Thermodynamic analysis and optimization of Allam cycle with a reheating configuration
Introduction
In recent years, global warming has attracted widespread attention [1]. Carbon dioxide (CO2) produced by the burning of fossil fuels in the power industry is a major contributor to the greenhouse effect [2]. According to BP Statistical Review of World Energy 2019 [3], global primary energy consumption grew at a rate of 2.9% in 2018, led by natural gas and renewables. Meanwhile, carbon emissions grew by 2.0%, which was the fastest rate for seven years. Therefore, decarbonization is an urgent task for the power sector, and it is the easiest way to reduce carbon emissions over the next 20 years [4].
Oxy-combustion is considered to be one of the most promising technologies tackling CO2 emissions from fossil fuel-fired power plants [5], which was first proposed by Abraham et al. in 1982 to increase oil recovery [6]. The fuel is burned in oxygen produced by the air separation unit (ASU), and most of the flue gas is recirculated into the combustor to moderate the combustion temperature [7]. Since the flue gas mainly consists of CO2 and water (H2O), CO2 can be easily separated after removing water by condensation [8]. Although the ASU consumes a lot of energy, the oxy-combustion power plant can maintain similar or even higher net electrical efficiency compared to air-fired power system [9].
The oxy-combustion system using gaseous fuel (e.g. natural gas) and high-purity oxygen is also called “oxy-turbine” system. A variety of oxy-turbine layouts have been proposed over the past few decades, such as the Graz cycle [10] and the clean energy system (CES) [11] using water as the temperature moderator, and the semi-closed oxy-combustion combined cycle (SCOC-CC) [12], the MATIANT cycle [13] and the NET Power cycle (also called Allam cycle) [14] using CO2 as the temperature moderator.
The Allam cycle is a trans-critical CO2 oxy-combustion cycle, which was developed by Allam et al. [15] of NET Power company. Oxygen at about 99.5% purity is used for the combustion with gaseous fuel (e.g. natural gas or syngas) in the combustor of the Allam cycle, which may achieve near-zero carbon emissions [16]. The developers evaluated that the net efficiency of the natural gas-based Allam cycle can reach 59%. A 50 MWth natural gas-based Allam cycle demonstration project, led by NET Power, is currently in operation in La Porte, Texas [17].
The International Energy Agency Greenhouse Gas R&D Program [18] (IEAGHG) conducted a literature review of the main natural gas-fired oxy-combustion systems, and compared the thermodynamic and economic performance of the SCOC-CC cycle, the Allam cycle, the S-Graz cycle and the CES cycle. Scaccabarozzi et al. [19] performed a detailed thermodynamic analysis and optimization of the natural gas-based Allam cycle. The highest net efficiency of 54.8% was obtained using a black-box numerical optimization algorithm. Then Zaryab et al. [20] designed and compared 4 part-load control strategies for the Allam cycle. The results showed that the increased off-design efficiency can reach up to 4.7 percentage points using advanced control strategies compared to the strategy employing only variable compressor guide vanes. Zhao et al. [21] designed the Allam cycle coupled with coal gasification process and conducted a parametric analysis of the effect of key cycle variables on the cycle performance. Later, they [22] proposed a novel dual expansion coal-fired Allam cycle layout, introducing the heat from the air compressor intercooler and low temperature syngas to balance the heat capacity between the two sides of the regenerator. The results showed that the net efficiency can be up to 42.1%.
Zhu et al. [23] proposed a modified Allam cycle (Allam-Z cycle) without any compressors, which used the cold energy of liquid oxygen and liquified natural gas to facilitate the CO2 liquefaction. The maximum efficiency of the Allam-Z cycle was 50.87%. Furthermore, they [24] proposed an Allam-ZC cycle, which combined supercritical water gasification of coal and supercritical carbon dioxide power cycle. And the exhaust gas of the turbine was used to supply heat for the coal gasification and the backpressure of the turbine was set to around 7.2 MPa in order to eliminate the CO2 compressors. Rogalev et al. [25] developed a 335 MW supercritical carbon dioxide turbine with single flow, double casing construction for the Allam cycle. Mitchell et al. [26] used an adiabatic compression of a bypass stream to balance the heat capacity of two sides of the regenerator, so that the net efficiency of the Allam cycle within this modification can reach 58%. Furthermore, the cycle efficiency can be temporarily increased to 66.1% using liquid oxygen storage (LOX) to decouple oxygen production from electricity generation. Hervás et al. [27] conducted an exergoeconomic analysis of the Allam cycle and obtained the net electrical efficiency and exergetic efficiency of 53.9% and 50.1%, respectively.
