Evaluating the Effect of Inert Supports and Alkali Sodium on the Performance of Red Mud Oxygen Carrier in Chemical Looping Combustion

Abstract

Chemical looping combustion (CLC) is an advanced technology with inherent CO 2 capture in which a solid oxygen carrier circulates between an air reactor and a fuel reactor. For coal-fueled CLC, the existence of solid impurities requires the oxygen carrier not only to have good reactivity but also to be contaminant-resistant, low-cost, and readily available. Therefore, the development of cost-effective and well-performing oxygen carriers is very meaningful for the coal-fueled CLC process. Natural red mud, a byproduct from the aluminum industry, was found to function well as an oxygen carrier and has also been found to have in situ coal catalytic gasification behavior. A thorough study on the longterm cyclic performance of red mud with coal char in a fluidized reactor was conducted in this work. For the purpose of comprehensively understanding the functions of inert supports as well as the sodium content in red mud, the effect of various inert oxides (Al 2 O 3 , SiO 2 , TiO 2 , and CaO) and the addition of sodium was evaluated. It has been proven that inert supports, Al 2 O 3 , SiO 2 , and TiO 2 , have a positive effect on both the reduction and oxidation reactivity of iron-based oxygen carriers by developing a porous structure in the particle. Al 2 O 3 and SiO 2 show the ability to stabilize the reactivity of iron oxide with a gaseous reductant (CO), even under fluidized conditions. Both Al 2 O 3 and TiO 2 can assist in maintaining the mechanical strength of the oxygen carrier after many cycles in a fluidized-bed reactor. The addition of sodium (Na) to red mud does not exhibit much effect on the reactivity of OC with CO as the fuel. However, it can significantly enhance the char gasification rate due to its catalytic function. Additionally, interaction between the active iron oxide and inert supports or sodium in the form of red mud at high temperatures leads to the formation of spinel phases. The growth of spinel phases results in the reduction of the oxygen carrying capacity. However, it helps fix sodium as a relatively stable chemical compound (NaAlSiO 4 or NaFe 0.25 Al 0.75 O 2 ). Both inert supports and sodium in natural red mud play critical roles in the performance of red mud as an oxygen carrier from either physical or chemical aspects.

Bauxite waste with low Fe 2 O 3 and high Na concentration as a promising oxygen carrier in chemical looping combustion

“…Figure displays SEM images of OC #3 before and after reaction testing. The fresh OC (Figure 5A,B) was porous on the surface and covered with small angular particles; this morphology is similar to that observed in OCs produced from other red mud sources . However, the particle sizes of OC #3 were smaller than those in synthetic Fe 2 O 3 ‐based OC with Al 2 O 3 .…”

“…This process could be accelerated and promoted at a higher calcination temperature. In addition, the crystal phase transition occurs under high temperatures (ie, γ‐Al 2 O 3 transforms to α‐Al 2 O 3 which completes at >1050°C), and the formation of aluminum spinel compounds (ie, NaAlSiO 4 shown in the XRD results) increases the mechanical strength and stability of red mud OCs …”

“…The relative intensities of the NaAlSiO 4 peaks show subtle change with respect to the Fe 2 O 3 peaks after reaction testing, but other significant changes were not observed. Hence, the crystalline phases of the red mud were not altered to any large degree during reaction testing; in addition, NaAlSiO 4 has been considered to be a stable species during conditions of OC reaction testing …”

“…One main concern is the feasibility of using RM with low Fe 2 O 3 content as an effective OC. Another concern is figuring out the stability of Na in such a high Na content condition, which is not included in the existing investigations . Moreover, this study on the utilization of RM wastes in CLC may help huge amounts of indigenous Chinese RM find a way out and play a positive role in environmental protection.…”

“…They also proposed an RM OC to serve as a multi-functional OC, which transported oxygen and heat for solid fuel gasification, while its reduced form was used as a catalyst for syngas reforming for auto-thermal solid-fueled chemical looping gasification (SF-CLG) technologies. 32 Bao et al 33 evaluated the functions of inert supports (Al 2 O 3 , SiO 2 , TiO 2 , and CaO) as well as the sodium content in RM and proved that inert supports have a positive effect on the reactivity by developing a porous structure in the OC particle. Ortiz et al 34 analyzed the performance of an RM OC with respect to gas combustion using a simulated PSA off-gas, methane, and syngas and found that RM OC had high oxygen transport capacity (2.4 wt.%) and reactivity for H 2 , CO, and O 2 but a noticeably lower reactivity for CH 4 at 950 C. Mendiara [35][36][37] and Abad 38 et al compared five potential OCs (ie, ilmenite, bauxite waste, hematite, copper ferrite, and synthetic material) for the iG-CLC with coal and found that bauxite waste and hematite minerals presented the best performance for iG-CLC based on the results obtained.…”