Computational investigation of hydrodynamics,coal combustion and NOx emissions in a tangentially fired pulverized coal boiler at various loads

This work presents a computational investigation of hydrodynamics, coal combustion and NOx emissions in a tangentially fired pulverized coal boiler at different loads (630, 440 and 300 MW; relative loads of 100%, 70% and 48%) to clarify the effect of load change on the furnace processes. A computational fluids dynamics model was established; the flow field, temperature profile, species concentration and NOx emissions were predicted numerically; and the influence of burner tilt angles was evaluated. Simulation results indicate that a decrease in boiler load decreases the gas velocity, attenuates the airflow rotations, and increases the tangent circle size. The high-temperature zone and flame moved toward the side walls. Such behaviors impair air–fuel mixing, heat transfer and steady combustion in the furnace. In terms of species concentrations, a decrease in boiler load increased the O₂ content, decreased the CO content, and decreased the char burnout rates only slightly. A change in boiler load from 630 to 440 and 300 MW increased the NOx emissions from 202 to 234 and 247 mg/m³, respectively. Burner tilt angles are important in coal combustion and NOx emissions. A burner angle of –15° favors heat transfer and low NOx emissions (<185 mg/m³) for the current tangentially fired boiler.     

Retention mechanism of calcium ferrite and compositions of ash on selenium during chemical looping gasification

Selenium pollution by coal utilization is of increasing concern. Calcium-iron (Ca–Fe) oxygen carriers (OCs) and alkali metal ions have strong inhibitory effects on selenium, which can reduce the emissions of selenium vapor. The retention mechanisms of selenium by Fe₂O₃, CaFe₂O₄, Ca₂Fe₂O₅ and bottom ash are investigated during chemical looping gasification (CLG). Iron-based OC can oxidize H₂Se(g) to SeO₂(g); furthermore, lattice oxygen is released by Fe₂O₃, contributing to the formation of an Fe–O–Se structure to retain selenium and form selenite. Because calcium ferrite is poorly oxidizing, it cannot oxidize H₂Se(g), but the CaO produced when OCs are reduced can react with H₂Se(g) to form CaSe(s), and this process can be promoted by H₂S(g). The best retention rates reached 32.301% when Ca₂Fe₂O₅ was used. In the cyclic experiment, the selenium retention of the bottom ash gradually increases. Alkali metal ions in bottom ash are the main factor in retaining selenium. Ca²⁺ and Mg²⁺ do not easily vaporize due to their high melting points; therefore, their selenium retention is significantly better than that of K⁺ and Na⁺. This research provided a new idea for the removal of selenium by using OCs and bottom ash particles during CLG.     

Accelerating fully resolved simulation of particle-laden flows on heterogeneous computer architectures

An efficient computing framework, namely PFlows, for fully resolved-direct numerical simulations of particle-laden flows was accelerated on NVIDIA General Processing Units (GPUs) and GPU-like accelerator (DCU) cards. The framework is featured as coupling the lattice Boltzmann method for fluid flow with the immersed boundary method for fluid-particle interaction, and the discrete element method for particle collision, using two fixed Eulerian meshes and one moved Lagrangian point mesh, respectively. All the parts are accelerated by a fine-grained parallelism technique using CUDA on GPUs, and further using HIP on DCU cards, i.e., the calculation on each fluid grid, each immersed boundary point, each particle motion, and each pair-particle collision is responsible by one computer thread, respectively. Coalesced memory accesses to LBM distribution functions with the data layout of Structure of Arrays are used to maximize utilization of hardware bandwidth. Parallel reduction with shared memory for data of immersed boundary points is adopted for the sake of reducing access to global memory when integrate particle hydrodynamic force. MPI computing is further used for computing on heterogeneous architectures with multiple CPUs-GPUs/DCUs. The communications between adjacent processors are hidden by overlapping with calculations. Two benchmark cases were conducted for code validation, including a pure fluid flow and a particle-laden flow. The performances on a single accelerator show that a GPU V100 can achieve 7.1–11.1 times speed up, while a single DCU can achieve 5.6–8.8 times speed up compared to a single Xeon CPU chip (32 cores). The performances on multi-accelerators show that parallel efficiency is 0.5–0.8 for weak scaling and 0.68–0.9 for strong scaling on up to 64 DCU cards even for the dense flow (φ = 20%). The peak performance reaches 179 giga lattice updates per second (GLUPS) on 256 DCU cards by using 1 billion grids and 1 million particles. At last, a large-scale simulation of a gas-solid flow with 1.6 billion grids and 1.6 million particles was conducted using only 32 DCU cards. This simulation shows that the present framework is prospective for simulations of large-scale particle-laden flows in the upcoming exascale computing era.     

