Thermal and emission characteristics in a tangentially fired boiler model furnace

Tangentially fired furnaces are vortex‐combustion units and are intended for use as low NO formation furnaces. NO characteristics inside these furnaces depend on many parameters. The present study investigates numerically the problem of pollution in a real furnace of a 1699 MW tangentially fired boiler having 32 burners. NO formation contour maps in a tangentially fired furnace are presented. The study covered different combustion air temperature values, different fuel–air ratios and different cases of tripped burners. Available experimental measurements were used for validating the calculation procedure. The details of the temperature and NO fields were obtained from the solution of the conservation equations of mass, momentum and energy and transport equations for scalar variables in addition to the equations of the turbulence model. The equations governing the NO formation were solved to calculate the NO distribution. The simulation provided more insight on the correlation between the total NO concentration and the maximum furnace temperature and furnace average temperature. The results have shown that the furnace average temperature and NO concentration decrease as the excess air factor increases for a given air mass flow rate. As the combustion air temperature increases, furnace temperature increases and the thermal NO concentration increases sharply. The results show that the temperature distributions are significantly distorted by tripping any of the burners. The results show that tripping one or two burners either adjacent or opposite or tripping four results in regions of high temperature gases close to the walls. Heat absorptions in super‐heater and economizer are greatly influenced by combustion air temperature and excess air factor and are slightly influenced by burner tripping. Copyright © 2009 John Wiley & Sons, Ltd.     

CFD modelling of bed shrinkage and channelling in fixed-bed combustion

Combustion of fixed fuel beds in grate furnaces is common within production of heat and power from solid fuels. Available theoretical and experimental experience provides a solid base of knowledge on how a conversion model of a fuel bed, using Computational Fluid Dynamics (CFD), needs to be structured and solved. Most existing models, however, handle the conversion in one single dimension of constant bed properties; when observing a burning fuel bed in a grate furnace it becomes apparent that the fuel bed is neither homogeneous nor uni-dimensional. In this study, a two-dimensional model of the combustion of fixed fuel beds has been developed for the purpose of studying the influence of heterogeneous fuel-bed properties on the conversion. In the model, the available experience from fuel-bed modelling by means of the sub-models for fixed-bed conversion was structured into a fluid-flow scale and into a fuel-particle scale, in which new formulations describing the shrinkage of the fuel bed on a multi-particle scale was introduced. Both available and new sub-models were introduced into a pre-existing CFD-platform, in which the framework for simulating fluid flow in porous media was used to solve also the conversion related processes acting within the particle scales as well as within the multi-particle scales. The complete model was validated with good correspondence between available measurements of temperature and species concentration in a wood-char combustor. In addition, the modelled shrinkage was found to well describe the observed shrinkage of the fuel bed in a combustion experiment. Results of model simulations by using heterogeneous bed porosity show that a porous passage through the bed risks causing channelling in the fuel bed – a phenomenon common in modern grate furnaces and suspected to cause increased emissions of nitric oxides and unburned carbon compounds. The channelling tendency could, however, to a large extent be reduced by grates of higher flow resistance. The natural porosity increase attributable to the packing of particles onto a wall was shown to concentrate combustion disturbances close to the surface of the grate. Thus, larger changes in the porosity than caused by natural fuel packing against a wall are needed to give rise to channels that emerge through the fuel bed.     

Simulation of ash deposition in different furnace temperature with a 2D dynamic mesh model

Ash deposit on the heat exchangers reduces the heat transfer efficiency and even threatens the operation of the equipment. The tool of computational fluid dynamics (CFD) allows for better understanding of the deposit formation and the prediction of the process. This paper presents an improved CFD model to reproduce the growth of ash deposition on a temperature-controlled probe in a pilot-scale furnace with the commercial software Fluent16.0. Dynamic mesh technique is included to investigate the shape variation of the ash deposit during the deposit growth. The model is improved by taking the changing surface temperature of the deposition into consideration. The deposition efficiency, surface temperature and heat flux through the deposit are monitored as the iteration. Three cases are presented to investigate the influence of furnace temperature (1473 K, 1523 K and 1573 K). The results show that the deposition efficiency increases with the increasing surface temperature of the deposit while the mass flow of impaction decreases with the changing flow field. The growth rates of the deposit for the three cases are 0.064, 0.079 and 0.103 mm/min within the simulation time which is consistent with experiment results. The simulated surface temperature shows the same trend of the experimental values. The heat flux in the simulation decreases with a range of 38.2%, 50.3% and 50% for the three cases, respectively. This method of modelling can be used to predict the growth of deposit accurately.     

