Performance assessment of a subcritical Organic Rankine Cycle using pure and zeotropic mixtures of R1234ze(E), R1234yf, AND R134a with flue gas from a refinery boiler

The Organic Rankine Cycle (ORC) has gained widespread adoption in recent decades due to its efficiency in converting medium-temperature heat sources into power. Its simplicity, cost-effectiveness, and ease of maintenance make it a preferred choice for waste heat recovery applications, particularly in Rankine cycles. ORC systems utilize organic refrigerant working fluids with lower boiling points, enabling operation at lower heat source temperatures. However, to optimize performance, factors such as net power output, economic feasibility, environmental impact (zero ozone depletion potential and low global warming potential), and operational efficiency have led to the phase-out of certain working fluids. In recent years, hydrofluorocarbons (HFCs) and hydrofluoroolefins (HFOs) have been investigated as promising alternatives. Utilizing non-renewable industrial waste gases—especially flue gas from boilers—as a heat source can effectively lower global warming impact by reducing flue gas temperatures. This temperature reduction enhances the heat duty of the evaporator, subsequently increasing the net power output of ORC systems. This study simulates and assesses ORC performance using AspenPlus (V10), focusing on subcritical conditions to achieve near-critical pressure, thereby lowering equipment costs while preventing liquid droplet formation in the expansion process. The research examines the performance of pure and zeotropic mixtures of R1234ze(E) and R1234yf (environmentally friendly HFOs) and R134a (a widely used HFC) in an ORC system utilizing medium- temperature flue gas from a refinery boiler. Among the evaluated pure and binary zeotropic mixtures, the ternary mixture R1234ze(E)/R134a/R1234yf (0.7/0.2/0.1) achieved the highest net work output of 1470.42 kW, surpassing all other assessed configurations. The binary mixture R1234ze(E)/R134a (0.8/0.2) followed closely, with a net power output of 1468.18 kW. The superior performance of zeotropic mixtures stems from their enhanced heat recovery capability, as evidenced by a 73.3–76.6% reduction in flue gas outlet temperature to 65.41–74.51 oC. Additionally, the selected ternary zeotropic mixture exhibited a 10 kW higher system exergy output compared to R134a, with lower flue gas output exergy, indicating improved thermodynamic efficiency. Despite their superior thermal performance, zeotropic mixtures require a greater heat recovery demand due to lower flue gas output temperatures, leading to increased evaporator and condenser sizes. This, in turn, raises the Specific Equipment Cost (SPEC), impacting key economic indicators such as Net Energy (NE), Return on Investment (ROI), and Payback Period (PBP). The study highlights the trade-off between thermodynamic efficiency and economic feasibility when selecting working fluids for ORC applications. While zeotropic mixtures significantly enhance heat recovery and net power output, they also result in higher capital costs. Understanding these implications is crucial for assessing ORC systems in industrial waste heat recovery applications, balancing efficiency, sustainability, and cost- effectiveness.

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