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Review of the Recent Chemical Looping Process Developments for Novel Energy and Fuel Applications Behdad Moghtaderi*

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Priority Research Centre for Energy, Chemical Engineering, School of Engineering, Faculty of Engineering and Built Environment, The University of Newcastle, Callaghan, New South Wales 2308, Australia ABSTRACT: Driven by the need to develop cleaner and more efficient energy systems, an extraordinary array of chemical-loopingbased process concepts have been proposed and researched over the past 10 years. An overview of these technology options, particularly those proposed/developed over the last 3 years, is presented in this paper. The focus, however, would be primarily on process-related aspects of such advanced chemical looping concepts for novel energy and fuels applications rather than aspects such as oxygen carriers, redox properties, and solid circulation/transport, which have been adequately covered in several other reviews.

1. INTRODUCTION Sustainable energy arguably represents the most important technical challenge of this century and will undoubtedly play a major role in issues such as the global climate change, international politics, and commerce in the near future. Greater reliance on advanced low-emission energy technologies and improvements in the energy efficiency of existing fossil-fuel-based power generation assets are vital for a sustainable energy future.1,2 Chemical looping is one of several emerging technology options capable of facilitating the uptake of low-emission energy technologies and helping in a diverse range of applications for productions of fuels, chemicals, and electricity. As shown schematically in Figure 1, in the chemical looping process, a given reaction (e.g., A + B f C + D) is divided into multiple subreactions, with each being typically carried out in a separate reactor (see reaction R1). The link between subreactions is provided by shuttling the so-called solid intermediates (SIs), such as metal oxides (e.g., NiO, CuO, and FeO) or CO2 scavengers (e.g., CaO), between the reactors, where SIs are reduced and regenerated in a cyclic fashion through the progress of the subreactions. A þ SI1 f C1 þ SI2 B þ SI2 f D þ SI1 overall: A þ B f C þ D

minimized, while allowing for the separation of the undesired products (e.g., CO2) generated from the reactions to be accomplished with ease, yielding an overall efficient, economical, and low-emission process. For example, if chemical looping is applied as a combustion process, fuel combustion would takes place in the absence of nitrogen, ensuring that the main constituents of the flue gas are CO2 and water vapor, which can be easily separated from CO2 by cooling the exhaust gas and removing the condensed liquid water. Thus, fuel and air never mix, and CO2 does not become diluted by nitrogen. There is no or little energy penalty associated with the capture of CO2 when chemical looping combustion (CLC) is employed; an issue currently restricting application of CO2 capture options from dilute CO2 streams by solvents, such as amines, which require substantial energy for regeneration. It is this inherent ability for the separation of undesired products, such as CO2, which makes the chemical looping process an invaluable tool in low-emission technologies. Furthermore, the ability to incorporate a diverse range of intermediates provides chemical looping with an unprecedented versatility, enabling it to be employed in a wide range of applications, for example (1) chemical looping gasification (pre-combustion capture of CO2), (2) chemical looping reforming (CLR; pre-combustion capture of CO2), (3) CLC (in situ capture of CO2), (4) sorbent chemical looping for postcombustion capture of CO2, (5) chemical looping air separation for oxygen supply in oxy-fuel and integrated gasification combined cycle (IGCC) operations, and (6) chemical looping removal of ventilation air methane (VAM) in mining operations. The history of chemical-looping-based processes dates back to the mid-1940s,37 although the term “chemical looping” was first introduced in the literature in 1987 by Ishida et al.8 Gilliland and co-workers from the Massachusetts Institute of Technology (MIT) (Cambridge, MA) were the first researchers who

ðR1Þ

The key to successful implementation of any chemical looping process is recognition of the fact that chemical reactions between SIs and other chemical species involved in a given chemical looping process are predominately heterogeneous gas/solid reactions. Homogeneous solidsolid reactions are extremely slow. Therefore, in the case of condense phase reactants (e.g., solid or liquid species), chemical looping reactions would only be possible if the condense phase is gasified first and then its off-gas reacts with the SI particles or, alternatively, when the active ingredient in the SI (e.g., oxygen in the case of metal oxides) is decoupled and released into the gas phase before reacting with the condense phase reactant(s). Chemical looping processes can be designed in such a manner that the energy and exergy losses of the overall process are r 2011 American Chemical Society

Special Issue: 2011 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: August 31, 2011 Revised: October 10, 2011 Published: October 12, 2011 15

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Table 1. Major Focus Areas in Chemical Looping Research since 2000 sample and/or key publications3119

focus area overall mass, energy, and exergy analyses

2, 24, 25, and 70

properties of oxygen carriers

2656

(physical, chemical, and redox)

Figure 1. Schematic representation of the CLC concept.

gas leakage and hydrodynamics

57

reactor designs

18, 19, 58, 59, and 70

(conventional and novel)

employed the principles of chemical looping and patented a process for the production of synthesis gas at industrial scales using a chemical looping approach.9 Later, Lewis and Gilliland adapted this approach and developed a slightly different chemical-looping-based process for production of pure carbon dioxide.10,11 In 1983, Richter and Knoche proposed the principle of CLC for increasing the combustion efficiency.12 Ishida from Japan, who is also credited with the introduction of the technical term “chemical looping”, then proposed the use of CLC as a means of capturing carbon dioxide.8,13 These early efforts in the field of chemical looping inspired a generation of new researchers to focus on developing newer chemical looping processes for a variety of different applications since 1997. Some of the major highlights during this period include (but not limited to) the development of (1) mediator recycling integrating technology (MERIT) by Hatano in 1997,14 (2) unmixed reforming for natural gas/unmixed combustion for solid fuels by Lyon in 1999/ 2000,3 (3) hydrogen production by reaction integrated novel gasification (HyPr-RING) process by Lin and co-workers in 1999/2002,15,16 (4) novel CO2 separation system (later named sorbent energy transfer system) by Copeland in 2001,17 (5) production of H2 from coal by GE-EER in 2001,3 (6) one-step H2 with water splitting by ENI in 2003,3 (7) chemical looping with oxygen uncoupling by Lyngfelt in 2005,18 (8) chemicallooping steam reforming by Ryden in 2006,19 (9) syngas chemical looping for production of high-purity H2 by Fan and co-workers,20,21 and (10) chemical looping air separation by Moghtaderi in 2010.22 Numerous research papers have been published since 2000 covering various aspects of the chemical looping process (for example, refs 3119), and Table 1 provides a breakdown of key papers in major focus areas, such as (i) overall mass, energy, and exergy analyses,2,24,25,70 (ii) oxygen carriers, their redox properties, and reaction kinetics,2656 (iii) gas leakage between chemical looping reactors,57 (iv) conventional and novel reactor designs,18,19,58,59,70 (v) pilot plants and scale-up studies, particularly those reported by the Vienna University of Technology,60,61 Chalmers University of Technology,62 the Instituto de Carboquimica in Spain,63 and the Korea Institute of Energy Research,64 and (vi) operational issues, such as solid circulation/transport, defluidization, sulfur contaminants, and carbon deposition on carrier particles.29,32,33,37,45,46,51,6568 However, because of the global interest in CO2 emissions from fossil-fuel-based power plants, much of the literature on the chemical looping concept has been devoted to CLC applications, and the review papers by Hossain and de Lasa,4 Fang et al.,5 Guang and Tao,6 and Eyring and Konya69 provide very good summaries of the literature on CLC before 2007, although their coverage of more recent papers, especially those works with unconventional features, is rather limited. Apart from CLC, there has also been a surge of research

pilot plants and scale-up studies

6064

operational issues

29, 32, 33, 37, 45, 46, 51, and 6568

power generation

18, 24, 48, 49, 50, 52, 55,

(see Table 2 for details) polygeneration (see Table 3 for details)

and 7188 15, 19 21, 27, 70, 84, 8995, 97104, and 106109

novel energy applications

22, 58, 70, 110, and 111

(see Table 5 for details) reviews and book(s)

47, 69, and 70

publications since 2005 (particularly in 2010 and 2011) on nonCLC applications of the chemical looping concept, particularly for hydrogen production, reforming, and gasification. The excellent book and the review paper by Fan and co-workers summarize some of the relevant works.7,70 However, the emphasis of these reviews is primarily fossil fuels, and as such, the more recent research on non-fossil-based fuels and/or unconventional applications of chemical looping is not fully covered. Given the above background, it is clear that there is a gap between what has been presented in previous reviews/books and that of the literature, particularly in terms of fuels (i.e., fossil versus non-fossil) and application areas (i.e., CLC versus nonCLC). The review presented here is an attempt to narrow this gap by focusing more on exotic and advanced applications of the chemical looping concept for novel energy and fuel (both fossil and non-fossil) applications. However, the emphasis of the present review is process-related aspects of such advanced chemical looping concepts. Aspects such as oxygen carriers, redox properties, gas leakage, mass/energy/exergy analyses, and reaction kinetics, which have been adequately covered in several recent reviews/books,57,69 are not examined in any great details in this review. Also, it is reminded that rather than providing an exhaustive list of chemical-looping-related literature; we discuss those references and concepts that represent and/or describe key technological breakthroughs in the field of chemical looping.

2. CHEMICAL-LOOPING-BASED CONCEPTS/TECHNOLOGIES SUITABLE FOR POWER GENERATION The standalone generation of power (i.e., electricity) has been the focus of many chemical looping studies, and from the outset, many of the chemical-looping-based concepts and/or technology options were designed and tailored to maximize power generation from hydrocarbon fuel sources. Regardless of whether the fuel is gas, liquid, or solid, a fundamental component of such 16

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Table 2. Advanced Chemical-Looping-Based Concepts/Technologies Suitable for Power Generation Applications underlying chemical looping

distinctive

concept/technology

features/innovation

chemical looping with oxygen

feedstock

main

operating

product(s)

conditionsa

extends CLC to solid fuels

coal

power

T 8001200 °C;

CLOU with mixed oxides

CH4 and coal

power

T 750950 °C;

use of CaMn0.875Ti0.125O3

petroleum coke,

power

T 8001000 °C;

uncoupling (CLOU)

Shulman et al.73

P atmospheric

uncoupling (CLOU) chemical looping with oxygen

Lyngfelt et al.18,71

P atmospheric

uncoupling (CLOU) chemical looping with oxygen

reference(s)

as an oxygen carrier

Leion et al.74

P atmospheric

coal, and CH4

chemical looping combustion (CLC)

extends CLC to biomass fuels

biomass

power

T 740920 °C; P atmospheric

Shen et al.50

chemical looping combustion

extends CLC to biomass/coal

biomass and coal

power

T 740980 °C;

Gu et al.52

(CLC)

co-firing applications; uses

P atmospheric

mixture

coal as means of reducing the impact of biomass ash alkali content chemical looping combustion (CLC) of solid fuels using circulating fluidized beds chemical looping combustion using CaSO4

use of a novel circulating

coal and other solids

power

fluidized bed with three loop seals uses natural anhydrite as

T 8001000 °C;

Cao et al.55,72

P atmospheric coal and gas

power


oxygen carriers in CLC

Andrus et al.,75 Anthony et al.,76,77 and Song et al.49,78

pressurized chemical looping combustion (CLC)

examined the high-pressure

IGCC syngas

power

T 800 °C;

Garcıa-Labiano et al.48

P 0.13 MPa

redox properties of metal oxides

pressurized chemical looping combustion combined cycle

use of iron ore as an oxygen carrier

coal

power

T 5001150 °C; P 33.2 MPa

Xiao et al.24

turbine reheat is achieved using

natural gas

power

T 10001200 °C;

Naqvi et al.79

gas, solid, and

power

T 5001300 °C;

(PCLC-CC) chemical looping combustion combined cycle (CLCCC) chemical looping combustion

P 7 MPa

multi-stage chemical looping use of dynamically operated pack

(CLC) with stage combustion

bed reactors; no need

and oxygen transport (SCOT)

for solid transport

P atmospheric

liquid fuels

Chakravarthy et al.80 and Noorman et al.8183

gas, solid, and liquid fuels

power


Najera et al.84

integrated desulfurization

syngas

power

T 600900 °C;

Solunke et al.85

uses methanol as fuel;

methanol

power

T 1501200 °C;

chemical looping dry reforming (CLDR)

use of CO2 for regeneration of carrier

chemical looping combustion

particles P up to 3 MPa

(CLC) chemical intercooling gas turbine cycle combined

reduction reaction is

with CLC (CIGT/CLC)

supported by the heat from

gasification chemical looping

compressor intercooling uses the heat generated in the

combustion combined cycle

AR to gasify coal indirectly

(GCLC-CC)

and then burns the resulting

Zhang et al.86

P 0.11.3 MPa

coal

power

T 9501200 °C;

