Factors influencing the ignition of flames from air-fired swirl pf burners retrofitted to oxy-fuel

Combustion tests were undertaken in a vertical pilot-scale furnace (1.2 MWt) at the IHI test facility in Aioi, Japan, to compare the performance of an air fired swirl burner retrofitted to oxy fired pf coal combustion with the oxy fired feed conditions established to match the furnace heat transfer for the air fired case. A turn down test at a reduced load was also conducted to study the impact on flame stability and furnace performance.Experimental results include gas temperature measurements using pyrometry to infer the ignition location of the flames, flue gas composition analysis, and residence time and carbon burnout. Theoretical computational fluid dynamics (CFD) modelling studies using the Fluent 6.2 code were made to infer mechanisms for flame ignition changes.Previous research has identified that differences in the gas compositions of air and oxy systems increase particle ignition times and reduce flame propagation velocity in laminar systems. The current study also suggests changes in jet aerodynamics, due to burner primary and secondary velocity differences (and hence the momentum flux ratio of the flows) also influence flame shape and type.For the oxy fuel retrofit considered, the higher momentum flux of the primary stream of the oxy-fuel burner causes the predicted ignition to be delayed and occur further distant from the burner nozzle, with the difference being accentuated at low load. However, the study was limited to experimental flames being all Type-0 (low swirl with no internal recirculation), and therefore future work consider higher swirl flames (with internal recirculation) more common in industry.     

On-line monitoring of fuel moisture-content in biomass-fired furnaces by measuring relative humidity of the flue gases

Combustion of biomass for heat and power production is continuously growing in importance, because of incentives for replacing fossil energy resources with renewable ones. In biomass combustion, the moisture content of the fuel is an essential operation parameter, which often fluctuates for biomass fuels. Variation in moisture content complicates the operation of the furnaces and results in an uncertainty in the energy content of the fuel delivered to a plant. The fuel moisture-content in a furnace may be determined either by direct measurement on the entering fuel or by measuring the moisture and oxygen contents of the flue gases deriving the moisture content of the fuel. However, reliable methods of a motivated cost for the small to medium-scale furnaces are today not available. An exception is if the furnace is equipped with flue-gas condenser, which can be used to estimate the moisture content of the flue gases. A limitation of this method is, though, that not all furnaces have flue-gas condensers and that the measured signal has an inherent time delay.In this work, measurement of the relative humidity (RH) of the flue gases from a furnace is investigated as the central component in the on-line monitoring of the moisture content of the fuel in a furnace. The method was analysed with humid air in a laboratory environment and tested for accuracy and dynamical behaviour in two biomass-fired heat-production units, one circulating fluidised-bed boiler (CFB) and one grate furnace. The results show that the method, which is easy to calibrate on site, can be used to predict the moisture content of the biomass fuel in the grate furnace with very good precision (<4% error). Furthermore, the method detects variations in moisture content of the furnace flue gases due to changes in the moisture content of the combusted fuel within the order of seconds. Since the transport time of the flue gases from the furnace to the measurement position is of the same order of magnitude, the total time for detection of a change in the moisture content of the fuel is small enough for the signal to be used to control both the fuel feed and the combustion air in a grate furnace.     

Computational Modeling of Tangentially Fired Boiler(Ⅰ) Models,Flow Field and Temperature Profiles

In this work,an Eulerian/Lagrangian approach has been employed to investigate numerically the flowcharacteristics,heat transfer and combustion in a tangentially fired furnace.The RNG(Re-normalization group)κ-εmodel and a new method for cell face velocity interpolation based on a non-staggered grid system are employed.Toavoid pseudo-diffusion that is significant in modeling tangentially fired furnaces,attempts are made at improving thedifferential volume scheme.Some new developments on turbulent diffusion of particles are also taken into account.Thus,computational accuracy is improved substantially.     

Effect of Fuel Mixture Composition on Flame Vortex Structure

This paper studies the effect of propane–air mixture composition on the spontaneous structure of an inverted vortex flame during combustion of a gas injected to the lower surface of a plate inclined to the horizon. It has been established that the angular velocity of combustion products is determined by the orientation of the gas injection velocity vector with respect to the gravity direction, the velocity in the burner nozzle, and the fuel mixture composition. It has been shown that fuel composition changes cause restructuring of the velocity field in the vortex structure, which leads to concentration and temperature changes in the flame. Dependences of flame height and fuel combustion completeness on injectionrate and propane content are obtained.     

