Nickel−Copper Oxygen Carriers To Reach Zero CO and H₂ Emissions in Chemical-Looping Combustion

Abstract

Ni-based oxygen carriers allow to work at high temperatures (900-1100ºC) in a Chemical-Looping Combustion (CLC) process with full CH 4 conversion, although thermodynamic restrictions result in the presence of CO and H 2 in the gas outlet of the fuel reactor. On the other hand, Cu-based oxygen carriers allow complete fuel combustion to CO 2 and H 2 O, but the operating temperature is limited due to the low melting point of the metallic Cu. The objective of this research was to analyze the behavior of several Ni-Cu oxygen carriers to reduce or avoid CO and H 2 emissions during a CLC process working at high temperatures. Commercial Al 2 O 3 and  Al 2 O 3 were used as support to prepare by dry impregnation different oxygen carriers based on nickel and copper. The reactivity of these oxygen carriers was determined in a thermogravimetric analyzer (TGA). The effect of the mixture of NiO and CuO on the CO and H 2 generation was analyzed in a fixed bed reactor, and the gas product distribution during reduction/oxidation reactions was studied in a batch fluidized bed reactor working with CH 4 as fuel and diluted air for oxidation. The fluidization behavior of the oxygen carriers with respect to the attrition and agglomeration processes was also analyzed during the multi-cycle batch fluidized bed tests.The presence of CuO in the Ni-Cu oxygen carriers allows the full conversion of CH 4 to CO 2 and H 2 O in the batch fluidized bed reactor during the initial part of the reduction time, and this time depended on the CuO content of the oxygen carrier. TGA and X-ray diffraction studies indicated that CuO is used for the reduction reaction before NiO. In addition, it was observed that the presence of NiO stabilized the CuO and allowed to work at 950 ºC with Ni-Cu oxygen carriers with a high metal oxide utilization.

Progress in Chemical-Looping Combustion and Reforming technologies

“…Boudouard reaction. The carbon formation has been systematically observed in batch mode reactors -TGA, fixed bed or fluidized bed-for Ni-based particles because both reactions can be catalyzed by metallic nickel [144][145][146][147][148][149][150][151][152][153][154][155][156][157][158]. Carbon formation was strongly dependent on the availability of oxygen.…”

“…There is evidence that the reforming reactions (48) and (49) can be decisive in CH 4 conversion when Ni-based oxygencarriers are used. For Ni-based oxygen-carriers the unconverted products were H 2 and CO when the temperature was low [160] or the oxygen in the particles was depleted [145][146][147]153,162,172,215]. In some cases, even though methane was fully converted, H 2 and CO can be significantly high for low reactive oxygen-carriers [150,151,157].…”

Fe2O3 on Ce‐, Ca‐, or Mg‐stabilized ZrO2 as oxygen carrier for chemical‐looping combustion using NiO as additive

“…Son and Kim 28 applied NiO and Fe 2 O 3 in different ratios onto bentonite, using NiO to provide high reactivity and Fe 2 O 3 to improve the particle strength. Adánez et al 29 added CuO to a NiO-based oxygen carrier to improve the conversion of CO and H 2 , which is thermodynamically constrained for NiO.…”

Effect of support on reactivity and selectivity of Ni-based oxygen carriers for chemical-looping combustion

“…Ni-based oxygen carriers allow working at high temperatures (900-1100ºC) in a CLC process with full CH 4 conversion, although thermodynamic restrictions result in a small presence of CO and H 2 in the gas outlet of the fuel reactor [7. There are in the literature many works studying Ni-based oxygen carriers [5-20, and in some of these works alumina was used as support [5][6][7][11][12][13][14][15][16][17][18][19][20] that the Ni spinel (NiAl 2 O 4 ) was present when using high calcination temperatures (T> 800ºC) due to the solid-solid reaction of NiO with Al 2 O 3 . This aluminate can react with methane in the reduction reaction of the oxygen carrier, but the thermodynamics of this reaction show a low methane conversion to CO 2 +H 2 O at temperatures typical for the CLC process (gas conversion<95%, T>800ºC).…”