Performance prediction of a RPF‐fired boiler using artificial neural networks

In order to provide adequate engineering assistance and to improve the energy efficiency in process industries, it is crucial to evaluate the operational performance of a boiler in terms of its practical requirements, viz. temperature, pressure, and mass flow rate of steam. This study was aimed at assessing and optimizing the performance of a refuse plastic fuel‐fired boiler using artificial neural networks. A feed‐forward back propagation neural network model was developed and trained using existing plant data (5 months), to predict temperature, pressure, and mass flow rate of steam, using the following input parameters: feed water pressure, feed water temperature, conveyor speed, and incinerator exit temperature. The predictive capability of the model was evaluated in terms of mean absolute percentage error between the model fitted and actual plant data, while sensitivity analysis was performed on the input parameters by determining the absolute average sensitivity values. The higher absolute average sensitivity value of the incinerator exit temperature in comparison to that of feed water pressure, feed water temperature and conveyor speed suggested that the change of incineration exit temperature has a significant influence on the selected outputs (steam properties). Overall, the good results observed from this work demonstrate the fact that artificial neural networks can efficiently predict the data on steam properties and could serve as a good tool to monitor boiler behavior under real‐time conditions. Copyright © 2013 John Wiley & Sons, Ltd.     

Measurements and numerical simulations for optimization of the combustion process in a utility boiler

A three‐dimensional computational fluid dynamics code was used to analyse the performance of 550MW pulverized coal combustion opposite a wall‐fired boiler (of IEC) at different operation modes. The main objective of this study was to prove that connecting plant measurements with three‐dimensional furnace modelling is a cost‐effective method for design, optimization and problem solving in power plant operation. Heat flux results from calculations were compared with measurements in the boiler and showed good agreement. Consequently, the code was used to study hydrodynamic aspects of air–flue gases mixing in the upper part of the boiler. It was demonstrated that effective mixing between flue gases and overfire air is of essential importance for CO reburning. From our complementary experimental‐numerical effort, IEC considers a possibility to improve the boiler performance by replacing the existing OFA nozzles by those with higher penetration depth of the air jets, with the aim to ensure proper mixing to achieve better CO reburning. Copyright © 2004 John Wiley & Sons, Ltd.     

国产化大型循环流化床锅炉的运行分析

简要介绍了国产化DG450／9．81-1型循环流化床锅炉的主要设计参数及结构布置，通过对该型锅炉的运行分析，认为该型锅炉总体设计基本达到要求，运行稳定可靠，主要运行参数满足运行要求。对锅炉存在的问题也进行了简要分析，希望能够进一步完善大型循环流化床锅炉的设计、制造、安装及提高运行水准。     

Evaluation of a combustion model for the simulation of hydrogen spark-ignition engines using a CFD code

The present work deals with the evaluation of a combustion model that has been developed, in order to simulate the power cycle of hydrogen spark-ignition engines. The motivation for the development of such a model is to obtain a simple combustion model with few calibration constants, applicable to a wide range of engine configurations, incorporated in an in-house CFD code using the RNG k–? turbulence model. The calculated cylinder pressure traces, gross heat release rate diagrams and exhaust nitric oxide (NO) emissions are compared with the corresponding measured ones at various engine loads. The engine used is a Cooperative Fuel Research (CFR) engine fueled with hydrogen, operating at a constant engine speed of 600 rpm. This model is composed of various sub-models used for the simulation of combustion of conventional fuels in SI engines; it has been adjusted in the current study specifically for hydrogen combustion. The basic sub-model incorporated for the calculation of the reaction rates is the characteristic conversion time-scale method, meaning that a time-scale is used depending on the laminar conversion time and the turbulent mixing time, which dictates to what extent the combustible gas has reached its chemical equilibrium during a predefined time step. Also, the laminar and turbulent combustion velocity is used to track the flame development within the combustion chamber, using two correlations for the laminar flame speed and the Zimont/Lipatnikov approach for the modeling of the turbulent flame speed, whereas the (NO) emissions are calculated according to the Zeldovich mechanism. From the evaluation conducted, it is revealed that by using the developed hydrogen combustion model and after adjustment of the unique model calibration constant, there is an adequate agreement with measured data (regarding performance and emissions) for the investigated conditions. However, there are a few more issues to be resolved dealing mainly with the ignition process and the applicability of a reliable set of constants for the emission calculations.     

