Feasibility of fluidized bed reactor systems for pressurized chemical looping combustion of natural gas

Chemical looping combustion (CLC) of natural gas at atmospheric pressure has been successfully demonstrated at technical laboratory scale (120 kWth) since 2009 (1,2). Scale-up studies to 10 MWth have been presented (3,4), but no pilot or demonstration projects have been executed so far. Instead, researchers have recently concentrated their activities on CLC of solid fuels (5). One reason for this is the deployment challenge for natural gas CLC power plants. State-of-the-art gas-turbine combined cycle (GT-CC) plants reach high electric efficiencies up to 60% and are superior to steam cycle concepts even if post combustion CO2 capture is applied. On the other hand, all the early studies on CLC focused on efficiency increase for GT-CC plants through application of CLC (6,7). The present contribution therefore seeks to address this mismatch and to assess the practical potential of natural gas CLC for power production. A principal baseline is the application of atmospheric pressure CLC and steam cycle power generation reaching electric efficiencies up to 45% dependent on steam parameters and plant size. In order to reach higher electric efficiencies, pressurized CLC and gas turbine cycles would need to be implemented. Fluidized bed systems have been proposed for CLC (8) and preferred so far for they combine good heat management and continuous operation. If CLC is used in combination with gas turbines, at increased pressure, the operation of fluidized bed systems is challenging. Process configurations have been compared based on mass- and energy balances and basic design calculations have been carried out based on fluidization engineering methods. It turns out that high gas turbine efficiencies can only be reached if turbine inlet conditions are sufficiently high and relative pressure losses are within reasonable limits. Based on the results of the present work, the efficiencies of CLC based power generation cycles remainsignificantly lower than standard GT-CC efficiencies without CO2 capture. An optimal range of operating conditions can be identified for operation of a pressurized CLC plant with increased efficiency and design considerations for a dual circulating fluidized bed reactor system are reported. Such systems are characterized by solids transport ducts and loop seals of increased dimensions relative to atmospheric pressure systems. Accordingly, also the fluidization gas (steam) demand for loop seals is relatively increased for pressurized systems. The outcome of this work may serve as a general basis for techno-economic evaluation of pressurized CLC systems for power generation. Please click Additional Files below to see the full abstract

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, CaMn0.9Mg0.1O3d, 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

Three-dimensional full loop simulation of solids circulation in an interconnected fluidized bed

-D full loop CFD simulation of solids circulation is conducted in a complicated circulating-fluidized bed, which consists of a riser, a bubbling bed, a cyclone and a loop-seal. The effects of operating gas velocity, particle size and total solids inventory on the solids circulation rate are investigated based on the system pressure balance of an interconnected fluidized bed. CFD results indicate that the gas velocity in the riser plays a dominant role in controlling the solids circulation rate, whilst the gas velocity in the pot-seal influences in a narrow operating range. The solids circulation rate is strongly influenced by particle size and total solids inventory, but becomes insensitive to the operating conditions in the bubbling bed when the gas velocity is higher than the minimum fluidization velocity

