Chemical Looping Combustion Pilot Plant Research Articles

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.

Increasing energy efficiency in chemical looping combustion of methane by in-situ activation of perovskite-based oxygen carriers

CaMnO3-based perovskites are attractive for use as oxygen carriers in chemical looping combustion (CLC) because they are made from cheap materials (manganese, calcium). However, these materials show low reactivity with methane, and high oxygen carrier-to-fuel ratios (ϕ) are required to achieve complete fuel combustion. It is believed that energy efficiency in the chemical looping process would be improved if the reactivity of these materials were higher. In this work, the increase in reactivity of a perovskite (CaMn0.775Mg0.1Ti0.125O2.9-δ) material is evaluated during its in-situ activation under determined operating conditions in a 0.5 kWth CLC pilot plant. The combustion efficiency of methane over 60 h of continuous operation was analysed taking into consideration the effect of operating conditions on the activation process for increasing the reactivity of the perovskite. The material increased in reactivity as the ϕ ratio decreased to below 6, improving combustion efficiency. Moreover, the material was pre-activated by being fully reduced in the CLC plant. The conversion of methane with the pre-activated material was subsequently higher than the conversion achieved before the activation process, and almost complete combustion was achieved at a lower ϕ ratio. Characterization of the particles after the combustion tests showed good stability of the material with the operating time, although a negative effect on the crushing strength was observed when the particles were highly reduced. The impact of the perovskite material activation on the energy efficiency of the process is discussed.

Evaluation of a new DCFB reactor system for chemical looping combustion of gaseous fuels

A 120 kWth chemical looping combustion pilot plant, has been used to evaluate the performance of up to 10 different oxygen carrier materials within more than 1000 h of operation. It was one of the largest pilot plants, featuring a scale up ready design and auto thermal operation. Initially the plant was designed for nickel based oxygen carriers. During the evaluation of other materials, limitations of the reactor design caused by different design demands of the oxygen carriers have been identified. A new reactor system has been installed incorporating design adaptations to improve the reactor flexibility and performance. The new air reactor is narrower and allows higher gas velocities and high solid circulation rates, even at lower fuel loads. The upper loop seal, connecting the air reactor with the fuel reactor has been optimized to reduce a gas leakage between the reactors, identified in the old reactor system. The new fuel reactor consists of two sections, a lower part with a larger cross section and a narrower upper part. This allows larger specific inventories in the reactor and good gas/solid contact over the full reactor height. The new reactor system has been evaluated during a benchmark campaign, using a manganese based oxygen carrier which has also been used in the old unit. The results are reported in this work. The new design meets the design criteria, high solids circulation is possible, higher specific inventories can be operated and the gas leakage via the upper loop seal has been reduced significantly.

Unsteady three-dimensional theoretical model and numerical simulation of a 120-kW chemical looping combustion pilot plant

In this paper, reactive unsteady three-dimensional numerical simulations of a chemical looping combustion (CLC) unit are presented. The configuration is a 120-kW pilot plant working with perovskite, CaMn0.9Mg0.1O3−δ, as the selected oxygen carrier. Numerical simulations were performed using NEPTUNE_CFD code in the frame of an Euler-Euler approach by computing both gas and solid phases in an Eulerian technique, accounting for specific closures in order to model the interphase mass, and momentum and energy transfers. Heterogeneous reduction and oxidation (gas–solid) reactions were modeled by means of a grain model to account for the competing mechanisms of the chemical reaction onto the grain surface, the gaseous diffusion through the product layer around the grain, and the external transfer through the gas mixture surrounding the particle. Results from numerical simulations were assessed against experimental measurements and analyzed in order to acquire insight on the local behavior of reactive gas-particle flow in the CLC system. The theoretical/numerical tool developed in this work can be used for the design upgrade of crucial parts of the system in the stage of scaling-up from pilot to industrial plants.

