In-situ catalytic deNOx is a promising NOx control technology for circulating fluidized bed (CFB) boilers. In this application, matching the conditions between the catalyst and gaseous species is crucial. To understand this, a comprehensive computational particle fluid dynamics (CPFD) model was established; flow, combustion, and NOx emission characteristics in an industrial CFB boiler were elaborated; 20 catalysts with various sizes and densities were designed, and their degree of matching with the gaseous species was evaluated. The simulation results indicated that NOx was gradually produced at the bottom of the furnace and attained its maximum concentration at the elevation of secondary air; CO showed a high concentration in the bottom dense-phase zone; and the homogeneous NO-CO reaction is too weak to effectively reduce NOx. With catalyst application, the NO-CO reaction was evidently enhanced and the in-furnace NOx concentration decreased significantly. The 20 evaluated catalysts can be categorized as dipleg deposition, fluidization circulating, furnace suspension, and furnace deposition types. While the last three types of catalysts could match the spatial and temporal distribution of CO and NOx species well, the furnace suspension-type catalyst produced an optimal matching degree and maximum deNOx efficiency.

The ultra-low NOx emission requirement (50 mg/m3) brings great challenge to CFB boilers in China. To further tap the NOx abatement potential, full understanding the fundamentals behind CFB boilers is needed. To achieve this, a comprehensive CPFD model is established and verified; gas-solid flow, combustion, and NOx emission behavior in an industrial CFB boiler are elaborated; influences of primary air volume and coal particle size on furnace performance are evaluated. Simulation results indicate that there exists a typical core-annular flow structure in the boiler furnace. Furnace temperature is highest in the bottom dense-phase zone (about 950 °C) and decreases gradually along the furnace height. Oxygen-deficient combustion results in high CO concentration and strong reducing atmosphere in the lower furnace. NOx concentration gradually increases in the bottom furnace, reaches maximum at the elevation of secondary air inlet, and then decreases slightly in the upper furnace. Appropriate decreasing the primary air volume and coal particle size would increase the CO concentration and intensify the in-furnace reducing atmosphere, which favors for NOx reduction and low NOx emission from CFB boilers.

This paper presents a CFD modeling of deNOx process in a coal-fired power plant selective catalytic reduction (SCR) system, with focus on the transient hydrodynamics of multi-species flow and the influence of vortex on the deNOx process. For this purpose, a comprehensive CFD model is established, parameter study and model validation are performed, and the hydrodynamics, vortex evolution and species concentration distribution are numerically investigated. Simulation results indicate that many vortices with various scale/intensity/shape exist in the SCR system, causing apparent pressure pulsations and velocity fluctuations. High-intensity eddies are mainly distributed in the deflector group Ι, the NH3 nozzles, the static mixer, and the right part of the rectifying grille. The number of eddies decreases significantly with reducing the unit loads. Affected by vortex evolution, the NH3 concentration fluctuates in the SCR system, especially in the vertical flue. The deNOx process completes within 6 s, and the ammonia slip is less than 1.0 ppm, which well meets the requirement of industrial standards. In addition, the static mixer severely destroys the velocity uniformity but favors the mixing of NH3 and NOx. The rectifying grille improves the uniformity of flow field and species concentration field significantly.

为探究流场和组分浓度分布对SCR装置脱硝效率和氨逃逸的影响进而提升脱硝性能,基于ANSYS FLUENT软件平台、耦合多孔介质模型和E-R脱硝反应动力学模型,建立国内某660 MW燃煤机组SCR系统的三维计算流体力学模型;详细分析不同负荷下烟气速度分布、组分浓度分布、脱硝效率和氨逃逸等流动反应特性;进而针对分区喷氨和静态混合器对脱硝性能的影响进行模拟与优化。结果表明:当顶部导流板倾角与楔形弯头倾角相同时,流场分离现象消失,速度分布更加均匀。较之于均匀喷氨方式,分区喷氨可使NH_3浓度均匀性提高55%、出口NO浓度偏差降低50%。采用平板型混合器结合分区喷氨策略,催化剂入口截面NH3浓度偏差系数小于2.5%,出口截面NO浓度偏差系数小于8%,局部最大氨逃逸为1.82mg/m~3,脱硝效率提高了7%,实现SCR装置的稳定高效运行。

