ASME ORC 2015 3rd International Seminar on ORC Power Systems 12-14 October 2015, Brussels, Belgium PERFORMANCE EVALUATION OF NGCC AND COAL-FIRED STEAM POWER PLANTS WITH INTEGRATED CCS AND ORC SYSTEMS Vittorio Tola DIMCM, Dept of Mechanical, Chemical and Materials Engineering, University of Cagliari, Italy email:vittorio.tola@dimcm.unica.it 1
Introduction Carbon capture and storage (CCS) is an important tool for the power sector to reduce CO 2 emissions. CCS alone will account for 19% of the total CO 2 emissions reduction in 2050. Coal-fired steam and natural gas-fired power plants are expected to contribute to about 65 and 30 % of the total installed power generation capacity equipped with CCS. CO 2 capture from power plants is both very capital- and energy-intensive. CO 2 removal causes substantial efficiency penalties to the power plant: 1. Thermal energy, typically at 140 C, for driving the desorption reaction in the reboiler column. 2. Electrical energy for driving exhaust gas fans and solvent circulation pumps, 3. Electrical energy for compressing and pumping CO 2 to high pressure for transport and storage. Near-term CO 2 removal system based on chemical solvents are expected to reduce plant efficiency in the order of 8-12 percentage points at 90 % CO 2 capture. R&D projects are dedicated to the development of more efficient or less expensive capture processes. Exhaust gas recirculation (EGR) was proposed for NGCC to increase the CO 2 concentration and reduce costs. Efficiency losses are reduced by about 1% pt and capital costs for the capture unit are reduced by 20-30%. Little attention was given to recovering low temperature heat rejected from capture plants and the auxiliaries. This paper examines performance impact of recovering low temperature heat with an Organic Rankine Cycle (ORC) integrated with a post-combustion CO 2 capture for coal-fired steam plants and natural gas combined cycle. 2 The study is based on complex simulation models specifically developed through HYSYS TM and Gate- Cycle TM commercial softwares.
PC power plant Large scale PC (Pulverized coal-fired) steam power plant (400 MW) Superheated steam Reheated steam Flue gas Flue gas treatment CO 2 to compression and storage LP steam Flue gas Steam boiler HPT IPT LPT Air Coal to HP FWHs steam Deareator to HP FWHs to HP FWHs steam to LP FWs Condenser 3 HP FWHs condensate LP FWHs condensate Fuel chemical power input MW 1000.0 Coal mass flow kg/s 39.4 Coal lower heating value (LHV) MJ/kg 25.39 Air mass flow kg/s 359.8 HP/IP/LP steam temperature C 537.9/540.0/322.6 HP/IP/LP steam pressure MPa 25.0/3.4/0.7 Condenser steam temperature and pressure C/kPa 26.7/3.5 High/low pressure heat exchangers minimum ΔT C -1.5/1.5
Natural Gas Combined Cycle Large scale NGCC (Natural Gas Combined Cicle) plant (600 MW) Gas turbine Triple pressure steam cycle Flue gas Flue gas treatment CO 2 to compression and storage LP steam Flue gas NG HRSG gas turbine air steam turbine 4 Gas turbine Fuel chemical power input MW 1000.0 Natural gas mass flow kg/s 20.0 Exhaust mass flow kg/s 877.4 Exhaust temperature C 642.0 Steam cycle HP steam temperature/pressure C/MPa 565.0/16.3 IP steam temperature/pressure C/MPa 565.0/2.4 LP steam temperature/pressure C/MPa 311.0/0.45 Condenser pressure kpa 3.5
CO 2 removal system Complex configuration with CO 2 removal and compression sections Post combustion capture system based on 30 wt-% mixture of MEA and water purified gas Make up CO 2 H2O Exhausts Absorber C2 H1 LP steam C1 HTX Desorber Reboiler 5 CO 2 -rich solvent CO 2 -lean solvent
PC and NGCC reference power plants Reference PC and NGCC power plants PC NGCC Fuel chemical power input MW 1000.0 1000.0 Flue gas treatment requirements MW t 13.3 / Net power output MW 420.2 592.0 Net efficiency % 41.5 59.2 Flue gas mass flow kg/s 395.5 877.2 CO 2 molar fraction in flue gas % 15.0 4.1 CO 2 emitted by the plant kg/s 87.6 55.2 CO 2 specific emissions g/kwh 735.9 336.0 6
