Lime Shaft Kilns Research Articles

DEM-CFD coupled simulation of limestone calcination and fuel combustion in beam type lime shaft kiln

This paper presents the development and simulation analysis of a coupled DEM-CFD model of the gas–solid heat and mass transfer process in an industrial-scale beam lime shaft kiln, which uses Computational Fluid Dynamics (CFD) to simulate the combustion of gaseous fuels and the three-dimensional transport of mass, momentum and energy in the gas, and the Discrete Element Method (DEM) to simulate the movement of limestone particles and calcination reactions in the kiln are simulated using the Discrete Element Method. A beam lime kiln uses five burner beams staggered up and down to provide heat for the decomposition of limestone particles in the kiln, and natural gas is injected as fuel from the burners on the burner beams into the limestone bed of the kiln for combustion, while cooling air enters the kiln from the bottom air cap to cool the calcined products and then enters the calcination zone as secondary air to participate in the combustion. In this paper, a 250 m3 beam lime shaft kiln in normal production is used as the research object to simulate and analyze the data of temperature, gas and solidity field and limestone decomposition rate in the kiln. The simulation results show that the overall limestone calcination temperature in the kiln is low and the temperature distribution is uneven, and the decomposition degree of limestone near the kiln wall is higher than that in the center of the kiln. It's caused by the wall effect, the particle porosity is larger near wall, and combustion gas penetration is therefore enhanced in the vicinity. Meanwhile, the simulation results are in good agreement with the actual measured data, which verifies the reasonableness of the model for application in industrial-grade lime kiln.

Simulation of Reacting, Moving Granular Assemblies of Thermally Thick Particles by Discrete Element Method/Computational Fluid Dynamics

AbstractThe potential and limitations of the discrete element method (DEM) coupled with computational fluid mechanics (CFD) to simulate chemically reacting, moving granular material interacting with a fluid flow are summarized. A special focus is set on thermally thick particles, which requires to resolve the intraparticle transport and reaction processes. The aspect of complex particle shape is addressed, as shape may dominate the particle behavior in densely packed granular assemblies even more than the details of contact force laws. The fluid flow in the granular assembly is assumed to be a gas. The potential of DEM/CFD will be highlighted presenting three illustrative examples: a large‐scale lime shaft kiln with intermittent operation, an industrial‐size grate firing system for the incineration of municipal waste, and a small‐scale straw pellet stove.

Dem-Cfd Coupled Simulation of Limestone Calcination and Fuel Combustion in Beam Type Lime Shaft Kiln

Dem-Cfd Coupled Simulation of Limestone Calcination and Fuel Combustion in Beam Type Lime Shaft Kiln

Lime Shaft Kilns

The annual world lime production amounts to approximately 350 million tons. The most efficient and ecological way to calcine limestone and dolomite is the use of modern shaft kilns. The requirement to install new lime shaft kilns at a stagnating market growth is substantial because a significant amount of lime is still produced with outdated kiln models. More than three quarters of the world’s lime production is high reactive lime. The PFR lime kiln has established itself worldwide for this type of product. This article compares the advantages of modern shaft kilns with other kiln models and explains the functional principle of such lime kilns. It also shows new innovative ideas and explains in which areas further research and development is still required in order to meet the growing demands of the market.

Simulation of Reacting Moving Granular Material in Furnaces and Boilers An Overview on the Capabilities Of the Discrete Element Method

A brief overview is given on the capabilities and on the current limitations of the Discrete Element Method (DEM) coupled with Computational Fluid Mechanics (CFD) to simulate chemical reacting moving granular material. An approach to resolve the internal transport and reaction phenomena in particles of complex geometry is presented. Heat and mass transfer from and to the particles are accounted for as well as heat transfer between particles and between particles and a surrounding gas phase including radiation. Gas phase reactions outside the particle interact with inner particle processes.Examples will be shown to demonstrate the capabilities of DEM/CFD coupling. These examples are an industrial scale lime shaft kilns, the simulation of a domestic pellet stove and a grate firing system for the incineration of municipal waste. The advantages of a DEM/CFD approach will be highlighted but also the still existing drawbacks and limitations are discussed. The paper ends with an outlook on necessary developments to make DEM/CFD a standard engineering tool for chemically reacting granular material.