With deep peak-load regulations, utility boilers are frequently operated under variable/low load conditions. However, their hydrodynamics, combustion and NOx emission characteristics are uncertain and relevant theoretical guidance are lacking. For this purpose, a comprehensive CFD model including flow, coal combustion and NOx formation is established for a 630 MW tangentially fired pulverized-coal boiler, aiming at solving the problem of decreasing combustion stability and increasing NOx emission in low-load operation. Based on the grid independence and model validation, the flow field, temperature profile, species concentration profile and NOx emission are predicted, and the influences of angle/arrangement of burners are further evaluated. Simulation results indicate that under low-load conditions, residual airflow rotation still persists at the top of boiler regardless of how to adjust the angle/arrangement of burners. With tilting the burner angle upward, flame is more concentrated, combustion becomes more stable, and heat flux rises in the upper zone; the burner arrangement of ABDE gives more uniform temperature distribution in the combustion zone. CO species shows higher content in the combustion zone; the 0° tilt angle gives maximum CO content, followed by the 15° angle, and finally the −15° angle; compared to the ACDE and ABCE arrangement, the ABDE arrangement mode gives much lower CO contents. Burner tilt angle of −15° benefits for lower NOx emission (183 mg/m3) but goes against stable combustion; the burner arrangement mode of ABDE is optimal for the present boiler, which ensures both stable combustion and lower NOx emission (209 mg/m3).

Downward feed injection scheme is more promising than traditionally upper feed injection scheme for FCC riser reactors, however, its effects on the whole riser performance have not been elaborated. This study aims at CFD modeling of hydrodynamics and chemical reactions in an industrial-scale riser reactor, with focus on the influence of downward feed injection scheme. For this purpose, a CFD model, verified earlier in a real industrial riser reactor, is extended to the present work. The hydrodynamics, temperature profile and species concentration distribution in the riser reactor with the downward feed injection scheme are numerically studied and compared to those in the upper feed injection scheme. The results indicate that different from the smooth evolution in the upward feed injection scheme, the gas velocity, particle content and riser temperature in the downward injection scheme exhibit local maximum value in the feed injection zone. In the middle and upper zones of the riser reactor, the downward 45° and 60° injections show lower gas velocity and riser temperature than the upward 60° injection while the downward 30° injection shows an opposite trend. The downward feed injection scheme with an angle of 45° and a velocity of 60 m/s is optimal for the present industrial-scale riser reactor. Compared to the traditionally upper feed injection scheme, the new downward feed injection scheme could enhance the yields of the diesel and gasoline species by 0.93 and 0.29% point and reduce the yields of the dry gas and coke species by 0.61 and 0.96 unit.

This study presents a comprehensive comparision between 2D and 3D hydrodynamic modeling of a pseudo-2D turbulent fluidized bed with Geldart B particle. Based on the Euler-Euler approach and the EMMS-based drag model, 2D/3D CFD models are established, their sensitivities to the restitution coefficient and the specularity coefficient are analyzed, and the 2D/3D hydrodynamic simulations are performed and compared. The simulation results show that 3D simulations are more sensitive to the restitution coefficient and the specularity coefficient than 2D simulations. At the beginning of fluidization process, 2D simulation predicts greater bubble size and higher bed expansion than 3D simulation; as a complete fluidization is achieved, 2D model exhibits higher solid concentrations in the middle transition and the upper dilute-phase regions; the fluidization process in the 2D simulation develops more quickly than that in the 3D computation. Both the 2D and 3D models could capture the global flow behavior in the bottom dense-phase region of the turbulent fluidized bed reasonably. In the middle and upper regions, however, the 2D model overestimates the solid concentration and particle velocity while the 3D simulation gives better hydrodynamic prediction. For the present pseudo-2D turbulent fluidized bed with Geldart B particle, the bottom dense-phase region resembles 2D flow and 2D simulations may be adequate; however, the middle and upper regions exhibit 3D flow and full 3D simulations are needed.