Chemical Looping Combustion Process Research Articles

Evolution mechanism of functional groups during coal chemical looping combustion

Chemical looping combustion (CLC) plays a vital role in coal combustion which released pure CO2 in the exhaust gas with low energy consumption. Researchers have developed over 1000 oxygen carriers (OCs) to enhance the properties of OCs. This study focus on the microstructural changes of coal during CLC process which could provide the performance improvement direction for OCs. Results showed that the phenolic -OH group can easily be removed by CuO while the free -OH group and the self-associating -OH group are hard to be consumed by the lattice oxygen in CuO at 800 °C, showing good stability in CLC process. Aliphatic hydrocarbons could be consumed in the gas phase rapidly. The functional group of C=O maintains high reactivity with CuO which explains why the addition of CO2 promotes the reaction rate in coal CLC. But C=C group was nonpolar bond and hard to be consumed while the -CH2 and -CH3 group and C-O-C group showed good reactivity with CuO. Higher reaction temperature affords higher reactivity of coal CLC process. But the -OH group changed slowly at higher temperature which could because that higher temperature makes the organic big molecular of the coal char decomposed and produce more -OH group.

Performance of fuel reactor in Chemical Looping Combustion system with various metal oxide particle size and operating temperature

Chemical looping combustion (CLC) process is a promising technology for improving the thermal efficiency of a system with the potential of 100% carbon capture and no NOx formation. In the present research work, bubble hydrodynamics inside the fuel reactor of a CH4–fueled CLC system has been numerically investigated. The reaction kinetics has been incorporated into the reactive system of the fuel reactor by a user-defined function (UDF) during numerical analysis. The present study uses CuO as an oxygen carrier material and CH4 as a fuel in combustion processes. The bubble hydrodynamics in terms of development, growth, rise, and burst are visualized and analyzed the solid-gas molar fraction inside the fuel reactor. In the present work, authors have chosen different operating temperatures varying from 923 K to 1323 K. The fuel conversion rate has been observed to increase with the increased temperature. Different granular size of metal oxide particles varying from 100 µm to 400 µm has been considered to observe the effect of various grain size of metal oxide and it was observed that the fuel conversion rate is decreasing with increased granular size.

Macroscopic analysis of chemical looping combustion with ilmenite versus conventional oxides as oxygen carriers

AbstractOver 40% of global energy-related CO2emissions are due to the combustion of fossil fuels for electric energy generation. Albeit CO2capture and storage have been identified as promissory actions to mitigate its emissions, the problem separating N2and CO2remains. A very effective solution for the former problem is to obtain the combustion CO2as a pure molecule, which is possible using the Chemical Looping Combustion (CLC) technology, which uses a solid oxygen carrier to transport the oxygen from an oxidating media (regeneration reactor) to a reducing media (combustion reactor). One of the key issues to apply CLC is to find or develop some material, suitable from the kinetic and thermodynamic points of view, for the reduction-oxidation cycles taking place inside combustion and regenerator reactors. The evaluation of “oxygen carrier” candidates for CLC is based on reactivity (rates and conversions), resistance to carbon accumulation, and “regenerability”, which means the ability of the material for cyclic reduction and oxidation. Another challenging issue to use CLC processes is the loss of oxygen carrier; this problem involves the use of supported metals on materials, such as zirconia, Al2O3, etc. Preparation of this kind of supported carriers requires time, money, and equipment. Meanwhile, the natural mineral ore named ilmenite, which consists of a mixture of iron and titanium oxides, and do not need to be supported, has been seen as promising to increase CLC efficiency as oxygen carrier. In this work, the performance of ilmenite is compared with some other oxygen carriers used in CLC.

