Investigation of direct contact condensation for wet flue-gas waste heat recovery using Organic Rankine Cycle

Abstract

Low-temperature flue gases (<120°C) exiting industrial processes could be recovered for electricity generation and constitute an effective mean to reduce primary energy consumption and carbon dioxide emissions. In the wet flue gases, substantial heat can be recovered if water vapor contained in the gases is condensed. Technical options include indirect contact water vapor condensation recovery, where heat is transferred between the two fluids (typically flue gases and working fluid) using an intervening wall (typically fin-and-tube heat exchanger) and direct contact water vapor condensation recovery, which involves direct mixing between flue gases and cooling fluid (typically water) through a condensing unit. In this paper, the two recovery processes are investigated using ORC (Organic Rankine Cycle). While the indirect contact condensation is the most favorable heat recovery scheme concerning the net output power, the direct contact heat exchanger has received attention because there are no heat-transfer surfaces exposed to corrosion. In a direct contact water–vapor condensation, the inlet flue-gas wet-bulb temperature determines the operating temperature levels throughout the system and limits the circulating water temperature. The maximal net turbine power for the direct contact system is reached for a final water temperature nearby the entering wet bulb temperature of the flue gases. The temperature pinch is as low as 0.5K, which is possible with a direct contact heat exchanger.