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.

CO2 capturing methods: Chemical looping combustion (CLC) as a promising technique

This reports recent advances on CO₂ capturing methods, focusing on chemical looping combustion (CLC) as a promising technology to achieve this goal. Generally, there are three main methods to capture CO₂ resulting from fossil fuel combustion: post-combustion, oxy-combustion, and pre-combustion. In CLC, which is either classified as a pre-combustion method or as the fourth capturing method, the solid oxygen carrier provides the oxygen needed for combustion. This technique helps to avoid diluting the combustion effluent stream with the N₂ released from air and therefore, minimizes the requirement of CO₂ separation, a major cost of CO₂ capture. In addition, it minimizes the formation of NO_x that results when N₂ comes in contact with oxygen and fuels at high temperatures. The desired properties of oxygen carrier candidates for CLC are high reduction and re-oxidation rates, high oxygen capacity, good stability and fludiziability at high temperatures, friendly to the environment, and low cost. Transition metal oxides are common candidates for CLC. Most investigations in this field have examined the reactivity and stability of oxygen carriers but few investigations have focused on their reduction and re-oxidation reaction mechanisms. Researchers have proposed two mechanisms for these reactions, the nucleation-nuclei growth and unreacted shrinking core models. Despite numerous investigations of CLC, there is still a lack of knowledge in some of its aspects such as the underlying surface chemistry and the economic impact. This work critically reviewed all capturing methods of CO₂ with focusing on CLC process as a promising technology due to its ability to concentrate the resulted CO₂ and minimizes the separation cost. This work provides essential insight information into CLC technology and highlights its status and needs.

Assessment of the Redox Characteristics of Iron Ore by Introducing Biomass Ash in the Chemical Looping Combustion Process: Biomass Ash Type, Constituent, and Operating Parameters

Chemical looping combustion (CLC) is a potential CO2 capture and sequestration (CCS) technology that can easily separate CO2 and H2O without energy loss and greatly improve the efficiency of carbon capture. Due to the inherent defects of natural iron ore, such as low reactivity and poor oxygen carrying capacity, four kinds of biomass ashes (rape stalk ash, rice stalk ash, platane wood ash, and U. lactuca ash) that have different constituents of K, Na, Ca, and Si were applied to modify the redox performance of natural iron ore. The effects of biomass ash type, constituent, reaction temperature, H2O vapor flow rate, and redox cycle on the CLC process were assessed experimentally in a batch fluidized bed reactor system. Oxygen carrier physicochemical characteristics were determined by several analytical techniques. The results showed that rape stalk ash, platane wood ash, and U. lactuca ash with a high K content and high K/Si ratio significantly improved the reactivity and cycle stability of iron ore, even after 10 redox cycles, while rice straw ash with a low K/Si ratio showed an inhibitory effect due to the formation of bridge eutectics, which enhanced agglomeration. In a range from 800 to 950 °C, higher temperatures led to a much better ability to promote the CLC process than lower temperatures. A higher flow rate of H2O had little effect on the further promotion of the CLC process due to hydrogen inhibition. It is believed that the application of BA-modified iron ore oxygen carriers is an effective strategy to improve the CLC process.

Energetic Analysis of Different Configurations of Power Plants Connected to Liquid Chemical Looping Gasification