Exploring depolymerization mechanism and complex reaction networks of aromatic structures in chemical looping combustion via ReaxFF MD simulations

Uncovering the reaction mechanism of various fuels to chemical looping combustion (CLC), especially depolymerization of solids in a complex environment, is an important scientific issue. Aromatic structures are the backbone of solid fuels such as coal and biomass. However, their depolymerization mechanisms during CLC remain unclear, and few complex reaction networks have been established to date. This work presents molecular insights into reaction behaviors and mechanism for several aromatic structures in the fuel reactor of CLC. The distribution and evolution of reactants, important intermediates, and products in the CLC are studied in details by means of reactive force field molecular dynamics (ReaxFF MD) simulations. It is found that the reaction process of aromatic fuels reacted with Fe2O3 as oxygen carriers in CLC can be divided into four stages. The decomposition rate of fuel molecules is relevant to the reaction temperature and aromatic sizes. The higher of reaction temperature and the smaller aromatic sizes, the faster the conversion of fuel molecules. The amount of CO and H2O produced gradually increases during CLC process, and however shows a decreasing trend in the later stages. The higher the CLC temperature is, the more favorable the oxygen releases from the oxygen carrier lattice are, so that the fuel molecules can burn fully to produce more CO2. The fuel molecular structure has less effect on generation time of CO molecule, while more on that of H2O molecule. The initial reactions of aromatic structures in CLC involve competition between thermal cracking and oxidation reactions. Fuel molecules generally go through a thermal ring-opening step first, followed by continuous bonding dissociation and oxidation to produce numerous structural fragments. The reactions to aromatic structures become more complex with an increasing number of aromatic rings and a wider variety of intermediates. By visualizing the ReaxFF MD results, complex reaction networks of different aromatic molecules in CLC are systematically established.

Experiments on the Dynamic Behavior of the Chemical Looping Combustion Process

AbstractThe dynamic operation of thermal power plants becomes more and more important due to more diverse and volatile power sources. Load changes of biomass, bituminous coal and methane were conducted during six experimental runs in a chemical looping combustion pilot plant which combines power generation and CO2 capture. The resulting step responses of the hydrodynamics and gas concentrations were compared to each other to understand the transient behavior of the interconnected fluidized bed system. The influence of the fuel rate and solid circulation on the response time and intensity was assessed.

Emission characteristics of flue gas during the chemical-looping combustion process for multi-component solid waste

Abstract Solid waste has interactions with its flue-gas products during combustion, which offers the possibility of regulating its pollutant emissions. Especially, these interaction pathways would be clearer under anaerobic conditions when the chemical-looping combustion (CLC) process is used. The CLC experiments of multi-component solid waste were conducted on a homemade twin-bed reactor and the characteristics of flue gas were investigated for the effect of the mixing ratio of sewage sludge and polyvinyl chloride (PVC). The results indicated that the combustion efficiency was >99.9% for these CLC processes; the highest carbon-conversion rate was obtained at 96.3% for PVC with 60% sludge. The highest NO and SO2 emissions were 26% and 19%, respectively, when the sludge was mixed with 20% PVC. As the proportion of PVC blended into the sludge increased, the time when the concentration of NO in the flue-gas peaks moved backwards, while peak SO2 concentration moved forward. The general trend was to increase first and then decrease. In addition, there were multiple peaks in carbon emissions, corresponding to ~10%, 30% and ~70% of the carbon-conversion rate; nitrogen emissions reached 90% of total emissions before the carbon-conversion rate was 40%; sulphur emissions had a longer cycle and were mainly emitted between 10% and 60% of the carbon-conversion rate. The results are expected to provide a reference for solid-waste source suppressing to inhibit the generation of pollutants.

Pressurised Chemical Looping Combustion (PCLC): Air Reactor design

Bioenergy combustion with Carbon Capture and Storage (BECCS) is a key technology to achieve carbon negative emissions power generation. This can be achieved by coupling the biofuels combustion with CO2 capture and storage (CCS). The lowest cost for CCS corresponds at the moment to the Chemical Looping Combustion (CLC) process. This can use biofuels which can be gaseous (biomethane, biogas or syngas etc.), liquid (biodiesel, bioethanol, biobutanol and pyrolysis oils etc.) or solids (wood dust, charcoal dust, wood chips, wood pellets etc.) While plant design with gaseous and liquid biofuels would be simpler, plants using solid biofuels and based on two couple fluidisd beds would need the use of a third reactor named carbon stripper. In the specific case if we plan to couple a CLC plant with a turbo expander (to achieve the high efficiencies of a combined cycle power plant) we have to work with pressurized reactors. However, there are some technical barriers to the coupling of a chemical looping combustor with a turbo expander, such as: the operation of the combustor in pressurised conditions; the inventory balance among reactors; elutriated particles reaching the turbo expander. This explaind why there is no commercial plant at the moment capable to do this. The aim of this paper is to present a model for the dimensioning of an air reactor to be coupled to a turbo expander of the power of about 12 MWe. Based on this, the air mass flow can be obtained and the geometric parameters can be calculated, to have an air velocity which is needed to achieve the fast fluidization regime and to ensure a high conversion rate as well as particles and heat exchage among air and fuel reactor.

