SIMULATION OF MIXED-FEED LIME SHAFT KILN : PERFORMANCE ANALYSIS AND OPTIMAL DESIGN

Abstract

The process of endothermic decomposition of calcium carbonate (limestone) to calcium oxide (quick lime) is brought about at elevated temperatures usually exceeding 1173 K under atmospheric pressure conditions. A mixed feed lime shaft kiln is basically a solid fuel-fired moving bed reactor with the upward-flow of hot gases counter-current to the down-flow of a mixed-charge of limestone and fuel particles undergoing calcination and combustion, respectively. A kiln basically has three operating sections, namely, the preheating, the burning and the cooling zones. The feed consisting of limestone and fuel particles is loaded into the kiln at the top, and the calcined product is withdrawn from its bottom. Air at ambient conditions enters at the bottom and the hot kiln (exhaust) gases leave from the top of the kiln. The literature survey revealed that although the most sophisticated lime shaft kilns have been developed to obtain high productivity, lime quality and thermal efficiency, yet the design of a suitable kiln for a new situation using the basic princip les involved in lime burning remains uncertain owing to the complex nature of the process and the uncertainties involved in the operating practices. No published information is available on mathematical modelling and simulation of the mixed-feed limeshaft kilns. A field survey conducted for the study of lime kilns in India revealed that a major fraction of lime is being produced in the mixed-feed lime shaft kilns in the building and small scale process (iii) industries for capacities generally in the range of 5 to 15 tonnes of quick lime per day. Presently these kilns are designed using thumb rules, based on past experience. For example, the relevant Indian Standards suggest a height-to-diameter ratio in the ran<*&of 3 to 4 2 corresponding to a superficial lime output rate of 2.5 tonnes/(m ) (day) for an efficient mixed-feed vertical lime shaft kiln. , In view of the importance of the mixed-feed lime shaft kilns for lime manufacture in India, the present investigation was undertaken to develop a detailed mathematical model for simulating the performance of such kilns. The effect of different design and operating parameters on the performance of these kilns was then studied to establish optimal design and operating criteria. Initially an attempt was made to evolve some state-of-the-art process engineering correlations for the mixed feed lime shaft kilns based on published information and the experience gained during the field survey and investigations on some commercially operating kilns. The energy requirements for the preheating and calcination of limestones were correlated with their chemical compositions. For a continuously operated kiln the times required for the preheating of limestone particles, their calcination and cooling of the product quick lime were correlated in terms of the particle diameters and the superficial lime output rates. The heights of the three zones of the lime kiln were estimated through the knowledge of the retention times for the respective zones. On account of the generalized practice (iv) of semi-continuous kiln operation followed for these types of low capacity mixed-feed kilns, a correlation was evolved to estimate the increase in the overall retention time by duly accounting for the time lag of manual over mechanized operation, working (8-hourly) shifts per day, the number of charges or discharges effected per shift, and the average time for each charge or discharge. Simulation of the mixed-feed lime shaft kiln required a detailed study of the phenomenological behaviour of the process and careful analysis of the design and operating parameters. The important design parameters identified are : Nominal lime production rate, Superficial lime output rate, wall thickness of brick masonry shaft and the exhaust gas temperature. The operating parameters of significance are : the size of limestone particles, the size and composition of the fuel particles, limestone-to-fuel ratio in the mixed-feed (input) and excess air fraction. The shrinking core model has been assumed for the decomposition of a limestone sphere undergoing calcination. At a given instant, there Is a central core of undecomposed calcium carbonate surrounded by a shell of calcium oxide with the reaction occurring at the Interface between the core and the shell. Rate of conduction of heat through the lime layer to the limestone core boundary has been taken to be the calcination rate controlling mechanism. The surface of stone exchanges heat with the fuel and (v) gases by conduction, convection and radiation mechanisms. The undecomposed limestone core is assumed to remain at the calcination temperature (1173 K) after the commencement of the calcination process irrespective of the core size. The net heat received at the surface of the limestone-lime particle Is consumed for the dissociation of calcium carbonate and partially for the heating up of the lime layer. The stone particle conduction equation was solved using the usual quasi-steady state assumption for any specified surface temperature and limestone conversion. Development of the model equation with due consideration for the sensible heat