Fuelling power plants by natural gas: An analysis of energy efficiency, economical aspects and environmental footprint based on detailed process simulation of the whole carbon capture and storage system

Highlights

• Simulations provide reliable carbon capture and storage process data.

• Potassium carbonate is more energy-intensive than amine process.

• Natural gas power plants with carbon capture and storage degrade energy indicators.

• Renewables revealed as the best solution in terms of levelized cost of energy.

• Carbon capture and storage does not guarantee to limit the impact on global warming.

Abstract

In view of the low-carbon transformation of the power sector, natural gas-fired power generation is the only technology, among fossil resources, that will continue to provide an important source of flexibility for the power system in the coming years. However, the carbon dioxide emissions produced by the operation of such plants call for carbon capture and storage equipment, whose deployment needs to be assessed and compared with the main renewable technologies: wind power and photovoltaics. This work uses process simulation in order to assess two different carbon capture processes: a traditional one, based on monoethanolamine, and an innovative one, based on hot potassium carbonate. Process simulation is also used for the transportation of carbon dioxide to the sequestration site. Mass and energy balances from the simulations are then used for the calculation of the Energy Return on Energy Invested, the Levelized Cost of Energy and as inputs for the Life Cycle Assessments of both alternative designs. The life cycle analyses of the considered power technologies exhibited higher contributions due to fossil-based power plants towards climate-related impact categories, while renewable sources were revealed to be more burdensome for the exploitation of mineral resources. The calculated Energy Return on Energy Invested for gas-fired power plants with carbon capture and storage is between 5.2 and 12.4, comparable with the values of photovoltaics and wind power. On the other hand, their Levelized Cost of Energy is between 10.2 and 20.0 eurocent per kilowatt-hour, much higher than that of renewables. The conclusion is that, at present, the sustainability of gas-fired power stations equipped with carbon capture and storage should be carefully considered and not taken for granted.

Introduction

After two years of growth, global emissions were unchanged in 2019 even though the world economy has grown by 2.9% [1], primarily thanks to the expansion of renewable sources in the power sector. Nevertheless, still about 80% of global carbon dioxide (CO₂) emissions originate from the energy sector [2]. In this respect, gas-fired power generation is the third largest source of CO₂ emissions, accounting for 7.1 Gt in 2018 [3], more or less a quarter of the total emitted by the energy sector [4]. Since the increasing share of variable generation of photovoltaics and wind power needs to be followed by an increase in the flexibility of the power system to balance supply and demand, the future share of gas-fired power plants seems to remain important, as shown in Table 1 [4]. Therefore, in order to reduce the CO₂ emissions of gas-fired plants, a number of them could be retrofitted with Carbon Capture and Storage (CCS). Nonetheless, this technology has not been successfully implemented on a large scale yet.

According to the state of the art, research on CCS and its application to power plants has been prolific over the last decade, with a stable rate of 90–100 documents published per year [5]. The physical properties impacts on CCS processes are well described in [6]. Numerous studies in the literature carried out detailed assessments of CCS processes, based on results from process simulations [7]. For example, several works evaluated the performance of chemical absorption with MonoEthanolAmine (MEA, C₂H₇NO) and membrane-based CCS, concluding that the former is more energy-intensive and generates higher environmental impacts [8]. Despite its apparent advantages, however, membrane-based CCS [9] is still at the development stage, while chemical absorption has an advanced technological maturity. Recently, the use of hot potassium carbonate (HPC, K₂CO₃) has attracted attention as a possible alternative solvent, characterized by higher stability, low toxicity, non-volatility, and the same technology readiness as MEA [10]. Most of the works are focused on the assessment of CCS technology coupled with coal-based power plants [11]. However, to reflect the current situation and the strategic plans towards a smooth energetic transition (future trends reveal that coal will cover only 2% of the peak demand, while natural gas will cover 26%, as shown in Table 1), natural gas-fired plants need also to be analysed, since they are characterized by higher and more diluted flue gases (FG) flow rates. To the best of the authors’ knowledge, no studies on HPC-based CCS applied to gas-fired plants are present in the literature. In addition, most of the works do not address the problem from a thorough and interdisciplinary approach that evaluates energetic, economic and environmental factors based on rigorous and coherent material and energy balances and costs data, but rather focus on the analysis of single aspects.

For these reasons, the aim of the present work is to analyse and assess the sustainability of natural gas-fired power plants without and with a CCS system, by combining detailed process simulation (PS) with an evaluation of the Energy Return on Energy Invested (EROEI), the Levelized Cost of Energy (LCOE) and the Life Cycle Assessment (LCA). The results obtained are then compared using the same metrics as are used for photovoltaics and wind power.

All the data needed to evaluate the EROEIs, LCOEs and LCAs are provided by PS, with material and energy balances calculated using reliable and validated models, taking as references existing processes coupled with an existing real natural gas power generation plant. Process simulation is a mature tool that solves material and energy balances for chemical [12] and biochemical [13] processes: its use is consolidated in the phase of process design, optimization, and feasibility studies. Indeed, the outcomes of the desired process must be evaluated in terms of energy consumption, the cost of energy and emission of greenhouse and pollutant gases to limit the industrial environmental impact. Moreover, PS can provide an energetic and an economic evaluation of the given process, as well as sensitivity analysis of the most relevant independent variables of the process itself. By combining PS with EROEI estimation, LCOE calculation and the LCA procedure, it is possible to obtain target values for all the desired indicators and impact categories. In this way, process engineers can evaluate the sustainability of the proposed design at an early stage using a rigorous process simulation-based analysis [14].

