Effects of cold temperature and main compressor intercooling on recuperator and recompression cycle performance Weiland, Nathan T.; White, Charles W.; O'Connell, Andrew C. In: 2nd European sCO2 Conference 2018 This text is provided by DuEPublico, the central repository of the University Duisburg-Essen. This version of the e-publication may differ from a potential published print or online version. DOI: https://doi.org/10.17185/duepublico/46085 URN: urn:nbn:de:hbz:464-20180827-131257-1 Link: https://duepublico.uni-duisburg-essen.de:443/servlets/DocumentServlet?id=46085 License: This work may be used under a Creative Commons Namensnennung 4.0 International license. 2nd European Supercritical CO2 Conference August 30-31, 2018, Essen, Germany 2018-sCO2.eu-111 EFFECTS OF COLD TEMPERATURE AND MAIN COMPRESSOR INTERCOOLING ON RECUPERATOR AND RECOMPRESSION CYCLE PERFORMANCE Nathan T. Weiland* Charles W. White Andrew C. O’Connell National Energy Technology Laboratory KeyLogic KeyLogic Pittsburgh, PA, U.S.A. Fairfax, VA, U.S.A. Pittsburgh, PA, U.S.A. Email: [email protected] nuclear, concentrated solar, and fossil-fueled power generation. ABSTRACT In most of these studies, the supercritical CO2 recompression Brayton cycle (sRBC) is employed due to its inherently high The performance of supercritical CO2 (sCO2) power cycles efficiency, resulting from both an effective thermal recuperation in general can be improved by reducing the cold sCO2 scheme, and a high average temperature of heat addition at the temperature in order to increase the sCO2 density at the inlet to primary heater. This leads to cycle and plant efficiencies that are the compressor. This reduces the specific power required for typically higher than steam Rankine cycles at comparable compression and increases the net power and thermal efficiency operating conditions, as well as the possibility of smaller of the sCO2 cycle. For similar reasons, the addition of main turbomachinery due to the high overall pressure and low pressure compressor intercooling typically improves the specific power ratio relative to steam cycles. of a recompression cycle, reducing the sCO2 mass flow, and thus Figure 1 shows a simplified diagram of the sCO2 the required cycle size and cost, for a given power output. The recompression Brayton power cycle. In this diagram, heat enters effects of intercooling on cycle efficiency are more complex and the cycle from a generic heat source through a primary heat require consideration of temperature distributions along the exchanger (PHX) and is delivered to the turbine (T). The recuperator train. partially expanded sCO2 then cools as it heats the cold high- The present study investigates the effects of cold sCO2 pressure CO2 in the high temperature recuperator (HTR) and low temperatures, compressor inlet pressures, and number of main temperature recuperator (LTR). On exiting the LTR a portion of compressor intercooling stages, on the efficiency and specific the cooled expanded CO is diverted to the recycle compressor power of recompression Brayton cycles. In particular, it is 2 (RC) before joining the high-pressure CO2 exiting the hot end of shown that the addition of main compressor intercooling the LTR and then entering the cold end of the HTR. The non- typically leads to an internal temperature pinch point within the bypass portion of the cooled CO exiting the hot side of the LTR low temperature recuperator. Cycle operation strategies for 2 is then cooled in the primary CO2 cooler and compressed in the handling this internal pinch point are discussed, and apply to main compressor (MC) before entering the cold end of the LTR. recompression cycles for all applications, as well as other split- The high-pressure CO2 is heated in the LTR and HTR before flow cycle types, including partial cooling cycles. Further, entering the PHX, completing the cycle. internal pinch points and their remediation strategies are also considered for the main CO2 cooler, as well as for the flue gas heat exchanger, as may be employed in economized recompression cycles for fossil energy applications and cascade- style cycles for waste heat recovery applications. Upfront consideration of these remediation strategies is essential in order to determine attainable cycle operating conditions and component sizing requirements for the sCO2 cycle. INTRODUCTION Figure 1: Recompression sCO power cycle Much work has been completed in recent years on closed 2 sCO2 power cycles for a variety of applications, including 1 DOI: 10.17185/duepublico/46085 Background where the pressure ratio between intercooled main compressor As with any power cycle, the efficiency of the cycle can be stages is optimized [6]. increased by either increasing the hot source temperature or Study Objectives decreasing the cold source temperature. A significant feature of The present study investigates the effects of cold sCO these power cycles is their operation in the vicinity of the CO 2 2 temperatures, compressor inlet pressures, and number of main