65t/h抛煤机锅炉飞灰回燃装置改造的实践

针对抛煤机链条炉飞灰可燃物含量的问题，将平板式分离器改为槽形分离器，同时在炉膛敷设回燃带，提高燃烧温度。运行结果表明：锅炉平均出力提高5～10t/h，产汽标煤耗下降8％。     

CFD modeling of heat transfer and fluid flow inside a pent-roof type combustion chamber using dynamic model

A numerical work has been performed to analyze the heat transfer and fluid flow in a pent-roof type combustion chamber. Dynamic mesh model was used to simulation piston intake stroke. Revolution of piston (1000 ≤ n ≤ 5000) is the main governing parameter on heat and fluid flow. k–ε turbulence model was used to predict the flow in the cylinder of a non-compressing fluid. They were solved with finite volume method and FLUENT 12.0 commercial code. Velocity profiles, temperature distribution, pressure distribution and velocity vectors are presented. It is found that the inclined surface of pent-roof type of combustion chamber reduces the swirl effect and it can be a control parameter for heat and fluid flow.     

Bump环强化柴油混合过程的数值模拟研究

对双模式HCCI燃烧,促进其主喷阶段燃油的混合速率至关重要。采用CFD数值模拟方法研究了一种新型燃烧室设计——BUMP燃烧室对直喷柴油机喷雾燃烧过程的影响。结果表明,bump环扰动缸内气流运动产生复杂的流谱,形成强烈的湍流。燃油喷雾撞壁后,bump环剥离壁面射流形成二次空间射流,减少了燃油在燃烧室壁面的沉积,湍流混合速率大大增加。自燃着火时刻,BUMP燃烧室内有38．2％的燃油处于碳烟生成门槛之外,而对比燃烧室仅为28．9％。数值模拟解释了BUMP燃烧室同时降低NOx和碳烟排放的实验现象。此外,模拟还发现燃油混合速率对喷油定时非常敏感,存在一个高湍流混合速率曲轴转角区间。     

Compositional and structural study of ash deposits spatially distributed in superheaters of a large biomass-fired CFB boiler

Recognizing the nature and formation progress of the ash deposits is essential to resolve the deposition problem hindering the wide application of large-scale biomass-fired boilers. Therefore, the ash deposits in the superheaters of a 220 t/h biomass-fired CFB boiler were studied, including the platen (PS), the high-temperature (HTS), the upper and the lower low-temperature superheaters (LTS). The results showed that the deposits in the PSs and HTSs were thin (several millimeters) and compact, consisting of a yellow outer layer and snow-white inner layer near the tube surface. The deposits in the upper LTS appeared to be toughly sintered ceramic, while those in the lower LTS were composed of dispersive coarse ash particles with an unsintered surface. Detailed characterization of the cross-section and the initial layers in the deposits revealed that the dominating compositions in both the PSs and the HTSs were Cl and K (approximately 70%) in the form of KCl. Interestingly, the cross-section of the deposition in the upper LTS exhibited a unique lamellar structure with a major composition of Ca and S. The contents of Ca and Si increased from approximately 10% to approximately 60% in the deposits from the high temperature surfaces to the low temperature ones. It was concluded that the vaporized mineral matter such as KCl played the most important role in the deposition progress in the PS and the HTS. In addition, although the condensation of KCl in the LTSs also happened, the deposition of ash particles played a more important role.     

Safe heating-up of a full scale SOFC system using 3D multiphysics modelling optimisation

Heating-up strategies of full scale solid oxide fuel cell (SOFC) systems still affect the safe operation of the system and incorporation of the technology into the global energy sector. To ensure rapid start-up times whilst retaining the structural reliability of the SOFC system components, requires a safe heating-up operation. To master a controlled heating-up stage, detailed understanding of the component interaction and multiphysics within a fuel cell system is required. State of the art dynamic fuel cell system modelling comprises sub-models of the assembly, or is based on empirical nature. However, invaluable information of the multiphysics inside the system is lost. Therefore, it is of paramount importance to understand and improve the knowledge of the detailed processes, occurring within the interacting components. The effect of integrating different electrical heater cartridges at different locations has been thoroughly investigated to optimise the heating-up of the system. The study utilises a previously developed and experimentally validated full scale three dimensional planar type SOFC system model to mitigate experimental costs and shed light on the details, occurring within the system. A comparison to a simplified variant of the model has been added to shed light on its effect on the results.     