Computational modeling of pulverized coal combustion processes in tangentially fired furnaces

In this work, an Eulerian/Lagrangian approach has been employed to investigate numerically flow characteristics, heat transfer and combustion processes in a tangentially fired furnace. A new method of cell face velocity interpolation for non-staggered grid system is employed to avoid pressure oscillation. Grid-independence tests have been conducted. To avoid pseudo-diffusion that is significant in modeling tangentially fired furnaces, some attempts have been made at improving the finite-difference scheme. The standard k-ε model performs well in predicting flows without swirling or without sharp change within the calculated region. But for tangentially fired boiler furnaces, where swirling flow is very marked, we must resort to other more valid, more efficient turbulent models to gain accuracy. In this paper, we try to use RNG k-ε model as an alternative to the standard k-ε model. Comparisons have been made between standard k-ε and RNG k-ε models. Some new developments on turbulent diffusion of particles are taken into account for improving computational accuracy, and probability error is also discussed. Finally, temperature deviation is studied numerically so as to gain deeper insight into tangentially fired furnaces.     

Computational study of the combustion process and NO formation in a small-scale wood pellet furnace

This paper presents a computational study of the combustion process of wood pellets in a small-scale grate fired furnace. The objectives were to obtain detailed information on the combustion characteristics and NO formation in the furnace, and to examine the effect of secondary air on the combustion process. The simulation results were compared with experimental data in terms of flame temperature and distributions of species concentrations, including CO and NO. It was shown that the combustion process is strongly controlled by the inflow turbulence from the secondary and tertiary air jets. The combustion process is not sensitive to the bed combustion process in the present test case. The high speed air flow from the secondary and tertiary air inlets ‘destroys’ the history of the effluent volatile gases from the fuel bed. Different paths for the NO emission were investigated, including the thermal NO, the fuel-NO and NO from the N₂O intermediate mechanisms. The fuel-NO path is responsible for the rapid NO increase and the high NO peak near the fuel bed. Fuel-NO is rather low far downstream owing to the rather low nitrogen content in the fuel (less than 0.1% on mass basis), and the de-NO_x reactions with NH₃. NO is likely formed from the N₂O intermediate mechanism far downstream.     

Monitoring and characterisation of pulverised coal flames using digital imaging techniques

Quantification of physical properties of fossil fuel fired flames is becoming increasingly important to achieve in-depth understanding and subsequent optimisation of combustion processes. This paper presents investigations into the geometrical and luminous properties of pulverised coal flames using an advanced flame monitoring system. The system combines optical sensing, digital image processing and computing techniques and is designed to measure most physical parameters of a flame. Experimental trials of the system have recently been conducted on a 1 MW_th coal-fired combustion test facility. The flame in the furnace was visualised continuously using the imaging system and then characterised in terms of geometrical and luminous parameters. Results obtained over a range of combustion conditions demonstrate that the system is capable of characterising the flames both qualitatively and quantitatively. The correlations between the flame parameters measured and the corresponding furnace data, including furnace load, mass flow rate of primary air and particle size, are identified and analysed.     

The effect of air preheating on the combustion of solid fuels on a grate

Combustion of solid fuels on a grate is widely used. Mostly, the combustion behaviour is explained by the classical theory of Rogers. However, that theory cannot explain the combustion process when primary air preheating is applied. Solid fuel grate combustion is studied by experiments in a pot furnace. Experiments with and without primary air heating are described. These are compared with conclusions learnt from real plant experiments. It was found that the pot furnace experiments have a limited value in explaining the combustion behaviour of solid fuels on a grate. In order to be able to explain the results from practice an quantitative extension of Rogers' theory for the case with air preheating is presented.     

On the effects of firing semi-anthracite and bituminous coal under oxy-fuel firing conditions

