Fluidized Bed Chemical Looping Combustion Research Articles

A novel moving bed chemical looping process with integration of combustor heat exchangers for hydrogen production: Process simulation and techno-economic analysis

In this study, using natural gas as the feedstock, a new scheme – a chemical looping combustion system with a moving bed reducer, coupled with the steam methane reforming (MCLC-SMR), is proposed and simulated for hydrogen production. This suggested process configuration is compared with other hydrogen production systems including the fluidized bed chemical looping combustion system coupled with the steam methane reforming (FCLC-SMR), the chemical looping partial oxidation system combined with the water-gas-shift reaction (CLPO-WGS), the 3-reactor chemical looping system for hydrogen generation (CLHG-3R), the conventional steam methane reforming (SMR) and auto-thermal reforming (ATR) systems with CO2 capture. The simulation results indicate that the particle circulation rate in MCLC-SMR is 64% less than in FCLC-SMR. MCLC-SMR also shows the best system performance regarding cold gas efficiency (79%) and effective thermal efficiency (75%). The economic assessment indicates that MCLC-SMR can achieve the lowest levelized cost of hydrogen (LCOH) of $1.34/kg at 30 ton/hr H2 production capacity, and CLHG-3R can achieve the lowest LCOH of $2.06/kg at 1 ton/hr H2 production capacity among the six hydrogen production methods. At 30 ton/hr H2 production capacity, the chemical looping reactors in MCLC-SMR are 37% cheaper than in FCLC-SMR. The sensitivity analysis also indicates that MCLC-SMR remains the most economical hydrogen production method over a wide range of price parameters (the prices of particles, chemical looping reactors, natural gas, and electricity).

Modelling of SO2 and NOx Emissions from Coal and Biomass Combustion in Air-Firing, Oxyfuel, iG-CLC, and CLOU Conditions by Fuzzy Logic Approach

Chemical looping combustion (CLC) is one of the most advanced technologies allowing for the reduction in CO2 emissions during the combustion of solid fuels. The modified method combines chemical looping with oxygen uncoupling (CLOU) and in situ gasification chemical looping combustion (iG-CLC). As a result, an innovative hybrid chemical looping combustion came into existence, making the above two technologies complementary. Since the complexity of the CLC is still not sufficiently recognized, the study of this process is of a practical significance. The paper describes the experiences in the modelling of complex geometry CLC equipment. The experimental facility consists of two reactors: an air reactor and a fuel reactor. The paper introduces the fuzzy logic (FL) method as an artificial intelligence (AI) approach for the prediction of SO2 and NOx (i.e., NO + NO2) emissions from coal and biomass combustion carried out in air-firing; oxyfuel; iG-CLC; and CLOU conditions. The developed model has been successfully validated on a 5 kWth research unit called the dual fluidized bed chemical looping combustion of solid fuels (DFB-CLC-SF).

Investigation of biomass alkali release in a dual circulating fluidized bed chemical looping combustion system

Chemical looping combustion (CLC) of biomass is a promising technology for power generation with integrated carbon capture. In CLC, alkali content of biomass poses potential issues of bed agglomeration, as well as heat exchanger fouling and corrosion. The fate of biomass alkalis was investigated in a dual-interconnected circulating fluidized bed CLC system. Experiments were conducted in oxygen carrier aided combustion (OCAC) and CLC modes. Ilmenite and braunite oxygen carriers and three biomass fuels (wood pellets, wood char, straw pellets) were tested. Flue gas alkali emissions in the air reactor (AR) and fuel reactor (FR) were measured with a surface ionization detector (SID). Results showed that CLC operation yields gas-phase alkali emissions that are up to 15 times higher than in comparable OCAC operation. Results analysis concluded that increased alkali emissions in CLC arise from the steam atmosphere in the FR, whereby steam accelerates the decomposition of alkali compounds in the biomass. Retention of alkalis in the condensed phase was found to be >97% for ilmenite and >92% for braunite CLC operation. Up to 60–80% of the retention was attributed to fuel ash formation. The residual retention was attributed to absorption of alkalis by the oxygen carriers. Absorption likely occurred mainly through formation of alkali manganates and silicates in braunite, and formation of alkali silicates, aluminosilicates, manganates, and titanates in ilmenite. Gas-phase alkali emissions in the AR, although less than in the FR, were found to occur due to combustion of unconverted fuel carried over from the FR to the AR.