Reactive two–fluid model for chemical–looping combustion – Simulation of fuel and air reactors

Chemical–looping combustion (CLC) is a promising oxyfuel carbon capture technology consisting of two interconnected fluidized beds with a metal oxide as a bed material/oxygen carrier. The particles, which circulate between both reactors, firstly absorb oxygen in the air reactor (AR) and secondly provide the oxygen for the combustion in the fuel reactor (FR). In this work, CFD simulation models for fuel and air reactor of the world’s second–largest CLC pilot plant at Technische Universität Darmstadt have been developed. An in–house thermochemical reaction model is combined with the two–fluid model implemented in ANSYS–FLUENT to simulate the gas-solid interaction of the FR and AR of the 1 MWth chemical–looping combustion pilot plant. All reactions, including the pyrolysis of coal, the gasification and oxidation of char and the reactions of the oxygen carrier are included by customized user defined functions (UDF). Stable, transient simulations of 55 s (FR) and 50 s (AR) were carried out. The FR simulation is showing a good agreement towards experimental measurements for the pressure drop. However, deviations occur in the gas composition between the simulation and the experimental data. Good agreement is obtained for the gas composition and the temperature in the AR. Slight discrepancies occur in the pressure profile between the numerical and experimental data. Possible reasons for deviations are discussed and appropriate solutions provided.

Development and validation of a 1D process model with autothermal operation of a 1 MWth chemical looping pilot plant

Chemical Looping Combustion (CLC) has been extensively investigated up to pilot scale. Despite promising results, this CO2 capture technology has to be further developed towards a potential industrial application. In order to accelerate the scale up of the process, it is essential to simulate the CLC using a process simulation software. In this study, a process simulation model of the CLC pilot plant at Technische Universität Darmstadt is developed employing Aspen Plus®. The air reactor and the fuel reactor are integrated in the process model to calculate the conversion of the oxygen carrier and gases. The solids distribution and also the chemical conversion of gases and solids are implemented by user-defined functions programmed with FORTRAN. These codes are based on mathematical equations for fluidized beds and kinetic data of the chemical reactions taken into consideration. For this reason, dedicated thermal gravimetric analysis (TGA) experiments were carried out to determine both kinetics of redox reactions of oxygen carrier and gasification kinetics of the coal used in the 1 MWth CLC unit. The one-dimensional solids distributions and gas/solid mixing are calculated from empirical correlations according to Kunii and Levenspiel. The models are validated by means of experimental data obtained from the 1 MWth CLC pilot plant with ilmenite as oxygen carrier and Russian hard coal (“Taldinsky”) as fuel. Among others, pressure profiles and gas concentrations at reactor exits are calculated for various operational points with autothermal operation. The results show good agreement with the experimental data. The process model was applied to optimize the operational conditions of the pilot plant.

Performance of an iron based oxygen carrier in a 120 kWth chemical looping combustion pilot plant

Chemical looping combustion is an unmixed combustion process with inherent separation of CO2. CLC technology has the capability to generate concentrated CO2 as further use for carbon capture and storage and thus reduce CO2 emissions. Since the use of cheap and non toxic oxygen carriers is favourable, iron is a potential candidate as active component.The performance of iron oxide impregnated on alumina (called Fe17) as oxygen carrier is investigated. Fe17 is tested in a 120kWth chemical looping combustion pilot plant at TU Wien. Various tests are carried out, the influence of active solid inventory, temperature, used gaseous fuel and oxygen carrier to fuel ratio are in the focus of the investigation. Post-experimental tests with thermogravimetric analysis, X-ray fluorescence analysis, particle size distribution and scanning electron microscope are used to characterize Fe17. The oxygen carrier shows attrition and a loss of iron. This leads to a drop of fuel conversion. The energy based fuel conversion is approx 40% to 60 %. For good fuel conversion, high inventories are needed in both reactors. Furthermore, high solid circulation rates and a high iron content on the particles show better fuel conversion.

Performance and degradation mechanisms of CLC particles produced by industrial methods

A promising copper based oxygen carrier produced using industrially-relevant manufacturing equipment has been aged in IFPEN’s 10 kWth Chemical Looping Combustion pilot plant. Methane combustion was performed at 900 °C for 160 h with an OC/fuel ratio of 1.2. While full methane conversion to CO2 and water was achieved in the early stage of the experiment, fast deactivation was observed. The oxygen carrying particles were thoroughly characterized at various stages of the ageing experiment, from which a pathway leading to deactivation is proposed.