Traditional wisdom has lead to the design of a boiler being dictated by its fuel. Typically, lignite requires a large boiler to accommodate the moisture content and ash behaviour and anthracite needs a design with a long residence time to allow for complete combustion. Thus the result is that different boiler designs are required for different fuel types. This work demonstrates that it is possible to fire under oxy-fuel firing conditions different fuels in potentially a single combustion environment. In the present work a short series of scoping tests firing Russian semi-anthracite under air and oxy-fuel firing conditions on the RWEnpower Combustion Test Facility (CTF) have been performed and result compared to firing a South African bituminous coal. An IFRF swirl burner was used. The purpose behind this work was to determine whether oxy-fuel firing offered the potential for firing a wider range of coal qualities on a swirl stabilised burner than is conventional showing that stable combustion can be achieved with semi-anthracite as with bituminous coal. In this short communication, it is shown that this is possible. Flame photographs of the Russian semi-anthracite coal fired on air and under oxy-fuel firing conditions at recycle ratios of 75%, 72% and 68% were taken. The air firing condition produced a non-luminous flame in the near burner region. For oxy-fuel firing at 75% recycle ratio, the flame is also non-luminous and more so that the air firing case. When the recycle ratio is reduced from 75% to 68% the flame becomes increasingly luminous and at 68% an intense flame was observed well anchored into the burner quarl. Radiative heat flux measurements were taken with the Russian semi-anthracite coal at 68% recycle ratio and compared to the South African bituminous coal at 68% recycle ratio and on air. In general the peak in radiative heat flux for the Russian semi-anthracite at 68% recycle ratio compared to the South African bituminous coal on air is slightly higher reflecting the effect of oxygen enrichment and the higher calorific value of the semi-anthracite. It can also be observed that the location of the peak in radiative heat flux with Russian semi-anthracite coal at 68% recycle is displaced downstream. In the near burner region, the radiation intensity is lower for the Russian semi-anthracite at 68% recycle ratio compare to South African bituminous coal at 68% recycle ratio and on air reflecting the lower (but not insignificant) intensity of combustion in this region for the Russian semi-anthracite coal.     

Numerical modelling and simulation of pulverized solid‐fuel combustion in swirl burners

A finite‐volume numerical model for computer simulation of pulverized solid‐fuel combustion in furnaces with axisymmetric‐geometry swirl burner is presented. The simulation model is based on the k ? ε single phase turbulence model, considering the presence of the dispersed solid phase via additional source terms in the gas phase equations. The dispersed phase is treated by the particle source in cell (PSIC) method. Solid fuel particle devolatilization, homogenous and heterogeneous chemical reaction processes are modelled via a global combustion model. The radiative heat transfer equation is also resolved using the finite volume method. The numerical simulation code is validated by comparing computational and experimental results of pulverized coal in an experimental furnace equipped with a swirl burner. It is shown that the developed numerical code can successfully predict the flow field and flame structure including swirl effects and can therefore be used for the design and optimization of pulverized solid‐fuel swirl burners.     

Experimental study of the burner for FAA fire test: NexGen burner

The NexGen (Sonic) burner is the new burner developed by the Federal Aviation Administration, FAA, to replace old oil burners used for the required fire certification tests on power plant‐related materials, as it provides the capability to control both air and fuel flow rates. During a fire test, the burner is supposed to simulate a certain fire condition, so the flame properties should be robust and repeatable. The NexGen burner can achieve this due to the precise fuel and air controls. However, the current calibration criterion (ISO2685:1998 and AC20‐135) may not be good enough to ensure consistent flame properties. In the presented work, the sensitivity of the burner performance to air and fuel flow rate, as measured by the temperature and heat flux for calibration purposes, was studied. Additionally, the influence of the turbulator and the thermocouple size used for flame calibration was also studied. The impact of varying fuel/air ratio and thermocouple sizes was studied by conducting fire tests on aluminum samples, to show the inadequacies in the current calibration standards.     

The redesign of chain grate stoker links to reduce pollutant emissions: aerodynamic design

Pollutant emissions from chain grate stoker furnaces may be attributable, in the main, to poor combustion air distribution to the fuel burden. A better distribution of air may be achieved by modifying the aerodynamic features of the individual grate links. A combination of flow visualisation, computational modelling and velocity measurements is used to aid in the redesign of a traditional link design. The flow improvements to the redesigned link are demonstrated.     

CONTROL FOR LUMINOUS FLAME FIRING IN GLASS FURNACES*

Luminous-flame firing of natural gas in glass furnaces has been accomplished inefficiently and with, difficulty for many years in an attempt to secure the advantages of the luminosity characteristics. Luminosity in the gas flame is now secured by retardation of combustion between overlying layers of air and gas, the resulting flame being particularly adapted for glassmelting. A successful method for controlling the length and shape of the luminous flame in a glass furnace involves the mixing of spent gases from the flue with the incoming fuel gas.     

热解燃烧链条炉低NOx排放特性的数值模拟

利用Fluent软件,对功率为1.4 MW的新型热解燃烧链条炉的NOx排放特性进行了数值模拟,其中,煤热解产生的还原性可燃气简化为CH4,采用添加元素N的乙烯-空气混合物模拟链条炉排半焦层燃烧及其生成的NO. 数值计算结果表明,在过量空气系数为1.2、再燃比为30%的燃烧条件下,热解燃烧比传统燃烧可降低NO排放14.6%. 热解燃烧链条炉由于热解气的再燃作用,在炉膛中形成一局部还原区,可较有效地降低NOx排放,证明了热解燃烧技术的可行性. 增大再燃比和减小炉排前段风室配风量可提高出口NO还原率,减小炉膛前拱长度和前后拱间距会使NO还原作用增强.     

