Coal-fueled Chemical Looping Combustion Research Articles

Semi-continuous Operation of Chemical Looping Combustion of Coal Using a Low-Cost Composite Oxygen Carrier

To reach the carbon peak by 2030 and achieve carbon neutrality by 2060, the implementation of low-carbon combustion approaches for fossil fuels (especially coal, a high-carbon fuel) is urgent in China. Chemical looping combustion (CLC) has been considered as one of the most promising technologies for low-carbon, efficient, and clean utilization of coal. However, a low-cost and high-performance oxygen carrier (OC) holds the bottleneck that constrains the industrial application of coal-fueled CLC. In this paper, a low-cost composite OC named Cu13.0Red87.0@C (with the mixing mass ratio of copper ore to red mud being 13.0:87.0, except for a 20 wt % cement bonder) was first prepared by the hydroforming method and then systematically evaluated by designing CLC tests in a semi-continuous fluidized bed using lignite as fuel. The results showed that this composite OC exhibited superior combustion performance in comparison to pure red mud. Moreover, it was found that increasing the temperature could significantly improve the average carbon capture efficiency but had a relatively small positive effect on the average CO2 yield. Additionally, with the elevation of the oxygen/fuel ratio, the resulting average carbon capture efficiency and average CO2 yield both tended to increase. Generally, the semi-continuous unit can simulate the continuously operated fuel reactor with an adjustable bed inventory and solid circulation rate, and it is easy to attain smooth operation of ca. 30 min, achieving both carbon capture efficiency and a CO2 yield of ca. 90%.

Thermodynamic evaluation and modelling of an auto-thermal hybrid system of chemical looping combustion and air separation for power generation coupling with CO2 cycles

Chemical looping technologies have been demonstrated to have favorable economy for CO2 separation and high exergy efficiency. In order to realize the energy cascade utilization, a hybrid system combing autothermal coal-fueled chemical looping combustion (CLC), chemical looping air separation (CLAS), supercritical CO2 Brayton cycle and transcritical CO2 cycle is constructed for high-efficiency power production and CO2 separation. The proposed hybrid system is first modeled and simulated under fundamental condition to investigate the basic performance and exergy efficiency of each component. Then the thermodynamic evaluations of the system performance are conducted under different conditions with five variables. Finally, using these simulation results as training samples, a neural network model (NNM) of the system is constructed to predict the power output under variable working parameters. As the energy is used in different subsystems according to its grade, the net power efficiency of this hybrid system can reach to a high value of 64.59%. The established NNM is accurate for power prediction under random conditions with correlation coefficient of 0.9931.

Study on the performance of the purified CaSO4 oxygen carrier derived from wet flue gas desulphurization slag in coal chemical looping combustion

Flue gas desulphurized slag was collected as the raw material and purified using the multiple steps of sour purification procedures, which was further prepared as CaSO4 oxygen carrier (OC). Reaction characteristics of the purified desulphurized slag OC with the selected lignite as well as release of the different gaseous sulfur species from the CaSO4 side reactions were investigated in a lab-scale fixed bed reactor. Several main influencing factors, including reaction temperature, OC excess number ϕ and the cycle numbers, were focused. The experimental results indicated that the purified CaSO4 OC was of high reactivity and promising to be applied in the coal-fueled chemical looping combustion process as OC. Furthermore, according to the pros and cons of the three influencing factors on the carbon conversion versus the gaseous sulfur release, the optimized reaction conditions were determined as 900°C and the OC excess number around 1.0. Finally, under this optimized reaction condition, the effect of the cycle numbers was evaluated and found that the continuous release of gaseous sulfur from the CaSO4 side reactions deteriorated the reactivity of the purified CaSO4 OC and its reaction stability was diminished with the five cycles.

