Identification of operational regions in the Chemical-Looping with Oxygen Uncoupling (CLOU) process with a Cu-based oxygen carrier

Abstract

Chemical-Looping with Oxygen Uncoupling (CLOU) is an alternative chemical-looping process for the combustion of solid fuels with inherent CO 2 capture. The CLOU process demands a material as oxygen carrier with the ability to decompose with O 2 release at suitable temperatures for solid fuel combustion, e.g. copper oxide. This article presents an experimental method to determine the maximum oxygen generation rate of an oxygen carrier as well as to determine the minimum solid inventory that must be used in the fuel reactor. The method here proposed can be used as basis for comparison of the use of different oxygen carriers or type of coals. In this work, the combustion of coal by using a promising Cu-based oxygen carrier prepared by the spray drying method was tested. The oxygen carrier (Cu60MgAl) was composed of 60 wt.% CuO and MgAl 2 O 4 was used as supporting material.Experiments were carried out in a batch fluidized-bed reactor at temperatures ranging from 900 to 980 ºC. Three different regions were identified depending on the oxygen carrier to coal mass ratio. For oxygen carrier to coal ratios higher than 50 (Region I), coal was fully converted to CO 2 and H 2 O. In addition, an excess of oxygen was present in the flue gases, which was close to the equilibrium concentration. When this ratio was in the range 50-25 (Region II), the concentration of oxygen was decreasing whereas some CO was observed as the only unconverted gas. Further decrease in the oxygen carrier to coal ratio below 25 (Region III) caused the depletion of oxygen in the exhaust gases but CO remained as the only Con formato: Inglés (Estados Unidos) 2 unconverted gas. CH 4 or H 2 were never detected at the reactor outlet in any case and agglomeration problems were never observed. These regions were related to the solids inventory in the fuel reactor by the rate of oxygen generation calculated in every case. A maximum rate of oxygen generation for the oxygen carrier was determined as kg O 2 /s per kg of oxygen carrier, which increased with the temperature from 2.1·10 -3 at 930 ºC to 2.8·10 -3 at 980 ºC. From these values, the estimated solids inventory in the fuel reactor was changed from 39 at 930 ºC to 29 kg/MW th at 980 ºC. The results obtained in this work showed that in the CLOU process it is possible to reach full conversion of the solid fuel with very low solids inventory and avoiding the oxygen polishing step.

Kinetic analysis of a Cu-based oxygen carrier: Relevance of temperature and oxygen partial pressure on reduction and oxidation reactions rates in Chemical Looping with Oxygen Uncoupling (CLOU)

“…11) is lower than the solids inventory needed to reach a CO 2 capture higher than 95% without a carbon stripper (»600 kg/MW th ). This fact agree with previous results obtained from the analysis of the oxygen carrier reactivity [25] and results obtained in a CLOU unit [8,10]. It can be concluded that the kinetic parameters determined together with the simulation tool developed in this work, configure a useful instrument for the design and optimization of CLOU process.…”

“…Thus, the kinetic model here proposed properly prediced the reaction rates when the O 2 concentrations was close to the equilibrium, where previous literature models had failed [15]. This condition is required in the fuel reactor of a CLOU unit to avoid unconverted compounds exiting the fuel reactor [8,25].…”

“…Preliminary results showed that this material has adequate values of reactivity and oxygen transport capacity in fluidized-bed conditions [9,25]. High combustion rates with complete combustion to CO 2 and H 2 O were obtained with this material using a low solids inventory in the fuel reactor of a CLOU unit burning different types of coal and biomass [8,10].…”

“…Table 2 shows the main properties of this material, which were presented in previous works [2,[8][9][10][11]25].…”

“…This condition is reached when the oxygen concentration in Eq. (20) is considered to be zero, corresponding to the asymptotic limit in which all the O 2 released is consumed by the fuel and there is not an excess of O 2 in gases [25].…”