Heat transfer in a direct-fired rotary kiln: II. Heat flow results and their interpretation

Heat flow rates from the gas to the wall and to the bed have been derived from temperature profiles measured in the UBC pilot rotary kiln. The experimental gas-to-wall heat flux has been found to agree closely with theoretical predictions based on a simple radiative model consisting of a grey gas surrounded by a grey surface at uniform temperature. Under identical conditions, the gas-to-solids heat flux is up to ten fold greater than the heat flux between the gas and wall. The gas-to-solids heat flux is a function of the solids feed rate at low throughputs but is constant at higher throughputs. At the lowest feed rates and corresponding rotation speeds heat flow to the bed is limited by mixing of the solids. Heat transfer control changes to the gas side of the bed at higher feed rates and rotation speeds. Under the latter conditions convection appears to account for the major fraction of heat flow to the bed. The heat flow measurements relate directly to observations of the bed motion which can be conveniently characterized by a “bed behavior” diagram. The low heat flux to the bed at low feed rates is due to a slumping action at the surface and concomitant poor mixing. The high heat flux obtained at high feed rates coincides with the observation of a rolling bed. Burden-side and gas-side convective heat transfer coefficients, calculated from the heat flux data, have values ranging from 700 to 1200 and 120 to 240 w/m²K respectively.     

Heat transfer in a direct-fired rotary kiln: II. Heat flow results and their interpretation

Metallurgical and Materials Transactions B - Heat flow rates from the gas to the wall and to the bed have been derived from temperature profiles measured in the UBC pilot rotary kiln. The...     

Heat transfer in a direct-fired rotary kiln: I. Pilot plant and experimentation

The characterization of heat flow processes in direct-fired rotary kilns requires detailed measurements of gas, solids and wall temperatures. This paper describes the construction, instrumentation and operation of a 5.5 m long x 0.406 m inside diam kiln designed for such measurements. The heating of inert sand was chosen for experimental study. Methods of calculating heat flows among solids, wall and gas from the measured axial and radial temperatures are presented and the heat balance calculations and other necessary checks on the validity of the data are given. The effects of the kiln operating variables on heat flow rates, and the implications of the results for modelling and scale-up to large kilns are discussed in Part II.     

Heat transfer in a direct-fired rotary kiln: I. Pilot plant and experimentation

Metallurgical and Materials Transactions B - The characterization of heat flow processes in direct-fired rotary kilns requires detailed measurements of gas, solids and wall temperatures. This paper...     

Limestone calcination in a rotary kiln

An experimental study of the calcination of limestone has been carried out in a highly instrumented pilot-scale rotary kiln. Local gas, solids, and wall temperatures and pct calcination have been measured under a range of operating conditions to determine the influence of limestone type, feed rate, rotational speed, inclination angle, and particle size on calcination and heat flow in the kiln. Thus, it has been found that the local calcination is dependent primarily on the solids temperature and hence on heat transfer. Of the variables studied, the limestone feed rate has the strongest effect on the temperature and calcination fields, whereas inclination angle and rotational speed are relatively less important. The different limestones studied exhibited significant differences in heat-absorption capacity and calcination temperature which may be related to their radiative properties. Increasing particle size over a range of 0.75 to 3.5 mm resulted in an increase in both heat transfer to the bed and calcination.     

The rotary kiln: An investigation of bed heat transfer in the transverse plane

Rotary kilns are commonly employed to thermally process granular materials. While kiln rotation promotes particle mixing and heat transfer, it also leads to de-mixing through segregation of finer or denser particles, creating a “kidney” within the bed. Experience and experimental evidence indicate that rotation-induced mixing is insufficient to eliminate radial thermal gradients within the bed, but it is unclear as to whether particle segregation plays a significant role in the development of these gradients. This article presents experimental data obtained by heating sand of varying particle-size distributions within a 0.4-m ID batch rotary kiln to bed temperatures up to 775 °C. The results suggest that segregation has little influence on heat transfer within the bed and that radial thermal gradients are primarily the result of inadequate particle mixing. In order to scale up the experimental results, a mathematical model for heat transfer within the transverse plane was developed. A key variable for the model is the mixing-induced conductivity applied to the active layer, for which an empirical value is derived by fitting model predictions to the experimental data. Based on several assumptions for scaling of mixing conductivity with kiln size, model predictions for radial thermal gradients are presented for industrial-scale kilns in the 2- to 4-m ID range.     

