活性石灰生产过程工艺条件的优化

生产活性石灰首先要有优质的石灰石、合格的燃料和先进的石灰窑等基本条件,其次,石灰石煅烧过程工艺条件的优化也至关重要。石灰煅烧质量取决于以下因素:①石灰石的块度、煅烧时间和煅烧温度的关系;②燃料块度与石灰块度的关系;③石灰窑的热效率与燃料配比的关系;④石灰质量与风量的关系;⑤生石灰运输与贮存方式选择;⑥窑气的净化与综合利用。     

Mass transfer in rolling rotary kilns: a novel approach

A novel approach to modeling mass transfer in rotary kilns or rotating cylinders is explored. The movement of gas in the interparticle voids in the bed of the kiln is considered, where particles move concentrically with the geometry of the kiln and gas is entrained by these particles. The approach considers a differential section along the length of a rotary kiln where the gas concentration in the freeboard is assumed to be uniform in that section. A reactor modelling approach has been used to derive effectiveness factors for the bed as a function of bed fill, reaction kinetics and rotation speed. In many cases, the entrained gas becomes depleted within the bed, leading to a simplified model for the bed effectiveness factor. Experimental data confirms the validity of this model for slower rates. At faster rates, mass transfer can be much higher than the model predicts, indicating that other mechanisms, such as dispersion or diffusion are also important in these conditions.     

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.     

Simulation of Drying Using a Kiln Model

A mathematical model was developed for simulating a convective batch lumber drying process. The model incorporates mass and heat transfer relationships within the lumber stack, as well as thermodynamic properties of the wood and drying air. It takes into account the change of air properties along the stack and its effect on the mass and heat transfer parameters. The model relies on a drying rate function that is an empirical correlation based on single-board tests. A drying rate function for western hemlock (Tsuga heterophylla) lumber was developed. The drying rate function was obtained based on experiment results from 500 small boards dried over a range of conditions used in commercial practice. The model was first validated against data available in the literature and then against large batches of hemlock dried in a laboratory kiln. In both cases, the model output was in good agreement with the average moisture content, the drying rates, and the temperatures measured in the larger batches.     

Burning and nodulization process of clinker in the rotary kiln as viewed from the fine textures of the constituent minerals

Portland cement clinkers from the rotary kiln vary with their grain size in both chemical compositions and microscopic textures, according to the wide variation in the burning and nodulization environment in the kiln. The dense interior of large clinker nodules, first formed at high heating rates on and near the surface of the moving raw mix mass, is enriched with K₂O and SO₃ and made up from coarse-grained components of the raw mix. The alite crystals consist mostly of the M₃ phase with inclusions in the core. By contrast, the porous exterior, formed inside the mass at lower heating rates and firing temperatures, is less in K₂O and SO₃ content and made up from fine-grained components of the raw mix. The alite crystals are generally zoned with M₁ occurring in the core. Clinker nodules of medium size, similar in both the chemical composition and the fine textures of alite, are formed concurrently with the exterior of large nodules. Fine clinker nodules come from the core of the mass where the radial motion is stagnant and are formed, due to the large temperature gradient in the mass, at low heating rates and firing temperatures. Dust components comprise, besides small fragments of clinkers, separate alite and belite grains in quantity, indicating that they are separated mostly in the quenching cooler from the porous surface layers of the large clinker nodules. K₂O and SO₃, as well as the fine textures of alite, are useful as an indicator of the progress of firing and nodulization in the kiln.     

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 (CO₂) 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.     

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.     

USE OF DYNAMIC MODELLING FOR THE DESIGN OF BATCH-MODE DEHUMIDIFIER DRYERS

The performance of batch-mode dehumidifier dryers undergoes significant changes during the drying process due to the range of temperature and humidity conditions experienced. Consequently steady-state methods are inapplicable for determining the performance of such kilns. In this paper a dynamic dehumidifier kiln model is used to estimate the drying speed, energy use and annual income for a batch-mode dehumidifier kiln. The model has been validated at both the dryer and dehumidifier levels, and the product is Pinus radiata, an easy-to-dry plantation soft-wood. The results show that control limits for the evaporating and condensing temperatures must be well matched to maximise the drying speed in the constant rate period. In the falling rate period, it is important not to place a lower limit the relative humidity unnecessarily. The use of supplementary heat to accelerate the kiln start-up is shown to have only a small influence on the kiln performance.     

