| The quality classification and cascade utilization of low-rank coal is the strategic development direction of clean and efficient coal utilization,and the main utilization methods are coal gasification and coal pyrolysis.The semi-coke and fly ash produced during pyrolysis and gasification can be reused as fuel.The volatile of these fuels are generally less than 10%,called as ultra-low volatile carbon-based fuel,which have many challenges(e.g.difficulty in the ignition,poor combustion stability,low combustion efficiency,high pollutant emission).Preheating combustion technology can realize the stable combustion and low NOx emission of non-combustible solid fuel.Based on this technology,an inner cycle preheating unit(ICPU)with built-in separation and return device is proposed,which is suitable for stable preheating of fuel,in order to realize clean and efficient utilization of ultra-low volatile fuel and promote the application of preheating combustion technology in the engineering field.In order to study the operation characteristics of ICPU,the complicated gas-solid flow characteristics in ICPU are studied in this topic with experiment and numerical calculation.Secondly,the feasibility of ICPU is verified in the preheating combustion experiment,and preheating,combustion and NOx emission characteristics of ultra-low volatile fuel are investigated.Such research can provide basic data and theoretical support for design,operation and engineering applications of ICPU.The main research work and conclusions are as follows:(1)Based on ICPU cold test rig,the influence of superficial air velocity(Ug)on the operation and gas-solid flow characteristics of ICPU are studied.The full loop of riser-separator-return valve-riser can be established in ICPU.The negative pressure difference in circulation loop mainly depends on the pressure drop of separator.Particles in return valve show the bottom dense and upper dilute,and center dilute and wall dense non-uniform distribution.With the increase of Ug,the particle concentration in return valve increases,and the discharge pattern of return valve converts from continuous discharge to intermittent discharg.The intermittent discharge caused by the failure of return valve to continuously maintain the pressure balance of circulation loop is the main factor of the instability of ICPU.The pressure and ECT measurement can realize the real-time monitoring of the gas-solid flow state in return valve.(2)Based on ICPU cold test bench,the influences of outlet opening,height of return valve and Ug on the operation and gas-solid flow characteristics of return valve were studied.The air leakage and solids choking will occur under the inappropriate outlet opening of return valve.Increasing the height of return valve can improve the accumulating pressure capacity,and the design height should be 2.2 times of the minimum height of maintaining the pressure balance.If the return capacity of return valve can’t match large loop circulation flux,solids accumulates to form defluidization choking,and the pressure balance of circulation loop is difficult to establish.(3)Based on a built-in gas-solid separator test rig,the influence of inlet structure and operating parameters on the performance of separator are studied.The pressure drop and collection efficiency of single-inlet separator are higher than those of multi-inlet separator,and the pressure drop and collection efficiency of the separator with inlet guide structure are higher than those of the separator without inlet guide structure.Considering the pressure drop and collection efficiency with grey correlation method,the four-inlet separator with inlet guide structure is the preferred structure,and an inlet speed of 20 m/s is appropriate.(4)3D full-loop CPFD simulation on ICPU is carried out based on MP-PIC methodology and BarracudaTM platform.Particles in riser show the bottom dense and upper dilute axial distribution,and "core-annular" radial distribution with dilute center and dense wall.With the increase of Ug,the gas-solid flow in return valve transitions from dilute phase-coexistence of dilute and dense phase-dense phase accumulation,and the circulation flux firstly increases and then decreases.With the decrease of outlet opening,the return valve transitions from air leakage,stable solids sealing and choking state,the gas-solid flow transitions from coexistence of dilute and dense phase to dense phase accumulation.(5)Based on 2 MW inner cycle preheating unit test platform and 16 MW inner cycle preheating unit test platform,the preheating,combustion and NOx emission characteristics of ultra-low volatile fuel were studied.ICPU can preheat the fuel up to 850℃ continuously and stably.69.3%of fuel nitrogen is released in the preheating process,and the ICPU has a strong potential of the reduction of fuel nitrogen.The conditions for reducing NOx emissions are strong gasification,strong mixing and long reaction time.With the decrease of secondary air ratio,NOx emissions decrease first and then increase.Reducing the momentum ratio of inside secondary air to outside secondary air,multi-layer and delayed arrangement of tertiary air can reduce NOx emission effectively.The lowest NOx emission reached 67 mg/m3(@6%O2). |