Xiang et al.87

P 2.5 MPa

syngas in FR; gas products run a combined cycle chemical looping combustion

extends CLC to liquid fuels

liquid fuels

T 950 °C;

Hoteit at al.88

P atmospheric

(CLC) a

power

Only for chemical looping unit operations.

chemical looping options is a CLC unit, where the heat released from chemical reactions is used for subsequent power generation. It is therefore not surprising to see the high level of attention paid to such systems over the past decade and, in particular, in the last 5 years. Table 2 summarizes the major achievements made in

CLC research and development since 2005 and briefly outlines some of the distinctive features of relevant technologies developed for gaseous, solid, and liquid fuels. As noted in section 1, chemical looping processes for condense phase reactants (e.g., solid or liquid fuels) cannot proceed 17


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unless either the condense phase reactants are converted into the gas phase for subsequent gas/solid reactions at the surface of SI particles or, instead, when the SIs release their active ingredients (e.g., O2) into the gas phase for subsequent gas/solid reactions at the surface of the condense-phase reactant. Because of the technical difficulties associated with both approaches, the early CLC-based systems for power generation were predominately designed for gaseous fuel feedstock. However, it was recognized very early that the wider acceptance of the CLC hinges upon extending the concept to fossil- and organic-based solid fuels, in particular, coal. It is, therefore, not surprising that much of the advancement made in this field since 2005 is related to solid fuel applications. Such advancements are specially considered to be important in the context of carbon capture and storage (CCS) because of the inherent ability of CLC for CO2 separation/ capture. As seen from Table 2, one of the major technological milestones achieved during this period has been the development of the chemical looping with oxygen uncoupling (CLOU) process by Lyngfelt and co-workers from Chalmers University of Technology (Gothenburg, Sweden).18,71 When metal oxides were incorporated with reversible redox properties (e.g., Cu, Co, and Mn), the CLOU process enables the gas-phase release of oxygen in the fuel reactor, in turn allowing the CLC concept to be directly applied to solid fuels. The technique does not involve any homogeneous reactions between the solid fuel and oxygen carrier particles. Instead, a three-step chemical looping process is employed and carried out using two interconnected reactors, namely, an air reactor and a fuel reactor. In the air reactor, the metal oxide captures oxygen from the air (step 1, oxygen coupling), while the release of oxygen by metal oxide particles (step 2, oxygen decoupling) and the subsequent reaction of this oxygen with fuel particles (step 3, combustion) take place in the fuel reactor. Prior to the CLOU process, the alternative methods available for CLC of solid fuels were all indirect and included (1) the ex situ gasification approach, where the solid fuel is gasified first in a separate reactor and then the resulting off-gas is fed into a gas-type CLC system, and (2) the in situ gasification approach, where the solid fuel is gasified in the fuel reactor, while the off-gas is combusted by the oxygen carrier particles in the same reactor.72 The former approach has some operational advantages; for example, oxygen carrier particles are not mixed with particles of the fuel (e.g., coal). As a result, there is no need for separating oxygen carriers from solid fuel particles, which could be a difficult task. However, in an ex situ approach, the gasifier is a completely standalone reactor (i.e., the combustion and gasification processes are not fully integrated) and, hence, requires expensive auxiliary units to provide the necessary heat for the endothermic gasification reactions. The latter approach (in situ gasification) also has some operational advantages, chiefly among them, no requirement for the gasifier or the air separation unit (ASU) given that the heat of gasification is primarily provided by circulating oxygen carrier particles from the combustor. However, the in situ approach suffers from a number of shortcomings. For instance, the oxygen carrier and fully reacted fuel particles (ash) have to be separated from each other at the end of each cycle. Moreover, the unburnt carbon formed during the process may deposit on the surfaces of oxygen carrier particles and deactivate them. The unburnt carbon may also end up in the air reactor, thus, lowering the CO2 capture efficiency of the system. Among the above issues, the separation of ash and

oxygen carrier particles from each other is perhaps the most challenging task in in-situ-gasification-based CLC processes. While not trivial, such solidsolid separations can be best achieved using mechanical rather than chemical means. On the basis of their operational principles, solidsolid separators are typically classified into systems based on (1) selective barriers, (2) phase density, (3) fluid/particle mechanics, and (4) surface and electrical characteristics. Examples of solidsolid separators are screens, air and wet classifiers, centrifugal classifiers (e.g., cyclones), jigs (shakers), tables, spiral concentrators, flotation cells, dense-medium separators, magnetic separators, and electrostatic separators. Of these systems, the wet classifiers, flotation cells, and dense-medium separators are not suitable for CLC operations because they require a liquid medium (e.g., water) to operate. Other systems, especially cyclones and electrostatic separators, can be potentially employed in CLC operations with relative ease. The CLOU process shares some of the difficulties associated with the in situ gasification approach in terms of separating oxygen carrier particles from ash and unreacted fuel particles, as well as carbon deposits and carrier deactivation. Also, only a limited number of metal oxides can match the reversible redox requirements of the CLOU process. However, the advantages of the CLOU process, such as its simple hardware and direct oxidizer/fuel contact, far outway its disadvantages. The concept of CLOU was first discussed in a Swedish patent by Lyngfelt and co-workers.18 Its application to solid fuels was the subject of a series of journal and conference publications by Lyngfelt, Mattisson, Leion, and co-investigators.71 Given the limited range of metal oxides with reversible redox properties available for CLOU applications, in 2009, Shulman and coinvestigators73 examined the viability of using mixed oxides of Mn/Fe, Mn/Ni, and Mn/Si in the CLOU process. With the exception of Mn/Si oxygen carriers, all investigated mixed oxides showed satisfactory performance under operating conditions pertinent to the CLOU process and were particularly able to reoxidize at 900 °C in an atmosphere of 10% O2 and 90% N2. The ease of reoxidization varied significantly among mixed oxides studied by Shulman et al.,73 with Mn/Si species being the most difficult materials. The work on mixed oxides for potential applications in the CLOU process was continued by Leion and co-workers,74 who studied Mn, Ti, and Ca as oxygen carriers. Specifically, these Swedish researchers74 investigated the effectiveness of CaMn0.875Ti0.125O3 in the CLOU process. Through a comprehensive experimental campaign, the effectiveness of CaMn0.875Ti0.125O3 oxygen carriers was verified and promising results were obtained from tests performed in both thermogravimetric analyzer (TGA) and fluidized-bed experimental setups. In particular, no or little CO was detected during solid fuel reactions. More importantly, at temperatures around 1000 °C, the enthalpy of reaction for CaMn0.875Ti0.125O3 was found to be exothermic, implying that the redox reactions in both fuel and air reactors may progress with little external heat input (if any). The performance of CaMn0.875Ti0.125O3 oxygen carriers could also be optimized in a relatively simple manner by varying the porosity of oxygen carrier particles through adjustment of the sintering temperature during the manufacturing of granulates. Application of CLC to solid fuels other than coal has also been the subject of several studies. Among them, the works by Cao et al.,55,72 Shen et al.,50 and Gu et al.52 deserve a special mention because they investigated the CLC of a variety of solid fuels, 18


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including biomass, solid waste, and biomass/coal mixtures. All of these studies adopted an in situ gasification approach for combustion of solid fuels. Cao and co-workers55,72 used a novel circulating fluidized-bed reactor combined with three loop seals to carry their investigations, while two interconnected fluidizedbed reactors were employed in Shen et al.50 and Gu et al.52 studies. Shen and co-investigators50 experimentally examined the conversion of biomass carbon to CO2 in the fuel reactor. Their results showed that an increase in the fuel reactor temperature led to higher levels of CO production during biomass gasification than from the oxidation of CO by iron oxide. Cao et al.55 and Shen et al.50 have also identified the alkali content of biomass ash to be an impediment in successful application of the CLC concept to biomass fuels. Gu et al.52 proposed co-firing a small percentage of coal with biomass to alleviate the effect of the biomass ash alkali content. The approach resulted in limited success. The use of calcium sulfate (CaSO4) as an alternative oxygen carrier to transitional metal oxides in CLC of solid fuels has also been the subject of several studies.49,7578 These investigations were primarily motivated by the higher oxygen carrying capacity (Ro) of CaSO4 compared to those of typical metal oxides (0.4706 for CaSO4, as opposed to 0.2212, 0.2011, and 0.1001 for NiO, CuO, and Fe2O3, respectively) and the abundance of calcium sulfate in nature, hence, its low cost. The first use of CaSO4 was reported by Alstom Power, Inc.75 for CLC of coal, although no information was disclosed in the literature. Anthony and coworkers76,77 also carried out a series of modeling studies using the Aspen Plus process simulation package and verified the feasibility of using calcium sulfate in CLC of solid fuels, particularly coals with high sulfur contents. Song et al.49,78 also investigate the use of calcium sulfate using a combined theoretical and experimental approach. Previous studies have identified a number of thermodynamic limitations for CaSO4, which could potentially result in the unwanted release of sulfur and incomplete conversion of the fuel. Moreover, the relatively poor mechanical strength of calcium sulfate limits its operational life under repeated cycles in typical CLC systems. Several methods have been proposed in the literature to deal with the issue of unwanted sulfur release when CaSO4 is used as an oxygen carrier in CLC systems, including the approach recommended by Song and co-workers49,78 in using fresh limestone in the CLC reactors as a means of sulfur removal. Pressurized CLC systems have also been investigated by several research groups in the context of coal-fired IGCC48 and other types of coal-based combined cycles,24 which rely on the ex situ gasification approach for CLC of coal. The key drivers behind the interest in pressurized CLC systems have been (1) the relatively slow coal gasification process, which limits the overall coal combustion and plant efficiency, (2) the mismatch between the low operating temperatures for CLCs (because of the low melting points of oxygen carrier particles and/or coal ash) and the elevated gas turbine inlet temperatures required to maintain high net plant efficiencies, and (3) recognition of the fact that, in a high-pressure CLC plant, CO2 can be captured as a high-pressure gas, minimizing the energy demand for further compression of CO2 and, thereby, higher net plant efficiencies. Unfortunately, the high-pressure TGA studies conducted by Garcıa-Labiano et al.48 revealed that an increase in the operating pressure of a CLC system considerably lowers the reaction rates of common oxygen carriers, such as Ni, Cu, and Fe oxides. Moreover, the internal structure and reactivity of oxygen carriers

are heavily influenced by the total pressure, to the extent that the kinetic properties of oxygen carriers have to be determined at the actual pressure and cannot be extrapolated from the data corresponding to the atmospheric pressure.48 The results reported by Xiao et al.24 were more promising and showed that Companhia Valedo Rio Doce (CVRD) iron ore can perform relatively well when subjected to repeated redox cycles at elevated pressures. However, the high-pressure experiments carried out by Xiao et al.24 were limited to 5 bar (500 kPa), which is far below the suitable range of operating pressures for IGCC and other combined cycle applications. As such, the feasibility of using pressurized CLCs in conjunction with combined cycles still remains in doubt. Apart from the progress in application of CLC to solid fuels, a number of other innovations in the area of CLC have been reported in the literature in recent years. Several of these innovations are briefly discussed in this section, beginning with the concept reported by Naqvi and Bolland79 for chemical looping combustion combined cycle (CLCCC) of natural gas. A CLCCC plant essentially comprises a CLC reactor unit, an air turbine, a CO2 turbine, and a steam cycle (see Figure 2). The base-case configuration shown in Figure 2a can deliver net plant efficiencies of about 52% (at an oxidation temperature of 1200 °C) with close to 100% CO2 capture without the need for any post-combustion capture unit. The multi-stage reheat versions of the CLCCC cycle (Figure 2b) can achieve similar thermal and CO2 capture efficiencies at lower oxidation temperatures (∼1000 °C) and, as such, possess much smaller energy footprints. However, the results by Naqvi and Bolland79 suggest that, among multi-stage reheat options, the single-reheat configuration (Figure 2b) exhibits much better performance characteristics, while the double- or triple-reheat cycles result in marginal efficiency improvement, as compared to the single-reheat configuration. Another innovation in the field of CLC, which can be equally applied to other chemical-looping-based processes, is the use of dynamically operated pack-bed reactors.8083 This approach also known as “stage combustion with oxygen transport80 (SCOT)” or “manifold switching reactors80 (MSR)” eliminates the need for particle transport between the fuel and air reactors. Unlike conventional chemical looping systems, in SCOT- or MSR-based systems, carrier particles are not circulated between the two reactor assemblies and the cycle is completed by switching over the reacting gases from one reactor assembly to the other. Each reactor, therefore, functions periodically as a fuel reactor and an air reactor. As a result, the oxygen carrier particles are not subjected to cyclic moves from one reactor to another, and hence, their physical integrity can be maintained over much larger repeated redox cycles. This opens up a pathway for a much wider use of low mechanical strength yet highly effective oxygen carriers, such as CaSO4, in chemical looping applications. While the above approach certainly resolves many of the issues associated with particle transport in CLC systems, the use of packed beds lowers the degree of mixing in chemical looping reactors and may ultimately lead to the formation of undesirable hot spots in the reactors. Until these practical issues are fully resolved, a limited use for dynamically operated pack-bed reactors in CLC applications is anticipated. Of other CLC innovations in recent years, the works reported by Najera et al.,84 Solunke et al.,85 Zhang et al.,86 Xiang et al.,87 and Hoteit et al.88 deserve special mention. Najera et al.84 proposed and studied the feasibility of chemical looping dry 19


