A simple model for explosive combustion of premixed natural gas with air in a bubbling fluidized bed of inert sand

The combustion of premixed natural gas and air has been studied in a bubbling fluidized bed of inert particles. The temperature of the solids was carefully monitored, using 8 thermocouples, immersed in the bed at different heights. The observed temperature profiles were used to find the height above the distributor at which most of the combustion occurred and on this basis a clear distinction could be made between combustion above the bed and inside the bed. The region where most of the heat of combustion is evolved depends on the average bed temperature. If this temperature is low, the gases burn above the bed or just under its upper surface, but at higher temperatures the process is located close to the distributor. Rapid fluctuations in the measured temperature and pressure indicate that the process inside the bed is not a steady one. The model developed here assumes that combustion takes place inside bubbles of premixed gases, as they move through the bed. A detailed chemical kinetic model was used to calculate the induction period for ignition. The model can predict the height above the distributor at which bubbles should ignite and explode. Comparison of the experimental results with the modeling calculations indicates that the course taken by the process depends on temperature. At the lowest temperatures, the gases burn above the bed. In the high temperature range, where the bubbles ignite is determined by the induction period. At intermediate temperatures the location of the reaction is determined by the depth of the bed and bubble size, with ignition spreading from above the bed to bubbles, which are about to leave, but are still in the bed. That bubbles explode at different heights up the bed is reflected in the acoustic signals registered above and below the bed. The associated changes in the composition of the flue gases are also very characteristic.     

Bubble induced heat transfer in gas fluidized beds

The influence of gas bubbles on heat transfer in gas fluidized beds has been investigated. A platinum wire has been used as a heat-transfer probe and the aggregative gas fluidized bed has been simplified by generating a single continuous stream of gas bubbles into an incipiently fluidized bed. It has been found that in the case of aggregative gas fluidized beds of small particles operating below the radiative temperature level, transient conduction into the emulsion phase is responsible for at least 90% of heat transfer and that the remainder is contributed by the superimposed gas convection. A theoretical model of the bubble induced heat transfer has been developed. Finally, experimental justification for the concept of the property boundary layer introduced in [2] is presented.     

Experimental investigation of the two-phase theory in a fluidized-bed combustor

Though the two-phase theory of fluidization is well-accepted, no direct experimental measurements of the different gas concentrations predicted to occur in bubble and particulate phases could be found in the literature. For the first time, theoretical predictions of these different gas concentrations have been validated experimentally, using a combined oxygen/bubble probe. Based on the two-phase theory, a mathematical model was developed for the combustion of a batch of char particles in a fluidized-bed combustor. The experimental oxygen concentration in the particulate phase as a function of time was well predicted by the model. Slight discrepancies for the bubble phase values were eliminated when low-oxygen-concentration bubbles were excluded from the data, attributed to some char combustion occurring in bubbles being contrary to the model assumption. The temperature difference between char and bed particles (ΔT) was the only adjustable parameter in the model. A value of 20°C fitted the burnoff times measured by visual observation of the top of the bed, for both 5 and 10 g char batch masses. Model predictions of the oxygen concentrations were not sensitive to ΔT during the first half of burnoff, when mass transfer controlled the combustion rate, so the mass transfer processes were predicted correctly by the model effectively with no adjustable parameter. The ΔT value of 20°C was significantly lower than experimental measurements of maximum burning char particle temperatures, reported to be 70°C for the small-diameter bed particles used in this work. The discrepancy was attributed to two factors: (i) the decrease in char particle temperature towards the end of the burnoff, when kinetics significantly affected the combustion rate; and (ii) a lower char particle temperature in the particulate phase than in the bubble phase, with experimental char particle temperature measurements biased towards the higher bubble phase values. It was inferred: (i) that the maximum values of ΔT measured experimentally are too high for calculation of the char particle combustion rate during the kinetic-controlled latter stage of burnoff and (ii) that reported values of the heat transfer coefficient from burning char particles to the particulate phase deduced from these particle temperature measurements may have been underestimated.     

