Investigations On Combustion Characteristics And Scaling Of Supercritical CO₂ Circulating Fluidized Bed Boiler

Nowadays,coal-fired power generation occupies a dominant position in China,and improving power generation efficiency is the key technology to achieve energy conservation and CO₂gas emission reduction.The development of new and efficient power cycle generation system is one of the important technical ways to break through the bottleneck of coal-fired power development.Supercritical CO₂（S-CO₂）power cycle has broad development prospects due to its advantages including simple system structure,high cycle efficiency,compact elements,and environmentally friendly.However,circulating fluidized bed（CFB）boiler,as one of the two most widely used coal-fired devices,faces challenges in thermodynamic analysis,heat surface arrangement,and coordination of the boiler with other components to accommodate this new cycle.Little is known about the different hydrodynamics and combustion characteristics of the S-CO₂ CFB boiler compared with the conventional water steam one.Especially in industrial applications,due to the insufficient grasp of many more complicated combustion laws,it encounters certain difficulties in the equipment amplification design,structural parameter optimization,and other aspects.This paper deeply studied the combustion characteristics and scale-up regularity of the S-CO₂ CFB boiler by numerical simulations.A 3D model of 600 MW CFB boiler with a novel annular furnace and built-in flue for S-CO₂power cycle was constructed.Based on the Multiphase Particle-in-Cell（MP-PIC）method under the Eulerian-Lagrangian method framework,the gas solid hydrodynamics in the 600MW coal-fired S-CO₂ CFB boiler were revealed from the perspective of the full loop numerical simulation.The bed flow pattern was divided into six modes according to the gas-solid fluidization regimes,and the flow transition phase diagram was drawn to reveal the flow transition regularity and the gas-solid flow structure in each mode.The fluidization air velocity range that is the most suitable for the S-CO₂ CFB boiler was determined.Within this range of fluidization air velocity,the effects of operating air velocity and particle size on the gas-solid flow characteristics were investigated,further revealing the operating characteristics of coal-fired S-CO₂ CFB boiler.A new dynamic boundary treating method based on the simulation of the combustion process in furnace side coupling with the heat transfer process in working fluid side was proposed,which overcame the previous shortcomings of isolating the two models from each other.The heat transfer process was predicted based on the polynomial fitting calculation model,and the combustion process was predicted based on MP-PIC numerical simulation by establishing 3D computational fluid dynamics（CFD）model.The pyrolysis model of coal,gas phase combustion model,as well as basic reactions in the carbon combustion and gasification models were involved.And meanwhile,the generation and removal models of main gaseous pollutants（NO,N₂O,SO₂）were introduced,as well as the convection and radiation heat transfer model between particles,gas phase,and heating surfaces.Coupling simulations were implemented by an iterative convergence process mediated by the S-CO₂ cold-wall heat-flux to reveal the different heat transfer performance of the working fluid side,combustion characteristics,heat transfer characteristics,and pollutant emission characteristics of the furnace side under the S-CO₂ dynamic boundary compared with conventional water steam heat transfer boundary.Based on the above research method,the scale up study of the coal-fired S-CO₂ CFB boiler was conducted.Multi-directional comparisons of the boiler structure,gas-solid flow characteristics,combustion characteristics,heat transfer characteristics,and gas emission characteristics of three scale S-CO₂ CFB boilers with five boiler capacities,namely lab-scale（0.1 MW）,pilot-scale（10 MW）,and industry-scale（100 MW,300 MW,600 MW）were conducted.Furthermore,the effects of boiler load and bed inventory on boiler gas-solid hydrodynamics and combustion characteristics were examined.The scale up regularity of S-CO₂ CFB boiler on two aspects of boiler capacity and operating parameters was revealed,and the scale up fitting formulas for combustion efficiency and carbon conversion rate were also proposed.Guiding suggestions were given for the industrial application of coal-fired S-CO₂ CFB technology.Moreover,a scale-up prediction model of the combustion characteristics in S-CO₂ CFB boiler based on Adaptive Particle Swarm Optimization（APSO）Support Vector Machine（SVM）was proposed.The model performed the adaptive processing of the own parameters of the particle group algorithm,and then integrated the adaptive particle group algorithm with the support vector machine,which solved the"precocious"convergence problem of the particles being easy to gather at a certain position in the process of mode recognition.The novel APSO-SVM model improved the prediction accuracy of boiler capacity,carbon conversion rate,and NOx emission compared with traditional PSO-SVM and SVM models,and the scale-up regularities of combustion characteristics in the amplification process of boiler capacity and operating parameters were effectively predicted.The complicated and huge workload in the numerical simulation process was simplified,which laid the application foundation of S-CO₂ CFB boiler in industry with precision credibility.