小直径回转窑的改进设计

介绍了小直径回转窑的结构及改进设计后的生产实践。经过改进,回转窑的使用寿命延长,检修费用大大降低,设备生产利用率显著提高。     

熟料烧结回转窑传热传质数值模型

建立了窑长为90m的氧化铝熟料回转窑的传热传质数学模型.采用化学动力学方法研究了烧结反应吸、放热对窑内温度分布的影响,提出了氧化铝与碳酸钠的反应为窑内熟料烧结反应结束的标志.结果表明,熟料反应约在距离窑头85 m处完全结束,各反应速率随温度的升高而增大,喷煤量的增加对预热、烧结带的影响大于其他各带区.     

转杯粒化工艺高温熔渣换热数值模拟

转杯粒化器是高温熔渣离心粒化余热回收系统中最关键的部件,它与高温熔渣接触后的换热和温度分布直接影响到转杯的安全运行。由于熔渣温度超过1 500℃,现有的试验测温手段受到了很大限制,因此,从数值模拟出发,研究高温熔渣在转杯上流动换热特性以及凝固渣壳的形成规律,结果表明,渣壳厚度与转速呈负相关,转杯表面热流密度以及对应的壁面温度均与渣壳厚度呈负相关,转杯边缘的渣壳厚度最薄,同时温升速率及温度在转杯内部最高。通过在转杯表面设置耐火材料能显著降低转杯不锈钢内部温度,从而起到对不锈钢本体热防护的作用。     

Residence time distribution and material flow studies in a rotary kiln

Experiments were conducted in a rotary kiln containing ilmenite particles to study the residence time distribution (RTD) of low-density particles, holdup, and bed depth profile. The variables include feed rate of solids, slope and rotational speed of the kiln, type and size of the tracer, and dam height. Correlations are presented for mean residence time, dispersion number, holdup, and steady-state throughput of solids in terms of the process variables. A simple method is proposed to estimate the dam height that gives rise to a flat profile of solids bed along the length of the kiln.     

回转窑供电装置改造

本阐述了回转窑供电装置存在的缺陷，亦即改造过程和改造后的效果。     

Mathematical modeling of pneumatic char injection in a direct reduction rotary kiln

A one-dimensional steady-state mathematical model is proposed for direct reduction process in rotary kilns akin to the SL/RN process. The model takes into account pneumatic coal char injection from the discharge end of the kiln to supplement the heat availability. The model is based on material and energy conservation principles, and the empirical equations for kinetics and heat transfer are obtained from the literature. Predictions are carried out for both iron oxide reduction and ilmenite beneficiation processes. Improvement in the performance was predicted with pneumatic char injection.     

Benefits of oxygen enrichment in sponge iron rotary kiln emerging from data-driven modelling

Optimal oxygen enrichment conditions for sponge iron rotary kiln have been successfully explored on an industrial scale using a data-driven model. A multi-objective optimisation by genetic algorithm (MOGA) is employed to find the favourable conditions. The objective function for MOGA is derived from neural networks using pre-processed operational data. From industrial experimentations guided by the optimum conditions predicted by the present model, it emerged that when the coal fines injection is maintained at 1.75?tph and the oxygen enrichment is 8 Nm³?t^?1 of sponge iron, a reduction in the specific air requirement from 2609 to 2150?Nm³?t^?1 was obtained, while the end-zone bed temperature remained under control at 1132°C. These conditions resulted in a reduction of specific coal consumption by 6%, an enhancement in the sponge iron production by 6% and an increase in the rotary kiln campaign life from 50 to 100 days.     