Enrichment of iron from bauxite waste in chemical looping combustion

AbstractThis work reports a method and findings related to the enrichment of iron from bauxite waste (red mud) using chemical looping combustion (CLC). According to the results from X‐ray Diffraction, X‐ray Fluorescence, and scanning electron microscope–energy‐dispersive X‐ray spectroscopy, uniformly distributed iron in the red mud particles migrated outward to the surface during the 148 redox cycles in a bubbling fluidized bed reactor, in which the size‐reduced iron‐oxide particles due to attrition are concentrated in filters. The concentrations of iron oxide in raw particles and attrition fines are 43 wt.% and 83–87 wt.%, respectively. Since the attrition in CLC is inevitable, this recycled iron oxide can be a valuable by‐product to compensate for the cost of CLC, whereas the bed materials continue to participate in the CLC process. Moreover, the abundant bauxite waste can be processed in an eco‐friendly manner for integrated power generation, carbon capture, and iron recovery by the proposed strategy in this work.

Investigation on the Redox Properties of a Novel Cu-Based Pr-Modified Oxygen Carrier for Chemical Looping Combustion.

CO2 levels in the atmosphere are growing as a result of the burning of fossil fuels to meet energy demands. The introduction of chemical looping combustion (CLC) as an alternative to traditional combustion by transporting oxygen emphasizes the need to develop greener and more economical energy systems. Metal oxide, also defined as an oxygen carrier (OC), transports oxygen from the air to the fuel. Several attempts are being made to develop an OC with a reasonable material cost for superior fuel conversion and high oxygen transport capacity (OTC). This study aims to synthesize a potential OC using the wet impregnation method for the CLC process. Thermogravimetric analysis (TGA) was used to determine the cyclic redox properties using 5% CH4/N2 and air as reducing and oxidizing gases, respectively. The 10CuPA-based OC retained a high OTC of about 0.0267 mg O2/mg of OC for 10 cycles that was higher than 10CuA-based OC. Furthermore, the oxygen transfer rate for 10CuPA-based OC was relatively higher compared to 10CuA-based OC over 10 cycles. In comparison to 10CuA-based OC, the 10CuPA-based OC presented a steady X-ray diffraction (XRD) pattern after 10 redox cycles, implying that the phase was stably restored due to praseodymium-modified γ alumina support.

Evaluation of the effect of pressure and heat transfer on the efficiency of a batch fuel reactor, using Iron-based Oxygen Carrier with a CFD model

Coupling a Chemical Looping Combustor fed with biofuels with a turbo expander is a promising Negative Emissions Technology (NET) to realize climate neutral targets in China and Europe. This is also an example of Bioenergy with Carbon Capture and Storage (BECCS) technology. To realize it, we need a Pressurized Chemical Looping Combustion process (PCLC). In this work, a Eulerian-Lagrangian hybrid model is developed in Barracuda-VRTM software, incorporating chemical reactions to predict the performance of a Fuel Reactor using Fe2O3 as oxygen carrier and syngas as fuel, under different pressures, ranging from 1 bar to 20 bars. The model predicted the conversion efficiency of syngas reduction using an iron-based oxygen carrier (Fe2O3/Al2O3). The results show, that the increase in pressure promotes the conversion of CO and inhibits the conversion of H2. When the two gases are considered together, the increase in pressure promotes the reaction between syngas and Fe2O3 and reduces the demand for Fe2O3 oxygen carrier per unit of syngas Lower Heating Value and so also the inventory of the reactor. Increasing temperatures promotes both the reaction of H2 and CO with Fe2O3. Dealing with CO conversion, this is more affected by pressure changes and temperature changes than H2. This represents important information for Fuel Reactor design, scale up and optimization. Further validation is neded in batch and continuous pressurised plants.