Electrical power (MW) Thermal energy (MJ/kg CO2 ) CO 2 removal rate effects on electrical power and thermal energy requirements 10 9 8 7 6 5 4.5 4 CO 2 removal rate 70% 90% Electrical power (MW) 5.3 6.1 Thermal energy (MJ/kg CO2 ) PC 3.50 3.75 5 4 3.5 3 2 Thermal Electrical PC NGCC 3 NGCC CO 2 removal rate 70% 90% 1 Electrical power (MW) 8.1 9.0 0 0.7 0.72 0.74 0.76 0.78 0.8 0.82 0.84 0.86 0.88 0.9 CO 2 removal rate 2.5 Thermal energy (MJ/kg CO2 ) 3.72 4.58 7
CO 2 (g/kwh) CO 2 removal rate effects on CO 2 captured and emitted Reference CO 2 emission : PC 736 g/kwh NGCC 336 g/kwh 8 850 800 750 700 650 600 550 500 450 400 350 300 250 200 150 100 50 0 PC NGCC 0.7 0.72 0.74 0.76 0.78 0.8 0.82 0.84 0.86 0.88 0.9 CO 2 removal rate CO 2 captured CO 2 emitted PC CO 2 removal rate 70% 90% CO 2 captured (g/kwh) 608 820 CO 2 emitted (g/kwh) 261 91 NGCC CO 2 removal rate 70% 90% CO 2 captured (g/kwh) 271 376 CO 2 emitted (g/kwh) 116 42
Plant net power (MW) Net plant efficiency reduction (%) CO 2 removal rate effects on power and efficiency Reference net efficiency: PC 41.3% NGCC 59.2% 600 550 90%CO 2 net efficiency: PC 33.3% NGCC 47.5% 36 33 Power reduction Amine heat requirements 77.2% Capture system electrical cons. 7.7% CO 2 compression 15.1% 500 450 400 PC NGCC 30 27 24 PC CO 2 removal rate 0% 70% 90% 350 21 Net power (MW) 420.2 354.8 337.7 300 250 200 150 Power Efficiency 18 15 12 9 Net efficency decrease (%) NGCC * 15.6 19.6 100 6 CO 2 removal rate 0% 70% 90% 50 3 Net power (MW) 592.0 513.2 475.4 9 0 0.7 0.72 0.74 0.76 0.78 0.8 0.82 0.84 0.86 0.88 0.9 CO 2 removal rate 0 Net efficency decrease (%) * 13.3 19.7
Exhaust Gas Recirculation (EGR) Air NG G Cleaned Exhaust G HRSG CO 2 Capture Unit Exhaust Gas Recirculation (EGR) Liquid H 2 O CO 2 Exhaust gas partially extracted at the end of HRSG Exhaust gas cooled down to near-ambient conditions in an EGR cooler Exhaust gas recirculated back to the compressor of gas turbine EGR penalizes compressor performance: Higher temperature of air-flue gas mixture Higher heat capacity of more humid recirculated gas 10 EGR improves CO 2 capture efficiency and reduces capture equipment costs: Increase of CO 2 concentration in the flue gas Reduced size of CO 2 capture unit Reduced energy requirements and equipement costs for the amine system
Plant net power (MW) Net plant efficiency (%) EGR influence on plant power and efficiency EGR Variables: EGR ratio (0-0.5) NGCC-CCS with 90% CO 2 capture EGR cooler outlet temperature =30 C 500 50 495 490 485 Power Efficiency 49.5 49 48.5 480 475 470 465 460 455 450 48 47.5 47 46.5 46 45.5 45 0 0.1 0.2 0.3 0.4 0.5 EGR ratio 11
Available low temperature sources Different sources of available low temperature heat from solvent-based CO 2 removal systems Clean Flue Gas Cooler Rich Solvent CO 2 Condenser Lean Solvent A Exhaust gas cooler Flue gas Absorber HX Stripper Reboiler C B C Amine reboiler condensate cooling Stripper condenser Steam Cold, Dirty FlueGas Rich Solvent Lean Solvent Hot water A B Cold water 12
ORC arrangements Organic Rankine Cycle Heat exchanger Turbine G Pump Condenser Different ORC arrangements for simultaneous heat recovery in single ORC loop I serie II parallel III cascade G G G G 13
ORC fluids Carbon dioxide (CO 2 ) N-Butane Penta-fluoro-propane (R245fa) Using CO 2 allows synergies in the fluid handling or safety infrastructure with the CO 2 capture system, low costs for the organic fluid and a better match with the exhaust gas cooling curve CO 2 shows some disadvantages (a higher pump work) which leads to a low plant performance, due to the supercritical conditions, and potentially higher equipment costs due to the very high operating pressure. Temperature ( C) Pressure (bar) Pressure (bar) Pressure (bar) CO 2 N-Butane R245fa 10 44.9 1.5 0.8 20 57.3 2.1 1.2 30 72.1 2.8 1.8 40 - * 3.8 2.5 50-4.9 3.4 60-6.4 4.6 70-8.1 6.0 80-10.1 7.8 90-12.5 10.0 100-15.3 12.6 * CO 2 critical point at 73.8 bar and 31 C 14
