Study On Agglomeration Trend Of Biomass Fluidized Bed Combustion Based On Inter-particle Mechanical Analysis

As a mature fluidized combustion technology, CFB is widely used in biomass combustion owe to its good fuel adaptability, high efficiency and low emission levels. However, as the special characteristics of biomass fuel, agglomeration is often found in the boiler, which can easily lead to the defluidization of the boiler. This research is focused on the agglomeration formed mechanism and how to detect the forming of agglomeration. We can easily find that the defluidization is directly caused by the driven force of fluidizing gas is lower than the interparticle flow resistance. With the funding of the national nature science foundation of China, we take the bed material out from the fluidized bed to research its interparticle mechanical characteristic, expecting to quantitatively conduct the agglomeration detection in the fluidized bed.Our research is started with considering the bed material as pseudo-fluid and studying its relationship between stress and strain. In order to get the rheological characteristics of the bed material, we discussed the suitability of inclined cute method, Cone plate method and rotating coaxialcylinder method. Finally, we build our experiment platform by the guidance of single cylinder method. After a series of experiments in the test bench, we obtained the stress and strain relation of Quartz sand at room temperature and found it is not corresponded with the change rule of conventional fluid. With the aid of quadratic polynomial widely used in the analyze of mudslide, we found it can describe the constitutive relationship of the quartz sand appropriately. Due to the shear of intensive particles appears great difference with the conventional fluid, in order to focus the core problem in the process of agglomeration, we started to pay attention to the interparticle mechanics properties build on the foundation of the discrete particle in granular material, studying the interparticle mechanics under change of granule concentration. Through the study we found that small particle size, smooth particle surface, good sphericity can obviously decrease the force chain strength in the shearing of bed material and increase the time required from the natural grain piles up to packing transition, which are the key factors to influence the fluidization performance of the bed material. Through a series of burning experiments in bubbling fluidized bed, we studied the influence of bed material particle size, combustion temperature and characteristics of fuel on the agglomeration. By the analysis of the bed material after burning, we found three different agglomeration forming mechanism. By the burning of rice straw and tobacco straw, we got the bed material after burning in different conditions. Through the analysis of the bed material in the test bench, we find the agglomeration trend can be can be well reflected for the rice straw bottom slag, but not obvious to the tobacco straw bottom slag. We also found that spherical bed material behave great difference with irregular bed material in the process of agglomeration. For the former the contact is limited to the contact point between spherical particles, so it needs very strong adhesion to stick together. For the latter, the contact can happen between surfaces of the particles, so very small particle surface viscous can form great adhesive force between particles, agglomerates faster.