Evaluation of waste heat recovery technologies for the cement industry

Abstract

Copyright © 2020 The Authors. Cement is the world’s most widely used construction material. In 2019, global production amounted to 4086 MT, of which Colombia contributed 12.59 MT. The main component of cement is Clinker and it appears as an intermediate product in the manufacturing process that is produced in a kiln system at sintering temperatures. Such a process exhibits high environmental impacts due to both elevated emissions of Carbon Dioxide and fuel consumption and it is inherently prone to thermal inefficiencies, as heat losses to the surroundings, because of the large flow rates and high temperatures. In this work, the waste heat obtained from the cooling of a high-temperature gas effluent from the rotary kiln in a Colombian cement plant is analysed for its potential use either to dry wet raw material (limestone) or to generate electricity through an ORC. Material, energy and exergy balances for the steady-state were assisted with simulations in Aspen Plus V.10 software. Exergo-economics analysis followed the traditional approach using the net present value (NPV) of the investment as decision criteria. To achieve a holistic view of the waste heat recovery scenario a sensitivity analysis is carried out varying the outlet temperatures of the hot gases for various working fluids in the ORC. Results showed that the best alternative, NPV = 0.37 MUSD at market conditions of electricity and fuel sale price, delivers a maximum of 3.77 MW of electricity with a thermal efficiency of 15.96% and an exergy efficiency of 37.52% using Cyclo-Pentane as working fluid. None of the dryer units attained a positive NPV and were discarded. However, the highest moisture reduction in the solids stream was 5.67% at T = 120∘C. The option of placing a drying unit immediately after an ORC to completely cool down the gases was economically analysed for ORC cases with best NPV, T= 150∘C and T = 180∘C. But no substantial improvement was found over using the ORC alone. The possibility to improve the simple ORC performance is explored through the inclusion of an internal heat exchanger, such recuperated cycle outperforms its simpler configuration in terms of thermal and economic performance delivering 4.1 MW of net work with an NPV = 0.42 MUSD, a rate of return of 15.58% and a payback time of PB = 6.07 years. This is 8.75% more work with 13.51% better economic performance than the simple ORC.