Energy cost minimization in grate/kiln induration
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I.
INTRODUCTION
IMPROVEMENTS in plant design, particularly in the recovery of sensible heat from pellet cooling air, have significantly reduced fuel consumption of the grate/kiln pellet induration process. Of particular note is the work of LKAB and Allis-Chalmers in modernizing LKAB's plant at Svappavaara 1'2 and in the design of a new plant at Kiruna, Sweden. 1 Substitution of fuel oil or natural gas by cheaper fuels has occurred in many plants. Coke, charcoal, or coal may be added to the ore prior to pelletizing3'4 to provide some of the required energy, or coal may be combusted by a burner in the kiln. 5 The formulation of efficient operating policies, however, continues to be an important aspect of plant economics. Choosing operating conditions that will minimize energy consumption is not straightforward, and widely-divergent views are held. As for the straight-grate process, 6 mathematical simulation may be used to advantage to investigate effects on energy consumption. Some results, from a partly developed model, have been reported by Young, Cross, and Gibson. 7 The present paper concerns studies of acid pellet production from hematite ores, undertaken in collaboration with the B.H.P. Steel International Group-Long Products Division. Results are given of simulations exploring the effects of changing most of the conditions likely to influence energy usage. While these are specific to the B.H.P. plant at Whyalla, South Australia, they may indicate the magnitude of potential savings at other plants. More important, however, is the insight the simulations give into interactions within the process and effects these have on energy consumption. Details of the grate furnace, kiln, and cooler sub-process models, an overview of the way in which they are linked into a dynamic model of the complete process, and discussion of the validation of this model, are given elsewhere. 8'9
J.A. THURLBY is Principal Research Scientist, Division of Mineral Engineering, Commonwealth Scientific and Industrial Research Organisation, Clayton, Victoria, Australia. Manuscript submitted June 24, 1986.
METALLURGICAL TRANSACTIONS B
II.
SIMULATION RESULTS
The process is described in the companion papers) '9 The flow diagram is reprinted here (Figure 1) for convenience. A base case against which the effects of various changes in operating conditions can be determined is also discussed elsewhere. 9 In summary, this case is for the production of 163 tonnes/h of acid pellets, substantially below the capacity of the plant simulated and requiring only two of the three available cooling fans. The kiln is fired with fuel oil only, and no burners are used in the preheat furnace. Coke is included in the green balls as a supplementary fuel, at 1.0 wt pct of dry ore.
A. Individual Effects With a view to determining likely directions for optimization, simulations involving changes to individual operating variables were run. To achieve a consistent basis for comparison of cases, changes were made to the control scheme. 9 The maximum calculated pelle
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