Modeling and Off-Design Performance Analysis of a Screw Expander-Based Steam Pressure Energy Recovery System in a Combined Heat and Power Unit

[Image: see text] A screw expander-based heating system is proposed based on a 330 MW combined heat and power unit to recover the extraction steam pressure energy. EBSILON Professional software was applied to model the proposed system, and the thermal performance of the traditional system and the new system was compared under different operating conditions. The results show that under the designed heating condition, the standard coal consumption rate of the new system is reduced by 4.74 g/kWh, and the exergy efficiency of the heating process is increased by 17.29%. When the extraction steam pressure changes with the load within a range greater than 0.1 MPa, there is an optimal operating range for the new system, which is related to the built-in pressure ratio of the screw expander and the extraction steam pressure difference, and the distribution of the optimal operating range varies with the supply-water temperature. The heating process exergy efficiency of the two systems shows opposite trends under off-design conditions. Additionally, under the condition of meeting the extraction steam pressure, the method for increasing the output power of the screw expander by adjusting the butterfly valve to increase the extraction steam pressure only increases the standard coal consumption rate of the system. Furthermore, through an application case study, the annual economic benefit of the new system is expected to be ¥ 4.15 million, and the payback period is 5.02 years.