Material stability assessment of R-1234ze(E) as a working fluid for supercritical organic Rankine cycle

https://doi.org/10.1016/j.jiec.2021.01.023Get rights and content

Highlights

  • Corrosion of materials from HFO-1234ze(E) refrigerant exposure was studied.

  • Scale layer formation and C and F penetration were observed in SS304 and SS316.

  • Erosion of surface and hole formation in bulk were observed in copper and bronze.

  • Excellent corrosion resistance was observed in SS630 and Inconel 718.

  • Segregation of MgF2 layer at the subsurface region was observed in Al6061.

Abstract

Supercritical organic Rankine cycle (SORC) is considered one of the most promising candidates for producing electricity from medium- to low-temperature heat resources. Various types of alloys are considered potential materials for constructing the main SORC components. The decomposition of a working fluid, R-1234ze(E) (trans-1,3,3,3-tetrafluoropropane), in the high-temperature region of the SORC loop produces hydrogen fluoride (HF), which affects the stability of the alloys. In this work, the corrosion behavior of seven selected materials under supercritical R-1234ze(E) conditions was examined. In the turbine inlet region of the designed SORC loop (180 °C, 5 MPa), the formation of a thick scale layer (10–40 nm) and penetration of F in the subsurface region were observed in the SS304 and SS316 samples after seven days of exposure. In the case of copper and bronze, the erosion of the surface and hole formation at the bulk phase were caused by the deep penetration of HF. In contrast, SS630 and Inconel 718 exhibited excellent corrosion stability without changing the chemical environment of their subsurface regions. In the case of Al6061, preferential segregation of the MgF2 layer in the subsurface region was observed.

Introduction

The organic Rankine cycle (ORC) is one of the most promising techniques to convert low- (<150 °C) to medium-temperature (150–300 °C) heat to electricity because of its high efficiency, high reliability, and easy maintenance [1], [2], [3], [4]. These advantages of ORC make it particularly suitable for use in geothermal, solar, and waste heat recovery [5], [6], [7], [8], [9]. Compared to the conventional steam Rankine cycle, working fluids for ORC are organic molecules that have much lower critical temperature and pressure than those of water [10]. Among the ORCs, the supercritical organic Rankine cycle (SORC) has received considerable attention owing to its high thermal efficiency and good thermal match between the working fluid and the heat source with an appropriate choice of the working fluid [11], [12], [13], [14], [15], [16]. Potential working fluids include hydrocarbons (HCs), hydrofluorocarbons (HFCs), hydrofluoroolefins (HFOs), and hydrochlorofluoroolefins (HCFOs). Considering their low global warming potential (low-GWP), low toxicity, low flammability, and environmental benignity, HFOs such as R-1234yf (2,3,3,3-tetrafluoropropene), R-1234ze(E) (trans-1,3,3,3-tetrafluoropropene), and R-1336mzz(Z) (cis-1,1,1,4,4,4-hexafluoro-2-butene) are considered the most favorable working fluids for SORC [17]. Among of the HFOs, R-1234ze(E) is considered a highly promising alternative to currently used high-GWP HFC refrigerant, R-134a (1,1,1,2-tetrafluoroethane), in broad applications such as HVACR (heating, ventilation, air conditioning and refrigeration) [18], [19], [20], high-temperature heat pump systems [21], [22], and ORC systems [23], [24], [25] owing to its excellent drop-in energy performance, similar thermodynamic properties with R-134a, and much better environmental-friendliness. Li et al. [26] investigated thermodynamic performance and optimization of subcritical and transcritical ORC systems using R-1234ze(E) for 100–200 °C heat sources, and the results were compared with R-245fa (1,1,1,3,3-pentafluoropropane) and R-600a (2-methylpropane) as baselines. This study concluded that the maximized system net power output of R-1234ze(E) was the largest for the approximately 100–167 °C heat sources without the outlet temperature limit, which was 31.4% larger than that of R-245fa and 25.8% larger than that of R-600a. Despite of the high performances of R-1234ze(E), the presence of highly reactive Cdouble bondC bonds in the molecular structure of R-1234ze(E) could be a major challenge in its industrial applications as the working fluid in high-temperature ORC systems. The plausible thermochemical decomposition at the high-temperature region of ORC loop could damage component materials of the ORC system [15], [27], [28]. Therefore, the decomposition of R-1234ze(E) and potential corrosion of the materials used in the manufacturing components of the ORC (turbines, high-temperature heat exchangers, pumps), particularly under supercritical conditions, should be carefully addressed.

Because material compatibility with the working fluid in the SORC is dependent on the chemical stability of the working fluid and its decomposition products, the decomposition mechanisms of working fluids have been previously explored. Over 20 reaction pathways for the decomposition of R-1234yf were proposed based on the density functional theory (DFT) [29]; at the initial stage of the decomposition, the formation of HF and CF4 was observed. A study on the decomposition of R-1234yf and R-1234ze(E) suggested that the formation of HF by elimination can be the main decomposition reaction that produces a variety of isomeric C3HF3 species [30]. An investigation of the thermal stability of R-1234ze(E) indicated the formation of a yellow-to-brown liquid and coke under high-temperature and high-pressure conditions (e.g., >200 °C, >10 MPa) [15], while R-1234ze(E) was relatively stable in the turbine inlet region (176 °C and 5.2 MPa) of our designed SORC loop [31]. A study of plausible decomposition mechanisms of R-1234ze(E) under mild and harsh conditions using the DFT indicated that the initial intramolecular elimination of HF plays a key role in the decomposition and re-polymerization of R-1234ze(E) [27]. Because of its high corrosiveness, acidity, and reactivity, HF has a detrimental effect on various metallic and alloy materials, and thus HF produced by the decomposition of the HFO-based working fluid can cause corrosion and degradation of materials used in the main components of the SORC such as turbines and high-temperature heat exchangers.