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Figure 2. Simplified process flow diagram of the CLCCC: (a) base-case configuration and (b) multi-stage reheat version.79

reforming (CLDR), where CO2 captured during the CLC process is employed for regeneration of oxygen carrier particles (Figure 3). When air was replaced with CO2 as the oxidant, Najera and coinvestigators84 effectively proposed CLDR as an alternative to CLC. Through a comprehensive series of thermodynamic and reaction kinetic studies, Najera et al.84 demonstrated the feasibility of the CLDR approach for any carbon-based fuel, although

nanostructured oxygen carrier particles were required to achieve the desirable reaction rate in the reduction of CO2 to CO. Solunke et al.85 developed a suit of nanocomposite CuBHA oxygen carriers with high H2S removal capacity. On the basis of these nanocomposite carriers, Solunke and co-workers85 proposed a novel CLC process capable of simultaneous CO2 and sulfur capture. The process was successfully demonstrated at 20


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laboratory scale for in situ desulfurization of syngas, although, in principle, the process can be applied to high sulfur content solid fuels. Zhang et al.86 reported on the first applications of CLC with a liquid fuel feedstock. In their studies, Zhang and co-investigators86 proposed an innovative chemical intercooling gas turbine cycle combined with CLC (CIGT/CLC) for combustion of methanol and dimethyl ether, which are considered by many as alternative energy carriers to coal and natural gas. In this process, the thermal energy required for reduction of iron oxide oxygen carriers is supplied by the intercooling heat of the cycle compressor, while the compressed air is heated by the oxidation reaction of iron oxides (Figure 4). Preliminary modeling results from a series of Aspen Plus process simulations showed that the CIGT/CLC cycle can achieve thermal efficiencies as high as 56.8% with 90% CO2 capture. This is almost 10% higher than a conventional combined cycle with the same CO2 capture efficiency. Xiang et al.87 proposed a novel combined cycle based on slurry coal gasification and CLC of the product gas. This process known as gasification chemical looping combustion combined cycle (GCLC-CC) incorporates a gas-based CLC unit into a gas/ steam turbine combined cycle, as shown in Figure 5. Coal slurry enters a pipe-type gasifier immersed in the CLC air reactor, in turn, absorbing the heat released from the oxidation of oxygen carrier particles. The syngas produced from the gasification process is then cleaned and fed into the fuel reactor of the

CLC unit. The hot exhaust gases from the CLC fuel reactor are then used to run a gas turbine combined steam cycle for power generation. The steam in the flue gas is condensed and separated continuously during the process, and the resulting CO2 is compressed for subsequent storage. Results indicate that the GCLC-CC process can achieve thermal efficiencies of about 44% [lower heating value (LHV)] and 90% CO2 capture efficiency. Hoteit, Gauthier, and co-investigators extended the application of CLC to liquid fuels.88 These researchers employed a laboratory-scale fluidized-bed setup to investigate the cyclic contact between liquid fuels (e.g., dodecane and heavy oil no. 2) and oxygen carriers during both reduction and oxidation half cycles. The internal diameter of the fluidized-bed reactor was 20 mm, and particles of NiAl0.44O1.67 were used as the oxygen carrier. The system was operated at 950 °C in the pulse mode, where 12 g of the liquid fuel was injected into the reactor at regular intervals. Regardless of the type of liquid fuel used, experiments showed that almost complete fuel conversion could be achieved during the reduction half cycles, while full or partial combustion could be achieved during the oxidation half cycle depending upon the amount of oxygen available in the fluidized bed.88

3. ADVANCED CHEMICAL-LOOPING-BASED CONCEPTS AND/OR TECHNOLOGIES SUITABLE FOR POLYGENERATION APPLICATIONS Many chemical looping schemes have been devised to produce more than one product and, as such, are suitable for the socalled “polygeneration” applications. As shown in Table 3, often the main product in the polygeneration-type chemical looping processes is hydrogen, which can be subsequently used for power generation (i.e., as a secondary product) or alternatively as a feedstock in a range of other fuel and/or chemical manufacturing processes. While some polygeneration-type chemical looping schemes are based on concepts, such as HyPr-RING,15,89 by far, the variants of the CLR1921,27,70,90104 concept have formed the basis of the majority of polygeneration chemical looping processes developed in recent years (see Table 3). For this reason, CLR-based schemes are discussed ahead of HyPr-RING and other relevant concepts in this section.

Figure 3. Schematic representation of the CLDR process.84

Figure 4. Simplified process flow diagram of the CIGT/CLC cycle.86 21


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Figure 5. Simplified process flow diagram of the GCLC-CC process.87

Ryden and co-workers19,27 were the first group who proposed a fully fledged CLR-type process (more specifically CLATR) for the production of hydrogen and power with CO2 capture. Their process flow diagram is shown schematically in Figure 7. As illustrated, the major unit operations are an air reactor (AR), a fuel reactor, which also contains the reformer tubes (FR/SR), a high-temperature watergas shift reactor (WGS), condensers for removal of steam (COND), and a pressure swing adsorption unit (PSA). The process essentially resembles a conventional steam-reforming plant, in which the furnace has been replaced by a CLC unit. Energy for the endothermic reforming reactions is provided by indirect combustion that takes place in the air and fuel reactors. The process proposed by Ryden and coworkers19,27 features a number of attractive characteristics. For instance, the off-gas is rich in H2, which exhibits high reactivity with oxygen carriers. Moreover, almost 100% CO2 capture efficiencies can be achieved, and because of low temperatures maintained in the air reactor, the formation of thermal NOx is negligible. Xiang et al.90 adopted the CLSR variant of the CLR process (Figure 6c) for simultaneous production of hydrogen and power from coal. In this process, which is shown schematically in Figure 7, the coal feedstock is gasified first in a shell-type gasifier and then the resulting synthesis gas is fed to a CLSR unit similar to that described earlier in Figure 6c, where syngas is converted to H2O and CO2 using iron oxide oxygen carriers. The H2-rich stream from the CLSR unit is expanded in a turbine for electricity generation. The heat content of the hydrogen stream exiting the turbine is further recovered in a heat recovery steam generator (HRSG) before hydrogen is compressed for further use. Similarly, the hot CO2/steam stream from the CLSR unit is expanded in a dedicated turbine for electricity generation, and its waste heat is recovered in the HRSG unit. The CO2/steam stream is then put through a multi-stage compression and heat-exchange process for removal of the condensate (i.e., water) and pressurization of CO2 for storage/sequestration. Xiang et al.90 assessed the performance characteristics of their process using the Aspen Plus process simulation software. The results show that very highpurity H2 (99.9%) can be produced using the proposed process.

3.1. CLR-Based Polygeneration Schemes. The working

principle of various CLR pathways has been shown schematically in Figure 6. All CLR-type processes are based on the cyclic reduction and oxidation of either metal oxide oxygen carriers or CO2 scavengers in the presence of steam. The CRL processes are typically carried out by exchanging the carrier particles between two interconnected fluidized-bed reactors, although in some studies,90 dynamically operated pack-bed reactors have been employed. The main goal in CLR processes is to produce hydrogen or synthesis gas (i.e., a mixture of H2 and CO) rather than heat and power. For this reason, the air/fuel ratio is kept low to prevent the complete oxidation of fuel to CO2 and water. As shown in Figure 6, the major pathways for CLR are (a) chemical looping partial oxidation (CLPO) and autothermal reforming (CLATR) (Figure 6a), (b) chemical looping CO2 acceptor reforming (CLCAR) (Figure 6b), and (c) chemical looping steam reforming (CLSR) (Figure 6c). In both CLPO and CLATR cases (Figure 6a), the fuel is provided with sub-stoichiometric oxygen to generate synthesis gas. However, for CLATR, a reformer gas (typically either steam or CO2) is also introduced into the fuel reactor. In the CLCRA case (Figure 6b), the cyclic carbonization and calcination of a CO2 scavenger, such as CaO, is used to improve the selectivity of hydrogen in the product gas stream. In the CLSR process (Figure 6c), the fuel is completely oxidized and carrier particles are fully reduced in the fuel reactor. Reduced particles are then directed to the steam reactor, where pure H2 is produced by oxidization or regeneration of particles with steam. Among the above alternative pathways, the CLSR process is more attractive for many applications mainly because the production of hydrogen in CLSR is divided into two separate steps of “fuel oxidation” (i.e., carrier particle reduction) and “steam reduction” (i.e., carrier particle oxidization). This allows for the production of high-purity H2 to proceed without any post-processing, such as watergas shift reaction or pressure swing adsorption (PSA).80 Also, in terms of energy demand and, thereby, cost effectiveness, the CLSR process is very appealing, because it requires relatively low levels of energy input given that the reactions between steam and metal oxide carrier particles are typically exothermic. 22


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Table 3. Advanced Chemical-Looping-Based Concepts/Technologies Suitable for Polygeneration Applications underlying chemical looping concept/technology chemical looping reforming (CLR) chemical looping reforming (CLR)

main distinctive features/innovation uses chemical looping autothermal

feedstock

operating conditionsa T 8501000 °C;

natural gas H2 (power)

reforming (CLATR) ex situ coal gasification

product(s)

H2 (power)

(shell gasifier) followed by

Ryden et al.19,27

P atmospheric T 788815 °C;

coal

reference(s)

Xiang et al.90

P 3.2 MPa

chemical looping steam reforming (CLSR) chemical looping reforming (CLR)

H2 production using the threereactor chemical looping steam

natural gas H2 (power)

T 449831 °C; P 1.82.2 MPa

Chiesa et al.101

T 5001150 °C;

Fan et al.,20,21,70

P 33.2 MPa

Xiang et al.,98

reforming (3RCLSR) chemical looping

ex situ coal gasification followed by

reforming (CLR);

H2 production using the three-

also known as syngas

reactor chemical looping

chemical looping (SCL)

steam reforming (3RCLSR)

chemical looping reforming (CLR); also known as coal direct chemical looping (CDCL) chemical looping reforming (CLR); also known as

modified version of the SCL where

coal H2 (power)

and Cormos99 T 700950 °C;

coal H2 (power)

the gasifier has been eliminated; coal gasification takes place

Fan et al.104 and

P 3 MPa

Gnanapragasam et al.106

not provided

Kobayashi et al.100

T 7001000 °C;

Moghtaderi and

P up to 7 MPa

Zhang95,107

in the reducer parallels the design of the CDCL

biomass H2 (power)

process and extends it to biomass

biomass direct chemical ooping (BDCL) chemical looping reforming

uses a novel 3RCLSR to generate

(CLR); also known as integrated gasification

(i) H2/steam for ex situ gasification of coal and

chemical looping

(ii) combustion of coal gas

coal, biomass, and other solid fuels

power (H2)

combustion (IGCLC) chemical looping reforming (CLR)

H2 production by CLSR in a

T 900 °C;

coal char H2 (power)

single reactor; reduction of iron

Yang et al.92

P atmospheric

oxides by coal char chemical looping reforming (CLR) chemical looping reforming (CLR)

uses periodically operated pack

T 4001200 °C;

syngas H2 (power)

bed reactors in conjunction with H2 production using CLSR uses a three-loop-reactor system

P 3 MPa T 800950 °C;

coal H2 (power)

for implementation of the

Solunke and Veser97

P atmospheric

Wolf et al.102 and Kulkarni et al.103

chemical looping CO2 acceptor reforming (CLCAR) chemical looping reforming (CLR); also

incorporates the CLCAR process

T 800950 °C;

biomass H2 (power)

and applies it to biomass fuels

Acharya et al.91

P atmospheric

known as sorbent chemical looping gasification (SCLG) chemical looping reforming

combines the CLCAR process with

(CLR); also known as

the use of construction and

sorbent chemical looping

demolition waste as the source

gasification (SCLG) chemical looping reforming (CLR) chemical looping reforming

other solid fuels

H2 (power)

Moghtaderi94

P atmospheric

of CO2 sorbent use of composite Ce, Fe, Cu, and

T 600900 °C;

CH4 syngas (chemicals)

Mn oxides as carrier particles uses combined steam reforming,

(CLR); also known as

dry reforming, and partial

CLC combined reforming

oxidation in conjunction

(CLCCR)

T 800950 °C;

coal, biomass, and

P atmospheric T 4001000 °C;

propane power (N2 and

He et al.93

Kale et al.108

P atmospheric

syngas)

with CLC; uses CaSO4 oxygen carriers 23


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Table 3. Continued underlying chemical looping concept/technology hydrogen production by reaction integrated novel

main distinctive features/innovation integrated slurry coal

feedstock

product(s)

coal H2 (power)

gasification

operating conditionsa

reference(s)

T 600850 °C;

Lin et al.15 and

P 0.13 MPa

Nakagaki89

gasification (HyPr-RING) dual chemical looping (DCL)

HyPr-RING based; first loop

T 600850 °C;

coal

for CO2 capture by Li4SiO4

H2 (power)

Nakagaki89

P up to 3 MPa

sorbent, second loop for gasification and heat the first loop gas-conditioning chemical looping (GCCL)

combines a conventional methanol synthesis process

CH4 plus CO2 methanol and

Zeman and Castaldi109

dimethyl ether

with a gas CLC a

T 200650 °C; P not specified

Only for chemical looping unit operations.

