Kiln Length Research Articles

Analysis of the Impact of Rdf Combustion on Cement Clinker Quality in a Superstoichio-Metric Oxy-Fuel Atmosphere by Cfd Simulations

ABSTRACT Superstoichiometric oxygen combustion (i.e. the oxygen content during combustion is larger than required for the complete conversion of the fuel), also called pure-oxy-fuel, is a novel concept to reduce CO2 emissions from the cement clinker production. In the current contribution, we examine in a simulation study the influence of superstoichiometric combustion in combination with the use of refuse-derived fuel (RDF) of different composition and particle size on process parameters and cement clinker quality in the rotary kiln. A comparison with a standard fuel, namely lignite, and a conventional air-fired kiln is presented. The specific models developed to determine the flight and combustion behavior of RDF particles, which are of complex shape and significantly larger than pulverized coal, are described. The simulations conducted show that some RDF fractions react only partially in the gas phase due to their relative large size. Unburnt RDF particles reach the clinker bed, where they continue to react. In addition, for RDF, gas-phase temperature and the clinker temperature are different along the kiln length compared to coal, influencing the local mineralogical reaction in the clinker bed. However, in summary, similar clinker qualities could be obtained also compared to a standard air-fired kiln.

Transient operation effects on the thermal and mechanical response of a large-scale rotary kiln

Rotary kilns are central equipment in material processing operations such as oxide ore reduction, clinker manufacturing, and hazardous waste incineration. This work focuses on an industrial-scale rotary kiln for Ferronickel production via the Rotary Kiln-Electric Furnace (RKEF) process. Rotary kilns are prone to mechanical failure exacerbated by thermal effects, mainly during transient operation, such as preheating and cooling. However, few studies have tackled the impact of the transient operation on the stress state in the rotary kiln wall. Therefore, we evaluate the thermal and mechanical behavior of the rotary kiln wall during preheating. Actual operation data and a thermal model enable to obtain the temperature distribution along the kiln length. This temperature distribution is the starting point for calculating the stress state of the shell and refractory. The maximal thermal gradient occurs at 10 m from the hot end, with temperatures at the outer and inner surfaces of 200 °C and 820 °C, respectively. The von Mises stress shows a maximum of 121 MPa at the wheel position in the hot region of the refractory and a range of 30–60 MPa in the shell. In addition, the compressive stress is higher than the ultimate compressive strength of the refractory. However, it does not necessarily represent refractory detachment due to the plastic deformation undergone by the refractory under high temperatures. This work paves the way towards developing a tool for accurately predicting rotary kiln mechanical failure, which can help optimize operating conditions and assist in programming maintenance.

Computer Simulation of Heat Transfer in Alumina and Cement Rotary Kilns

Abstract This paper presents computer simulation of heat transfer in alumina and cement rotary kilns. The model incorporates radiation exchange among solids, wall, and gas, convective heat transfer from the gas to the wall and the solids, contact heat transfer between the covered wall and solids, and heat loss to the surrounding as well as chemical reactions. The mass and energy balances of gas and solids have been performed in each axial segment of the kilns. The energy equation for the wall is solved numerically by the finite-difference method. The dust entrainment in the gas is also accounted for. The solution marches from the solids inlet to the solids outlet. The kiln length predicted by the present model of the alumina kiln is 77.5 m as compared to 80 m of the actual kiln of Manitius et al. (1974, “Mathematical Model of an Aluminum Oxide Rotary Kiln,” Ind. Eng. Chem. Process Des. Dev., 13(2), pp. 132–142). In the second part, heat transfer in a dry process cement rotary kiln is modeled. The melting of the solids and coating formation on the inner wall of the kiln are also taken into account. A detailed parametric study lent a good physical insight into axial solids and gas temperature distributions, and axial variation of chemical composition of the products in both the kilns.