Modeling of the Chemical Looping Combustion of Hard Coal and Biomass Using Ilmenite as the Oxygen Carrier

This paper presents a 1.5D model of a fluidized bed chemical looping combustion (CLC) built with the use of a comprehensive simulator of fluidized and moving bed equipment (CeSFaMB) simulator. The model is capable of calculating the effect of gas velocity in the fuel reactor on the hydrodynamics of the fluidized bed and the kinetics of the CLC process. Mass of solids in re actors, solid circulating rates, particle residence time, and the number of particle cycles in the air and fuel reactor are considered within the study. Moreover, the presented model calculates essential emissions such as CO2, SOX, NOX, and O2. The model was successfully validated on experimental tests that were carried out on the Fluidized-Bed Chemical-Looping-Combustion of Solid-Fuels unit located at the Institute of Advanced Energy Technologies, Czestochowa University of Technology, Poland. The model’s validation showed that the maximum relative errors between simulations and experiment results do not exceed 10%. The CeSFaMB model is an optimum compromise among simulation accuracy, computational resources, and processing time.

Experimental demonstration of pressurized chemical looping combustion in an internally circulating reactor for power production with integrated CO2 capture

This study presents an experimental demonstration of pressurized chemical looping combustion (CLC) in an internally circulating reactor (ICR). The ICR concept is a novel alternative to the conventional interconnected fluidized bed CLC configuration as it eliminates all cyclones, loop seals and solids transport lines, and it can be pressurized in a single pressure shell. Stable operation with high fuel conversion was established for about 40 h of operation at pressures up to 6 bar, achieving reasonable CO2 purity and capture efficiency (up to 97%). The solids circulation rate was found to increase with increasing the operating pressure at a constant fluidization velocity with no effect on CO2 capture and purity. The experimental campaign also examined the effects of solids inventory and fluidization velocities in the air and fuel reactors. The CO2 purity and capture efficiency were most sensitive to the solids inventory, whereas the solids circulation rate was most sensitive to the air reactor fluidization velocity and the solids inventory. A correlation for solids circulation rate was derived from the collected experimental data, thus providing a robust tool for designing an ICR system for pressurized operation. This correlation can assist in further scale-up and demonstration of the ICR concept in commercial scale.

Geopolymer-composites with thermomechanical stability as oxygen carriers for fluidized bed chemical looping combustion with oxygen uncoupling

Oxygen carriers based on geopolymer composites with enhanced thermal and mechanical stability have been synthesized and been proposed for chemical looping combustion with oxygen uncoupling. Geopolymer synthesis belongs to green chemistry and ensures chemical and mechanical properties similar to those of high temperature sintering. Suitable metal oxides, such as combinations of Cu, Mn and Fe oxides, have been used as active fillers in geopolymer matrices. Thermogravimetric analysis conducted at 900 °C pointed out the excellent performance of the CuO-based oxygen carrier, and the overall performance trend of CuO > CuMn-oxides > MnFe-oxides. The performance of the CuO based material (around 1.0% wt. of transferred O2) was confirmed in tests carried out in a fluidized bed at 900 °C, where a slightly lower carrying capacity, and a remarkable oxidation rate index (37.1 ± 1.4%) were obtained. Tests of wood-char combustion confirmed the ability of the CuO-based oxygen carrier to operate in a real chemical looping combustion process. Moreover, attrition and crushing tests demonstrated that the mechanical resistance of geopolymer-based oxygen carriers was comparable to that of materials currently used for fluidized bed application.