Figure 6. Alternative pathways for CLR (note that MeO and Me denote metal oxide and reduced metal).58

Furthermore, the net thermal efficiency of the process was found to be strongly affected by the steam conversion rate. Accordingly, the net thermal efficiency varied between 53.17 and 58.33% when the steam conversion rate was increased from 28 to 41%. The nominal net thermal efficiency was estimated to be 57.85% for a steam reactor temperature of 815 °C at a steam conversion rate of 37%. The corresponding exergy efficiency was 54.25%, and the losses were found mainly to be due to gasification and HRSG unit operations. Chiesa et al.101 adopted a version of the CLSR process for hydrogen/power production from natural gas, which was slightly different from the general layout shown earlier in Figure 6c. In their interpretation of the CLSR process, Chiesa and coworkers101 employed a three-step reforming process for the production of hydrogen based on the well-know steam/iron process devised and patented by Huebler and co-workers105 in 1969. The reforming process by Chiesa and co-workers is carried out using a three-reactor setup similar to that shown in Figure 8 and, for this reason, is commonly referred to as the three-reactor chemical looping steam reforming (3RCLSR) process. With reference to Figure 8, in the 3RCLSR process, the oxygen-rich hematite (Fe2O3) is first reduced to wuestite (FeO) by oxidizing the natural gas in the fuel reactor (FR) according to the endothermic reaction represented by reaction R2. The wuestite is then fed into the steam reactor, while the CO2 content of the flue gas is taken for storage after water condensation. The exothermic reaction R3 between wuestite and steam in the steam

Figure 7. Simplified process flow diagram for the CLR scheme proposed by Xiang et al.:90 (1) gasifier, (2) fuel reactor, (3) steam reactor, (4) CO2/steam turbine, (5) hydrogen turbine, (6) supplementary firing, (7) particulate removal unit, (8) desulfurization unit, (9) compressor, (10) cooler, and (11) separation unit.

reactor (SR) forms magnetite (Fe3O4) and hydrogen, of which the latter represents the main product of the 3RCLSR process. To maintain the thermal balance of the overall process, the magnetite particles are then taken to the air reactor (AR), where they are oxidized and regenerated to the hematite state (reaction R4). 4Fe2 O3 þ CH4 f 8FeO þ 2H2 O þ CO2 ,

356:5 kJ=mol

ðR2Þ 12FeO þ 4H2 O f 4Fe3 O4 þ 4H2 ,

þ 199:3 kJ=mol ðR3Þ

4Fe3 O4 þ 3O2 f 6Fe2 O3 ,

þ 314:6 kJ=mol

ðR4Þ

The 3RCLSR process is very attractive for hydrogen production, and in comparison to conventional reforming processes, does not require large and expensive components, such as watergas shift reactors, CO2 removal and H2 purification units, reformers, and ASUs.101 The 3RCLSR process also substantially differs from the three-loopreactor systems proposed in the literature for hydrogen production 24


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based on the CLCAR concept (see Table 3).102,103 In the 3RCLSR process, only metal oxides are used, whereas in three-loop-reactor systems, a sorbent material, such as calcium oxide, is employed to absorb CO2 from the reactor, in which the fuel is reformed to CO.101 As a result, the 3RCLSR process features fewer solid handling issues and greater flexibility in operational aspects of the reactors. For this reason, many of the more recent CLR-based processes have been inspired by the 3RCLSR concept and incorporated it as their core technology platform.20,21,95,98100 For example, Fan et al.,20,21,70 Xiang et al.,98 and Cormos99 investigated the feasibility of ex situ coal gasification followed by H2 and power production using the 3RCLSR concept. The process developed by Fan et al.,20,21 which is commonly referred to as syngas chemical looping, has been successfully demonstrated for conversion of coal into high-purity hydrogen and/or power with integrated CO2 capture. The SCL process shown schematically in Figure 9 combines a syngas generation and cleanup system (essentially a commercial coal gasifier fitted with a gas treatment plant) with a 3RCLSR unit comprising the reducer (or FR), the oxidizer (or SR), and the combustor (or AR). Coal is first gasified into raw syngas in a commercial gasifier, and after some treatment, the syngas is fed into the 3RCLSR unit

for further processing. The key reactions now are Fe2 O3 þ CO=H2 f 2FeO þ H2 O=CO2

ðinthereducerÞ

ðR5Þ 3FeO þ H2 O f Fe3 O4 þ H2 4Fe3 O4 þ O2 f 6Fe2 O3

ðintheoxidizerÞ

ðinthecombustorÞ

ðR6Þ ðR7Þ

Hydrogen purity was found to decrease because of the carbon deposition from the Boudouard reaction, which was catalyzed by the reduced hematite particles at a low-temperature zone in the reducer gas inlet.20 Nominal hydrogen purities of about 99.8% were achieved using the SCL process with minimal formation of carbon deposits on oxygen carrier particles when composite iron oxide particles were employed.20 Laboratory experiments using a 2.5 kWth bench-scale unit and a 25 kWth pilot plant were carried out over extended periods of time, and the results were compared to theoretical predictions using the Aspen Plus software tool.70 Results showed that, under identical conditions, the SCL process is capable of delivering thermal and CO2 capture efficiencies higher than those corresponding to more conventional coal gasification processes (see Table 4). Fan et al.104 developed a modified version of their SCL process by eliminating the syngas generation and cleanup unit (i.e., gasifier) from the overall scheme. In this process, called coal direct chemical looping (CDCL), the coal feedstock is directly reacted with oxygen and iron oxide particles in the reducer (fuel reactor; see Figure 10). Studies by Gnanapragasam et al.106 revealed that the elimination of the gasifier and its accessories, such as ASU, significantly lowers the energy footprint of the CDCL process to the extent that the conversion rate of the solid fuel rises by a factor of 50. This, in turn, reduces the required solid inventory and the physical dimensions of the reactors. Generally, to produce a given amount of hydrogen, the CDCL requires less steam than the SCL process. Moreover, the CDCL results in a higher hydrogen/CO2 ratio than that of the SCL process. Perhaps the biggest difference between the two processes is the methane reduction reaction. Even with the conversion of methane in the syngas to hydrogen, the overall production of H2 in the SCL process is still less than hydrogen produced by the CDCL process.84 Kobayashi et al.100 extended the application of the CDCL concept to biomass fuels. Their process, which is referred to as

Figure 8. Schematic representation of the 3RCLSR process.101

Figure 9. Simplified process flow diagram for the SCL process.70 25


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the biomass direct chemical looping (BDCL) process, very much parallels the design implemented by Fan et al.104 and Gnanapragasam et al.106 in the CDCL process (see Figure 10). The preliminary studies by Kobayashi and co-investigators100 showed that the BDCL process with its high conversion efficiency and relatively low cost of electricity production (∼$60/MWh) can potentially compete with conventional energy generation systems. However, several factors, including fuel-handling issues (e.g., particle size, moisture, and tar), have to be considered before a complete feasibility analysis can be performed, to assess the technoeconomic viability of the BDCL process.100 Inspired by the 3RCLSR concept, Moghtaderi and Zhang95,107 have recently proposed a new chemical looping scheme for conversion of carbonaceous solid fuels, such as coal, biomass, and municipal solid waste (MSW), to hydrogen, power, and other value-added products. The process, which is referred to as integrated gasification chemical looping combustion (IGCLC), resolves the issues associated with both ex situ and in situ approaches while maintaining their best features. In the IGCLC process, the fuel gasification takes place in the absence of oxygen carriers (similar to the ex situ approach). However, the gasification process is fully integrated with chemical looping reactions because the gasifying agent and the heat required for the gasification process are produced using a three-step chemical loop similar to that used in the 3RCLSR concept. The basic steps in the IGCLC process are (i) generation of H2 from steam, (ii) fuel gasification in the presence of the H2/steam mixture, (iii) combustion of the fuel off-gas originated from the gasification of the solid fuel particles, and (iv) regeneration of oxygen carriers. Because of the need for high-purity hydrogen, the IGCLC

process can work best with metals with multiple oxidation states. Among these, Fe-based oxides are preferred primarily because (1) Fe has three oxides, namely, wuestite (FeO), hematite (Fe2O3), and magnetite (Fe3O4), (2) Fe-based oxides are abundant in nature and are of low cost, (3) oxides of Fe show relatively good redox properties, (4) oxides of Fe have good mechanical and thermal stability, and (5) Fe-based oxides are nontoxic and environmentally benign. As shown in Figure 11, in a Fe-based IGCLC process for coal, an equimolar steam/hydrogen mixture is first generated in a steam-reforming reactor (SR) through chemical reactions between steam and particles of wuestite (FeO). Some of the hydrogen/steam mixture is then fed into a gasification reactor (GR), where the solid fuel is gasified (reformed). The remaining part of the hydrogen/steam mixture is then used in a combined cycle for power generation and/or undergos additional processing for the removal of condensate and production of high-purity hydrogen. Meanwhile, the gaseous fuel mixture resulting from the gasification process then flows to a gaseous FR, where the fuel off-gas mixture from the gasification process (e.g., H2, CH4, and CO) is oxidized into CO2 and steam by hematite (Fe2O3) particles. During this process, hematite is mostly reduced to

Table 4. Comparison of Performance Characteristics of SCL and Conventional Coal Gasification Processes70 conventional

SCL

coal to hydrogen IGCC (power only) SCL coal feed rate (tons/h)

132.9

132.9

132.9

132.9

CO2 capture (%)

90

90

100

100

H2 production (tons/h)

14.4

0

0

15.6

net power (MW)

2.1

321

365

26

thermal efficiency (% HHV)