逆向射流燃烧技术研究进展

逆向射流燃烧技术是可同时适用于燃气和燃煤领域的高效低污染燃烧技术,逆喷结构和射流流速比决定了其流场特性。笔者综述了逆向射流燃烧技术在燃气和燃煤领域的发展历史、研究现状和发展趋势。在燃气领域,逆向射流主要起稳定火焰作用,具有良好的燃料-空气混合条件,形成一个近似均匀的热流场,避免燃烧过程中出现局部热点,但目前仅为一种为燃气轮机和飞机发动机提供的探索性技术,工程应用还需克服燃料和空气在一个狭小空间里的流场合理控制以及从简化装置到工程放大等问题。在燃煤领域,对于煤粉燃烧器,逆向射流可形成一个可控组分、大小、形状和位置的回流区,且将煤粉直接送进回流区,还可控制煤粉在回流区内的停留时间,该技术与传统火焰稳定方式相比,火焰稳定能力更强、停留时间更长、污染更低,更适用于低阶煤的高效燃烧,目前,逆向射流燃烧技术耦合其他稳燃、低氮技术为煤粉高效清洁利用发展提供了新方向,且已有实际工程应用,但对于其机理研究不够深入,限制了其进一步发展与推广。对于电站锅炉,部分一次风或燃尽风逆向偏转射入炉内,可缓解四角切圆燃烧方式下炉膛出口烟气的烟速和烟温偏差,目前主要是燃尽风反切的工业应用,但如何合理控制燃尽风反切角度、反切动量以及反切层数等关键问题还需进一步研究。     

Combustion of residual steel gases: laminar flame analysis and turbulent flamelet modeling

In recovery combustion systems operating in the steel industry, energy is provided by boilers burning residual gases of blast furnace and coke oven. To help understand combustion of this particular type of fuels, a numerical study is conducted where the major chemical properties of steel gas flames are collected. The chemical composition of representative fuel and oxidizer steel gas is varied over a large range in calculations using detailed chemistry and complex transport properties. The chemical equilibrium compositions, premixed flame speeds and diffusion flame extinction strain rates are determined. The advantages and shortcomings of the use of vitiated air emerge, and its introduction into the boiler appears as an interesting alternative to reduce NO_x emission. The detailed information obtained with laminar flame calculations is also introduced in flamelet turbulent combustion modeling. Reynolds Averaged Navier Stokes (RANS) simulations of a test case burner are performed and some comparisons between numerical predictions and experimental results are presented.     

Gas/particle flow characteristics of a centrally fuel rich swirl coal combustion burner

A three-component particle-dynamics anemometer is used to measure the characteristics of two-phase gas/particle flows in the near-burner region for a centrally fuel rich swirl coal combustion burner using a gas/particle two-phase test facility. Velocities, mean particle diameters and particle-volume flux profiles were obtained. The primary air and glass beads partially penetrate the central recirculation zone and are then deflected radially. At the center of the central recirculation zone, the mean radial velocities and tangential velocities are low and a zone of high particle-volume flux and large particle size is formed. The influence of gas/particle flow characteristics on combustion is analyzed.     

Experimental study of multispecies gas combustion in a several-section porous media burner

Experiments with combustion of diluted liquefied petroleum gases used as a fuel premixed with air are performed. From the experiment results, one can see that low-heat-value gases are capable of stable burning in a porous media burner. Distributions of species concentrations and flame temperature are measured. Based on these data, the flame is found to be most stable if the equivalence ratio is equal to 0.8. To improve the burner performance, experiments with different characteristics of porous media are performed. Optimal parameters of porous media are confirmed by subsequent numerical simulations. It is demonstrated that the properties of combustion in the porous media burner are superior as compared to those in the free flame burner.     

数字式燃烧控制技术在加热炉炉温控制上的应用

某钢铁公司加热炉采用PLC炉温控制系统，根据加热炉的上、下部烧嘴布置及烧嘴本身的特点，加热炉上部区的温度采用传统的空／煤气流量双交叉限幅控制(比例控制)，而下部区的温度控制采用数字式燃烧控制技术。重点介绍了加热炉下部区的温度控制方式、脉冲数字烧嘴工作时间控制，以及火焰长度控制。