Kiln Control

The detailed approach to kiln control depends on the design of kiln, the market being supplied and the culture of the organisation. The following general comments are offered as basis for consideration in a particular context. 

Before developing a control philosophy, it is important to answer two questions. 

It should be noted that the response time of lime reactivity, which partially reflects conditions in the calcining zone, may be less than the residence time of the burden in the kiln, whereas that of % CaCOs, which reflects the time-temperature history throughout the kiln, may be longer than the residence time. 

In the context of the principles of calcination of limestone (chapter 15), the objective of kiln control is to produce a time-temperature profile such that the calcination and sintering of the various sizes of particles produce the required product quality in terms of % CaCOa, reactivity and, in some cases, % sulfur. This is achieved by varying the fuel to stone/lime ratio, the air to fuel ratio and the residence time. 

Section 15.4 and Table 15.3 illustrate the challenge of achieving an adequate degree of calcination of the larger particles of limestone fed to the kiln, without over-burning the smaller particles. 

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Table 4-2. Effect of Burning 9 to 10 percent TDF In a Gas and Oil Co-fired Dry Process, Rotary Cement Kiln Controlled by an ESP20 Ash Grove Cement, Durkee, Oregon...

Note A wet process coal-fired cement kiln controlled by an ESP. 

Figure 4-11. Percent change in emissions when burning 15% TDF in a gas-fired rotary lime kiln controlled by a venturi scrubber.18...

As kiln control involves a number of operators working on shifts, it is preferable that, as far as possible, they all work to the same set of procedures. This has two principal advantages. 

Sampling for kiln control purposes should be done at a frequency within the shortest response time of the kiln. For example, if the response time to reactivity were 12 hours, routine samples should be taken every 8 hours and more frequently during periods of disturbed operation. 

A danger with shaft kiln control is that changes are made too frequently. In principle, the kiln should be allowed to respond fully to a change — whether it be a shut-down or an adjustment to the settings — before a further change is made. It is recognised that, in practice, such an approach may not always be feasible. In addition, the effects of routine events, such as shut-downs, may be countered by a prescribed response such as an increase in heat input per tonne of stone for a specified period. 

The advent of computers has transformed kiln control (e.g. [16.57,16.58]), but the following sections concentrate on the instruments which enable individual parameters to be measured with the required precision. It should be noted that, in most cases, consistency (i.e., precision) is more important than accuracy. 

As pointed out by Mordn (2001), TDAL reflects the average drying rate in the kiln directly. He suggests that monitoring of the TDAL provides a method of adaptive kiln control, with the TDAL set at a constant value at constant wet-bulb temperature after an initialization period, followed by a period at constant dry-bulb temperature. 

The benefits of even a relatively simple kiln-control system are illustrated in an example quoted by Culpepper (1990). The incorporation of a programmable logic controller at one site increased the overall grade recovery from 70.7% to 81.9%, with a reduction in energy costs of 43% for steam and 10% for electrical power. The total saving represented a 1.2-years payback on the investment. 

Moren, T., 2001. Adaptive kiln control systems based on CT-scanning and industrial practice, in Proceedings of the Seventh International lUFRO Wood Drying Conference, Tsukuba, Japan, pp. 48-53. 

Perre, P. and Martin, M., 1994. Drying at high temperature of heart-wood and sapwood Theory, experiment and practical consequence on kiln control, Drying Technol., 12(8) 1915-1941. 

I. Umbers and P. King, An analysis of human decision-making in cement kiln control and the applications of automation, Int. J. Man Mach. Stud., 12 11-23, 1980. 

S. Sheridan and P. Skjoth, Automatic kiln control at Oregon Portland cement company s Durkee Plant utilizing fuzzy logic, IEEE Trans. Ind. AppL, Ia-20 562-568, 1984. 

HISTORY OF ONO S THEORIES OF KILN CONTROL THROUGH MICROSCOPY... 

Ono s method and theory of kiln control were introduced to the Western world by Mau (1975). In that same year Ono conducted a seminar for North American cement-company personnel in Hawaii, where he taught the details of his theories and method of kiln control with powder-mount microscopy. The dissemination of Ono s technique to the Western world was largely due to this seminar. Since that time Ono s theories and method of clinker interpretation have been subjects of research in laboratories of many North American cement companies, the Portland Cement Association, and in Europe. Mau (1979) reported on the routine application of the Ono technique in Hawaii and strongly supported Ono s Method and theories, stressing their use to control burning temperature. 

Frederick, J., "Panel on Kiln Control," Proceedings ofthe Seventh International Conference on Cement Microscopy, International Cement Microscopy Association, Ft. Worth, Texas, 1985, p. 97. 

Ortbosificates. See silicate structures. Orton Cones, pyrometric cones (q.v.) made by the Edward Orton Jr. Ceramic Foundation, Columbus, Ohio. They are made in two sizes 2Vt in. (63.5 mm) high for industrial kiln control, and 1 l/8th in. (29 mm) high for P.C.E. testing. For nominal equivalent softening temperatures see Appendix 2. Osborn-Shaw Process. See shaw process. Osmosis. See electro-osmosis. 

It is evident from these relationships why it is preferable to feed a kiln with larger size particles as in the limestone calcination process so as to take advantage of the radiation effect enabled by the interparticle spacing. Through Equation (8.26), the effect of mixing on effective thermal conductance is achieved through two thermal diffusivities. For kiln control purposes, we can recast Equation (8.25) in the form... 

The sound levels emitted by the lifter zone change quite distinctly with variations in the running of the kiln and often correspond (with sometime lag) to the variations in the burning zone temperature. With a high fines content the sound levels, measured at a distance of 1 m from the cooler, may vary by about 20 dB (A). If these variations in sound are utilized as a criterion for kiln control, it should be remembered, however, that a high proportion of fine particles may, under certain raw material conditions, also be formed as a result of "over-burning" the clinker. 

For instance, of what use is a mathematical kiln control model ensuring minimum energy costs if, to achieve its stated purpose, such a model requires undisturbed operation of the whole plant for periods of more than 24 hours ... 

The benefits of even a relatively simple kiln-control system are illustrated in an example quoted by Culpepper (1990). The incorporation of a... 

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