Transient response of granular bed motion in rotary kiln

A non linear model proposed by Perron and Bui (1994), which focuses on the dynamics of slow mode to predict the transient response induced by step changes in operating variables in the motion of the granular bed in a rotary kiln, was validated for different experimental conditions. The variables include feed rate of solids, inclination and rotational speed of the kiln, and dam height. The validity of the model for step changes in two or more variables at a time was also verified.     

变径炭素回转窑内物料平均停留时间的计算

在前人厚度变化公式基础上推导出适用于窑尾扩径和增设翻料装置的炭素回转窑内物料平均停留时间的计算公式,同时建立了实验室回转窑模型,进行了冷态实验。将物料停留时间的计算值与变径回转窑模型实验测试值作了比较。结果表明,公式计算值与实验测试值比较吻合,误差范围在-15％～12％内,并且计算中不需要引入修正系数;回转窑转速和斜度的增加均使物料平均停留时间缩短,而窑尾挡料圈高度的增加可以使平均停留时间缩短,也可以使平均停留时间延长。     

Mean residence time and hold-up of solids in rotary kilns

The influence of operational variables on the mean residence time (MRT) and hold-up of the solids in rotary kilns has been investigated by many researchers. However, their reports on the influence of the feed rate on MRT, for example, are not consistent with one another. In this study, experiments are conducted on a 400 mm ID pilot kiln with a cylindrical discharge dam. Sand is used as the testing solids. MRT and the hold-up are measured under various feed rates and rotation speeds. Experimental results indicate that MRT increases slightly with the feed rate, and that the gradient is smaller at high rotation speeds. The hold-up shows a near-linear increase with the mass of feed per kiln rotation. The experimental data are then used to test equations provided by different researchers for the prediction of MRT. Comparisons show that, results of the empirical equation from Sullivan, J.D., Maier, C.G., Ralson, O.C. [1927. Passage of solid particles through rotary cylindrical kilns. U.S. Bureau of Mines Technical Paper No. 384.] agree well with measurements for the cases of low kiln loadings, while the numerical model of Saeman, W.C. [1951. Passage of solids through rotary kilns: factors affecting time of passage. Chemical Engineering Progress 47, 508-514.] gives good predictions over the entire range of parameters considered.     

Oxygen enrichment in rotary lime kilns

The operation of a pilot-scale rotary lime kiln, fired with natural gas, was studied to determine the effects of enriching the combustion air with oxygen. The variables of interest were the temperature, calcination and heat flow patterns within the kiln as functions of the fuel rate, oxgyen concentration and limestone feed rate. All tests were conducted with essentially a stoichiometric fuel/oxygen ratio and employed two different limestones. Data are presented to show the increase in kiln throughput that can be achieved using oxygen enrichment. The results are interpreted using detailed measurements of axial solids, gas and wall temperatures and axial calcination profiles. These permit the evaluation of the driving forces for the various heat exchange processes, and of local heat flow to the solids bed.     

The dependence of the elastic properties of silica/alumina materials on the conditions used for firing

The dependence of the elastic properties of a range of powder compact samples has been measured as a function of firing variables. It was found that both Young's modulus and Poisson's ratio are particularly sensitive to the peak temperature and the time for which the peak temperature is maintained, over a range of these variables for which density is not significantly affected. The material investigated is used industrially for the manufacture of wall tiles. Firing trials conducted in an industrially operated tunnel kiln have indicated that sufficient variation in firing conditions exists, in the cross-section of the tunnel kiln, to cause significant variation in the values of Young's modulus and Poisson's ratio of bodies fired in different positions in the kiln. Microstructural examination of bodies produced to have very similar densities but vastly different values of Young's modulus and Poisson's ratio has indicated that the dependence of Young's modulus and Poisson's ratio on firing conditions can be explained by the extent of sintering within the ceramic matrix.     

Pyrolysis of oil sand from the Whiterocks deposit in a rotary kiln

A rotary kiln reactor was evaluated for thermal recovery of oil from Utah oil sands. A series of continuous-flow pyrolysis experiments was conducted. Process variables investigated included temperature (748–848 K), solids retention time (10–27 min) and sweep gas flow rate (1.27–2.83 m_s³ h⁻¹). The results indicated that the pyrolysis temperature and the solids retention time were the two most important variables affecting the liquid and gas yields. The liquid yield (C₅⁺]) decreased and the gas yield (C₁–C₄) increased with increasing temperature. The liquid yield increased with decreasing solids retention time, while the gas yield decreased. No significant effect of the sweep gas flow rate on the product distribution and yields was observed. The quality of the bitumen-derived liquids was significantly better than that of the bitumen. A preliminary process kinetics model which conforms to the observed trends was proposed.     