Sulfur Evolution and Capture Behavior by a Solid Waste of Red Mud during Chemical Looping Combustion of Petroleum Coke

The sulfur evolution behavior during the chemical looping combustion (CLC) process of petroleum coke was investigated in a fluidized bed reactor with a low-cost solid waste of red mud as the oxygen carrier. Effects of temperature, oxygen carrier, and potassium catalyst addition on sulfur release were investigated, and H2S oxidization kinetics with the red mud and the effect of elements in the red mud on sulfur conversion were evaluated in detail. The red mud oxygen carrier had a promoting effect on H2S generation and thus enhanced sulfur conversion. The addition of potassium remarkably accelerated carbon conversion due to its catalytic effect. Meanwhile, it promoted H2 generation and then enhanced the generation of H2S. The addition of potassium into the process could weaken the release of gaseous sulfur but had no obvious negative influence on the reactivity of the red mud oxygen carrier. The experiment H2S interaction with the red mud suggested that the conversion of H2S to SO2 decreased with increasing temperature. The red mud has a low ability to oxidize H2S to SO2 but a high reactivity to capture sulfur-containing gases. Solid sulfur-containing products such as CaSO4 and Fe3S4 as well as K2S2 in the case of K addition into the process were detected.

Impact of Particle Size Polydispersity on Chemical Looping Combustion of Biomass under a Continuous Injection Mode

Particle size polydispersity considerably influences the chemical looping combustion (CLC) of solid fuels. In the present work, a three-dimensional multiphase particle-in-cell (MP-PIC) method was established and applied to study the full-loop CLC of biomass under a continuous injection mode. After model validation, the gas–solid flow characteristics, solid circulation rate (SCR), particle mixing in a fuel reactor (FR), and biomass leakage at different oxygen carrier (OC) particle size distribution (PSD) widths and biomass densities were systematically explored. The results showed that OC particles significantly stratified within the FR and both loop seals. Moreover, the fluctuation amplitude of the SCR decreased as the OC PSD width increased. Small biomass particles were more likely to leak. The number fraction of total leaked biomass particles first decreased and then increased as the OC PSD width increased. In the FR, small OC particles were primarily distributed in the upper part of the bed, and large particles were mainly distributed in the lower part. In addition, within the test range, the magnitude of the density difference between biomass and OC particles positively correlated with the likelihood of biomass leaking. These observations are all beneficial for the design and optimization of the CLC process of biomass.

Hydrotalcite-Derived Copper-Based Oxygen Carrier Materials for Efficient Chemical-Looping Combustion of Solid Fuels with CO2 Capture.

Chemical-looping combustion (CLC) is a promising technology that utilizes metal oxides as oxygen carriers for the combustion of fossil fuels to CO2 and H2O, with CO2 readily sequestrated after the condensation of steam. Thermally stable and reactive metal oxides are desirable as oxygen carrier materials for the CLC processes. Here, we report the performance of Cu-based mixed oxides derived from hydrotalcite (also known as layered double hydroxides) precursors as oxygen carriers for the combustion of solid fuels. Two types of CLC processes were demonstrated, including chemical looping oxygen uncoupling (CLOU) and in situ gasification (iG-CLC) in the presence of steam. The Cu-based oxygen carriers showed high performance for the combustion of two solid fuels (a lignite and a bituminous coal), maintaining high thermal stability, fast reaction kinetics, and reversible oxygen release and storage over multiple redox cycles. Slight deactivation and sintering of the oxygen carrier occurred after redox cycles at an very high operation temperature of 985 °C. We expect that our material design strategy will inspire the development of better oxygen carrier materials for a variety of chemical looping processes for the clean conversion of fossil fuels with efficient CO2 capture.

Applying a new configuration for thermal integration of ethane cracking and CLC processes to enhance the ethylene and hydrogen productions

In this study, a new method for applying the chemical looping combustion (CLC) process instead of the furnace to provide the required heat for the ethane thermal cracking reactor has been suggested. It has been proposed that the new configuration of the ethane thermal cracking process be performed in the three separate series reactors. The structure of these new integrated reactors consists of three concentric cylinders. In the internal cylinder with a small diameter, the thermal cracking process is performed, while in the middle and external sides of new reactors, the oxidation and reduction processes of oxygen carriers are executed, respectively. The essential thermal energy for the endothermic ethane thermal cracking process is provided by the exothermic oxidation reaction in the air reactor. Furnace removal prevents nitrogen oxide emission, and a significant amount of carbon dioxide is recovered in the CLC process. According to the results of this new configuration, the ethylene and hydrogen production yields rise by about 12% and 15%, respectively, compared to the conventional ethane cracking reactor. Finally, the effects of design parameters such as inlet feed temperatures of the air, fuel and cracking reactors, as well as the mass flow rates of ethane and oxygen carrier on the performance of ethane thermal cracking have been investigated.