Mixed convective heat transfer characteristics and mechanisms in structured packed beds

The structured packed bed is considered a promising reactor owing to its low pressure drop and good heat transfer performance. In the heat transfer process of thermal storage in packed beds, natural convection plays an important role. To obtain the mixed convective heat transfer characteristics and mechanisms in packed beds, numerical simulations and coupling analyses were carried out in this study on the unsteady process of fluid flow and heat transfer. A three-dimensional model of the flow channel in the packed bed was established, and the Navier–Stokes equations and Laminar model were adopted for the computations. The effects of the driving force on fluid flow around a particle were studied in detail. The differences in velocity and density distributions under different flow directions due to effect of the aiding flow or opposing flow were intuitively demonstrated and quantitatively analyzed. It was found that the driving force strengthens the fluid flow near the particle surface when aiding flow occurs and inhibits the fluid flow when opposing flow occurs. The boundary layer structure was changed by the natural convection, which in turn influences the field synergy angle. For the aiding flow, the coordination between the velocity and density fields is higher than that for the opposing flow. By analysis the effects of physical parameters on mixed convective heat transfer, it is indicated that with an increase in the fluid-solid temperature difference or the particle diameter, or a decrease in the fluid temperature, the strengthening or inhibiting effect of natural convection on the heat transfer became more significant.     

Development and implementation of an IOT based instrumentation system for computing performance of a tractor-implement system

A high precision and compact IOT based digital instrumentation setup to measure, display and record various tractor and implement system performance parameters was developed and installed on a 28.3 kW Tractor. The setup was capable of continuous monitoring and wirelessly transmitting tractor-implement performance parameters on a cloud platform such as engine speed, radiator fan speed, fuel consumption, draft, forward speed, lift arm angle, wheel slip, wheel slip, PTO speed, geo-location/position of the tractor, choking of seeds in the implement and vibrations experienced by the implement. For precision measurements, commercial transducers used in the system were calibrated and assessed under both static and dynamic conditions. The average calibration constant for fuel consumption, forward speed, lift arm angle and load cell were 0.00009804 L/pulse, 0.01610306 km/h/pulse, 0.056 mA/degree and 0.2575 mV/kN respectively. The system based on DataTaker DT 85 Data logger connected to a micro-computer through transducers capable of transferring data wirelessly was installed on John Deere 5038 tractor and was tested with a Spatially Modified No-Till Drill in agricultural field with varied implement depth.     

Improved operator comfort and off-road capability through pendulum arm technology

The Nordic forest industry requires just-in-time wood deliveries. Operations must continue regardless of season, weather and terrain. Soil compaction and deep ruts must be avoided while providing high performance and a reasonable working environment for operators.The Xt28 pendulum arm forwarder is a full-size concept forwarder with six hydrostatic propelled wheels on pendulum arms built on a three-piece frame connected with two articulation joints. The Xt28 concept machine was tested according to Skogforsk standard machine tests. Rut depth test focused on soil interaction where rut depth was measured related to number of passes. Machine dynamics were measured using standardized test track with focus on operator comfort.The project proved the potential of pendulum arm technology in off-road transportation. Automatic pendulum arm levelling, equalized ground pressure between wheels and improved operator comfort through reducing adverse vibrations and roll angles, simultaneously reducing dynamic forces transferred to the forest floor. Pendulum arm technology improves travel speed in adverse terrain, providing unparalleled side slope capability and enhanced productivity.     

