Development And Numerical Simulation Of Venturi-type Circulating Fluidized Bed Quicklime Dry Digestion Unit

The dry-method circulating fluidized bed flue gas desulfurization?CFB-FGD?process has been widely applied in the field of flue gas desulfurization in the small and medium-sized coal-fired power plants and steel sintering industries widely due to its advantages of simple process system,less occupied land area,low investment and operation cost,high desulfurization efficiency and no secondary pollution of wastewater.Hydrated lime?Ca?OH?₂?is the most commonly flue gas desulfurizer due to its wide source of raw materials,good activity and easy comprehensive treatment and utilization of by-products.How to prepare the hydrated lime is one of the key contents in the development and application of CFB-FGD process.In this paper,the dry-method CFB-FGD project of a sintering flue gas of 350 m²sintering machine in an iron and steel enterprise is selected,and the process and device of a Venturi-type circulating fluidized-bed dry-lime hydration?VCFB-DH?were developed based on the dry-method hydration of quicklime and particle fluidization principle.The structure of Venturi-type hydrator was optimized by numerical simulation,and based on the established pilot plant of VCFB-DH,the cold and hot test scheme and its evaluation method of quicklime hydration in the pilot VCFB-DH are put forward.It provides a basis for the application and development of dry flue gas desulfurization technology.The main work and conclusions of this paper are as follows:According to the requirements of super-clean emission of SO₂ in flue gas of sintering machine,taking the 350 m² sintering flue gas CFB-FGD process as an object,combined with the operating experience of CFB-FGD and the dry hydration characteristics of quicklime,the design of VCFB-DH process system was carried out.Based on the working process and basic assumptions of the system,the material balance and energy balance of 10 t/h VCFB-DH process were calculated,and the performance evaluation of the process system is carried out in combination with the actual operation.The results showed that:?1?The VCFB-DH process is mainly composed of quicklime feeding,vertical dry hydrator,dust removal and recycle,slaked lime storage,process water and its atomization,flue gas supply and instrument gas supply,the vertical dry hydrator is the key part of the process.?2?As far as the 10 t/h VCFB-DH process is concerned,the consumption of quicklime is 10 t/h,the output of hydrated lime is 12.60 t/h,the consumption of atomized water is 2.60 t/h,and the exothermic quantity of quicklime is 9048.9 MJ/h,the total input heat of the unit is 6503.9 MJ/h,in which the input heats of quicklime,atomized water for hydration and flue gas account for 0.7%,0.3%and 99.0%respectively,and the total output heat of the unit is 11084.2 MJ/h,of which,the heat from flue gas is the main output heat,accounting for 87.0%of the total output heat.The heat from slaked lime and the heat from cylinder are relatively small,3.4%and 9.6%respectively.?3?In order to ensure the hydration quality of quicklime,spray water atomization was used to keep the flue gas at designed temperature.At the inlet flue gas temperature of80?,the amount of atomized water used for cooling flue gas was 1.67 t/h and the total amount of atomized water was 4.27 t/h.Based on the process design of VCFB-DH,the key components and systems of vertical dry hydrator,quicklime feeding,dust removal and recycling were designed.The results showed that:?1?the vertical dry hydrator consists of flue gas inlet,Venturi tube,upper and lower cone,cylinder body and flue gas outlet.The total height of the dry hydrator is 26.25 m,the cylinder body is 220 m,the Venturi tube is composed of seven-hole sub-Venturi,the total height of each sub-Venturi is 0.9 m,the diameter of inlet and outlet is 0.44 m,the diameter of throat is 0.22 m,the diameter of flue gas inlet and the width of flue gas outlet are 1.2 m.?2?The volume of quicklime silo is?7000×3.3H and the height of cone section is 7 m,the effective volume is 170 m³,the bottom of the raw lime silo is connected with DN250 feeding star row and feeding chute?Model B300/AS200?,and the top of the raw lime silo is equipped with a pulse bag top dust collector?model HMC48B?.?3?The two-stage cyclones with the cylinder type 2X6H and the material quality 345 is selected,and the corresponding feeding and returning devices are arranged,at the same time,a hydrated lime silo with 7000×13.5 H,7 m high cone section and 600 m³ effective volume has been prepared.Based on the numerical simulation platform of Fluent,flow characteristics of the flue gas in vertical dry hydrator was carried out,the influence of deflector of smoke chamber and cylinder structure on the distribution characteristics of resistance and flow field of dry hydrator is analyzed.The results showed that:?1?Compared with the original venturi-type digestor,the axial flow field distribution shows that the symmetry of the flow field in the reactor barrel has been improved by adding deflectors in the flue gas chamber,with the addition of deflectors in the flue gas chamber,the phenomenon of smoke segregation caused by the asymmetry of flow field at the outlet of Venturi tube is effectively avoided,which is beneficial to the back mixing of particles in the hydrator,and the movement of some hydrate products towards the wall is avoided,the wall sticking phenomenon is reduced,thus the efficiency of hydration reaction is improved.At the same time,under the condition of different inlet gas velocity,the pressure difference between inlet and outlet of the reactor gradually increases with the increase of inlet gas velocity.?2?Compared with the original venturi-type hydrator,adding a throat with suitable size in the hydrator body will not only not destroy the flow field in the hydrator,but also help to strengthen the secondary efflux of the hydrator,and improve the efficiency of the hydrate reaction.With the increase of the number of throat,the loss of resistance increases slightly by 5%at the same inlet velocity,and the pressure difference between inlet and outlet decreases with the increase of the throat's diameter,the flow velocity of flue gas in the hydrator decreases gradually.In consideration of the flow field and resistance in the hydrator,the best performance is obtained when the size of the throat in the hydrator body is 1200 mm.