accumulation In the lime layer, and accounting for variations in surface temperature and fractional conversion along the axial position in the kiln are the novel features Incorporated In the proposed model. The proximate analysis of coal provides a basic framework for describing its combustion behaviour in a mixed-feed lime shaft kiln. The coal particles get preheated initially to 373 K and thereupon the demoisturlzation starts. The volatile matter is assumed to be driven off linearly with temperature when the fuel temperature lies in the range of 673-1173 K. Instantaneous combustion of the volatile matter is generally assumed when the fuel temperature becomes equal to greater than 948 K, the assumed value of auto-Ignition temperature for the volatile matter! The volatile matter released In the fuel temperature range of 673 to 948 K Is assumed to escape unburnt resulting in the partial loss of thermal efficiency. (vl) The ignition of the fixed carbon content of a coal particle has been assumed to commence only after the process of devolatllization is completed. The kinetics of shrinking particle size model has been assumed for combustion of the fixed carbon content of the fuel alongwith simultaneous segregation of ash from the fuel particle. An equation to describe the combustion behaviour of the fixed carbon content of the coal particle was used which accounts for the surface reaction rate and oxygen mass transport resistances. When the fuel temperature reaches 1173 K (assumed ash fusion temperature) and above, the fusion of the ash particles is assumed to result in the formation of a porous ash layer around the shrinking core of fixed carbon Interspersed with the fine ash particles. The necessary changes In particle surface areas before and after the ash fusion and increase In the density of the ash layer on account of its shrinkage with increase in the fuel temperature are also Incorporated in the model. Detailed analysis and modelling of the preheating, burning and cooling zones of the coal fired lime shaft kiln are carried out with some realistic assumptions. Comprehensive material and energy balances have been written for a differential volume element of axial width AZ at a given axial position (Z) of the kiln in due cognizance of the conditions prevailing in a particular zone of the kiln under (vii) consideration. The complicated interphase modes of the heat and mass transfer processes with or without chemical reactions, the relevant heat transfer coefficients, effectiveness factors for estimating active heat exchange surface areas, variations in specific heats with temperature and composition, variations in gas emissivitles for gas-solid radiant heat exchange, etc., are properly accounted for in the analysis of a particular zone. In addition, the bulk movement of gaseous components from one phase to another at appropriate temperatures and their contributions in the energy balances are properly incorporated in the proposed models. The temperatures of bulk gas, limestone and the fuel particles, fractional degree of combustion of the fuel and fractional degree of calcination of the limestone particles are the important output parameters obtained from the solution of the simulation model after applying suitable convergence criteria Thus, a rigorous, uniquely stable and efficient system has been developed for analyzing the behaviour of a coal fired lime shaft kiln. The important design and operational parameters investigated, their average values for the base conditions, and their ranges of variation are given in Table-6.1 (Chapter-6). The effect of variations in the design and operational parameters on the performance of the kiln are extensively discussed. Temperature and conversion profiles pertaining to the base conditions are carefully analyzed. Only one parameter is varied at a time for (viii) parametric sensitivity analysis by choosing the maximum and the mini mum values of the ranges investigated apart from the base conditions, maintaining all the other parameters at the values corresponding to the base conditions. The results of the senstivity analysis are presented in Tables-6.4 through 6.6 (Chapter-6). The aim of the present investigation was largely to develop a comprehensive, uniquely stable and an efficient simulation system for the mixed feed lime shaft kiln, a stupendous task in itself, and not to carry out rigorous optimization studies for such kilns. But parametric sensitivity analysis le d to the identification of the following optimum and most appropriate design and operating conditions from the thermal performance point of view for a nominal lime production rate of 10 tpd : (i) Partical size of limestone (d ) =0.100 m (11) Ratio of particle size of fuel to size of limestone (d /d ) =0.5 Kf *s *•» (ill) Superficial lime output rate (P ) = 3.0 t/(m2) (day) 8 (iv) Limestone to fuel ratio (X ) =5.8 (v) Fractional content of volatile matter in fuel (X-J - as low as possible, but not exceeding 0.25 (vi) Excess air based on total combustion of fixed carbon and total volatile matter (X J (ix) - 10 percent (vil) Wall thickness of masonry shaft (W ,) excluding fire bricks = 0.46 m (viii) Exhaust gas temperature (T ) = = 813 K o For the above conditions the effective kiln height-to-its-internal diameter ratio in around 6.5.