Modelling study of two chemical looping reforming reactor configurations: Looping vs. switching

Autothermal Chemical Looping Reforming (CLR) is a promising technology for hydrogen production with integrated CO2 separation. Conventional CLR employs two fluidized beds (fuel and air reactors) with an oxygen carrier circulating between them. In this way, CLR supplies heat to the endothermic reforming reaction while avoiding fuel/nitrogen mixing. This configuration can achieve steady operation and low gas leakages between reactors, but has some drawbacks. The complex interconnected configuration is challenging to scale up, especially under the pressurized conditions required for high process efficiency. Moreover, the external circulation of particles through cyclones and loop seals increases reactor costs and imposes a narrow operating window. These challenges can be circumvented by carrying out the reduction/oxidation reactions in a single bubbling/turbulent fluidized bed alternatively fed with fuel and air. This gas switching (GS) concept has been demonstrated experimentally (1) and thermodynamically (2) for chemical looping combustion (CLC) and can be extended to CLR. The primary drawbacks of the GS concept are the undesired mixing between fuel and nitrogen after the gas feed switch and the need for high temperature valves at the reactor outlet. The objective of this paper is to compare the conventional CLR configuration against the GS configuration using a generic phenomenological model. This model is based on the probabilistic approach (3) which makes it applicable to the fluidization regimes used in both concepts. Steady state (looping) and transient (switching) simulations are completed and results are compared in terms of important variables such as methane conversion and CO2 separation efficiency. REFERENCES 1. Zaabout, A., Cloete, S., Johansen, S. T., Sint Annaland, M. van, Experimental Demonstration of a Novel Gas Switching Combustion Reactor for Power Production with Integrated CO2 Capture. Industrial & Engineering Chemistry Research, 2013. 52(39): p. 14241-14250. 2. Cloete, S., Romano, M. C., Chiesa, P., Lozza, G., Amini, S., Integration of a Gas Switching Combustion (GSC) system in integrated gasification combined cycles. International Journal of Greenhouse Gas Control, 2015. 42: p. 340-356. 3. Abba, I. A., Grace, J. R., Bi, H. T., Spanning the flow regimes: Generic fluidized-bed reactor model. AIChE Journal, 2003. 49(7): p. 1838-1848

Design optimisation of a bench-scale chemical-looping reactor by means of flow modelling

Zsfassung in dt. SpracheIn a circulating fluidised bed (CFB) system consisting of two separate reactors, an oxygen carrier in the form of a metal oxide is transported from the air reactor (AR) to the fuel reactor (FR). The metal oxide is reduced by the fuel and then transported to the AR where it is re-oxidised by the air. The main advantage of these technologies is that the combustion gas is kept separated from the rest of the flue gas, and so the CO2 [CO tief 2] can be captured easily.This thesis represents the experimental work carried out for the further development of a compact laboratory scale reactor system for CLC and CLR. Based on earlier investigations on an existing cold flow model, different design improvements with the predominating aim to reduce gas leakage between the two reactors were carried out and the fluid dynamics of two similar reactor concepts were studied experimentally. The solid circulation rate and the gas leakage were investigated against variation of the amount of solid inventory, the fluidisation rate of the air reactor and the fluidisation rate of the fuel reactor.Additionally, the effects of geometrical modifications of the downcomer and the slot section were studied and the measured values were compared with the original model.In einem zirkulierenden Wirbelschichtsystem (CFB) bestehend aus zwei separaten Reaktoren, wird ein Metalloxid welches als Sauerstoffträger fungiert vom Luft Reaktor (AR) zum Brennstoff Reaktor (FR) transportiert. Dort wird das Metalloxid durch den zugeführten Brennstoff reduziert und zum AR zurücktransportiert, wo es durch den Luftsauerstoff reoxidiert wird. Der Hauptvorteil dieser Technologien liegt darin, dass das entstehende Rauchgas separat vom restlichen Abgas geführt wird, wodurch das CO2[CO tief 2] leichter abgetrennt werden kann.Die vorliegende Arbeit repräsentiert die experimentelle Arbeit zur Weiterentwicklung eines kompakten Reaktorsystems im Labormaßstab für CLC und CLR. Basierend auf früheren Untersuchungen an einem existierenden Kaltmodell wurden verschiedene konstruktive Verbesserungen mit dem vorherrschenden Ziel der Reduktion der Gasleckagen durchgeführt. Dazu wurde das Strömungsverhalten des Feststoffes in zwei ähnlichen Reaktorkonzepten experimentell untersucht. Die Zirkulationsrate des Feststoffes und die Gasleckage wurden messtechnisch erfasst und gegenüber der Menge des eingesetzten Feststoffes, des Volumenstromes im Luft Reaktor und des Volumenstromes im Brennstoffreaktor variiert.Zusätzlich wurden die Auswirkungen von geometrischen Veränderungen im Bereich des SLOTS und des DOWNCOMERS untersucht und die gemessenen Werte denen des Originalmodells gegenübergestellt.6