| Black liquor?BL?is a kind of waste liquid from chemical pulp process of papermaking,which contributes approximate 90% of COD?Chemical Oxygen Demand?of paper industry.Therefore,the treatment of BL is the key to solve the pollution problems of wastewater from papermaking.The BL mainly consists of alkali lignin,hemicellulose,degradation of carbohydrates and some inorganic compounds from the chemicals used in the pulping process.Thus,the BL is also a potential energy source.Nowdays,the fluidized bed alkali recovery technology is a well-established and widely used method for BL treatment.In this system,the BL is burned in a fluidized bed to recover the energy and chemicals.However,the problem of bed material agglomeration was found during the straw BL combustion due to the fact that the straw BL has higher contents of Cl,K and Si.In addition,the traditional alkali recovery process is complicated and will cause additional energy consumption,which give high capital costs.In view of the above problems,the direct causticizing technology of straw BL in fluidized bed was put forward to inhibit bed agglomeration and simplify the recovery process.In this thesis,the experiments of reed BL combustion in a fluidized bed were conducted under the conditions of different temperatures and bed material.And based on the optimum operating conditions,the direct causticizing experiment was conducted.In addition,the effects of leaching technology on removal of K and Cl from wheat straw and rice straw were studied.The last,a three dimensional model with the same size of test bed was created f or the numerical simulation of reed BL combustion in fluidized bed.The conclusions that can be drawn are as follows :Fisrtly,the effects of calcium-based zeolite on inhibiting agglomeration,release of K and Cl and emissions of NOx and SO2 were investigated as reed BL burned in a fluidized bed,using calcium-based zeolite as bed material.The results show that the calcium-based zeolite has strong effect on inhibiting agglomeration,the fluidization is still in good condition after 4h of com bustion at 700-850°C.Cl was mainly released in the form of HCl above 700°C and calcium-based zeolite caused lower K release due to the formation of K3Al?SO4?3,KAl?SO4?2 and K2OAl2O3-SiO2 eutectic compounds.Secondly,the effects of limestone on inhibiting agglomeration,release of K and Cl and emissions of NOx and SO2 were investigated as reed BL burned in a fluidized bed at 700-900°C,using limestone as bed material.In addition,the products in the direct causticizing process were identified,and the recovery rate of alkali was measured.The results show that limestone has better effect on inhibiting agglomeration than calcium-based zeolite.In addition,the direct causticizing system successfully recovers Na OH when using limestone as bed material.More than 90.1% of sodium fed into the system can be recovered.Thirdly,detailed experiments with removal of Cl and K from wheat straw and rice straw with leaching methods were conducted.The effects of leaching temperature,leaching time and acid or alkali on the removal rate of Cl and K were studied.The results show that more than 95.18% of Cl and 85.12% of K can be removed from wheat straw and rice straw by leaching technology,respectively.Higher temperature can promote the removal rate of K and Cl.The 0.25mol/L Na OH solution had slight effect on removal of K and Cl.0.25mol/L citric acid can improve the removal rate of K.Finally,based on the optimal operating parameters used in experiments,the combustion of reed black liquor in fluidized bed was numerically simulated with the Euler-Euler method including pyrolysis,homogeneous reactions and heterougeneous reactions inside the furnace.The distributions of velocity,concentration and static pressure,turbulent characteristics,species concentrations and gas temperature distributions were obtained,and the simulated results were then compared with the experimental values to testify the combustion model developed in this study. |
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