3D-DEM-CFD simulation of heat and mass transfer, gas combustion and calcination in an intermittent operating lime shaft kiln

This work presents DEM-CFD simulations of the transient processes occurring in an industrial scale PFR-kiln. DEM allows the numerical simulation of the moving and reacting limestone bed in the kiln and is coupled with a 3-dimensional CFD simulation describing the interstitial gas phase.A PFR-kiln consists of two vertical shafts and a connecting crossover channel. The two shafts periodically switch their function at regular intervals of about 15 min. While one shaft calcines the product in parallel flow with gas temperatures above 900 °C, the other preheats the stones in counter flow.The model has been applied to an industrial PFR-kiln of 18 m height. A realistic particle size distribution of the limestone with particles ranging from 50 to 90 mm has been set. Methane combustion provides the heat for calcination and is simulated by a two-step mechanism. Simulation results show a nearly uniform temperature distribution in the calcination zone but significant inner particle temperature gradients. The calcination degree depends on the particle location within the kiln and decreases towards the outer kiln walls. Measured and simulated temperatures are compared. Maximum temperature values as well as its characteristic oscillation induced by the periodic kiln operation could be reproduced by the simulation, especially keeping in mind the difficulties of thermocouple measurements under the harsh conditions in industrial reality.

Coupled three dimensional DEM–CFD simulation of a lime shaft kiln—Calcination, particle movement and gas phase flow field

In lime shaft kilns the limestone is heated in counterflow with the flue gas from multiple burners, commonly fired by fossil or alternative fuels. After preheating, the limestone enters the calcination zone with gas temperatures above approximately 900°C. To achieve high thermal efficiencies and calcination degrees, a homogeneous gas flow through the packed bed is essential. Spatial pressure differences and energy sinks and sources resulting from the calcination process and the fuel combustion make it even more difficult to predict the actual three-dimensional temperature and flow distribution in the kiln. Furthermore the particle size distribution is important for the system design, because limestone with larger size needs significantly higher residence time in the calcination zone for full conversion.This work presents the application of a novel 3-dimensional particle-mechanics based, numerical tool on an industrial scale. The tool allows the simulation of a moving and reacting limestone bed in a shaft kiln coupled with the 3-dimensional CFD-simulation describing the interstitial gas phase. Convective heat transfer between gas phase and particles, radiative and contact heat transfer between particles as well as calcination reaction are accounted for. With this combined approach the calcination of the limestone bed can be simultaneously described on the system scale while resolving detailed thermo-chemical processes on the particle scale.

Measurement and Simulation of Lime Calcination in Normal Shaft Kiln

This article presents a one-dimensional mathematical model to simulate the lime-burning process in normal shaft kilns. The model comprises ordinary differential equations derived from the principal conservations of mass and energy. A shrinking core approach is employed to describe the mechanisms and to calculate the decomposition rate of limestone particles. The model is used to simulate the temperature of the gas and the solid, the lime-burning degree, the pressure drop, and the heat loss by kiln shell. Operating conditions, such as kiln throughput, energy consumption, and stone size, are also investigated by the model. Experiments were carried out to measure temperatures in industrial kilns. Maximum temperatures were observed in the region of 1.0–2.0 m above the burner level. The kiln temperature is about 1600°C. Comparisons of the experiments and simulations show that the temperatures measured in the kilns are very close to the solid temperatures predicted by simulations. The results of simulations and those of experiments are in good agreement. The results of this study can be direct utility for the regulation and optimization of industrial lime shaft kilns.

Energy and exergy assessments of a lime shaft kiln

Calcium oxide (CaO), commonly known as lime or quicklime, is an energy intensive product. In order to produce lime, vertical shaft kilns are widely used in the lime industry. The objective of this work is to analyze the energy and exergy consumption of the calcination process in vertical shaft kilns, in order to identify the factors affecting fuel consumption. Data on energy and exergy consumption and losses throughout the calcination process are given for two shaft kilns. In the process, the energy efficiency is found to be higher than the exergy efficiency, e.g. 71.6% and 40.8% for the energy and exergy efficiency of one of the kilns. Results also showed that the most irreversible processes taking place in the kiln are the exergy destruction due to fuel combustion and the exergy destruction due to internal heat and momentum transfer both accounting for about 40% of the efficiency loss. Moreover, the main exergy loss through the boundaries of the kiln is the exergy loss with the exhaust gases contributing with more than 10% of the efficiency loss. Improvements on both the energy and the exergy efficiencies can be achieved through a better control of the operational parameters of the kiln (ratio limestone/fuel supply, excess of combustion air, size and size distribution of the limestone fed to the kiln and exit temperature of quicklime flow). The present study proposes a tool for the analysis of energy and exergy utilization of the calcination process in limekilns, and also provides some energy conservation measures.

