生物质洁净能源研究中的流化床动力学模型

分别对最小流化床、鼓泡流化床和腾涌流化床及相应的全混模型、鼓泡模型、气泡汇集模型等加以综述 ,分析其优缺点 ,并在此基础上提出动力学模拟研究的新思路 .根据流化床内在的本质———流化态的不同 ,将流化床分为最小流化床、鼓泡流化床和腾涌流化床三种 .总结了前人针对各种流化床提出的全混模型、鼓泡模型、气泡汇集模型等思想 ,建议今后可以在以下几个方面进行深入研究 :⑴使得模型更有普适性 .⑵由于气泡有效直径尚不能在理论上求得 ,可以在理想气泡直径变化公式的基础上 ,加入非线性化学的计算 .⑶确定不同情况下的参数 ,使得工作更有延续性 ,也使得模型更加具有生命力 .⑷从高压的角度去进行模型的计算 ,并得到相应的试验数据支持 .

An Important Topic in Biomass Clean Energy:Dynamic Models of Fluidized Beds

There are two kinds of gasifier, the fixed bed and fluidized bed. The latter is paid much attention because of its advantages as a high intensity of gasification, a well-distributed temperature, easy controlling and the capacity for continuous and large-scale productions. So it is a matter of primary importance to simulate the chemical and physics phenomena in the fluidized bed gasifier by the dynamic model. The substance of building kinetic model is to write a set of dynamic equations that fit one kind of fluidized bed gasifier, and to confirm the dynamic parameters in it. Through the kinetic modeling study of gasification in the fluidized bed gasifier, we can not only get some reference data for designing the gasifier, but also predict the trend of some important variables such as the heat value, the quantity of product gas, the components of product gas and the efficiency of energy transformation. The researchers classify the fluidized bed gasifiers by the fluidized velocity of liquid or gas. For different fluidized bed gasifiers, different researchers have built a lot of dynamic models. Though the kinetic equations and the parameters used in the models are different, the methods to resolve the questions have a common ground. Firstly, by considering the geometrical structure of gasifier and analyzing its symmetrical degree, the dimension of the model can be decided. Then, the phases of materials in the gasifier are also considered and several important hypotheses are made. Lastly, as they are difficult to simulate and have less importance the complicated terms are neglected. The governing equations and the hypotheses compose the mathematical model of biomass gasification kinetics. After confirming the boundary conditions and the initial values, one result can be obtained. Then the result is compared with the experimental data and the degree of satisfactory agreement indicates whether the model is right and whether the hypotheses are reasonable. Currently, many kinetic models, each of which has a distinct standpoint, have been established for different fluid beds. But none of these models has been recognized because there are not enough experimental data to confirm them. Further validation is required. We classify and summarize the kinetic models that exist currently by the types of fluid beds, modeling hypotheses, etc. Although the predicted values of existed models tally with experimental data well, the models have many shortages. Advanced research can be proceeded with in several ways: (1) Model should be adjusted to apply in more gasifiers. (2) Because the virtual diameter of bubble cannot be calculated, calculation of non-linear chemistry can be taken into account based on the formula of diameter transformation of ideal bubble. (3) The values of several parameters in different conditions should be confirmed. (4) The factor of high pressure can be added into the model, and more experimental data to prove the validity of model is needed.