Investigation of Different NiO/NiAl₂O₄ Particles as Oxygen Carriers for Chemical-Looping Combustion

Abstract

Chemical-looping combustion is a combustion technology, where CO 2 is separated from the rest of the flue gases without an energy-consuming gas-separation process. The combustion is performed in two reactors, with metal oxide particles circulating between them, transferring oxygen from the combustion air to the fuel. Particles of NiO, supported by NiAl 2 O 4 , have been reported earlier as excellent oxygen carriers for this process. The aim of the present investigation is to verify that commercially available raw materials can be used to produce oxygen carrier particles with properties suitable for the technology. A total of 36 oxygen carrier materials were prepared by freeze granulation and investigated with respect to parameters important for chemicallooping combustion. The reactivity of the particles was investigated in a small fluidized bed reactor by exposing them cyclically to CH 4 and 5% O 2 in N 2 , at 950 °C. Although defluidization occasionally occurred for some materials, it was clear that the gas conversion and the reactivity were generally high. An addition of Ca(OH) 2 to the oxygen carriers increased the strength and thus reduces the risk of fragmentation and attrition in a chemical-looping combustion device. An addition of MgO enhanced the fuel conversion early in reduction, which seemed to be restricted because of the limited amounts of metallic Ni. An increased sintering temperature generally resulted in harder particles of higher density; however, the risk of defluidization seemed to increase for such particles. Carbon formation was only detected when the oxygen carriers were highly reduced and the fuel conversion was incomplete, i.e., at conditions not expected in a real chemical-looping combustion device. Two of the investigated particles, NOV1T1400 and NOV2T1400, displayed a combination of high reactivity and strength as well as excellent fluidization behavior and should be feasible for use in a chemical-looping combustion unit.

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.…”

“…As a result, the product selectivity for methane conversion changes during the oxygen-carrier reduction. During the early seconds of NiO reduction the product gas showed unconverted CH 4 [149,157,161,162,181,215], which disappeared as the reduction reaction progressed.…”

Hybrid density functional theory description of N- and C-doping of NiO

“…On the other hand, investigations of NiO/NiAl 2 O 4 as oxygen carriers for chemical-looping combustion to fabricate high effective gaseous fuels have been performed in several papers [16,17].…”

Reduction and oxidation kinetics of nickel-based oxygen-carriers for chemical-looping combustion and chemical-looping reforming

“…and   1 dt dX NiO (22) where  is the time for complete conversion of the NiO present in the particle. The  values can be obtained from the slope of the curve X NiO versus time.…”