Abstract
In the present work, the steady-state behavior of a Chemical Looping Combustion process of interconnected fluidized bed reactors is simulated. The simulations have been carried out in two different scales, 50 kWth and 100 MWth . Attrition model derived from small scale laboratory experiments has been employed for the prediction of the process behavior in terms of attrition and Oxygen Carrier loss. Information on Oxygen Carrier characteristics and reaction kinetics were taken from literature. Realistic circulation mass flows of Oxygen Carrier particles are obtained and Oxygen Carrier losses are quantified. The large scale process looses significantly more Oxygen Carrier than the small scale process based on the same amount of thermal energy produced. Incomplete conversion in the air reactor could be identified as a critical point. Another issue is the fuel gas bypassing the Oxygen Carrier particles through bubbles in the large scale process which leads to lowered fuel conversions. The simulations indicate that a similar performance of a pilot scale and a large scale process is not guaranteed due to the scale-up effect on fluid dynamics. Furthermore, the simulations allow an assessment of the influence of the quality of the solids recovery system on the Oxygen Carrier loss. The distribution of the losses between possible origins is investigated and different changes in the solids recovery system are discussed regarding their potential to decrease the Oxygen Carrier loss. For example, the addition of a second-stage cyclone after the air reactor of the large scale process reduces the Oxygen Carrier loss significantly.
Highlights
Chemical Looping Combustion (CLC) is a carbon capture and storage technology that can be employed to obtain a stream of almost pure CO2 at the end of a power plant fired with fossil fuels
The particle sizes in this study lie in between those of Circulating Fluidized Bed (CFB) combustors and FCC units. To reach such values during the simulations the air reactor had to be limited in height. This might lead to incomplete conversion of the Oxygen Carrier (OC) in the air reactor and this could be observed in the simulations
The solids are almost completely converted in the fuel reactor (Xs = 99.8% in large scale / Xs = 94.7% in small scale) while after the air reactor 52.3% of the solids are still in reduced state (44.2% for the small scale)
Summary
Chemical Looping Combustion (CLC) is a carbon capture and storage technology that can be employed to obtain a stream of almost pure CO2 at the end of a power plant fired with fossil fuels. A major concern in Chemical Looping Combustion is the mechanic stability of the Oxygen Carrier particles since the particles will be repeatedly subjected to stresses in a process of coupled fluidized beds. The mechanical stability of the Oxygen Carrier particles is a key to a successful CLC process These demands towards Oxygen Carrier particles favor the use of synthetic OC particles. In either case it is desirable to have attrition resistant Oxygen Carrier particles and the loss through the gas-solids separation devices (e.g. cyclones) have to be minimized. The aim of this investigation is to study the Chemical Looping Combustion process from the particle technology point of view. Simulations have been carried out in two scales: pilot scale and an up-scale to a 100 MWth process
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