Long-lasting Cu-based oxygen carrier material for industrial scale in Chemical Looping Combustion

One of the most important current objectives of the Chemical Looping Combustion (CLC) technology for gaseous fuels lies in scaling-up the aforementioned technology in the short term from 100kWth to 10MWth scale. In order to meet this challenge, the commercial availability of suitable multi ton-scale oxygen carrier materials at competitive price is needed. In this work, a Cu-based oxygen carrier prepared by the impregnation method using a commercial alumina as support, referred as Cu14γAl_Commercial, has been developed and evaluated in a 500Wth CLC pilot plant during the combustion of CH4 at two different temperatures, i.e., 800°C and 900°C. The outstanding results obtained in terms of both combustion efficiency and mechanical stability have shown that the Cu14γAl_Commercial impregnated oxygen carrier can be selected to upscale CLC technology for gaseous fuels.

Extended Euler–Euler model for the simulation of a 1 MWth chemical–looping pilot plant

In chemical–looping combustion (CLC), the oxygen–carrier circulates between two interconnected fluidized beds. Experiments can provide substantial insight into the hydrodynamic behaviour of CLC, but are limited to the scale of the investigated pilot plant. Furthermore, the availability of detailed experimental information from pilot plants is difficult because of the lack of accessibility, the harsh environment and the costs of measuring devices. Computational fluid dynamics (CFD) offers a significant contribution to the direct measurements. A recent literature review shows that Euler–Euler CFD–based model has not yet been applied to chemical–looping combustion of coal. Therefore, a three–dimensional, transient simulation model for the fuel reactor of CLC has been developed. Besides the calculation of gas–solid flow, the model includes an accurate description of kinetic models for pyrolysis, heterogeneous reactions and related heat and mass transfer processes. The extended Euler–Euler/thermochemical reaction model is validated with measurements obtained from the world's second–largest CLC pilot plant (1 MWth) erected at TU Darmstadt. The numerical results suggest that the pressure drop in the fuel reactor, the exiting flue gas composition as well as the solid fuel conversion can be predicted reasonably. The validated model is of relevance for further feasibility and scale–up studies of CLC.

Design and operation of a 1 MWth chemical looping plant

Chemical looping combustion (CLC) is an efficient combustion technology with inherent separation of CO2. A metal oxide is used to transport oxygen from air to the fuel, thus avoiding direct contact between fuel and air. The CLC process imposes a very low energy penalty and low CO2 capture costs. The largest CLC pilot plant worldwide with a nominal power of 1MWth has been erected at Technische Universität Darmstadt. This paper presents the layout of the 1MWth pilot plant and first operational results using ilmenite and hard coal as fuel. The fuel reactor was fluidized with a mixture of air and steam, so that partial CLC operation was achieved. Conversion of coal was gradually shifted from combustion to gasification by decreasing the air ratio from 1 to 0.55 in the fuel reactor, leading to production of unconverted gases at the fuel reactor exit. The oxygen demand required for fully oxidizing the unconverted gases varied between 12 and 17. Relating the unconverted gases to the remaining 45% of the fuel that have not been oxidized by air, the oxygen demand would be in the range of 26–38%. A system for oxygen injection to fully convert the unconverted gases in the flue gas duct was proven to work successfully.

Modeling of CH4 combustion with NiO/NiAl2O4 in a 10 kWth CLC pilot plant

This paper presents a reactor model for combustion of CH4 with NiO/NiAl2O4 oxygen carriers in the Fuel Reactor of Chemical Looping Combustion (CLC) process. The model hydrodynamics is based on the two phase bubble - emulsion concept of bubbling fluidized beds, validated by residence time distribution tests conducted in a 10kWth equivalent cold flow prototype. The model kinetics is based on the literature and adjusted based on experimental results from a 10kWth CLC pilot plant. Results show the necessity of including both non-direct catalytic and non-catalytic direct reaction of gases on the oxygen carriers. The model shows good agreement with experimental results if some kinetic and hydrodynamic modifications are considered, including impact of gas adsorption by falling particles in the bubble phase on the gas exchange coefficient and thermodynamic limitation of CO and H2 combustion.