Evaluation of coal conversion pathways in fluidized bed chemical looping combustion with oxygen uncoupling (CLOU)

Chemical looping with oxygen uncoupling (CLOU) is a variant of chemical looping combustion that uses oxygen carriers that release gaseous O2 in the fuel reactor to efficiently combust solid char. There are many different pathways by which fuel carbon can be converted in a CLOU system, including reaction of volatiles, oxidation of char and gasification of char followed by conversion of the gaseous products. Therefore, there is competition between volatiles and solid char for both gaseous and metal bound oxygen. This study presents an experimental method of analysis to better understand the conversion pathway and fate of fuel carbon in a fluidized bed reactor system. The analysis is applied to a Cu-based CLOU carrier in a bench-scale reactor, feeding either coal or coal char in small batches. The effect of coal particle size and reactor temperature on the relative contributions of different carbon conversion pathways is evaluated. Under the conditions studied, higher temperatures and smaller char particles were more effectively converted by gaseous O2 released by the CLOU carrier, resulting in less unconverted char. However, there was more unconverted volatiles exiting the reactor from small coal particles and the resulting char had a higher tendency to exit the fuel reactor unconverted. Larger char particles relied more on conversion by gasification and subsequent oxidation of combustible products. Results for other systems will depend on operating conditions and properties of the fuel, but conversion pathways can be systematically evaluated using the method described.

Color Indistinction Method for the Determination of Devolatilization Time of Large Fuel Particles in Chemical Looping Combustion

Chemical looping combustion (CLC) is one of the promising fuel conversion technologies for carbon capture with low energy penalty. Devolatilization is an important physical phenomenon occurring during solid fuel CLC. Devolatilization behavior influences fragmentation, combustion rate, emission, and particulates generation in fluidized bed CLC (FB-CLC), thus a critical input for its design. Existing visual techniques for determining devolatilization time cannot be applied in CLC conditions because of its flameless combustion nature. In the present study, a new, simple, and quick technique called “color indistinction method” (CIM) is proposed for the determination of devolatilization time (τd) in FB-CLC, where the end of devolatilization is inferred from the disappearance of fuel particle in a hot fluidized bed. Single-particle devolatilization studies in FB-CLC are conducted to determine the devolatilization time using CIM for two types of fuels, viz., coal and biomass (Casuarina equisetifolia wood), of si...

Conversion of Coal in a Fluidized Bed Chemical Looping Combustion Reactor with and without Oxygen Uncoupling

Chemical looping combustion (CLC) is a carbon capture technology that involves circulating metal-based oxygen carrier particles between two reactors, where the metal is alternately oxidized by air and then reduced by fuel as the fuel combusts. In the variant of CLC known as chemical looping with oxygen uncoupling (CLOU), the oxygen carrier releases gaseous O2 in the fuel reactor, allowing for a rapid reaction with solid fuels. This study focuses on improving the understanding of mechanisms responsible for carbon conversion of coal and coal char in a bench-scale fluidized bed reactor using oxygen carriers with and without oxygen uncoupling properties. Effects of coal particle size and temperature on the rate of carbon conversion are evaluated in order to quantify performance and to aid in design and modeling of large-scale fully circulating chemical looping systems. The data confirms that conversion with a copper-based CLOU carrier is faster and results in lower CO and CH4 concentrations than the natural o...

Primary Fragmentation Behavior of Indian Coals and Biomass during Chemical Looping Combustion

Devolatilization and fragmentation are important physical phenomena occurring during solid fuel chemical looping combustion (CLC). Primary fragmentation during devolatilization strongly affects the rate of fuel conversion, emissions, and fine particulates generation in a fuel reactor of a fluidized bed CLC unit, thus forming a critical design input. The present study focuses on investigating the primary fragmentation behavior of large coal and biomass (wood) particles during the devolatilization phase of CLC. Three types of coals (two Indian coals, one Indonesian coal) and one type of Casuarina wood of three sizes in the range of 8–25 mm, at different fuel reactor bed temperatures (800, 875, and 950 °C) are studied for primary fragmentation. Iron ore with 64% Fe is used as the oxygen carrier bed material, with steam as the fluidizing medium in the fuel reactor. The fragmentation behavior is expressed in terms of the number of fragments, fragmentation index, frequency of fragmentation, and particle size di...