Process simulation of activated carbon production using a rotary kiln

The rotary kiln used for the activation of charcoal is simulated using mass and energy balances to obtain the temperature distributions of environmental gas and solid in the kiln. The computed results are used to find the optimal operation condition. In finding the optimal gas temperature for the reductive gas environment necessary to the charcoal activation, the outcome gives the amount of fuel required and the effects of other operational variables, such as feed and steam rates. While the fuel and feed rates give large variation of gas and solid temperatures, the activation steam and moisture and volatile contents in feed do not affect the temperatures significantly. It was found that the temperature distribution has a similar pattern to that of coal pyrolysis because of the similarity of heat transfer mechanism.     

THE USE OF THE PENETRATION MODEL FOR THE DISSOLUTION OF LIMESTONE IN THE CO2-WATER SYSTEM

The penetration model was implemented for the dissolution of limestone in the CO₂-water system. The model includes the acid-base reactions of the carbonate species as well as the autoprotolysis of water. It was also assumed that there is no surface resistance to the dissolution of the solid. This assumption restricts the use of the model to those conditions where the dissolution rate is limited by the rate of mass transfer. When using the model, only the hydrodynamics of the water solution need to be experimentally determined and put in terms of the model's hydrodynamic parameter. All other model inputs are either physical constants or known bulk concentrations. Dissolution experiments, performed on a rotating cylinder system, were used to test the ability of the model to predict the dissolution rate of limestone in an aqueous solution. Of special significance was the ability of the model to predict the dissolution rate at different pH-values, CO₂ partial pressures, temperatures and hydrodynamic conditions. An explicit finite differences method was used to deal with the system of non-linear partial differential and algebraic equations, which arose from the implementation of the penetration model. This investigation has shown that the limestone dissolution process in the mass transfer controlled region, can be modelled and described by the penetration model. The penetration model accurately describes the effects of all parameters investigated, including the enhancement effect from CO₂ (up to a factor of 10 compared with dissolution in a CO₂-free atmosphere) and temperature. The penetration model has also been compared with the film theory model. The comparison of the two models shows that the penetration model yields a better correlation to the experimental data in a CO, atmosphere. In a CO₂-free atmosphere the models are almost identical. However, the penetration model is computationally more difficult. A numerical procedure for solving the penetration model has been designed. This procedure includes a method of dealing with the unusual boundary conditions at the surface.     

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。     

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.     

CHARACTERISTICS OF PYROMETRIC CONES1

For the first time the characteristics of pyrometer cones have been determined under definite and reproducible conditions of heating. The temperatures of the “end points” of a complete set of cones together with their bending intervals were measured at carefully controlled rates of heating in clean air and in various kiln gases. Heated at 20°C per hour in furnace gases free from SO₂, the end points of cones 015 to 01 are somewhat higher than those observed when the cones were heated in clean air. Cones 022 to 016 and 1 to 42 were acted upon only slightly by kiln gases free from SO₂. There is more significance in the demonstration of the utility of measuring the end points of the cones under accurately controlled and known conditions of heating than in the numerical results obtained.     

回转窑煅烧烧结石灰的探索

在采用回转窑煅烧烧结石灰的过程中,在破碎、运输等环节经常出现堵料问题。对两种石灰石矿和石灰进行了深入的分析,采用适宜的石灰石来煅烧烧结石灰解决了堵料问题,对采用回转窑煅烧烧结石灰具有指导意义。     

Mathematical modeling of an in-line low-NOx calciner

The reduction of the NO_x content in in-line-calciner-type kiln systems can be made by optimization of the primary firing in the rotary kiln and of the secondary firing in the calciner. Because the optimization of calciner offers greater opportunities the mathematical modeling of this reactor is very important. A heterogeneous, dynamic mathematical model for an in-line low-NO_x calciner based on non-isothermal diffusion-reaction models for char combustion and limestone calcination has been developed. The importance of the rate at which preheated combustion air was mixed into the main flow was particularly studied. The results of the simulations indicate that the external heat and mass transfer to the char particles is not limiting. Internal diffusion of O₂, CO, NO and CO₂ is important especially in the reducing zone and the first part of the oxidizing zone of the calciner and the internal heat transport limitation is significant for the endothermic limestone calcination. The rate at which preheated combustion air is mixed into the main flow directly influences the coal combustion rate, and thereby through the rate of heat release from combustion, it also influences the calcination rate and the temperature profile. The mixing rate has some influence on the CO concentration profile and an important influence on the overall degree of fuel-N to NO conversion.     