57.8

32.1

36.5

64.1

Figure 11. Simplified process flow diagram for the IGCLC process.95

Figure 10. Simplified process flow diagram for the CDCL or BDCL processes.100 26


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wuestite, which is fed back to the SR reactor. The oxidation of wuestite particles in the SR reactor results in the formation of magnetite (Fe3O4) as well as the steam/hydrogen mixture. The solid product from the SR reactor (i.e., magnetite) is then transported to an AR, where magnetite particles are oxidized to form the hematite needed in the FR. The IGCLC process as outlined above has several key advantages over ex situ, in situ, and CLOU approaches described earlier. First, the solid fuel is able to transform to gaseous products in a thermoneutral manner, and as such, the gasification process does not require extra oxygen or air to generate heat by combusting a portion of the solid fuel. An ASU unit is also no longer necessary. Second, the direct contact between oxygen carrier particles and the solid fuel is avoided in the IGCLC process, and thus, unlike the CLOU or in situ methods, the deactivation of oxygen carrier particles by carbon deposits and ash is eliminated in the IGCLC process. This would minimize the likelihood of carbon combustion and, hence, unwanted CO2 formation in the AR unit, in turn, maximizing the overall CO2 capture efficiency of the system (note that ideally the exhaust gas stream from the AR unit should only contain N2, while that of the FR unit should be rich in CO2). The thermodynamic feasibility of the IGCLC process has recently been assessed through a comprehensive set of chemical equilibrium calculations carried out with the aid of the Aspen Plus process simulation software.95,107 It has been found that the IGCLC process is feasible, and there are no thermodynamic barriers to its operation. The use of the product gas (i.e., steam/ hydrogen mixture) from the SR unit for gasification of coal is a viable option, and the gasification process can be made autothermal (i.e., no need for external heat) provided that the reaction temperature is greater than 1023 K and the pressure is fixed at about 70 bar. The gasification temperature and, hence, the performance of the gasifier can be significantly improved by either increasing the hydrogen/steam to carbon ratio or preheating the steam/hydrogen mixture using the hot flue gas from the AR unit before the mixture is fed into the GR unit. The energy performance of an IGCLC-type plant is much greater than a conventional power station with or without CCS measures. The IGCLC on average delivers about 36% higher net plant efficiency than a conventional coal-fired power plant without any CCS measures. This value rises to about 80% if the conventional plant is retrofitted with CCS measures.95,107 Despite its attractive features, the implementation of the CLSR concept through a 3RCLSR-based scheme in a real industrial setting would inevitably require a large inventory of unit operations, including at least three reactors. Driven by the desire to lower the complexity of the CLSR process, Yang et al.92 proposed a novel CLSR-based method for the production of hydrogen in a single reactor, where the reduction of iron oxides is achieved by oxidization of coal char. The main objective of the study by Yang et al. was to verify the feasibility of direct hematite reduction by char (i.e., without a gasifying agent) based on CLC principles, H2 production by the steam/iron process, and the oxidation of Fe3O4 to Fe2O3 by normal air. To achieve this, Yang et al.92 employed an innovative fluidized-bed reactor shown schematically in Figure 12 and conducted a comprehensive set of experimental studies. By and large, the approach was found to be feasible. In particular, chars made from a Chinese lignite coal were used successfully as a reducing material for the reduction of Fe2O3 to FeO and Fe. However, no information was provided on

Figure 12. Schematic diagram of the Yang et al.92 experimental setup: (a) reaction system and (b) details of the fluidized-bed reactor.

technical issues associated with running three competing and somewhat complex reactions involving solid fuels and metal oxides in a single reactor (e.g., carbon deposition, ash effect, ash separation, etc.). Another group motivated by the desire to lower the hardware complexity of the CLSR process was Solunke and Veser,97 who studied the feasibility of using periodically operated pack-bed reactors. This approach, which parallels the SCOT and MSR methods described earlier in section 2, avoids oxygen carrier attrition associated with solid circulation by incorporating a set of periodically operated fixed-bed reactors, where gas rather than solid particles are exchanged between the chemical looping reactors. The approach adopted by Solunke and Veser97 conceptually allows for the complete decoupling of thermodynamics and kinetics given that the two half steps of the watergas shift reaction are conducted in different half cycles of the process. Solunke and Veser97 reported that their reactor configuration exhibited a remarkable robustness against changes in operating conditions, such as throughput and dilution of the steam feed. No major heat accumulation and hot spots were also detected in the reactor, although this requires further investigation because hot spot formations are inherent to fixed-bed reactors. Among the variants of the CLR concept, the use of the CLCAR has been rather limited primarily because of issues related to the mechanical integrity of sorbents. Of the relevant studies reported in the literature, the works and/or concepts by Acharya et al.,91 Moghtaderi,94 Wolf et al.,102 and Kulkarni et al.103 deserve special mention. A three-loop-reactor system for implementation of CLCAR was proposed and studied by Wolf et al.102 and Kulkarni et al.,103 where natural CaCO3/CaO sorbents were employed. As noted earlier, the above three-loop process features solid handling issues and limited flexibility in operational aspects of the reactors. To avoid these issues, Acharya et al.91 developed a single-loop CLCAR-based process for gasification of biomass and potentially other solid fuels (coal, MSW, etc). This process, which is referred to in this review as the sorbent chemical looping gasification (SCLG) process, is shown schematically in Figure 13. The singleloop configuration is much simpler than other chemical-loopingbased gasification processes,102,103 where two or more chemical loops and, consequently, more than two reactors are used. A single-loop configuration lowers operational cost and complexity, as well as the capital plant cost. 27


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Figure 13. Schematic diagram of the SCLG process for biomass fuels. The major unit operations are (1) bubbling-bed gasifier, (2) circulating fluidizedbed regenerator, and (3) cyclone.91

As seen in Figure 13, the centerpiece of the process is a set of two interconnected fluidized beds, comprising a bubbling fluidized gasifier, a circulating fluidized-bed regenerator, and a cyclone. Fuel gasification takes place in the gasifier unit in the presence of steam and particles of the CaO sorbent. The hydrogen-rich syngas product is then taken for power generation and/or further processing. Meanwhile, CaCO3 from the gasifier is transported to the CFB regenerator, where CaO is regenerated by calcinations of CaCO3 particles. CO2 gas and CaO are then separated out from each other in the cyclone. The theoretical energy analysis and experimental investigation of the above SCLG process showed that, under ideal conditions, the system efficiency can be as high as 87.49%, with biomass as fuel.91 A sensitivity analysis for the plant efficiency, however, revealed that, by varying carbon capture efficiency, the overall plant efficiency may drop to levels as low as 71%. The impact of lowering the regeneration efficiency was more severe because it was found that, at 80%, changing the regeneration efficiency from 100 to 50% lowers the overall plant efficiency to about 57%.91 Moghtaderi94 also adopted the SCLG approach and developed a chemical-looping-based gasification process (see Figure 14) for thermal conversion of carbon-based fuels to high-purity combustible gases (e.g., hydrogen, synthesis gas, and low-molecularweight hydrocarbons, such as methane). Moghtaderi’s process too is carried out in a cyclic fashion by continuous carbonation and calcination of a CO2 sorbent. Similar to the SCLG process by Acharya et al.,91 the carbonation subprocess (i.e., sorption of CO2) in Moghtaderi’s process94 is not only exothermic but also

Figure 14. Schematic representation of the CDW-based SCLG process.94

separates CO2 from other gaseous species formed during the gasification process. As a result, the process features an inherent ability to resolve the two undesirable characteristics of conventional gasification technologies, namely, (i) external heating to compensate for the endothermic nature of the gasification reaction and (ii) generating substantial quantities of greenhouse emissions, especially CO2 and CH4. Moreover, Moghtaderi’s process94 also features a key innovation, which enables it to make a more effective and robust use of the CO2 sorption process. Here, concrete and demolition waste (CDW) rather than naturally occurring calcium-based materials is used as the source of the CO2 sorbent. Natural forms of CaO or calcined rocks, which are predominately employed as sorbents in conventional gasification processes, are mineral catalysts that contain oxides of alkaline earth metals. They can be classified 28


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Figure 15. Yield of major gas species during the SCLG of MSW.94 Figure 16. Weight loss because of CO2 release during the SCLG of MSW.94

according to their CaO/MgO content into limestone (>50), dolomitic limestone (450), calcite dolomite (1.54), and dolomite (1.5). The activity of calcined rocks can be increased by increasing the Ca/Mg ratio, decreasing the grain size, and increasing the active metal content, such as iron. However, calcined rocks suffer from a range of problems during prolonged operations in SCLG-type systems, most notably particle attrition (because of weak mechanical strength) and deactivation.91 Given the variability and inhomogeneous nature of the organic- and fossil-based fuels, attrition and deactivation of calcined rocks would lead to greater demands for fresh CaO-containing sorbents, in turn, increasing the operational costs if the CO2 sorption process is employed in a SCLG-type process. These problems can be avoided using CDW, which exhibits very good reactivity (see below) and does not suffer from attrition issues associated with low mechanical strength. CDW is also cheap and readily available in large quantities all around the world. Moghtaderi and his team carried out a series of theoretical and experimental studies to assess the feasibility of the SCLG process and the use of CDW in it. Equilibrium calculations were performed using the software package COSILAB (Combustion Simulation Laboratory), version 2.3, developed by SoftPredict (Rotexo Beteiligungs-GmbH, Germany). Calculations were performed for MSW as a representative of inhomogeneous and difficult fuels. The chemical structure of MSW was assumed to be CH1.6O0.8 based on typical composition in Newcastle (Australia). The equilibrium results confirmed the feasibility of the CDWbased SCLG process and showed that endothermic gasification reactions can be sustained using the heat generated by exothermic reactions, such as watergas shift reaction, CO2 sorption (carbonation), and char combustion. Results also highlighted that the carbonation reaction (CaO + CO2 f CaCO3) limits the progress of the entire process. It was also found that maintaining a pressure difference between the gasifier and calciner was essential because of the increase in the equilibrium pressure of CO2 with an increasing temperature [P = 4.137 107 exp(20 474/T)]. As a result, the calcination process advances only if the partial pressure of CO2 is lower than its equilibrium pressure. Thus, the calcination process has to be carried out at the lowest possible pressures, while the carbonation reaction has to be conducted at high pressures (i.e., pressurized gasification). Experiments were also carried out to examine the kinetic barriers of MSW and CDW reactions in the SCLG process. Experiments were conducted in a TGA at atmospheric pressure over a range of temperatures between 650 and 1000 °C.94 Figure 15 shows the yield of major gas species (H2, CO, and CO2) as a function of the bed material during a typical

carbonationcalcination cycle at 750 °C. As seen, a bed of inert sand results in the lowest yield of H2 and highest yields of CO and CO2. A limestone bed, on the other hand, leads to the highest H2 yield and lowest CO and CO2 yields. This can be attributed to the fact that the catalytic effect of limestone shifts the equilibrium of the steam gasification and watergas shift reactions to the left, resulting in more H2 and CO2 production. However, much of the CO2 is adsorbed to the surface of the limestone through the carbonation of CO2 by CaO. The bed of CDW exhibits a very similar behavior to limestone, although because of its lower CaO content, the yields of H2, CO, and CO2 are different from those of limestone. The influence of the CaO content of the bed material on its effectiveness is demonstrated in Figure16, where the percentage of weight loss because of CO2 release is plotted against time during the calcination of pure bed materials (i.e., no char particle was present). Clearly, the limestone bed loses more weight, indicating that it had adsorbed more CO2 during its carbonation phase. However, the difference between limestone and CDW is not significant, and as Figure 15 illustrates, CDW provides very reasonable levels of H2, CO, and CO2 production. This combined with the fact that the effectiveness of CDW particles are not severely eroded over repeated carbonationcalcination cycles makes the CDW an attractive sorbent for the proposed process. However, the sorption capacities of CDW samples studied here appear to be less than their theoretical capacities. Theoretically, 1 kg of CDW with 67% CaO content should generate 1.2 kg of CaCO3, in turn, releasing 0.53 kg of CO2 during the calcination stage. This roughly equates to a weight loss of about 44%. However, the weight loss associated with CDW in Figure 15 is 20%, which is only 45% of the theoretical value. The discrepancy between the theoretical and actual sorption capacities can be partly attributed to the pore structure of CDW particles but more importantly to changes in the pore structure, morphology, and composition of particles during repeated carbonationcalcination cycles. Of other innovations in the area of CLR, which have been recently reported in the literature, the works by He et al.93 and Kale et al.108 should be briefly outlined here because of their potentially broader impact. He et al.93 studied the use of composite Ce, Fe, Cu, and Mn oxides as carrier particles in the CLR of methane. The motivation behind this study was to see if the strong redox properties of ceria-based oxygen carriers can be exploited in CLR applications. Composite ceria-based oxygen carriers for methane CLR modified with iron, copper, and 29


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Figure 17. Schematic representation of the CLCCR process.108

manganese oxides were prepared by co-precipitation, and their redox properties were investigated using TGA and fixed-bed reactor setups. The fresh and reacted composite oxygen carriers were also characterized using scanning electron microscopyenergy-dispersive spectrometry (SEMEDS) and X-ray diffraction (XRD) techniques. The experiments revealed that doping of ceria-based oxygen carriers with transition-metal oxides can improve the oxygen-donation capabilities of ceria. The CeFeO, CeMnO, and CeCuO oxygen carriers exhibited high CO and H2 selectivity at temperatures above 800 °C. In particular, the conversion of methane to syngas with a H2/CO molar ratio of about 2:1 was achieved with a CeFeO composite oxygen carrier at temperatures between 800 and 900 °C, while the CeMnO and CeCuO oxygen carriers were able to convert methane to syngas with a H2/CO molar ratio of much greater than 2:1 at temperatures around 850 °C. The composite Ce-based oxygen carriers showed higher reactivity than conventional transitional metal oxides typically used in CLR applications. Moreover, the reactivity of composite Cebased oxygen carriers did not degrade after repeated redox cycles. Kale et al.108 proposed a novel chemical looping scheme, which integrates a conventional CLC system with a combined reforming unit for generation of power and syngas from propane.