Dynamic simulation of fluidized bed chemical looping combustion process with iron based oxygen carrier

Chemical looping combustion is a promising energy conversion technology for fossil fuel combustion with inherent carbon dioxide separation and minimum energy losses. A full understanding of the dynamic behavior is of paramount importance to successfully implement this technology at commercial scale. In this work, a dynamic mathematical model has been developed to simulate interconnected fluidized bed reactors of a 1 MWth syngas-based chemical looping combustion process with iron based oxygen carrier. The model consists of mass and energy balances equations as well as the equations that describe the hydrodynamics and heat transfer processes. The particles distribution inside the fluidized bed reactors is described by a 1.5-D hydrodynamic model. The model showed two zones in axial direction (an upper lean zone and a bottom dense zone) and a horizontal separation between core region and wall layer. Different heat transfer mechanisms have been taken into account: convection between gas phase and the oxygen carrier particles; convection of the moving particle phase with wall and radiation. The developed model was used to predict (in space and time): gas and solid velocity distribution, gas composition distribution, behavior of oxygen carrier, and temperature profiles inside the air and fuel reactor. The dynamic behavior of the chemical looping combustion process was studied by step and ramp input changes in the input gas flow rate.

Improvement of “Near-term” Fluidized Bed Chemical Looping Combustion for Power Generation

Chemical looping combustion (CLC) is regarded as the “second-generation” CCS for power generation, and it has been mainly focused on to be operated on two interconnected fluidized bed. With the previously abundant and relative matured experience in operating CLC in fluidized bed, power generation from fluidized bed CLC is a “near-term” solution for practical utilization of CLC. In this work, we proposed a novel process by thermally coupled fluidized-bed CLC with advanced supercritical CO2 cycle (CLC-sCO2). From the preliminarily results of thermodynamic performance, approximately 6.4 percent points of net electricity efficiency is benefited from this novel process as against that of conventional CLC-steam cycle process, reaching 48.13% close-match to that of a conventional NGCC process with CO2 capture (49.38%). To further elevate thermal efficiency of CLC-sCO2, improvements are conducted by operating a low-pressure fluidized bed CLC and replacing one-stage supercritical CO2 compressor into two-stage intercooling compressors, as can be expected, approximately 2% of additional thermal efficiency is earned in an improved CLC-sCO2 system. Finally mapping of future CLC power station including “near-term”, “long-term” and future period is included.

Polyhedral meshing as an innovative approach to computational domain discretization of a cyclone in a fluidized bed CLC unit

Chemical Looping Combustion (CLC) is a technology that allows the separation of CO2 , which is generated by the combustion of fossil fuels. The majority of process designs currently under investigation are systems of coupled fluidized beds. Advances in the development of power generation system using CLC cannot be introduced without using numerical modelling as a research tool. The primary and critical activity in numerical modelling is the computational domain discretization. It influences the numerical diffusion as well as convergence of the model and therefore the overall accuracy of the obtained results. Hence an innovative approach of computational domain discretization using polyhedral (POLY) mesh is proposed in the paper. This method reduces both the numerical diffusion of the mesh as well as the time cost of preparing the model for subsequent calculation. The major advantage of POLY mesh is that each individual cell has many neighbours, so gradients can be much better approximated in comparison to commonly-used tetrahedral (TET) mesh. POLYs are also less sensitive to stretching than TETs which results in better numerical stability of the model. Therefore detailed comparison of numerical modelling results concerning subsection of CLC system using tetrahedral and polyhedral mesh is covered in the paper.

