Part-Load Performance Analysis of a Modular Biomass Boiler With a Combined Heat and Power Industrial Rankine Cycle and Supplementary sCO₂ Brayton Cycle

The addition of a supplementary high-efficiency cycle integrated with an existing steam power cycle may increase energy efficiency and net generation. In this article, part-load performance and operation of a modular biomass boiler with an existing industrial Rankine steam heat and power cycle and supplementary supercritical CO₂ (sCO₂) Brayton cycle are analyzed. The aim is to leverage the high efficiency of the sCO₂ cycle by retrofitting sCO₂ heaters in the existing biomass boiler, increasing net power output and thermal efficiency. With nominal load performance previously investigated, understanding part-load performance and operation is vital to determining cycle feasibility. A quasi-steady-state one-dimensional thermofluid network model was used to simulate the integrated cycle performance for loads ranging from 100% to 60%. The model solves the mass, energy, momentum, and species balance equations, capturing detailed component characteristics. Two control methodologies are explored for the sCO₂ Brayton cycle, namely inventory control and inventory control combined with throttling valve control. Inventory control is selected as the better-performing control strategy for load following, maintaining high thermal efficiency across partial loads. At 60% load, the sCO₂ compressor operates near the pseudo-critical point, leading to a sharp decrease in sCO₂ cycle capacity, which requires careful management of inventory control. Two sCO₂ heater configurations are investigated, namely a single convective-dominant heater, and a dual heater configuration with a radiative and a convective heater. The single heater configuration is preferred to minimize adverse impacts on the Rankine cycle superheaters.