Influence of particle residence time distribution on the biomass pyrolysis in a rotary kiln

Rotary kilns are widely used in industry to perform several different operations. When numerically modeling such systems, a common simplification is to neglect the residence time distribution of particles. The present study numerically investigates the influence of particle residence time distribution on the pyrolysis of biomass in a co-current heated rotary kiln. First, the model (balance equations, reaction scheme, flow model, and heat transfer mechanisms) is developed. The applied particle flow model (Seaman's model) is then validated by comparing the calculated mean residence time τ and bed height to the results of a discrete element method (DEM) simulation of an example case. The mean residence time (τ = 552 s) and the bed height (H(z=0) = 52 mm) of the Seaman's model was found to be in good agreement with the results of the DEM simulation (τ = 542 s, H(z=0) = 50 mm).A detailed discussion of the residence time distribution's influence on the simulated conversion for the example case is then presented. We performed 54 simulations with various Péclet numbers between 1 and 100 and showed that the product yield significantly changes for Péclet numbers below 10, that is, neglecting residence time distribution (RTD) may not be a true assumption in all operating conditions.Finally, performing a comprehensive parameter study, the RTD's influence under various operating conditions and kiln designs is investigated. The varied parameters are the kiln diameter (D = 0.1–1 m) the ratio of particle to kiln diameter (d/D = 5 × 10−3–40 × 10−3), the ratio of kiln length to kiln diameter (L/D = 1–10), the kiln's inclination angle (β = 0.1–8∘), the Froude number (Fr = 10−3–10−2), the rotational Reynolds number (Re = 102–16 × 103), and the Péclet number (Pe = 5–100). Using more than 13000 simulations, we show that in one third of the investigated cases the change of product yield is higher than 2% when changing from plug flow behavior to high axial dispersion (Pe = 5). For cases where the influence of the RTD on the final product yield is small, conditions along the kiln might still change significantly. The biggest change of the product yield from plug flow behavior to high axial dispersion was found to be 5.5 wt% for the following conditions: D = 1 m, d/D =5 × 10−3, L/D = 1, β = 0.1, Fr = 10−2 and Reω = 16,000.

Techno-economic assessment of a rotary kiln shell radiation waste heat recovery system

In this work, the feasibility of implementing a waste heat recovery system based on the capture of radiation emitted from the surface of a rotary kiln is evaluated by coupling CFD analysis and process modelling including mass, energy, and exergy balances. It is found a potential heat recovery of up to 4980 kW of heat with an annulus absorber panel which extends 30 m of the kiln length and reaches an average temperature of up to 240 °C. Such heat could be used to generate 864.25 kWe of electricity through a Recuperated ORC with a thermal efficiency ηth = 17.35% and an exergetic efficiency ηexg = 48.62% for a total saving of 16.6 MJe per tonne of clinker. An economic feasibility for this recovery alternative is highly dependent on the electricity price in the cement plant location. It is observed for markets with electricity prices exceeding 0.1 $/kWh, the return on investment could reach values of 5% corresponding to a NPV close to 0.06 MUSD.

Simulation of the distribution of air flows and fuel combustion products in a channel of a tunnel kiln

A characteristic problem of the operation of tunnel kilns with high overlap is the output of products with low quality and a decrease in the energy efficiency of the burning process as a whole. Therefore, the object of research is the process of the flow of kiln gases through the channel of the tunnel kiln, the change in velocity of which was studied depending on the geometric parameters of the tunnel.In the course of studies of the dependence of the velocity distribution of the flow of kiln gases along the channel of the kiln on its geometric characteristics, a numerical simulation method in a simplified 2D formulation using the OpenFoam open code using the k--shear stress turbulence model is used. The velocity fields of kiln gases are obtained for three options for the height of the channel: the base with a vault height of 2 m, with a reduced vault height along the entire length of the tunnel and with a reduced tunnel height only in the burning zone. Analysis of changes in flow rates shows that the most effective would be to reduce the height along the entire kiln length, while changing the arch height in the burning zone will hardly affect the uniformity of velocities in the heating zone. Reducing the arch height also minimizes the likelihood of a return air flow in the cooling zone in the area from burning to the place of air extraction for drying. A lower overlap in the cooling section will increase the intensity of heat removal and, accordingly, reduce its loss with the product in order to use it for drying products.The presented simulation results make it possible to note that a change in the height of the overlap will lead to an increase in aerodynamic drag and, consequently, a pressure drop. This will require additional energy costs for driving draft engines and the possibility of increasing temperatures in the space under the cars.