A 1.5D model of a complex geometry laboratory scale fuidized bed clc equipment

This paper describes experiences in modeling of a laboratory scale fluidized bed Chemical Looping Combustion (CLC) equipment.The simulations are carried out by 1.5D model completed by the use of Comprehensive Simulator of Fluidized and Moving Bed equipment (CeSFaMB).The studies, calculations and model validation are performed for a CLC unit. The facility constitutes two cycles: the main cycle and the internal cycle with air reactor (AR) and fuel reactor (FR), respectively.The paper is focused on fluid dynamics in a cold model of a CLC unit. Simulations showed, that the performed 1.5D model correctly describes the fluidization dynamics and can be applied to study the fluidized bed CLC unit operation.The described investigations are a reference point for further simulations of hot CLC test rig since they constitute the first part of the work scheduled in the Polish-Norwegian Research Programme: “Innovative Idea for Combustion of Solid Fuels via Chemical Looping Technology”, funded by the National Centre for Research & Development in Poland.

Reduction kinetics of hematite as oxygen carrier in chemical looping combustion

Chemical looping combustion (CLC), which has the characteristic of CO2 inherent separation, is a novel fossil fuel utilization technology for CO2 capture with low cost and low energy efficiency penalty. The reduction kinetics of oxygen carrier (OC) is one of the most important issues for CLC reactor design and computational fluid dynamics (CFD) simulation. In this work, well-organized isothermal experiments in a thermogravimetric analyzer (TGA) were first carried out to obtain the kinetics of a hematite as OC reduced by carbon monoxide. The reduction process was identified as two sequential stages, which correspond to the reduction of Fe2O3 to Fe3O4 and Fe3O4 to FeO, respectively. As obtained from the experimental result, the first stage is controlled by gas diffusion in the boundary layer on particle surface while the second stage is mainly controlled by heterogeneous chemical reaction. For the second stage, the activation energy, E, is 110.75kJ/mol and the reaction order, n, is 1.5. Then, the same batch of hematite was tested by reacting with the reducing gas of 10vol.% CO balanced by N2 in a fluidized bed reactor. The different reaction characteristics in TGA and fluidized bed reactor were analyzed. This work provides a deep understanding on the applicability of the OC reduction kinetics obtained by TGA to the reactor design and CFD simulation of a fluidized bed CLC rig.

An Eulerian Approach to Computational Fluid Dynamics Simulation of a Chemical-Looping Combustion Reactor With Chemical Reactions

Chemical-looping combustion (CLC) is a next-generation combustion technology that shows great promise in addressing the need for high-efficiency low-cost carbon capture from fossil fueled power plants. Although there have been a number of experimental studies on CLC in recent years, computational fluid dynamics (CFD) simulations have been limited in the literature. In this paper, simulation of a CLC reactor is conducted using the Eulerian approach in the commercial CFD solver ansys fluent based on a laboratory-scale experiment with a dual fluidized bed CLC reactor. The solid phase consists of a Fe-based oxygen carrier while the gaseous fuel used is syngas. The salient features of the fluidization behavior in the air reactor and fuel reactor beds representing a riser and a bubbling bed, respectively, as well as the down-comer, are accurately captured in the simulation. This work is among the few CFD simulations of a complete circulating dual fluidized bed system for CLC in 3D in the literature. It highlights the importance of 3D simulation of CLC systems and the need for more accurate empirical reaction rate data for future CLC simulations.

Experimental investigation of fluidized bed chemical looping combustion of Victorian brown coal using hematite

An experimental investigation on chemical looping combustion (CLC) of Victorian brown coal is carried out in a fluidized bed of hematite. The aim of this study is to understand the feasibility of Victorian brown coal CLC as very little technical information is currently available on the process using these coals. The in situ CLC experiments are first performed using a thermogravimetric analyzer (TGA). The TGA results show good performance of hematite as oxygen carrier over five multiple re-dox cycles under CO2 gasification environment. Therefore, further investigation is performed using a bench-scale fluidized bed that operates in a batch mode cyclically with reduction in CO2 environment, and oxidation in air. Several tests have been conducted to assess the impact of different temperatures, particle size of hematite and CO2 concentration in a flowing fluidizing gas medium. It is observed that the hematite particles of 100–150μm performed best with respect to carbon conversions that show an increasing trend with increasing temperature and CO2 concentration in the fluidizing gas.