Fault simulation of boiler heating surface ash deposition in a power plant system

The simulation model of a power generation system was developed based on EASY5 simulation platform. The performances of the power plant under the conditions of the furnace slagging and ash deposition of the heating surfaces in the boiler were simulated. The results show that the simulation model can reasonably reflect the characteristics of the power plant when each component is under fault conditions. Through fault simulation, the change of the performance parameters can be obtained, which can be used in fault diagnosis system as the diagnosis criterion for expert system.     

3D transient multiphysics modelling of a complete high temperature fuel cell system using coupled CFD and FEM

Full commercialisation of the solid oxide fuel cell (SOFC) technology faces many technological challenges that prevent the incorporation of the technology into the global energy sector. The effort to increase the transient thermomechanical reliability of the interacting fuel cell components and the associated fuel cell system requires a comprehensive understanding of the complex multiphysics, occurring within the system. State of the art dynamic fuel cell system modelling comprises sub-models of the assembly, or is based on empirical nature. The present study introduces a transient, coupled 3D computational fluid dynamics/computational solid mechanics model of a complete solid oxide fuel cell system and its experimental validation. The model includes all system components; namely the fuel cell stack, afterburner, pre-reformer, air pre-heater and the auxiliary components. All components are presented in their real geometrical resolution. The capabilities of the 3D system level model are demonstrated by simulating the heating-up process and the critical system locations susceptible to thermomechanically induced stress, over time.     

CFD modelling and experimental validation of cell performance in a 3-D planar SOFC

Fuel cells can be used to provide power for most electrical or electronic devices designed for operation from batteries or from conventional utility power sources. In this study, a three dimensional Computational Fluid Dynamics (CFD) simulation model has been developed and experimentally tested for an anode-supported planar SOFC that has bipolar plated for corrugation which serving as a gas channel and current collector. Experiments were performed on planar cross-flow type at different reactant flow rates, cell temperatures and pressures. In the experimental analysis, values varied from 0.12 L/min to 2 L/min for reactant and from 700 °C to 800 °C SOFC cell temperature. Thereby divergent operating parameters about cell parameters have been addressed. The conservation equations of momentum, energy and mass types are solved with the ANSYS FLUENT software in the proposed model. The maximum power density measured as 6 kW/m² under optimum working conditions. The results also show that the current density and the inlet velocity of fuel gassed are the main parameters that drive the fuel utilization and the total conversion efficiency. All the experimental and numerical findings, which were in good agreement with each other, showed that for Current density – Potential difference characteristic of SOFC cell graphs.     

LES modelling of hydrogen release and accumulation within a non-ventilated ambient pressure garage using the ADREA-HF CFD code

Computational Fluid Dynamics (CFD) has already proven to be a powerful tool to study the hydrogen dispersion and help in the hydrogen safety assessment. In this work, the Large Eddy Simulation (LES) recently incorporated into the ADREA-HF CFD code is evaluated against the INERIS-6C experiment of hydrogen leakage in a supposed garage, which provides detailed experimental measurements, visualization of the flow and availability of previous CFD results from various institutions (HySafe SBEP-V3). The short-term evolution of the hydrogen concentrations in this confined space is examined and comparison with experimental data is provided, along with comments about the ability of LES to capture the transient phenomena occurring during hydrogen dispersion. The influence of the value of the Smagorinsky constant on the resolved and on the unresolved turbulence is also presented. Furthermore, the renormalization group (RNG) LES methodology is also tested and its behaviour in both highly-turbulent and less-turbulent parts of the flow is highlighted.     

