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Chemical-looping combustion (CLC) is a combustion technology with inherent separation of the greenhouse gas CO2. The technique involves the use of a metal oxide as an oxygen carrier which transfers oxygen from combustion air to the fuel, and hence a direct contact between air and fuel is avoided. Two inter-connected fluidized beds, a fuel reactor and an air reactor, are used in the process. In the fuel reactor, the metal oxide is reduced by the reaction with the fuel and in the air reactor; the reduced metal oxide is oxidized with air. The outlet gas from the fuel reactor consists of CO2 and H2O, and almost pure stream of CO2 is obtained when water is condensed. Considerable research has been conducted on CLC in the last years with respect to oxygen carrier development, reactor design, system efficiencies and prototype testing. In 2002 the process was a paper concept, albeit with some important but limited laboratory work on oxygen carrier particles. Today more than 600 materials have been tested and the technique has been successfully demonstrated in chemical-looping combustors in the size range 0.3 - 50 kW, using different types of oxygen carriers based on the metals Ni, Co, Fe, Cu and Mn. The total time of operational experience is more than a thousand hours. From these tests it can be established that almost complete conversion of the fuel can be obtained and 100% CO2 capture is possible. Most work so far has been focused on gaseous fuels, but the direct application to solid fuels is also being studied. Moreover, the same principle of oxygen transfer is used in chemical-looping reforming (CLR), which involves technologies to produce hydrogen with inherent CO2 capture. This essay presents an overview of the research performed on CLC and CLR highlights the current status of the technology.
Chemical looping combustion (CLC) typically employs a dual fluidized bed system (circulating fluidized bed process) where a metal oxide is employed as a bed material providing the oxygen for combustion in the fuel reactor. The reduced metal is then transferred to the second bed (air reactor) and re-oxidized before being reintroduced back to the fuel reactor completing the loop.
Isolation of the fuel from air simplifies the number of chemical reactions in combustion. Employing oxygen without nitrogen and the trace gases found in air eliminates the primary source for the formation of nitrogen oxide (NOx), producing a flue gas composed primarily of carbon dioxide and water vapor; other trace pollutants depend on the fuel selected.
Chemical looping combustion (CLC) uses two or more reactions to perform the oxidation of hydrocarbon based fuels. In its simplest form, an oxygen carrying species (normally a metal) is first oxidised in air forming an oxide. This oxide is then reduced using a hydrocarbon as reducer in a second reaction. As an example, a nickel based system burning pure carbon would involve the two redox reactions:
2Ni(s) + O2(g) â†’ 2NiO(s)
C(s) + 2NiO(s) â†’ CO2(g) + 2Ni(s)
If (1) and (2) are added together, the reaction set reduces to straight carbon oxidation - the nickel acting as a catalyst only i.e.:
C(s) + O2(g)â†’ CO2(g)
CLC was first studied as way to produce CO2 from fossil fuels, using two interconnected fluidized beds. Later it was proposed as a system for increasing power station efficiency. The gain in efficiency is possible due to the enhanced reversibility of the two redox reactions; in traditional single stage combustion, the release of a fuel's energy occurs in a highly irreversible manner - departing considerably from equilibrium. In CLC, if an appropriate oxygen carrier is chosen, both redox reactions can be made to occur almost reversibly and at relatively low temperatures. Theoretically, this allows a power station using CLC to approach the ideal work output for an internal combustion engine without exposing components to excessive working temperatures.
The figure below illustrates the energy exchanges in a CLC system graphically, and shows a Sankey diagram of the energy fluxes occurring in a reversible CLC based engine. Studying this figure, a heat engine is arranged to receive heat at high temperature from the exothermic oxidation reaction. After converting part of this energy to work, the heat engine rejects the remaining energy as heat. Almost all of this heat rejection can be absorbed by the endothermic reduction reaction occurring in the reducer. This arrangement requires the redox reactions to be exothermic and endothermic respectively, but this is normally the case for most metals. Some additional heat exchange with the environment is required to satisfy the second law; theoretically, for a reversible process, the heat exchange is related to the standard state entropy change, Î”So, of the primary hydrocarbon oxidation reaction as follows:
Qo = ToÎ”So
Figure: Sankey diagram of energy fluxes in a reversible CLC system.
However, for most hydrocarbons Î”So, is a small value and, as a result, an engine of high overall efficiency is theoretically possible.
