Design, energy efficiency, and CO2 emissions analysis of a power generation process of coking dry gas reforming coupled with solid oxide fuel cell and organic Rankine cycle

https://doi.org/10.1016/j.enconman.2023.116655Get rights and content

Highlights

  • Hybrid systems of the SOFC-ORC fueled by the CDG are proposed and modeled.

  • Electrical efficiencies of the new systems increase from 45.45 to 56.89 and 63.73%.

  • CO2 emissions of the proposed systems decrease from 467 to 373 and 333 kg/MWh.

  • The air excess ratio is an important factor affecting system technical performance.

Abstract

Coking dry gas is a kind of refinery dry gas, which is rich in H2 and light hydrocarbon resources, but it’s often used as fuel, resulting in inefficient utilization of dry gas resources and a large amount of air pollutants emissions. Solid oxide fuel cell can efficiently convert the chemical energy of hydrogen into electricity. Therefore, this work establishes a model of a power generation process of coking dry gas reforming coupled with the solid oxide fuel cell and organic Rankine cycle. Process simulation is used to analyze the influence of key operating parameters on the system output performance. The obtained results indicate that the net electrical efficiency of the proposed system is increased from 45.45 % of the coking dry gas direct combustion for power generation system to 56.89 %. In terms of environmental impact, the CO2 emissions of the new system is 373.31 kg/MWh, 20.12 % lower than that of the direct combustion process. Besides, the effect of air excess ratio on system performance is evaluated and the net electrical efficiency of the proposed system increases to 63.73 % as the air excess ratio decreases from 2.0 to 1.1. The above results show that the proposed hybrid power system can provide an effective way for efficient and clean utilization of coking dry gas.

Introduction

Refinery dry gas (RDG) is a by-product of the secondary processing of crude oil such as hydro-treating, continuous catalytic reforming, fluid catalytic cracking, delayed coking, etc. It contains a large amount of hydrogen, C1, C2, and other light hydrocarbon resources [1], [2]. Due to the separation difficulty caused by the complex composition of dry gas, companies are searching for ways to broaden applications and increase the economic value of the dry gas [3]. Some refineries recycle dry gas for heat and power cogeneration to improve the economic benefit of the whole plant. Yangzhou Petrochemical Plant designed a cogeneration project, which uses a medium-temperature and medium-pressure gas boiler equipped with a steam turbine (ST) for mixed combustion of the RDG and bituminous coal to meet the needs of steam and electricity. The average power and superheated steam generation of the project could reach 18 million kWh/y and 85 kt/y [4]. Yongping Refining Plant used the RDG as fuel for steam and then power generation, which can ensure the steam supply of the whole plant [5]. Eldean et al. proposed three schemes for thermal desalination and power generation using the RDG: combined with steam generators, combined with the organic Rankine cycles (ORC) and steam generators, and the use of gas turbine (GT) cycles, the exergy analysis results show that the third scheme has the highest exergy efficiency of 62.37 % [6].

Solid oxide fuel cell (SOFC), as an efficient power generation technology, has attracted more and more attention in recent years. The energy conversion efficiency of the SOFC can reach more than 60 % through electrochemical reactions by converting chemical energy of fuel into electricity [7]. In addition, the SOFC can be combined with other thermodynamic cycles such as the GT to further improve energy conversion efficiency by recovering heat from exhaust gas since it operates at a high temperature of 873–1273 K [8], [9]. Leal et al. established a hybrid system combining direct internal reforming SOFC and a GT, and its thermodynamic and exergy efficiencies were up to 62.1 % and 58.7 % [10]. Peng et al. used medical waste gasified-based syngas as fuel for electricity and thermal energy cogeneration system integrating with the SOFC-GT and supercritical CO2 power cycle, and the system’s overall net electrical and exergy efficiencies could reach about 59.30 % and 57.56 % [11]. Wongchanapai et al. made a sensitivity analysis of key operating parameters of a biogas-fueled SOFC-micro GT hybrid cogeneration system to determine the optimum operating conditions [12]. Hajabdollahi and Fu conducted a thermal simulation of the SOFC-GT cogeneration plant and optimized the system to obtain the maximum exergy efficiency and minimum total cost [13]. Zhao et al. developed a novel process that combined the chemical looping hydrogen generation and the SOFC-GT to realize efficient power generation and CO2 separation and capture, and the power generation efficiency could reach more than 65 % [14].

The exhaust gas of the megawatt-scale SOFC/GT hybrid system contains a lot of low-grade heat energy, which is often utilized ineffectively [11]. The ORC is a mature technology, which can be applied to geothermal energy, fuel energy, solar energy, and waste heat sources [15]. In addition, the ORC technology has the advantages of good security, low maintenance cost, and a wide application range [16]. Many studies have introduced the ORC as the bottom cycle in the SOFC to improve system efficiency. Ebrahimi et al. proposed an integrated system combined with biomass gasification, SOFC-GT, Stirling engine, and ORC technologies, which achieved total exergy and electrical efficiencies of 51.56 % and 62.88 % [17]. Emadi et al. established a novel hybrid system composed of the SOFC-GT unit and a dual-loop ORC unit for the cogeneration, which obtained maximum exergy efficiency of 51.6 % through multi-objective optimization based on genetic algorithm [18]. Tian et al. developed a new type of trigeneration system integrating ammonia-water absorption chiller, SOFC, and ORC to produce cooling, heat, and power, the calculation results showed the total electrical and exergy efficiencies of the integrated system were 52.83 % and 59.96 % [19].