Figure 17 illustrates the block diagram of the CLC combined reforming (CLCCR) process proposed by Kale et al.108 The combined reforming unit is essentially a catalytic reactor that combines steam reforming, dry reforming, and partial oxidation for the production of a rich syngas product stream. As seen from Figure 17, the reactants for the combined reforming unit are fuel (propane here), oxygen, and the CO2/steam mixture originating from the outlet of the CLC unit. CaSO4 oxygen carriers were found to be the most suitable carriers for the CLCCR process. Thermodynamic assessment of the process verified its feasibility in the temperature range of 400783 °C at atmospheric pressure. It was found that the CLCCR process can generate syngas mixtures with a H2/CO ratio of 3 or more, which is extremely attractive for petrochemical manufacture. Moreover, the process can produce a considerable amount of heat in its CLC unit, which can be subsequently used for power generation. Thermodynamic analysis shows that the best operating condition for the combined reforming unit is the thermoneutral temperature of 702.12 °C, yielding 5.98 mol of syngas per mole of propane feed. 3.2. Other Polygeneration Schemes. One of the early chemical-looping-based schemes developed for combined hydrogen and power generation from coal with CO2 capture is 30


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Figure 18. Schematic of the (a) HyPr-RING concept and (b) simplified HyPr-RING process diagram.15

the HyPr-RING process developed and patented by the National Institute of Advanced Industrial Science and Technology of Japan (AIST).15 HyPr-RING is a process in which the CO2sorbent calcium hydroxide Ca(OH)2 is directly added into a coal gasifier and the CO2 generated during the gasification process is separated from the hydrogen product and fixed as CaCO3 (Figure 18). The HyPr-RING process is very similar to the CLCAR process outlined earlier in section 3.1. However, in the HyPr-RING process, the gasification is conducted in aqueous phase and the calcium hydroxide sorbent is produced in situ within the gasifier by reacting dry CaO with feedwater. Furthermore, much of the heat necessary for gasification of coal slurry in the HyPr-RING gasifier is provided by the exothermic reaction between CaO and H2O. In contrast, the CLCAR process is conducted under a non-aqueous environment, and the heat necessary for gasification is supplied by the exothermic reaction between CaO and CO2. The main reactions taking place in the gasification unit of the HyPr-RING process are CaO þ H2 O f CaðOHÞ2 , C þ H2 O f CO þ H2 , CO þ H2 O f CO2 þ H2 ,

109 kJ=mol

ðR8Þ

þ 132 kJ=mol

ðR9Þ

41:5 kJ=mol

CaðOHÞ2 þ CO2 f CaCO3 þ H2 O, C þ 2H2 O þ CaO f CaCO3 þ 2H2 ,

69 kJ=mol 88 kJ=mol

Figure 19. Schematic representation of the DCL concept inspired by the HyPr-RING process.89

slurry, and the recirculation gas-blown fluidized bed. The CO2 absorption unit and the gasifier are separated to recover pure CO2 as the sorbent is being regenerated. Heat management between the gasification and CO2 absorption reactors is quite feasible given that the exothermic and endothermic reaction heats can be exchanged fairly easily using conventional heat exchangers. However, preliminary results by Nakagaki89 suggest that the partial oxidation of coal at temperatures corresponding to CO2 absorption by lithium silicate may suffer from some limitations and/or technical barriers. Further studies on the oxygen carriers suitable for promoting the partial oxidation reaction of coal (e.g., composite oxides with lattice oxygen) are needed before the energy penalty for CO2 separation in a DCL system can be accurately estimated. One of the very few research and development studies on chemical looping polygeneration schemes for the production of products other than hydrogen and/or power is that by Zeman and Castaldi.109 These researchers proposed and studied the gasconditioning chemical looping (GCCL) process for the production of methanol and dimethyl ether from a mixture of methane and CO2.109 As shown in Figure 20, the GCCL process in effect combines a conventional methanol synthesis process with a gas CLC. The GCCL process has three main unit operations,

ðR10Þ ðR11Þ ðR12Þ

The overall reaction is exothermic, indicating that, in theory, there is no need for external heat. Also, CO2 fixation enhances reactions R9 and R10 for H2 generation. Inspired by the HyPr-RING process, Nakagaki89 proposed a dual chemical looping (DCL) version of the process, where, in the first loop, CO2 is captured by the novel lithium orthosilicate (Li4SiO4) sorbent and, in the second loop, coal is gasified and partially supplies the heat required for the first loop (Figure 19). Copper oxide is used as the oxygen carrier in the second loop. The CO2 absorption capability of lithium silicate has already been proven experimentally. The non-equilibrium characteristics of lithium silicate around 650 °C are particularly attractive for high-purity hydrogen production. The schematic of the DCL process is shown in Figure 19. The gasification subprocess is realized using reactants, such as steam, oxygen carriers, coal 31


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namely, the reformer, the chemical loop, and the fuel synthesis reactor (Figure 20). The process has been designed to produce syngas mixtures containing equaimolar CO and H2 components derived from a 1:1 mixture of CO2 and CH4.109 Gas conditioning is performed using the chemical loop, which is located between the reformer and the fuel synthesis reactor. The role of the chemical loop (i.e., gas-conditioning step) is to change the stoichiometry of the feed stream prior to reactants entering the synthesis reactor and to allow for any CO2 formed because of the watergas shift reaction to be retrieved and recycled back to the reformer for synthesis into methanol. The products of the metal oxide reduction reactions taking place in the chemical loop are all solid carbon, which has to be separated from reduced metal oxides. This is achieved by steam gasification of carbon deposits, which results in the formation of CO and additional H2.109 The CO2 and hydrogen produced during chemical looping gas conditioning and reforming of methane are forwarded as reactants to the fuel synthesis reactor, where methanol (and higher carbon fuels) is produced. Zeman and Castaldi109 performed a comprehensive analysis of the GCCL process under a variety of different scenarios and found out that, for the range of conditions considered, the emission profile of methanol produced via the GCCL process

varied between 0.475 and 1.645 mol of CO2 per mole of methanol. It was shown that the upper bound could be lowered to 0.750 by applying CCS and/or using renewable heat sources for the reforming process. While the study by Zeman and Castaldi109 was limited in scope, it nevertheless showed that the GCCL process provides an effective pathway to incorporate CO2 into fuels independent of electrolytic hydrogen.

4. ADVANCED CHEMICAL-LOOPING-BASED CONCEPTS AND TECHNOLOGIES TAILORED FOR NOVEL ENERGY-RELATED APPLICATIONS In addition to chemical looping processes and/or schemes developed in recent years for power generation or polygeneration applications (see Tables 2 and 3), several other chemicallooping-based processes have been proposed for more exotic and specific applications in the energy and mining sectors (Table 5). We commence our discussion here with the process proposed and studied by Fan et al.70 Fan and his team at The Ohio State University (OSU) in collaboration with researchers from OSU are developing a process that combines a chemical loop with a solid oxide fuel cell. This process, which is shown schematically in Figure 21, integrates a coal direct chemical looping reducer with a direct carbon fuel cell. In this system, referred to as chemical looping solid oxide fuel cell (CLSOFC), reduced metal particles from the chemical looping reducer are directly fed into a solid oxide fuel cell capable of direct processing of solids. Reduced particles are then introduced into the fuel cell, where they react with oxygen or air at temperatures between 500 and 1000 °C. The loop is completed by recycling the oxidized particles back to the reducer. Preliminary studies of the CLSOFC process show that the system can produce 45 mA/cm2 at 0.4 V when Fe-supported metal oxides are employed at 800 °C. While the results are very promising and exciting, much work is still needed to verify the techno-economic viability of the process.70 We now turn our focus to chemical looping processes under development at the Centre for Energy at The University of

Figure 20. Principles of fuel synthesis via GCCL.109

Table 5. Advanced Chemical-Looping-Based Concepts/Technologies Tailored for Novel Energy-Related Applications chemical looping concept/technology chemical looping solid oxide fuel cell (CLSOFC) chemical looping air separation (CLAS)

distinctive features/innovation combines a chemical loop with a solid

feedstock coal

application and/

operating

or product

conditionsa T 5001000 °C;

fuel cells

Fan et al.70

P not specified

oxide fuel cell provides the first chemical-looping-based

reference(s)

high-purity oxygen T up to 1000 °C;

air

Moghtaderi22

P atmospheric

concept for air separation; uses steam as the reducing medium

integrated chemical looping air separation (ICLAS)

CLAS-based process specifically tailored

air

oxygen and CO2 mixture

for oxy-fuel power plants; uses recycled

T up to 1000 °C; P atmospheric

Moghtaderi et al.110

flue gas as the reducing medium chemical looping removal of

provides two alternative chemical-looping-

ventilation air methane

based processes for neutralization of the

(CLR-VAM)

ventilation air methane in mining operations

miniaturized chemical looping first miniaturized chemical looping steam steam reforming (μCLSR)

ventilation air methane (VAM) liquid or

reformer for hydrogen enrichment of liquid

gaseous fuels

VAM destruction in mining

T up to 1400 °C; P atmospheric T 700900 °C;

on-board fuel upgrading

Moghtaderi111

Moghtaderi58

P atmospheric

or gaseous fuels; incorporates the manifold switching reactor concept a

Only for chemical looping unit operations. 32


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Figure 22. Schematic of the CLAS process.22 Figure 21. Schematic of the CLSOFC concept.23

method, known as chemical looping air separation (CLAS),22 is a general technique for tonnage production of high-purity oxygen, while the second method [integrated chemical looping air separation (ICLAS)]110 is a modified version of the CLAS process specifically tailored for oxy-fuel applications. When the concept of oxygen decoupling is incorporated into the two-step redox reaction, CLAS is able to separate oxygen from normal air.22 As Figure 22 illustrates, the CLAS process works in a cyclic fashion by continuous recirculation of metal oxide particles between a set of two interconnected rectors, where oxidation (O2 coupling; see reaction R13) and reduction (O2 decoupling; see reaction R14) of carrier particles take place. During this cyclic process, oxygen is taken from air in one reactor, carried by particles, and then released in a second reactor. The system therefore consists of two reactors linked together through a loop seal to prevent gas leakage from one reactor to another. Air is fed into the oxidation reactor, so that the incoming reduced carrier particles can be regenerated to a higher oxidation state. The regenerated carrier particles, in turn, are transported back to the reduction reactor, where oxygen decoupling occurs in the presence of steam. The mixture of steam and oxygen exiting from the reduction reactor is passed through a condenser, so that steam can be fully separated from O2. The product oxygen can be then compressed for storage and delivery or directly fed to another process for on-site use.

Newcastle, Australia (Table 5). These processes are being developed as part of a broad R&D portfolio of chemical looping technology options for the energy and mining sectors. Much of the relevant research activities are being supported by the NSW Clean Coal Council, the ANLEC R&D, Xstrata Coal Pty Ltd., Newcastle Port Corporation, Moits Pty Ltd., and the Australian Research Council. 4.1. Air Separation22. Oxygen is the second largest volume chemical produced in the world, with a 30% share of the global industrial gas market. The global demand for oxygen in 2011 is forecast to be 950 billion cubic meters, with an annual growth rate of about 6%. It has major commercial applications in the metallurgical industry, chemical synthesis, glass manufacturing, pulp and paper industry, petroleum recovery/refining, and health services. Emerging markets for oxygen include advanced power generation systems, such as IGCC, oxy-fuel combustion, and solid oxide fuel cells (SOFCs). Among these, oxy-fuel combustion is particularly an attractive low-emission technology because of its inherent ability for in situ separation of CO2. However, oxyfuel combustion requires oxygen and, thereby, an ASU to function effectively. Conventional ASU units (e.g., cryogenic systems) may consume between 10 and 40% of the gross power output of a typical oxy-fuel plant and constitute 40% of the total equipment cost (about 14% of the total plant cost). Oxygen is commonly produced at industrial scales by air separation using cryogenic distillation and adsorption-based technologies [PSA and vacuum-PSA (VPSA)]. Advanced technologies, such as membrane separation [e.g., ion-transport membrane (ITM)] and in situ air separation, are also being developed for small-volume point-of-use oxygen generation. Cryogenic processes are generally expensive, owing to the energy intensity of their air compression subprocess. Similar to the cryogenic methods, air compression is a key step in the adsorption-based air separation methods, and as such, the specific power consumptions of PSA and VPSA plants are not much lower than their cryogenic counterparts. Membranes have been in commercial use for several decades, but much of their past applications have been in liquidliquid and liquidsolid separation. The use of membranes for large volumetric gas flow rates, such as those in air separation, has not yet been demonstrated. Membrane systems also suffer from a high cost of manufacture. There is a need for a simpler and more cost-effective air separation technology with a much smaller energy footprint and lower capital cost than conventional and emerging air separation methods. In recognition of this need, our group has been developing two chemical-looping-based methods for air separation. The first