A lime shaft kiln diagnostic expert system based on holographic monitoring and real-time simulation

In this paper, a shaft kiln (diameter 4 m and height 21 m) for processing lime is investigated. Monitoring equipment was used to collect information about the lime production process. From this data a dynamic simulated mathematic model of lime production was obtained. The model is based on the equations of mass balance, constitutive balance, reaction dynamics and energy. This model allowed the concept of holographic monitoring to be developed. From the analysis of the production process, direct experience of operating the plant, and a number of parameters obtained from real-time detection and off-line numerical simulation results of the furnace process, a real-time diagnostic expert system for the lime shaft furnace was constructed. The benefits of using the system are that equipment failures are greatly reduced, the output of the lime shaft kiln is increased by about 25%, the concentration of carbon dioxide in the exhaust is increased by about 10%, the decomposition rate of limestone is increased by 6%, and the coke ratio decreased by 20%.

Exergy-based indicators to evaluate the possibilities to reduce fuel consumption in lime production

A new way to evaluate the energetic performance of lime shaft kilns is proposed. Two new exergy-based indicators are introduced for the evaluation, one to assess the exergy efficiency of limekilns and the other indicator to assess the effectiveness of the exergy consumption of the dissociation reaction. The combination of both indicators provides a clear picture of the energetic performance of the process, highlighting the main potentialities for fuel saving (fuel consumption represents about 50% of total production costs). The validity of the proposed assessment is examined using some operating data measured in a commercial lime factory. Results show that introduction of exergy-based indicators in the assessment improves the evaluation of the energy consumption of the calcination process. In this way the impact of the process losses in the fuel consumption is better addressed.

Cleaner production in a small lime factory by means of process control

An approach to Cleaner Production in a small factory of hydrated lime is presented. The work aims to improving fuel and limestone utilization and reducing waste production in the plant. Efforts mainly focus on the calcination stage, where around 50% of the production costs due to fuel consumption occur. To fulfill this objective, new indicators are proposed to evaluate the performance of the lime shaft kiln, in order to develop an operational control method for the calcination process. The systematic use of the developed operational control method is a powerful tool in the daily operation of a small lime factory, since it allows a more efficient calcination process with reduced environmental impact and production cost.

A coupled fluid dynamic-discrete element simulation of heat and mass transfer in a lime shaft kiln

In the present study heat and mass transfer related to the chemical conversion of limestone to quicklime in a shaft kiln are investigated by means of a coupled numerical scheme for gas and solid phase transport. The three-dimensional transport of mass, momentum and energy in the gas phase is modelled by computational fluid dynamics (CFD), while a discrete element method (DEM) is employed for the mechanical movement and the conversion reactions of the solid material. The DEM simulation readily describes the mechanical and thermal particle-to-particle interactions of a large number of differently sized particles. Novel aspects addressed in this work are the simultaneous effects of inner particle heat-conduction and pore-diffusion of the gaseous product of the calcination reaction (CO 2) modelled by a shrinking core approach. Simulations of laboratory scale experiments of single reacting spheres show good agreement with the measured conversion rates. Simulations of an idealised vertical shaft kiln including pressure drop calculations demonstrate the suitability of the proposed approach for the modelling of industrial scale systems.

Effect of Kiln Gas Recirculation on Fuel Consumption Rate and on Kiln Gas Temperature in Fuel Fired Lime Shaft Kiln

Effect of Kiln Gas Recirculation on Fuel Consumption Rate and on Kiln Gas Temperature in Fuel Fired Lime Shaft Kiln

Derivation of the Mathematical Expressions for Calculating Thermotechnologically the Principal Quantities on fuel fired Lime Shaft Kiln as a Function of Decarbonation Temperature of Limestone

Derivation of the Mathematical Expressions for Calculating Thermotechnologically the Principal Quantities on fuel fired Lime Shaft Kiln as a Function of Decarbonation Temperature of Limestone

Fundamental of the thermotechnological treatment on the lime shaft kiln

Fundamental of the thermotechnological treatment on the lime shaft kiln

Design of lime shaft kilns

Calculation methods are given for the design of the preheating zone, the burning zone and the cooling zone. The heat transfer problem in the burning zone of gas fired kilns is solved by numerical methods. Residence times in the three zones are calculated. The influence of production rate and stone diameter on residence time is shown. Heat losses through the wall are calculated.

Experimental Study on the Aerodynamic Flow Patterns of Gases Associated with the Type of Liquid Fuel-Fired Lime Shaft Kiln

Experimental Study on the Aerodynamic Flow Patterns of Gases Associated with the Type of Liquid Fuel-Fired Lime Shaft Kiln

Remains of Lime Shaft Kiln

Remains of Lime Shaft Kiln

On the Ability of a Lime Shaft Kiln used Heavy Oil as Fuel

On the Ability of a Lime Shaft Kiln used Heavy Oil as Fuel