Experimental investigation on an advanced thermosiphon-based heat exchanger for enhanced waste heat recovery in the steel industry

Industry sector within the European Union (EU) accounts for approximately 25 % of final energy use, where the steel industry accounts for 10 % of the total energy consumption in the industry sector. The steel industry and similar process industries are facing significant challenges to reduce their greenhouse gas emissions due to recent climate change legislations. One method to achieve this, is via the implementation of waste heat recovery systems. The paper presented focuses on a steel plant located in Slovenia, where significant amounts of thermal energy are lost through exhaust gases from a natural gas furnace. The novel multi-sink gravity-assisted Heat Pipe Heat Exchanger (HPHE) aims to recover and reuse waste heat and generates two useful heat sinks. The novel HPHE consists of air and water heat sink sections with an average energy recovery efficiency of 47 %. The recovered energy from the air section provides preheated combustion air to the burners, whereas the recovered energy from the water section opens the possibility for district heating. The thermosyphons in the exhaust-air section were arranged in a counterflow arrangement with Dowtherm A and distilled water as the working fluid, whereas the exhaust-water sections were arranged in a crossflow, with distilled water as the working fluid. The novel HPHE features a bypass, allowing complete flexibility for the end user to deactivate the exhaust-water section. To ensure replicability of the HPHE, a theoretical model has been developed and validated through experimental results, the model exhibited a good agreement with the results within an error of 15 %. Both air and water sections recovered 1677 MWh and 753 MWh annually, operating at 8050 and 5750 h respectively. The implementation of the HPHE equates to an overall reduction in CO2 emissions of 334 tCO2 per annum. Moreover, the unit highlights a benchmark for the technology due to its readiness within industry due to a reported Return on Investment (ROI) of under 10 months.