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Research Papers: Gas Turbines: Microturbines and Small Turbomachinery

Efficiency Improvement in Precombustion CO2 Removal Units With a Waste–Heat Recovery ORC Power Plant

[+] Author and Article Information
Carsten Trapp

e-mail: c.trapp@tudelft.nl

Piero Colonna

e-mail: p.colonna@tudelft.nl
Process and Energy Department,
Energy Technology Section,
Delft University of Technology,
Leeghwaterstraat 44,
2628 CA Delft, The Netherlands

It contains a character (A: lower toxicity; B: higher toxicity) and a number (1: no flame propagation; 2: lower flammability; 3: higher flammability).

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the Journal of Engineering for Gas Turbines and Power. Manuscript received February 29, 2012; final manuscript received October 17, 2012; published online March 18, 2013. Assoc. Editor: Paolo Chiesa.

J. Eng. Gas Turbines Power 135(4), 042311 (Mar 18, 2013) (12 pages) Paper No: GTP-12-1065; doi: 10.1115/1.4023121 History: Received February 29, 2012; Revised October 17, 2012

This paper presents an analysis about recovering low-grade thermal energy from a precombustion CO2 capture process as part of an integrated gasification combined cycle (IGCC) power plant by means of organic rankine cycle (ORC) turbogenerators. The distinguishing feature of this system is the thermal energy source that is a syngas-water mixture, which is cooled from a temperature of approximately 140 °C, and partly condenses due to the heat transfer to the ORC primary heat exchanger. This study explores various types of ORC power systems for this application. The performance of commercially available ORC units is used as a benchmark and compared to the performance of two types of tailor-designed ORC power plants. The working fluid has a major influence on system performance and other technical and economic factors. The effect of selecting a fluid from the hydrocarbon and refrigerant families are therefore investigated, targeting the maximum net power output. In addition to pure fluids, two-component mixtures are also considered. The use of mixtures as working fluids in subcritical heat-recovery ORC systems allows for a better match of the temperature profiles in the primary heat exchanger and the condenser due to the temperature glide associated with phase-transition, leading to lower irreversibilities within the heat exchanging equipment. In order to further improve the thermal coupling between the cooling heat source and the heating of the working fluid, the supercritical cycle configuration is also studied. The performance of the three categories of systems, depending on working fluid and cycle configuration, i.e., systems based on (i) commercially available units, (ii) tailor-designed subcritical cycle, (iii) tailor-designed supercritical cycle, are analyzed in terms of net power output, second law efficiency, and component-based exergy efficiencies. The analysis shows that an improvement of 38.0% in terms of net power output compared to the benchmark system can be achieved by an optimized supercritical ORC power plant using an R134a/R236fa mixture as the working fluid. It is estimated that the total power consumption of the considered exemplary CO2 capture plant can be reduced by approximately 10% with the optimal ORC system. In this study, particular attention is focused on the semi-empirical optimization approach, in order to avoid unnecessary computations, and general guidelines are provided.

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Figures

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Fig. 1

Simplified process flow diagram of a precombustion CO2 capture island suitable for integration into an IGCC power plant

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Fig. 2

(a) Configuration of ORC power plants. (b) Subcritical and supercritical ORC processes in the T-s thermodynamic plane, in the case of pure working fluids (R245fa)

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Fig. 3

T-s diagram of base case ORC process for working fluid R245fa

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Fig. 4

Temperature profile of the primary heat exchanger in a subcritical cycle with the working fluid (a) R245fa and (b) R236fa

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Fig. 5

Saturation line in the T-s diagram of potential working fluids for subcritical ORC systems

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Fig. 6

Calculated net power output as a function of the evaporation temperature for subcritical ORC power plants using the same working fluids as in Fig. 5

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Fig. 7

Condensing process of (a) pure working fluid R236fa and (b) nonazeotropic mixture R236fa/R152a

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Fig. 8

Cycle diagram in the T-s thermodynamic plane of the optimized subcritical configuration in case of a R236fa (0.6)/R152a (0.4) mixture as the working fluid

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Fig. 9

Cycle diagram in the T-s thermodynamic plane of the optimized supercritical configuration in case of an R134a (0.65)/R236fa (0.35) mixture as the working fluid. The almost perfect thermal coupling of the triangular thermodynamic cycle with the thermal energy source and sink are notable.

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