Bulk oxygen conduction kinetics of iron oxides on the chemical looping combustion

Oxygen-ion conduction is generally considered the rate-determining step during the chemical looping combustion process, which is vital for the design of the oxygen carriers. However, the redox reaction is heavily influenced by gas diffusion, making it very difficult to derive convincing intrinsic kinetic data. This work aims to establish a methodology to evaluate the intrinsic kinetics of oxygen carriers for chemical looping combustion by using an ultra-thin oxygen conducting membrane to exclude the influence of gas diffusion. From mass balance and Fick's second law, a mathematical kinetics model describing the intrinsic oxygen conduction performance was established. The intrinsic activation energy took value of 77.84 kJ/mol for pure Fe2O3. The kinetics parameters can be used for the high throughput design and screening of oxygen carriers.

Chemical looping combustion of lignite using iron ore modified by foreign ions: Alkaline-earth and transition metal ions

In the chemical looping combustion (CLC) of lignite, oxygen carrier reactivity is the key to combustion performance. In this paper, the effects of particle size and temperature upon the CLC of lignite are investigated in a fixed bed reactor using iron ore as the oxygen carrier. Under optimized working conditions, the iron ores are loaded with alkaline-earth metals (K, Na and Ca) and transition metals (Ni, Mn and Cu) in a mass ratio of 10%, and their effect upon its CLC reactivity are studied. The carbon conversion rate, carbon capture rate, CO2 capture rate, CO2 selectivity, and other parameters are obtained. The size of lignite particles had a direct effect on the carbon capture rate of the fuel reactor. The particle size with the best effect on carbon and CO2 capture for both the Yunnan and Yizhong lignite is 80–200 mesh. The increase in temperature effectively promotes the reaction between oxygen carrier and volatiles, thereby resulting in an increase in CO2 concentration. The capacity of the oxygen carrier to transform the lignite is effectively enhanced by the modification with Cu, K, Na, Ca and Mn. In particular, the CO2 selectivity is greatly improved (up to 94.84%) for the Cu-modified oxygen carrier. These results provide important references for the CLC process, especially for the design of the oxygen carrier.

Chemical looping combustion of lignite using iron ore: C-gas products (CO2, CO, CH4) and NOx emissions

This work investigates the feasibility of lignite in chemical looping combustion (CLC) with an iron ore oxygen carrier. In this paper, C-gas products (CO2, CO, CH4) and NOx emission from the CLC process of lignite based on bubbling fluidized bed were studied. The effects of different atmospheres—Ar, 80%Ar–20%CO2, or 80%Ar–20%H2O—on the distribution of C-gas products and NOx emission were investigated. The important characteristic parameters of CLC, including carbon conversion degree, carbon capture ratio, CO2 capture ratio and NOx generation ratio, were obtained. Compared with pure Ar atmosphere, both 20%CO2 and 20%H2O atmospheres promote the gasification of char in the fuel reactor stage. In addition, the carbon conversion degree, carbon capture ratio, and the CO2 capture ratio are increased to 80%, 95% and 60%, respectively. In these 3 atm, the NOx generation in the fuel reactor stage was dominated by NO. Compared with the Ar atmosphere, the 20%CO2 atmosphere causes a close NOx generation ratio, while the 20%H2O atmosphere has a NOx generation ratio of 13.79%. Compared with the lignite combustion under two O2 concentrations (18% and 45%), NOx emission in CLC is reduced by 12% and 50%, respectively.