Drying timber in a solar kiln using an intermittent drying schedule of conventional laboratory kiln

The purpose of this study was to apply an intermittent drying schedule developed from a conventional kiln to a solar kiln. Implementing this experiment could help better understand the oscillation of the temperature inside a solar kiln and timber quality during drying progress. The theoretical recharge and discharge curves were used to predict the temperature inside the solar kiln using experimental data obtained previously using a solar kiln. The surface and internal checks were measured using ImageJ freeware, and the development of the Moisture Content (MC) profile was assessed by coring and slicing method for the Eucalyptus delegatensis boards during drying. The results showed that the recharge and discharge model can predict the temperature with less than 2?°C error from the experimental data in the solar kiln. The total drying time to 12% MC was 87?days for the solar kiln. The drying rate was equivalent to the conventional kiln decreasing at an average rate of 0.2% per day. The surface check formation was found when the MC gradient between the core and the case of the board was greater than 42% at 9?days of drying in the solar kiln and conventional laboratory kiln. The applied drying schedule used in the solar kiln was successful and offered similar drying time. However, the oscillation of temperature in the intermittent drying will require further improvement to get closer conditions in a solar kiln.     

Comprehensive Study on the Solids Acceleration Length in a Long CFB Riser

Systematic experimental work was conducted to investigate the solids acceleration length in a 0.10 m i. d., 16 m long circulating fluidized bed (CFB) riser with fluid catalytic cracking (FCC) particles over a wide range of operating conditions. A more feasible method is proposed to determine the acceleration length from the measured axial profiles of pressure gradient (or apparent solids holdup). With this new method and large amounts of experimental results, a clear picture of the variation of the acceleration length with both solids circulating rate and superficial gas velocity is obtained. It is found that the acceleration length increases generally with increasing solids flow rate and/or decreasing gas velocity. However, the particular variation patterns of the acceleration length with operating conditions are quite different in different operation ranges. Especially under the conditions near or at the accumulative choking, the acceleration length extends rapidly with increasing solids flow rate and/or decreasing gas velocity, and sometimes takes up the whole riser height. Reasonable explanations are provided for the observed variation patterns of the acceleration length.     

Convective heat transfer in a rotary kiln

An experimental study of convective heat transfer from hot air to the solid charge and walls in a non-fired rotary kiln is reported. Ottawa sand was heated by passing it counter-current to a flow of preheated air in a 2.5 m × 0.19 m I.D. rotary kiln. Axial temperature profiles of gas, wall and solids were measured. Local and average convective heat transfer coefficients from gas to solids and from gas to wall were determined assuming plug flow of gas and solids. Solid feed rates to 1750 kg/m² h and air rates to 3300 kg/m² h were investigated at rotational speeds to 6 r/min, holdup ratios to 17% and gas temperatures from 350–590 K. The gas/solids convective coefficient was found to depend on the gas through-put and to a lesser extent on solids holdup and rotational speed. Over the range tested, the angle of kiln inclination, solids throughput and particle size showed no significant effect on heat transfer. Gas/wall coefficients were about a factor of ten below gas/solid coefficients. Heat transfer results are compared to the limited data available in the literature, and to commonly used equations. Correlations of the experimental data on gas/solids, and gas/wall coefficients are presented; data from the literature on the wall/solids heat transfer coefficient are summarized.     

Rotary cement kiln coating estimator: Integrated modelling of kiln with shell temperature measurement

Coating thickness protection in the burning zone of a rotary cement kiln during operation is important from the viewpoint of the kiln productivity. In this paper, an integrated model is presented to estimate the coating thickness in the burning zone of a rotary cement kiln by using measured process variables and scanned shell temperature. The model can simulate the variations of the system, thus the impact of different process variables and environmental conditions on the coating thickness can be analysed. The presented steady‐state model derived from heat and mass balance equations uses a plug flame model for simulation of gas and/or fuel oil burning. Moreover, the heat transfer value from shell to the outside is improved by a quasi‐dynamic method. Therefore, at first, the model predicts the inside temperature profile along the kiln, then by considering two resistant nodes between temperatures of the inside and outside, the latter measured by shell scanner, it estimates the formed coating thickness in the burning zone. The estimation of the model was studied for three measured data sets taken from a modern commercial cement kiln. The results confirm that the average absolute error for estimating the coating thickness for the cases 1, 2, and 3 are 3.26, 2.82, and 2.21 cm, respectively.     