To date, there have been only a few studies on the material compatibility of working fluids for ORC systems. For example, the study on material compatibility between refrigerants (R-1234yf, R-1234ze(E), R-245fa, and R-134a) and polymers at low temperatures of 25–75 °C indicated that polytetrafluoroethylene is the most suitable polymeric material that was not severely affected by the decomposition products of the fluids [32]. Dai et al. [33] presented compatibility between various refrigerants (n-pentane, R-245fa, hexamethyldisiloxane (MM), and HFE7100 (1,1,1,2,2,3,3-heptafluoro-3-methoxy-propane)) and selected test materials (Al and Cu) for 100 h of exposure time at a SORC-relevant condition (250 °C, 4 MPa). The exposure of MM and n-pentane to Al and Cu caused very small changes in their mass, hardness, and tensile strength, indicating acceptable compatibility. Contrarily, the fluorinated working fluids (R-245fa and HFE7100) significantly affected Al and Cu at 250 °C, which was possible due to the decomposition and elimination of fluorine species during the compatibility test. Because of the presence of aluminum oxide layer, the Al sample was less affected as compare to the Cu sample. Later, Dai et al. [34] investigated material compatibility for high-temperature ORC system with MM as the working fluid at 300 °C for 10 days. The mass, hardness, and tensile strength changes of the SS304 sample were negligible as compare to the Cu sample, suggesting that SS304 can be suitable to be used in an evaporator of the high-temperature ORC system with MM as the working fluid.

In this study, we investigated the corrosion behavior of candidate alloy materials used in main SORC components under the supercritical R-1234ze(E) condition. The tested materials include SS304 (used as a turbine nozzle, rotor, and casing), SS316 (heat exchanger plate and flange), SS630 (high-temperature components of SORC where SS304 and SS316 may not be suitable), Inconel 718 (turbine shaft), copper (brazed plate heat exchanger), bronze (turbine labyrinth seal), and Al6061 (turbine shroud). The corrosion behavior of the selected materials has been studied under supercritical carbon dioxide (scCO2) and supercritical water (scH2O) conditions because of the potential use of scCO2 as a working fluid in the Brayton cycle [35] and carbon capture and storage applications [36], [37]. In the case of scH2O, the corrosive nature of supercritical water oxidation, which has been widely used to decompose highly recalcitrant organic contaminants [38], and the great potential of supercritical water gasification to produce renewable hydrogen from biomass [39] make it necessary to investigate materials that are resistant to corrosion under reaction conditions. Herein, the corrosion behavior of selected materials was investigated under supercritical R-1234ze(E) conditions (5 MPa and 180 °C, which was the designed condition of the turbine inlet region of the SORC [15], [31]) and also under harsh conditions (200 °C and 10 MPa) to accelerate corrosion.

Section snippets

Materials and pretreatment

High-purity R-1234ze(E) was purchased from Honeywell (USA). The alloy materials used in this study were provided by the Korea Institute of Machinery and Materials (KIMM, South Korea). The composition of the alloys is listed in Table 1. Each specimen was forged into a 10 mm × 10 mm × 1 mm coupon with an exposed area of approximately 1 cm2. Prior to the corrosion test, the sample coupons were washed using acetone (99.99% purity, Daejung Chemical Company, South Korea) and then rinsed with

Mass variations

The change in the weight of the sample coupons prior to and after the corrosion test at 180 °C and 5 MPa for 7 d is shown in Fig. 1. The weight gain of the coupons exposed to R-1234ze(E) was in the range of 0.05–0.20 g cm–2. Coupon sample Al6061 exhibited the lowest weight gain, while Inconel 718 and SS304 exhibited the highest weight gain. Despite the large weight gain of metal samples exposed to scH2O (in the order of 0.5.40 g cm–2 [40], [41], [42]) and scCO2 (in the order of 0.002–10 g cm–2

Conclusions

The corrosion stability of selected alloy materials employed in the manufacturing of supercritical organic Rankine cycle (SORC) components using the decomposed products of the working fluid (trans-1,3,3,3-tetrafluoropropane, R-1234ze(E)) was investigated. The materials examined in this study included SS304, SS316, SS630, Inconel 718, copper, bronze, and Al6061. When SS304 and SS316 were exposed to the condition of the highest temperature and highest pressure region of our designed SORC loop

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by a Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the financial resources of the Ministry of Trade, Industry & Energy (MOTIE), Republic of Korea (No. 20172010105960). Additional support was provided by a grant from the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) from the financial resources of the MOTIE, Republic of Korea (No. 201820101066550),

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