same original humidity content. For the four kinds of straws, the slope of the dry curve of corn is always the highest and the effect is also the best, no matter what the drying temperature is if only at certain one. The constant temperature drying process can be divided to three stages, pre-heating stage, stable speed drying one and down-speeding one. For the first, it fits parabola equation. Here the speed of water spreading is much higher than drying speed. For the second, it fits linear equation, here the two speeds are the same. For the third, it fits power equation. Here the speed of water spreading is much lower than drying speed.In the constant temperature drying experiment, the material added to the crucible every time was supposed as volume element. Then computation on relative parameter in the model was done according to assumption and experiential relational expression. According to the drying speed, the constant temperature drying process was divided into four stages, pre-heat stage, constant-speed stage, first down-speed stage and second down-speed stage. For the down-speed stages, the beginning point for the first one is the free humidity content 18% on the surface of the straw. And the beginning point for the second one is the free humidity content 0% on the surface of the straw. On the basis of the supposed volume element, according to the humidity content value on the surface of the straw, numerical calculation was done using different drying stage model. On the pre-heat, constant speed and first down-speed stages, the humidity content of the straw kept high. It fit the character of wet section in this article to establish drying model. So wet section equation was adopted for simulation. On the second down-speed stage, evaporating section in the drying model and drying section equation were adopted for simulation. At this time, the evaporating frontier was reducing and the drying of the combined water were the main reasons why the drying speed was deceasing further. The contrast between computation value and experiment value was done under the same original humidity content and different drying temperature or under the same drying temperature and different original humidity content. The result was that the two fit well. So it's proved that the mathematical model established in this article is correct. This provides theoretical and experimental evidence for the exploration of the general discipline for straw drying.