回转窑与不同煅烧气氛下的立窑熟料混合配制水泥的性能研究

在相同的水化条件下,通过水化程度分析、水化液相分析、XRD分析、DTA等方法研究了回转窑与不同煅烧气氛下的立窑熟料混合配制水泥的水化特性;结果表明:不管回转窑与何种煅烧气氛下的立窑熟料混合水化,其1d水化速率并非2种水泥的简单加权,而是有明显加快,其中以回转窑与还原料混合配制的水泥1d水化加快最明显;水泥浆体中的水化产物种类并未改变.同时,按GB175-92规定的实验方法测定了回转窑与不同煅烧气氛下的立窑熟料混合配制水泥的强度,结果表明:水泥强度并非2种单一水泥的简单加权,3d抗压和抗折强度有明显增长趋势.     

回转窑热工状况影响参数的数值模拟

以某炭素厂φ2.3 m×55 m的回转窑作为研究对象,运用流体模拟软件Fluent 和Matlab 分别对回转窑内气体空间和物料料层内的传热过程进行了模拟,预测了回转窑参数如二、三次供风量和位置、翻料装置、物料填充率、窑转速等对窑内温度场的影响。结果表明合理布置二、三次风供风量和位置、设定翻料装置的尺寸和个数能够有效提高窑内温度,进而提高煅后焦质量;合理增加物料填充率和提高窑转速既能够提高回转窑产能,又能够保证煅后焦质量。     

ON FLOW-INDUCED KINETIC DIFFUSION AND ROTARY KILN BED BURDEN HEAT TRANSPORT

The cross-section of a partially-filled cylindrical kiln rotating on its horizontal axis and processing granular solids produces a shear zone (active layer) at the free surface which grows with the kiln's rotational rate. The active layer, although relatively thin, compared with the rest of the bed burden, drives all physical/chemical reactions. This is because of the high rate of surface renewal which, in turn, promotes heat exchange between the exposed surface and the higher temperature freeboard gas. Unlike packed beds, particulate diffusion induced by the flow of granules, adds a significant component to the overall heat transfer in the bed. Problem formulation and modeling of heat conduction using flow fields derived from experiments suggest that at slow kiln speeds the diffusion effect may not be recognized due to long term duration of particle contacts and hence packed-bed heat conduction models may provide adequate characterization. However, at moderate and high kiln speeds particle collisions are short-termed and kinetic diffusion contributes to the effective thermal conductivity by as much as tenfold thereby resulting in well-mixed conditions and a homogeneous bed temperature. Industrial processing ramifications such as kiln speed control and product quality are discussed hereafter.     

Recycling of CRT panel glass as fluxing agent in the porcelain stoneware tile production

In the present work, the feasibility to substitute feldspar raw material in a porcelain stoneware body with Panel Cathode Ray Tube (CRT) glass was investigated. A standard batch and a composition, where 35 wt.% Na-feldspar was substituted by CRT glass, were sintered at different temperatures in the range of 1000–1250 °C. The degree of the densification was studied by evaluation of the closed and total porosity, while the sintering rate was estimated by non-isothermal dilatometric measures. The variation of the crystalline phase composition was evaluated by XRD analysis. From the preliminary study other ceramic samples with different percentages of CRT glass (i.e. 2.5, 5 and 10 wt.%) were prepared and fired in industrial kiln. The sintering parameters, the microstructure and the mechanical properties were measured and compared with the standard composition.     

A Model for Residence Time Distribution of Solids in a Rotary Kiln

A five‐parameter model for residence time distribution of low‐density particles in a bed of high‐density particles in a rotary kiln is derived based on combinations of flow regimes in several sub‐regions. The parameters of the model are the number of turnover stages, the cascade volume fraction, the Peclet number, the dead volume fraction in cascade region, and the cross‐flow ratio. The effect of feed rate of solids, rotational speed and inclination of the kiln on the model parameters is evaluated.     

矿化垃圾筛上物替代水泥窑燃料的工业性试验研究

采用螺旋输送的方式将再次加工后的热值为12500kJ/kg以上、含氯量为0.13%左右的矿化垃圾投入水泥窑分解炉焚烧以替代水泥燃料,结果表明:添加垃圾后,除NOx实测浓度和排放速率显著下降外,水泥窑窑尾废气测点中总悬浮颗粒物(TSPs)、HCl、HF、SO2、CO污染物实测浓度和排放速率均显著上升,而且波动很大。2.5t/h的垃圾投加量对窑系统的影响较小,主要工艺参数均正常,无较大幅度的波动,但分解炉喂煤量调整较为频繁。入窑热生料中的S含量无明显变化,而K、Cl的含量显著升高,但均在可控范围内。添加垃圾后烧制的水泥熟料,其抗压强度、抗折强度、标准稠度用水量均有所降低,但变化不显著;初凝时间和终凝时间有所升高,熟料饱和比显著下降。     

