Studies On Influence Mechnism Of Water Vapor On Limestone Desulfurization In Fluidized Bed Boiler

The in-furnace desulfurization with limestone addition has been widely used in the coal-fired circulating fluidized bed?CFB?boiler,which will at least reduces 80% of SO₂ emissions.When the limestone sorbent is introduced into the above-the-flame region of a CFB furnace.It firstly undergoes calcination resulting in formation of highly reactive porous CaO which is subsequently sulfated by SO₂ to form CaSO4.The whole reaction is called indirect desulfurization.There is as high as about 15²0% of water vapor in the flue gas after coal combustion under air atmosphere in the CFB boiler.And there are also minor impurities in natural limestone such as Si O₂,Al2O3 and Fe2O3.The existence of water vapor and impurities will influence limestone desulfuration,not only for calcination,but also sulfurization of CaO calcined.Therefore,it is important to study the influence of water vapor and impurities on limestone desulfurization under fluidized-bed combustion.Meanwhile,so far it has been found that the degree of sulfurization is usually low under typical conditions of CFB combustion,only ranging from 25% to 45%.The related studies in this thesis also aims to find effective measures to improve the utilization of limestone sorbent.At the beginning,the impacts of water vapor and impurities on the limestone calcination and sulfurization of CaO calcined in a micro fluidized bed reactor analyzer?MFBRA?are studied.The apparent activation energy under different operating conditions was firstly calculated.The results show that reaction temperature,water vapor and impurities all have influences on the indirect desulfurization of limestone.For calcination,the apparent activation energy decreases slightly in the presence of water vapor and it does not change with the impurities.For the sulfurization of CaO calcined,both water vapor and impurities decrease the apparent activation energy.In order to understand the influence mechanism of water vapor and impurities on the limestone calcination and sulfurization of CaO calcined,the two reactions are studied in this thesis,respectively.Firstly,the influences of water vapor on limestone calcination were studied in a laboratory-scale fluidized bed reactor?FBR?.Scanning electron microscope-energy dispersive spectrometer?SEM-EDS?,confocal scanning laser microscope?CSLM?,mercury intrusion porosimetry?MIP?,N2 adsorption instrument and X-ray diffraction?XRD?techniques were employed to analyze the micro-morphology,pore structure and crystal structure of the CaO calcined respectively.The results show that water vapor improves calcination rate and shortens the reaction time and those influences are stronger for higher impurity limestone.The testing results of micro-morphology and pore structure mean that the existence of water vapor improves sintering and growth of grains.The results of XRD give further evidence to the previous conclusion.These tests and analysis give rise to the mechanisms behind the impacts of water vapor on limestone calcination: the binding ability of H2O?g?to active site O* in CaO*-CO₂?Ca CO3?is stronger than that of CO₂.H2O?g?tends to replace CO₂ on the active site to increase the release of CO₂ in calcination.Water vapor also accelerates sintering,most possibly in the initial stage when sintering neck is formed.There exist two possibilities: H2O?g?molecules are absorbed on the active site of Ca-O* to promote the formation of the sintering neck of CaO by interaction between H2O?g?molecules?such as hydrogen bond?.Water vapor can also act as a solvent to improve the solid state diffusion from surface to sintering neck which also benefits the fusion and growth of minicrystals.Then the influences of water vapor and impurities on sulfurization of CaO calcined are studied in this thesis.Sulfurization experiments were conducted with CaO calcined in the FBR mentioned above.SEM et al.were also used to analyze the mechanism.The results show that the existence of water vapor could influence CaO sulfurization significantly.Under a certain concentration,the existence of water vapor not only increases the reaction rate,but also decreases reaction time.And under the experimental conditions of this thesis,there is an optimal water vapor concentration for the reaction.The testing results show that the existence of water vapor could improve sintering as well as fusion and growth of grains of sulfurization products,while the pore size could also increase.Based on the above results,the mechanisms behind the impacts of water vapor on CaO sulfurization are proposed: the solid-state ions diffusion processes for reactant CaO and product CaSO4 are both enhanced by water vapor,but the paths are different.The former belongs to surface diffusion without influencing the crystal structure,while the latter is more inclined to volume diffusion because crystal defects have been formed due to water vapor.Finally,in order to verify the above conclusions,the impacts of water vapor on limestone desulfurization during coal combustion are studied in a bench-scale fluidized-bed combustor?FBC?.Meanwhile,there is a certain amount of alkali and alkali earth metals?AAEMs?compounds within coal,especially in low-rank coal,such as Zhundong lignite.Therefore,the influence of AAEMs on the reaction is also studied in this thesis.The impacts of SO₂ release and removal during coal combustion were studied in the experiments.SEM,inductively coupled plasma-atomic emission spectrometry?ICP-AES?and XRD techniques were employed to analyze the micro-morphology and chemical compositions of deposited ash to understand the mechanism.The results show that water vapor and AAEMs within coal accelerate the limestone desulfurization during coal combustion.Zhundong lignite has a stronger propensity of self-desulfurization.The grain size of coal ash grows with water vapor addition,and this phenomenon is further intensified under the condition of desulfurization or Zhundong lignite combustion.The mechanism is probably that water vapor and AAEMs improve the solid state diffusion during desulfurization and improve growth of ash grains.The results above show that the mediate water vapor concentration could improve limestone desulfurization.In order to obtain the optimal desulfurization efficiency,it is necessary to maintain a proper water vapor concentration in the furnace.Under the experimental conditions of this thesis,it's about 2-15%.For CFB boilers the circulating ash returning wind may be used to adjust the atmosphere in the furnace.Besides,the high-AAEM coals,such as Zhundong lignite mentioned in this thesis,usually behave a strong self-desulfurization ability,thus the value of Ca/S should be determined prudently in these cases.