Energy and Heat Balance Optimization for CHP Plants with CO2 Capture: Modeling and Sensitivity Analysis

Sammanfattning

This thesis investigates how the integration of post-combustion carbon capture (PCC) systems would affect combined heat and power (CHP) plant performance, using DWSIM, an open- source process simulation tool. A case study on a CHP plant in southern Sweden was conducted, with models simulated for both baseline and CCS-integrated operation. DWSIM proved accurate within a relative error of under 3 % for key components such as turbines and condensers, demonstrating its potential as a cost-effective alternative to commercial simulators. Two carbon capture strategies were analyzed using steam extracted before the high-pressure (HP) turbine: the Split Stream Path (SSP) with a constant reboiler heat demand of 3.7 MJ/kg CO2, and the Separation Rate Path (SRP) with a variable duty of 3.3–3.7 MJ/kg CO2. SRP showed reduced steam use at partial capture rates but at 90 % capture, both strategies performed similarly due to equal reboiler demand. Different steam extraction points for supplying reboiler heat to the CO2 capture system were also analyzed. Four different steam extraction points were evaluated: before the HP turbine, after the HP turbine and condenser, before the HP condenser, and after the low-pressure (LP) turbine. To reach suitable steam conditions for the reboiler, auxiliary units like compressors and coolers were added. A sensitivity analysis assessed power and heat losses, impacts on district heating, and technical trade-offs. An economic analysis was conducted to compare different steam extraction points based on trade-offs between electricity and heat revenue losses, CO2 savings under the EU ETS, and CO2 transport and storage costs. All cases were evaluated under a 90 % capture rate. Among the four extraction strategies, Case 3 proved the most economically favorable, achieving the shortest payback time of 12.1 years due to its lower electricity revenue loss, despite slightly higher heat losses. This highlights the importance of selecting the right extraction point to maximize economic performance. Different technologies with lower reboiler duties from literature showed potential to reduce steam extraction and cut energy losses significantly. Integrating CCS in CHP plants requires changes like optimized steam extraction, turbine adjustments, and condensate return. Overall, CCS integration is feasible with targeted modifications, and DWSIM is a reliable tool for such modeling.