Experimental and theoretical investigation on unburned coal char burnout in a pilot-scale rotary kiln

Oxidation reactivity studies are imperative for improving carbon re-burn technologies and valuing the heat content of unburned carbon within coal combustion ashes. Non-isothermal, thermal gravimetric analysis (TGA) was used to examine the oxidation kinetics of unburned carbon in coal combustion fly ashes having different particle size distributions; TGA results were related to combustion efficiencies as measured in a bench-scale rotary kiln. The activation energy and pre-exponential factor were determined for the chemically-controlled reaction regime; the transition temperatures between chemically-controlled and partially diffusion-controlled combustion regimes were obtained for unburned carbon particles of different sizes. After the oxidation reaction rates were evaluated, the residence time distribution (RTD) of fly ashes in the rotary kiln were experimentally measured and the mean residence times related to process parameters, including the rotating velocity and kiln inclination. By comparing these results with an advective-dispersive model, the axial dispersion coefficient of fly ashes was determined. The reaction rates obtained by thermal analyses and the RTDs were used to predict combustion efficiencies within the kiln and oxidation conditions of unburned carbon using various processing options.     

Online measurement and control of solids moisture in fluidised bed dryers

Currently, two main methods are used to take online measurement of the solids moisture in fluidised bed dryers, namely microwave resonance and near infrared spectroscopy. In this paper, a new online approach to solids moisture measurement of batch fluidised bed dryers by electrical capacitance tomography (ECT) is presented for the first time. Based on online measurement of solids moisture, it is possible to implement feedback control and process optimisation of batch fluidised bed drying processes, aiming to increase the operation efficiency and to improve product quality. A twin-plane ECT sensor with eight electrodes in each plane is mounted in the bottom of a glass fluidisation chamber. From the adjacent electrode pairs, the water content of the solids is estimated based on the correlation between the moisture content and the permittivity value. To reduce measurement error, the effect of temperature on moisture measurement is compensated. The fluidisation velocity is estimated by a semi-empirical function based on the measured water content. The acquired information is sent to a controller to adjust the air flow rate of the fluidised bed dryer. To validate the moisture measurement by ECT, a mathematical model has been developed, based on the measured temperature and relative humidity of the outlet air. The Landweber iteration method is applied to reconstruct images. The averaged solids concentration along the radial direction at different fluidisation conditions is given and compared with results by the linear back-projection (LBP) method. Results from batch drying processes with online measurement and feedback control are given and compared with no feedback control. To compare the operation efficiency, the thermal efficiency is considered and the results show the possibility of online control and optimisation of the fluidised bed drying processes, based on online measurement of solids moisture by ECT. Some challenges and future work are discussed.     

Hydrodynamic Characteristics of a Pilot-Scale Screw Conveyor Dryer

This article presents results of an experimental study on the flow characteristics of a pilot-size screw conveyor dryer (SCD). In particular, the effects of granular solids flow rate (15.2–206 kg/h) and screw speed (10.8–28 rpm) on the residence time distribution (RTD) were studied using sand as the model material. The RTD was measured using a dynamic step change in the solids flow rate. Parameters such as solids holdup, degree of fullness, mean residence time, and uniformity of the discharge flow were studied. The solids holdup and hence the degree of fullness was found to decrease with increase in the screw speed and decrease with the solids flow rate. The mean residence time was found to decrease with an increase in the screw speed, as expected. The screw speed and the solids flow rate strongly affected the discharge uniformity. An optimum value of the degree of fullness was observed with regard to the solids flow rate. Over the operating range examined, the solids flow pattern was close to plug flow, as indicated by high values of N and Pe number.     

Hydrodynamic Characteristics of a Pilot-Scale Screw Conveyor Dryer

This article presents results of an experimental study on the flow characteristics of a pilot-size screw conveyor dryer (SCD). In particular, the effects of granular solids flow rate (15.2-206 kg/h) and screw speed (10.8-28 rpm) on the residence time distribution (RTD) were studied using sand as the model material. The RTD was measured using a dynamic step change in the solids flow rate. Parameters such as solids holdup, degree of fullness, mean residence time, and uniformity of the discharge flow were studied. The solids holdup and hence the degree of fullness was found to decrease with increase in the screw speed and decrease with the solids flow rate. The mean residence time was found to decrease with an increase in the screw speed, as expected. The screw speed and the solids flow rate strongly affected the discharge uniformity. An optimum value of the degree of fullness was observed with regard to the solids flow rate. Over the operating range examined, the solids flow pattern was close to plug flow, as indicated by high values of N and Pe number.