At present, the total amount of dry gas in China’s refineries accounts for about 5 % of the domestic crude oil processing capacity [20]. The processing capacity of delayed coking in China is about 112,000 kt/y, and the coking dry gas (CDG) produced accounts for about 26 % of the national dry gas [21], [22]. The CDG mainly consists of H2, CH4, and C2H4, and the CH4 concentration is as high as 50–60 % [3], [23], [24]. For domestic petrochemical plants, hydrogen-rich gas can be produced by the CDG reforming process, which can be used as the fuel of the SOFC for power generation [25], [26]. There are few studies on the power generation of the RDG-fueled SOFC systems. Rezaie et al. proposed a mixed integer nonlinear programming (MINLP) model for turbine cycle power generation, SOFC power generation, and ammonia production using surplus hydrogen from the refinery, and conducted a sensitivity analysis from an economic perspective [27]. However, this study has not conducted process modeling, parameter optimization, system efficiency, and environmental performance analysis of the SOFC power generation system.

The modeling of the SOFC integrated system mainly includes process simulation and mathematical simulation on the basis of thermodynamic and dynamic methods. Chen et al. used Aspen Plus software and numerical calculation to simulate the hybrid system composed of six subsystems including coal gasification, chemical looping hydrogen production, SOFC-GT, steam cycle, and CO2 compression, which achieved 43.53 % of power generation efficiency under the condition of zero CO2 emissions [28]. Abbasi et al. introduced a combination process of the SOFC and biomass gasifier carried out by Aspen Plus, which obtained the optimized power generation efficiency of 65.7 % through a sensitivity analysis [29]. Sadeghi et al. analyzed a natural-fuel tri-reforming and SOFC hybrid system from the view point of thermoeconomics, and the electrical efficiency could reach 56.94 % by single-objective optimization [30].

In order to utilize the CDG more efficiently, this study proposes a CDG-fueled power generation system, in which hydrogen from CDG reforming is used as the power source of the SOFC/GT and the ORC is used for waste heat recovery. The proposed process is denoted as CDG-SOFC-ORC. Based on the conservation of energy and mass, the simulation software of Aspen Plus is used to model and calculate the output performance of the studied processes. The effects of the steam to carbon (S/C) ratio of the steam reforming (SR) unit and reactor size of the water gas shift (WGS) unit on technical performance of the hybrid system are analyzed and optimized. After then, the mass integration and energy coupling of the hybrid system has been studied and two heating schemes for the SR unit are conducted by studying the air excess ratio of the SOFC. Finally, the power input, power generation, electrical efficiency, and CO2 emissions of the CDG direct combustion (CDG-DC) for power generation system is compared to illustrate the advantages and disadvantages of the proposed systems and provide reference for the deep development and utilization of the CDG.

Section snippets

System description and assumptions

The process flow diagram and Aspen flowsheet of the CDG-DC for power generation system are shown in Fig. 1 and Fig. A1, respectively. The CDG and air are firstly burned in combustion chamber after being pressurized by a compressor, and its high-temperature exhaust gas is then used for primary power generation through a GT. The heat release from the combustion chamber is also used to generate high-pressure and high-temperature steam, which is finally used for secondary power generation through a

Process modeling and assumption

This section describes the modeling details of the important units. The CDG feed flow supplied to the SR unit is set at 3000 kmol/h and its composition is shown in Table 1, sourced from Ref. [23].

Parameter analysis

This section verifies the models of the SR, WGS, and SOFC units, and analyses the influence of parameter changes on the heat duty and performance of the hybrid power system by studying the S/C ratio of the SR unit, the reactor size of the WGS unit, and the air excess ratio of the SOFC.

Energy and environment analysis

After parameter analysis and optimization, material and energy balance data are obtained through process simulation. The thermodynamic properties of each state point of the CDG-DC system and CDG-SOFC-ORC system with the air excess ratio at 2.0 are summarized in Table A1, Table A2, while their CDG input, CO2 emissions, power consumption, power generation, and net electrical efficiency are shown in Table 4. The heat recovery and steam generation of the CDG-SOFC-ORC integrate all heat sources and

Conclusions

In the present paper, novel CDG-SOFC-ORC hybrid systems for power generation is proposed through Aspen Plus software. The output performance of the new systems is optimized by variable analysis of key parameters and compared with the CDG-DC power generation system. This work indicates that the superior S/C ratio of the SR unit is 3.7. When the length to diameter of the WGS reactors ratio is set to 2.0, the superior length of the HT-WGS and LT-WGS reactors are 0.5 and 2.0 m.

The thermodynamic

CRediT authorship contribution statement

Huiju Cao: Writing – original draft, Software, Validation, Data curation. Dong Xiang: Conceptualization, Supervision, Writing – review & editing, Funding acquisition. Lingchen Liu: Investigation, Methodology. Mengqing Liu: Software. Peng Li: Investigation.

Declaration of Competing Interest

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

The financial support from the National Natural Science Foundation of China (No: 22078001, 21706001), the Anhui Provincial Natural Science Foundation (No: 1808085QB46) are gratefully acknowledged.

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