Evaluating Pressure Swing Adsorption as a CO₂ separation technique in coal-fired power plants

Highlights

• Pressure Swing Adsorption (PSA) is evaluated as a CO₂ separation technique.

• The layout of coal-fired power plants operating with PSA is defined and modeled.

• Process simulations of coal-fired power plants are proposed.

• PSA shows to be outperformed by absorption in a post-combustion scenario.

• PSA attains performance close to absorption in a pre-combustion scenario.

Abstract

The paper provides with a first assessment on the suitability of Pressure Swing Adsorption (PSA) as a valid option for Carbon Capture and Storage (CCS) in coal-fired power plants. A full-plant analysis of an Advanced SuperCritical (ASC) pulverized coal plant and of an Integrated Gasification Combined Cycle (IGCC) plant, operating with a PSA unit, is presented. The systems selected aim to represent the most diffused options for coal-based power generation, respectively in a post- and pre-combustion application of CO₂ separation. The definition of the PSA process is tailored for the two different scenarios considered, starting from the adsorbent selected (zeolite 5A and activated carbon, respectively for post- and pre-combustion). The objective is to investigate the competitiveness of PSA with respect to the benchmark technology for CCS, namely absorption. In order to consider the different aspects measuring the effectiveness of a CO₂ separation technique, the performance of the power plants is evaluated in terms of CO₂ separation performance, energy efficiency and footprint of the technology. The post-combustion scenario analysis shows that PSA can be competitive with regard to the separation and the energy performance. PSA is able to match the CO₂ separation requirements, and the relative energy penalty is slightly lower than that resulting from amine-absorption. Despite that, the footprint of the PSA unit demonstrates to be way larger than that related to absorption and unlikely acceptable. PSA in the pre-combustion scenario returns more encouraging results, approaching the outcomes achieved with absorption both in terms of CO₂ separation performance and plant energy efficiency. The footprint, even though significantly larger than with absorption, appears to be reasonable for actual implementation.

Introduction

The atmospheric concentration of carbon dioxide (CO₂) has increased by 40% since pre-industrial times, and recently passed the 400 ppm milestone. CO₂ is regarded as the main responsible for the atmospheric greenhouse effect, which is producing the warming of the climate system. It is extremely likely that human influence has been the dominant cause of the observed warming (IPCC, 2013). One possible mitigation action for stabilizing the atmospheric CO₂ concentration, while continuing exploiting fossil fuel resources, is Carbon dioxide Capture and Storage (CCS). CCS consists in separating CO₂ from large anthropogenic point sources, such as thermal power plants, compressing it for transportation and permanently storing it in underground geological formation. There are different types of CO₂ capture systems: post-combustion, pre-combustion and oxyfuel combustion (IPCC, 2005). Many different techniques have been proposed for capturing CO₂. These includes: chemical or physical absorption, adsorption, reactive solids, membranes, cryogenic processes (Ebner and Ritter, 2009). To date, all commercial CO₂ capture plants are based on absorption for separating CO₂ (Herzog et al., 2009), as it is the most mature and well understood technology. However, its large scale deployment is hindered by the large power consumption, which negatively affects the energy efficiency of the plant. That, summed to other concerns related to the solvent toxicity and to the potentially high corrosion rate, makes advisable to investigate alternatives. In the current work, Pressure Swing Adsorption (PSA) process is analyzed as an option for post- and pre-combustion CO₂ capture. PSA is a cyclic process. During the adsorption step, the CO₂ present in the feed gas stream is fixed on the surface of the selected adsorbent. Following, the regeneration of the bed is carried out by a pressure swing operation. The potential advantage connected to this process is the absence of any thermal energy duty during the regeneration step. Adsorption processes have been successfully employed for CO₂ removal from synthesis gas for hydrogen production (Cen and Yang, 1986, Ribeiro et al., 2008, Ribeiro et al., 2009, Yang and Lee, 1998, Yang et al., 1997). With regard to CCS applications, PSA process suitability has to be proven yet. A large number of studies have been done in order to assess PSA processes operating in the condition typical of post- (Agarwal et al., 2010, Choi et al., 2003, Chou and Chen, 2004, Ishibashi et al., 1996, Kikkinides et al., 1993, Ko et al., 2005, Liu et al., 2011a, Mehrotra et al., 2010, Na et al., 2001, Na et al., 2002, Nikolic et al., 2008, Plaza et al., 2010, Reynolds et al., 2006, Takamura et al., 2001, Tlili et al., 2009) and pre-combustion (Casas et al., 2013, Schell et al., 2013) applications. A significant lack was found in the analysis of more comprehensive systems (The Future of Coal, 2007), where the PSA process is integrated with the rest of the plant. Few works deal with the understanding of such complex arrangements. In post-combustion applications, only preliminary studies have been carried out, whose results can be considered partial (Panowski et al., 2010) and/or focusing on a particular side of the topic (e.g., economic considerations) (Ho et al., 2008). In pre-combustion applications, more thorough analyses have been performed. Liu and Green (2014) evaluated the applicability of PSA as CO₂ removal technology in an Integrated Gasification Combined Cycle (IGCC). They simulated a warm PSA process based on a tailored adsorbent, able to perform at elevated temperature. The results achieved are in line with those of a Selexol absorption process. Other studies investigated the performance of Sorption Enhanced Water Gas Shift (SEWGS), an innovative CO₂ capture process for pre-combustion applications, applied to both IGCC (Gazzani et al., 2013) and Natural Gas Combined Cycle (NGCC) (Manzolini et al., 2011). In either case the outcome appears to be extremely promising. The objective of this paper is to provide a full-plant analysis of coal-fired plants implementing CO₂ capture by a cold PSA process, meaning that the process takes place at temperature levels suitable for many of the most common adsorbents. Coal was selected as fuel because of its higher emission index (higher CO₂ emission per unit of energy released). Further, coal utilization is predicted to increase in the future, under any foreseeable scenario (The Future of Coal, 2007). Thus, CCS will become a critical tool in order to enable a sustainable exploitation of coal. Two plant configurations were considered, respectively to account for a post- and a pre-combustion scenario. Post-combustion CO₂ capture is implemented by integrating a PSA process into an Advanced SuperCritical (ASC) pulverized coal plant. Pre-combustion CO₂ capture is implemented by integrating a PSA process into an Integrated Gasification Combined Cycle (IGCC) plant. First, the layout of the thermal power plant, to be coupled with the CO₂ capture unit, is defined and modeled. Following, the modeling of the PSA process is presented resulting in a dynamic computational model. The procedure for the choice of the optimal PSA process configuration is outlined. A full-plant analysis is then provided for both the scenarios. Simulations were also implemented for the reference case without CO₂ capture and for the case with CO₂ capture based on an absorption process. A plant-level comparison is carried out, returning the competitiveness of PSA process with regard to another technique of decarbonization (i.e., state-of-the-art absorption processes). The performance of the system is evaluated on three levels, namely CO₂ separation performance, energy efficiency and footprint of the technology.