Fully resolved simulations of viscoelastic suspensions by an efficient immersed boundary-lattice Boltzmann method

An efficient immersed boundary-lattice Boltzmann method (IB-LBM) is proposed for fully resolved simulations of suspended solid particles in viscoelastic flows. Stress LBM based on Giesekus and Oldroyd-B constitutive equation are used to model the viscoelastic stress tensor. A boundary thickening-based direct forcing IB method is adopted to solve the particle–fluid interactions with high accuracy for non-slip boundary conditions. A universal law is proposed to determine the diffusivity constant in a viscoelastic LBM model to balance the numerical accuracy and stability over a wide range of computational parameters. An asynchronous calculation strategy is adopted to further improve the computing efficiency. The method was firstly applicated to the simulation of sedimentation of a single particle and a pair of particles after good validations in cases of the flow past a fixed cylinder and particle migration in a Couette flow against FEM and FVM methods. The determination of the asynchronous calculation strategy and the effect of viscoelastic stress distribution on the settling behaviors of one and two particles are revealed. Subsequently, 504 particles settling in a closed cavity was simulated and the phenomenon that the viscoelastic stress stabilizing the Rayleigh–Taylor instabilities was observed. At last, simulations of a dense flow involving 11001 particles, the largest number of particles to date, were performed to investigate the instability behavior induced by elastic effect under hydrodynamic interactions in a viscoelastic fluid. The elasticity-induced ordering of the particle structures and fluid bubble structures in this dense flow is revealed for the first time. These simulations demonstrate the capability and prospects of the present method for aid in understanding the complex behaviors of viscoelastic particle suspensions.     

Bottom bed regimes in a circulating fluidized bed boiler

This paper extends previous work on the fluidization regimes of the bottom bed of circulating flyidized bed (CFB) boilers. Pressure measurements were performed to obtain the time-average bottom bed voidage and to study the bed pressure fluctuations. The measurements were carried out in a 12 MW_th CFB boiler operated at 850°C and also under ambient conditions (40°C). Two bubbling regimes were identified: a “single bubble regime” with large single bubbles present at low fluidization velocities, and, at high fluidization velocities, an “exploding bubble regime” with bubbles often stretching all the way from the air distributor to the surface of the bottom bed. The exploding bubble regime results in a high through-flow of gas, indirectly seen from the low average voidage of the bottom bed, which is similar to that of a stationary fluidized bed boiler, despite the higher gas velocities in the CFB boiler. Methods to determine the fluidization velocity at the transition from the single to the exploding bubble regime are proposed and discussed. The transition velocity increases with an increase in particle size and bed height.     

Effect of secondary air on NO emission in a 440 t/h circulating fluidized bed boiler based on CPFD method

The 440 t/h circulating fluidized bed boiler was numerically simulated by the Computational Particle Fluid Dynamics (CPFD) method. The combustion characteristics of circulating fluidized bed boiler and the effect of secondary air on NO emission were investigated. The full-scale three-dimensional model of a 440 t/h circulating fluidized bed boiler was established. The rationality of the grid was validated by the experimental data of material layer resistance. The accuracy of the simulation was validated by measuring the temperature of each measuring point in the dense phase area. The combustion conditions in the furnace under different setting modes were simulated. The effects of secondary air rates on NO formation in fluidized bed were predicted. The results show that when the secondary air rate increases to 27%, the proper secondary air rate has a positive effect on the inhibition of NO generation, and the proper strengthening of the central air supply will improve the permeability of the secondary air and make the combustion more uniform and stable. When the secondary air rate increases to 33%, excessive improvement of air classification and central air distribution will affect the stability of circulating fluidized bed operation. Therefore, air classification and strengthening of central air supply can be used together to inhibit the generation of NO.     