Synthesis, characterization, and kinetic study of nanostructured copper-based oxygen carrier supported on silica and zirconia aerogels in the cyclic chemical looping combustion process

In this study, copper-based oxygen carriers (OCs) confined in the structure of silica and zirconia aerogels were in situ synthesized by a simple sol–gel method, and the performance of the prepared oxygen carriers was investigated in the chemical looping combustion (CLC) process. The effects of different support types and copper loadings (20–60 wt%) were studied on the physicochemical properties and catalytic behavior of OCs during 15 reduction–oxidation cycles at the temperature of 800 °C. Zirconia-supported oxygen carriers exhibited high reactivity and stability over the cycles. However, the OCs supported on silica showed lower oxygen capacity than the theoretically expected values. This is due to the decomposition of copper oxide (CuO) to Cupric (Cu2O) during the reaction along with the collapse and fuse of mesoporous silica at high calcination and reaction temperatures which leads to a significant loss of oxygen-carrying capacity. The kinetics of reduction/oxidation steps in the CLC process was studied using a modified grain model considering grain size distribution in the OC particles based on their pore size distribution profile and “pore to sphere” factor. The synthesized oxygen carriers were characterized by Brunauer-Emmet-Teller (BET), Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX).

Techno-economic analysis of a chemical looping combustion process for biogas generated from livestock farming and agro-industrial waste

Biogas is a renewable fuel that is generated from the decomposition of organic matter through the process of anaerobic digestion. The compound that provides biogas with its energy value is CH4, which accounts for between 40% and 75% of the gas, with CO2 being the other main component of this fuel. Although the biogas valorization process is carbon neutral, the overall process can become carbon negative with the application of CO2 capture technologies, such as chemical looping combustion (CLC). This work presents a techno-economic analysis of a 5 MWth plant for the generation of biogas from livestock farming and agro-industrial waste to produce electricity + heat or heat only (base cases), or for further treatment in a CLC unit (CLC cases), where the captured CO2 is converted into a raw material, e.g. for the production of greenhouse crops. Results from the comparative study of five different oxygen carriers used in CLC are also presented.The profitability of the base cases was evaluated by means of the Levelized Cost of Electricity (LCOE) and Levelized Cost of Thermal Energy (LCOTE) metrics, whereas the economic performance of the CLC cases was measured by the Levelized Cost of CO2 (LCOCO2). LCOE gave a value of €139.90/MWh for the base case, and LCOTE was €56.50/MWh for the modified base case, where biogas was combusted in a boiler to produce only heat. With regard to the CLC cases, LCOCO2 values ranged between €148.60 per tonne for a highly reactive, synthetic Cu-based oxygen carrier and €177.10 per tonne for ilmenite.Finally, by comparing the economic results obtained in this work with those found in the literature for conventional biogas production plants and greenhouse crop production processes, it can be concluded that the implementation of CLC technology in biogas plants has the potential to become a profitable and environmentally sustainable option for the valorization of organic waste because the CO2 generated in the fuel reactor of the CLC unit can be considered a marketable, high value-added product for the niche market of the greenhouse farming of horticultural crops.

Interaction mechanism between CO and H2S over CuFe2O4 oxygen carrier during chemical-looping combustion: A DFT study

During the chemical-looping combustion process, CO as an important part of syngas and H2S as a common sulfur compound inevitably coexist in the fuel. Based on the density functional theory and periodic structure model, the interaction mechanism between CO and H2S over the O-defective CuFe2O4 surface was comprehensively investigated. The results show that the adsorption energies of adsorbates on the O-defective surface are in the order of S* (−205.42 kJ/mol) > HS* (−191.37 kJ/mol) > CO (−44.88 kJ/mol) > H2S* (−28.97 kJ/mol). CO adsorption over the O-defective surface is dominant as compared to H2S adsorption. The adsorption energies of CO in the presence of H2S, HS* and S* are − 73.21, −255.98 and − 105.35 kJ/mol, respectively. The adsorption strength of CO over the O-defective surface was enhanced in the presence of HS* or S*. The presence of CO* will greatly limit the H2S dehydrogenation process, especially for the second dehydrogenation step. The energy barriers of CO oxidation with the participation of H2S*, HS* and S* are 69.66, 51.42 and 14.19 kJ/mol, respectively. COS may be produced form the reaction between CO and S* with an energy barrier of 27.13 kJ/mol, following the Eley–Rideal reaction mechanism. CuFe2O4 oxygen carrier exhibits good anti-sulfur ability because the reaction of CuFe2O4 with CO and H2S kinetically prefers the CO oxidation pathway rather than the COS formation pathway.