Olumide Olumayegun, Meihong Wang, Eni Oko. Process and Energy Systems Engineering Group University of Hull

Transcription

1 11 th European Conference on Coal Research and its Application (ECCRIA 11) THERMODYNAMIC PERFORMANCE EVALUATION OF SUPERCRITICAL CO 2 CLOSED BRAYTON CYCLES FOR COAL-FIRED POWER GENERATION WITH POST- COMBUSTION CARBON CAPTURE Olumide Olumayegun, Meihong Wang, Eni Oko Process and Energy Systems Engineering Group University of Hull

2 Outline Background & Motivation Contributions of Study Process Configurations and Description Steady State Modelling in Aspen Plus Results and Discussions Conclusions Acknowledgement 2

3 Background & Motivation Coal-fired power plants are still playing significant role in meeting world energy demands However, electricity generation from coal-fired power plants constitute the largest source of CO 2 emissions In the UK, emissions from electricity generation account for around a quarter of the UK total t Data Source: DECC,

4 Background & Motivation Coal-fired Power Generation with Post-combustion CO 2 Capture 42% 39% efficiency S-CO Steam Power Closed 2 Brayton Cycle Plant Coal Fuel Coal+Air Combustion CO 2 Capture Possible replacement of steam cycle with s-co 2 cycle Efficiency improvement Options for CO 2 emission reduction from power plant CO 2 capture 4

5 Background & Motivation Supercritical CO 2 Closed Brayton Cycle Power Plant Supercritical steam cycles represent the state-of-the-art in coalfired power generation Supercritical steam power plant increases efficiency i based on an increase in main steam conditions (supercritical pressure & high TIT) However, achievable efficiency improvement is limited by material technology S-CO 2 cycle is promising for further efficiency improvement at current conditions of pressure and temperature Other advantages: Simpler than steam cycle, smaller footprint, applicable to other energy sources (nuclear, solar, geothermal, waste heat) 5

6 Contributions of Study S-CO 2 cycle for coal, biomass and bottoming cycle applications in literature Manente and Lazzaretto (2014): S-CO 2 Brayton cycles comprising topping (recompression cycle) and bottoming (simple cycle) cycles for small to medium biomass plant as alternative to reciprocating IC or organic Rankine cycle engines Efficiency 10%-point higher than existing biomass power plant Le Moullec (2013): Conceptual study and design of coal-fired power plant built around a S-CO 2 power cycle and 90% post-combustion CO2 capture Double reheat eat (3 turbines), 3 recuperators Improve heat energy utilisation by cold CO 2 bleeding from two locations and 2 stages of combustion air preheating 15% reduction in levelised cost of electricity and 45% reduction in cost of avoided CO 2 Mecheri and Le Moullec (2016): Investigates coal-fired S-CO 2 cycle from thermodynamic consideration by comparing effects of number of reheat, number of recompression and advanced flue gas economiser configurations Improve heat energy utilisation by transferring flue gas heat to a fraction of cold CO 2 and preheating secondary air until C The plant net efficiency is higher than supercritical and ultra-supercritical steam plant by 5.3%-point and 2.4%-point respectively Kim et al. (2016): Compared thermodynamic performance of nine S-CO2 Brayton cycle layouts as bottoming cycles to a topping cycle of landfill gas fired gas turbine plant Concluded that recompression cycle not suitable for bottoming cycle application 6

7 Contributions of Study In this study. Adapts S-CO 2 cycles for efficient heat utilisation of pulverised coal-fired furnace by using a topping and bottoming S-CO 2 cycles, which were never explored before in the literature for coal-fired power plant Investigates alternative S-CO 2 cycle layouts as bottoming cycle to main S-CO 2 cycle New concept of S-CO 2 cycle layout for residual heat energy utilisation Integration of post-combustion CO 2 capture with coal-fired S-CO 2 cycle power plant 7

8 Process Configurations and Description Simple recuperator closed Brayton cycle S-CO 2 cycles take advantage of increased density around critical region by operating the compressor inlet close to the critical point The baseline closed Brayton cycle is the simple regenerative closed Brayton cycle Rapidly varying fluid properties around the critical point leads to mismatch of heat capacity in the recuperator (pinch point problem) Hence it is difficult to achieve high efficiency in simple s-co 2 Brayton cycle 8

9 Process Configurations and Description Recompression S-CO 2 cycle layout Temperature e In the recompression cycle, the problem of heat capacity mismatch is resolved by splitting the flow into two streams Of all the layouts, recompression layout gives the highest efficiency with a relatively simple configuration 9

10 Process Configurations and Description Partial heating cycle layout Matching of the heat capacities achieved by splitting the flow at the compressor outlet A component count of different layouts showed that only the simple cycle and the partial heating cycle layout are simpler than the recompression cycle layout Hence, this cycle consider only the simple cycle, the recompression cycle, partial heating and a new concept of S- CO 2 cycle layout 10

11 Process Configurations and Description New concept Single recuperator recompression cycle layout Similar to the recompression layout except that the HTR was eliminated leaving only one recuperator Flow is split into two streams s just like the recompression cycle to balance the heat capacities between the recuperator hot stream and cold stream 11

12 Process Configurations and Description Integration of single reheat recompression cycle with coal-fired furnace Hot flue gases T2 A T4 G HP LP RC T12 T13 T11 PREC T14 MC RADRHT CHT CRHT ECORHT ECOHT T3 T1 T5 HTR T18 T6 T17 T16 LTR T9 T7 T10 T15 T8 PCC Reboiler Coal RADHT Hot flue gases B Air Recompression S-CO 2 cycle adopted due to its superior performance when compared to other layouts The performance is further improved with a single stage of reheat Due to high level of recuperation, the flue gas leaves the furnace at relatively high temperature (about 500 C) 12