Mex Oy2 ðsÞ þ O2 ðgÞ f Mex Oy ðsÞ oxidation or O2 coupling

ðR13Þ Mex Oy ðsÞ f Mex Oy2 ðsÞ þ O2 ðgÞ reduction or O2 uncoupling

ðR14Þ From an energy-efficiency point of view, the CLAS process is quite efficient because of its low energy demands. This is partly due to the fact that the theoretical net heat released over reactions R13 and R14 is zero. Therefore, in theory, the heat transported by the incoming carrier particles into the reduction reactor must be sufficient to support the endothermic reaction R14. Furthermore, under steady-state operation, much of the heat required for the production of steam and preheating of air is offset by the heat contents of the superheated steam stream leaving the reduction reactor and the reduced air stream exiting from the oxidizer. As Figure 22 shows, this is achieved by exchanging (i) the sensible heat between various streams in a series of heat exchangers and (ii) the latent heat of phase change in a combined steam condenser/boiler unit. The additional thermal energy required to carry out the CLAS process can be 33


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oxygen (from the ASU), and recycled flue gas are co-fed into the boiler and the mixture is combusted at high temperatures. The heat generated from the combustion process runs a steam cycle, which, in turn, converts the thermal energy into electricity. The use of recycled flue gas here is an important and integral part of the oxy-fuel combustion process because firing pure oxygen in a boiler would result in excessively high flame temperatures, which may damage the boiler. Therefore, the mixture must be diluted by mixing with recycled flue gas before it can be fed into the boiler. Given the need for recycled flue gas in oxy-fuel combustion and considering the high energy demand for steam generation in a CLAS-type process, recycled flue gas rather than steam is employed in the ICLAS process during the reduction phase (Figures 23 and 24). This innovative use of the recycled flue gas in the ICLAS process (i) lowers the overall energy footprint of the air separation process and, hence, operational costs to levels well below those of the CLAS process, (ii) simplifies the hardware required for chemical looping air and, thereby, reduces the capital cost for the ASU in an oxy-fuel power plant, and (iii) leads to a more effective integration of the ASU with the oxy-fuel plant because of better use of material and energy streams (see Figure 24). The working principle of the ICLAS process is similar to that of the CLAS process, but the ICLAS process is executed in a distinctly different way. Both CLAS and ICLAS processes works in a cyclic fashion by continuous recirculation of metal oxide particles between a set of two interconnected rectors, where oxidation (O2 coupling; see reaction R13) and reduction (O2 decoupling; see reaction R14) of carrier particles take place. In both processes, air is first fed into the oxidation reactor for separation of oxygen from air through the oxygen coupling

provided by electrical power. Our mass and energy balance calculations15 carried out using the HYSYS process simulation package suggest that the heat/power demand for the CLAS process is much lower than that required in cryogenic systems. The specific power for the CLAS process varies between 0.041 and 0.053 kWh/m3n, with an average value of 0.045 kWh/m3n. This is about 11% of the specific power of a conventional cryogenic system (i.e., 0.4 kWh/m3n). More advanced cryogenic systems set to enter the market by 2012, however, are expected to approximately reach a specific power of 0.3 kWh/m3n. Such specific powers are still 7 times greater than the average specific power for the CLAS process. The ICLAS process110 is a step-change improvement over the CLAS process and has been specifically tailored for ease of integration with oxy-fuel-type power plants running on organic(e.g., biomass) or fossil- (e.g., coal, gas, oil, etc.) based fuels. For example, consider an oxy-fuel coal-fired power plant, where coal,

Figure 23. Schematic of the ICLAS process.110

Figure 24. Schematic of an oxy-fuel coal-fired power plant retrofitted with an ICLAS unit.110 34


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process (i.e., regeneration of reduced carrier particles) and then the oxidized (i.e., regenerated) particles are reduced in a reduction reactor to release oxygen via the oxygen decoupling reaction. However, in the ICLAS process, the reduction and, hence, oxygen decoupling process takes place in the presence of recycled flue gas and not steam. The mixture of oxygen and recycled flue gas exiting the reduction reactor is then directly fed into the boiler of the oxy-fuel plant (Figure 24). The use of flue gas rather than steam not only eliminates the need for steam generation but also implies that condenser units for separation of O2 from steam are no longer required. This reduces the number of unit operations and, thereby, capital cost, as well as operational and running costs. From the energy-efficiency point of view, the heat transported by the incoming carrier particles into the reduction reactor is sufficient to support the endothermic oxygen decoupling process. In practice, though, some heat must be supplied to the reduction reactor to compensate for heat losses to the surroundings. However, unlike the CLAS process, no additional heat is also required for generation of superheated steam in the ICLAS process. Moreover, much of the required heat duty is offset by using the flue gas stream, which is already hot. Our preliminary calculations110 suggest that the heat demand for the ICLAS process is ≈0.03 kWh per cubic meter of oxygen produced (i.e., 0.03 kWh/m3n), which is about 30 and 90% less than those of the equivalent CLAS- and cryogenic-type processes, respectively. The successful execution of the CLAS and ICLAS processes largely depends upon oxygen carriers capable of reacting reversibly with gaseous oxygen at high temperatures. This additional thermodynamic constraint is a means of differentiating oxygen carriers feasible for the CLAS and ICLAS processes from those only suitable for common redox applications. Numerous studies have been carried out on oxides of transitional metals, such as Fe, Cu, Co, Mn, and Ni, as potential candidates for redox applications. Thermodynamically, oxides of Cu, Mn, and Co are more promising for oxygen decoupling (i.e., CLAS and ICLAS processes) because of their ability to reversibly react with oxygen. However, any other metal oxide, solid oxides, or their mixtures with reversible oxygen decoupling properties can be employed in the CLAS and ICLAS processes. 4.2. VAM111. Release of fugitive methane (CH4) emissions from ventilation air in coal mines is a major source of greenhouse gas (GHG) emissions (the greenhouse impact of methane is 21 times greater than that of CO2). Approximately 64% of methane emissions in coal mine operations are the result of VAM. There are two alternative strategies for mitigation of VAM in mining operations, namely, (i) use of VAM as an energy source and (ii) destruction of VAM through an oxidation process. To date, the implementation of the former strategy has found to be difficult primarily because (1) the volume of the gas mixture is large (can be as high as 600 m3/s), (2) the methane concentration in the mixture is dilute (0.11%, v/v), and (3) the concentration of methane and the flow rate of the gas mixture are variable. The latter strategy (i.e., VAM destruction) has been found more attractive and easier to adopt, although technologies based on the VAM destruction approach are technically feasible when the methane concentration in air exceeds a minimum requirement and economic performance is not an issue. Such systems often need additional fuel for continuous operation. Current estimates suggest that the additional fuel intake may be at least 0.9% to maintain the methane concentration at suitable levels for prolonged operations. Examples of VAM mitigation systems based

Figure 25. Schematic of the 3S-CLRVAM process.111

on the destruction strategy are120,121 (1) TFRR (VOCSIDIZER, MEGTEC), (2) CFRR (CANMET), (3) catalytic monolith reactor (CMR; CSIRO), (4) catalytic lean-burn gas turbine (CLBGT; CSIRO, Ingersol-Rand), (5) recuperative lean-burn gas turbine (RLBGT; EDL), and (6) ventilated air methane regenerative after burner (VAM-RAB; Corkys Pty Ltd.). The processes under development at The University of Newcastle for chemical looping removal of ventilation air methane (CLRVAM) provide an advanced technology platform for treatment of VAM and can be employed as part of any use and/or destruction mitigation strategies. These chemical-looping-based methods do not suffer from the shortcomings outlined earlier and, hence, are prospective technology options for mitigation of VAM. While preliminary theoretical and experimental research initiatives in this area have led to promising results,111 more comprehensive R&D activities are needed to support its feasibility. Broadly speaking, the CLRVAM process comprises three main steps, namely, (1) hydrogen production, (2) combustion of VAM in the presence of hydrogen, and (3) regeneration of metal oxides. These steps can be executed by employing one of the following two alternative approaches: (1) a three-step chemical loop (3SCLRVAM) or (2) a two-step chemical loop (2S-CLRVAM). The former leads to a low/medium-temperature oxidation process during the combustion of the VAM/H2 mixture, while the latter results in high-temperature combustion. The 3S-CLRVAM is more complex but has a much lower energy footprint than the 2S-CLRVAM version. These alternative approaches are described below. 4.2.1. 3S-CLRVAM. A three-step chemical loop is incorporated into the 3S-CLRVAM process to fully integrate the hydrogen production, combustion, and regeneration steps (Figure 25). Because of the need for high-purity hydrogen, the 3S-CLRVAM process can work best with metals with multiple oxidation states. Among these, iron (Fe) is preferred primarily because (i) Fe has three oxides, namely, wuestite, hematite, and magnetite, (ii) Febased oxides are abundant in nature and are of low cost, (iii) oxides of Fe show relatively good redox properties, (iv) oxides of Fe have good mechanical and thermal stability, and (v) Fe-based oxides are nontoxic and environmentally benign. The main goal in step 1 is to produce high-purity hydrogen. The working principle of the chemical-looping-based process 35


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employed in this step is based on the cyclic reduction and oxidation of suitable metal oxide oxygen carrier particles, for example, FeO to Fe3O4. This is carried out by exchanging the oxygen carrier particles between the three interconnected reactors involved in the process, as shown schematically in Figure 25. In step 2, the mixture of H2 transferred from the hydrogen generator and the incoming VAM stream are combusted together in the presence of metal oxides. This ensures non-flaming oxidation of the fuel mixture at moderate to low temperatures (e.g., 500600 °C). Also, hydrogen is used in step 2, so that the overall fuel/oxygen ratio [i.e., (H2 + CH4)/O2] is increased. This ensures the complete combustion of the fuel/air mixture in a robust manner, leading to generation of CO2 and H2O. Step 2, as shown in Figure 25, is carried out in the presence of fully regenerated metal oxide particles, which are at their highest oxidation state (i.e., hematite, Fe2O3, in the case of iron). Much of the oxygen required for the combustion process (i.e., fuel oxidation) is provided by MeO(III) because they are more reactive than air. However, additional oxygen may be consumed from the ventilation air, and as a result, slight reduction in the O2 concentration of the ventilation air may be observed. During step 2, MeO(III) is reduced to MeO(I), which is the lowest oxidation state for the metal employed in the process. For example, Fe2O3 (hematite) reduces to FeO (wuestite) in the case of iron. The reduced metal, MeO(I), is then fed back to the hydrogen generator. Step 2 provides a very effective means of dealing with variations and/or fluctuations in the methane concentration. For instance, when the methane concentration in VAM is low, the addition of H2 shifts the oxidation process toward full completion. While for situations where the methane concentration in VAM increases to levels close to the explosion limit, the flows of H2 and MeO(III) can be stopped and steam is redirected to the combustor rather than the hydrogen generator. This not only brings the hydrogen production to an end but also significantly dilutes the concentration of the fuel/air mixture in the combustor, ultimately preventing any potential explosion. In step 3, the gaseous mixture exiting from the combustor and particles of MeO(II) (e.g., Fe3O4 in the case of iron) from the hydrogen generator are fed into the third reactor (regenerator). Here, the metal at its intermediate oxidation state is oxidized by the incoming air/steam/CO2 mixture from the combustor and reaches its highest oxidation state. 4.2.2. 2S-CLRVAM. In step 1 of the 2S-CLRVAM process, hydrogen is generated by chemical looping steam reforming (H2O + Me f MeO + H2), although a metal oxide with reversible reduction/oxidation (redox) properties is preferred (e.g., Co, Mn, and Cu). The main goal in step 1 is to produce high-purity hydrogen. The working principle of the chemicallooping-based process employed in this step is based on the cyclic reduction and oxidation of suitable metal oxide oxygen carrier particles (e.g., Cu/CuO). This is typically carried out by exchanging the carrier particles between two fluidized-bed reactors. In step 2, H2 is transferred from the hydrogen generator to the combustor, so that the overall fuel/air ratio [i.e., (H2 + CH4)/air] is increased and brought to levels close to the minimum flammability limit. Similar to the three-step version, this ensures the complete combustion of the fuel/air mixture in a robust manner, leading to generation of CO2 and H2O and slight reduction in the O2 concentration of the ventilation air (i.e., reduced air). This step provides a very effective means of dealing with variations and/or fluctuations in the methane concentration. For instance,

