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Study on Characteristics of Fly Ash Particles in Biomass Vortex Combustion

Author: ZengLeiZuo
Tutor: ZuoChunJiang
School: Zhejiang University
Course: Engineering Thermophysics
Keywords: biomass vortex combustion PM fly ash alkali metal submicron particle
CLC: TK6
Type: Master's thesis
Year: 2014
Downloads: 16
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Abstract

As a commonly used combustion mode of wood powder biomass, suspension combustion is characterized by high combustion intensity and good controllability. Vortex combustion is a relatively new branch of suspension combustion. In this combustion organization mode, the rotation flow of fuel particle can effectively utilize furnace height. Moreover, efficient combustion can be achieved with a compact structure. With investigating the ash particle characteristics of previous research, it is found that the ash formation mechanism in the biomass fixed bed and fluidized bed combustion has been comprehensively understood. This thesis is mainly to explore the ash particle characteristics including ash particle size distribution, morphology and chemical properties in the biomass vortex combustion.Firstly, we conducted the pure biomass combustion experiments with the new laboratory biomass vortex combustion boiler, including sawdust, rice straw, wheat straw, corn stalk and waste wood chips, and then the analysis of different biomass submicron particle emissions and morphology and chemical characteristics of fly ash particles were completed. The results show that the particle emissions were dominated by submicron particles in biomass vortex combustion, and the morphology of submicron particle was mainly irregular crystalline partciles. The detected chemical compounds of PM1were KC1, K2SO4and NaCl. Compared with herb fuels, woody fuels contained lower concentration of the submicron particles and the total fly ash particles, and it was proved that biomass fuel properties significantly affect the ash particle emission. The fly ash particle size distribution was bimodal in the combustion of different biomass fuels. The results show that the fine particle mode concentration of woody fuels varied between200and400mg/Nm3, and for the herb fuels it varied between400and1200mg/Nm3, but the fine particle mass ratio in total ash paticels was higher than herb fuels. In addition, higher temperature and primary/secondary air ratio can lead to increase emission of submicron particles, and the appropriate excess air coefficient of biomass combustion can lead a lower PM1emission.Secondly, the S/Cl mole ratio in various as particles of different sizes increased by adding a certain concentration of SO2in combustion atmosphere. Furthermore, increase the concentration of SO2in flue gas can promote the K2SO4formation in submicron particles and increase the PMt emission concentration, at the same time more chloride exhausted in the form of gas phase HC1. Besides, the fine particle mode concentration increased by adding three bituminous coal additive in the waste wood chips combustion, and the formation of K2SO4in PM1was mainly influenced by the sulful content in the coal base additives. Co-combustion with higher sulphur content of bituminous coal can lead to higher emission of PM1, and K/Na, S/Cl mole ratio in submicron particles.The results show that gas phase K2SO4dominates in the nucleation with the combustion of woody fuel under the high temperature, then gas phase KC1occurs heterogeneous condensation on the original nucleation particle surface and leads to the formation of submicron particle. As for the herb fuels, the gas phase K2SO4and KC1may occur simultaneously saturated nucleation under the relative lower temperature and then the submicron particles grow up with homogeneous condensation of more KC1vapour. Massive experiments indicate that vortex combustion mode is suitable for the combustion of the woody fuels, but in dealing with the herb fuels of high ash and alkali concentration a lower temperature of the primary air combustion region must be maintained. Finally, compared with other combustion organization mode, PM2.5-10contains especially high concentration of alkali salt, and with numerous submicron particle agglomeration and condensation in PM1-10under high temperature is the typical characteristic of biomass vortex combustion.

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