Abstract
Chemical looping combustion is especially competitive for electrical power generation with integrated CO2 capture when it is operated at high temperatures (1000–1200 °C) and high pressures (15 bar or higher). For these demanding conditions, dynamically operated packed bed reactors have been proposed, providing a good alternative to fluidized bed technology. This work addresses the importance of including the formation and reduction kinetics of spinel compounds to proper predict the packed bed reactor performance by validating a pseudo-homogeneous packed-bed reactor model to describe the redox kinetics of a CuO/Al2O3 oxygen carrier with experiments in a lab-scale packed bed reactor setup. A grain model describing the reaction kinetics of all solid species, including both spinel compounds (CuAl2O4 and CuAlO2), was included in a particle model and used to develop correlations for the effectiveness factor as a function of the particle conversion in order to account for internal solids concentration profiles and mass transfer limitations. The particle effectiveness factors were subsequently included in the source terms of the component mass balances of the reactor model accounting for all the reactions of the spinel compounds. Cyclic experiments (oxidation with air and reduction with a H2-N2 mixture) have been carried out in a lab-scale packed bed reactor with a 12.5 wt% CuO/Al2O3 oxygen carrier at different temperatures ranging from 600 to 1000 °C. The experimental results are well described by the packed bed reactor model, only when including the developed particle effectiveness factors to fully account for the kinetics of the formation and reduction of the spinel compounds. The results confirm that it is neccesary to include a detailed description of the redox kinetics at the particle level to be able to accurately estimate the breakthrough time, cycle time, final amount of Cu present in the bed and the temperature rise in the reactor after reduction/oxidation reactions for packed-bed chemical looping combustion with a CuO/Al2O3 oxygen carrier.
Highlights
To reduce anthropogenic CO2 emissions and combat climate change Chemical Looping Combustion (CLC) technologies have been proposed [1,2]
A one-dimensional, pseudo-homogeneous packed-bed reactor model with effectiveness factors for the gas-solid reactions developed with a detailed particle model embedding a pseudo-homogeneous grain model was developed and validated with experiments carried out in a lab-scale packed-bed reactor setup for chemical looping combustion using CuO/ Al2O3 as oxygen carrier
The importance of accurate gas-solid kinetics including the spinel compounds was demonstrated by comparison with a conventional simplified SCM ignoring spinel compounds, where the simulations with the SCM resulted in a mismatch of the predicted breakthrough time and the amount of Cu present in the bed after the reduction, whereas the model that fully accounts for the spinel kinetics quite adequately describes the breakthrough curves measured in the lab-scale packed-bed reactor
Summary
To reduce anthropogenic CO2 emissions and combat climate change Chemical Looping Combustion (CLC) technologies have been proposed [1,2]. Interconnected fluidized bed reactors were proposed for CLC [6,7,8,9,10,11] In this reactor concept, the particles are transported between an air reactor (typically a riser), where hot air is produced via the oxidation of the particles, and a regeneration unit (typically a bubbling fluidized bed), where the particles are reduced with methane or syngas while producing a concentrated CO2 exhaust stream. An additional energy input is required to transport the particles, and a cyclone is required to separate the particles from the hot air stream This particle separation is difficult considering the required, extremely harsh process conditions (high pressure and very high temperature) and the fact that even fines (resulting from inevitable particle attrition in the fluidized bed reactors) need to be removed to protect the downstream gas turbine
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