Techno-economic analysis of oxy-fuel power plant for coal and biomass combined with a power-to-gas plant
Introduction
Increased energy demand coupled with a decrease in fossil fuel energy supply in the current energy market has resulted in greater use of renewable energies, such as solar and wind power, in power production. However, power production using renewable energy suffers from the problem of intermittent energy production depending on external factors, such as weather. Two methods exist to address this challenge: utilizing distributed power, which preferentially uses power produced from the renewable energy source, and links with the energy storage device (Blaabjerg et al., 2017). In the case of the latter energy storage method, chemical, electrochemical, mechanical, and electromagnetic, and thermal storage methods have been investigated (Aissou et al., 2015; Divya & Østergaard, 2009; Rekioua et al., 2014; Rekioua & Matagne, 2012; Swierczynski et al., 2010). In Germany, 40% of total electric power was supplied from renewable energy in 2018, and the renewable energy usage ratio is expected to reach 65% by 2030, showing active progress in the technical development of energy storage systems (Kreeft, 2018). In Europe, an analysis of energy storage capacity demand was conducted for the case in which future-generation energy sources will be renewable (Cebulla et al., 2017). Because of its relatively high output variability, renewable energy requires a measure to prevent curtailment and enable stable electricity usage, and Power-to-gas (PtG) technology has recently attracted interest as an energy storage method (Chatzivasileiadi et al., 2013; Sterner, 2009). PtG technology presents the advantage of larger-capacity electricity storage for longer time periods compared with existing energy storage devices (Götz et al., 2016).
PtG technology may be used to store and utilize hydrogen generated from water electrolysis using electric power produced from renewable energy sources (Specht et al., 2010; Vandewalle et al., 2015). The generated hydrogen can be stored after pressurization or converted back to electric power using a fuel cell or hydrogen combustion engine whenever necessary. Moreover, hydrogen can be used not only as an energy source for electricity and transportation, but also as a raw material for chemical industries and for the synthesis of various hydrocarbon fuels, including methane (Ghaib et al., 2016). Owing to such advantages, 128 PtG technology-related research projects on converting and storing the surplus power obtained from renewable energy into hydrogen and gas forms are already being conducted in Europe (Hank et al., 2018; Wulf et al., 2018). Fig. 1 shows various application fields in which energy generated through a PtG system may be utilized.
However, the PtG technique for storing renewable energy sources remain limited due to economic feasibility, in terms of both operation and establishment; thus, research is required to enhance the energy efficiency and economic feasibility of process.
In previous studies on the PtG technique, the usefulness of actual operating plants that produce hydrogen using a renewable energy source was analyzed (Glenk & Reichelstein, 2019; Qadrdan et al., 2015). However, the water electrolysis method using pure water has the advantage of allowing the acquirement of high-purity oxygen while generating hydrogen. Conversely, in this method, if 7–8 kg of oxygen generated per 1 kg of hydrogen could be utilized where needed, it would have a significant effect on the profitability security of PtG technology. Therefore, the utilization of pure oxygen is imperative for the integration of PtG technology.
An oxy-fuel combustion power plant system is a potential situation for the utilization of pure oxygen (Buhre et al., 2005). The creation of combustion power plants that use pure oxygen presents several environmental advantages, such as the adoption of low-grade fuel, less nitrogen oxide generation, and ease of carbon dioxide capture; thus, if high-purity oxygen generated from water electrolysis was utilized, the economic and environmental value of PtG technology should increase.
A functioning oxy-fuel combustion power plant system continuously operates a separate air separation unit (ASU) to be supplied with high-purity oxygen. However, if surplus oxygen is provided by water electrolysis, the equipment cost of the system will be reduced (Bailera et al., 2015) (Fig. 2).
To this end, this study describes an environment-friendly PtG-based power-generation system linked with a combustion power plant using both coal and biomass, which generates hydrogen and utilizes an oxygen by-product produced by the PtG plant. For the case in which hydrogen is generated through water electrolysis and the oxygen by-product is utilized in the combustion power plant system, the optimal operating conditions were investigated and energy efficiencies were compared. Furthermore, an economic analysis was conducted, considering electricity and equipment costs, and the value of the proposed model was reviewed.
Section snippets
Electrolyzer process modeling
In terms of water electrolysis performed using renewable energy, the production of hydrogen and oxygen is enabled through chemical reactions, as shown in reactions (1) and (2) (Kopp et al., 2017). The variables required for the Polymer Electrolyte Membrane Electrolysis (PEM) analysis performed on this process are tabulated in Table 1, and the process simulation was conducted using the ASPEN HYSYS V.10 software (ASPEN, 2019). The RK-soave suitable for high-pressure phase equilibria estimation
PtG hybrid oxy-coal combustion boiler
Fig. 4 presents the main mass stream of the oxy-fuel combustion boiler process modeled in this study. In the process simulation, the process energy efficiency of each case was compared, and during this comparison, heat loss was not considered. The analysis was conducted for three cases. In Case 1, an oxy-fuel combustion boiler using ASU was analyzed. In Case 2, hydrogen was produced during water electrolysis in the PtG plant, and the oxygen by-product was added to the oxy-fuel combustion
Design sensitivity analysis
The levelized cost of hydrogen (LCOH) was estimated using the total revenue requirement (TRR) method developed based on the approach selected by the Electric Power Research Institute (EPRI). The TRR is the cost that needs to be reclaimed annually through sales profit to compensate for the costs of system operation and other factors during plant operation.
Based on individual equipment costs and design reference values determined by heat and material balance analysis results (deduced using the
Conclusion
In this study, a PtG plant that utilizes renewable energy sources was modeled and a linked system using an oxy-fuel combustion boiler power plant was evaluated. In the performance analysis, a current of 107 MW was supplied through the PEM water electrolysis process, and the high-purity oxygen generated in the PtG was supplied to the oxy-fuel combustion boiler and used to operate the power-generation turbine of the steam cycle. For the single oxy-fuel combustion boiler, the conditions of the
CRediT authorship contribution statement
The manuscript was written through the contributions of all authors. All authors approved the final version of the manuscript.
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.
Acknowledgment
This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (No. 20192810100071).
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