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 intensityofgasification, awell-distributed temperature, easy controlling and the capacityfor continuous and large-scale productions. So it is amatterof 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 confirmthe dynamic parameters in it. Through the kinetic modeling studyofgasification in the fluidized bed gasifier, we can notonlyget some reference data for designingthe 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 consideringthe 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 manyshortages. Advanced research can be proceededwith in severalways: (1) Model should be adjusted to apply inmore gasifiers. (2) Because the virtual diameterof bubble cannot be calculated, calculation of non-linear chemistry can be taken into account based on the formula of diametertransformation of ideal bubble. (3) The values of several parameters in different conditions should be confirmed. (4) The factorof high pressure can be added intothe model, and more experimental data to prove the validity of model is needed.