A numerical approach for the combined analysis of the dynamic thermal behaviour of an entire ceramic roller kiln and the stress formation in the tiles

The numerical analysis of thermal and fluid dynamics behaviour of an industrial roller kiln used for manufacturing ceramic tiles is carried out and combined with the analysis of the mechanical stresses formed in the final ceramics product. The developed numerical approach is able to address the energy efficiency, the fuel consumption as well as the pollutant emissions and the quality of the final product. The model of the ceramic kiln is based on a lumped and distributed parameter model and accounts for the heat and mass transfer phenomena that take place in the real components under actual operating conditions of the systems. Models for the simulation of the different components that are used for the kiln functioning are included in the modelling, such as the burners, the fan, the valves and the control system. The numerical approach demonstrates to predict accurately the temperature distribution of both the tiles and the hot gases along the kiln length. Numerical results are validated against experimental measurements carried out on a real kiln during regular production operations. Additionally, the calculated temperature profile of the tiles is employed to predict the mechanical stresses that form in the ceramic product within the kiln. A thermomechanical model is adopted to determine the curvature and residual stresses in the tiles and particular care is devoted to the final stresses that remain at the end of the kiln since they affect the quality of product.The developed numerical approach demonstrates to be an efficient tool for investigating different control strategies to optimize the kiln thermal behaviour as well as the tile quality. On the basis of the fluid-dynamics and thermomechanical numerical approaches, a modified operating strategy for the kiln's cooling section is proposed to minimize the tiles' residual stresses and the modified cooling profile resulted to be in the operating range of the real kiln.

The Development of a Process Control System for the Production of Partially Reducing Nickel Oxide in a Tubular Rotary Kiln

This article describes the results of a control system development by the process of nickel oxide reducing to obtain partially reducing nickel oxide (PRNO) in a tubular rotary kiln (TRK). The purpose of the recovery in TRK is to obtain an enlarged, maximally recovered product before the anode electric fusion, since such an electrically conductive product, as partially reduced nickel oxide (PRNO), allows to reduce the electricity and reducing agent consumption during the process of electric smelting, which is economically advantageous. TRK is an object with distributed parameters. The reducing product quality is determined by the temperature profile along a kiln length. The development of a temperature profile control system is required, ensuring the receipt of a specified quality product for the objects with distributed parameters. The cell model was used in the work, corresponding to the 5 technological zones of the process, the dynamic model of the object was constructed and the dynamic behavior of the object was studied with step perturbations and the use of 3 different control systems. The analysis of the obtained data made it possible to establish that the best control results are obtained by a fuzzy regulator use.

Computer Simulation of Heat Transfer in a Rotary Lime Kiln

In the present work, a steady-state, finite difference-based computer model of heat transfer during production of lime in a rotary kiln has been developed. The model simulates calcination reaction in the solid bed region of the rotary kiln along with turbulent convection of gas, radiation heat exchange among hot gas, refractory wall and the solid surface, and conduction in the refractory wall. The solids flow countercurrent to the gas. The kiln is divided into axial segments of equal length. The mass and energy balances of the solid and gas in an axial segment are used to obtain solids and gas temperature at the exit of that segment. Thus, a marching type of solution proceeding from the solids inlet to solids outlet arises. To model the calcination of limestone, shrinking core model with surface reaction rate control has been used. The output data consist of the refractory wall temperature distributions, axial solids and gas temperature distributions, axial percent calcination profile, and kiln length. The kiln length predicted by the present model is 5.74 m as compared to 5.5 m of the pilot kiln used in the experimental study of Watkinson and Brimacombe (1982, Watkinson, A.P. and Brimacombe, J. K., “Limestone Calcination in a Rotary Kiln,” Metallurgical Transactions B, Vol. 13B, pp. 369–378). The other outputs have been also satisfactorily validated with the aforementioned experimental results. A detailed parametric study lent a good physical insight into the lime making process and the kiln wall temperature distributions.