Investigation of a bubbling fluidized bed incinerator for low calorific fuels

Abweichender Titel laut Übersetzung der Verfasserin/des VerfassersZsfassung in engl. SpracheDurch die Verbrennung von Klärschlamm kann die darin gespeicherte Energie wiedergewonnen und das Volumen der zu lagernden Reststoffe minimiert werden. Durch die Robustheit des Prozesses ist die Verbrennung in stationären Wirbelschichten dafür sehr gut geeignet. Die Verbrennungseigenschaften und die Zusammensetzung des Klärschlamms unterscheiden sich allerdings stark von konventionellen festen Brennstoffen wie z.B. Kohle. Der hohe Wasser- und Aschegehalt des Schlamms erfordert die Zufeuerung von Stützbrennstoffen. Des Weiteren ist der Anteil der flüchtigen Bestandteile (bis zu 80%) viel höher als bei Kohle.Bei der Verbrennung in Wirbelschichten wird der Klärschlamm nicht zur Gänze im Wirbelbett umgesetzt. Ein Teil der flüchtigen Bestandteile verbrennt erst im darüberliegenden Freiraum. Ziel der Arbeit war es, mit Hilfe von Versuchen an einer Industrieanlage jene Mechanismen zu bestimmen, die den Ort der Freisetzung und den der Verbrennung der flüchtigen Bestandteile beeinflussen, und zu bestimmen, wie diese Parameter verändert werden müssen, damit der Anteil der Wärmefreisetzung im Wirbelbett vergrößert wird. Das Hauptaugenmerk wurde dabei vor allem auf die Querschnittsbelastung sowie auf die Fluidisierungsgeschwindigkeit gelegt. Die Versuchsdaten wurden mit einem Modell, basierend auf Massen- und Energiebilanzen, ausgewertet. Der Reaktor wurde dazu in zwei Bilanzzonen (Wirbelbett und Freiraum) unterteilt.Die Versuche haben gezeigt, dass die Fluidisierungsgeschwindigkeit signifikant ist und sich eine Erhöhung positiv auf die Wärmefreisetzung im Wirbelbett auswirkt. Mit den gewonnenen Erkenntnissen wurde das Bilanzierungsmodell um eine Parametergleichung zur Vorhersage des Brennstoffumsatzes in der Bettzone erweitert. Simulationen mit dem erweiterten Modell zeigen, dass weitere Maßnahmen wie die alternative Vorwärmung der Verbrennungsluft mit Hochdruckdampf sowie die Anreicherung der Verbrennungsluft mit Reinsauerstoff weiteres Optimierungspotential bergen. Die Erhöhung des Trockensubstanzgehalts im Klärschlamm hat ebenfalls signifikanten Einfluss auf den Stützbrennstoffbedarf.Incineration of sewage sludge is an established technology for recovering its energy content and for reducing the volume for landfilling. Because of its robust character, bubbling fluidized beds are often chosen for this purpose. However, composition and combustion characteristics of sewage sludge are completely different from traditional solid fuels like coal. Due to the high water and ash content, autothermal combustion is not possible and the use of secondary fuels is necessary. Another aspect of the characteristics of sewage sludge is the high content of volatile matter (up to 80%) During the combustion of sewage sludge in a fluidized bed incinerator, only a part of the carbon conversions takes place in the bed region. The rest is converted in the freeboard region above the bed surface. The purpose of this master thesis was to find out which parameters influence this partial heat release phenomenon and how they have to be changed to increase the heat release in the bed region. To identify these parameters, experiments were carried out on an industrial scale plant.The focus in the experiments was on the influences of the specific fuel power (fuel power per bed surface) and the fluidization gas velocity.The results were processed in a steady state mass and energy balance model. In the model, the height of the fluidized bed reactor was divided into two different zones, the bed and the freeboard region.The results of the experiments show, that the fluidization gas velocity has a significant impact on the heat release in the bubbling bed. With the knowledge gained from the experiments, the mass and energy balance model was extended by a parameter equation describing the partial heat release according to several parameters. A simulation based on the parametric model shows that several other measures like alternative air preheating, oxygen enrichment of the combustion air and reduction of the water content of the sewage sludge can bring further improvements.7

Chemical-Looping Coal Combustion – Results from the ACCLAIM project

This work concerns the first 22 months of the 30-month ACCLAIM project. The project has involved both experimental activities in CLC pilots of 1.5 kW, 10 kW and 100 kW, as well as laboratory investigations and studies in a cold-flow model. Furthermore, investigations have been made using modelling with different approaches and with different aims.The main result of the pilot operation is that several low-cost materials should be able to improve gas conversion significantly as compared to previously tested ilmenite. Promising low cost materials include iron and manganese ores. Two manganese ores were evaluated by operation in a 10 kW CLC reactor system. These materials are called Sinfin, and Mangagran. Both materials performed well with respect to gas conversion, and oxygen demand was clearly lower as compared to ilmenite. The production rate of fines suggested an expected lifetime of around 300 h for one of the manganese materials, Sinfin, which is a distinct improvement as compared to the Buriturama ore previously tested in the 10 kW unit.Further, the fate of fuel contaminants like sulphur and nitrogen has been investigated. Models to describe fluidization and to predict conversion have been developed and are validated against operational data. Mathematical modelling and cold-flow modelling show possible ways of increasing process performance by modification of process or reactor design.A 100 kW CLC unit was operated with a mixture of ilmenite and a Brazilian manganese ore called Buritirama, which had been tested in a previous project and had been found to be much more reactive than ilmenite, although concerns had been raised regarding the attrition resistance. The mixture of ilmenite and Buritirama gave significant improvements in gas conversion in comparison to ilmenite

Chemical-Looping Coal Combustion – Results from the ACCLAIM project

This work concerns the first 22 months of the 30-month ACCLAIM project. The project has involved both experimental activities in CLC pilots of 1.5 kW, 10 kW and 100 kW, as well as laboratory investigations and studies in a cold-flow model. Furthermore, investigations have been made using modelling with different approaches and with different aims. The main result of the pilot operation is that several low-cost materials should be able to improve gas conversion significantly as compared to previously tested ilmenite. Promising low cost materials include iron and manganese ores. Two manganese ores were evaluated by operation in a 10 kW CLC reactor system. These materials are called Sinfin, and Mangagran. Both materials performed well with respect to gas conversion, and oxygen demand was clearly lower as compared to ilmenite. The production rate of fines suggested an expected lifetime of around 300 h for one of the manganese materials, Sinfin, which is a distinct improvement as compared to the Buriturama ore previously tested in the 10 kW unit. Further, the fate of fuel contaminants like sulphur and nitrogen has been investigated. Models to describe fluidization and to predict conversion have been developed and are validated against operational data. Mathematical modelling and cold-flow modelling show possible ways of increasing process performance by modification of process or reactor design. A 100 kW CLC unit was operated with a mixture of ilmenite and a Brazilian manganese ore called Buritirama, which had been tested in a previous project and had been found to be much more reactive than ilmenite, although concerns had been raised regarding the attrition resistance. The mixture of ilmenite and Buritirama gave significant improvements in gas conversion in comparison to ilmenite