带有翻料装置的回转窑内衬结构的数值模拟

炭素回转窑内物料料层过厚,则物料煅烧不透．为解决此问题可在回转窑内衬增设翻料装置,即将原来的平滑窑内衬改为带有导流槽的凸凹内衬．为研究该内衬结构对窑内热工状况的影响,应用计算流体力学软件Fluent对窑内流场、温度场、浓度场等作了耦合计算．结果表明,翻料装置结构的不同对窑内流场、温度场、浓度场的影响不同;合理翻料装置的安装,可以加速挥发分的燃烧,提高窑内温度,以达到解决料层过厚导致物料煅烧不透的问题又不增大炭质烧损的目的．     

针状焦煅烧回转窑风口布置的仿真优化

应用CFD模拟软件,对回转窑内部的气相流场进行了数值模拟,得到了相应工况下窑内气体的速度场、温度场以及挥发分浓度场的模拟结果.通过对比分析,获得回转窑的最佳风口布置方案.     

氧化铝回转窑火焰图像分析与模糊控制

在烧结法生产氧化铝过程中,为监控回转窑内部燃烧状况,提高熟料质量,采用了数字图像采集与图像处理的方法,并通过对燃烧火焰和烧成熟料图像的量化分析与滤波,得到了火焰强度和熟料等级2个数字化的特征值,为实现回转窑的自动控制提供了可靠的依据。     

氧化铝熟料回转窑干法烧结数值分析

建立60t·h^-1氧化铝熟料回转窑综合传热、传质数学模型.应用数值方法,研究喂入料浆的水分含量对窑内热过程的影响,分析得到熟料烧结方式由湿法转变为干法时回转窑具有提高产能、降低能耗的潜力.结果表明:当料浆水的质量分数降至20%时,回转窑生产能力可比实际工况提高24%,相应燃煤单耗降幅可达18%.随料浆含水量再递减,则产能和燃煤单耗的平均变化量显著减少,改变幅度逐渐趋缓.     

镁质熔剂性球团回转窑结圈探析

为充分掌握熔剂性球团回转窑结圈行为,依托链篦机回转窑生产线,分别生产了碱度0.3和碱度1.0两种球团,对原料性能和生产过程进行分析,并对回转窑结圈物的宏观形貌、微观结构和成分组成进行分析。结果表明,生产碱度0.3的球团,回转窑结圈率低于碱度1.0的熔剂性球团,其结圈物结构较致密,结圈原因主要是由于堆积的粉末经过高温固结同时产生部分低熔点的液相。而碱度1.0的熔剂性球团结圈物强度较低,呈层片状结构,其结圈原因主要是由于焙烧温度相对较低,球团粉率较高,形成粉末堆积。     

A heat-transfer model for the rotary kiln: Part I. pilot kiln trials

A model for rotary kiln heat transfer, which accounts for the interaction of all the transport paths and processes, is presented in a three-part series. In this first paper, the pilot kiln facility is described, and the significant results from the heat-transfer trials are identified. Limestone, Ottawa sand, and pctroleum coke were heated using a range of firing rates, while other operating variables were held nearly constant. Measurements were made to obtain the net rates of heat transfer for the bed material, freeboard gas, refractory wall, and, unique to the study, the radial heat flux at the inside refractory surface as a function of circumferential position. High rates of net heat input to the bed material, occurring very near the feed end, were found to decline quickly with distance, and for an inert bed, leveled out at a value well below the rate of loss through the kiln wall. The onset of an endothermic bed reaction resulted in sharp increases in both the temperature cycling at the inside refractory surface and the net heat input to the bed, but no corresponding jump in the kiln wall heat loss. The temperatures of the bed material and inside refractory surface always were coupled closely, even in the presence of bed reaction. Regenerative heat transfer from the covered wall to the contacting bed material was not a major component of the net input to the bed, and for the inert bed, negative regeneration was en-countered beyond the kiln midpoint.