Numerical simulation of heat transfer during production of rutile titanium dioxide in a rotary kiln

This paper presents a computational heat transfer model of a rotary kiln used for the production of rutile titanium dioxide by the calcination of paste-like hydrous titanium dioxide. The work details the modelling of several chemical reactions occurring in the solid bed region along with turbulent convection of gas, radiation heat exchange among hot gas, refractory wall and the solid surface, and conduction in the refractory wall. Finite-difference techniques are used and the steady state thermal conditions are assumed. The kiln is divided into axial segments of equal length. The solution is of marching type and proceeds from the solid inlet to the solid outlet. The direction of gas flow is opposite to that of the solids. Mass balance of each species in the solid charge, and mass and energy balances of the solid and gas in an axial segment are used to obtain solids and gas temperatures, and species concentration at the exit of that segment. The kiln length predicted by the present model is 45.75m as compared to 45m of an actual kiln reported by Ginsberg and Modigell (2011). The steady-state axial gas and solid temperature profiles have been also satisfactorily validated with the numerical results of the aforementioned paper. The output data consist of refractory wall temperature distribution, the axial solids and gas temperature profiles, axial solids composition profile, the length required for drying of the solid charge and the total kiln length required to achieve 98% conversion of anatase TiO2 to rutile TiO2. A detailed parametric study with respect to the controlling parameters such as percent water content (with respect to dry solids), solids flow rate, gas flow rate, kiln inclination angle and kiln rotational speed lent a good physical insight into the rutile-TiO2 production process in a rotary kiln.

English

3 ABSTRACT: Two gas samples and two mazout samples which used as fuels in Cement Companies were collected. These studied fuel samples were analyzed for compositional analysis and calculation of their calorific values via capillary gas chromatography connected with TCD and FID according to standard methods. These fuels need to have an appropriate chemical energy content which depends on the type of components and their organic content. There are a number of factors that promote the use of fuels in cement kilns. Of these factors, the most notable are: the high temperatures developed, the appropriate kiln length, the long period of time the fuel stays inside the kiln and the alkaline environment inside the kiln. The fuels used should fall within the extreme values of parameters such as: minimum heating value, maximum humidity content, and maximum content of heavy and toxic metals. It has been found that the net and gross heat values of the used natural gas fuel are 1014 Btu/ft 3 and 1032 Btu/ft 3 respectively. Mazout fuel exhibits net and gross heat values between 40313 and 43565 kJ/kg, these date reflect the preferred natural gases as fuels not only due to the higher calorific values but also it is friendly environment.

CFD Analyses of Syngas-Fired Industrial Tiles Kiln Module

Industrial kilns for ceramic tiles production demand thorough control of the firing parameters to ensure uniform product quality. A given temperature profile must be imposed along the kiln length, while spanwise temperature profile should be as uniform as possible at the tiles level at any location. Due to special needs in emerging markets, interest is growing towards the use of gases produced by gasification processes as an alternative to methane. This requires specific burner design and proper re-calibration of the firing parameters. In the present work, computational fluid dynamics is used to analyse an industrial kiln section for different fuels, nominal burner powers, and burner nozzle diameters. The results are given in terms of temperature and velocity fields in the kiln room, and temperature distributions over the tiles floor. It is shown that a sensible combination of the three parameters investigated can lead to satisfactory results, even with gases having poor heating value.