Although proposed as a means of increasing efficiency, in recent years, interest has been shown in CLC as a carbon capture technique. Carbon capture is facilitated by CLC because the two redox reactions generate two intrinsically separated flue gas streams: a stream from the oxidiser, consisting of atmospheric N2 and residual O2, but sensibly free of CO2; and a stream from the reducer containing CO2 and H2O with very little diluent nitrogen. The oxidiser exit gas can be discharged to the atmosphere causing minimal CO2 pollution. The reducer exit gas contains almost all of the CO2 generated by the system and CLC therefore can be said to exhibit 'inherent carbon capture', as water vapour can easily be removed from the second flue gas via condensation, leading to a stream of almost pure CO2. This gives CLC clear benefits when compared with competing carbon capture technologies, as the latter generally involve a significant energy penalty associated with either post combustion scrubbing systems or the work input required for air separation plants. This has led to CLC being proposed as an energy efficient carbon capture technology.
Actual operation of chemical-looping combustion with gaseous fuels was demonstrated in 2003, and later with solid fuels in 2006. Total operational experience in pilots of 0.3 to 120 kW is more than 4000 h. Oxygen carrier materials used in operation include oxides of nickel, copper, manganese and iron.
A closely related process is Chemical-Looping Combustion with Oxygen Uncoupling (CLOU) where an oxygen carrier is used that releases gas-phase oxygen in the fuel reactor, e.g. CuO/Cu2O. This is helpful for achieving high gas conversion, and especially when using solid fuels, where slow steam gasification of char can be avoided. CLOU operation with solid fuels shows high performance.
Chemical Looping can also be used to produce hydrogen in Chemical-Looping Reforming (CLR) processes.
Comprehensive overviews of the field are given in recent reviews on chemical looping technologies.
In summary CLC can achieve both an increase in power station efficiency simultaneously with low energy penalty carbon capture. Challenges with CLC include operation of dual fluidized bed (maintaining carrier fluidization while avoiding crushing and attrition), and maintaining carrier stability over many cycles.
The research about chemical looping at the at Vienna University of Technology http://www.chemical-looping.at/start.asp
From a general point of view, "chemical looping" refers to a process where a chemical reaction takes place within two different reactors, and a reactive solid material circulates (loops) between both reactors to drive this chemical reaction. Depending on the respective species, transported by the looping solids and on the obtained process product, different chemical looping applications can be identified. The most important technologies, currently under investigation within research activities around the world, are shown in the next figure.
Figure: Chemical Looping principle
The research focus at Vienna University of Technology lies on chemical looping combustion and reforming with main activities in the sectors of reactor hydrodynamics, detailed reactor modelling, process modelling and process demonstration.
Chemical looping combustion for CO2 capture
One of the most promising capture technologies is chemical looping combustion (CLC). While other capture technologies exhibit the need of cost and energy extensive gas-gas separation steps, CLC separates CO2 inherent to the process via unmixed combustion. Oxygen is transported in terms of an oxygen carrier (generally a metal oxide) from the combustion air stream to the fuel stream and thus air nitrogen and fuel are never mixed (Figure). The chemical reactions with oxygen carrier take place in two separated reactors. Oxidation of the oxygen carrier is performed within the so called air reactor, which is supplied with combustion air and reduction takes place inside the fuel reactor, where fuel is fed into the system. The total amount of heat released from the two reactors equals the heat released from ordinary combustion of the fuel fed. Most of the proposed CLC applications are using well-established boiler technology very similar to (dual) fluidized bed boilers, which also means that costs can be assessed with great accuracy.
Figure: CLC concept for capturing CO2
In conclusion, CLC features up to 100% CO2 capture efficiency, a highly concentrated stream of CO2 ready for sequestration, no NOx emissions, and no costs or energy penalties for gas separation. Furthermore CLC uses well-established boiler technology, which means that costs can be assessed with great accuracy. Therefore, CLC is estimated to achieve CO2 capture cost reductions of 40 to 50% compared to today's best available carbon capture technologies, namely post combustion amine and oxyfuel combustion.
Challenges that CLC faces
The main challenges that CLR faces are effective dust removal and fluidized bed operation at elevated pressure. However, comparing this outlook with state-of-the-art reforming technologies, the problems arising with CLR still have a better chance to be solved, because of the:
availability of suitable particles from commercial raw materials
availability of suitable dual circulating bed technology
improvements in dust removal technology (medium and high temperature filters, etc.).