Investigation on lateral dispersion characteristics of fuel particles in dense phase zone of a large-scale circulating fluidized bed boiler under combustion conditions

The dispersion characteristics of fuel particles in the dense phase zone in circulating fluidized bed (CFB) boilers have an important influence on bed temperature distribution and pollutant emissions. However, previous research in literature was mostly on small-scale apparatus, whose results could not be applied directly to large-scale CFB with multiple dispersion sources. To help solve this problem, we proposed a novel method to estimate the lateral dispersion coefficient (D_x) of fuel particles under partial coal cut-off condition in a 350 MW supercritical CFB boiler based on combustion and dispersion models. Meanwhile, we carried out experiments to obtain the D_x in the range of 0.1218–0.1406 m²/s. Numerical simulations were performed and the influence of operating conditions and furnace structure on fuel dispersion characteristics was investigated, the simulation value of D_x was validated against experimental data. Results revealed that the distribution of bed temperature caused by the fuel dispersion was mainly formed by char combustion. Because of the presence of intermediate water-cooled partition wall, the mixing and dispersion of fuel and bed material particles between the left and right sides of the furnace were hindered, increasing the non-uniformity of the bed temperature near furnace front wall.     

Analysis of gas–solid flow and shaft-injected gas distribution in an oxygen blast furnace using a discrete element method and computational fluid dynamics coupled model

Ironmaking using an oxygen blast furnace is an attractive approach for reducing energy consumption in the iron and steel industry. This paper presents a numerical study of gas–solid flow in an oxygen blast furnace by coupling the discrete element method with computational fluid dynamics. The model reliability was verified by previous experimental results. The influences of particle diameter, shaft tuyere size, and specific ratio (X) of shaft-injected gas (SIG) flowrate to total gas flowrate on the SIG penetration behavior and pressure field in the furnace were investigated. The results showed that gas penetration capacity in the furnace gradually decreased as the particle diameter decreased from 100 to 40 mm. Decreasing particle diameter and increasing shaft tuyere size both slightly increased the SIG concentration near the furnace wall but decreased it at the furnace center. The value of X has a significant impact on the SIG distribution. According to the pressure fields obtained under different conditions, the key factor affecting SIG penetration depth is the pressure difference between the upper and lower levels of the shaft tuyere. If the pressure difference is small, the SIG can easily penetrate to the furnace center.     

Particle/Turbulence Interactions, Mass Transfer and Gas/Solid Chemistry in a CFBC Riser

Gas/solid chemistry in the upper, dilute region of a circulating fluidised bed combustor (CFBC) riser is addressed. The limitations of turbulent mixing are illustrated by the example of the heterogeneous NO/CO/char reaction, relevant in CFB combustion of nitrogen-containing solid fuels. The mass transfer of the gaseous reactants to the char surface is determined, and how the conversion is influenced by the degree of mixing of the multiphase system by turbulent dispersion. Particle/turbulence interactions are taken into account by a (Lagrangian) frequency spectrum of velocity fluctuations, which determine the dispersion of the char particles described here with the Tchen–Hinze model. Chars from solid fuels characterised by fuel ratio (FR) ranging from 0.1 (wood) via 0.5 (peat) and 1 (coal A) to 2 (coal B) were considered. The effective rate of the NO/CO/char reaction is determined as a function of the size and type of the char particle, temperature, particle concentration, reactor dimensions and fluidization velocity, at atmospheric pressure. It was found that for this case the effective gas/solid conversion rate in the upper, dilute region of the riser is much lower than the gas/solid chemistry, mainly due to mass transfer limitations for char particles with sizes of typically 300 μm. The concentration of NO at the char particle surface is only a few % of that in the bulk gas phase. Strong influences were found for particle size and temperature, whilst the fluidisation velocity and the reactor size have only a small influence. It is concluded that for a typical CFB riser, for particles larger than approx. 20 μm, mass transfer has a stronger influence on the heterogeneous NO/CO/char reduction mechanism than the ``unmixedness' due to particle eddy dispersion limitations. It is recommended that this or a similar approach to turbulent dispersive mixing is implemented in CFD codes when these are used for boiler and furnace calculations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Particle-scale and sub-grid drag models coupled CFD for simulating the CO methanation in a CFB riser

