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Title: A thermodynamic analysis of alternative approaches to chemical looping combustion

Abstract

In this article, we review and clarify some of the points made by previous authors regarding chemical looping combustion (CLC). While much of the recent interest in chemical looping combustion has been associated with carbon sequestration, our primary interest here is its potential to increase the thermodynamic efficiency of converting fuel chemical energy into useful work. We expand on several points about the details of CLC that we feel have not previously been sufficiently explored, and suggest alternative (and possibly more practical) approaches that exploit some of the same thermodynamic concepts. We illustrate our key points with {First} and {Second} Law analyses of ideal conceptual processes, which in addition to {CLC} also include isothermal, non-equilibrium, preheated combustion and combustion with thermochemical recuperation. Our results suggest that a significant portion of the potential efficiency benefit of CLC might be achieved without the need to handle and transport large quantities of solid oxygen storage material. Exploitation of this fact may lead to higher efficiency approaches for power generation from hydrocarbon fuels combustion.

Authors:
 [1];  [1];  [1]
  1. ORNL
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Fuels, Engines and Emissions Research Center (FEERC); National Transportation Research Center
Sponsoring Org.:
USDOE Office of Energy Efficiency and Renewable Energy (EERE)
OSTI Identifier:
1023298
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Journal Article
Resource Relation:
Journal Name: Energy & Fuels; Journal Volume: 25; Journal Issue: 2
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; CARBON SEQUESTRATION; COMBUSTION; EFFICIENCY; HYDROCARBONS; OXYGEN; POWER GENERATION; STORAGE; THERMODYNAMICS; TRANSPORT

