Organic Rankine cycle (ORC) power systems are being increasingly deployed for waste heat recovery and conversion to power in several industrial settings. In the present paper, we investigate the use of working-fluid mixtures in ORC systems operating in combined heat and power mode (ORC-CHP) with shaft power provided by the expander/turbine and heating provided by the cooling-water exiting the condenser. The waste-heat source is a flue gas stream from a refinery boiler with a mass flow rate of 560 kg/s and an inlet temperature of 330 °C. When using working fluids comprising normal alkanes, refrigerants and their subsequent mixtures, the ORC-CHP system is demonstrated as being capable of delivering over 20 MW of net shaft power and up to 15 MW of heating, leading to a fuel energy savings ratio (FESR) in excess of 20%. Single-component working fluids such as pentane appear optimal at low hot-water supply temperatures, and fluid mixtures become optimal at higher temperatures, with the combination of octane and pentane giving an ORC-CHP system design with the highest efficiency. The influence of heat demand intensity on the global system conversion efficiency and optimal working fluid selection is also explored