SIMULATION OF DRYING IN A BATCH LUMBER KILN FROM SINGLE-BOARD TESTS

A deterministic model was developed to perform a board-by-board simulation of a forced convective batch lumber kiln. Individual board properties may be input and dryer operating parameters varied. The drying rates are empirical correlations based on single-board laboratory tests. The model incorporates the thermodynamic properties of the wood and gas, as well as mass and energy balances within the lumber stack. It also accounts for differences in heat and mass transfer resulting from position and changing gas properties throughout the dryer. The rate of drying predicted by the model and the final moisture content distribution were verified by weighing boards in a batch kiln before, during, and after drying. The application of the model is illustrated by simulating four common scenarios.     

THE EFFECT OF DRYING TEMPERATURE ON THE MECHANO-SORPTIVE BEHAVIOR OF RED OAK LUMBER

Matched units of 3.2 cm thick red oak lumber were dried simultaneously in a steam heated and dehumidification kiln. The two pairs of runs are designated Set I and Set II. The objective was to compare the mechano-sorptive behavior and board shrinkages while using the recommended U.S. Forest Products Laboratory schedule with the steam kiln and comparatively low temperature drying in the dehumidification kiln.

Drying rates in Set I and Set II were comparable for the two kilns up to approximately 450 hours, which illustrated the dependence of the drying rate on the relative humidity of the kiln air rather than its temperature. Subsequently, the stepwise increases in dry bulb temperature for the steam kiln were accompanied by accelerated drying.

Less compression set developed in the interior mechano-sorptive slices for the dehumidification kiln runs. On an average, the maximum compression set for the core slices from the dehumidification kiln was about S0% of that for core slices from the steam heated kiln. Simultaneously the surface slices from the dehumidification kiln developed more tension set than those from the steam heated kiln. Board width shrinkage at the end of drying, at the same average moisture content, was greatest for the steam kiln. The greater shrinkage is attributed to greater compression set due to the higher drying temperatures. These results support McMillen)s explanation for the effect of drying temperatures upon sets and the shrinkage of red oak lumber.     

EFFECTS OF SAWING PATTERN ON LUMBER DRYING: MODEL SIMULATION AND EXPERIMENTAL INVESTIGATION

The sawing pattern of lumber affects the drying rate due to transverse permeability differences. These permeability differences are considered in a single board drying model which is able to investigate the drying rates for boards with varying growth ring angles. For the drying of Pinus radiata lumber, the model predicts that the quartersawn boards need longer drying time than the flatsawn boards. The drying time difference was 10-15% of the total drying time for conventional temperature (CT) drying and accelerated conventional temperature (ACT) drying, but was less significant for high temperature (HT) drying. In the simulation of a kiln stack drying, a sawing pattern factor was introduced to the relative drying rate function, which reflected the effects of the growth ring angle and the drying temperatures. The modified kiln wide drying model was used to predict the drying rates for a kiln stack consisting of entirely flatsawn boards and a kiln stack consisting of entirely quartersawn boards. Drying tests were conducted using stacks of mixed flatsawn and quartersawn Pinus radiata sapwood boards. In the tests, three drying schedules were used which included CT, ACT and HT drying. The experimental results agree closely with the model predictions and thus, after further validation, the drying models can be used to predict commercial kiln drying of boards with different sawing patterns.     

THE EFFECT OF DRYING TEMPERATURE ON THE MECHANO-SORPTIVE BEHAVIOR OF RED OAK LUMBER

ABSTRACT

Matched units of 3.2 cm thick red oak lumber were dried simultaneously in a steam heated and dehumidification kiln. The two pairs of runs are designated Set I and Set II. The objective was to compare the mechano-sorptive behavior and board shrinkages while using the recommended U.S. Forest Products Laboratory schedule with the steam kiln and comparatively low temperature drying in the dehumidification kiln.

Drying rates in Set I and Set II were comparable for the two kilns up to approximately 450 hours, which illustrated the dependence of the drying rate on the relative humidity of the kiln air rather than its temperature. Subsequently, the stepwise increases in dry bulb temperature for the steam kiln were accompanied by accelerated drying.

Less compression set developed in the interior mechano-sorptive slices for the dehumidification kiln runs. On an average, the maximum compression set for the core slices from the dehumidification kiln was about S0% of that for core slices from the steam heated kiln. Simultaneously the surface slices from the dehumidification kiln developed more tension set than those from the steam heated kiln. Board width shrinkage at the end of drying, at the same average moisture content, was greatest for the steam kiln. The greater shrinkage is attributed to greater compression set due to the higher drying temperatures. These results support McMillen)s explanation for the effect of drying temperatures upon sets and the shrinkage of red oak lumber.     

石灰窑技术及发展方向

简述了活性石灰的新技术、影响石灰石煅烧质量的因素、窑控制技术及故障排除方法,以及石灰粉料的应用,指出了石灰行业面临的大好形势和发展特点。