A mathematical model for exit gas composition in a 10 MW fluidized bed coal combustion power plant

A mathematical model has been developed for the oxygen mass balance for a 10 MW fluidized bed coal combustion power plant operated at Jamadoba (TISCO, India). Assuming the three phase theory of fluidization, the fluid bed is considered to consist of a number of equivalent stages in series. Within each stage, an exchange of gas takes place between the bubble, cloud-wake, and emulsion phases. An effective chemical reaction rate of char combustion has been derived considering the single film theory of char combustion for shrinking particles. The model has been used to predict the consumption of oxygen in the fluidized bed combustor, the outlet gas composition, variation of oxygen concentrations in different phases and also the variation of average oxygen concentration along the bed height. Model predictions were compared with plant data, and reasonable accuracy was obtained.     

Mathematical modeling of a continuous fluidized bed dryer

In this work, a numerical simulation of a fluidized bed dryer based on the two-phase theory of fluidization has been proposed. Solid particles in the emulsion phase are considered lumped and in the bubble phase is assumed plug while in the emulsion phase is considered totally mixed. Moreover, the bubble size variations along the bed height are taken into consideration. Influence of the superficial gas velocity, input temperature of drying gas, particle size and mean residence time of solid particles on the drying process have been reported. The simulation results show an improvement to the prediction of other models that consider uniform size bubble and other simplification assumptions.     

流化床煤燃烧与气化过程中的辐射换热

流化床反应器中颗粒与颗粒之间的传热在一定程度上决定了化学反应的速率及反应的中间历程。本文通过对气固流化床乳化相中颗粒群结构的进一步认识，建立了颗粒间的辐射换热模型，比较了不同颗粒直径、不同床层温度水平及不同流化工况下颗粒间辐射换热与通过气膜导热份额的大小，并预测了流化床反应器中反应颗粒与惰性床料之间的温差，对于流化床反应器选择合理的运行工况和进行操作参数优化具有参考价值     

循环流化床传热系数的计算模型

本文在循环流化床流动模型的基础上建立了传热模型，流动模型根据实际运行情况考虑了颗粒的宽筛分，并把床层在轴向上分为密相床和稀相床两部分。在密相床内，传热按照鼓泡床传热微型进行计算；在稀相床内，传热模型建立在颗粒团更新的假设基础上，根据假设，床层由颗粒浓度很低的上升稀相和相对颗粒浓度较大的颗粒团两部分组成，两部分交替地与床壁面接触，床层和受热面间局部换热系数和颗粒浓度及两部分接触壁面的份额有关。模化结     

A two phase model of high temperature steam-only gasification of biomass char in bubbling fluidized bed reactors using nuclear heat

A two phase biomass char steam gasification kinetic model is developed in a bubbling fluidized bed with nuclear heat as source of energy. The model is capable of predicting the temperature and concentration profiles of gases in the bubble, emulsion gas and solid phases. The robust model calculates the dynamic and steady state profiles, as well as the complex parameters of fluidized bed. Three pilot scale gasifiers were simulated in order to see the effect of the H/D ratio and the bed heating dynamics in the gasification kinetics, these parameters are found to be really important in order to enhance the water-gas shift reaction, and consequently, the hydrogen production. For the system modeled, hydrogen is the principal product of the steam-only gasification, as reported in the literature data. The carbon dioxide yield seems to be smaller than the ones in other works, but these differences are due principally to the energy source (no combustion is conducted) and that char (no oxygen in the solids) was used as the carbon source.     

Thermal analysis of a fluidized bed drying process for crops. Part I: Mathematical modeling

Development of a comprehensive mathematical model to simulate the simultaneous heat and mass transfer processes in a bubbling fluidized bed is described. Although the model is applicable to a wide range of particles, wheat is chosen as an example. In the development of the model, the commonly used two‐phase theory is not used because of its insensitivity to the particle group used in the bed. Instead, a new hydrodynamic model is developed for each specific particle group. The behaviour of bubbles in a bed of group D particles (wheat) is modelled with the consideration that they grow in size as they rise in the bed, but are of the same size at any height in the bed. The voidage of bubbles, particles and interstitial gas is modelled separately. A newly developed expression to determine the minimum fluidization velocity of wet particles is used. The model considers the presence of different phases inside the bed, and their physical variation along the bed. The interstitial gas phase, the bubble phase, and the solid phase are modelled separately. The drying mechanism for the solid phase is considered in two stages: the falling rate, and the constant rate, with appropriate temperature and moisture diffusion coefficients and wall effects. The simultaneous heat and mass transfer processes during the drying process including the internal and external effects are modelled for each phase. A set of coupled nonlinear partial differential equations is employed to accurately model the drying process without using any adjustable parameters. A numerical code is developed to solve the governing partial differential equations using a control volume‐based discretization approach. Piecewise profiles expressing the variation of dependent variables between the grid points are used to evaluate the required integrals. Copyright © 2000 John Wiley & Sons, Ltd.     