Computer Simulation of Drying of Food Products With Superheated Steam in a Rotary Kiln

The present work reports a computer simulation study of heat transfer in a rotary kiln used for drying and preheating food products such as fruits and vegetables with superheated steam at 1 bar. The heat transfer model includes radiation exchange among the superheated steam, refractory wall and the solid surface, conduction in the refractory wall, and the mass and energy balances of the steam and solids. The gas convection is also considered. Finite-difference techniques are used, and the steady state thermal conditions are assumed. The false transient approach is used to solve the wall conduction equation. The solution is initiated at the inlet of the kiln and proceeds to the exit. The output data consist of distributions of the refractory wall temperature, solid temperature, steam temperature, and the total kiln length. The inlet of the kiln is the outlet of the gas (superheated steam), since the gas flow is countercurrent to the solid. Thus, for a fixed solid and gas temperature at the kiln inlet, the program predicts the inlet temperature of the gas (i.e., at the kiln exit) in order to achieve the specified exit temperature of the gas. In the absence of experimental results for food drying in a rotary kiln, the present model has been satisfactorily validated against numerical results of Sass (1967, “Simulation of the Heat-Transfer Phenomena in a Rotary Kiln,” Ind. Eng. Chem. Process Des. Dev., 6(4), pp. 532–535) and limited measured gas temperature as reported by Sass (1967, “Simulation of the Heat-Transfer Phenomena in a Rotary Kiln,” Ind. Eng. Chem. Process Des. Dev., 6(4), pp. 532–535) for drying of wet iron ore in a rotary kiln. The results are presented for drying of apple and carrot pieces. A detailed parametric study indicates that the influence of controlling parameters such as percent water content (with respect to dry solids), solids flow rate, gas flow rate, kiln inclination angle, and the rotational speed of the kiln on the axial solids and gas temperature profiles and the total predicted kiln length is appreciable. The effects of inlet solid temperature and exit gas temperature on the predicted kiln length for carrot drying are also shown in this paper.

Simulation and Optimization of Drying of Wood Chips With Superheated Steam in a Rotary Kiln

The present work reports a computer simulation and optimization study of heat transfer in a rotary kiln used for drying and preheating wood chips with superheated steam at 1 bar. A rotary kiln employed for drying and preheating wet solids consists of a refractory lined cylindrical shell mounted at a slight incline from the horizontal plane. The kiln is slowly rotated about its longitudinal axis. Wet solids are fed into the upper end of the cylinder, and during the process, they are dried and heated by the counter-current flow of the hot gas. Finally, it is transferred to the lower end, where it reaches the desired temperature and is discharged. The heat transfer model includes radiation exchange among hot gas, refractory wall and the solid surface, transient conduction in the refractory wall, and mass and energy balances of the hot gas and solids. A finite-difference based computational heat transfer approach is used. A univariate search method has been used to obtain the minimum kiln length with respect to various kiln operating parameters subject to a constraint on the inlet gas temperature. The parametric study lent a good insight into the physics of the drying process in a rotary kiln. The optimization study reveals that for the same predicted kiln length, lower inlet steam temperature can be used, which will result in saving of energy cost.

Calculation of Limestone Burning in a Coke-Fired Kiln

A mathematical model of the steady-state process of limestone burning in a kiln is proposed in the one-dimensional approximation. The model is developed on the basis of the laws of conservation of mass and energy with regard for the kinetics of physicochemical transformations and is a set of ordinary differential equations. If the geometric parameters of the kiln are known, then by setting the conditions at the inlet and outlet, i.e., the boundary conditions, and solving the boundary-value problem, one can determine the thermochemical and hydrodynamic situation in the kiln. In particular, the model allows one to find the kiln length distributions of the compositions and temperatures of the solid and gas phases, the mass flow rates, and the volume contents at various feed rates and initial temperatures of air, raw material, and fuel and different coke and limestone particle sizes.

