Proposal of a thermally-driven air compressor for waste heat recovery

The industrial sector accounts for one third of the global energy use, mainly due to the energy-intensive industries. Waste heat recovery plays a major role among the advances that can lead to potential savings in these industries. The present work proposes an air compressor that generates industrial compressed air in a novel manner only by recovering heat from exhaust gases, not by consuming electric power, and employing readily available technologies transferred from other sectors. The proposed system is an externally-heated open-loop Brayton cycle operating with air in which a fraction of the compressed air from the compressor is delivered as product, while the remainder is heated up and processed in the expander. In its turn, the expander drives only the compressor and not also an electric generator as in conventional cycles. The system is simply realized combining a single- or a two-stage turbocharger from marine reciprocating engines and a recovery heat exchanger. In single-stage, it can deliver compressed air at a pressure up to 500 kPa, while in two-stage over 1 000 kPa. Here, the two-stage configuration is applied to a container glass manufacturing plant and simulated accurately with the code Aspen Plus and Aspen EDR. This work demonstrates that the system could be realized with proven technology from other industrial sectors. It indicates also that 2 342 m3/h of compressed air at 800 kPa can be produced from the exhaust gas at 12 000 m3/h (referred to 0 °C and 101.325 kPa) and 560 °C. The performance is affected strongly by the ambient air condition and the furnace load with variations of the compressed air rate ranging from -92% up to +52% in the extreme conditions. The intercooling power is typically around just 35% of the waste heat recovery and, in any case, always lower than 50%. The gross and net equivalent electric efficiencies are in the range 12-16% and 10-14%, respectively, similarly to conventional heat recovery power plants of same size. The performance can be further improved by employing the hot air from the expander as preheated combustion air or for cogeneration. Ultimately, with respect to conventional plants, the system is a simpler technology operating with a harmless fluid, requiring a lower cooling power and a smaller footprint.