The main drawbacks and future research and prospects are remarked. Environmental Engineering Research ; 19 4 : Published online: December 31, Abstract This study presents a review on Chemical looping combustion CLC development, design aspects and modeling. Introduction The global warming effect is leading to life threatening problems for human and other living creatures all around the world. Nearly all the developing and developed countries use fossil fuels to supply their energy requirements. The CO 2 concentration in the atmosphere has increased strongly over the few past decades as a result of world dependency on fossil fuels for energy production [ 1 — 3 ] and global atmospheric concentration of CO 2 increased from a value of ppm to ppm [ 1 , 2 , 4 ].
Therefore, there is an urgent demand to develop technologies for reduction of emissions of this gas. CO 2 capture and storage CCS is an appropriate option to reach this objective.
The purpose of CCS technologies is to produce a concentrated stream of CO 2 from industrial sources, transport it to a suitable storage location and then store it away from the atmosphere for a long period of time [ 1 , 3 , 5 , 6 ]. So far, mainly three technologies for CO 2 capture are under consideration for industrial and power plants applications: pre-combustion, oxy-fuel combustion and post combustion.
All of these methods are energy-intensive and lead to considerable plant low efficiency.
Pre-combustion CO 2 capture is a process where the carbon in the fuel is separated, or removed, before combustion process. Instead of burning coal or natural gas in a combustion plant, the fuel can be converted to hydrogen and CO 2 prior to combustion. The CO 2 can then be captured and stored, while the hydrogen is combusted to produce power. Oxy-fuel process needs pure oxygen for burning the fuel; consequently demands for energy to separate oxygen from the air. Post combustion needs energy to remove the CO 2 diluted with other gases in the power plant exhaust stream.
Great efforts are made in the recent years to develop new low-cost CCS technologies. Chemical looping combustion CLC is an appropriate alternative to reduce economic cost of CO 2 capture from combustion of fossil fuels in power plants [ 1 — 4 ]. Chemical-Looping term is used for cycling processes that use a solid metal oxide as oxygen-carrier for supplying the required oxygen for fuel conversion.
The fuel reduces the solid metal oxide. To complete the loop, the solid metal oxide must be re-oxidized before to a new cycle begins. The final purpose of fuel conversion can be the combustion or hydrogen production. For combustion purposes, the chemical looping process takes place in two steps, while air and fuel are kept away from each other in two separate reactors referred to as air and fuel reactors.
Here, the combustion air oxygen and fuel direct contact is avoided. Therefore, the CLC can be called as unmixed combustion process. The first step, in fuel reactor, the oxygen supplied to fuel by the oxygen carrier, where the fuel reduces the metal oxide by taking up the required oxygen. In the second step, this reduced metal oxide re-circulates into the air reactor where it is re-oxidized by air.
Since, the air and fuel are not in direct contact, the combustion gases are not diluted with N 2 , consequently separation of CO 2 is not required [ 1 — 3 , 7 — 9 ]. This avoids extra separation cost of CO 2 from flue gases. CLC process uses gaseous or solid materials as fuels.
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In CLC of gaseous fuels, the oxygen carrier reacts directly with a fuel such as natural gas, refinery gas, etc. A short description of these processes is presented here. In the Syngas-CLC process, the solid fuel is gasified in a gasifier to form a syngas and then the oxygen carrier come into contact with this syngas in the air reactor. Although the fuel fed to CLC is gaseous, the primary fuel is in solid state. In iG-CLC process the gasification and combustion can occur in a unique reactor.
The oxygen carrier reacts with gasification products of solid fuel generated inside fuel reactor. The CLOU method is basically different from the two previous ones. CLOU uses the oxygen carriers which are able to release gaseous oxygen for the combustion of solid fuel [ 1 , 6 ]. In this review, the important challenges related to CLC process with respect to experimental and modeling studies, especially on gaseous fuels are presented.
This article covers the main recent findings on CLC process through available experimental and theoretical studies as follows in section 2, a short review on process development is presented. Section 3 covers the status of the development of oxygen carrier materials and discusses main achievements in this field.
The reactor design aspects are discussed in section 4. Advances on mathematical modeling of CLC process and the main progresses in this field are presented in section 5. The most important kinetic models are mentioned in this section, as well. Section 6 includes the drawbacks and future prospects of CLC process.
