Experimental Investigation And Prevention Mechanism Study Of ASH Deposition On Convective Heating Surfaces In CFB MSW Incinerator

Municipal Solid Waste（MSW）incineration has been widely used in China owing to its advantages of significant reduction in volume,recovery of energy and complete disinfection.Compared to more traditional incinerators such as grate furnaces and rotary kilns,fluidized bed combustion has shown to be a versatile technology,capable of burning practically any waste combination such as garbage in China with high moisture and low calorific value,with lower emissions.However,MSW is a very complex and heterogeneous fuel,containing several individual material fractions with different physical and chemical characteristics.It is easy to form ash on the heating surface during incineration.Ash-related problems like deposit formation may have implications for heat transfer rates and hence decrease the efficiency of the boilers or even render the incinerators unmanageable.Therefore,ash depositon and prevention mechanism of ash on convective heating surface in MSW Circulating Fluidized Bed（CFB）incinerator is of great significance for the development and promotion of fluidized bed waste incineration technology.Ash samples were collected from a MSW CFB incinerator,which included loose-like fouling and ash deposits from the economizer and evaporating heating exchanger.The composition,microstructure,particle size distribution,surface analysis and the element analysis of those samples were studied with Scanning Electron Microscope（SEM/EDX）,X-ray fluorescence（XRF）,and Laser particle analyzer etc.Study results indicate that economizer loose-like fouling contains a high proportion of small and irregularly shaped particles while there are some agglomeration particles among evaporating heating exchanger fouling.The particle size of most fouling of these two kinds is between 0～100μm.The ash fusion temperature of economizer fouling is 80℃lower than that of evaporating heating exchanger.Besides,the main mineral phase contained in the ash deposits is CaSO₄.Compared to ash deposits formed on the economizer surface,evaporating heating exchanger deposits contain of a relatively higher CaO and SO₃ but lower Al₂O₃ and SiO₂.The content of Ca and S from the inner layer of the ash is gradually reduced,while the content of Al and Si is gradually increased;the content of K,Na,Fe and Cl in the inner layer of the ash is higher than that of the other layers.A study was performed where experiments were carried out to examine the ash deposition characteristics in a full-scale MSW CFB incinerator,using a newly designed deposit probe that was fitted with six thermocouples and four removable half rings.The influence of probe exposure time and probe surface temperature（500,560,and 700°C）on ash deposit formation rate was investigated.The results indicate that the deposition mass and collection efficiency achieve a minimum at the probe surface temperature of 560°C.Ash particles are deposited on both the windward and leeward sides of the probe by impacting and thermophoretic/condensation behavior.The major inorganic elements present in the ash deposits are Ca,Al and Si.Compared to ash deposits formed on the leeward side of the probe,windward-side ash deposits contain relatively higher Ca and S concentrations,but lower levels of Al and Si.Among all cases at different surface temperatures,the differences in elemental composition of the ash deposits from the leeward side are insignificant.However,as the surface temperature increases,the concentrations of Al,Si,K and Na in the windward-side ash deposits increase,but the Ca concentration is reduced.The commercial software Fluent was used to simulate ash deposition on the tube bundles.The influence of flow parameters such as fly ash particle size,flue gas velocity and probe surface temperature on the characteristics of fly ash paticles accumulation is studied.The results show that when the flue gas velocity increases from 4 m/s to 10 m/s,the mass of particles deposited on the windward side increases gradually,but when the flue gas velocity increases to 12 m/s,the deposition decreases.With the increase of the size range of fly ash,the deposition rate of fly ash decreases.The deposition rate of fly ash decreases first and then increases with the increase of the wall temperature.Generally,there is a certain gap between the simulation and experimental results,so deposition model needs further improvement.A study was performed where experiments were carried out to examine the ash depositon characteristics in a bench-scale Circulating Fluidized Bed（CFB）.The influence of ash particle sizes,flue gas temperatures and heat exchange tube surfaces temperature（500,550,600 and650℃）on ash deposit formation rate was investigated.The results indicate that with the increase of particle size,the content of CaO and SO₃ in the loose-like fouling ash gradually decreased,while the content of SiO₂ and Al₂O₃ increased gradually.The contents of the alkali metals Na and K and the halogen Cl are the highest in the ash with a smaller particle size.The ash deposition is mainly rich in elements such as Ca,S,Si and Al.The content of CaO and SO₃in the ash is higher than that in the fouling ash,while the content of Al₂O₃ and SiO₂ is lower.The deposition mass increases as the temperature of the flue gas increases,and decrease as the particle size range of the fly ash increases.The effect of particle size on ash deposition is significant.Large-size ash particles are difficult to deposit while small-size ash particles are easy to deposit.The deposition mass achieve a minimum at the probe surface temperature of600℃.The content of CaO and SO₃ decreases as the probe surface increases,while the content of refractory Al₂O₃ and SiO₂ increases as the wall temperature increases.The numerical simulation results show that the deposition rate of fly ash tends to decrease with the increase of the particle size range,and most of the particle size on the windward side is under 10μm.Fine particles under 10μm are more significantly affected by the turbulence and thermophoresis on the leeward side;the higher the flue gas temperature is,the faster the deposition rate is.What’s more,the deposition rate of fly ash particles is related to the probe surface temperature,and the deposition rate of fly ash on the windward side decreases first and then increases with the increase of the tube wall temperature.CaO,Al₂O₃,SiO₂ and kaolin were added into ash samples in order to study the effect of mineral additives on melting properties of ash.HSC Chemistry and FactSage were used to explain the reason of the change of ash melting properties.With the increase of SiO₂ and Al₂O₃blending ratio,the melting temperature of mixed ash increase too.The increasing of Al₂O₃/SiO₂can promotes the melting point;With the increase of CaO content,the melting points of ash increase greatly,which is due to the phase shift of ash from low-melting-point feldspar phase zone to pseudo-wollastonite phase zone with higher melting-point.Kaolin can also increase the melting point of ash,but at a ratio of more than 10%,the increase of melting temperature is not significant.Comparing the four additives,all of them can improve the melting temperature of blending ash.The effect of CaO is the most obvious.The information gathered by these investigations reveal the ash depositon and prevention mechanism of ash on convective heating surface in CFB MSW incinerator,which has certain guiding significance for the development and improvement of fluidized bed incineration technology.