PERFORMANCE EVALUATION OF NGCC AND COAL-FIRED STEAM POWER PLANTS WITH INTEGRATED CCS AND ORC SYSTEMS

asme-orc2015 Tracking Number 119

A performance assessment of natural gas-fueled combined cycle (NGCC) power plants and coal-fired steam power plants, both equipped with a CO2 removal system and integrated with an Organic Rankine Cycle (ORC), was performed. For large scale power plants (in this paper a fuel chemical power input equal to 1000 MWt was assumed as reference for both NGCC and steam plant), post-combustion CO2 removal systems based on chemical solvents like amines are expected to reduce the net plant efficiency between 9-12 percentage points at 90% overall CO2 capture. For NGCC+CCS power plants, to improve the capture efficiency and reduce capture equipment costs, exhaust gas recirculation (EGR) has been firstly proposed, assuring a gain of plant overall efficiency in the range of 1-1.5 percentage points. The recovery of low temperature heat, available from the solvent-based CO2 removal systems and related process equipment, can be performed in order to further increase the plant efficiency. In particular low temperature heat is available in flue gas coolers that are required upstream of the CO2 capture unit and, in case of NGCC also for exhaust gas recirculation. Gas at the hot end of the syngas coolers shows a temperature in the order of 80-100 °C for the NGCC and of about 120 °C for the steam plant. Additional available low temperature heat sources are the amine condenser of the CO2 desorption column, which operates at around 100-110 °C and the amine reboiler water cooling that reaches temperatures of 130-140°C. The thermal energy of these various sources could be utilized in different low-temperature heat recovery systems. This paper evaluates low temperature heat recovery by means of an Organic Rankine Cycle (ORC) that can convert heat into electricity at very low temperatures. By producing additional electrical power by the ORC, the global performance of the above mentioned power plants can be improved. This study shows that the integration of CCS with the steam plant allows to recover a larger amount of waste heat in comparison to NGCC (more than 200 MW versus 110 MW). As a consequence, integrating ORC technology with a post-combustion capture system leads to an increase of efficiency of about 1-1.5 percentage points for the NGCC plant and of about 2 percentage points for the steam plant, depending on the amount of low temperature heat available. Different ORC configurations (in series, in parallel or cascaded) were analyzed for thermal energy recovery. Among several organic fluids available and analyzed, N-Butane was assumed as organic operating fluid. Optimum cycle operating temperatures and pressures were identified in order to evaluate the most efficient approach for low temperature heat recovery.