The diffusion and chemical reactions inside the catalyst particles and the heterogeneous flow structure in the computational cells are key factors to affect the accuracy of the coarse-grid simulation in circulating fluidized bed (CFB) methanation reactors. In this work, a particle-scale model is developed to calculate the effective reaction rate considering the transient diffusion and chemical reactions in the particle scale, i.e., the scale of the single catalyst particle. A modified sub-grid drag model is proposed to consider the effects of the meso-scale and chemical reactions on the heterogeneous gas-solid interaction, where the meso-scale is between the single particle and the whole reactor and featured with the particle cluster. Subsequently, a coupled model is developed by integrating the particle-scale and modified sub-grid drag models into CFD. Moreover, the coupled model is validated to achieve accurate predictions on the CO methanation process in a CFB riser. Notably, the coupled model can be performed with a coarse grid (∼58 times particle diameter) and a large time step (0.005 s) to accelerate the simulation. By simply changing the reaction kinetics, different gas-solid catalytic reaction systems can be simulated by using the coupled model.     

Spectroscopic research on infrared emittance of coal ash deposits

This paper deals with thermal radiation characteristics of ash deposits on a pulverized coal combustion boiler of an electric power plant. Normal emittance spectra in the near to medium infrared (2.5–25 μm) region and total normal emittances were measured on four kinds of ground ash deposits. Measurements were conducted in the 570–1460 K temperature range which is common for boiler furnaces, by both heating and cooling the ash samples, with the aim to study the effect of their thermal history. Dependence of emittance on wavelength, temperature and chemical composition was studied, too. Samples were tested for transparency (opacity) to verify the accuracy of results. It was determined that the thicknesses used for the ash powders are opaque for infrared radiation for thicknesses in the order of a millimeter. Tests have shown that spectral emittance increases with an increase of wavelength with a characteristic pattern common for all samples. Spectral normal emittance increases strongly with temperature at shorter wavelengths and remains high and unchanged at longer ones. Emittance spectra are not very sensitive to chemical composition of ashes especially beyond λ ≈ 5 μm. With an increase of temperature, total emittance of the powdered sample decreases to a minimum value around 1200 K. Further temperature rise induces an increase of total emittance due to sintering in the ash. On cooling, the emittance increases monotonically following the hysteresis. Quantitative directions for evaluating thermal radiation characteristics of ash deposits for the merits of the safety design of boiler furnaces were proposed. That comprises correlating the experimentally obtained emittance spectra with curves of simple analytical form, i.e., a continuous function of minimum emittance vs. wavelength. The proposed method can be extended to other specimens from the same furnace and used to determine correlations for thermal calculation of old and design of new furnaces – with similar geometry and combusting similar coal. The method is potentially applicable to completely different boiler furnaces combusting different coal, and the authors recommend running the tests with new deposit samples. The data will then be applicable to the thermal design of a whole new class of furnaces, having similar geometry and combusting similar coal. This is expected to greatly enhance the accuracy and precision of thermal calculation as well as the efficiency of thermal design of steam boilers.     

Effects of particle size and concentration on bubble coalescence and froth formation in a slurry bubble column

A new approach for simulating the formation of a froth layer in a slurry bubble column is proposed. Froth is considered a separate phase, comprised of a mixture of gas, liquid, and solid. The simulation was carried out using commercial flow simulation software (FIRE v2014) for particle sizes of 60–150 μm at solid concentrations of 0–40 vol%, and superficial gas velocities of 0.02–0.034 m/s in a slurry bubble column with a hydraulic diameter of 0.2 m and height of 1.2 m. Modelling calculations were conducted using a Eulerian–Eulerian multiphase approach with k–ε turbulence. The population balance equations for bubble breakup, bubble coalescence rate, and the interfacial exchange of mass and momentum were included in the computational fluid dynamics code by writing subroutines in Fortran to track the number density of different bubble sizes. Flow structure, radial gas holdup, and Sauter mean bubble diameter distributions at different column heights were predicted in the pulp zone, while froth volume fraction and density were predicted in the froth zone. The model was validated using available experimental data, and the predicted and experimental results showed reasonable agreement. To demonstrate the effect of increasing solid concentration on the coalescence rate, a solid-effect multiplier in the coalescence efficiency equation was used. The solid-effect multiplier decreased with increasing slurry concentration, causing an increase in bubble coalescence efficiency. A slight decrease in the coalescence efficiency was also observed owing to increasing particle size, which led to a decrease in Sauter mean bubble diameter. The froth volume fraction increased with solid concentration. These results provide an improved understanding of the dynamics of slurry bubble reactors in the presence of hydrophilic particles.     