13 Process Configurations and Description Utilisation of flue gas residual heat A major drawback of coupling closed Brayton cycle to coal-fired furnace is the significant loss of heat through the hot flue gas leaving the furnace If this exiting flue gas is not utilised, it will represent the main cause of inefficiency in the power plant Several options exist for utilising waste heat of flue gases from combustion processes: Combined heat and power systems some early operated coal-fired closed Brayton cycle were used to generate electricity and produce heat for industrial heating (Oberhausen and Kashira plant) Preheating part or all of the working fluid prior to main heat addition in the furnace Bottoming cycle Echogen (USA) in the process of commercialising S-CO2 cycle as bottoming cycle utilising waste heat Preheating the incoming combustion air common practice in conventional coal-fired plants Adopted in this study 13

14 Process Configurations and Description Overall plant configurations (Case A Simple cycle bottoming) 14

15 Process Configurations and Description Overall plant configurations (Case B Partial heating cycle bottoming) 15

16 Process Configurations and Description Overall plant configurations (Case C New concept-single recuperator recompression cycle bottoming) 16

17 Steady State Modelling in Aspen Plus A model of the three cases of coal-fired S-CO 2 cycle power plant with PCC was developed in Aspen Plus for comparison among the cases as well as with a benchmark coal-fired supercritical steam power plant The plant systems/components modelled include coal mill, fans, preheaters, pulverised coal-fired furnace, ash removal components, flue gas desulfurization, s-co2 cycles and PCC unit 17

18 Steady State Modelling in Aspen Plus Summary of assumptions and design point parameters Parameter/variable Value Coal feed ( 0 C/bar/kg/s) 15/1.01/51.82 Air ( 0 C/bar) 15/1.01 Excess air (%) 20 Maximum cycle pressure (bar) 290 HP & LP turbines inlet temperature ( 0 C) 593 Compressor inlet pressure (bar) 76 Compressor inlet temperature ( 0 C) 31 Gas-CO 2 TTD ( 0 C) 30 Preheater hot outlet temperature ( 0 C) 116 RecuperatorTTD ( 0 C) 10 Turbine isentropic efficiency (%) 93 Main compressor isentropic efficiency (%) 90 Recompression compressor isentropic efficiency (%) 89 Fan isentropic efficiency (%) 80 Generator efficiency (%) 98.4 Ash distribution, fly/bottom ash (%) 80/20 18

19 Results and Discussions Stream tables Case A Stream P (bar) T ( 0 C) m (kg/s) Coal Air Pry air Sec. air Pulv. Coal + air A B C D E F Flue to PCC T T T T a1,b1,c a2,b2,c b8 Case B P (bar) T ( 0 C) m (kg/s) Case C P (bar) T ( 0 C) m (kg/s)

20 Results and Discussions Summary of plants performance Parameters Case A Case B Case C No PCC With PCC No PCC With PCC No PCC With PCC HHV, MJ/kg Input heat value, MJ Heat transferred to cycle (topping/bottoming), g), MW / / / / / / Furnace efficiency, % Preheater duty, MW Gross electric power / / / / / / (topping/bottoming), g), MWe Cycle efficiency (topping/ 50.56/ 40.09/ 50.64/ 40.05/ 50.64/ 40.37/ bottoming), % Overall cycle efficiency, % Auxiliaries power, MW Net electric power, MWe Overall plant net efficiency, % 20

21 Results and Discussions Distribution of the input heat value About half of the heat input transferred as radiant heat in the furnace Total loss of heat is about 12% i.e. furnace efficiency about 88% Hence, the addition of bottoming cycle enables efficient i utilisation i of furnace heat About 12% of the input heat value was recovered in the bottoming cycle heater 21

22 Results and Discussions Single reheat supercritical steam turbine power plant, 24.1MPa/593 0 C/593 0 C Efficiencies of the S-CO 2 cycle power plants are about 3% point higher than the benchmark steam power plant At the same preheated air level, partial heating has the highest h efficiency i The new concept permits higher air preheating level, and better performance at the high preheating level 22

23 Conclusions A concept of coal-fired power plant using S-CO 2 closed Brayton cycles as power conversion systems and integrated with 90% post-combustion CO 2 capture has been evaluated The S-CO 2 cycles were adapted for efficient utilization of furnace heat by addition of bottoming cycle and air preheating The thermodynamic performance evaluation highlights the promising potential of S-CO 2 cycle for coal-fired power plant application (about 3% efficiency point higher than conventional steam power plant) Case C (the newly developed layout as bottoming cycle) allows the highest level of air preheating, thereby improving the pa plant net efficiency ce cy There is need to consolidate these results by validating the performance of the coal-fired S-CO 2 cycle power plant 23

24 References Kim, M.S., Ahn, Y., Kim, B. and Lee,,J J.I. (2016), 'Study on the supercritical CO2 power cycles for landfill gas firing gas turbine bottoming cycle', Energy, vol. 111, p DOI: Le Moullec, Y. (2013), 'Conceptual study of a high efficiency coal-fired power plant with CO2 capture using a supercritical CO2 Brayton cycle', Energy, vol. 49, no. 0, p DOI: Manente, G. and Lazzaretto, A. (2014), 'Innovative biomass to power conversion systems based on cascaded supercritical CO2 Brayton cycles', Biomass and Bioenergy, vol. 69, no. 0, p DOI: Mecheri, M. and Le Moullec, Y. (2016), 'Supercritical CO2 Brayton cycles for coal-fired power plants', Energy, vol. 103, p DOI: gy 24

25 Acknowledgements EU FP7 Marie Curie (R-D-CSPP-PSE PIRSES- GA ) School of Energy & Environment Southeast University, China GE Power 25

26 Questions 26

27 THANK YOU!