燃用宽筛分煤循环流化床锅炉燃烧模拟计算

建立了循环流化床锅炉炉膛颗粒燃烧和脱硫反应模型。该模型考虑了炉膛下部为高颗粒浓度的密相区和上部为低颗粒浓度稀相区的特征,模拟计算给出了烟气温度,热流密度和各气体成分（Ｏ２,Ｃ２Ｏ,ＣＯ,Ｈ２Ｏ和Ｓ２Ｏ）的轴向分布,模拟计算结果的趋势是合理的。     

Gas combustion in shallow fluidised beds

Combustion of premixed gas and air in shallow fluidised beds is described and some of the limits set by gas velocity and particle size are discussed.There is a lower limit of gas velocity below which heat transfer back to the distributor plate causes excessive preheat leading to pre-ignition and to temperatures well above the equilibrium level. This condition can cause particle sintering and, at worst, distributor failure. A graph showing a solution of the problem is presented.It is shown that there is a lower limit to particle size, which depends upon the particle density, below which stable combustion will not occur. This limit is set when enough of the gas/air mixture bypasses the bed without burning to prevent the bed from reaching combustion temperatures. The experimental observations are explained in terms of the two-phase theory of fluidisation by postulating that the fuel which passes through the dense phase of the fluidisation is totally burnt, while that passing up through the bed in bubbles either does not react or reacts too late for its combustion heat to be transferred to the bed.A quantitative model based upon these two ad-hoc assumptions is shown to provide reasonable agreement with the observed results and possible refinements of the simplified theory, to make it more rigorously based, are indicated.     

Experimental research of radiative heat transfer in fluidized beds

A new method to measure the radiative heat transfer in fluidized beds was presented. Experiments were carried out on a 0.8 th⁻¹ fluidized bed combustion boiler. The residual slag of fired coal was operated in a fluidized bed at room temperature. As the radiative heat transfer at room temperature is insignificant, its contribution at high temperatures might be obtained by the comparison of experimental results at high and low temperatures. On experimental study, a radiative contribution was given as a function of bed temperature and particle size. The results were compared with those in other references.     

两相流动对流化床燃烧行为的影响

循环床锅炉沿床高的烟气浓度及燃烧份额分布测试结果证明,鼓泡流化床和循环流化床的重要差异表现为密相区燃烧行为的根本不同,由于床料平均粒径较低,循环床密相区的流动不同于鼓泡床,导致气固两相之间的传质阻力增加,从而影响燃烧反应,密相区的燃烧行为表现为欠氧。循环床锅炉沿床高乃至分离器都有燃烧反应发生,建立了考虑气固相间传质阻力的流化床密相区燃烧模型,并与实际循环流化床锅炉的测试数据比较,计算结果与测试值比较吻合。     

Two phase biomass air-steam gasification model for fluidized bed reactors: Part I—model development

A two-phase model capable of predicting the performance of fluidized bed biomass air-steam gasification reactor during dynamic and steady state operations was developed based on the two phase theory of fluidization. Material and energy balances were taken into consideration and the minimization of free energy technique was used to calculate the gas mole fractions. The fluidized bed was divided into three zones (jetting, bubbling and slugging) and the mass and heat transfer coefficients were calculated for each zone in both bubble and emulsion phases. The model includes the hydrodynamics, transport and thermodynamic properties of fluidized bed. The finite element method was used to solve the partial differential equations. The input variables of the computer program included fluidization velocity, steam flow rate and biomass to steam ratio. The model is capable of predicting the bed temperature, gas mole fractions, higher heating value and production rate.     

Does co burn in a fluidized bed?—a detailed chemical kinetic modeling study

One important issue in fluidized bed combustion is the effect of the fluidized particles on the homogeneous oxidation of both the volatiles and CO from char combustion. These surfaces are thought to quench the radicals and thus lower the rate of combustion significantly. In this work the quenching of the radicals is introduced in a detailed chemical kinetic mechanism to model the effect of the solids on the oxidation of CO in the particulate and bubble phases. In this way the effect of particle concentration, particle diameter, temperature, air-to-fuel ratio, and water concentration has been studied. The results do confirm that oxidation rates are significantly reduced in the particulate phase, although no complete suppression was found. In the bubble phase the effect of heterogeneous radical quenching following mass transfer into the particulate phase and also quenching on the solids at a bubble’s surface is small. No critical (minimum) bubble diameter for the ignition of CO in the bubble phase was found under the investigated conditions. A comparison with the literature on the oxidation of CH₄ in an incipiently fluidized bed confirms the predictions for the oxidation of CO.     

