1. Introduction
Increasing greenhouse gas emissions are one of the main reasons for the frequent occurrence of global extreme weather. CO
2 comprises approximately 80% of those emissions [
1]; therefore; reducing CO
2 emissions to cope with global climate change is one of the greatest challenges facing mankind in energy production today. A promising technology for power generation that reduces CO
2 emissions is oxyfuel combustion (OC) with carbon capture. In this type of power plant; the fuel is burned in an O
2/CO
2 atmosphere; producing flue gas consisting mainly of CO
2 and H
2O [
2].
As a near-zero-emission clean coal power generation technology that can directly capture CO
2, OC technology is a commercially feasible technology for large-scale CO
2 capture [
3]. However, large-scale carbon capture coal-fired generating units using OC technology need to use an air separation unit and CO
2 purification and compression unit. The high energy consumption incurred by these additional types of equipment leads to a decrease in the overall generating efficiency of the units and an increase in operating costs, which are the main obstacles to their large-scale use [
4]. Therefore, it is necessary to conduct a comprehensive and in-depth analysis of the relevant operational factors that affect those obstacles. These analyses are important for reducing unit energy consumption, promoting large-scale use of OC technology, and reducing CO
2 capture cost.
Extensive work has been done on operation energy consumption and techno-economic analysis of OC and CO
2 capture units. Han [
5] and Jin [
6] used an exergy analysis method to analyze the thermodynamic performance of an air-separation oxygen system and a CO
2 capture system. They also proposed ways to reduce the operating energy consumption of the systems. Tola [
7] and Oboirien [
8] did a techno-economic analysis on the operating energy consumption, equipment investment, power generation cost, and CO
2 emission reduction cost of coal-fired generating sets that used OC and a CO
2 capture system. They concluded that the CO
2 emission rate was reduced by 10 times after the power plant was modified with oxyfuel technology, and 27% to 29% of the energy was used to capture CO
2. The energy loss was related to the nature of coal. Fu [
9] proposed applying process heat to the Rankine cycle of regenerative steam to increase the power generation of the steam turbine in an OC power plant. Yan [
10] did a comprehensive sensitivity analysis on a 600MW supercritical steam oxyfuel combustion power generation system using dry, semidry, and wet flue gas recycling modes. After evaluating the changes in eight main operating parameters, he concluded that air leakage had the greatest influence on the output energy of the system and that the most practical flue gas recirculation mode was the semidry type. Gładysz [
11] and Koiwanit [
12] adopted the method of total life cycle cost of coal-fired units before and after the modification of OC equipment investment, power generation cost, and CO
2 emission reduction cost, and put forward a different method of life cycle assessment. Jin [
13], Kong [
14], and Han [
15], through modeling and process simulation, analyzed and evaluated OC and CO
2 capture generator sets from the perspective of thermal dynamics and thermal economics. They then proposed the best value of oxygen concentration and an optimization scheme for reducing system energy consumption. Escudero [
16] proposed technology for an oxyfuel coal-fired power plant that uses a high-concentration oxidizer of over 40% and integrates waste heat into the new steam cycle to minimize energy consumption. The net efficiency of their power plant was significantly higher than that of a benchmark oxyfuel plant, and the energy penalty could be reduced from 10.5% to 7.3%. Gładysz [
17] and Ziębik [
18] used cumulative exergy loss, local exergy loss, and a thermodynamic system to devise a perfect degree of cumulative exergy efficiency. They analyzed three techno-economic indexes such as the OC, the CO
2 capture generating set, and the thermodynamic performance of the system. Lei [
19] proposed a new OC process. Its capital investment and the operation costs of the air separation unit and the circulating fan were lower. The variation of flue gas composition in the flue gas combustion furnace was small, and when the ratio of air to oxygen was 2 or 3, the economy of the OC was better. Gao [
20] used a Peng–Robinson (PR) equation of state model on the basis of the actual gas state equation to establish a method using a bias function and a CO
2 compressed air separation unit to calculate the operation energy consumption of a purification unit.
Previous studies have mostly analyzed the influence of one or several single operating factors on the energy efficiency indicators of an OC power generation system. For example, Jin [
13] analyzed the influence of oxygen concentration on the net efficiency. Kong [
14] and Han [
15] analyzed the influence of oxygen purity on net efficiency. Yan [
10] undertook a comparative analysis and evaluation on the influence of such parameters as flue gas recirculation rate, air leakage rate, pressure rise of fans, flue gas condensation temperature, oxygen concentration, and flue gas circulation model on the net efficiency. However, in a uniform experiment or simulation platform, the main operation parameters, such as the oxygen purity in the air separation unit, the oxygen concentration and excess oxygen coefficient in power generation unit, and the recirculation rate of dry flue gas in boiler flue gas for quantitative analysis of the operating energy indexes of in-depth research has yet to carry out in-depth research. These operating economic indexes include the net standard coal consumption rate and the net electrical efficiency, among which the net standard coal consumption rate can comprehensively reflect the overall operating energy consumption level of a power generation system.
Moreover, due to a large number of influencing parameters in the study of thermodynamic characteristics of an OC power generation system, each parameter has a certain range of variation. How to consider the interaction between parameters requires a comprehensive analysis of each parameter’s collocation, so the calculation will be very large. Therefore, the single factor analysis method with fixed other parameters was mostly used in previous studies. However, this method cannot determine the optimization order of each factor and the influence of the interaction between each factor on the system performance.
An orthogonal design method is a design method to study multi-factors and multi-levels. This method selects representative points from the overall test according to the orthogonality for the test, which can solve the coupling effect of multi-factors and reduce the number of tests [
21,
22].
At present, the orthogonal design method is often used in the design and parameter optimization of different power generation systems and cycles. For example, Wang [
23] and Liu [
24] established the thermodynamic model of the organic Rankine circulation system, and through the analysis of the orthogonal design results, the optimal level composition of system parameters to improve the thermal and economic performance of the system was given. Xi [
25] analyzed the experimental system of a new organic Rankine cycle based on the range analysis method of orthogonal design and obtained the optimal and the worst constitutions of six indexes. Chiu [
26] conducted an experimental study on the reaction conditions of methanol steam reforming, obtained the influence of each reaction condition on the MSR process, and obtained the optimal combination of each control factor. Chen [
27] and Terzioğlu [
28], respectively, analyzed the performance of the thermoelectric generator by the orthogonal design method and obtained the influence relationship of material and geometric parameters on system performance. However, the application of an orthogonal design method to the optimization of operating parameters in an OC power generation system has not been carried out yet.
The purpose of this study is to take the net standard coal consumption and the net electrical efficiency of the system as specific energy efficiency indicators, study the sensitivity characteristics of various single factors under a unified platform, and obtain the results of comprehensive sensitivity analysis of operating parameters to improve the thermal performance and economic performance of the OC power generation system through orthogonal design method, range analysis, and variance analysis for the first time. This study will provide theoretical guidance for the engineering design and optimization of an OC power generation system.