Effects of ash fouling on heat transfer during combustion of cattle biomass in a small‐scale boiler burner facility under unsteady transition conditions

Combustion of cattle biomass (CB) as a supplementary fuel has been proposed for reducing emissions of NO_x, Hg, SO₂, and nonrenewable CO₂ in large coal‐fired power plants; however, its high ash content resulted in fouling and slagging problems when the CB was co‐fired with coals during small‐ and pilot‐scale tests. Ash depositions during combustion of the CB as a reburn fuel were investigated using a 30 kW_t (100 000 Btu h^?1) boiler burner facility with water‐cooled heat exchangers (HEXs) under unsteady transition conditions and short‐term operations. Two parameters were used to characterize the effects of the ash deposition: (1) Overall heat transfer coefficient (U) and (2) Burnt fraction (BF). A methodology was presented and empirically demonstrated for the effects of ash deposition on heat transfer under unsteady transition conditions. Experiments involving ash deposition during reburning the CB with coals were compared with experiments involving only ash‐less natural gas. It was found that the growth of the ash layer during reburning the CB and coals lowered the heat transfer rate to water in the HEXs. In low‐temperature regions, the thin layer of the ash deposition promoted radiation effects, while the thick layer of the ash deposition promoted the thermal resistance of the ash layer. A chemical analysis of the heavy ash indicated that the BF increased when a larger fraction of the CB was used in the reburn fuels, indicating better performance compared with coal combustion alone. However, the results of ash fusion temperature indicated the ash deposited during combustion of the CB and coals was more difficult to remove than the ash deposited during coal combustion alone. Copyright © 2010 John Wiley & Sons, Ltd.     

A 7 year long measurement period investigating the correlation of corrosion,deposit and fuel in a biomass fired circulated fluidized bed boiler

The present investigation involves a unique, 7 year (2001–2007) long study of corrosion and deposits on superheater tubes in a biomass fired circulated fluidized bed boiler. These measurements are correlated against the different fuels used over this period. In the earlier years, the boiler was run with a mixture of different biomass fuels and peat. In later years, recycled wood was introduced into the fuel mix. The deposit growth rate approximately doubled when the recycled wood content of the fuel was increased to 10–20%. Small amounts of chlorine and zinc were found both in the recycled wood and in the deposit layer. These elements together with alkali metals from the biomass, have the potential to form sticky compounds that increase the deposit growth rate. The corrosion rate of the superheater tubes varied over the study period. A number of possible explanations for this phenomenon are discussed.     

The black smoke control in a chain‐grate stoker‐fired boiler at the time of ignition and restart of the combustion

The black smoke is always emitted from the chimney in the chain‐grate stoker‐fired boiler at the time of ignition at ambient temperature and restart of the combustion after temporary flameout in China. The purpose of the work is to reduce the black smoke emission. A laboratory fixed‐grate model has been used to simulate the combustion of coal in chain‐grate stoker‐fired boiler. The CO and O₂ concentration in the flue gas have been measured with a flue gas analyser, and the black smoke emitted from the chimney has been screened with Charge Couple Device (CCD) video camera. Power 2^# coal, sized at 5–25 mm, has been fired in the fixed‐grate model. The secondary air has been used to enhance the turbulence in the furnace after the numerical simulation. The results of experiments show that the emission of the black smoke at the time of ignition of the coal at ambient temperature is more serious than that of restart of the combustion after the temporary flameout for the case of the temperature; the secondary air is helpful for reducing the black smoke emission for enhancing the intensity of the turbulence in the furnace; selection of coal particle size is necessary, the smaller the size of the coal is, the more serious the black smoke emission is, and the effect of reducing the black smoke emission with the secondary air is more evident with smaller‐size coal. The industrial test has been employed to study the effect of the air demand. It indicates that rational stoichiometric air/fuel ratio is helpful for reducing the black smoke emission in the restart of the combustion after the temporary flameout. Copyright © 2006 John Wiley & Sons, Ltd.     

CFD modeling and consequence analysis of an accidental hydrogen release in a large scale facility

In this study, the consequences of an accidental release of hydrogen within large scale, (>15,000 m³), facilities were modeled. To model the hydrogen release, an LES Navier–Stokes CFD solver, called fireFoam, was used to calculate the dispersion and mixing of hydrogen within a large scale facility. The performance of the CFD modeling technique was evaluated through a validation study using experimental results from a 1/6 scale hydrogen release from the literature and a grid sensitivity study. Using the model, a parametric study was performed varying release rates and enclosure sizes and examining the concentrations that develop. The hydrogen dispersion results were then used to calculate the corresponding pressure loads from hydrogen-air deflagrations in the facility.