Development and testing of ilmenite granules for packed bed chemical-looping combustion

Abstract Chemical-looping combustion (CLC) is a promising technology that integrates power production and CO 2 capture with a low energy penalty. CLC has been successfully demonstrated using interconnected fluidized bed reactor systems. However, high pressure operation allows the use of inherently more efficient power cycles than low pressure fluidized bed solutions. With the aim to work at elevated pressures, dynamically operated packed-bed reactors have been proposed for CLC. In a packed-bed CLC reactor bigger oxygen carrier particles are used to avoid very large pressure drops and the required mechanical properties of these particles are quite different from the properties needed in a fluidized bed CLC system. In this work different particles have been developed and tested for application in a packed-bed CLC reactor. Ilmenite was chosen as base material because of its good reactivity with syngas, the natural availability and related low cost. Different ilmenite pellets with different composition and shape were developed and their mechanical properties before and after thermal and chemical cycling were analysed and compared. The reactivity of the particles and the influence of different reaction conditions were also studied in a thermogravimetric analyser. It was found that the thermal and chemical stresses produced an important deterioration of the mechanical properties of the pellets. The type of additive used during the production process showed an important effect on the mechanical properties of the granules. Only the granules produced with Mn 2 O 3 as additive demonstrated acceptable suitable mechanical properties after thermal and chemical cycling.

Overview of Chemical-Looping Reduction in Fixed Bed and Fluidized Bed Reactors Focused on Oxygen Carrier Utilization and Reactor Efficiency

A model-assisted comparison of two types of chemical-looping (CL) reactors (fixed bed and fluidized bed), with the same oxygen carrier loading and fuel capacity, is carried out to examine performance and efficiency of CL Reducers, operating with methane as the feedstock and nickel oxide as the oxygen carrier. The study focuses on the reduction step of chemical-looping combustion (CLC), for which the reactor efficiency and fuel utilization are crucial in terms of economics and carbon capture efficiency. Process models (a three phase dynamic model for bubbling fluidized beds and a two dimensional homogeneous model for fixed beds) and reaction kinetics developed and validated in previous studies are used. A fluidized bed chemical-looping combustion Reducer is compared to a fixed bed equivalent reactor, scaled-up from a smaller experimental reactor, constrained to bed height to reactor diameter ratios that prohibit excessive temperature and pressure drops across the bed. Through a detailed comparison, CLC operated in the fluidized bed reactor is shown to deliver superior performance, i.e., uniform temperature and pressure distribution; high methane conversion (> 95%) and carbon dioxide selectivity (> 95%) sustained for longer reduction periods; negligible carbon formation (< 2 mol% C basis); and better efficiency in oxygen carrier utilization.

Ni based oxygen carrier over γ-Al2O3 for chemical looping combustion: Effect of preparation method on metal support interaction

This study deals with the development of a Ni based oxygen carrier (OC) over La-modified and Co-doped γ-Al2O3 suitable for a fluidized bed chemical-looping combustion (CLC) process. La is found to stabilize the bulk phase transformation of γ-Al2O3 whereas Co minimizes the formation of irreducible bulk nickel aluminate. Apart from these beneficial effects, the reducibility and the structural properties of the prepared OCs are found to be modified by the different preparation methods used. The N2 adsorption isotherm shows that γ-Al2O3 retains its structural integrity in only one preparation method. Reducibility as determined by consecutive temperature programmed reduction and oxidation techniques resembles the chemical properties of δ- and θ-Al2O3 for the other two preparation methods. However, no bulk phase change is detected for the three OCs using XRD. This suggests that the observed changes occur mainly at the molecularly thin surface layers of the OCs which display structural properties (geometric, electronic, coordination) significantly different from those of the bulk. The prepared OCs are also tested under CLC operating condition of the Chemical Reactor Engineering Center (CREC) fluidized Riser Simulator, using multiple reduction/oxidation cycles. Reactivity results obtained show expected reducibility, oxygen carrying capacity and stability with the three different preparation techniques.