ReviewCO2 capture from pre-combustion processes—Strategies for membrane gas separation
Introduction
Anthropogenic induced climate change is very likely driven by increasing atmospheric carbon dioxide levels (IPCC, 2007), caused by the world's dependence on fossil fuels. Currently, the capture of carbon dioxide from large point sources, such as power plants, is considered a viable option to reduce emissions, since it allows storage opportunities such as geo-sequestration. There are three main strategies for CO2 capture from fossil fuel based power plants (Steeneveldt et al., 2006, Thambimuthu et al., 2005). In post-combustion, CO2 is captured from the exiting flue gas, while in oxy-fuel combustion air is replaced by oxygen in the combustion process producing mainly H2O and CO2 which are readily separated. The third strategy, the focus of this article, is pre-combustion capture, where fossil fuels are reformed into synthesis gas (syngas) comprised mainly of hydrogen and carbon monoxide. This is then further converted to more hydrogen through the water-gas shift (WGS) reaction, resulting in high pressure CO2 and H2. Separation of these two components allows for the storage of CO2, while H2 can be used for a number of processes, such as power generation. Pre-combustion capture has a number of advantages, such as the production of a carbon-free fuel and capture of CO2 at high pressure (Audus et al., 1998, Thambimuthu et al., 2005).
Membrane technology holds great potential for both CO2 and H2 separation within such gasification processes for a number of reasons. The simplicity of the approach, the removal of gas through a selective film, ensures it has high-energy efficiencies, small equipment footprint and therefore low capital cost compared to conventional separation processes (Bracht et al., 1997, Shelly, 2009). Gas separation membranes have been commercially proven in the sweetening of natural gas (removal of CO2 and H2S) and are commonly used for H2 recovery in refineries. This review focuses on the inclusion of membrane gas separation in pre-combustion carbon capture as part of the integrated gasification combined cycle (IGCC) from a technical and design perspective. Initially, an overview of the IGCC process is provided, highlighting where membranes can be of most advantage. This is followed by a brief review of membranes for H2 and CO2 separation, as well as their potential to drive the water-gas shift as membrane reactors. The review then explores potential improvements to the IGCC process as a result of incorporation of membrane gas separation. This is supported by an examination into the economics of membrane technology in IGCC. The review finishes with a discussion on the future direction of this field.
Section snippets
Integrated gasification combined cycle
Gasification is a method of generating synthesis gas (syngas) from heating carbonaceous fuels, primarily coal, with reactive gases, such as air or oxygen, often in the presence of steam (Thambimuthu et al., 2005). The syngas components, hydrogen and carbon monoxide, can be used to generate power (combined cycle) or as a feed source for the synthesis of a range of chemicals, such as Fischer–Tropsch reactions and ammonia. If hydrogen is the desired component, the syngas can be further reformed
Membrane types
There are a number of comprehensive reviews that investigate the types of membranes generally available for gas separation (Adhikari and Fernando, 2006, Armor, 1998, Baker, 2002, Maier, 1998, Paglieri and Way, 2002, Powell and Qiao, 2006, Shao et al., 2009a, Stern, 1994). In this section, only a brief overview of H2- and CO2-selective membranes is presented to familiarize the reader with the membrane options available for use in the IGCC process. This overview considers first membranes that
Water-gas shift membrane reactors
In a fixed bed WGS reactor, the equilibrium CO conversion is a function of both the temperature and the H2O/CO ratio (also known as the steam to carbon ratio S/C), as shown in Fig. 3 (Xue et al., 1996). The exothermic nature of the forward WGS reaction means that high conversions are obtained at low temperatures, while increasing the amount of water present also produces more H2. However, from an economic view, a H2O/CO ratio between 0.9 and 1.5 has been claimed to be the most beneficial (
Membrane processing strategies and economics
For membrane based pre-combustion capture to be viable, the process must be economical compared with alternative CO2 capture technologies (Spillman and Grace, 1989). This includes WGS-MR relative to a standard WGS reactor, with solvent scrubbing or other CO2 removal technology, as well as the modifications required to an IGCC process for incorporation of the membrane separation.
Future directions
This review has shown that while membranes are prospective for pre-combustion capture, there remain a number of key issues that must be resolved before the technology can be considered competitive with solvent absorption processes.
At the laboratory scale, more research is required into high performance membranes that are firstly inexpensive and secondly resilient to syngas conditions. The minor components in the syngas can have an important influence on membrane performance (Scholes et al., 2009
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