Evaluation of performance of copper converter slag as oxygen carrier in chemical-looping combustion (CLC)

In this study, copper converter slag sample was utilized as oxygen carrier (OC) for Chemical Looping Combustion (CLC) of synthetic syngas. The CLC cycles were substantiated in a bench-scale fluidized bed reactor by altering the inlet atmosphere between fuel and air consecutively. The tests were conducted at 4 times the minimum fluidization velocity, and 1123 K (850 °C) reduction half-cycle temperature (TRHC) for 50 cycles. The mean fuel gas conversion values obtained during 30 s of RHC time (tRHC) were as; 78% for 1:1 CO in nitrogen, 84–89% for syngas, and 97% for 1:1 H2 in nitrogen. Copper converter slag sample was stable and sufficiently reactive throughout the tests, moreover, the reactivity increased substantially by the consecutive CLC cycles. The increase of the porosity and the formation of the copper ferrite during the CLC cycles were found to be the main contributors to the activation of the copper converter slag. It is concluded that the evaluation of copper converter slag in the CLC process is both an economical option for OC supply (especially in coal-fueled CLC processes where some OC loss is inevitable), and an environmentally favorable alternative for recycling copper converter slag unless wasting the precious metals in it.

Evaluation of a bauxite cement-bonded Fe-based oxygen carrier during a hundred of cycles under coal-fueled chemical looping combustion conditions

The development of cost effective oxygen carriers (OCs) is one of the key factors for chemical looping combustion (CLC) to become a viable technology for controlling CO2 emissions from fossil fueled power plants, especially those using coal. Although bauxite cement-bonded Fe-based OCs have been identified as a potential candidate for CLC implementation, few studies have examined their long-term stability and process effectiveness under conditions of coal fueled power. Hence, this study examined long-term effectiveness of a bauxite cement-bonded Fe-based OC for conversion of coal and production of CO2-enriched off-gases under cycled processing typical to CLC technology. During reduction and oxidation cycles, the exhaust gas and samples of the OC were extracted from a laboratory scale, coal burning fluidized bed reactor to enable their characterization and provide insight into the process performance and physical attributes of the OC conducive to its application. The results show that the bauxite cement-bonded OC maintained high reactivity and regeneration capacity, and the mechanical strength integrity to withstand the mechanical and chemical stresses associated with fluidized bed and solids transfer typical to CLC.

Activation of ilmenite as an oxygen carrier for solid-fueled chemical looping combustion

Ilmenite ore is one of the promising oxygen carriers (OCs) used for coal-fueled Chemical-Looping Combustion (CLC) for electricity generation and CO2 capture. However, the low reactivity and natural activation of ilmenite OC are two major constrains impeding its application. This effort is to improve ilmenite OC’s performance by introducing a small amount of foreign elements, including alkali or alkaline earth metals (K and Ca) and transition metals (Cu, Mn, and Ni). Coating and re-granulation methods were used to prepare OCs where ilmenite ore was the primary constituent. The reactivity, transport capacity, and selectivity of these ilmenite-based OCs with wet syngas, as well as their performances in coal char-fueled CLC were investigated using a TGA, fixed- and fluidized-bed reactors. The addition of K-element significantly improved the OC’s reactivity with wet syngas and coal char. The strong catalytic function for WGSR from K-added OCs was found to play a vital role. Cu-coating hindered effectively Fe-element segregation on the surface of ilmenite OC during cyclic reaction, and the OC structural integrity was well maintained. Ca-, Ni- and Mn-added ilmenite OCs did not show promising prospects. The gasification rate and combustion efficiency could be respectively correlated to the reactivity and selectivity of different OCs except for the K-added samples.

Evaluating the Effect of Inert Supports and Alkali Sodium on the Performance of Red Mud Oxygen Carrier in Chemical Looping Combustion

Chemical looping combustion (CLC) is an advanced technology with inherent CO2 capture in which a solid oxygen carrier circulates between an air reactor and a fuel reactor. For coal-fueled CLC, the existence of solid impurities requires the oxygen carrier not only to have good reactivity but also to be contaminant-resistant, low-cost, and readily available. Therefore, the development of cost-effective and well-performing oxygen carriers is very meaningful for the coal-fueled CLC process. Natural red mud, a byproduct from the aluminum industry, was found to function well as an oxygen carrier and has also been found to have in situ coal catalytic gasification behavior. A thorough study on the long-term cyclic performance of red mud with coal char in a fluidized reactor was conducted in this work. For the purpose of comprehensively understanding the functions of inert supports as well as the sodium content in red mud, the effect of various inert oxides (Al2O3, SiO2, TiO2, and CaO) and the addition of sodium w...