Figure 26. Schematic of the 2S-CLRVAM process.111

when the methane concentration in VAM is below the lower flammability limit, the addition of H2 shifts the oxidation process toward the generation of a stable flame. While for situations where the methane concentration in VAM increases to levels close to the explosion limit, the flow of H2 can be stopped simply by directing steam to the combustor rather than the hydrogen generator. This not only brings the hydrogen production to an end but also significantly dilutes the concentration of the fuel/air mixture in the combustor, ultimately preventing any potential explosion. In step 3, the gaseous mixture exiting from the combustor and the MeO from the hydrogen generator are fed into the third reactor (regenerator). Here, the reduced air is enriched by decoupling the oxygen from MeO simply by controlling the partial pressure of oxygen using a process similar to that used in the CLAS process. This ensures that the mixture exiting the regenerator only contains normal air, steam, and carbon dioxide. The steam in the outlet can be separated out by condensation, and the resulting water can be reused as make-up water in the steam generation process (not shown in Figure 26). 4.3. Miniaturized Reformers for On-Board Fuel Upgrading58. The hydrogen enrichment of gaseous and liquid fuels on-board stationary and mobile combustion systems is a topic of significant interest in many applications, such as internal combustion engines (ICEs), liquid- and/or gas-fired boilers, gas turbines, and microcombustors (for potential use in medical devices, military equipment, cell phones, notebook computers, and other similar electronic devices). The interest in hydrogen enrichment of fuels is largely driven by the fact that the addition of small quantities of hydrogen into a premixed fuel mixture can greatly extend the lean and dilution limits of the mixtures with simultaneous benefits of increasing the overall thermal efficiency and reducing emissions. For example, in the case of ICEs, the hydrogen enrichment of the air/fuel mixture can improve the overall thermal efficiency of the engine by as much as 20% and lower its NOx and CO2 emissions by 98 and 20%, respectively.122,123 The improvements in thermal efficiency and emission reductions because of hydrogen enrichment can greatly 36


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Figure 27. Schematic representation of the miniaturized CLSR fuel reformer.58

help to reduce the GHG footprints of many industrial sectors that primarily rely on combustion-based systems. For instance, according to the International Energy Agency, the use of ICEs in the transportation sector alone is responsible for 18.4% of the global CO2 emissions and about 22% of the national GHG inventory in Australia. If the hydrogen-enrichment concept is deployed across even 10% of the transportation sector, the global GHG emissions can be reduced by 100 million tons per year, which is equivalent of removing 20 million average cars off the roads each year. While the concept of on-board hydrogen enrichment of fuels has been investigated in the past, because of the unresolved technical issues associated with on-board storage of hydrogen, a variety of on-board fuel reformers have been developed for in situ production of H2.58 However, these on-board reformers achieved limited success primarily because of (i) the low purity of the product gas stream in terms of its H2 content, (ii) unwanted sudden changes in the rate and composition of the product gas stream because of perturbations in the waste heat recovery process under load, and (iii) large physical dimensions and heavy weights of conventional catalytic reformers.58 The fuel reforming technology under development at The University of Newcastle (Australia) is an attempt to resolve the above shortcomings by integrating the principles of chemical looping steam reforming and process miniaturization into a unified platform. The miniaturized chemical looping steam reforming (μCLSR)-based fuel reformer under development in Newcastle is quite novel and, to the best of author’s knowledge, has neither been investigated in the past nor been studied anywhere in the world. The system, as shown schematically in Figure 27, incorporates a pair of identical microreactor assemblies, similar to that shown in Figure 28, each comprising 272 microreactors packed with metal oxide particles.58 Unlike conventional chemical looping systems, carrier particles are not circulated between the two reactor assemblies and the cycle is completed by switching over the steam and fuel inlet streams from one reactor assembly to the other (i.e., MSR approach).58 Each assembly, therefore, functions periodically as a FR and SR. The changeover of the flow is achieved using a manifold switching system (dashed arrows in Figure 27).58 Fuel heater and steam generator units have also been fitted to the reformer upstream of the manifold switching system. The fuel heater is used to preheat the fuel and/or convert any liquid fuel to vapor. The required fuel for the process is obtained from the main combustion system to which the chemical looping reformer is attached (e.g., an ICE). The pure H2 produced by the fuel reformer is directed to the main combustion system for on-board use. The mixture of CO2 and steam, which is inherently separated from H2 in the product

Figure 28. Picture of a prototype microreactor assembly.58

gas stream, can be further processed in several alternative ways. For example, steam can be separated from CO2 by condensation in a heat exchanger, where fuel is preheated. The separated CO2 can then be exhausted or captured by chemicals in an absorber. The condensed water can also be recycled for steam generation. Alternatively, CO2 in the CO2/steam mixture can be separated first in an absorber, and then the CO2-free mixture is recycled to the CLSR process.58 The design of the microreactor assemblies of the fuel reformer is based the so-called plate/stack architecture, where microreactors are embedded in a number of planner structures (i.e., plates) stacked on top of each other to form the reactor assembly. To achieve a desired throughput, the reformer can be scaled up or down simply by changing the number of reactor plates in each stack or alternatively the number of pairs of microreactor assemblies.58 The author has successfully used these types of plate/stack systems in his past studies.124,125 The configuration most suited to the miniaturized CLSR concept is the zigzag shape to provide a better contact between particles and reacting gases and, hence, optimize the residence time and physical dimensions. The miniaturized reactor assembly in Figure 28 incorporates the above configuration and comprises 272 zigzag-shape microreactors (0.05 m long with a 200 100 μm cross-sectional area) organized in 16 reactor plates, each with 17 microreactors. Experiments were conducted on the Fe3O4/FeO metal oxide system under pure methane and pure steam environments in a TGA and a prototype-miniaturized microreactor. Experimental results show that, during a typical fuel oxidation step, the concentration of methane in the product gas stream gradually decreases while Fe3O4 is being reduced to FeO. However, on or about a fractional conversion of 60%, the slope of the CH4 plot sharply increases because of catalytic effects of FeO on methane decomposition. Similarly, the H2 plot associated with the steamreforming step picks up rapidly and reaches a maximum of 98% at a fractional conversion of 30%. The conversion times of steam and fuel in the microreactor were generally shorter than conversion times obtained in the TGA system. The experimental results provided two vital pieces of information: (i) the chemical looping steam-reforming cycle is technically viable, and (ii) the performance of the process at microscales needs to be further understood before high-throughput miniaturized reformers could be designed and built. 37


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5. CONCLUSION Chemical looping is a versatile reaction engineering concept that can be adopted in a wide range of applications, particularly those in the general field of energy and fuel technology. Despite their attractiveness, the majority of chemical-looping-based technologies are currently in the R&D phase, with no demonstration and/ or commercial plants in operation. The current status will likely remain unchanged unless a number of outstanding issues that hamper the widespread deployment of chemical looping technologies are resolved. These are (1) technical issues associated with large-scale chemical looping operations, most notably particle transport, oxygen carriers, and heat management, (2) insufficient understanding of the process design, scale-up, and optimization of chemical looping processes for large-scale systems,70 (3) inadequate focus on chemical looping technologies for conversion of organic- and fossil-based solid fuels, especially coal, (4) insufficient attention in exploring and developing polygeneration chemical looping schemes, and (5) lack of interest in exploiting opportunities where chemical looping systems can be integrated and/or retrofitted to existing unit operations and/or plants rather than being used simply as replacement technology options. While major R&D efforts are required to resolved the above issues, as this review shows (see Tables 2, 3, and 5), the right steps are being undertaken by the research community, industry, and funding agencies around the world to accelerate the large-scale deployment and commercialization of chemical looping technologies. As noted in a number of review papers, valuable research work is being undertaken in developing more effective, environmentally friendly, durable, and cheaper oxygen carriers.47,69,70 In this context, the focus on composite and mixed oxygen carriers is of significant importance because it opens up new pathways for designer-made oxygen carriers with superior performance. The issues of particle transport and circulation are also being addressed through direct fundamental research on particle technology aspects of chemical looping processes as well as developing novel reactor concepts, such as SCOT and MSR (see Table 2). The issues related to heat management by and large remain unresolved, and closer attention is required in this area, perhaps through more fundamental thermofluid research on exergetic optimization of chemical looping processes. The approach adopted by Chakravarthy et al.80 is quite valuable in this regard. The process design, scale-up, and optimization issues still remain unanswered, although by far, we have a greater understanding of these issues thanks to the valuable knowledge gained through excellent pilot-scale studies by the Vienna University of Technology,60,61 Chalmers University of Technology,62 the Instituto de Carboquimica in Spain,63 and the Korea Institute of Energy Research,64 who have been carrying out comprehensive chemical looping campaigns over prolonged operational hours in the past few years. Pleasingly, there has been a considerable shift in focus toward developing chemical looping technology options for solid fuels, particularly coal. As discussed in this review, numerous research undertakings are currently underway all around the world to develop novel chemical looping schemes or tailor existing schemes for coal conversion. What is even more encouraging is that a great majority of these coal-focused chemical looping processes are being developed as polygeneration concepts, where more than one product is produced. Often the production of H2

and power underpins these polygeneration concepts, and for this reason, CLR-based technologies has attracted most of the attention (see Table 3) given that hydrogen production can be maximized when reforming processes (particularly steam reforming) are employed. Unfortunately, much less attention has been paid in recent years to exploit the synergies between the established technologies and chemical looping concepts and to investigate if chemical looping systems can be integrated and/or retrofitted to existing industrial plants. Chemical looping processes, such as CLSOFC,70 ICLAS,110 and μCLSR,58 are among a handful of examples in which the above synergies have been fully exploited. Clearly, more needs to be done in this area.

’ AUTHOR INFORMATION Corresponding Author

*Telephone: +61-2-4985-4411. Fax: +61-2-4921-6893. E-mail: [email protected].

’ ACKNOWLEDGMENT The author acknowledges the financial support of The University of Newcastle (Australia), the NSW Clean Coal Council, the ANLEC R&D, Xstrata Coal Pty Ltd., Newcastle Port Corporation, Moits Pty Ltd., and the Australian Research Council (ARC Linkage Grant LP100200871, 20102012) for the work presented in this paper. ’ REFERENCES (1) Birol, F. World Energy Outlook 2010; International Energy Agency: Paris, France, 2010. (2) Metz, B.; Davidson, O. R.; Bosch, P. R.; Dave, R.; Meyer, L. A. Climate Change 2007: Mitigation of Climate Change, Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change; Cambridge University Press: Cambridge, U.K., 2007; ISBN: 978-0-521-88011-4. (3) Lyngfelt, A. Oil Gas Sci. Technol. 2011, 66 (2), 161–172. (4) Hossain, M.; de Lasa, H. I. Chem. Eng. Sci. 2008, 63, 4433–4451. (5) He, F.; Li, H.; Zhao, Z. Int. J. Chem. Eng. 2009No. 710515, DOI: 10.1155/2009/710515. (6) Jin, H. G.; Hong, H.; Han, T. Chin. Sci. Bull. 2009, 54 (6), 906–919. (7) Fan, L. S. Chemical Looping Systems for Fossil Fuel Conversions; John Wiley and Sons: New York, 2010; ISBN: 978-0-470-87252-9. (8) Ishida, M.; Zheng, D.; Akehata, T. Energy 1987, 12, 147–154. (9) Gilliland, E. R. Production of industrial gas comprising carbon monoxide and hydrogen. U.S. Patent 2,671,721, 1946. (10) Lewis, W. K.; Gilliland, E. R.; Sweeney, M. P. Chem. Eng. Prog. 1951, 47, 251–256. (11) Lewis, W. K.; Gilliland, E. R. Production of pure carbon dioxide. U.S. Patent 2,665,972, 1954. (12) Richter, H.; Knoche, K. ACS Symp. Ser. 1983, 235, 71–85. (13) Ishida, M.; Jin, H. Energy 1994, 19, 415–422. (14) Lin, S. Y.; Suzuki, Y.; Hatano, H.; Harada, M. Proceedings of the 16th International Pittsburgh Coal Conference; Pittsburgh, PA, 1999. (15) Lin, S.; Suzuki, Y.; Hatano, H. Japanese Patent 29791-49, 1999. (16) Lin, S.; Suzuki, Y.; Hatano, H.; Harada, M. Energy Convers. Manage. 2002, 43, 1283–1290. (17) Copeland, R. J.; Alptekin, G.; Cesario, M.; Gebhard, S.; Gershanovich, Y. A novel CO2 separation system. Proceedings of the First National Conference on Carbon Sequestration; Washington, D.C., 2001. (18) Lyngfelt, A.; Mattisson, T. Trestegsfo rbranning for avskiljning av koldioxid. Swedish Patent, 2005. 38


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