Integrated Performance of a Modular Biomass Boiler With a Combined Heat and Power Industrial Rankine Cycle and Supplementary sCO₂ Brayton Cycle

In this paper, the integrated performance of a modular biomass boiler with an existing industrial Rankine steam heat and power cycle and a supplementary supercritical-carbon dioxide (sCO₂) Brayton cycle is analyzed. The aim is to leverage the high efficiency supplementary sCO₂ cycle to increase net generation and energy efficiency from the existing biomass boiler. Two sCO₂ heater configurations situated within the flue gas flow path are investigated, namely a single convective-dominant heater, and a dual heater configuration with a radiative and a convective heater. A quasi-steady-state 1D model was developed to simulate the integrated cycle, including detailed component characteristics for the Rankine and Brayton cycles. The model solves the mass, energy, momentum, and species balance equations. The system is analyzed for three cases: (i) the existing Rankine cycle without the sCO₂ integration, (ii) with the single convective-dominant sCO₂ heater configuration, and (iii) the dual sCO₂ heater configuration. The results show the required rate of overfiring for the sCO₂ configurations, with a 15.3% increase in fuel flowrate resulting in an additional 21.2% in net power output. The model quantifies the impact of the sCO₂ heaters, with reduced heat uptakes for downstream boiler heat exchangers. Furnace water wall heat uptake increased due to overfiring, offsetting the reduced heat uptakes at downstream evaporative heat exchangers. The dual configuration has more impact on Rankine cycle operation due to the radiative sCO₂ heater placement in front of the second superheater, absorbing some of the direct radiation from the furnace.