Experimental study of copper modified manganese ores as oxygen carriers in a dual fluidized bed reactor

Chemical-looping combustion (CLC) is a developing CO2 capture technology. CLC makes use of the repeated oxidation/reduction reactions of metal oxide (oxygen carrier, OC) to separate CO2 from fuel combustion and to obtain a pure CO2 stream suitable for compression and storage. Low cost materials, such as natural ores, are required for coal-fueled CLC because the lifetime of the oxygen carrier (OC) is lowered by side reactions with the fuel ash or carryover losses. In this study, five manganese ores were examined as oxygen carriers using CO as the fuel gas in a laboratory batch fluidized bed reactor. All five of the ores were impregnated by copper nitrate solution to evaluate the reactivity enhancement of copper impregnation. The period with full CO conversion can be enhanced 2–100 times for different ores in the single fluidized bed test, which indicated that the Cu impregnation may be a general method to enhance the reactivity of manganese ores. Finally, one manganese ore and the corresponding Cu-modified particles were tested in a dual fluidized bed reactor. The attrition rates of both materials were measured as 0.13wt.%/h during the 88h operation in the dual fluidized bed. Both the manganese ore and the Cu-impregnated ore exhibited stable and high reactivity during the continuous test in the dual fluidized bed reactor, even at a low temperature (∼310°C). Copper impregnation had no obvious influence on the attrition property of the manganese ore.

Use of Pyrite Cinder as an Iron-Based Oxygen Carrier in Coal-Fueled Chemical Looping Combustion

Selection of low cost oxygen carriers with abundant reserves while being environmentally benign is preferred in the chemical looping combustion (CLC) process. Pyrite cinder is characterized as a waste material and poses potential environmental risk while having issues associated with disposal. In this study, pyrite cinder was utilized as a potential iron-based oxygen carrier. The reactivity, recyclability, and attrition behavior of pyrite cinder were evaluated in a laboratory scale fluidized bed reactor. The oxygen carrier to fuel ratio, steam concentration in the fluidization gas, fuel particle size, and temperature on the reactivity of pyrite cinder were investigated. The attrition behavior of pyrite cinder under both inert and reacting conditions was evaluated. The chemical and physical analyses of pyrite cinder confirmed it as a ready source of oxygen carrier. It displayed sufficient reactivity to convert char gasification products to CO2 and H2O. The performance of the system was found to be improved with respect to the carbon conversion rate and gasification rate under the following conditions: higher oxygen carrier to fuel ratio, higher steam concentration in the fluidization gas, smaller fuel particle size, and higher temperature. Cyclic redox tests of pyrite cinder over 20 cycles revealed that it behaved steadily with a stable CO2 yield being achieved. Additionally, pyrite cinder exhibited good resistance to sintering and agglomeration. The attrition behavior of pyrite cinder under inert conditions showed that the collisions of pyrite cinder particles with each other and with reactor wall at high superficial fluidization velocity was the predominant factor influencing its attrition behavior. The cyclic attrition tests showed that the attrition rate was higher in the initial cycle, but this reduced as the redox cycles progressed. It can be inferred from this study that pyrite cinder is a suitable iron-based oxygen carrier for CLC of coal while alleviating the environmental problems associated with its disposal.

Development of a Cost-Effective Oxygen Carrier from Red Mud for Coal-Fueled Chemical-Looping Combustion

Coal-fueled chemical-looping combustion (CF-CLC) is an advanced process for highly efficient coal combustion with in situ CO2 capture. The feasibility of applying CF-CLC is heavily dependent on the suitability of the oxygen carrier (OC). In this study, the abundant red mud bauxite waste produced from Bayer’s alumina production process is proposed as a cost-effective oxygen carrier using a direct granulation method. A screening test was carried out to select the best OC from samples prepared at various calcination temperatures. The regeneration and reduction behavior of red mud OCs were extensively investigated in a simulated CLC process while considering the influence of water vapor and reaction temperature. A synthetic iron-based OC was used as a reference to evaluate the performance of the red mud OCs. The fresh and used OC particles were characterized by XRD, SEM, and BET analyses to interpret the behavior of the solid particles. The leaching behavior of the raw red mud and the OC samples was tested using simplified leaching tests to assess the feasibility of disposal of the spent red mud OCs in a landfill.

