Lime Kiln Process presents one of the most challenging measurement and control solutions in the Pulp and Paper mills. The complex dynamics and multi-variable nature of the lime kiln process, with its non-linear reaction kinetics, long time delays and variable feed characteristics, make the process inherently difficult to operate efficiently.
During its operation, many interconnected variables must be considered and control actions must be designed to meet multiple and sometimes conflicting objects. An additional difficulty is the reliability and/or accuracy of some of the measurements that are needed for even the basic controls.
The operation may also be upset by severe disturbances and certain process variables must be maintained within predefined constraints in order to ensure safe operation of the process and to protect the environment.
Control of the kiln process is thus, in many respects, a demanding task and therefore most of the kilns have been and are still operated without supervisory-level control system.
Economical pulp mill operation dictates the recovery of the spent cooking chemicals and dissolved wood compounds that are removed from the digester. This chemical recovery is performed in the recovery boiler and causticizing processes. The causticizing process, in turn, requires reliable source of calcium oxide (CaO). In the causticizing reaction, calcium oxide (CaO) is converted to calcium carbonate (CaCO3). Calcium carbonate is the major constituent of lime mud. The Lime Recovery System is designed to take the calcium carbonate (CaCO3) and convert it back to calcium oxide (CaO). This lime is again used in the slaking and causticizing processes. The conversion reaction is known as calcining or lime burning.
CaCO3 (s) + Heat --- > CaO (s) + CO2 (g) Calcining Reaction
Lime produced in this fashion is known as reburned lime. This reburned lime must be of high quality as determined by its ability to slake as described above. Calcium oxide (CaO) slakes with water and is converted to calcium hydroxide (Ca(OH)2).
Reburned lime produced in the lime kiln is a solid, and is whitish/grayish green in color. It forms in small pebbles when correctly calcined. Impurities enter the mill with the wood, water and makeup chemicals. They tend to build up in the lime and lime mud. Typically, the reburned lime will be about 82 - 90% calcium oxide (CaO) with 10% - 18% impurities including 2-6% residual carbonate (CaCO3).
Lime mud is separated from white liquor in the white liquor clarifier/filter and mud washer. It will enter the Lime Kiln System from the storage tanks at about 40% solids, will then be concentrated to about 74-79% solids content at Precoat Filter before entering the kiln and contains about 90% CaCO3 and 10% impurities.
In the lime kiln, calcium carbonate (CaCO3) is converted to calcium oxide (CaO). This reaction, known as calcination, requires an exposure to temperatures in excess of 1500 F. Exposure to these temperatures results in the liberation of carbon dioxide gas (CO2). The calcination reaction is: CaCO3 (Solid) + Heat --- > CaO (Solid) + CO2 (Gas).
Lime mud enters the lime kiln at 74-79 solids and 21-26% water. Lime mud is calcium carbonate plus impurities. The water is evaporated and the lime is heated to at least 1500 F for calcination to begin. The calcination reaction goes faster at temperatures in excess of 1500 F. Due to reaction time limitations, calcining temperatures of about 1800 F will be used. Gas or oil is burned in the kiln to supply the heat. At some times NCGs (Non-Condensable-Gases) are also contributed. The calcining temperature is a key variable in determining lime quality.
The lime produced in the lime kiln is used in the causticizing process. The causticizing and lime recovery processes are extremely interdependent. Quality variations in the lime kiln create problems in the causticizing process. These causticizing problems then will in turn create problems with the lime kiln and digester operation. System upsets eventually smooth out, but it takes time. It is, therefore, important to carefully control the processes to minimize disturbances.
Lime Kiln Advanced Control
Lime Kiln Control Objectives
OptiLime-2100 Solution optimizes the entire lime kiln process. Its primary objective is to increase capacity to achieve higher production rates, while simultaneously producing more consistent lime at higher quality—all while achieving lower overall energy consumption and reduced environmental impact
Lime Kiln Controls
Lime Kiln Temperature Profile Control is the most important part of the advanced control scheme. The objective of the temperature profile control is to provide optimal conditions for all four process zones: drying, heating, calcining and burning. This control compensates for changes in lime mud moisture, production rate and filling degree.
The hot-end temperature target is the most important control parameter. The hot-end advanced temperature control is vital part of the reduction of the variability of the lime residual carbonate level. The midsection and end-temperatures are used for monitoring the temperature profile and preventing over-temperature excursions. The calcination process is stabilized by controlling the temperature profile in the lime mud as it is flowing through the kiln.
The primary controllers in OptiLime-2100 are Model Predictive Controller (MPC) and Statistical Process Controller (SPC). These Controllers are located in Delta-V based industrial PC with connection to DCS utilizing a built in OPC link.
Residual Carbonate module maintains the quality of the
reburned lime in the optimum range by means of small adjustments to
the target value of the hot-end temperature. The adjustments are
calculated on the basis of the results of the laboratory analysis.
Production module determines the optimal production rate on the basis of the lime mud storage level together with the kiln process conditions. It also carries out adjustments to the kiln rotational speed in order to increase the maximum sustainable production rate.
Energy and TRS module adjusts the target value for the excess oxygen and cold-end temperature in order to ensure low emission levels while the heat losses are forced down. It also adjusts the wash water rate in the lime mud clarifier/ Precoat Filter so that adequate washing of the mud is ensured.
Limits module is used to protect personnel, equipment and the environment in the case of severe disturbances and/or abnormal process conditions. It also handles large deviations from the target values by means of stepwise changes in the set points.
Stabilizer module maintains controlled variables close to the target values by means of small feed back corrections to the set points of all the major manipulated variables in the process.
Controllers module maintains smooth operation of the process during pending production rate changes. It alters set points of the major manipulated variables according to the models obtained primarily from process data.
Manufacturer: R.E. Hodges; Auburn, Alabama
The DURALYZER-NIR™ Lime Process Analyzer utilizes an extremely simple sample extraction
technology somewhere conveniently located between the kiln outlet
and lime bin. Due to no requirements, whatsoever, for any kind of
sample preparation, a simple water flow arrangement carries the lime
pebble/powder sample to the analyzer, the effect being the same as
the slaking reaction in a Slaker. The end result is a dissolved lime
sample, of which the Residual Carbonate (CaCO3) can be measured by
the Reflective NIR principle.
The Lime process analyzer has been designed for ease of operation and minimization of short and long-term maintenance requirements. This has a net effect of minimizing the overall cost of ownership. Scheduled maintenance requirements have been kept to a minimum and include periodic bulb replacement in the light source enclosure, sample cell optics cleaning.