Although the Allam cycle has attracted much attention due to the high efficiency and low cost in recent years, it still has some disadvantages at current technical level as follows: (1) the specific energy consumption of pure oxygen is high [28]. (2) the temperature and pressure of the combustor is relatively high [29]. (3) the heat integration of ASU results in low operational flexibility of the system.
As stated above, although some studies have been conducted on the Allam cycle, the net power output of those system is not large, and the heat integration of ASU is needed to supply extra heat for the power cycle. Allam et al. [14] proposed a concept of the Allam cycle with a low-pressure reheat, and estimated that the cycle efficiency can be up to 57.5%. Although the efficiency is lower than that of the Allam cycle without reheat calculated by the developers, the overall power output is increased from 295 MWe to 745 MWe. However, the authors did not show any specific schemes of the Allam cycle with a low-pressure reheat, nor did they conduct a detailed analysis of this cycle. There are no other studies on the Allam cycle with reheat in the open literature. Therefore, a novel and detailed layout of the Allam cycle with a reheating configuration is proposed to increase the scale of the net power output and decouple the ASU from the power cycle in this study. The effects of the key cycle variables on the performance of the Allam cycle with reheat are investigated in detail. And parametric optimization is performed by nonlinear optimization with the mesh adaptive direct search algorithm (NOMAD) to obtain the optimum net cycle efficiency of the system. The results of this study are of great significance for the decarbonization of the power sector and the reduction of oxy-combustion system costs in the future.
Section snippets
Construction of the Allam cycle with reheat
The detailed schematic of the Allam cycle with reheat proposed in this study is shown in Fig. 1. In order to expand the scale of power generation, a set of reheating components are added to the original Allam cycle, including a low-pressure combustion chamber (Combustor 2), a low-pressure turbine (Turbine 2), a low-pressure natural gas compressor and oxygen compressor, as shown in Fig. 1. The detailed description of the system is as follows: the oxidant stream (stream 18c) and the high-pressure
Thermodynamic model of the novel Allam cycle with reheat
The model of the Allam cycle with reheat was developed and calculated in the commercial software Aspen Plus V11 [30]. The main modules of the present study are combustors, turbines and regenerator, etc.
Outlet temperature of Combustor 1
The effect of the outlet temperature of Combustor 1 (To,comb1) on the net cycle efficiency (η) and net specific work (ω) is presented in Fig. 4. To,comb1 varies in the range of 1050-1300 °C. The other parameters, assumptions and constraints are kept constant (same values as those reported in base case).
As can be seen in Fig. 4, ω increases gradually as To,comb1 increases from 1050 °C to 1300 °C, and it reaches the peak of 582.2 kJ/kg when To,comb1 is equal to 1300 °C. Because To,comb1 is
Optimization method
The black-box approach is used to conduct the optimization of the Allam cycle with reheat. That is, the simulation process of Aspen Plus is regarded as a black-box function, and then NOMAD is used in the MATLAB software [41] to optimize the black-box function. NOMAD can solve non-differentiable and global nonlinear programs and is an excellent solver for derivative free optimization [42].
The optimization is conducted for maximizing the net cycle efficiency. The decision variables are as follows:
Conclusions
A novel layout of the Allam cycle with a reheating configuration was proposed in this study. The effects of the key cycle variables on the cycle performance were investigated by means of sensitivity analyses. Then the Allam cycle with reheat was optimized in order to maximize the cycle efficiency by NOMAD algorithm. The main conclusions drawn from this study are as follows:
- (1)
Tmin and To,comb2 have significant effects on the cycle performance of the Allam cycle with reheat, while the effect of pmax
CRediT authorship contribution statement
Wen Chan: Conceptualization, Methodology, Software, Data curation, Visualization, Writing - original draft, Formal analysis. Xianliang Lei: Conceptualization, Writing - review & editing, Methodology. Fucheng Chang: Writing - review & editing, Methodology. Huixiong Li: Supervision, Resources, Writing - review & editing.
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.
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