Investigation of a Coupled Fuel Reactor for Coal-Fueled Chemical Looping Combustion

This work presents an experimental investigation and modeling study of a fuel reactor designed for in situ gasification chemical looping combustion (iG-CLC). The fuel reactor is designed to couple a bubbling fluidized-bed reactor and a downer reactor. The downer reactor can enhance gas–solid contact in the freeboard region and eliminate the unburned gases from the fuel reactor. The fuel reactor consists of an annular bubbling fluidized bed and a center-located riser. Cold-model experiments were conducted. The solids circulation rate and gas bypassing between the annular fluidized bed and the riser were studied. Experiments in a hot downer reactor were performed, and a two-dimensional model was developed and validated with experimental data. The verified model shows that a downer reactor with proper solids dispersion can eliminate more than 78% of the fuel gas exiting the fuel reactor with an overall solids dropping rate of 8 kg/m2·s.

Pressurized chemical-looping combustion of coal using an iron ore as oxygen carrier in a pilot-scale unit

Chemical looping combustion (CLC) of coal is established as a new concept for inherent CO2 separation with the advantages of low cost and high conversion efficiency. A pilot-scale unit consisting of two fluidized bed reactors was constructed and operated to study the performance of pressurized chemical-looping combustion (PCLC). Experiments were conducted using Shenhua bituminous coal as fuel and MAC iron ore as oxygen carrier at three different operation pressures (0.1, 0.3 and 0.5MPa) and the unit has been operated for totally 19h with steady coal-feeding, about 13.5h of which were realized under stable operation. The results showed that PCLC of coal demonstrated many remarkable advantages over the conventional CLC of coal, in terms of the improvement of carbon conversion in the fuel reactor, CO2 concentration of exhaust gas and combustion efficiency. At the operating pressure of 0.5MPa, the CO2 concentration, carbon conversion and combustion efficiency reached very high values of 97.2, 84.7 and 95.5% respectively. The potential of low-cost iron ore as oxygen carrier for commercial coal-fueled CLC unit was also examined. The loss of oxygen carrier due to fine particles leaving the reactors increased with the increase of operation pressure and no agglomeration of oxygen carrier particles occurred during these tests. The oxygen carrier used in PCLC tests were also characterized by SEM-EDX and BET analysis to further emphasis on the effects of operation pressures and runtime.

Effect of Temperature on Reduction of CaSO4 Oxygen Carrier in Chemical-Looping Combustion of Simulated Coal Gas in a Fluidized Bed Reactor

Chemical-looping combustion (CLC) is a promising combustion technology for gaseous and solid fuel with efficient use of energy and inherent separation of CO2. The concept of a coal-fueled CLC system using calcium sulfate (CaSO4) as oxygen carrier is proposed in this study. Reduction tests of CaSO4 oxygen carrier with simulated coal gas were performed in a laboratory-scale fluidized bed reactor in the temperature range of 890−950 °C. A high concentration of CO2 was obtained at the initial reduction period. CaSO4 oxygen carrier exhibited high reactivity initially and decreased gradually at the late period of reduction. The sulfur release during the reduction of CaSO4 as oxygen carrier was also observed and analyzed. H2 and CO conversions were greatly influenced by reduction temperature. The carbon deposition ratio was found to be quite low. The oxygen carrier conversion and mass-based reaction rates during the reduction at typical temperatures were compared. Higher temperatures would enhance reaction rates and result in high conversion of oxygen carrier. An XRD patterns study indicated that CaS was the dominant product of reduction and the variation of relative intensity with temperature is in agreement with the solid conversion. The slight content of CaO in reduced oxygen carrier at high temperatures was due to the formation of SO2 and H2S during the reduction period. ESEM analysis indicated that the surface structure of oxygen carrier particles changed significantly from impervious to porous after reduction. Slight agglomeration of small grains occurred for reduced particles at 950 °C. EDS analysis also demonstrated the transfer of oxygen from the oxygen carrier to the fuel gas and a certain amount of sulfur loss and CaO formation on the surface at higher temperatures. The reduction kinetics of CaSO4 oxygen carrier was explored with the shrinking unreacted-core model. The apparent kinetic parameters were obtained, and the kinetic equation well predicted the experimental data. Finally, some basic considerations on the use of CaSO4 oxygen carrier in a CLC system for solid fuels were discussed.