Radiation modelling in oxy-fuel combustion scenarios

Coal combustion was simulated in a lab-scale furnace and a full scale utility boiler at air-burn, oxy-combustion with dry- and wet-flue gas recycles. Good agreement was obtained between the numerical predictions and experimental measurements. The study confirmed that certain dry and wet recycle ratios imitate the temperature and heat transfer characteristics found in air combustion. The performances of five grey models to predict the radiative properties of gases in the simulations were examined. Emissivity correlations developed for purely CO₂ media were not found to be suitable to predict the property at low H₂O/CO₂ ratios encountered during dry-recycle. Therefore, a new total emissivity correlation and a weighted-sum-of-grey-gases (WSGG) model were formulated to address the shortcomings and inaccuracies in existing grey gas models under oxy-firing. The new WSGG model with 4 band intervals was found to perform well in test cases that were representative of air- and oxy-firing conditions in boilers.     

Flow behavior of high-temperature flue gas in the heat transfer chamber of a pilot-scale coal-water slurry combustion furnace

The flow characteristics of high-temperature flue gas are important in the heat transfer of coal-water slurry (CWS) combustion furnaces. The flow field of a 250 kg/h vertical-type slag tap cyclone furnace was non-intrusively investigated, using two-dimensional particle-image velocimetry (2D PIV). The method was verified using traceable fly ash particles in high-temperature flue gas. The flow field of the flue gas was analyzed with a time-averaged method, based on which the effects of excess air ratio and loading were investigated. The flue gas separated by a gas separator maintained good rigidity near the furnace wall, rather than eroding the heating surface. Numerical simulations validated the reliability of PIV under the actual circumstances within the furnace. This study provides guidelines for applying 2D PIV in analyzing flue gas in thermal test boilers.     

Ultra-low-emission steam boiler constituted of reciprocal flow porous burner

This experimental study examined a low-emission steam boiler in which the filtration combustion technology was employed. This new boiler concept is consisted of a reciprocal flow porous burner, in which a combustion wave propagates along the reactor length. The boiler’s burner is filled up by an inert porous material, which leads to a stable burning of ultra-lean fuel/air mixtures, operating below flammability limits of conventional burners. In reciprocal filtration combustion, the reaction zone travels back and forth along the length of the burner, maintaining a typical trapezoidal temperature distribution favorable to the energy extraction. Embedding heat exchangers into the ends of the porous bed results in an alternative low-emission high-efficiency boiler. The heat re-circulation inside the porous matrix and the low degree of thermal non-equilibrium between the gas and the solid phases result in ultra-low levels of CO and NO_x. Over an equivalence ratio range from 0.20 to 1.0 and a gas flow velocity range from 0.2 to 0.6 m/s, burning the technical methane, the developed prototype has reached efficiencies superior to 90% and NO_x and CO emission levels lower than 1.0 and 0.5 ppm, respectively.     

Combustion and stabilization characteristics of a branched flame under Helmholtz-type excitations

 The combustion and pollution characteristics of the newly rediscovered “branched flame” are experimentally investigated using a Helmholtz-type excitation. Under specific excitation conditions, high-amplitude Helmholtz excitation induces side jet ejection, which leads to a branched flame. Intense combustion and enhanced heat transfer due to strong oscillation of the flame and hot gases of the branched flame increase the heating effectiveness and fuel saving. Strong velocity oscillation results in accumulation of jet fluid ahead of the ring structure for generation of the side jet. In the side-jet evolution, the strong entrainment of the ring vortex in the initial stages followed by the early growth of the streamwise vortical structures greatly shortens the route to mixing transition of fuel and air in the upstream region of the flame. This enhanced premixing process of the side jet leading to high F probability, which is defined as the probability of the presence of a premixture of fuel and air with concentration within the flammability limits, and low strain rate has significant implications for the stabilization of the branched flame. NOx emission indices for the branched flames can be 30% higher and CO emission indices 50% lower than the unexcited case. Received: 5 June 2000 / Accepted: 21 March 2001