Theoretical And Experimental Studies On Biomass Fluidized Combustion

As a brand-new biomass direct combustion technology, biomass circulating fluidized bed combustion technology has come to the popularization and extension stage. This article introduces the mainstream technologies of biomass combustion in China. Here, the present situations, existing problems and development prospects of these technologies, as well as the efficient solution and probable improvement methods are discussed. Therefore, this article is providing the technical advice by making through research in the fields of:1) shrinkage and fragmentationcharacter,2) mixing and separating character of biomass and bed material,3) thermal conduct character,4) reaction dynamic character and5) deposition problem.Aiming at collecting the information of the shrinkage, deformation and fragmentation of biomass particles during the combustion process, a small sized biomass semi-char fragmentation research system is built. Utilizing this test system and "fire immediately extinguish method", the size distribution of semi-char particles are studyed and analyzed, and the law of biomass char fragmentation is expressed into a combination of equations. Finally, a mathematical model is set up with the data from our experiments, which is tally with the actual situation and meets the practice.Initial stage of this project, the cold phase modeling theory is adopted and the test rig is built in which the "small-room sampling method" is developed by our own research group. Two main aspects influencing the biomass heat release are extracted during combustion:1)kinds of biomass and2)size distribution of bed material. By arranging these two facts, several different combinations are obtained. Under different combinations, the way of separation characteristics between the fuel and bed materials is discussed. The influence of different combination on mixing and separating character is explored. By a series of experiments, the law of these parameters influence on the distribution of fuel either along the height or along the section direction of the furnace is investigated, and the rules of interaction among particle phase, bed material phase and gas phase of biomass are explored. Our particular interests are on the influence of multiple factors to the admixture, such as the variety of biomass and distribution inside the furnace. On the second stage, a0.5MW biomass CFB facility is involved. With the understanding of cold multiphase flow, the influence of different combination factors to thermal conduct coefficiency in dilute region is analyzed. The study focuses mainly on the bed temperature, fuel diversity, the ratio of primary and secondary air and exceed air ratio. Based on scientific experiments, the existing theoretical calculation method of thermal conduct coefficiency in dilute region is verified, come up with rational correction coefficient, which help instructing the industrial design of biomass CFB boiler.With the aid of thermal gravity analysis, the temperature of devolatization, ignition temperature and the difficulty of char burn out of typical biomass fuels are analyzed. Armed with these data, the dynamic parameters (such as activation energy) is obtained, the kinetic equations of biomass combustion are built, preparing for the establishment of a mathematical model for biomass combustion in a CFB boiler.On the basis of the previous research and data, and taking the mathematic model of coal CFB boiler as a reference, a mathematical model for biomass CFB is built up. Under ideal conditions, it provides the computer simulation of combustion of biomass in CFB. Therefore, it is able to predict the parameters of temperature distribution of furnace, the gas phase density and pollutants discharge. Just as expected in the beginning of this project, this model may theoretically guide the practical application of biomass fluidized combustion technology.Last but not least, the study focuses on one of the practical problems of real biomass direct combustion for power project on agricultural and forest residues-the deposition problem. The deposition significantly reduces the efficiency of thermal conduction and as a result, has been cost a loss to the electricity generating efficiency. The data in the field work is collected and analyzed. The forming process of deposition investigated. Our main interest is the influence of multiphase flow in biomass CFB on deposition formation.