Citation Formats

Chakravarthy, Veerathu K, Daw, C Stuart, and Pihl, Josh A. A thermodynamic analysis of alternative approaches to chemical looping combustion. United States: N. p., 2011. Web. doi:10.1021/ef101336m.
Chakravarthy, Veerathu K, Daw, C Stuart, & Pihl, Josh A. A thermodynamic analysis of alternative approaches to chemical looping combustion. United States. doi:10.1021/ef101336m.
Chakravarthy, Veerathu K, Daw, C Stuart, and Pihl, Josh A. 2011. "A thermodynamic analysis of alternative approaches to chemical looping combustion". United States. doi:10.1021/ef101336m.
@article{osti_1023298,
title = {A thermodynamic analysis of alternative approaches to chemical looping combustion},
author = {Chakravarthy, Veerathu K and Daw, C Stuart and Pihl, Josh A},
abstractNote = {In this article, we review and clarify some of the points made by previous authors regarding chemical looping combustion (CLC). While much of the recent interest in chemical looping combustion has been associated with carbon sequestration, our primary interest here is its potential to increase the thermodynamic efficiency of converting fuel chemical energy into useful work. We expand on several points about the details of CLC that we feel have not previously been sufficiently explored, and suggest alternative (and possibly more practical) approaches that exploit some of the same thermodynamic concepts. We illustrate our key points with {First} and {Second} Law analyses of ideal conceptual processes, which in addition to {CLC} also include isothermal, non-equilibrium, preheated combustion and combustion with thermochemical recuperation. Our results suggest that a significant portion of the potential efficiency benefit of CLC might be achieved without the need to handle and transport large quantities of solid oxygen storage material. Exploitation of this fact may lead to higher efficiency approaches for power generation from hydrocarbon fuels combustion.},
doi = {10.1021/ef101336m},
journal = {Energy & Fuels},
number = 2,
volume = 25,
place = {United States},
year = 2011,
month = 1
}
  • Iron oxide (Fe{sub 2}O{sub 3}) or in its natural form (hematite) is a potential material to capture CO{sub 2} through the chemical-looping combustion (CLC) process. It is known that magnesium (Mg) is an effective methyl cleaving catalyst and as such it has been combined with hematite to assess any possible enhancement to the kinetic rate for the reduction of Fe{sub 2}O{sub 3} with methane. Therefore, in order to evaluate its effectiveness as a hematite additive, the behaviors of Mg-modified hematite samples (hematite –5% Mg(OH){sub 2}) have been analyzed with regard to assessing any enhancement to the kinetic rate process. Themore » Mg-modified hematite was prepared by hydrothermal synthesis. The reactivity experiments were conducted in a thermogravimetric analyzer (TGA) using continuous stream of CH{sub 4} (5, 10, and 20%) at temperatures ranging from 700 to 825 {degrees}C over ten reduction cycles. The mass spectroscopy analysis of product gas indicated the presence of CO{sub 2}, H{sub 2}O, H{sub 2} and CO in the gaseous product. The kinetic data at reduction step obtained by isothermal experiments could be well fitted by two parallel rate equations. The modified hematite samples showed higher reactivity as compared to unmodified hematite samples during reduction at all investigated temperatures.« less
  • This paper is the first in a series of two, where we present the concept of a CLC process of solid fuels using a circulating fluidized bed with three loop seals. The riser of this circulating fluidized bed was used as the oxidizer of the oxygen carrier; one of the loop seals was used as the reducer of the oxygen carrier and the separator for ash and oxygen carrier, and the other two loop seals were used for pressure balance in the solid recycle process. Pressure profiles of recycled solids using this process are presented in detail. For the developmentmore » of an oxygen carrier, we focused on the establishment of a theoretical frame of oxygen transfer capability, reaction enthalpy, a chemical equilibrium, and kinetics. Analysis results indicated that Cu-, Ni-, and Co-based oxygen carriers may be the optimum oxygen carriers for the CLC of solid fuels. Thermodynamic analysis indicated that CO{sub 2} can be concentrated and purified to at least 99% purity for the gas-solid reaction mode or even higher for the solid-solid reaction mode on the basis of the selected oxygen carriers. A Cu-based oxygen carrier is the choice that has the potential to make the reducer self-sustaining or autothermal because of its exothermic nature during reduction. This would be beneficial for simplifying the operation of the reducer. The tendency of the Cu-based oxygen carriers to agglomerate can be eliminated by decreasing the operating temperature in the CLC system. In the second part of the series, we will evaluate the reduction kinetics of selected Cu-based oxygen carriers by coal and other 'opportunity solid fuels' using a simultaneous differential scanning calorimetry-thermogravimetric analysis to simulate a microreactor, using an X-ray diffractometer and a scanning electron microscope to characterize the solid residues, and a thermogravimetric analysis coupled with mass spectra to characterize the evolved gas compositions. 46 refs., 5 figs., 2 tabs.« less
  • Cited by 3
  • Chemical-looping combustion (CLC) is a combustion technology for clean and efficient utilization of fossil fuels for energy production. This process which produces sequestration ready CO2 systems is a promising technology to be utilized with coal gasification systems. In the present work, chemical-looping combustion has been studied with an oxygen carrier, NiO/bentonite (60 wt % NiO) for the gasification systems utilizing simulated synthesis gas. Global reaction rates of reduction and oxidation as a function of conversion were calculated for oxidation-reduction cycles utilizing the thermogravimetric analysis (TGA) data on multicycle tests conducted with NiO/bentonite at atmospheric pressure between 700 and 900 °C.more » The rate of reduction increased slightly with an increase in temperature, while the rate of oxidation decreased at 900 °C. The effect of particle size of the oxygen carrier on CLC was studied for the particle size between 20 and 200 mesh. The rates of reactions depended on the particle size of the oxygen carrier. The smaller the particle size, the higher the reaction rates. The multicycle CLC tests conducted in a high-pressure flow reactor showed stable reactivity for the production of CO2 from fuel gas at 800 and 900 °C and full consumption of hydrogen during the reaction. The data from a one cycle test on the effect of the pressure on the performance with NiO/bentonite utilizing the tapered element oscillating microbalance (TEOM) showed a positive effect of the pressure on the global rates of reduction-oxidation reactions at higher fractional conversions. The X-ray diffraction (XRD) analysis confirmed the presence of the NiO phase in NiO/bentonite with the oxidized sample in the highpressure reactor and Ni phase with the reduced sample. The presence of a small amount of NiO in the reduced sample detected by X-ray photoelectron spectroscopy (XPS) may be due to its exposure to air during sample transfer from the reactor to XPS. Scanning electron microscopy (SEM) analysis showed no significant changes in morphology of NiO/bentonite reacted in the temperature range 700-800 °C in an atmospheric TGA for 10 oxidation-reduction cycles, but some loss of surface area and porosity was observed at 900 °C. This effect was found to be greater with increase in the particle size of the oxygen carrier.« less
  • Chemical looping combustion (CLC) is a flameless two-step fuel combustion that produces a pure CO2 stream, ready for compression and sequestration. The process is composed of two interconnected fluidized bed reactors. The air reactor which is a conventional circulating fluidized bed and the fuel reactor which is a bubbling fluidized bed. The basic principle is to avoid the direct contact of air and fuel during the combustion by introducing a highly-reactive metal particle, referred to as oxygen carrier, to transport oxygen from the air to the fuel. In the process, the products from combustion are kept separated from the restmore » of the flue gases namely nitrogen and excess oxygen. This process eliminates the energy intensive step to separate the CO2 from nitrogen-rich flue gas that reduce the thermal efficiency. Fundamental knowledge of multiphase reactive fluid dynamic behavior of the gas–solid flow is essential for the optimization and operation of a chemical looping combustor. Our recent thorough literature review shows that multiphase CFD-based models have not been adapted to chemical looping combustion processes in the open literature. In this study, we have developed the reaction kinetics model of the fuel reactor and implemented the kinetic model into a multiphase hydrodynamic model, MFIX, developed earlier at the National Energy Technology Laboratory. Simulated fuel reactor flows revealed high weight fraction of unburned methane fuel in the flue gas along with CO2 and H2O. This behavior implies high fuel loss at the exit of the reactor and indicates the necessity to increase the residence time, say by decreasing the fuel flow rate, or to recirculate the unburned methane after condensing and removing CO2.« less