Effects of N-Butane pressure on ORC performance Parameter Range Vapor fraction at the turbine 1 entry Pressure at the turbine entry 6-12 bar Temperature at the turbine entry 58-88 C Pressure at the condenser 2.5 bar Temperature at the condenser 25 C Minimum ΔT at the heat 5 C exchanger Amine reboiler (6 bar) Amine reboiler (12 bar) Stripper condenser (6 bar) Stripper condenser (12 bar) 15 N-Butane Pressure (bar) Temperature ( C) 4 42.2 6 57.6 8 69.6 10 79.4 12 88.0 14 95.5
Waste heat recovered (MW t ) Waste heat recovered (MW t ) Waste Heat Recovery (WHR) An increase of ORC maximum pressure reduces waste heat recovery WHR from condenser is the highest one and constant up to 10 bar. High T of saturated water (140 C) allows to a slightly decrease of WHR from reboiler WHR from exhaust gas is low and possible only at very low pressure. 220 PC 220 NGCC (40% EGR) 200 200 180 160 140 180 160 140 Total Reboiler Condenser Exhaust gas 120 100 80 60 40 20 0 Total Reboiler Condenser Exhaust gas 120 100 80 60 40 20 0 16 6 7 8 9 10 11 12 ORC maximum pressure (bar) 6 7 8 9 10 11 12 ORC maximum pressure (bar)
ORC net power (MW) ORC net power (MW) ORC net efficiency ORC net efficiency ORC net power and efficiency An increase of ORC maximum pressure increases ORC efficiency 7.1% of ORC efficiency for a ORC maximum pressure equal to 6 bar. 11.9% of ORC efficiency for a ORC maximum pressure equal to 12 bar. Due to ORC efficiency trend, maximum ORC power is obtained at about 10 bar. PC 6 7 8 9 10 11 12 NGCC (40% EGR) 6 7 8 9 10 11 12 0.2 0.2 0.16 0.16 0.12 0.12 0.08 0.08 0.04 0.04 24 0 24 0 20 16 20 16 Total Reboiler Condenser Exhaust gas 12 8 Total Reboiler Condenser Exhaust gas 12 8 4 4 17 0 6 7 8 9 10 11 12 ORC maximum pressure (bar) 0 6 7 8 9 10 11 12 ORC maximum pressure (bar)
Main plant performance PC-CCS Maximum ORC net power (MW) 21.6 Power increase vs reference plant (%) 6.4 Total WHR (MW) 203.1 ORC net efficiency (%) 10.7 NGCC-CCS Maximum ORC net power (MW) 12.3 Power increase vs reference plant (%) 2.6 Total WHR (MW) 112.0 ORC net efficiency (%) 11.0 PC NGCC Gross power of reference plant MW 454.6 625.2 Net power of reference plant MW 420.2 592.0 Net power of plant + CCS MW 337.8 475.5 Net power of plant + CCS + ORC MW 359.4 487.8 Net efficiency of reference plant % 41.5 59.2 Net efficiency of plant + CCS % 33.3 48.8 Net efficiency of plant + CCS + ORC % 35.5 50.1 18
Conclusions This paper analyses option to recover low-grade heat from CO 2 capture processes for both natural gas combined cycles (NGCC) and Pulverize coal-fired (PC) steam power plants by using Organic Rankine Cycle (ORC) technology. Three different potential waste-heat sources are identified: exhaust gas cooler, amine rebolier condensate cooling and stripper condenser. Most appropriate ORC system layouts are discussed: serie, parallel and cascade. Parallel layout has been chosen. N-Butane has been chosen as an ORC working fluid. Maximum pressure equal to 10 bar leads to best results. Exhaust gas recirculation (EGR) was also considered for enhancing performance of NGCC integrated with CO 2 capture processes. ORC technology integrated with PC-CCS allow to increase net power output of about 22 MW, whereas the integration with NGCC-CCS limits the increase to about 13 MW. Globally an overall power plant net efficiency improvement potential of 1.3 percentage point (NGCC) and of 2.2 percentage point (PC) is estimated. An economic analysis should be performed in order to evaluate costs of integration. 19
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