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They introduced the concept of oxygen carrier, with the possibility to use different fuels to reduce it and the use of two interconnected fluidized bed for the solid circulation. The principles of CLC process to increase thermal efficiency in fossil fuel power plants is proposed in the early eighties [ 1 , 9 ].
Ishida, et al. In a CLC-based power plant the outlet gas stream of air reactor drives gas air turbine while the exhaust gas from the fuel reactor derives a CO 2 turbine. The exhaust from the air turbine is circulated through a heat recovery steam generator to produce steam, which is used in the low pressure steam turbine to generate extra power.
Finally, the concentrated CO 2 stream is compressed for transportation and sequestration [ 1 ]. Anheden and Svedberg [ 13 ] performed a detailed energy analysis for two different CLC gas turbine systems. In the first one, methane was used as fuel and NiO as oxygen carrier, while in the second system; the used fuel was gasified coal CO and H 2 and Fe 2 O 3 was used as oxygen carrier. However, taking into account the advantages of inherent CO 2 separation, a CLC-based process offers higher overall energy efficiency. The application of CLC process at larger scales is the next step in developing of this technology.
Lyngfelt, et al. There is an increasing interest in developing CLC technology because of the advantages described before. The CLC processes have about 3, hours of operational experience in continuous plants of different size. Considering that the experimental experience of this process is less than 10 years old, it is assumed that the process development status is very successful [ 1 ]. Oxygen Carriers The large scale application of CLC process greatly depends on the availability of suitable oxygen carriers [ 2 , 16 ].
The selection of oxygen carrier is considered as one of the most essential issues of the process [ 2 , 16 , 17 ]. In fact the amount of the bed material in each reactor and solid circulation rates between reactors mainly depend on oxygen carrying capacity of oxygen carrier material. The most important features of suitable oxygen carrier can be listed as follows:.
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A number of metal oxides are investigated as potential oxygen carriers for CLC process. In general, it is considered that transition metal oxides like nickel, copper, cobalt and manganese are among the feasible candidates to be used as an oxygen carriers [ 1 , 2 , 16 , 18 , 20 — 22 ]. Initially the bulk metal is investigated as the oxygen carrier.
Good reactivity is observed in the first several cycles. Although bulk materials have low cost, their fast degeneration and low reactivity mainly due to the particle agglomeration and changes of crystalline form of these materials make them unsuitable for application in CLC process.
To overcome these drawbacks, an inert material can be added into the oxygen carries [ 22 ]. These inert materials act as a supports which can provide a vast specific area and appropriate pore structure for reaction. These inert support can act as a binder for increasing the mechanical strength and attrition resistance, as well [ 16 , 22 , 23 ]. These materials enhance the reactivity, durability and fluidizability of the oxygen carrier particles [ 2 , 16 ].
One of the important features of an oxygen carrier is oxygen transport carrying capacity also called oxygen ratio which is defined by the following Eq. Different oxygen carriers may be reduced to different states depending on the metal dispersed on the support materials. It should be noted that, only the transformation from hematite to magnetite may be applicable for industrial CLC systems [ 23 ].
Obviously, the inert support decreases the oxygen transport capacity of the oxygen carriers [ 22 , 23 ]. Different proportions of active metal oxide and inert support can yield different kinds of oxygen carriers [ 22 ]. The oxygen carrying capacity depends on both the active metal oxide content x MeO and the type of the metal oxide [ 23 ].
Normally, the support material is considered to be inert with respect to any phase in the reaction. However, the experimental findings reveal that the inert support would react with the active carrier to form a complex chemical compound. Therefore, the oxygen carrier capacity is corrected by active metal content factor x MeO in oxygen carrier, as Eq. The oxygen transport capacity of some oxygen carriers has been reported in Table 1.
In order to select a suitable oxygen carrie, the thermodynamic aspects and physical properties like particle size, density, active surface area, pore volume, crushing strength, anti-agglomeration properties and melting point must be considered [ 2 , 22 ]. Thus, these materials may not be suitable oxygen carriers. Johansson, et al. They reported that, in general, nickel particles are the most reactive, followed by manganese.
The Fe particles are hard with low reactivity.
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