Research On Near-Wall Burning And Flowing Characteristics Of Solid Particulate Fuel In Swirling Conditions

Pulverzied coal combustion is widely used in all kinds of industrial boilers and kilns.But the discharging of afterburnt fly ash and dust are the main source of PM2.5 and will cause air pollution.How to properly deal with the fly ash and dust remains unsolved.Clean coal combustion technologies have been listed in the Medium and Long Term Science and Technology Plan of China(2006-2020).In the present reasech,the swirling coal combustion technology has been integrated with the near-wall burning technology.The emphasis have been laid on the new clean pulverized coal combustion technology using the flowabilities of molten slag under high temperature,which can be used in the small and medium sized burners and kilns.It is essential to understand the aerodynamic and combustion features under swirling near-wall burning conditions.For slagging coal combustion,it is also critical to understand the distribution and heat transfer features of the slag layer so that ash can be captured instantly and discharged in the form of molten slag.The combustion efficiency and pollutant discharging features can also be improved.This research can be referred to the optimization of clean coal combustion technology as well as the designing and retrofitting of the swirling pulverized coal combustor.There are four research achievements in the present thesis as follows:1.It is essential to generate a favorable swirl-reverse airflow field for efficient and clean coal combustion.In this thesis,a brand new twin swirl combustion technology with conical staggered inlets was proposed.CFD simulations were used to study the intensities and interation mechanism between swirling and reversing of confined airflows with different inlet layouts and flux ratios.The optimized Twin Swirl Combustor(TSC)were designed based on the simulation results and the experiment results proved that the swirling and recirculation flow field by primary and secondary swirling airflows can meet the demand of the pulverized coal combstion and fly ashes can be properly captured by combustor wall and be removed.2.The hybrid RANS/LES methods were used to simulate the large-scale vortex structures and instantaneous flowing features to further understand the mechanism of unsteady features during swirling combustion.The calculation accuracy of hybrid RANS/LES methods has been improved compared with that of RANS methods.The simulation strategies of the hybrid RANS/LES mothods were generated.Based on the simulation strategies,hybrid RANS/LES methods were used to study the aerodynamic flowfiled of TSC.It is proved that with the hybrid RANS/LES methods,higher simulation resolution can be obtained and the large scale vortex structures in the TSC can be observed.The statistical results based on the transient results of the hybrid RANS/LES methods are close to the steady state RANS results.3.For the better understand of combustion and slagging features in reacting conditions,a comprehensive slagging model with real time data interaction features was set up and coupled with the existing coal combustion models to improve the combustion and ash capture efficiency of the TSC.The whole process of colliding,deposition and flowing of the ash particles was analyzed.The Coupled Coal-Slagging(CCS)simulation framework was then estabilished and validated to calculate the combustion characteristics and slagging features of the TSC.Results by different equivalence ratios show distinct zonal combustion features along axial and radial directions.Highly efficient clean coal combustion was realized.The distribution and heat transfer features of the slagging layer with different equivalence ratios were obtained and results prove that ash capture efficiency of the TSC is remarkable.4.Oxy-fuel combustion is one of the most promising and feasible coal combustion technologies for carbon capture in the current state.The twin swirl combustion technology was combined with oxy coal combustion technology.The combustion characteristics as well as the slagging features of the TSC under oxy combustion conditions were studied and compared with the normal air combustion.Based on the criteria of same oxygen supply,comparative results show that highly efficient combustion can still be realized under oxy combustion condition.Ignition delay was observed when oxy combustion was applied.The ignition position and combustion intensities could be adjusted by the oxy ratio.A decrease of about 3% was observed for the ash capture efficiency of oxy combustion,compared with that of air combustion.It is feasible of retrofitting of oxy combustion for the current TSC facilities.The combustion efficiency of the TSC is higher than 95%.Ash capture rate is from 88% to 91%.The TSC,which based on the twin swirl clean pulverized coal combustion technologies,can be widely used in small and medium sized industrial burners and kilns,while the cost is much lower than the IGCC and gasifiers.A new research area of clean coal combustion has been established for the technologies used in the TSC.