Experimental Investigation of Fe–Ni–Al Oxygen Carrier Derived from Hydrotalcite-like Precursors for the Chemical Looping Gasification of Biomass Char

Abstract

Chemical looping gasification (CLG) of biomass uses the lattice oxygen of oxygen carriers to convert biomass into syngas with a low tar content, high heating value, and low price. It is of key importance to exploit well-dispersed and thermally stable oxygen carriers for the CLG process. In the current work, a series of oxygen carriers with varied Fe and Ni molar ratios were synthesized from hydrotalcite compound precursors (HTlcs), which made the metallic elements mix at the molecular level. Consequently, highly dispersed complex metal oxygen carriers can be achieved after precursor calcinations. CLG of biomass char was carried out in TGA and a fixed bed reactor accompanied by various physical and chemical analyses for the fresh and used oxygen carriers. The result manifested the HTlcs crystalline form, which was formed in the precursors and produced the Fe0.99Ni0.6Al1.1O4 compound after calcination, suggesting that a high degree dispersion of the multimetal oxygen carrier was synthesized. The main H2 uptake and CLG reactivity of the oxygen carriers were related to its higher metal dispersion and synergistic effect between Fe and Ni. Accordingly, there was an optimum Fe/Ni ratio of 4:1 in oxygen carriers at which the oxygen carrier can achieve better CLG reactivity. Also, the oxygen carrier to biomass char mass ratio of 7:3 provided a maximum weight loss of 35.59% and a largest mass loss rate of 2.46 wt %/min, suggesting higher lattice oxygen releasing efficiency. CO exhibited a higher generating rate in the CLG reactions owing to its higher reaction activation energy with lattice oxygen [O], while H2 was more prone to being consumed. The morphological analysis of fresh and regenerated samples exhibited that the oxygen carrier was reduced to the Fe3Ni2 alloy phase after the CLG process, and the lattice oxygen can be fully recovered in an air atmosphere. Although the BET surface displayed a decreased trend in the regenerated oxygen carriers, serious sintering was not observed in the samples, and the main metallic crystallized phases were still maintained.

Chemical Looping Combustion of Coal in China: Comprehensive Progress, Remaining Challenges, and Potential Opportunities

“…Mixed oxides, which integrate the merits of different components and weaken the disadvantages of single phases, may attain a synergetic effect in reaction and provide better performance in comparison to that of individual metal oxides. With this regard, different mixed oxides, such as Cu–Fe, − Ni–Fe, − Co–Fe, Mn–Fe, , and Ca–Fe, have been synthesized and tested as oxygen carriers in chemical looping processes.…”

Advanced Chemical Looping Materials for CO2 Utilization: A Review

“…Due to its low price and non-toxicity, Fe is advantageous from an economic and environmental point of view compared to Ni. Aiming to combine the complementary advantages of these two constituent metals, many efforts have been made to develop bimetallic Fe-Ni oxygen carriers via two different main strategies, physical [108,133] and chemical [23,108,109,110,111,112] mixing. Johansson et al [133] reported that the addition of only 3 wt % NiO in the bed of a Fe-based oxygen carrier was sufficient to reach a very high CH 4 conversion in CLC process.…”

Developing Oxygen Carriers for Chemical Looping Biomass Processing: Challenges and Opportunities

“…[ 69–71 ] In specific cases, biochars have been used as the reducing agents in TGA. [ 72 ] As shown in Figure 5, OTCs measured by TGA vary from 1 wt% (e.g., in the case of iron oxide‐based LD slag) to 12 wt% (e.g., in the case of 60 wt% NiO supported on MgAl 2 O 4 ). Under chemical looping operations, other rate limiting factors such as intraparticle mass transfer resistance and product layer diffusional resistance could further reduce the practically available OTC, [ 22,73–75 ] unless the issues above can be adequately addressed through improved OC designs.…”

“…studied Fe‐Al‐Ni OCs using a combined TGA‐XRD approach, which directly established the correlation between the DTG peaks during TPR and the specific phase transformations. [ 72 ] Their results shows that the two DTG peaks could be assigned to the reduction of Fe 0.99 Ni 0.6 Al 1.1 O 4 to Fe 1– x O and the subsequent reduction to metallic Fe.…”