critical point (31 °C, 7.37 MPa) at the cold side of the cycle. For compressor intercooling stages on the efficiency and specific the sRBC, the benefits of reducing the cold sCO temperature are 2 power of indirect sCO power cycles. In cases where colder particularly advantageous. Typically, the sRBC efficiency is 2 sCO temperatures or main compressor intercooling cause maximized when the CO entering the main compressor is at or 2 2 temperature pinch point problems in the heat exchangers, options just above its saturation pressure, as shown below. In this regime, for resolution of these issues are explored, as well as their impact lowering the CO temperature increases its density and reduces 2 on efficiency and other cycle parameters. the specific power required for compression. This study is novel in its approach to resolution of internal Lowering the cooler CO temperature has two additional 2 temperature pinch points in the LTR, which are shown to result benefits for the sRBC performance. It allows for an increase in from main compressor intercooling. Further, the effect of cold the LTR effectiveness and it makes low temperature heat sCO temperature on heat exchanger performance, plant recovery from process sources or the low temperature region of 2 efficiency and specific power is included in this study for both the heat source a more attractive option. Both of these effects can condensing and non-condensing CO cycle operation. improve the efficiency of a power plant based on the sRBC: the 2 The recompression Brayton cycle is studied in this work, former by directly increasing the power cycle efficiency and the although portions of the analysis apply to partial cooling and latter by increasing the quantity of heat harvested by the cycle. cascade cycles as well. Thus the work is relevant to a wide For similar reasons, the addition of main compressor variety of sCO power applications, including nuclear, intercooling typically improves the specific power of a 2 concentrated solar, fossil, and waste heat sources. recompression cycle, reducing the sCO2 mass flow, and thus the required cycle size and cost, for a given power output. [1] However, the effects of intercooling on cycle efficiency are more METHODOLOGY complex and require consideration of temperature distributions along the recuperator train. Modeling Approach Several studies have explored the benefits of reducing the The thermodynamic performance of the plant concepts sCO2 temperature at the cooler. An early study by Wright et al. described in this paper are based primarily on the output from a [2] projected a 4-5 percentage point increase in plant efficiency steady-state system model developed using Aspen Plus® for a nuclear light water reactor with an sCO2 power cycle, by (Aspen). The individual unit operations models are the same as moving to condensing cycle operation. An experimental portion those used in the NETL oxy-coal indirect sCO2 cycle study [1], of this study proved the feasibility of this concept by which included a circulating fluidized bed (CFB) boiler and demonstrating condensed CO2 operation of a radial compressor carbon capture enabled by oxycombustion. Details on the design and gas cooler that were designed for gas phase operation near basis, assumed feed compositions, and state point tables can be the CO2 critical point. A later study on sCO2 power cycles for found in the NETL oxy-CFB indirect sCO2 cycle study [1]. This air-cooled sodium fast reactor nuclear applications shows study includes component capital cost estimations, total plant improvement in cycle performance as the compressor inlet cost calculations, and cost of electricity analyses [1], although temperature is decreased, as well as variability in performance these are not considered in this work. with compressor inlet pressure. [3]. Similar studies in the For the boiler and flue gas components of the process, the nuclear application space have shown that an optimal Aspen Physical Property Method PENG-ROB was used, which compressor inlet pressure exists for maximizing efficiency as the is based on the Peng-Robinson equation of state (EOS) with the compressor inlet temperature is varied. This optimal pressure is standard alpha function [7]. This is consistent with the property typically at [4] or slightly above the pseudo-critical pressure for methods used in other NETL systems studies of power plants CO2 [5]. with CFB or pulverized coal combustion heat sources. A recent NETL report examined the cost and performance Accurate modeling of sCO2 power cycles requires high of a baseline coal-fired oxy-CFB power plant with carbon accuracy in determining the physical properties of CO2, capture, incorporating the sRBC [1]. The results showed that the particularly near its critical point of 31 °C and 7.37 MPa. The CO2 plant offered a significantly higher efficiency and lower Span-Wagner EOS [8] is the most accurate property method COE than a plant employing a Rankine cycles at operating available for processes consisting of pure CO2.