Resources Recycling ISSN:2765-3439(Print) 2765-3447(Online) 자원리싸이클링

Preliminary rotary kiln design and economic assessment have traditionally relied on mass and heat balances, which are useful for rapidly estimating fuel consumption and gross heat losses. However, heat-balance analysis focuses on the quantity of energy and does not directly evaluate differences in thermodynamic quality and availability among fuel combustion heat, preheated-feed sensible heat, shell heat loss, etc. In this study, a previously reported β-cristobalite rotary-kiln preheating design case was revisited by adding an exergy-based interpretation to the heat-balance calculation, and a simple numerical model was developed for this purpose. Using the reported no-heat-recovery and cyclone-preheater cases, the increase in feed sensible heat due to preheating was approximately 869 kW, whereas the reduction in fuel combustion heat was approximately 953 kW. The heat-basis substitution ratio was therefore about 0.91. When the same conditions were converted into exergy terms, however, the increase in feed physical exergy was approximately 642 kW-ex, whereas the reduction in fuel chemical exergy was approximately 1000 kW-ex, giving an exergy-basis substitution ratio of only about 0.64. In addition, because the reported shell heat-loss fraction was only about 4% of the total heat input, a sensitivity analysis was performed by varying the shell heat-loss fraction from 4% to 10%, 20%, and 30%. The results showed that the required fuel combustion heat, fuel heat saving, and exergy-basis substitution ratio varied with the assumed shell heat-loss fraction even under the same feed-preheating condition. These findings indicate that preheated-feed sensible heat and fuel combustion heat are not thermodynamically equivalent, even when they are expressed in the same kW unit. Therefore, techno-economic assessment related to heat-energy saving and feed-preheating design in rotary kilns should be based on heat balance but complemented by exergy-flow analysis.