In this article, a thermodynamic study was conducted on the energetic and exergy performance of a new configuration of liquid chemical looping gasification (LCLG) plant integrated with a power block to assess the overall performance of the system including exergy partitioned in syngas and first law efficiency (FLE). LCLG is a relatively new concept for the production of high-quality synthetic gas from solid feedstock such as biomass. As the temperature and pressure of the looping system are high, there is thermodynamic potential to co-produce chemical products, power and heat. Hence, in the present work, three different configurations of a power cycle were thermodynamically assessed. In the first proposed power cycle, the produced syngas from the gasifier was combusted in a combustion chamber and the exhausted gases were fed into a gas turbine. In the second and third proposed power cycles, the hot air was directly fed into a gas turbine or was used to produce steam for the steam turbine combined cycle. The processes were simulated with Aspen Plus and Outotec HSC chemistry software packages. The influence of different operating parameters including temperature and pressure of the air reactor and type of oxygen carrier on the first law and exergy efficiency (exergy partitioned in synthetic gas) was assessed. Results showed that the FLE for the proposed gas turbine and steam turbine combined cycles was ~33% to 35%, which is within the range of the efficiency obtained for the state-of-the-art power cycles reported in the literature. Results also showed that lead oxide was a suitable oxygen carrier for the LCLG system, which can be integrated into a steam turbine combined cycle with an FLE of 0.45, while copper oxide showed an FLE of 0.43 for the gas turbine combined cycle.

Numerical Investigation of Solid-Fueled Chemical Looping Combustion Process Utilizing Char for Carbon Capture

The in-depth understanding of the gas–solid flow and reaction behaviors, and their coupling characteristics during the chemical looping combustion (CLC) process has the guiding significance for the operation and optimization of a chemical looping combustor. A three-dimensional numerical model is applied to investigate the char-fueled CLC characteristics in a fuel reactor for efficient CO2 separation and capture. Simulations are carried out in a bubbling fluidized bed fuel reactor with a height of 2.0 m and a diameter of 0.22 m. The initial bed height is 1.1 m, and hence the height–diameter ratio of the slumped bed is five. The oxygen carrier is prepared with 14 wt% of CuO on 86 wt% of inert Al2O3. In the process of mathematical modeling, a Eulerian-Eulerian two-fluid model is adopted for both of the gas and solid phases. Gas turbulence is modeled on the basis of a k–ε turbulent model. The reaction kinetics parameters are addressed based upon previous experimental investigations from literature. During the simulation, the gas–solid flow patterns, composition distributions, and reaction characteristics are obtained. Moreover, the effects of solids inventory and fluidizing number on the reaction performance are elucidated in-depth. The results have shown that the reaction rates have close relationship with the flow patterns and the distributions of gas concentrations. Compared to the steam-char gasification over sand, the application of char-fueled CLC can effectively promote the conversion of gasification products. In addition, higher CO2 concentration at the outlet can be achieved by increasing the initial solids inventory or decreasing the fluidizing number. Some calculated values are verified by the previous data, indicating that the current three-dimensional models are reasonable to study the process mechanism of char-fueled CLC.

Effect of hematite addition to CaSO4 oxygen carrier in chemical looping combustion of coal char

Catalytic mechanism of synergy effect of iron oxide in gasification of char and in the reduction of CaSO₄ oxygen carrier.

Materials for Chemical-Looping with Oxygen Uncoupling

Chemical-looping with oxygen uncoupling (CLOU) is a novel combustion technology with inherent separation of carbon dioxide. The process is a three-step process which utilizes a circulating oxygen carrier to transfer oxygen from the combustion air to the fuel. The process utilizes two interconnected fluidized bed reactors, an air reactor and a fuel reactor. In the fuel reactor, the metal oxide decomposes with the release of gas phase oxygen (step 1), which reacts directly with the fuel through normal combustion (step 2). The reduced oxygen carrier is then transported to the air reactor where it reacts with the oxygen in the air (step 3). The outlet from the fuel reactor consists of only CO2 and H2O, and pure carbon dioxide can be obtained by simple condensation of the steam. This paper gives an overview of the research conducted around the CLOU process, including (i) a thermodynamic evaluation, (ii) a complete review of tested oxygen carriers, (iii) review of kinetic data of reduction and oxidation, and (iv) evaluation of design criteria. From the tests of various fuels in continuous chemical-looping units utilizing CLOU materials, it can be established that almost full conversion of the fuel can be obtained for gaseous, liquid, and solid fuels.