Techno-economic analysis of oxy-fuel power plant for coal and biomass combined with a power-to-gas plant

Highlights

• Surplus oxygen utilized in the oxy-fuel boiler requires no separate air separation.

• Oxy-fuel coal and biomass power plant combined with PtG plant had high net power generation.

• Levelized H₂ cost of oxy-fuel plant with PtG plant was approx. $3.67/kg-H₂.

• Facility capacity factor and electricity price were the main factors in the economic efficiency.

• An increase in equipment capacity factor can increase the economic feasibility.

Abstract

Power-to-gas (PtG) is a promising technology for the production of hydrogen via electrochemical reactions in water electrolyzers to manage intermittent power generation from renewable energy sources, such as wind and solar energy. Water electrolyzers generate hydrogen and highly purified oxygen as by-products. In this study, process analysis was performed on oxy-fuel combustion in a coal and biomass power plant and a PtG plant, which uses highly purified oxygen obtained from a water electrolyzer. The power-generation efficiency of an oxy-fuel power plant supplied with oxygen through an air separation unit and that of an oxy-fuel power plant supplied with oxygen in a polymer electrolyte membrane (PEM) water electrolysis process were also compared. Furthermore, we analyzed a combined system comprising an oxy-fuel power plant and PtG plant and found the resulting net power generation and power-generation efficiency to be higher than those of the individual oxy-fuel power plant that was supplied with oxygen through an air separation unit. This process efficiency was further improved when applying biomass to fuel the oxy-fuel combustion boiler rather than coal, and in cases where the steam-water is in a supercritical condition. Comparing with other methods, power generation using biomass is environment-friendly and sustainable. Finally, an economic analysis of hydrogen production showed that the levelized hydrogen cost was deduced to approximately $3.67/kg-H₂, which is similar to the 2030 target value for hydrogen economy revitalization roadmap of the Korean government of 4000 KRW/kg-H₂.

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.