The accelerating growth of electricity demand and the associated costs necessitate looking for potential waste heat recovery solutions, particularly in the manufacturing industry. A significant potential for efficient waste heat recovery is observed in the cement manufacturing industries. In the context of emerging economies, such as India, the estimated potential of the cement industries for waste heat power generation is around 1100 MW, a significant 22%, of the total waste heat potential in the country. In a typical cement manufacturing plant, two exhaust streams i.e. the preheater exhaust and the cooler exhaust, are potential sources of heat recovery. The exhaust gases from the preheater are typically in the temperature range of 200˚C - 450˚C depending upon the number of stages in the preheater used (typically varies from 3-6 in current scenarios). Increasing the number of stages in the preheater results in a need to increase the production capacity of the plant, in order to keep the manufacturing process economically viable. This heat is typically lost as waste heat to the ambient, can act as a moderate quality heat source for a thermodynamic bottoming power cycle. Based on the source heat temperature, one of the two potential options, the supercritical CO2 Brayton (sCO2) cycle or the Organic Rankine cycle (ORC) is primarily deployed. The proposed study focuses on modeling and optimization of these thermodynamic cycles for power generation from the preheater exhaust of 1 MTPA cement plant using genetic algorithm. The present study carries out a comprehensive optimization process in which all the decision variables including compressor pressure ratio and inlet temperature, pinch point temperature difference, turbine inlet temperatures and mass flow rate of the working fluid are optimized simultaneously. The prime objective is to maximize the power output derived from the preheater exhaust in the bottoming cycle. A comparative thermodynamic analysis of the ORC with the RC (simple Recuperated cycle) and RRC (Recompressed- Recuperated cycle) configurations of the S-CO2 Brayton cycle is presented based on the number of preheater stages employed in the cement manufacturing industry. Preliminary results suggest that ORCs are superior with respect to RC and RRC configurations for S-CO2 cycles for number of pre-heaters ranging between 3 and 6. It is also observed that for low number of preheater stages and the chosen mass flow rate, the RRC configuration yields higher power output than the RC configuration. Additionally, the study suggests that the choice of the optimal cycle configuration has a direct correspondence with the mass flow rate of the preheater exhaust gas. It is interesting to note that the optimum turbine inlet temperature does not reach its maximum allowed value and the cycle is not completely recuperated. The study is concluded by observing that the deployment of ORC cycles as compared to the steam Rankine cycles and S-CO2 cycles, leads to a significant enhancement in the thermal efficiency as well as the power output of the system.

Thermodynamic Analysis of Waste Heat Recovery Systems in Large Waste Heat Generating Industries

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2021/volume/84898

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