With reference to the energetic aspect, the appropriate calculation of the EROEI [15] serves as a reasonable proxy for the biophysical utility of any particular energy source to society [16]. However, EROEI values vary considerably from study to study depending on the data used for the estimation of the energy contributions involved [17] and of the EROEI of fuels [18]. Recent studies calculated the value of the EROEI when a CCS technology is coupled to electrical energy production processes from fossil fuels [19], highlighting all the necessary data for a reliable estimation of the effect of CCS on the EROEI [20].

As far as the environment is concerned, considerable effort has been spent on evaluating the impacts related to electricity production. A review of the life cycle of renewable energy sources [21] and a specific one on electricity generation [22] are available in the literature. Regarding life cycle assessments of CCS, early works dealt with the emission from the MEA capture process [23] and with the installation of MEA capture plant in series at a natural gas combined cycle power plant [24] or a pulverized coal-fired power plant [25]. LCA studies now envisage other promising technologies within their scopes [26], e.g. separation membrane [9], oxy-fuels [27], alternative solvents [28], power supply [29] and fuels [30], or chemical looping configurations based on pre-combustion [31] or post-combustion [32] carbon capture, aiming at identifying the best technology for future industrial applications. Among the most investigated CCS technologies, the comparison between MEA and HPC has already been studied from the LCA standpoint. Process simulation tools have been used to obtain life cycle inventories of each carbon capture process in terms of mass and energy balances, but for coal-fired power plants only. Many papers have used AspenPlus™ as process simulation software. Zhang et al. modelled different post-combustion technologies, i.e., an MEA-based system, a gas separation membrane process and a hybrid membrane-cryogenic process [8]. Urech et al. compared three solvents (monodiethanolamine, hot potassium carbonate and Selexol™) for a pre-combustion carbon capture application [33]. Wang et al. combined MEA-based process simulation with power plant steam cycles obtained using Cycle-Tempo software [29]. Grant et al. compared post-combustion carbon capture using either hot potassium carbonate or MEA [34]. Besides AspenPlus™, another software called Pro Treat® has been employed for comparing potassium carbonate (bio-catalyzed and traditional) and amine-based carbon capture technologies [35]. Most of these simulations do not take into consideration the shared infrastructures and operations (i.e., transportation and storage of CO₂), preventing the comparison of their results with other electricity generation technologies. A number of publications compare coal-fired or natural gas-fired power plants equipped with carbon capture systems to other electricity generation sources. For instance, Turconi et al. reviewed the existing LCA studies dealing with hard coal, lignite, natural gas, oil, nuclear power, hydropower, photovoltaics, wind and biomass in order to identify the averaged environmental impacts of each technology [36]. Hertwick et al. performed a comparative LCA study, including several photovoltaics technologies, concentrating solar power, hydropower from different reservoirs, wind power onshore and offshore, various coal and natural gas technologies with and without CCS [37]. Gibon et al. combined existing data for coal, natural gas, ground-mounted and roof-mounted photovoltaics, concentrating solar power, hydropower, geothermal and wind power with original life cycle inventories covering biopower technologies with and without CCS and nuclear power [38]. In general, these works show a general increase of the environmental footprints for traditional technologies in the majority of impact categories. However, none of these works compare complete CCS systems employing different technologies, as well as fossil- and renewables-based power generation technologies. The method proposed in this paper aims to fill this gap, using detailed process simulations to provide a comprehensive cradle-to-gate LCA study of a traditional natural gas-fired power plant without and with two CCS technologies (MEA and HPC), a photovoltaic plant and a wind turbines system.

In addition, also the few relevant scientific papers discussing economic aspects focus on coal-fired plants [39], and often consider a small nominal net power output related in most cases to pilot plants [40]. Moreover, the technical reports available in the literature usually do not consider the CCS option [41] or they do not include the transportation and storage of CO₂. With the method proposed in this work, the total plant cost obtained from the PS is directly used as a key input for the calculation of the LCOE.

With respect to the state of the art, the main advance of this paper is the use of detailed process simulation of CCS processes tailored to a real Natural Gas Combined Cycle (NGCC) plant, taken as representative of electrical energy production from natural gas. AspenPlus™ is employed for comparing two CO₂ capture processes: (i) a traditional one, well known in the petrochemical industry, based on MEA and (ii) an innovative one based on HPC. In both cases, a simulation of the transport and preparation for storage of CO₂ is also evaluated. Accordingly, the novelty of this paper is twofold: on the one hand, this is the first time a novel process for CCS based on HPC is simulated and optimized in order to predict the possibility of coupling it with a real NGCC power plant. On the other hand, this paper shows how process simulation results in terms of mass balance, energy balance and equipment cost evaluation can be successfully used for the a priori estimation of relevant indicators such as the EROEI, LCOE and LCA, which could also be valuable for defining long-term strategies for the development of national and international energy systems.

The paper is organized as follows: section 2 presents the methods used in this paper including the details of the process simulation, the calculation of the EROEI, LCOE and LCA; section 3 provides and discusses in detail the results obtained with the methods previously described; lastly, some final considerations are reported in section 4.