Modeling of single coal particle combustion in O2/N2 and O2/CO2 atmospheres under fluidized bed condition

A one-dimensional transient single coal particle combustion model was proposed to investigate the characteristics of single coal particle combustion in both O₂/N₂ and O₂/CO₂ atmospheres under the fluidized bed combustion condition. The model accounted for the fuel devolatilization, moisture evaporation, heterogeneous reaction as well as homogeneous reactions integrated with the heat and mass transfer from the fluidized bed environment to the coal particle. This model was validated by comparing the model prediction with the experimental results in the literature, and a satisfactory agreement between modeling and experiments proved the reliability of the model. The modeling results demonstrated that the carbon conversion rate of a single coal particle (diameter 6 to 8 mm) under fluidized bed conditions (bed temperature 1088 K) in an O₂/CO₂ (30:70) atmosphere was promoted by the gasification reaction, which was considerably greater than that in the O₂/N₂ (30:70) atmosphere. In addition, the surface and center temperatures of the particle evolved similarly, no matter it is under the O₂/N₂ condition or the O₂/CO₂ condition. A further analysis indicated that similar trends of the temperature evolution under different atmospheres were caused by the fact that the strong heat transfer under the fluidized bed condition overwhelmingly dominated the temperature evolution rather than the heat release of the chemical reaction.     

循环流化床锅炉热力计算方法探讨

循环流化床锅炉与鼓泡流化床相比 ,有许多新的特点 ,床内的燃烧和受热面的传热过程很复杂。由循环倍率决定的颗粒浓度是循环流化床锅炉的一个重要参数 ,它对传热强度、燃烧与燃尽、过热器的汽温、负荷的调节范围以及分离器的设计等都产生重要影响。以 65t/h油页岩流化床锅炉的热力计算为例 ,从循环倍率入手 ,着重讨论了循环倍率以及由它决定的飞灰浓度对床内燃烧传热及过热器传热的影响     

流化床密相区流动特性的数值模拟

流化床内气固两相流动一直是实验研究和数值模拟的热点。基于Eulerian双流体模型,本文建立了流化床内的气固两相流动模型,采用FLUENT软件对流化床密相区两相流动特性、床内气泡的产生运动和爆裂等特性进行了数值模拟。模型中,将颗粒相看作是连续介质,建立与气相相同形式的数学模型;采用了离散介质动力理论,引入颗粒温度来描述固相粘性应力,并用气固曳力进行气固两相耦合。模拟得到了气泡产生、运动和爆裂的变化过程,与实验结果相一致。采用不同的曳力模型对流化床稠密两相流动进行了模拟,与Kuipers实验对比,结果表明采用Gidaspow曳力模型描述流化床稠密两相流动特性更准确。     

燃煤循环流化床模型与试验研究

利用循环流化床内气－固两相流动等基础方面的研究成果,根据本文床内气固浓－淡流动模型,建立适用不同结构参数的循环流化床燃烧模型,考虑了床内气体、固体颗粒的返混、循环过程,以及煤燃烧、ＮＯ的生成和分解、颗粒磨损等因素。在循环流化床燃烧试验台上进行实验研究,模型仿真结果和实验数据吻合良好,表明气固两相浓－淡流动模型所建立的循环流化床燃烧系统模型可以正确地模拟循环流化床的燃烧过程。     

A Numerical Investigation of Heat Transfer Mechanisms in Gas-Solid Fluidized Beds Using the Two-Fluid Granular Temperature Model

In this work, the two-fluid granular temperature model is used to investigate the heat exchanged between a heated wall and a gas-solid fluidized bed. Numerical simulations were performed in 2-D and 3-D fluidized beds using a solid phase effective thermal conductivity correlation based on the granular temperature. The heat exchange in the bubbles' wake is investigated by tracking the train of bubbles that rises along the heated wall. Large heat transfer coefficients were obtained in the rear wake region of bubbles due to relatively larger granular temperature there and intense particle circulation.