Use of alternative fuels in the Polish cement industry

Alternative fuels are made up of mixtures of different wastes, such as industrial, municipal and hazardous wastes. These fuels need to have an appropriate chemical energy content which depends on the type of components and their organic content. An industry that is particularly well suited to the employment of alternative fuels is the cement industry. There are a number of factors that promote the use of alternative fuels in cement kilns. Of these factors, the most notable are: the high temperatures developed, the appropriate kiln length, the long period of time the fuel stays inside the kiln and the alkaline environment inside the kiln. There are a number of countries that use their own alternative fuels in cement plants. These fuels have different trade names and they differ in the amounts and the quality of the selected municipal and industrial waste fractions used. The fuels used should fall within the extreme values of parameters such as: minimum heating value, maximum humidity content, and maximum content of heavy and toxic metals. Cement plants in Poland also use alternative fuels. Within the Lafarge Group, the cement plants owned by Lafarge Poland Ltd. have initiated activities directed at promoting the wider use of alternative fuels. There are a number of wastes that can be incinerated as fuel in cement plants. Some that can be mentioned are: selected combustible fractions of municipal wastes, liquid crude-oil derived wastes, car tyres, waste products derived from paint and varnish production, expired medicines from the pharmaceutical industry and others. The experience gained by the cement plants of Lafarge Cement Poland Ltd confirms that such activities are economically and ecologically beneficial. The incineration of alternative fuels in cement plants is a safe method for the utilisation of waste that is ecologically friendly and profitable for the industrial plants and society alike.

Axial transport and residence time of MSW in rotary kilns: Part I. Experimental

Experiments on the influences of operational variables on the axial transport of both heterogeneous municipal solid waste (MSW) and homogenous sand are conducted in a continuous lab-scale rotary kiln cold simulator. Compared with sand, the residence time of MSW has a relatively large discrepancy with the ideal normal distribution due to the trajectory segregation of MSW components. The residence time at different axial zone is quite different due to the varied bed depth profile along the kiln length. MSW has a longer mean residence time (MRT) and a lower material volumetric flow (MVF) than sand because of the higher θ d than sand. The increment of both rotating speed and kiln slope reduces MRT, and increases MVF. Exit dam has a significant impact on the MRT and the influence of internal structure group consisting of various axial ribs and circular ribs is mainly determined by the height of circular ribs. Inside wall roughness also has effect on MRT through changing the bed regimes. For a case with the certain inlet and exit bed depths, the product of MRT and MVF holds at a constant within the limits of experimental errors in spite of the changing experimental variables.

Heat Transfer in the Non-reacting Zone of a Cement Rotary Kiln

This paper presents a steady-state heat transfer model for a rotary kiln used for drying and preheating of wet solids with application to the non-reacting zone of a cement rotary kiln. A detailed parametric study indicates that the influence of the controlling parameters such as percent water content (with respect to dry solids), solids flow rate, gas flow rate, kiln inclination angle and the rotational speed of the kiln on the axial solids and gas temperature profiles and the total predicted kiln length is appreciable.

The Effects of Rotary Kiln Operating Conditions and Design on Burden Heating Rates as Determined by a Mathematical Model of Rotary Kiln Heat Transfer

A mathematical model of heat transfer in a directly-fired rotary kiln is developed and used to examine the effects of operating and design parameters on burden temperature. The model includes a mean beam length radiation model and axial zoning. Conductive and convective heat transfer are also included. Radiation between immediately adjacent zones is permitted. Calculation of heat transfer rates is facilitated by the use of an electric circuit analogue. An iterative solution procedure is adopted to solve the energy balance equations. At the conditions examined, the model predicts that coflowlng gas and solid streams result in higher average burden temperatures than do counterflowing streams. The moisture level of the feed is predicted to be a key operating parameter. The effects of kiln length, burden residence time, firing rate, and flame length are also examined.

A new theory for a rotary-kiln heat exchanger

From theoretical considerations, two limits for the length required to achieve a prescribed temperature rise in rotary kilns are established. The “well-mixed” condition represents the lower limit (minimum length), while the “non-mixed” condition represents the upper limit (maximum length). The actual kiln length lies between these two limits. Procedures for the calculation of both limits are presented in the paper. Results from the “well-mixed” theory agree remarkably well with published data for an existing kiln.